Glucocorticoid receptor-mediated survival signaling in breast cancer

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Improving breast cancer cure rates will require a detailed molecular understanding of the anti-apoptotic mechanisms used by cancer cells to resist both chemotherapy and "targeted" treatments. Our laboratory has uncovered a novel anti-apoptotic signaling pathway in breast cancer that is initiated glucocorticoid receptor (GR) activation. Because glucocorticoids are physiological stress-induced hormones and the GR is ubiquitously expressed in breast epithelium, identifying the underlying mechanisms of GR-mediated epithelial cell survival has important implications for advancing our knowledge of both cancer etiology and resistance to therapy. Currently, relatively little is known about the downstream events underlying GR- mediated anti-apoptotic signaling in human mammary epithelial cells (hMECs). Our laboratory has used large-scale microarray and bioinformatic analyses to characterize dynamic gene expression changes over 24 hours following GR activation in hMECs. Through these studies, we have identified MAP kinase phosphatase-1 (MKP-1) and serum and glucocorticoid inducible kinase-1 (SGK-1) as early transcriptional targets of the GR, and we have recently demonstrated the requirement for SGK-1 and MKP-1 activity in GR- mediated survival signaling. SGK-1 and MKP-1, via their potent kinase and phosphatase activities, in turn can regulate the activity of the transcription factors ELK-1 and FOXOSa. We hypothesize that GR-mediated induction of MKP-1 and SGK-1 alters ELK-1 and FOXOSa transcriptional activity, respectively, in turn causing key changes in anti-apoptotic gene expression. In this renewal application, we propose to continue these studies by identifying the specific mechanisms downstream of MKP-1 and SGK-1 induction that contribute to cell survival. In Aim 1, the GR/MKP-1/ELK-1 pathway will be defined by first validating the ELK- 1 putative targets (identified from gene expression studies) and then examining the role of these targets in GR-mediated cell survival. In parallel, Aim 2 will examine the GR/SGK-1/ FOXOSa pathway. In addition, the possible molecular "cross-talk" between these two pathways will be investigated. In Aim 3, a breast cancer xenograft model will be used to determine the in vivo role of SGK-1 and MKP-1 activity in anti-apoptotic signaling, gene expression and resistance to chemotherapy. The completion of these aims is expected to advance our understanding of anti-apoptotic signaling and therapy-resistance in epithelial cancers.