The Effects of Metformin on Pancreatic Cancer Metabolism

Abstract

Metformin (MET) is a widely used drug indicated for type-2 diabetes management. Interestingly, numerous epidemiological studies show that MET may confer protective benefits from overall cancer risk and cancer-related mortality. Various pre-clinical studies show that MET also exerts chemotherapeutic properties using doses that far exceed those for glycemic control. Currently, there are numerous ongoing clinical trials testing the chemotherapeutic advantages of MET treatment in various cancer types using doses that are within the therapeutic range for diabetes management. We sought out to determine whether therapeutic doses of MET for diabetes management is chemopreventive or chemotherapeutic. We have shown that such doses are chemopreventive at best since they were unable to decrease cell viability in BxPC-3 (wild type K-RAS) and MIA PaCa-2 (mutant K-RAS) pancreatic cancer (PDAC) cells. Through a targeted metabolomics approach using 1,2-¹³C₂-D-glucose as the sole tracer, we have shown that a therapeutic dose (100 μM) of MET that is prescribed for diabetes management inhibits glucose-derived new palmitate synthesis when acetyl-CoA is dedicated towards de novo fatty acid synthesis. This occurs when a) K-RAS mutation is present (in MIA PaCa-2 cells) and b) cholesterol in the form of the more water-soluble derivative cholesteryl hemisuccinate (CHS) is supplemented in the media to prevent acetyl-CoA from being directed towards cholesterol synthesis. Immnunoblot analyses showed that this phenomenon is regulated by decreased protein expression of the fatty acid synthase (FAS) enzyme. We also showed that chronic (every two days in 30 days) CHS and MET treatment decreased triglyceride intracellular concentrations in BxPC-3 and MIA PaCa-2 cells. These treatments exerted no effect on FASN gene expression, indicating that the lipid-inhibitory effects of MET in PDAC is regulated at the metabolic flux and protein expression levels. Finally, we interrogated the metabolic effects of MET treatment using uniformly-labeled glucose tracer (¹³C₆-D-glucose) and gas chromatography/mass spectrometry (GC/MS) using a LSL-K-rasᴳ¹²ᴰ/⁺;LSL-Trp53ᴿ¹⁷²ᴴ/⁺;Pdx-1-Cre (KPC) mouse model of PDAC. We showed that acute (5 days), high-dose treatment (250 mg/kg body weight administered intraperitoneally) of MET reversed the glycolytic metabolism of pancreatic tumor into an oxidative one in the presence of K-ras and Tp53 mutation. MET, regardless of K-ras status, inhibited glucose-derived acetate enrichment towards palmitate synthesis. Immunohistochemistry analysis of KPC pancreases revealed that a decrease in the tumor cell proliferation marker Ki67 and in the FAS protein was observed in KPC mice treated with MET. In summary, we have identified a mechanism of action of the popular anti-hyperglycemic drug MET against PDAC. It is an inhibitor of de novo fatty acid synthesis that is dependent on the K-RAS and metabolic status of the tumor. Future pre-clinical and clinical studies should take this into consideration when performing mechanistic studies on MET and when investigating the potential chemotherapeutic effects of this drug. It is also important to determine what dose of MET is chemotherapeutic in the clinic. As shown by other pre-clinical studies and ours as well, this dose exceeds the range for diabetes management.