PO Box 142181, Gainesville, FL 32614, USA
Correspondence to: Jay Pravda, MD, PO Box 142181,
Gainesville, FL 32614, USA. firstname.lastname@example.org
To propose a new pathogenesis called Radical Induction to
explain the genesis and progression of ulcerative colitis (UC). UC
is an inflammatory bowel disease. Colonic inflammation in UC is
mediated by a buildup of white blood cells (WBCs) within the colonic
mucosal lining; however, to date there is no answer for why WBCs
initially enter the colonic mucosa to begin with. A new pathogenesis
Induction Theory" is proposed to explain this and states that
excess un-neutralized hydrogen peroxide, produced within colonic
epithelial cells as a result of aberrant cellular metabolism,
diffuses through cell membranes to the extracellular space where it
is converted to the highly damaging hydroxyl radical resulting in
oxidative damage to structures comprising the colonic epithelial
barrier. Once damaged, the barrier is unable to exclude highly
immunogenic fecal bacterial antigens from invading the normally
sterile submucosa. This antigenic exposure provokes an initial
immune response of WBC infiltration into the colonic mucosa. Once
present in the mucosa, WBCs are stimulated to secrete toxins by
direct exposure to fecal bacteria leading to mucosal ulceration and
bloody diarrhea characteristic of this disease.
The WJG Press and Elsevier Inc. All rights reserved.
Key words: Ulcerative colitis; Radical induction; Oxidative
stress; Hydrogen peroxide
Pravda J. Radical induction theory of ulcerative colitis. World J
Ulcerative colitis (UC) is an inflammatory bowel disease
characterized by infiltration of white blood cells (WBCs) into the
colonic mucosa resulting in tissue destruction and recurrent bouts
of bloody diarrhea. The initial inflammatory reaction begins in the
rectal mucosa in over 95% of cases and may extend in a contiguous
fashion to involve the whole colon.
Often, young individuals in the prime of life are struck with this
disease whose course can be severely debilitating, unpredictable and
unrelenting. Treatment modalities are few and unsatisfactory with
total colectomy being the only option for individuals unresponsive
to the limited medical therapy currently available.
Since the history of medically treated UC is
characterized by lifelong repeated episodes of this disease, it
appears that no currently available medical therapeutic modality is
capable of addressing the fundamental disorder present and therefore
unable to alter the natural history of this condition.
Several immunologically oriented hypotheses
regarding the etiology and pathogenesis of UC have been advanced.
All remain ill defined, fall short of promoting a clear
understanding of this illness and lack predictive value for
therapeutic development. These postulates provide no basis for
individual risk factor profiling and are unable to explain the known
histological, biochemical, immunological and epidemiological
abnormalities associated with this disease.
WBCs found within the colonic mucosal lining
mediate the tissue injury in active UC; however, to date there is no
satisfactory answer for why these WBCs accumulate within the colonic
mucosa to begin with. It is the purpose of this paper to propose a
new evidenced based pathogenesis termed "Radical
Induction Theory" to explain the genesis of this initial influx
of WBCs which leads to UC.
The Radical Induction Theory of UC states that
excess un-neutralized hydrogen peroxide, produced within colonic
epithelial cells as a consequence of aberrant cellular metabolism,
diffuses through cell membranes to the extracellular space where it
is converted to the highly damaging hydroxyl radical, which is
capable of causing extensive oxidative damage to structures
responsible for maintaining the colonic epithelial barrier function.
Once damaged, the epithelial barrier is no longer
able to exclude highly immunogenic fecal bacterial antigens from
invading the normally sterile submucosal tissue. This antigenic
exposure provokes an initial immune response consisting of
WBC infiltration into the colonic mucosal surface in an attempt to "Plug
the hole" and prevent systemic bacterial invasion and fatal
sepsis. Once present within the mucosa, WBCs are stimulated to
secrete toxic substances by direct exposure to high concentrations
of fecal bacteria leading to mucosal ulceration and bloody diarrhea
characteristic of this disease.
It is perhaps among the greatest physiological wonders of evolution
that the most highly evolved immune system ever engendered can
remain unperturbed while surrounding the highest concentration of
bacteria on the planet, separated only by a tenuous sheet of tissue
one cell thick.
This unlikely truce describes the living
conditions of the normal human colon where the luminal concentration
of potentially pathogenic bacteria is estimated to be 1012
(one trillion) colony-forming units (viable bacterial cells) per
gram of colonic contents.
Previous attempts at creating an animal model
of UC have met with limited success. No current animal model is
and experimental attempts to create an animal model of human UC
using rectal instillation of toxic chemicals are inherently limited
in their ability to faithfully reproduce the disease due to complex
psychological, physiological, genetic, environmental and
immunological interactions that antecede and contribute to the
pathogenesis of this condition in humans.
What is clear from animal studies is that the
integrity of the colonic epithelial surface barrier is paramount in
maintaining immune quiescence within the colonic tissues and
preventing the colonic immune system from mounting an immune
response to the high concentration of bacterial antigen that is
poised to invade the normally sterile sub-epithelial environment.
Cellular mechanisms involved in maintaining the integrity of the
colonic surface barrier function may therefore be compromised early
on in the pathogenesis of UC. Dysfunction of a vital process
required to maintain mucosal integrity must therefore be an early
and necessary part of a sequential series of events ultimately
leading to deterioration of epithelial barrier function with
subsequent mucosal immune activation secondary to antigenic
penetration into the normally sterile colonic sub-mucosal tissues.
In other words, the additive effects of abnormal
cellular stressors focused on a common biochemical pathway are
acting in concert to disrupt an intracellular biochemical process
that contributes a required function necessary for maintaining
colonic surface barrier integrity.
The high incidence (over 50%) of spontaneous
improvement and relapse seen in UC
suggests a reversible disruption and the possibility of a
self-replenishing depletion syndrome affecting a crucial element
required for mucosal integrity.
In 1951 Science published an article entitled "A
New Concept of the Pathogenesis
of Ulcerative Colitis".
In this seminal publication, the authors demonstrated that patients
with UC have either completely absent or severely damaged colonic
epithelial basement membranes (BMs). An important observation was
that total destruction of BM was seen in the absence of any mucosal
inflammation (no WBCs present) and in many areas the BM was noted to
out". The authors ascribed an important pathogenetic role to
the BM destruction seen in colonic biopsy samples of their patients
The first real clue came from initial
observations that BM was destroyed in areas uninvolved in active
It was already known that UC was an inflammatory condition with
infiltration of WBCs (neutrophils) into the mucosal lining of the
colon and that these WBCs were capable of causing inflammation and
tissue damage. However, the presence of damaged BM in tissue areas
without inflammation suggested that a prior process anteceded the
The presence of "thinned
out" BM suggested that a gradual, non-immune mediated, erosion
had taken place. The authors also noted sections of epithelium which
away from seemingly intact BM. This suggested that the epithelial
cells themselves played a role in the process that led to the BM
alterations and their own (epithelial cell) detachment from the BM.
