The adaptation of cancer cells to serum deprivation is mediated by the mTOR-dependent cholesterol synthesis

Cancer cells are capable of sustaining survival independently of exogenous growth factors. How they adapt to serum deprivation was addressed in this study. Cancer cells exposed to serum-free medium showed a markedly reduced proliferation rate in a range of 30% during the first 24 hours that reached about 50% on the second day. Our transcriptomic analysis in two cancer celllines revealed that mRNAs encoding key enzymes of the cholesterol synthesis pathway were upregulated under serum deprivation. This induction of cholesterol biosynthesis represented critical adaptive response, as a pharmacological inhibition of the pathway with statin triggered robust apoptotic cell death accompanied by generation of a mitochondrial reactive oxygen species. The mechanistic target of rapamycin complex 1 (mTORC1), a master regulator of cell growth, is known to be engaged in controlling lipid biosynthesis. We detected the high polysomal and preribosmal peaks not only in serum containing medium but also under serum deprivation indicating a high rate of protein synthesis and ribosomal biogenesis independent of serum. Besides, the inhibition of mTOR kinase activity substantially reduced polysome abundance, with a more pronounced effect in serum deprived cancer cells. Notably, the mTOR kinase inhibition also prevented the upregulation of the cholesterol synthesis enzyme that established a direct link between mTOR activity, protein synthesis, and cholesterol biosynthesis. Together, our results show that cancer cells adapt to serum withdrawal by activating the cholesterol synthesis pathway through mTOR-dependent regulation of gene expression and protein synthesis, underscoring a critical mechanism of survival under serum withdrawal.