Mechanism of Metformin-dependent Inhibition of Mammalian Target of Rapamycin (mTOR) and Ras Activity in Pancreatic Cancer ROLE OF SPECIFICITY PROTEIN (Sp) TRANSCRIPTION FACTORS

Background: Metformin inhibits pancreatic cancer cell and tumor growth and down-regulated Sp transcription factors. Results: Inhibition of mTOR and Ras signaling by metformin is due to decreased expression of Sp-regulated insulin-like growth factor-1 receptor (IGF-1R) and epidermal growth factor receptor (EGFR), respectively. Conclusion: Metformin-induced down-regulation of Sp proteins affects multiple pro-oncogenic pathways. Significance: These results identify important metformin-induced anticancer activities. The antidiabetic drug metformin exhibits both chemopreventive and chemotherapeutic activity for multiple cancers including pancreatic cancer; however, the underlying mechanism of action of metformin is unclear. A recent study showed that metformin down-regulated specificity protein (Sp) transcription factors (TFs) Sp1, Sp3, and Sp4 in pancreatic cancer cells and tumors, and this was accompanied by down-regulation of several pro-oncogenic Sp-regulated genes. Treatment with metformin or down-regulation of Sp TFs by RNAi also inhibits two major pro-oncogenic pathways in pancreatic cancer cells, namely mammalian target of rapamycin (mTOR) signaling and epidermal growth factor (EGFR)-dependent activation of Ras. Metformin and Sp knockdown by RNAi decreased expression of the insulin-like growth factor-1 receptor (IGF-1R), resulting in inhibition of mTOR signaling. Ras activity was also decreased by metformin and Sp knockdown of EGFR, another Sp-regulated gene. Thus, the antineoplastic activities of metformin in pancreatic cancer are due, in part, to down-regulation of Sp TFs and Sp-regulated IGF-1R and EGFR, which in turn results in inhibition of mTOR and Ras signaling, respectively.