It also suggested that this process began in the interface between
the BM and epithelium. Since BM regeneration was noted after
successful treatment, it appeared that the process could be halted
and reversed. What this process was and what effector molecules, if
any, were involved could not be determined from histological studies
alone, and in 1951 there was no animal model of UC to experiment
with. However, there was a human model of this disease readily
available for study which provides a second clue.
The second clue came from a series of case
reports from dedicated clinicians over the span of several decades.
For many years during the 20th
century, hydrogen peroxide (H2O2)
enemas were routinely employed and recommended by physicians for the
evacuation of fecal impactions. However, in the 1930s reports began
to surface regarding the development of rectal bleeding and colitis
subsequent to the use of hydrogen peroxide enemas.
A fatal case of UC subsequent to hydrogen peroxide enema was first
recorded in 1948.
In 1951, Pumphery reports severe ulcerative proctosigmoiditis
following hydrogen peroxide enemas in two patients.
In 1960 Sheenan and Brynjolfsson
were able to reproduce acute and chronic UC by rectal injection of
rats with a 3% solution of H2O2.
This was the first animal model of UC and it mirrored the effects of
human UC. Microscopic examination of killed rats revealed colonic
mucosal ulceration and WBC (neutrophilic) infiltration, which was "sharply
delineated from adjacent normal mucosa". The mucosal
inflammation extended proximally over time.
It was noted that, in surviving rats, most of the
mucosal ulcerations were healed by 10 wk with the exception of some
ulcers which "were
located almost always in the left colon a few centimeters above the
anus". These three observations (sharp inflammatory tissue
delineation from normal tissue, rectal inflammatory persistence and
contiguous proximal extension) are also characteristic of human UC.
Despite the demonstrated adverse effects of
hydrogen peroxide it continued to be used as an enema and, in 1981,
Meyer reported three cases of acute UC after administration of
hydrogen peroxide enema and stated that "acute
ulcerative colitis appears to be a fairly predictable occurrence
after hydrogen peroxide enemas".
Even small amounts of hydrogen peroxide could cause human UC as was
reported by Bilotta and Waye in 1989
after experiencing an epidemic of hydrogen peroxide-induced colitis
in the GI endoscopy unit at their institution due to the inadvertent
instillation of hydrogen peroxide during colonoscopy. These results
indicate that, when in contact with the colonic mucosa, small
amounts of hydrogen peroxide can, in predisposed individuals,
produce a clinical and histological picture, which is
indistinguishable from spontaneously occurring primary idiopathic
The data presented thus far reveals that
epithelial cell "sloughing"
(detachment) from BM and BM erosion (in non-inflamed areas only
populated by colonic epithelial cells) are fairly characteristic
histological findings in UC suggesting that epithelial cells play a
role both in their own detachment and in BM erosion. Additionally,
clinical reports and experimental results reveal that UC is a "fairly
predictable" occurrence when hydrogen peroxide comes in contact
with rectal epithelium.
The third and final clue tying histological
observations of Levine
and Kirsner with the adverse clinical effects of hydrogen peroxide
enemas came by way of biochemical studies undertaken by
investigators in the early 1970s, which demonstrated that mammalian
cells are constantly generating hydrogen peroxide as a byproduct of
normal aerobic metabolism.
WHAT IS HYDROGEN PEROXIDE?
Hydrogen peroxide is a colorless, highly damaging oxidizing agent, a
powerful bleaching agent; used for wastewater treatment, and as an
oxidant in rocket fuels. H2O2
has a ubiquitous presence in cells and is continuously being
generated by the plasma membrane, cytosol and several different
sub-cellular organelles including peroxisomes, endoplasmic reticulum,
nucleus and by almost 100 enzyme systems[12-15].
Under normal conditions, 90% of H2O2
is generated as a toxic by-product of
mitochondrial electron transport chain (ETC) respiratory activity[14,16].
The mitochondrial ETC is a series of proteins that channel the flow
of electrons derived from ingested food into the synthesis of
adenosine triphosphate (ATP), which is used as a chemical energy
source for all energy requiring cellular processes.
The transfer of electrons through the ETC,
however, is not perfect. Up to 5% of electrons do not make it all
the way through the chain and fail to combine with oxygen to produce
electrons combine directly with molecular oxygen in the immediate
vicinity, instead of the next carrier in the chain, to form the
It is estimated that under normal conditions 2% of available oxygen
is converted to superoxide by ETC "leakage".
Superoxide spontaneously dismutates to H2O2
or undergoes enzymatic conversion to H2O2
at the site of production within mitochondria by the enzyme
superoxide dismutase (SOD) (EC 22.214.171.124)[12,14,17].
Complex I and III, of the ETC, are the source of electron leakage
leading to the eventual intracellular generation of hydrogen
is long lived and highly biomembrane
permeable and must be immediately neutralized at the site of
production to prevent diffusion throughout the cell or to the
Sophisticated enzyme systems exist expressly for this purpose. These
neutralizing anti-oxidant enzymes are catalase (E.C. 126.96.36.199) and
glutathione peroxidase (GPx, E.C. 188.8.131.52) with GPx responsible for
91% of H2O2
If allowed to accumulate H2O2
will diffuse from its site of production and generate hydroxyl
which is the most damaging and chemically reactive radical formed in
cellular metabolism. Hydroxyl will indiscriminately destroy
everything it encounters[17,25,26].
The hydroxyl radical is principally responsible for the cytotoxic
effects of oxygen in animals.
The iron catalyzed Haber-Weiss reaction (O2-+Fe+3→O2+Fe+2),
followed by (Fe+2+H2O2→Fe+3+HO+HO),
is considered to be the major mechanism by which the highly reactive
hydroxyl radical is generated.
Molecules interacting with hydroxyl radicals sustain severe damage
to the extent that the hydroxyl radical is able to crack
polysaccharides; nucleic acids and proteins.
is also able to peroxidize and destroy lipids that make up cell
Detoxification of hydrogen peroxide, the immediate precursor to
hydroxyl radical, therefore is crucial to normal cellular function
MECHANISM OF DISEASE
The above data suggests a link between intracellular hydrogen
peroxide production and UC. Since exogenously applied hydrogen
peroxide can cause UC in humans, and colonic epithelial cells
produce hydrogen peroxide, is it reasonable to speculate that excess
hydrogen peroxide generated by colonic epithelial cells may be
causing UC? How this may come about is suggested by the histological
work of Levine and Kirsner (above) which hints of an extracellular
process in the epithelial cell/BM interface causing epithelial cell
detachment by erosion of subjacent anchoring BM and destruction of
apical intercellular tight junctions (TJs). Together, these two bits
of data suggest that colonic epithelial cells produce excess
hydrogen peroxide, which exits the cell causing oxidative damage to
BMs and TJs, which are structures responsible for physical
epithelial integrity and barrier function. The resulting destruction
of the epithelial barrier allows luminal bacterial antigens to enter
the normally sterile submucosal layers of the colonic wall itself
initiating an immune response leading to UC.
