The role of Raf kinase inhibitor protein (RKIP) in health and disease.

Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding protein (PEBP) family. RKIP plays a pivotal modulatory role in several protein kinase signaling cascades. RKIP binds inhibits Raf-1-mediated phosphorylation of MEK through binding to Raf-1. Protein kinase C (PKC) phosphorylates RKIP, resulting in release of Raf-1 and activation of MEK and ERK. The phosphorylated RKIP binds to and inhibits G-protein-coupled receptor kinase, resulting in sustained G-protein signaling. The regulatory role that RKIP has in cell signaling is reflected in its role in physiology and pathophysiology. RKIP is involved in neural development, cardiac function and spermatogenesis and appears to have serine protease activity. In addition to its roles in physiology, dysregulated RKIP expression has the potential to contribute to pathophysiological processes including Alzheimer's disease and diabetic nephropathy. RKIP has been shown to fit the criteria of being a metastasis suppressor gene, including having decreased expression in prostate cancer metastases and restoring RKIP expression in a prostate cancer cell line diminishes metastasis in a murine model. Clearly, RKIP has multiple molecular and cellular functions. In this review, RKIP's molecular roles in intracellular signaling, its physiological functions and its role in disease are described.