World J Gastroenterol. 2002 April 15; 8(2): 224-229.
Published online 2002 April 15. doi: 10.3748/wjg.v8.i2.224.
Effect of cis-9, trans-11-conjugated linoleic acid on cell cycle of gastric adenocarcinoma cell line (SGC-7901)
Jia-Ren Liu, Bai-Xiang Li, Bing-Qing Chen, Xiao-Hui Han, Ying-Ben Xue, Yan-Mei Yang, Yu-Mei Zheng and Rui-Hai Liu.
Jia-Ren Liu, Bai-Xiang Li, Bing-Qing Chen, Ying-ben Xue, Yan-Mei Yang, Yu-Mei Zheng, Department of Toxicological Health, Public Health College, Harbin Medical University, Harbin 150001, Heilongjiang Province, China
Xiao-Hui Han, ICU of Cardiological Surgery, The Second Hospital, Harbin Medical University, Harbin 150001, Heilongjiang Province, China
Rui-Hai Liu, Food Science and Toxicology, Department of Food Science, Cornell University, Ithaca, NY 14853-7201, USA
Author contributions: All authors contributed equally to the work.
Correspondence to: Dr. Jia-ren Liu, 199 Dongdazhi Street, Nangang District, Harbin 150001, Heilongjiang Province,
Telephone: +86-451-3639459 Fax: +86-451-3641253
Received August 23, 2001; Revised September 1, 2001; Accepted September 5, 2001;
AIM: To determine the effect of cis-9, trans-11-conjugated linoleic acid (c9,t11-CLA) on the cell cycle of gastric cancer cells (SGC-7901) and its possible mechanism in inhibition cancer growth.
METHODS: Using cell culture and immunocytochemical techniques, we examined the cell growth, DNA synthesis, expression of PCNA, cyclin A, B1, D1, P16ink4a and P21cip/waf1 of SGC-7901 cells which were treated with various c9,t11-CLA concentrations (25, 50, 100 and 200 μmol•L⁻¹)of c9,t11-CLA for 24 and 48 h, with a negative control (0.1% ethane).
RESULTS: The cell growth and DNA synthesis of SGC-7901 cells were inhibited by c9,t11-CLA. SGC-7901 cells. Eight day after treatment with various concentrations of c9,t11-CLA mentioned above, the inhibition rates were 5.92%, 20.15%, 75.61% and 82.44%, respectively and inhibitory effect of c9,t11-CLA on DNA synthesis (except for 25 μmol/L, 24 h) showed significantly less 3H-TdR incorporation than that in the negative controls (P < 0.05 and P < 0.01). Immunocytochemical staining demonstrated that SGC-7901 cells preincubated in media supplemented with different c9,t11-CLA concentrations at various times significantly decreased the expressions of PCNA (the expression rates were 7.2%-3.0%, 24 h and 9.1%-0.9% at 48 h, respectively), Cyclin A (11.0%-2.3%, 24 h and 8.5%-0.5%, 48 h), B1 (4.8%-1.8% at 24 h and 5.5%-0.6% at 48 h)and D1 (3.6%-1.4% at 24 h and 3.7%-0% at 48 h) as compared with those in the negative controls (the expressions of PCNA, Cyclin A, B1 and D1 were 6.5% at 24 h and 9.0% at 48 h, 4.2% at 24 h and 5.1% at 48 h, 9.5% at 24 h and 6.0% at 48 h, respectively) (P < 0.01), whereas the expressions of P16ink4a and P21cip/waf1, cyclin-dependent kinases inhibitors (CDKI), were increased.
CONCLUSION: The cell growth and proliferation of SGC-7901 cell is inhibited by c9,t11-CLA via blocking the cell cycle, with reduced expressions of cyclin A, B1 and D1 and enhanced expressions of CDKI (P16ink4a and P21cip/waf1).