For hydrogen peroxide to be considered a primary etiologic agent in
the pathogenesis of UC, a logical pathogenetic chain of events
should be demonstrable starting from the generation of H2O2
within sub-cellular organelles to the eventual development of UC. H2O2
should possess distinct physicochemical attributes that render it
uniquely qualified, to the exclusion of other agents, to induce UC.
In effect it must be demonstrated that H2O2
can be produced in excess in colonic epithelial cells and this leads
to UC. H2O2
must also be capable of exiting colonic epithelial cells and be the
source of damage to colonic barrier function structures (BMs and TJs),
whose disintegration is important in the development of UC. Finally,
in order to have clinical relevance it follows that conditions
associated with UC must lead to excessive hydrogen peroxide within
colonic epithelial cells. UC associated
intracellular abnormalities such as impaired beta oxidation and
neoplastic transformation should also be readily explainable. The
following sections address these concerns.
1. Can H2O2
be produced in excess within colonic
epithelial cells and does this cause UC? The answer came by way of
genetic studies of knockout mice. These are mice that are
genetically engineered with a deletion of a certain gene in order to
isolate and study its effects. Knockout mice rendered genetically
devoid of GPx (the main hydrogen peroxide neutralizing enzyme)
spontaneously develop a crypt destructive colitis (mucosal
inflammation - similar to human UC) as early as 11 d of age with
extension to the proximal colon by d 15.
This indicates that, when the biological enzyme system needed to
neutralize hydrogen peroxide is hindered, the resulting increase in
un-neutralized colonic epithelial intracellular hydrogen peroxide
can lead to UC.
2. Can hydrogen peroxide egress from the cell?
This is important since H2O2
would need to exit the cell in order to
cause the severe BM damage seen during histological examination of
affected colonic tissue. It turns out that hydrogen peroxide is
freely and highly permeable through biological membranes
enabling its diffusion out of the cell from any site of excess
production within the cell. H2O2
therefore is capable of reaching both the
extracellular BMs and TJs from any intracellular location. H2O2's
production as a coupled consequence of fundamental cellular
metabolic processes plus its ability to pass through biomembranes
and produce damaging oxygen radicals far from its site of generation
is a unique combination of properties not possessed by any other
3. Can hydrogen peroxide damage BMs and TJs? It
has been reported that extracellular hydrogen peroxide can severely
damage BMs, TJs and colonic epithelial cell membranes by producing
hydroxyl radical via a metal catalyzed Haber-Weiss reaction.
Hydroxyl radical is able to damage proteins in BMs and TJs by
cleavage of peptide bonds, formation of intra- and inter-molecular
cross-linkages and oxidation of amino acids[14,30,31].
The mechanism has been identified as a site-specific metal ion
catalyzed oxidative damage and cleavage of amino acids and peptide
bonds by hydroxyl radical. The in vivo source of all hydroxyl
radical was identified as endogenously generated hydrogen peroxide.
can therefore disintegrate the micro-anatomical GI barrier
structures that maintain epithelial integrity (BMs and TJs).
Hydroxyl radical oxidizes and destroys everything it encounters
resulting in microscopic alterations, which increase mucosal
permeability allowing penetration of luminal proteins and antigens[31,33-38].
Using a well-characterized model of BM, Riedle
evaluated the in vitro effects of hydrogen peroxide induced
changes on interstitial matrix proteins and the consequences for the
integrity of the BM/matrix network.
The authors documented significant disintegration
of matrix structure with 15% of matrix proteins being released into
the incubation medium. This corresponded to seven times that was
seen in control conditions without hydrogen peroxide. Importantly,
extensive oxidative damage of individual amino acid residues (tryptophan)
was noted without any morphological change to the BM/matrix network.
The hydrogen peroxide-derived hydroxyl radical was found to be the
main reactive oxygen species responsible for matrix protein
disintegration. Laminin, a major BM structural protein, was also
released from BM when incubated with low concentrations of hydrogen
Hydrogen peroxide infused into rat renal artery
produced local H2O2-derived
oxygen radicals and subsequent marked glomerular protein leak
suggesting an increased porosity of the glomerular BM secondary to
oxidative damage of its constituent proteins.
In addition to BM damage, hydrogen
peroxide-derived oxygen radicals are also able to disrupt colonic
epithelial TJs. TJs are composed of thin bands of plasma-membrane
proteins that completely encircle the apical (luminal) region of
colonic epithelial cells and are in contact with similar thin bands
on adjacent cells. These intercellular protein junctions fasten
adjacent epithelial cells together forming a sealing gasket, which
prevents the passage of most dissolved molecules and bacterial
antigens from one side of the epithelial sheet to the other.
Since only a single layer of colonic epithelial
cells separates the bacterial laden luminal contents from the
subjacent lamina propria, the epithelial TJ constitutes the major
primary barrier that prevents luminal bacterial antigens from
gaining access to the effector immune cells and vasculature in the
normally sterile lamina propria.
Thus, the intestinal barrier function relies primarily on the
tightness of the epithelial layer to maintain impermeability with
sub-epithelial layers contributing a minor function.
Hydrogen peroxide, at a low concentration of 0.2
mmol/L, was reported to increase in vitro epithelial
monolayer permeability by disrupting paracellular junctional
In experiments to assess the effect of hydrogen peroxide on
intestinal permeability, Grisham et al. found a significant
increase in mucosal permeability after in vivo perfusion of
rat intestine with hydrogen peroxide.
Altered epithelial permeability is also a consistent effect of
hydrogen peroxide in other tissues including endothelial and renal
In studies to determine the in vitro
effect of oxidative stress on TJ integrity, Parrish et al.
studied the effect of chemically induced oxidative stress on the E-cadherin/catenin
protein complex, which is the principal intercellular TJ (zonula
adherens) anchoring protein. The authors bathed precision cut rat
liver slices with non-lethal concentrations of oxidant chemicals (diamide
and t-butylhydroperoxide), which penetrate the hepatocytes
and oxidize both intracellular-reduced glutathione and NADPH. This
depletes available glutathione stores and prevents the regeneration
of reduced glutathione. This causes oxidation from both these added
chemicals and any endogenously generated hydrogen peroxide. The
authors found that this level of oxidative stress disrupted the E-cadherin/catenin
cell-adhesion protein complex of the TJ.
4. Are BMs and TJs important in the pathogenesis
of UC? BMs together with colonic epithelial cells and the TJs that
bind them together are the micro-anatomical structures that comprise
the gastrointestinal barrier, which prevents fecal bacteria from
entering the sterile deeper layers of the colonic tissue and gaining
entrance to the blood stream.
In an early study, BMs were found to be absent or
severely damaged in UC.
In a subsequent report of 29 patients with UC, Jacobson and Kirsner
reported either completely destroyed or fragmented BM in all
subjects. Of note was the observation of "thinned"
out sections of BM consistent with a diffusible agent such as H2O2
causing membrane dissolution.
