Understanding the benefit of metformin use in cancer treatment

Abstract

Biguanides have been developed for the treatment of hyperglycemia and type 2 diabetes. Recently, metformin, the most widely prescribed biguanide, has emerged as a potential anticancer agent. Epidemiological, preclinical and clinical evidence supports the use of metformin as a cancer therapeutic. The ability of metformin to lower circulating insulin may be particularly important for the treatment of cancers known to be associated with hyperinsulinemia, such as those of the breast and colon. Moreover, metformin may exhibit direct inhibitory effects on cancer cells by inhibiting mammalian target of rapamycin (mTOR) signaling and protein synthesis. The evidence supporting a role for metformin in cancer therapy and its potential molecular mechanisms of action are discussed.

Figure 1

Direct and indirect effects of metformin on cancer. Metformin activates AMPK leading to stabilization of TSC2 and inhibition of mTORC1 signaling and protein synthesis. Metformin can also directly target mTOR independently of AMPK and TSC2. Systemically, metformin sensitizes tissues to insulin, reduces hepatic gluconeogenesis, and lowers circulating insulin levels, indirectly reducing receptor tyrosine kinase activation and PI3K signaling. AMPK = AMP-activated protein kinase; 4E-BP1 = eukaryotic initiation factor 4E-binding protein-1; LKB1 = liver kinase B1; mTORC1 = mammalian target of rapamycin complex 1; PI3K = phosphatidylinositol-3-kinase; PKB/Akt = protein kinase B; PTEN = phosphatase and tensin homologue deleted on chromosome 10; Rheb = Ras homologue enriched in brain; S6K = ribosomal protein S6 kinase; TSC2 = tuberous sclerosis complex 2.