Abstract P1-08-02: Metformin blocks de novo synthesis of cholesterol in triple negative breast cancer cells

Background: Metabolic shifts in cancer and stem cells are well recognized (the so called Warburg Effect). Tumor cells frequently demonstrate alterations in lipid metabolism. The anti-diabetic drug metformin has been shown to have broad anti-cancer activity, reducing both the incidence and mortality of several types of cancer. Metformin inhibits pro-oncogenic cell signaling and metabolic pathways, and has demonstrated unique anti-stem cell activity. We have reported that metformin is particularly active against triple negative (TN) breast cancer cells, inducing S phase arrest and apoptosis. Metformin down regulates de novo fatty acid synthesis, significantly reducing fatty acid synthase (FASN) and FASN protein expression. Mechanistically, these shifts appear secondary to a metformin associated up regulation of miR-193a, which directly targets the FASN 3"UTR (In press, Wahdan-Alaswad et al, Hormones and Cancer). Cancer cells require high cholesterol for membrane synthesis, rigidity, the formation of lipid rafts (membrane regions which organize critical receptor/signaling molecules including the epidermal growth factor receptor (EGFR)), and to provide precursor molecules for hormones and sterols. While de novo cholesterol biosynthesis and high systemic cholesterol levels have been associated with an increase in breast cancer risk and a worse prognosis for women with the disease, clinical studies of statins (drugs to inhibit the mevalonate synthesis pathway and thus block cholesterol synthesis) have shown mixed results. Silvente-Poirot S et al have suggested that the contradictory effects of anti-cholesterol strategies may be due to the complexity of the genes and enzymes controlling the "sterolome" (Science, 2014). Results: The effects of metformin on de novo cholesterol biosynthesis in the TN breast cancer cells, MDA-MB-231 and MDA-MB-468, were studied using expression profiling and miRNA microarrays. Metformin treatment for 24 h down regulated the expression of 21 genes encoding enzymes in the cholesterol biosynthesis pathway at high levels (up to 9 fold), which has been confirmed by qRT-PCR. Metformin treatment for 6 h also resulted in the upregulation of 17 miRNA, which are predicted to control many of the down regulated genes. These data suggest a post-translational inhibition of cholesterol biosynthesis, similar to what we have reported for fatty acid biosynthesis (see above). The down regulated enzymes include critical steps in complex cholesterol biosynthesis pathway occurring in the cytosol, endoplasmic reticulum and the nuclear envelope. Western blots have confirmed that miRNA 141 and miR192 reversed the metformin induced reduction in p-MEK1/2, p-Akt and pMAPK. Conclusions: Metformin regulates cancer cells by multiple pathways including reduction in cell growth, increases in apoptosis and alterations in cellular metabolism. The effects on the cholesterol pathway are both extensive and affect multiple downstream pathways including EGFR, Akt, and MAPK. Partial funding was provided by the Susan G. Komen Foundation. Citation Format: Ann D Thor, Zeying Fan, Reema Wahdan-Alaswad, Steve M Anderson, Jennifer K Richer, Susan M Edgerton. Metformin blocks de novo synthesis of cholesterol in triple negative breast cancer cells [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P1-08-02.