The authors also point out that destruction of BM was noted in the
absence of leukocytic infiltration, which is also consistent with a
diffusible agent of non-leukocytic (i.e. colonic epithelial cell)
Colonic epithelial BM structure and function was
also found to be seriously disturbed in active cases of UC. In a
study to determine the integrity of BM in active UC, Schmehl et
al., found no positive immunoreactivity for BM laminin in
affected colonic tissue. The authors concluded that the
three-dimensional network of colonic epithelial BM and its function
is seriously disturbed in active UC.
Using alternating current impedance analysis
Schmitz et al. found that epithelial resistance in UC is
strongly impaired and that this barrier defect was paralleled by a
decrease in TJ strand count.
Employing immunostaining with anti-human E-cadherin
and catenin antibodies, Karayiannakis
determined that E-cadherin expression was reduced in all cases of
active UC but in none of the inactive cases. (The E-cadherin/catenin
protein complex is the primary intercellular TJ (zonula adherens)
Altered a-catenin was also seen in all cases of
active UC but was not altered in any case with inactive disease.
Importantly, epithelial cells adjacent to mucosal ulcers showed loss
of E-cadherin and a-catenin, while epithelial cells distant from the
mucosal margin revealed normal E-cadherin and a-catenin expression.
The authors found that altered E-cadherin always coexisted with
abnormal a-catenin. This is consistent with an advancing margin of a
diffusible agent which is disrupting the E-cadherin/catenin complex
within the TJ.
TJ disruption was also found to correlate with
the progression of UC. In studies employing immuno-cytochemistry,
Western blotting and in situ hybridization, Jankowski found a
strong correlation between E-cadherin disruption and the progression
The authors showed that as disease activity progresses both
cytoplasmic and membranous E-cadherin expression are lost. The
authors propose that normal E-cadherin function is essential for the
maintenance of normal colorectal epithelium.
In agreement with the above studies, results are obtained with
genetically engineered chimeric mice expressing enterocytes with
either normal or non-functional cadherin. Mice expressing
non-functional cadherin developed an inflammatory bowel disease with
histological similarities to human UC, including cryptitis, crypt
abscesses and a neutrophilic infiltrate.
These inflammatory changes were confined to foci of epithelium
expressing non-functional cadherin, confirming the absence of an
If extracellular hydrogen peroxide, originating
from intracellular sources, is a causative factor in the breakdown
of colonic barrier (TJ) function, then an increase in epithelial
permeability should be an early manifestation in UC. Furthermore,
these early permeability lesions should be macroscopically normal
and occur in intact epithelium since the neutrophilic inflammatory
process has not yet begun.
Gitter et al.
found such early permeability increases in colonic tissue obtained
from individuals with early (Truelove I) UC. The authors found that
in seemingly intact epithelium there was a 35% increase in
conductivity (ionic permeability) in early/mild (Truelove I) UC
tissue samples with a 300% permeability increase in tissue samples
showing a moderate to severe inflammation. These areas of early
permeability increases correlated with foci of colonic epithelial
apoptosis. Both of these effects (i.e. increase in epithelial
permeability and induction of apoptosis) are known consequences of
tissue exposure to hydrogen peroxide.
This is in agreement with permeability studies in animal models of
UC, which demonstrated an increase in colonic mucosal permeability
to tracer molecules prior to the appearance of a visible
5. Are conditions that enhance production of hydrogen peroxide also
associated with UC? Elevated levels of intracellular hydrogen
peroxide can result from increased production or decreased
originates from two main intracellular
sources. One is the mitochondrial ETC and the other is the total sum
metabolic activity of nearly 100 oxidase enzymes distributed within
most sub-cellular organelles and cytosol[12-15,17,47,48].
neutralization is mainly accomplished via the enzymatic action of
GPx (E.C. 184.108.40.206) in conjunction with the anti-oxidant tri-peptide
co-factor glutathione (GSH-reduced). Selenium is a required
active-site co-factor for GPx enzymatic function. Oxidized
glutathione (glutathione disulfide, GSSG) is converted back to the
cytoprotective reduced state via the enzymatic activity of
glutathione-disulfide reductase (GDR) (E.C. 220.127.116.11-formally E.C.
18.104.22.168.), with electrons supplied by NADPH which itself is
generated by the pentose phosphate pathway (PPP). The chemical
equation for H2O2
neutralization and glutathione regeneration is as follows:
Cytoprotective (reduced) intracellular glutathione is the main
reducing agent available to colonic epithelial cells to neutralize
hydrogen peroxide. Although other redox couples such as
and reduced:oxidized thioredoxin participate in the maintenance of
the required intracellular reduction state critical to metabolic
function, the glutathione redox couple (GSH:GSSG) is a two to four
orders of magnitude more abundant than any other redox couple.
Glutathione also serves as a common reducing agent for other redox
and accounts for over 90% of intracellular H2O2
neutralization (catalase contributes a
small amount to overall H2O2
neutralization). Glutathione is therefore the main supplier of
reducing equivalents and a reliable indicator of the redox state of
can thus be abnormally elevated secondary to increased production or
impairment of any element or action required for its neutralization.
The following sections illustrate this concept.
Increased oxidase enzyme activity
Oxidase enzymes utilize molecular oxygen as an electron accepting
co-factor necessary for the enzymatic reaction to proceed. H2O2
is produced as an end by-product of these reactions. Thus, increased
oxidase enzymatic activity can contribute to the generation of
intracellular hydrogen peroxide. UC has been reported subsequent to
the administration of certain xenobiotics (i.e. vitamin B-6).
Vitamin B-6 is metabolized by pyridoxine 4-oxidase (EC 22.214.171.124),
which generates H2O2
as a by-product.
Increased electron transport activity
UC can develop subsequent to hyperthyroidism[51-55].
Hyperthyroidism is known to enhance ETC activity, which increases
hydrogen peroxide generation. On the other hand, cigarette smoking,
which inhibits ETC activity, is protective. Studies quantifying the
effect of cigarette tar on mitochondrial electron transport activity
report an 82% inhibition rate on whole chain respiration,
whereas cessation of cigarette smoking (which releases ETC
inhibition) is a powerful risk factor for the development of UC[2,57-59].
Colonocyte ETC activity can become a source of
if subjected to hypoxia and sudden re-oxygenation.
This process of hypoxia and re-oxygenation increases the activity of
the ETC due to the interim accumulation of reducing substrate
resulting in increased production of hydrogen peroxide. Local
colonic hypoxia/reoxygenation can be caused by stress. The following
section reviews mechanisms of stress-related increases in colonocyte
Psychological stress has long been recognized as an exacerbating
factor for UC. Dr. Burrill Crohn was well aware of the psychological
effects of stress on UC when, in the first issue of Gastroenterology
in January 1943, he reported the appearance of acute UC in a
16-year-old girl following a criminal rape, noting that "the
psycho-somatic aspect of this case was particularly significant".
