Metformin revisited: Does this regulator of AMP-activated protein kinase secondarily affect bone metabolism and prevent diabetic osteopathy

Figure 1 Metformin actions via osteoblasts are pro-osteogenic and anti-resorptive. Metformin is incorporated into osteoblasts, where it inhibits intracellular ROS production and induces AMPK phosphorylation/activation. This increases eNOS activity and NO production, promoting osteoblast proliferation. In addition, activated AMPK up regulates osteoblastic differentiation and mineralization via expression of Runx2 and SHP, while decreasing osteoclastic recruitment and bone-resorbing activity through a reduction in osteoblastic RANKL/OPG ratio. ROS: Reactive oxygen species; AMPK: AMP-activated protein kinase; OPG: Osteoprotegerin; RANKL: Reduced receptor activator of nuclear factor-κB ligand; SHP: Small heterodimer partner; RUNX2: Runt-related transcription factor 2; AMP: Adenosine monophosphate; ATP: Adenosine triphosphate; Met: Metformin.

Figure 2 Metformin induces an increase and redistribution of activated AMP-activated protein kinase in multinucleated osteoclasts. UMR106 osteoblasts and Raw 264.7 macrophages were co-cultured for 7 d, in the absence (A) or presence (B) of 500 mol/L metformin. Cells were fixed, permeabilized and incubated with an anti-phosphorylated AMPK antibody, followed by a FITC-conjugated secondary antibody (green). Nuclei were counterstained with propidium iodide (red). Cells were visualized with a Leica TSC SP5 AOBS confocal microscope. Metformin induced an increase in activated AMPK with a punctillate and predominantly cytoplasmic distribution. AMPK: AMP-activated protein kinase.

Figure 3 Effects of metformin on bone-derived cells. The results of several in vitro studies show that metformin modulates the phenotypic balance of bone marrow stromal cells (MSC) away from adipogenesis and towards osteoblastogenesis. In addition, metformin increases in vitro the bone-forming capacity of osteoblasts, while decreasing the recruitment and bone-resorbing activity of osteoclasts. ALP: Alkaline phosphatase; TG: Triglycerides; TRAP: Tartrate-resistant acid phosphatase; RANK: Receptor activator for nuclear factor κB; RANKL: RANK ligand.

Figure 4 Actions of metformin on bone metabolism - animal studies. Orally administered metformin promotes the osteogenic potential of bone MSC, increases the quality of bone tissue (improving its micro-architecture and mineral density) and facilitates the repair of bone lesions. In addition, metformin may prevent experimental diabetic osteopathy as well as ovariectomy-induced bone loss. MSC: Marrow stromal cells; BMD: Bone mineral density; p-AMPK: Phosphorylated AMP-activated kinase.