Mechanisms of Action of Metformin as an Anti-cancer Agent

Cancer is the second leading cause of death worldwide and epidemiological studies suggest the association of diabetes mellitus with an enhanced risk for multiple cancers. Metformin (1,1-dimethylbiguanide hydrochloride) is the most widely prescribed anti-diabetic drug. However, in addition to its anti-diabetic activity metformin exhibits antineoplastic effects by inhibiting development of tumors and also by inhibiting tumor growth, survival and metastasis.

Specificity protein (Sp) transcription factors (TFs) belong to the Sp/Kruppel-like family of transcription factors (KLFs). Sp1 and other Sp proteins are overexpressed in many tumors and regulate the expression of genes essential for cancer cell proliferation, growth, angiogenesis, and survival. Based on the reported metformin-induced activities in cancer cells and tumors, we hypothesize that the anti-neoplastic effect of metformin is due, in part, to downregulation of Sp transcription factors in cancer cells. Treatment of pancreatic cancer cells with metformin inhibited cell proliferation, induced apoptosis and also downregulated Sp1, Sp3 and Sp4 proteins and several prooncogenic Sp-regulated genes. Metformin also decreased microRNA-27a and induced the Sp repressor, ZBTB10, and disruption of miR-27a:ZBTB10 interaction by metformin was mediated by MAPK phosphatases 1 and 5 (MKP1 & MKP5).

Furthermore, we also demonstrated that treatment with metformin or downregulation of Sp TFs by RNA interference (RNAi) inhibited two major pro-oncogenic pathways in pancreatic cancer cells, namely insulin-like growth factor receptor (IGF-1R) mediated mTOR signaling and epidermal growth factor (EGFR)-dependent activation of RAS. Knockdown of IGF-1R and EGFR inhibited mTOR signaling and RAS activity respectively. Metformin also inhibited pancreatic tumor growth and downregulated Sp and Sp regulated genes in tumors in an orthotopic model.

We also investigated the antineoplastic effect of metformin in breast cancer cells. The effects of metformin in breast cancer cells were comparable to those observed in pancreatic cancer cells. In addition, metformin also decreased expression of ErbB2 in breast cancer cells overexpressing this oncogene. Treatment with metformin or downregulation of Sp TFs by RNAi decreased expression of ErbB2, YY1 and mTOR signaling. Results of this study have unraveled an important mechanism of action of metformin in cancer cells which will facilitate the design of clinical applications of metformin in various combination drug therapies.