During the 1950s, practitioners noticed the onset
and/or exacerbation of UC commonly occurring subsequent to emotional
Early observations of severely emotionally disturbed individuals
with UC reported resolution of the latter when the former was
More recently an association has been reported
between stress and UC disease activity[2,67].
Up to 40% of patients with UC report psychological stress as an
Life stress has been reported to be associated with both objective
and subjective aspects of activity in UC
and high long-term stress was found to triple the risk of disease
The importance of stress as an initiating factor can be seen in the
cotton-top tamarin, a small monkey found only in northwest Columbia
that spontaneously develops colitis when deprived of its native
habitat while in captivity. Affected animals will enter remission
when transferred to natural conditions indicating that the effects
of stress can be reversed.
The molecular basis of stress-induced exacerbation of UC can be
correlated to both increased H2O2
production and decrease H2O2
neutralization secondary to the effect of stress on electron
transport activity and cellular enzyme systems. These mechanisms may
find expression either through systemic or local effects of stress
on the colon as discussed below.
1. Acute systemic psychological stress increases
the amount of systemic circulating biogenic amines (catecholamines),
such as serotonin, epinephrine, nor-epinephrine and dopamine.
Mono-amine oxidase (EC#126.96.36.199), an enzyme present on the outer
surface of mitochondria within colonic epithelial cells, catalyzes
the oxidative deamination of both exogenous xenobiotic amines (i.e.
medications) as well as endogenous catecholamine stress hormones and
in the process reduces molecular oxygen to hydrogen peroxide.
The reaction catalyzed is RCH2+H2O+O2→RCHO
Stress therefore may increase H2O2
levels by providing additional metabolic substrate (endogenous
catecholamines) for mono-amine oxidase. Thus, individuals with
genetically diminished anti-oxidant (reductive, H2O2
neutralizing) capacity are at greater risk of developing UC when
exposed to acute stressful events.
2. Chronic systemic psychological stress, such as
depression, has been associated with circulating increased
Depression has been reported to precede the onset of UC
significantly more often than expected.
Depressive stress and anxiety, however, were found to be
significantly more common after the appearance of Crohn's
This suggests that physiological alterations present in depression
contribute to the appearance of UC in contrast to Crohn's
depression may be a psychological reaction to the appearance of the
Chronic depression, therefore, may result in
significant long-term increases in circulating endogenous
catecholamine levels, which may elevate intracellular colonocyte H2O2
when metabolized via mono-amine oxidase. Chronically depressed
individuals with marginal anti-oxidant capacity needed to neutralize
this excess H2O2
are at increased risk for development of UC.
3. Local colonic perfusion/reperfusion (hypoxia/reoxygenation)
can result from the effects of psychological stress on the colon. Stress-induced colonic spasm may result in local hypoxia and
re-oxygenation, which can lead to oxygen deprivation of dozens of
oxidase enzymes such as xanthine oxidase (XO)[74-76].
In a seminal study, Almy and Tulin
directly observed the effects of stress on the colonic mucosa of
seven healthy volunteer medical students.
The students were fitted with a metal helmet
containing 18 large screws that could be tightened against the head
to produce a painful distressing headache lasting 30 min during
which time visual colonoscopic evaluation of the sigmoid colon was
recorded. In each case the authors visualized severe colonic spasm,
which was sufficient to occlude the lumen. Marked mucosal hyperemia
and engorgement with intermittent blanching and flushing
(perfusion/reperfusion) was also noted. During periods of maximum
engorgement, gentle movement of the proctoscope caused a superficial
injury with hemorrhage. Nausea often accompanied visualized episodes
of colonic spasm. This study indicates that stress can cause severe
alterations in colonic function and predispose to colonic hypoxia
and reoxygenation (perfusion/reperfusion) injury (sequential
blanching and hyperemia with mucosal engorgement). Thus, local
stress-induced colonic spasm is mediated via the enteric nervous
system, which results in spastic contraction of colonic smooth
muscle leading to transient local tissue hypoxemia with subsequent
reoxygenation upon colonic smooth muscle relaxation.
XO is a prototypical example of an oxidase enzyme
that is affected by perfusion/reperfusion-induced oxygen
deprivation. XO catalyzes the conversion of hypoxanthine to uric
acid and in the process reduces molecular oxygen to hydrogen
peroxide. During hypoxia, XO activity is significantly reduced due
to unavailability of oxygen needed as an electron-accepting
co-factor for the enzymatic conversion (oxidation) of hypoxanthine
to uric acid. When oxygen is reintroduced (re-perfusion), an
increased substrate load leads to increased hypoxanthine metabolism
and hydrogen peroxide production. Stress-induced
perfusion/re-perfusion, therefore, results in additional H2O2
due to increased metabolism of oxidase enzyme substrate which, after
having accumulated during hypoxemia, undergoes amplified metabolism
upon re-oxygenation with concomitant increases in hydrogen peroxide.
Pre-treatment of mice with allopurinol (XO inhibitor) prior to
experimentally induced colonic ischemia/reperfusion
significantly attenuated leukocyte adhesion to colonic submucosal
Thus, in this model of murine colitis, inhibition of XO
significantly reduces WBC endothelial adhesion, a crucial early step
which is likewise present in the development of human UC.
Stress-induced colonic smooth muscle spasm with
hypoxia/re-oxygenation can also increase colonocyte electron
transport activity with concomitant increases in H2O2.
Rectal epithelial cells possess an ETC, which can become a source of
if subjected to hypoxia and sudden re-oxygenation.
DECREASED HYDROGEN PEROXIDE
Decreased glutathione peroxidase activity
Hydrogen peroxide is metabolized via the enzymatic action of GPx, a
selenium containing enzyme, which utilizes the anti-oxidant
tri-peptide co-factor glutathione to neutralize intracellular H2O2.
Genetic conditions which inhibit GPx or decrease glutathione
availability will lead to increased hydrogen peroxide levels.
Genetic research by Cho uncovered the existence of a "pathophysiologically
crucial IBD susceptibility gene" located on the small arm of
human chromosome 1 (1p36)[80,81].
This genetic locus codes for two enzymes that exert control on
One is methylenetetrahydrofolate reductase (MTHFR,
EC 188.8.131.52), which is a main regulatory enzyme of homocysteine
Molloy has reported that 17.5% of individuals with UC possess a
polymorphic variant of the MTHFR gene vs 7.3% of controls.
Polymorphic variants of MTHFR result in an elevation of serum
Nagano has shown that children with UC have elevated serum
homocysteine levels and concludes that elevated homocysteine may be
associated with the underlying basic pathophysiology of the disease.
Markedly elevated levels of tissue homocysteine have also been
reported in colonic mucosa of individuals with UC.
Elevated homocysteine will increase hydrogen peroxide production by
Hydrogen peroxide is generated during the
oxidation of homocysteine to homocystine[82,87].