Gastric cancer is common in China[1-12], and it is currently thought to be caused by environmental factors, with diet being an important modifying agent[13-17]. Its mechanism of prevention and treatment still makes it become a hot spot in this area[18-39]. Its anticancerous potential Dietary fat has been implicated as an enhancing agent in carcinogenesis by both epidemiological and animal studies. Consumption of meat, specifically animal fat, has been implicated in a number of disease processes[40-42]. However, several epidemiological studies have suggested an association between increased consumption of meat and fat and decreased risk of stomach, mammary and esophageal cancers[43,44]. Among the fatty acids, only the essential fatty acid, linoleic acid (LA), has been clearly shown to enhance mammary tumorigenesis[43]. However, isomeric derivatives of cis-9, cis-12-octadecadienoic acid (linoleic acid, LA) containing a conjugated double-bond system (conjugated linoleic acid, CLA) showed inhibitory effect on carcinogenesis in animal studies[45,46]. CLA has a mixture of positional (9/11 or 10/12 double bonds) and geometric (various cis/trans combinations) isomers of LA formed by rumen and colon bacteria. The ability of CLA to prevent mammary and other tumors in rodents has been identified and has been the subject of several reviews[43]. There are eight potential isomers of CLA, but the cis-9, trans-11 and trans-9, cis-11 isomers are thought to be active as potential antioxidant and anticarcinogenic agents. Therefore, it is of interest to investigate more extensively the anticancer activities of CLA.
In the present study, we investigated the effect of cis -9, trans -11-CLA (c9,t11-CLA) on the cell cycle of human gastric adenocarcinoma cells (SGC-7901).
c9,t11-CLA, a monoisomer of c -9, t 11-octadecadienoic acid with 98% purity, was obtained from Dr. Rui-Hai Liu (Food Science and Toxicology, Department of Food Science, Cornell University, Ithaca, NY, USA). The c9,t11-CLA was dissolved in 96 mL•L⁻¹ ethanol, and was then diluted to the following concentrations: 0, 25, 50, 100 and 200 μmol•L⁻¹.
Cell culture Human gastric adenocarcinoma cells (SGC-7901), purchased from Cancer Research Institute of Beijing (China), were cultured in RPMI 1640 (Gibco) medium, supplemented with calf serum 100 mL•L⁻¹, penicillin (100 × 103 u•L⁻¹) and streptomycin (100 mg•L⁻¹). The pH was maintained at 7.2-7.4, by equilibration with 5% CO2.The temperature was maintained at 37 °C.The cells were sub-cultured with a mixture Ethylenedinitrile tetraacetic acid (EDTA) and trypsin.
Cell growth curve The SGC-7901 cells were seeded in six 24 well plates (Nuc, Co.); each well contained 2 × 104 cells. After 24 h, the medium of different plates was replaced with media supplemented with c9,t11-CLA at different concentrations. On the next day, the numbers of cells of 3 wells from each plate were determined daily by using the trypan blue staining. The means were obtained on each of eight days and were used to draw a cellular growth curve. The inhibitory rates (IR) on the 8thday was calculated, as follows:
IR (%) = (Total Number of cells in negative control (8 d) - Number of cells in test groups (8 d))/(Total number of cells in negative control (8 d)) × 100%
[3H]-Labeled precursor incorporation SGC-7901 cells (5 × 104/well in 24 well plate) were cultured in appropriate medium for 24 h prior to beginning the experiment. The medium was, then, replaced with different concentrations c9,t11-CLA. After 18 and 42 h, the cells were incubated with [3H] thymidine (China Nucleus Institute, 0.5 μci/mL, 1.0 μci/well). After 6 h the cells were harvested with trysin/EDTA. Cells were collected in an acetic fiber filter with cellular collector and washed three times with PBS. The filter was dried overnight at 37 °C. The filter was transferred into liquid of scintillation (containing 1% po and 2% pop in xylene) and cpm value determined by liquid Scintillation Counter (LS6500, Beckmen Co.).
Cell samples
SGC-7901 cells were treated for 24 and 48 h with various concentrations of c9,t11-CLA and collected by centrifugation. Specimens were fixed immediately in 40 g•L⁻¹ formaldehydum polymerisatum and embedded in paraffin. Gastric cancer tissue from a patient served as a reference.
Primary antibodyies
To examine the proliferating cell nuclear antigen (PCNA) in cell proliferation and to determine cyclins (A, B1 and D1) and cyclin-dependent kinases inhibitors (P16ink4a and P21waf1) in cell cycle of SGC-7901, we used six primary antibodies: corresponding mouse monoclonal antibodies for cyclin B1 and D1, PCNA and P21waf1 and corresponding rabbit polyclonal antibodies for cyclin A and P16ink4a. PCNA, P16ink4a, and cyclin D1 were purchased from Calbiochem Co.USA; others from Zhongshan Co. China.
