RKIP: Much More Than Raf Kinase Inhibitory Protein FAHD AL-MULLA, 1 * MILAD S. BITAR, 2 ZAINAB TAQI, 3 AND KAM C. YEUNG 4 1 Faculty of Medicine, Department of Pathology, Kuwait University Health Sciences Centre, Safat, Kuwait 2 Faculty of Medicine, Department of Pharmacology, Kuwait University Health Sciences Centre, Safat, Kuwait 3 Department of Molecular Biology, College of Biological Science, University of Kuwait, Safat, Kuwait 4 Department of Biochemistry and Cancer Biology, College of Medicine, Health Science Campus, University of Toledo, Toledo, Ohio, U.S.A. From its discovery as a phosphatidylethanolamine-binding protein in bovine brain to its designation as a physiological inhibitor of Raf kinase protein, RKIP has emerged as a critical molecule for maintaining subdued, well-orchestrated cellular responses to stimuli. The disruption of RKIP in a wide range of pathologies, including cancer, Alzheimer’s disease, and pancreatitis, makes it an exciting target for individualized therapy and disease-specific interventions. This review attempts to highlight recent advances in the RKIP field underscoring its potential role as a master modulator of many pivotal intracellular signaling cascades that control cellular growth, motility, apoptosis, genomic integrity, and therapeutic resistance. Specific biological and functional niches are highlighted to focus future research towards an enhanced understanding of the multiple roles of RKIP in health and disease. J. Cell. Physiol. 228: 1688–1702, 2013. ß 2013 Wiley Periodicals, Inc. The PEBP-1 protein was initially identified in bovine brain (Bernier and Jolles, 1984; Schoentgen et al., 1987, 1992) and rat spermatozoa (Banfield et al., 1998; Simister et al., 2002). PEBP-1 is a member of the evolutionary conserved phosphatidylethanolamine-binding protein (PEBP) superfamily, which contains more than 400 members from variety of species from bacteria to plants (Serre et al., 2001; Odabaei et al., 2004). In 1999, Yeung et al. used the yeast-2 hybrid screen to identify PEBP-1 as a RAF-1 binding protein that could inhibit MEK phosphorylation and activation. The authors named it Raf Kinase inhibitory protein (RKIP; Yeung et al., 1999). This landmark study was the first to elucidate a role for PEBP-1/RKIP in a pivotal cellular signaling cascade. Earlier data had demonstrated that PEBP-1/RKIP participates in diverse functions in different species, organisms, and cell types. It was reported that Tfs1p, an RKIP homolog in yeast, inhibits carboxypeptidase Y that acts as an inhibitor of yeast Ras GAP and consequently up regulates Ras (Bruun et al., 1998). RKIP expression has been detected in all mammalian tissues, such as brain, liver, stomach, spleen and muscle of human, cow, rat, and chicken; it is mainly localized to the cytoplasm and inner periplasmic membrane (Bollengier and Mahler, 1988; Seddiqi et al., 1994; Frayne et al., 1999; Vallee et al., 2001). RKIP is most highly expressed in the secretory product of testicular germ cells (Saunders et al., 1995; Seddiqi et al., 1996), brain oligodendrocytes, and specific cortical and hippocampal neuronal cell layers (Frayne et al., 1999; Seddiqi et al., 1996). RKIP is a precursor to the hippocampal neurostimulating peptide (HCNP), which is involved in neuronal differentiation and acts as a factor in acetylcholine synthesis (Seddiqi et al., 1994; Mitake et al., 1995, 1996a,b). RKIP-knockout mice were utilized to delineate the role of RKIP in the development of brain functions. Interestingly, this mouse strain showed a mild olfaction deficient condition at the age of 10 months, linking RKIP expression to olfactory system development (Theroux et al., 2007). In mice, RKIP was shown to modulate the photic entrainment of the suprachiasmatic nucleus circadian clock through the Raf-MEK-ERK pathway (Antoun et al., 2012). Photons can apparently induce RKIP phosphorylation at the S153 residue in the suprachiasmatic nucleus (Antoun et al., 2012). In nematodes, PEBP is one of the membranous secreted proteins conferring a protective role against the host immunological responses (Morgan et al., 2006). Drosophila PEBP homologs, of which at least seven isoforms are known, are expressed in olfactory cells (Rautureau et al., 2009). Interestingly, PEBP1 overexpression has been shown to protect Drosophila against bacterial infection by enhancing the release of immunity-related proteins in their hemolymph (Reumer et al., 2009), and in plants, RKIP was found to be associated with growth and differentiation, transforming plants from the vegetative to reproductive growth phases (Banfield and Brady, 2000). Furthermore, RKIP is reportedly involved in Cep290- mediated photoreceptor degeneration and retinopathy. The immunolocalization and accumulation of RKIP and its partner protein Cep290 prevented cilia formation in zebrafish and cultured cells (Murga-Zamalloa et al., 2011). Therefore, the diverse phylogenic conservation and functions attained by RKIP in different species and tissues/cell-types underscores its functional importance and potential cardinal roles in cellular signaling (Table 1). RKIP: An inhibitory Modulator Protein Especially since its discovery as the only physiological endogenous inhibitor of the Raf-MEK-ERK signaling pathway, research has primarily focused on elucidating the mechanism of RKIP actions (Yeung et al., 1999); The Raf-MEK-ERK is a highly conserved signaling pathway that regulates cell growth, differentiation, migration, and apoptosis (Hagan et al., 2006). All authors have contributed in writing of this manuscript. Authors have no conflict of interest. Contract grant sponsor: Kuwait Foundation for The Advancement of Sciences (KFAS); Contract grant number: 2012-130-201. *Correspondence to: Fahd Al-Mulla, Faculty of Medicine, Molecular Pathology Unit, Department of Pathology, Health Sciences Center, Kuwait University, P.O. Box 24923, Safat 13110, Kuwait. E-mail: fahd@al-mulla.org Manuscript Received: 20 December 2012 Manuscript Accepted: 16 January 2013 Accepted manuscript online in Wiley Online Library (wileyonlinelibrary.com): 28 January 2013. DOI: 10.1002/jcp.24335 REVIEW ARTICLE 1688 Journal of Journal of Cellular Physiology Cellular Physiology ß 2013 WILEY PERIODICALS, INC.