Homocysteine also increases levels of the enzyme SOD.
SOD catalyzes the conversion of superoxide anion to hydrogen
peroxide and increased activity of this enzyme will result in
greater hydrogen peroxide generation. Homocysteine has been reported
to inhibit GPx activity
by 10-fold. This epistatic inactivation of GPx will increase
hydrogen peroxide levels and inhibition of GPx was shown to occur at
physiologic (9 mmol/L) concentrations of free homocysteine.
Decreased 6-phosphogluconate dehydrogenase activity
A second enzyme located at this locus (1p36.3) is 6-phosphogluconate
dehydrogenase (PGD) (EC 184.108.40.206). PGD is one of only two enzymes in
the PPP, which are responsible for production of NADPH, which is
crucial for the reduction of glutathione disulfide (GSSG) back to
reduced glutathione (GSH) in order to neutralize the continuous
production of H2O2
being generated within the cell. Without
NADPH to regenerate reduced glutathione, intracellular enzymes would
suffer irreversible oxidative damage from excess hydrogen peroxide
and cellular function would cease in minutes as apoptosis is
triggered. The PPP is the engine that drives H2O2
neutralization and there is no backup system. PGD exists in several
polymorphic forms with decreased activity ranging from 22% to 79% of
Decreased levels of glutathione have been reported as a result of a
PGD polymorphic enzyme.
The phenotypic expression of both these genes
conclusion of a
pathophysiologically crucial IBD susceptibility gene located
at 1p36. PGD activity is also lowered by exogenous factors, i.e.
antibiotics, dietary fat and ageing[96-98].
Studiesof normal appearing colonic mucosa report significant
inter-individual variation of enzymes involved in glutathione
synthesis and metabolism.
Individual variation was considerable at 8-fold for glutathione-S-transferase,
10 fold for GPx, 14-fold for gamma-glutamyl-transpeptidase and 5
fold for gamma-glutamylcysteine synthetase.
These large enzyme variations directly or
indirectly affect intracellular glutathione concentrations which
itself shows a16-fold variation between individuals placing certain
individuals at the very lowest range of H2O2
Decreased glucose-6-phosphate dehydrogenase activity
Epinephrine has been shown to stimulate H2O2
release by macrophages and to inhibit glucose-6-phosphate
dehydrogenase (G-6-PD, EC 220.127.116.11).
G-6-PD is a crucial enzyme in the PPP, which produces NADPH needed
to regenerate reduced glutathione, which is crucial in order to
neutralize (reduce) H2O2.
Inhibition of this enzyme by circulating epinephrine during
stressful events reduces the amount of NADPH generated by the PPP,
which may lead to increased intracellular H2O2
levels. Therefore, conditions of sustained stress can increase the
concentration of circulating endogenous catecholamines and boost
production of hydrogen peroxide by rectal epithelial cells by either
direct production of H2O2
or reduction in NADPH needed for H2O2
Increased cytochrome P450 enzyme activity
The cytochrome P450 enzyme system is responsible for the majority of
oxidation reactions of drugs and other xenobiotics.
One study reports that 56% of over 300 drugs tested are metabolized
via the cytochrome P450 (CYP) family of oxygenase enzymes present in
the endoplasmic reticulum.
CYP is mostly found in the liver but is also present in the
intestine. A typical CYP catalyzed reaction is as follows:
This reaction consumes NADPH, which is also used
in regeneration of reduced glutathione required to neutralize H2O2.
Excessive NADPH utilization in predisposed individuals with marginal
anti-oxidant capacity may contribute to increased H2O2
levels and the development of colitis associated with certain drugs.
In a prospective cohort study, Jowett
found that individuals who consumed the most alcohol tripled their
risk of UC relapse compared to those who drank the least. After
ingestion, alcohol is distributed to all cells of the body including
the rectal epithelial cells. Alcohol is enzymatically converted to
acetaldehyde by alcohol dehydrogenase. The acetaldehyde is
enzymatically converted to acetic acid by aldehyde dehydrogenase.
Both of these cytosolic enzymes utilize NAD+
to oxidize their respective substrates and generate NADH that
normally serves as an electron donor to the ETC. The increased
availability of NADH can activate the ETC and generate excess
Alcohol can also be metabolized in the
endoplasmic reticulum by cytochrome P450 2E1 depleting NADPH needed
for glutathione regeneration. Alcohol, thus, generates H2O2
and decreases production of glutathione needed for neutralization of
Alcohol inhibits GPx, a crucial enzyme that
and depletes mitochondrial glutathione.
Glutathione is not synthesized within mitochondria and must be
transported from the cytosol into mitochondria through mitochondrial
membranes. Alcohol inhibits active transport of glutathione into
leading to mitochondrial depletion of glutathione and H2O2
A relationship exists, therefore, between UC and
conditions that enhance H2O2
production. Furthermore, significant genetic variability in H2O2
neutralizing capacity confers greatest risk of developing UC to
those individuals with genetically low H2O2
neutralizing capacity and co-existence of any of several conditions
provoking increased production of H2O2.
Figure 1 illustrates this concept.
(PDF) Pathogenesis of UC: Individuals with UC are basically
normal. Polymorphic genes (MTHFR and PGD) and significant
inter-individual variability in anti-oxidant capacity places certain
individuals at the lower threshold of their physiological hydrogen
capability for any given environmental oxidant stress level. Oxygen
radical production is induced by environmental oxidant stressors (xenobiotics,
stress, smoking cessation, hypermetabolic states) interacting with
cellular metabolism. This process is called "Radical
Induction". Excess un-neutralized hydrogen peroxide generated
during the radical induction phase diffuses from intracellular
compartments of colonic epithelial cells, through the plasma
membrane, to the extracellular space. Free extracellular hydrogen
peroxide reacts with superoxide (O2)
in a transition metal catalyzed Haber-Weiss reaction to form
hydroxyl radical (HO) and hydroxide (HO) as follows: *O2+Fe+3
(followed by) Fe+2+H2O2→Fe+3+HO+HO.
Hydroxyl radical initiates oxidative damage to structures that
comprise the gastrointestinal barrier (epithelial TJs, BM and
epithelial lipid peroxidation) resulting in transient immune
activation, which ceases when the damage is repaired. Intermittent
immune activation by colonic bacterial antigens can lead to antibody
formation (p-anca) and extra-intestinal manifestations. When
oxidative damage to the GI barrier cannot be repaired in time to
prevent subjacent endothelial adhesion molecule expression, WBCs (neutrophils)
begin infiltrating into the damaged colonic epithelium in an effort
to prevent systemic bacteremia, which would otherwise result in
fatal sepsis. Resident mucosal neutrophils produce additional large
quantities of diffusible H2O2
oxidative tissue damage in adjacent colonic epithelial cells whose
anti-oxidant (GSH) levels have already been previously compromised
by radical induction. This results in a proximal "advancing
edge" of oxidative tissue damage, TJ disruption and lipid
peroxidation extending proximally from the rectum, a unique tissue
with high oxidant exposure secondary to fecal generated oxygen
radicals, maximum bacterial antigenic load and least anti-oxidant
defense as compared to the rest of the GI tract. The accompanying
intense neutrophilic diapedesis in a restricted area of epithelial
TJ disruption is followed initially by microscopic erythrocyte
extravasation eventually leading to frank hemorrhage as endothelial
junctions fail to close leading to bloody diarrhea and further
neutrophil infiltration characteristic of this disease. The process
is only temporarily halted when sufficient anti-oxidant (GSH)
capacity is encountered producing a clear line of demarcation
between diseased and normal tissue. This self-perpetuating and auto-stimulating cycle is called
propagation. *Both iron (Fe) and superoxide (O2)
are plentiful within the colonic lumen[108-110].
