The AMPK Modulator Metformin as a Novel Adjunct to Conventional Therapy in Rheumatoid Arthritis Patients

Rheumatoid arthritis (RA) is a chronic, multisystem inflammatory disorder of unknown etiology. It is characterized by polyarthritis and systemic inflammation. The inflammation in the hyperplastic synovium can destruct the structure of affected joints. Although the exact molecular mechanism underlying the pathogenesis of RA remains unknown, it is suggested that T helper cell (Th) 17 plays a central role. Interleukin (IL)-17, mostly secreted by Th17, has synergistic effects with tumor necrosis factor- (TNF-α), IL-1β and IL-6 in cases of RA. Metformin, a traditional antidiabetic medication, exerts glucose lowering effects by activating AMP-activated protein kinase (AMPK), a critical enzyme involved in the lipid and glucose metabolism. In addition to the antidiabetic effect, metformin has been shown to inhibit Lipopolysaccharide-Induced Inflammation (LPS)-induced inflammation by suppress NF-κB production, which is also regulated by AMPK . In addition, metformin activated AMPK may inhibit mammalian target of rapamycin (mTOR), which then regulate the differentiation of T cells in vitro and in vivo. In particular, AMPK has been reported to be associated with the inhibition of TH17 cells through suppressing the phosphorylation of mTOR and its downstream signal transducers, suggesting the therapeutic potential of AMPK agonists. Besides the imbalance between Th17 cells and Treg cells is responsible for the chronic inflammation in RA, synovium hyperplasia also plays a central role in the joint destruction. It has been reported that rapamycin, an inhibitor of mTOR, is able to significantly reduce fibroblastlike synovial cell invasion in RA via suppressing mTOR signaling pathway. These regulatory effects of AMPK on the inflammation, immune and fibroblast-like synovial cells have prompted the investigation on the effects of metformin on rheumatoid arthritis.