Immunocytochemical staining was performed on serial sections at room temperature, using the horseradish peroxidase method. The sections were deparaffinized in xylene and rehydrated through graded alcohol. The sections were incubated for 10 min at 95 °C in 10 mmol•L⁻¹ sodium citrate (pH6.0) buffer for PCNA staining. Endogenous peroxidases were inactivated by immersing the sections in hydrogen peroxide for 10 min, and then were incubated for 10 min with 100 mL•L⁻¹ normal goat serum in PBS to block non-specific binding. The sections were subsequently incubated overnight at 4 °C with relevant antibodies (1∶50 dilution) respectively. The next day, the sections were incubated with biotinylated anti-mouse or anti-rabbit IgG (Zhongshan Co. China) for 30 min, followed by peroxidase-conjugated streptavidin (Zhongshan Co.China) for 30 min. The chromogenic reaction was developed with DAB (diaminobenzidine) for 10 min, and all sections were counterstained with hematoxylin. Controls consisted of omission of the primary antibody. The Positive Rate (PR) was calculated as follows:
PR (%) = (Number of positive cells)/(Total number (2 × 104)) × 100
Statistical analysis
Analysis of data was performed using the student's t test or χ² test. A value of P < 0.05 is considered to be statistically significant.
Effect of c9,t11-CLA on SGC-7901 cell growth
As shown in Figure 1, Growth of the cells in various concentrations of c9,t11-CLA did not differ from the negative control within 3 d. After 3 d, SGC-7901 cells incubated in 25 and 50 μmol•L⁻¹ of c9,t11-CLA grew at a lower rate than the negative control. While in 100 and 200 μmol•L⁻¹ concentrations of c9,t11-CLA, cell proliferation was significantly inhibited. The inhibitory rate of various c9,t11-CLA concentrations were 5.9%, 20.2%, 75.6% and 82.4%, respectively.
Figure 1
Figure 1
Figure 1
Growth curve of SGC-7901 cells cultured in various concentration of c9,t11-CLA
Effect on DNA synthesis
The effect of CLA on isotope incorporation into SGC-7901 cells are presented in Table 1. SGC-7901 cells preincubated in media supplemented with various c9,t11-CLA concentration (except for 25 μmol/L, 24 h) incorporated significantly less 3H-TdR than did the negative control (P < 0.05 and P < 0.01, Table 1). The inhibitory rate (IR) displayed a dose-response relationship as the concentration of c9,t11-CLA increased.
Table 1
Table 1
Inhibitory effect of c9,t11-CLA on DNA synthesis in SGC-7901 cells (n = 6)
Cell proliferation
As shown in Figure 2, expression raues of PCNA (Figure 3) on SGC-7901 cells gradually decreased after SGC-7901 cells were incubated with different concentrations of c9,t11-CLA at various times. Moreover, SGC-7901s cell expressed significantly less PCNA than did the negative control (P < 0.01). The expression rate displayed a dose-response relationship as the concentrations of CLA increased.
Figure 2
Figure 2
Figure 2
Expression of PCNA on SGC-7901 cells treated with c9,t11-CLA
Figure 3
Figure 3
Figure 3
A: The expression of PCNA on SGC-7901 cells of the negative controls (immunocytochemistry staining SP method, original magnification × 400); B: The expression of cyclin A on SGC-7901 cells of the negative controls (immunocytochemistry staining SP method, original magnification × 400); C: The expression of cyclin B1 on SGC-7901 cells of the negative controls (immunocytochemistry staining SP method, original magnification × 400); D: The expression of cyclin D1 on SGC-7901 cells of the negative controls (immunocytochemistry staining SP method, original magnification × 400); E: The expression of P16inf4aon SGC-7901 cells of c9,t11-CLA group (100 μmol•L⁻¹) (immunocytochemistry staining SP method, original magnification × 400); F: The expression of P21cip/waf1 on SGC-7901 cells of c9,t11-CLA group (100 μmol•L⁻¹) (immunocytochemistry staining SP method, original magnification × 400)
Expressions of cyclin A, B1 and D1 and P16ink4a, p21waf1
The the expression of rates cyclin A, B1, and D1 (Figure 3)on SGC-7901 cells was decreased (Table 2) after SGC-7901 cell were incubated with different concentrations of c9,t11-CLA for 24 h and 48 h while cyclin-dependent kinases inhibitors (P16ink4a, and P21waf1) increased (Table 3; Figure 3).
