Integrating signals between cAMP and MAPK pathways in breast cancer.

Breast cancer is one of the most common malignancies in Western society. Localized breast cancer, before it spreads, can be cured by surgery. However, the high mortality rate associated with breast cancer is due to a propensity of the tumor to metastasize when the primary tumor is small or undetectable. Although steroid receptor status has been recognized as the most precise predictor of response to hormone therapy, a significant number of tumors expressing these receptors metastasize and patients do not respond to the antihormone therapy. The mechanism leading to breast cancer progression and resistance to the hormone therapy is not completely understood at the present time. Compelling evidence shows that hormone-bound steroid receptors in breast cancer cells activate complex signaling networks, which include MAPK- and G protein-dependent pathways. These responses, which occur within seconds or minutes after steroid administration, are not due to changes in gene expression. Depending on cell systems, steroid activation of these networks leads to different and profound effects on extra nuclear and nuclear events. In such a way steroids foster cell cycle, reduce apoptosis and stimulate cell migration of target cells. All these processes are deregulated in breast cancer. In this review we will discuss new aspects of signaling pathways activated by steroids and their integration with other pathways in breast cancer. Recent findings on the discovery of compounds specifically interfering in such a complex network will be presented.

Hormone-dependent nuclear export of estradiol receptor and DNA synthesis in breast cancer cells.

In breast cancer cells, cytoplasmic localization of the estradiol receptor alpha (ERalpha) regulates estradiol-dependent S phase entry. We identified a nuclear export sequence (NES) in ERalpha and show that its export is dependent on both estradiol-mediated phosphatidylinositol-3-kinase (PI3K)/AKT activation and chromosome region maintenance 1 (CRM1). A Tat peptide containing the ERalpha NES disrupts ERalpha-CRM1 interaction and prevents nuclear export of ERalpha- and estradiol-induced DNA synthesis. NES-ERalpha mutants do not exit the nucleus and inhibit estradiol-induced S phase entry; ERalpha-dependent transcription is normal. ERalpha is associated with Forkhead proteins in the nucleus, and estradiol stimulates nuclear exit of both proteins. ERalpha knockdown or ERalpha NES mutations prevent ERalpha and Forkhead nuclear export. A mutant of forkhead in rhabdomyosarcoma (FKHR), which cannot be phosphorylated by estradiol-activated AKT, does not associate with ERalpha and is trapped in the nucleus, blocking S phase entry. In conclusion, estradiol-induced AKT-dependent phosphorylation of FKHR drives its association with ERalpha, thereby triggering complex export from the nucleus necessary for initiation of DNA synthesis and S phase entry.