bWe report a negative feedback loop between the signaling protein phospholipase D (PLD), phosphatidic acid (PA), and a specific set of microRNAs (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 overcrowding, and nutrient starvation for 3 to 6 h increase expression even further. However, after prolonged (>12-h) starvation, PLD levels return to basal or lower levels. The mechanism for this is as follows. 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-epithelial-to-mesenchymal transition (post-EMT) cancer cells. Second, continued PA production in later starvation induces expression of PLD-targeting microRNA 203 (miR-203), miR-887, miR-3619-5p, and miR-182, which reduce PLD translation. We provide direct evidence for a feedback loop, whereby PLD induction upon starvation leads to PA, which induces expression of miRNAs, which 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 allow cells to survive nutrient deprivation, which normal cells cannot do.M icroRNAs (miRNAs) are short molecules of noncoding RNA, ϳ22 nucleotides in length, and have crucial roles in the regulation of many cellular processes, including development, proliferation, differentiation, apoptosis, and stress response (1, 2). Mature miRNA molecules associate with the Argonaute (Ago1 and Ago2) proteins and the RNA-induced silencing complex (RISC) (3, 4). Active miRNAs regulate expression of their target genes via association of an ϳ7-nucleotide-long stretch seed region with a complementary sequence in the target mRNA located in the 3= untranslated region (UTR). Binding of miRNAs to their target mRNAs along with the RISC complex mediates inhibition of translation initiation (5). miRNA involvement in cancer development and metastasis is the subject of intense research (6-10).Phospholipase D (PLD) has been implicated in cellular signals that suppress apoptosis and contribute to cancer cell survival (11-13). Through cell signaling, elevated PLD activity leads to activation of mammalian target of rapamycin (mTOR), a survival signal often hyperactivated in cancer (14, 15). Elevated PLD activity also subdued the tumor suppressors p53 and protein phosphatase 2A (12). Zheng et al. published a model for enhanced survival and migration signals in the developing tumor (16). In a developing tumor mass, cells inside the mass were subjected to hypoxia and nutrient and growth factor deprivation. It is proposed that cells that respond to stress by elevating PLD protein levels will survive presumably by gaining the ability to migrate. However, very little is known about PLD regulation at gene and protein leve...