Survival Signals Generated by Estrogen and Phospholipase D in MCF-7 Breast Cancer Cells Are Dependent on Myc

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Abstract
Estrogens, which have been strongly implicated in the development of breast cancer, enhance proliferation of mammary epithelial cells and, importantly, estrogen receptor (ER)-positive breast cancer cells. In the absence of serum growth factors, the ER-positive MCF-7 breast cancer cell line undergoes apoptosis. Estrogens, most commonly 17-β-estradiol (E2), can suppress apoptosis in MCF-7 cells deprived of serum. While E2 stimulated a short-term transient increase in Myc expression, E2 stimulated a sustained increase in Myc expression that was detectable at 48 h and pronounced at 5 days, the point where increased proliferation of MCF-7 cells in the absence of serum could be detected. The delayed increase in Myc expression was not dependent upon transcription of the Myc gene. Suppression of Myc expression reversed the survival effects of E2. The Myc-dependent survival signal generated by E2 was dependent upon basal levels of mTOR (mammalian target of rapamycin) and two upstream regulators of mTOR, phosphatidylinositol 3-kinase and phospholipase D (PLD). Stable elevated expression of PLD2 also increased Myc expression and provided a Myc-dependent survival signal in the absence of E2. These data provide evidence that E2 promotes survival signals in breast cancer cells through an mTOR-dependent increase in Myc expression. The data also suggest that elevated PLD expression, which is common in breast cancer, confers E2 independence.