UC has been reported subsequent to oral iron supplementation for the
treatment of anemia
and in association with conditions of copper overload (i.e. Wilson's
6. Is impaired beta oxidation and neoplastic
transformation a consequence of excess H2O2?
The preferred energy source for colonic epithelial cells is a
short chain 4-carbon fatty acid known as butyrate (SCFA). Most
butyrate is derived from colonic bacterial fermentation of
unabsorbed dietary fiber.
SCFAs are metabolized rapidly by beta oxidation and are the major
respiratory fuels of colonocytes.
Beta-oxidation is the anapleurotic process occurring within
mitochondria by which fats are broken down into two carbon units to
form acyl-CoA, which is the entry molecule for the Krebs (tricarboxylic
acid) cycle. The Krebs cycle generates NADH, which is used as a fuel
for ETC activity resulting in ATP production.
Inhibition studies carried out on beta oxidation
led Roediger and Nance,
to conclude that "a
suitable inhibitor of
beta-oxidation would have unimpeded entry into mitochondria of
colonic epithelial cells". Hydrogen peroxide is permeable
through biomembranes including the cell membrane and both the inner
and outer mitochondrial membranes. Hydrogen peroxide has been shown
to inhibit the beta-oxidation enzyme system of enzymes.
The last enzyme in the mitochondrial
beta-oxidation process is acetyl-CoA C-acyltransferase (EC
18.104.22.168)(ACT) also known as 3-ketoacyl-CoA thiolase or thiolase I.
This enzyme contains two active binding sites each of which includes
a conserved cysteine residue, which is crucial for enzymatic
Cysteine is an amino acid which has a thiol (hydrosulfide or
sulfhydril) group. The thiol group is a univalent radical (-SH)
which must be maintained in a reduced state in order for ACT to be
is capable of oxidizing these cysteine
residues and inactivating ACT.
Inhibition of beta-oxidation has been reported in
macroscopically normal and clinically quiescent UC.
Studies have shown that impairment of beta-oxidation is
significantly associated with increased colonic permeability
followed by clinical relapse to active UC within a few weeks.
Remission was associated with normal beta-oxidation.
This supports a separate and distinct diffusible
intracellular oxidizing agent (i.e. H2O2)
as the vehicle for both impaired beta-oxidation and subsequent
increase in colonic permeability. Appearance of diffusible
explains the abnormalities of butyrate
metabolism during active UC, which may resolve during remission and
reappear prior to subsequent activation of the disease since H2O2
would accumulate within mitochondria and other sub-cellular
organelles prior to extracellular diffusion and destruction of BMs
Patients with UC have an incidence of colorectal cancer (CRC) which
is up to 20 fold higher and 20 years younger than CRC in the general
UC associated CRC originates from dysplastic colonic epithelial
The mechanism of neoplastic transformation involves nuclear DNA
damage within colonic epithelial cells[122,123].
is known to cause oxidative DNA damage and treatment of colonic
epithelial cells with low concentrations of H2O2
has been shown to cause oxidative nuclear DNA damage[124-126].
Cells whose nuclear DNA is damaged by H2O2
generated free radicals can undergo neoplastic transformation[127,128].
Thus, active colonic inflammation will lead to cancer due to the
high amount of H2O2
generated by infiltrating neutrophils; however, studies have shown
that individuals with quiescent disease in "remission"
have the same risk of developing CRC as those with a more active
suggesting an intracellular mechanism (i.e. excessive H2O2)
which is present during "remission".
In fact, studies demonstrate that CRC in the setting of UC increases
the risk of CRC in non-colitic relatives by 80%
suggesting the anteceding presence of a genetically predisposing
genotoxic mechanism or agent such as H2O2.
5-Aminosalicylic acid (5-ASA) has been proposed
as a maintenance therapy for the prevention of CRC[131-133].
However, results are conflicting and recent studies show that
maintenance with 5-ASA does not protect against the development of
Once dissolved in the near neutral pH of colonic fluids 5-ASA
becomes a zwitterion with a positive charge on the protonated amino
group at one end and a negative charge on the dissociated carboxylic
group at the other end of the molecule. This promotes attraction
between the positively charged amino terminal of 5-ASA and
negatively charged surface membrane proteins favoring retention of
5-ASA on the exterior surface of colonic epithelial cells[135,136].
In vitro studies of 5-ASA have shown that the site of action
of 5-ASA is extracellular.
Its mode of action is that of an extracellular tetravalent reducing
agent capable of donating four electrons per molecule for H2O2
and oxygen radical neutralization.
5-ASA also sequesters ferrous ions (Fe+2)
possibly by electrostatic attraction to its (5-ASAs) negatively
charged terminal thereby inhibiting the hydroxyl generating
extracellular Haber-Weiss reaction.
extracellular site and mode of
action precludes it from affecting the intracellular pathogenetic
mechanisms leading to neoplastic transformation during disease
quiescence. While in "remission"
generated during the radical induction phase and exiting the cell
will be neutralized by luminal 5-ASA reducing the incidence of
clinical reactivation; however, excessive intracellular H2O2
is free to diffuse into the nucleus causing oxidative DNA damage
resulting in CRC in this clinically quiescent stage of disease.
The evidence presented in this paper points to excess hydrogen
peroxide diffusing out of colonic epithelial cells as the initiating
etiology of UC. Hydrogen peroxide (H2O2),
a highly toxic oxidizing agent and by-product of normal aerobic
cellular metabolism, is constantly being generated within all cells
including colonic epithelial cells and must be immediately
neutralized in order for the cells to survive.
The intracellular generation of H2O2
is determined by the metabolic and
respiratory activity of the cell; however, the anti-oxidant capacity
required to neutralize H2O2
resides within a genetically determined
fixed range for each individual with significant inter-individual
variability being expressed. This places a sub-group of individuals
at the lowest end of anti-oxidant (H2O2
neutralizing) capacity. It is this
sub-group of individuals that have the highest risk of developing UC
when exposed to oxidant stressors.
