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Regulation of Prolactin Receptor Stability and Signaling by Ras Oncogene
The pituitary hormone prolactin (PRL) plays a key role in development of mammary gland and breast cancer. Human breast cancer cells produce their own PRL and exhibit high levels of PRL signaling, which promotes proliferation and survival of these very cells. All effects of PRL on cells are mediated through engaging a PRL receptor (PRLr) on cell surface. Regulation of the levels of PRLr is paramount to control of the extent of cellular responses to PRL. Upon treatment of cells with PRL, its receptor undergoes ubiquitination, endocytosis and degradation in the lysosomes. Ubiquitination of PRLr depends on its phosphorylation on Ser349 and interaction with ƒÒ-TrCP E3 ubiquitin ligase. Breast cancers often exhibit constitutive activation of signal transduction pathways that depend on activity of proto-oncogene Ras. Our preliminary data show that activation of these pathways in breast cells results in accumulation of PRLr. We hypothesize that activation of Ras-dependent signal transduction pathways leads to impaired ubiquitination, endocytosis and degradation of PRLr. These aberrations, in turn, should result in accumulation of PRLr and augmentation of PRL-induced signaling in breast cells harboring activated Ras. To test our hypothesis we propose to: (1) investigate the effects of activated Ras as well as activators and inhibitors of signaling pathways that are downstream of Ras (including phosphoinositide 3-kinase and mitogen-activated protein kinases) on ubiquitination, endocytosis and degradation of PRLr; (2) determine how these active Ras and activators and inhibitors of its downstream mediators affect the extent of cellular responses to PRL. These studies will expand our knowledge on the mechanisms regulating the hormonal effects of PRL and will gain insight in the mechanisms underlying PRLr degradation and the role of Ras in regulating cellular responses to PRL. In addition, successful completion of these studies will likely shed light on the role of Ras and PRLr stability in pathogenesis of breast cancer and potentially provide the rationale for future development of combination of PRL and Ras inhibitors for therapy of breast cancer.
Growth and survival of breast cells depends on the actions of hormone prolactin (PRL), which is normally secreted by pituitary gland. This secretion peaks at pregnancy resulting in development of breast tissue and production of milk. Breast cancer cells utilize the effects of PRL for their own growth and survival. Breast cancer cells produce their own PRL, which plays a key role in development and progression of breast tumors.
All effects of PRL on cells are triggered by interaction of PRL with its receptor (PRLr) on the surface of cells. PRLr mediates PRL-induced signaling that leads to increased cell growth and survival. Normal cells limit the effects of PRL by the mechanism that works as follows. Cells treated with PRL remove PRLr protein from the cell surface through internalization of these proteins followed by their degradation. Such down regulation results in a decrease in PRLr levels, hence, another PRL molecule has less chances of engaging PRLr and initiate another round of signaling. This down regulation of PRLr in response to PRL is a pivotal mechanism limiting the extent of cellular responses to PRL. This mechanism relies on ubiquitination, which is conjugation of PRLr with a small protein ubiquitin.
Our preliminary data show that PRLr degradation is impaired in breast cancer cells that exhibit an abnormal activation of Ras oncogene. Ras-dependent pathways are widely activated in a majority of breast cancers. It is plausible that activation of Ras impedes down regulation of PRLr at some step (ubiquitination or/and internalization or/and degradation), and, therefore, enables tumor cells to enjoy the endless flow of signaling triggered by PRL, which is essential for growth and survival of breast cancer cells. Thus, it is important to understand the mechanisms that underlie the ability of Ras to stabilize PRLr and increase the efficiency of PRL signaling. To this end, we propose to (1) investigate the effects of activated Ras as well as activators and inhibitors of signaling pathways that are downstream of Ras on ubiquitination, internalization and degradation of PRLr; (2) determine how these active Ras and activators and inhibitors of its downstream mediators affect the extent of cellular responses to PRL. Successful completion of these studies will likely shed light on the role of Ras and PRLr stability in pathogenesis of breast cancer and potentially provide the rationale for future development of combination of PRL and Ras inhibitors for the treatment of patients with breast cancer.