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Role of Raf Kinase Inhibitory Protein in Prostate Cancer

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Dysregulation of growth signaling cascades is a key step in the initiation and malignant progression of prostate cancers. Among the main mediators of signals in proliferative cells are the mitogen-activated protein kinases (MAPKs), a super-family of highly homologous proline-directed serine/threonine kinases that participate in the transduction of growth and differentiation-promoting signals. The Ras/Raf kinase cascade that leads to activation of ERK1 and ERK2, two members of the ERK subfamily of MAPKs, is required for normal cell growth as well as neoplastic processes. Activation of this signaling cascade is correlated with prostate cancer progression and androgen-independence and thus is an important therapeutic target. One of the modulators of Raf kinase is Raf Kinase Inhibitory Protein (RKIP). Recent studies from our laboratory have shown that RKIP regulates Raf activation via interaction at S153;phosphorylation of S153 by activators of protein kinase C release RKIP, enabling subsequent activation of Raf. We have also shown that RKIP regulates mitotic progression in a number of cell types. An exciting new biologic function for RKIP in the suppression of prostate cancer metastasis has recently been reported. Specifically, immunohistochemical analysis of clinical tissue samples revealed expression of RKIP in primary prostate cancer but not in metastatic lesions. Functional in vivo studies showed that ectopic expression of RKIP suppressed invasion in vitro and metastasis in vivo in the well-characterized metastatic C42 human prostate cancer cell model. Conversely, decreasing the level of RKIP in nonmetastatic LNCaP cells promoted their invasive ability in vitro. Taken together, these studies implicate RKIP as a metastasis suppressor protein. We anticipate that identification of the mechanism by which RKIP signals to suppress metastasis will provide information necessary to understanding the molecular underpinnings of prostate cancer metastasis. The goal of this proposal is to relate RKIP structure to its function as a metastasis suppressor and then use this information to develop therapeutic reagents that modulate or mimic RKIP biologic function. Specifically, we plan to: 1) Identify the targets of RKIP action in prostate tumor cells;and 2) Generate inhibitors or potentiators of Raf-1 or RKIP. The reagents developed in these studies could be used in combination with other tumor therapies to suppress specific steps in the metastatic cascade.

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