CCAAT/Enhancer binding protein delta (c/EBPdelta) regulation and expression in human mammary epithelial cells: I. "Loss of function" alterations in the c/EBPdelta growth inhibitory pathway in breast cancer cell lines

"Loss of function" alterations in growth inhibitory signal transduction pathways are common in cancer cells. In this study, we show that growth arrest (GA) treatments--serum and growth factor withdrawal and growth inhibitory IL-6 family cytokines (Interleukin-6 and Oncostatin M (OSM))--increase STAT3 phosphorylation (pSTAT3), increase CCAAT enhancer binding protein delta (C/EBPdelta) gene expression and induce GA of primary, finite-lifespan human mammary epithelial cells (HMECs), and immortalized breast cell lines (MCF-10A and MCF-12A). In contrast, serum and growth factor withdrawal from human breast cancer cell lines (MCF-7, SK-BR-3, T-47D, and MDA-MB-231) for up to 48 h induced a relatively modest increase in pSTAT3 levels and C/EBPdelta gene expression and resulted in varying levels of GA. In most breast cancer cell lines, IL-6 family cytokine treatment increased pSTAT3 levels and C/EBPdelta gene expression, however, growth inhibition was cell line dependent. In addition to "loss of function" alterations in growth inhibitory pathways, breast cancer cell lines also exhibit "gain of function" alterations in growth signaling pathways. The Akt growth/ survival pathway is constitutively activated in T-47D and MCF-7 breast cancer cells. The Akt inhibitor LY 294,002 significantly enhanced T-47D growth inhibition by serum and growth factor withdrawal or IL-6 family cytokine treatment. Finally, we show that activation of the pSTAT3/C/EBPdelta growth control pathway is independent of estrogen receptor status. These results demonstrate that "loss of function" alterations in the pSTAT3/C/EBPdelta growth inhibitory signal transduction pathway are relatively common in human breast cancer cell lines. Defective activation of the pSTAT3/ C/EBPdelta growth inhibitory signal transduction pathway, in conjunction with constitutive activation of the Akt growth stimulatory pathway, may play a synergistic role in the etiology or progression of breast cancer.