Under appropriate conditions excess H2O2
generated due to the effect of oxidant
stressors on cellular metabolism may overwhelm the genetically
predetermined anti-oxidant/reducing capacity available within the
cell resulting in intracellular H2O2
accumulation. This process is called "Radical
Induction". Radical induction will initially manifest in the
rectum which has the lowest anti-oxidant defense (H2O2
neutralizing) capacity of the entire GI tract coupled
with the highest bacterial exposure[140-143].
This makes rectal epithelium especially vulnerable to oxidant
Hydrogen peroxide is freely permeable through
cell membranes and essentially forms one intracellular compartment. H2O2
generation is biochemically coupled to fundamental metabolic
processes such as ATP (energy) production and ETC activity in
addition to the enzymatic activity of nearly 100 intracellular
enzyme systems. H2O2
production is therefore very sensitive to
and will fluctuate with environmental influences that affect
respiratory activity (tobacco use, stress, hyperthyroidism) and
availability of specific enzyme substrate (i.e. xenobiotics,
alcohol, vitamin B-6). Excess un-neutralized H2O2
will diffuse from any intracellular location to the extracellular
space where it can be converted to the highly destructive hydroxyl
radical via a metal catalyzed Haber-Weiss reaction causing
significant oxidative damage to colonic epithelial TJs, BM and
epithelial biomembranes, which are micro-anatomical structures that
comprise the colonic barrier function. This in turn increases
colonic mucosal permeability to luminal antigens resulting in the initial
influx of WBCs (neutrophils) from the subjacent vasculature to the
mucosal surface. This is a normal and expected immune response to
antigenic exposure; a normal response to normal colonic flora.
Once present on the colonic mucosal surface,
exposure to high concentrations of fecal bacterial antigens
stimulates neutrophils to secrete their own tissue damaging oxygen
radicals. Neutrophil mediated tissue damage attracts additional
neutrophils from the subjacent intravascular compartment to the
mucosal surface. This process is repeated until sufficient tissue
anti-oxidant (glutathione) levels are encountered to temporarily
halt its progression, sharply delineating diseased from normal
This self-perpetuating process of tissue
destruction is called propagation and results in a proximally
advancing edge of contiguous oxidative tissue destruction in
adjacent epithelium whose anti-oxidant defense capacity has already
been previously compromised during the radical induction phase.
Continued inflammatory tissue destruction results in rectal bleeding
and bloody diarrhea characteristic of this disease.
Since Hale-White first coined the term "ulcerative
colitis" in 1888
we have learnt that the clinical phase of this disease which begins
with rectal bleeding is characterized by colonic mucosal
inflammation mediated by the accumulation of WBCs (mainly
neutrophils) within the colonic epithelium. Based on the
histological findings in this phase, UC is classified as an
inflammatory bowel disease. The mechanism of neutrophil-mediated
tissue injury responsible for colonic bleeding has been well
To date however there is no satisfactory answer
for why neutrophils accumulate within the colonic mucosa to begin
with. The Radical Induction Theory of UC provides an explanation for
this initial influx of neutrophils.
Radical Induction Theory states that H2O2
originating from colonic epithelial cells diffuses to the
extracellular space resulting in oxidative damage and dissolution of
intercellular TJs and BM, which are micro-anatomical structures that
maintain the GI barrier function. Once compromised, the GI barrier
can no longer exclude highly antigenic bacterial antigens from
invading the normally sterile deeper layers of the colonic wall
resulting in the initial influx of neutrophils to the mucosal
surface. Continued accumulation of neutrophils results in extensive
tissue damage and bleeding characteristic of this disease.
Radical Induction Theory implies two distinct
phases of UC. The first phase is operational prior to any colonic
bleeding. During this initial preclinical "Radical
Induction" phase colonocytes are induced to generate excessive
due to effects of oxidant stressors upon
cellular metabolism. Hydroxyl radical and H2O2
have very short half lives (nanoseconds
and seconds to minutes respectively) which limits their destructive
activity to intracellular molecules (i.e. DNA, enzymes) and local
extracellular structures immediately adjacent to the cell (i.e. TJs
and BM). Pro-inflammatory cytokines, however, may be carried to
distal sites to exert their effect. Thus, initial intermittent
extracellular diffusion of H2O2
from epithelial cells causes short lived local barrier compromise
and transient immune activation resulting in cytokine production and
distal extra-intestinal manifestations such as arthritis, uveitis,
skin manifestations (i.e. pyoderma gangrenosum) and p-anca type
antibodies. Continued oxidative insult to colonic barrier function
culminates in neutrophilic infiltration.
A second (clinical) phase begins with rectal
bleeding and signals further destructive GI barrier compromise
secondary to neutrophil-mediated tissue damage. Continued
stimulation of mucosal neutrophils by fecal bacteria converts the
condition into an auto-stimulating/self-perpetuating process termed
Patients will normally present for treatment
during the propagation phase, which will continue inexorably to the
inflammatory destruction of the colon without outside intervention.
Although clinical remission (cessation of rectal bleeding),
endoscopic remission (normal macroscopic mucosal appearance) and
histologic remission (no mucosal neutrophils) are important
milestones, the lack of metabolic remission precludes complete
reversal of this condition and predisposes to future reactivation
and neoplastic transformation.
Fifty years of research has not demonstrated any
antecedent immune vulnerability in patients with UC.
However, rather than a local mucosal immune dysfunction, high levels
found in other cell lines in individuals with UC
suggests that this condition is a systemic disease of oxidant stress
whose primary pathological manifestation is in the rectum, a unique
body site with minimal anti-oxidant defense, high continuous oxidant
stress and maximum bacterial antigenic exposure.
Thus, the crucial element required for mucosal
integrity mentioned earlier consists of the biochemical machinery
needed to detoxify hydrogen peroxide which, if allowed to
accumulate, can oxidize and disintegrate TJ proteins leading to
dissolution of barrier integrity and UC. Neutralization of
intracellular hydrogen peroxide, therefore, constitutes a vital
process whose dysfunction results in physical disintegration of
gastrointestinal barrier function.
GPx (E.C. 22.214.171.124) in conjunction with co-factor
glutathione, a self-replenishing tripeptide reducing agent, is
responsible for 91% of H2O2
neutralization. Factors that decrease the
activity of GPx or decrease the amount of available reduced
glutathione will lead to increases in intracellular hydrogen
peroxide which upon diffusion to the extracellular space will result
in oxidative disruption of TJs and BM whose integrity is required
for GI barrier function.
The oxidative stress to which UC patients are
exposed has exceeded their physiological antioxidant defense
mechanisms. The significant inter-individual variability in oxidant
neutralizing capacity places certain individuals at the lower
threshold of their physiological (H2O2)
reducing capability for any given
environmental oxidant stress level. As the environment becomes
increasingly toxic more individuals will succumb to its effects. UC,
a purely descriptive term, may be more accurately described by its
pathophysiology as oxidative colitis.
I wish to thank Drs. William W. Taylor and Charles E. Reed for their
continued encouragement and support. I am deeply indebted to
Kimberly D. Shepherd whose courageous existence made this work
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