Table 2
Table 2
Positive rates of cyclin A, B1, and D1 on SGC-7901 cells treated with c9,t11-CLA (%)
Table 3
Table 3
Positive rates of p16ink4a and p21waf1 on SGC-7901 cells treated with c9,t11-CLA (%)
CLA is a naturally occurring fatty acid in animal's food. Dietary sources of CLA include grilled beef, cheese, and related foods[47]. Another source of CLA is its endogenous generation via the carbon centered free radical oxidation of linoleic acid[45]. Over the past ten years, a number of research works of animal experiments have supported the observation that CLA is an effective chemopreventive agent of cancer, and that it can inhibit carcinogenesis of different tissues at different stages of induction by chemical agents[44,45]. Several investigators in our group have reported that c9,t11-CLA is an effective agent to prevent carcinogenesis[48,49] and cancer[50-52]. Zhu's study[48] demonstrated that c9,t11-CLA could significantly inhibit the mice forestomach neoplasia induced by B(a)P (50 mg·kg-1) in post-initiation in short term (23weeks).The incidences of tumors of mice in the B(a)P group, B(a)P with high dose CLA (5 μL·g-1) group and B(a)P with low dose CLA (2.5 μL·g-1) group were 100%, 60% and 69% respectively (P < 0.05). Xue's research[49] also indicated that the incidence of neoplasm in mouse forestomach in the B(a)P group, 75% pure c9,t11-CLA group, 98% pure c9,t11-CLA group and 98% pure t10, c12-CLA group were 100.0%, 75.0%, 69.2%, and 53.8%, respectively. This maybe due to an inhibition mitogen of activated protein kinase (MAPK)-a way to reduce carcinogenesis. The data from our research group suggested that c9,t11-CLA could inhibit proliferation of cancer cells, i.e. SGC-7901 cells[50] and MCF-7 cells[51,52], and induced cancer cell (SGC-7901) apoptosis[53]. Moreover, the inhibiting effect of c9,t11-CLA on SGC-7901 cell proliferation may be related to cell cycle.
As shown in Figure 1, c9,t11-CLA at various concentrations in 8 d reduced the proliferative activity of SGC-7901 cells and its inhibitory rates were from 5.92% to 82.44%, but the mechanism of such inhibition of c9,t11-CLA has not been clarified. However, we discovered that SGC-7901 cells supplemented with c9,t11-CLA incorporated significantly less [3H] thymidine than negative controls (shown in Table I). The inhibitory rates of, from 16.4% to 78.2% after incubating with c9,t11-CLA for 24 h and from 14.0% to 97.5% after 48 h displayed a dose-response relationship.In the meantime, we investigated further the expressions of PCNA and protein from cell cycle such as cyclins and cyclin-dependent kinase inhibitors (CDKI) on SGC-7901 cells treated with various concentrations of c9,t11-CLA. PCNA (proliferating cell nuclear antigen) plays an essential role in both the replication and repair of DNA, and is an essential component of the DNA replication machinery, acting as the processing factor for polymerases δ and ε. In addition to its role in replication, PCNA is not only required for base excision-repair of nucleotides, but also binds to cell cycle regulatory proteins such as P21 and Gadd45[54]. In this study, we discovered that the expression of PCNA on SGC-7901 cells gradually decreased with increasing concentrations of c9,t11-CLA in comparison with negative controls (showed in Figure 2). In other words, DNA replication lessens, thereby resulting in slower on SGC-7901 cell proliferation.
The fundamental task of the cell cycle is to ensure that DNA is faithfully replicated once during S phase and that identical chromosomal copies are distributed equally to two daughter cells during M phase. The cell cycle is a complex process, regulated by many factors, which can be divided into three groups: cyclins (A, B, D, E....H); cyclin-dependent kinases (CDK, including CDK1-CDKa-7); CDK inhibitors (CDKI, including P16 family and P21 family). They are balanced through mutual interactions. Uncontrolled cell proliferation is the hallmark of cancers which are the result of damage to genes that directly regulate their cell cycles. Using immunocytochemical technique to detect expressions of cyclins and CDKI, we demonstrated that the expressions of cyclin A, B1 and D1 on SGC-7901 cells treated with various concentrations c9,t11-CLA were reduced, whereas expressions of CDKI (P16in4a and P21waf1) increased, as compared with those of negative controls. Successive actions of CDKs promote cell-cycle progression in mammalian cells. Various cyclins bind and activate CDKs at specific times during the cell cycle.
