Mechanism of Metformin-dependent Inhibition of Mammalian Target of Rapamycin (mTOR) and Ras Activity in Pancreatic Cancer: Role of specificity protein (Sp) Transcription Factors

Vijayalekshmi Nair,Riyaz Basha,Maen Abdelrahim,Aline Rodrigues Hoffman,Sandeep Sreevalsan,Ala Abudayyeh,Stephen Safe

doi:10.1074/jbc.m114.592576

Abstract

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.

Highlights

Metformin inhibits pancreatic cancer cell and tumor growth and down-regulated specificity protein (Sp) transcription factors
Similar results were observed after treatment with 15 mM metformin, and L3.6pL cells were more sensitive than Panc28 cells
Immunostaining of pancreatic tumors and normal pancreatic tissue from an orthotopic mouse model using L3.6pL cells [31] showed that the enhanced staining of phosphomTOR in tumors was decreased in pancreatic tumors from mice treated with metformin (Fig. 1C)

Summary

Background

Metformin inhibits pancreatic cancer cell and tumor growth and down-regulated Sp transcription factors. Treatment with metformin or down-regulation of Sp TFs by RNAi 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. Studies in this laboratory reported a novel mechanism of action for metformin in pancreatic cancer cells This involved down-regulation of specificity protein (Sp) transcription factors Sp1, Sp3, and Sp4 and pro-oncogenic Sp-regulated genes such as bcl, fatty acid synthase (FAS), survivin, vascular endothelial growth factor (VEGF), and VEGF receptor 1 (VEGFR1) [31]. Treatment with metformin or silencing Sp transcription factors by RNAi decreased epidermal growth factor receptor (EGFR) expression, resulting in inhibition of Ras activity, which is regulated by EGFR in pancreatic cancer cells [37, 38]. We know show for the first time that metformin-dependent inhibition of both mTOR signaling and Ras activity is due to down-regulation of Sp transcription factors in pancreatic cancer

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION