Therapeutic influence of simvastatin on MCF-7 and MDA-MB-231 breast cancer cells via mitochondrial depletion and improvement in chemosensitivity of cytotoxic drugs

Background: Breast cancer is the most commonly diagnosed cancer worldwide with 2.26 million cases in 2020. Cancer heterogeneity is the major challenge before existing therapeutic modalities due to metabolic variability of the cells as Warburg and anti-Warburg both type of metabolic phenotypes has been reported as a major contributing factors for cancer progression, invasion, metastasis and relapse. Also, this metabolic variability is associated with chemo and radio-resistance and poor therapeutic outcomes. Therefore, in present study we put an attempt to understand how simvastatin exert its effects on two metabolically different cell types and second how this drug can affect mitochondrial biomass, mt-DNA and glycolysis in both the cell types. Methods: We have observed effects of simvastatin on MCF-7 (dependent more on OXPHOS) and MDA-MB-231 (TNBC; more glycolytic with defected mitochondria) cells alone and after simvastatin pre-treatment followed by cytotoxic drugs including cisplatin, doxorubicin, gemcitabine, vincristine. We have conducted MTT assay for viability, cell death detection assay, apoptotic morphology study, scratch assay, transwell migration assay, lactate estimation in media (glycolysis parameter), mt-DNA to n-DNA ratio, mitotracker red (for mitochondrial membrane potential) and mitotracker green staining (for mitochondrial biomass) and qPCR to study expression of mitochondrial transcription factors and apoptotic genes including PGC-1α, NRF-1, NRF-2, TFAM, Bcl-2 and Bax. Results: We observed that 20 μM simvastatin (SIM) was most efficient dose for MCF-7, whereas 12.5 μM for MDA-MB-231 cells. Simvastatin itself caused a significant decrease in viability, increased cell death, and diminished wound closure in scratch assay as well as inhibited transwell migration. Also, the cells pre-treated with simvastatin for 72 h followed by treatment with cytotoxic drugs for 48 h increased chemo-sensitivity of cisplatin (CIS), doxorubicin (DOX), gemcitabine (GEM) and vincristine (VIN). SIM alone and in pre-treatment followed by cytotoxic drug treatment studies, there was a significant decrease in mitochondrial biomass and mitochondrial membrane potential (MMP), but also decreased glycolysis as evidenced by decrease in lactate levels in culture media. For inhibition of migratory potential, it was in the following order: CIS ˃ VIN ˃DOX˃ GEM, which was in the same order to diminish mitochondrial functionality (mt-DNA/n-DNA ratio, mitotracker green staining and a significant decrease in the expression of transcriptional factors of mitochondrial biogenesis). Contrastingly a decrease in the same order was observed in lactate concentration independent to the mitochondrial loss, but probably via inherent ability of the drugs to reduce lactate and glycolysis. However, for cell death, apoptotic phenotype, diminished expression of Bcl-2 along with increase in Bax and loss of viability, the efficiency of simvastatin alone and in pre-treatment studies was in the following order: VIN ˃ DOX˃GEM˃CIS, which was supported by loss of fluorescence of mitotracker red, suggested decrease in MMP; marker of cell death. Conclusion: We conclude that by using different doses simvastatin can target different metabolic phenotypes of breast cancer cells and can also increase the chemo-sensitivity of cytotoxic drugs, so that they can work efficiently at lower doses which will ultimately diminish the cost and toxicity issues