Mammalian cyclin A activates CDK2[55] in S-phase and CDK1 (Cdc2) in G2- and M-phases. One important mechanism that enables sequential activation of cyclin-CDK complexes is the periodic synthesis and destruction of cyclins. Cyclin A expression starts late in G1-phase and is increasing through S- and G2-phase before the protein is degraded in M-phase. The cell cycle-dependent expression of cyclin B1 is critical for the proper timing of a cell's entry into mitosis which is dependent both upon the binding of CDK1 to cyclin B1, as well as a series of phosphorylation and dephosphorylation events. The cyclin B1 protein accumulates during interphase and peaks at the G2-M phase transition[56]. One of the crucial substrates of G1 phase CDK, including CDK4 in the complex with D-type cyclins (cyclin D1, D2 and D3), is Rb protein (pRb), which is the product of the retinoblastoma susceptibility gene. Rb protein plays an important role in the regulation of the G1 to S phase progression in normal cells and the function of pRb is regulated by phosphorylation. Thus, during the G0 and G1 phase, Rb protein is in an un- or underphosphorylated stage and binds to E2F family transcription factors. Cylin Ds/CDK4 becomes activated around the mid G1 phase, resulting in the accumulation of increasingly phosphorylated, inactive forms pRb. This causes the release of E2F family transcription factors which induce the expression of S-phase genes by positive regulation through E2F-binding sites (see Figure 4)[57]. It is also known that abrogation of the functions of Cylin A prevents entry into the S phase. From the beginning of the S phase Rb protein remains in the hyperphosphorylated inactive state until the end of M phase; such condition is thought to be due to both cylin A/CDK2 and Cylin A, B1/Cdc2 in catalyzing the phosphorylation reaction[58]. P16ink4a is the founder member of a family of proteins with the ability to inhibit CDK4 and the CDK4-related kinase CDK6.
Figure 4
Figure 4
Figure 4
The relationship between CDKI (P16 and P21) and cyclins in G1/S transition
The INK4 family is composed of four members in mammalian organisms: P16ink4a, P15ink4b, P18ink4c, and P19ink4 d. The four mammalian INK4 proteins have similar biochemical properties: All of them bind to CDK4 and CDK6 and inhibit the kinase activity of the CDK4-6/Cylin D complexes (Figure 4)[59]. The INK4 inhibitor causes G1 arrest indicating that the phosphorylation of pRb on residues specific for CDK4 (and possibly CDK6) is critical for G1/S progression. While P21CIP1/waf1family, comprisingP21Cip1/waf1, P27CIP1 and P57CIP2, bind to a variety of CDKs and cyclins, preferentially to cyclin/CDK complexes rather than monomeric forms and also inhibit performed active cyclin/CDK complexes (see Figure 4)[59]. In addition to its role as a CDKI, P21Cip/waf1 has been shown to block DNA replication by direct interaction with PCNA mentioned above. However, P21CIP1/waf1 does not inhibit the PCNA-dependent nucleotide excision-repair of DNA. In deed, DNA damage leads to an increase in the level of P53, and result in P21-mediated cell cycle arrest in the G1 phase, which persists until DNA repair is completed[60].Thus, it is proposed that P21Cip/waf1 plays an important role under such conditions as terminal differentiation and cell senescence.
In conclusion, c9,t11-CLA may inhibit cell growth and proliferation by a decrease in the expressions of cyclin A, B1 and D1 and an increase in that of CDKI (P16ink4a and P21Cip1/waf1) on SGC-7901 cells in comparison with the negative controls. This result suggested that the inhibition effect of c9,t11-CLA on SGC-7901 cell proliferation is related to the cell cycle. The whole mechanism of the action of c9,t11-CLA on SGC-7901 cell cycle further research.
Edited by Lu HM
Supported by the National Natural Science Foundation of China, No. 39870661
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