A Repertoire of MicroRNAs Regulates Cancer Cell Starvation by Targeting Phospholipase D in a Feedback Loop That Operates Maximally in Cancer Cells

Abstract

We report a negative feedback loop between the signaling protein phospholipase D (PLD), phosphatidic acid (PA) and a specific set of miRNAs during nutrient starvation of breast cancer cells. We show that PLD expression is increased in four breast cancer cell lines, and that hypoxia, cell over-crowding and nutrient starvation for 3-6 hrs increase expression even further. However, after prolonged (>12 hr) starvation, PLD levels are back to basal or lower levels. The mechanism for this is, first, during initial starvation, an elevated PA (the product of PLD enzymatic activity) activates mTOR and S6K, known to inhibit apoptosis and enhances cell migration especially in post-EMT cancer cells. Second, continued PA production at later starvation induces expression of PLD-targeting micro-RNAs: miR-203, 887, 3619-5p and 182, that reduce PLD translation. We provide direct evidence for a feedback loop, where PLD induction upon starvation leads to PA, which induces expression of miRNAs that in turn inhibits PLD2 translation. The physiological relevance for breast cancer cells is that as PA can activate cell invasion, then, due to the negative feedback, it can deprive mTOR and S6K of their natural activator. It can further prevent inhibition of apoptosis, and survive nutrient deprivation, which normal cells cannot do.