Raf Kinase Inhibitor Protein RKIP Enhances Signaling by Glycogen Synthase Kinase-3β

Abstract

Raf kinase inhibitory protein (RKIP) is a physiologic inhibitor of c-RAF kinase and nuclear factor kB signaling that represses tumor invasion and metastasis. Glycogen synthase kinase-3b (GSK3b) suppresses tumor progression by downregulating multiple oncogenic pathways including Wnt signaling and cyclin D1 activation. Here, we show that RKIP binds GSK3 proteins and maintains GSK3b protein levels and its active form. Depletion of RKIP augments oxidative stress-mediated activation of the p38 mitogen activated protein kinase, which, in turn, inactivates GSK3b by phosphorylating it at the inhibitory T390 residue. This pathway de-represses GSK3b inhibition of oncogenic substrates causing stabilization of cyclin D, which induces cell-cycle progression and b-catenin, SNAIL, and SLUG, which promote epithelial to mesenchymal transition. RKIP levels in human colorectal cancer positively correlate with GSK3b expression. These findings reveal the RKIP/GSK3 axis as both a potential therapeutic target and a prognosis-based predictor of cancer progression. Cancer Res; 71(4); 1334-43. Ó2011 AACR.

The role of Raf-1 kinase inhibitor protein in the regulation of pancreatic beta cell proliferation in mice

“…Apoptotic rates are comparable between Rkip1 −/− and WT P28 pancreases, implying that NF-κB-dependent survival pathways are not affected by Rkip1 deletion. On the other hand, RKIP1 deficiency does not alter levels of GSK3β in islets, as reported in other tissues [9], ruling out this possible mechanism. Future work should be aimed at delineating the signal cascades regulated by RKIP1 and their connection to beta cell mitogens during physiological beta cell growth.…”

“…In a similar way, deletion of Raf-1 does not result in decreased ERK1/2 phosphorylation in pancreatic islets, indicating that other Raf-1-dependent pathways may be operative in beta cells [25]. Additionally, RKIP1 may function through Raf-1 independent mechanisms involving NF-κB [8], GSK3β [9] or G protein-coupled receptors [11]. Apoptotic rates are comparable between Rkip1 −/− and WT P28 pancreases, implying that NF-κB-dependent survival pathways are not affected by Rkip1 deletion.…”

“…5f). RKIP1 also enhances GSK3β-dependent signalling through stabilisation of this kinase [9]. GSK3β is involved in β-catenin turnover and WNT/β-catenin signalling has been shown to regulate neonatal beta cell expansion [29].…”

“…However, upon phosphorylation by protein kinase C, RKIP1 can dissociate from Raf-1 and inhibit G protein-coupled receptor kinase 2 (GRK2) [7], a negative regulator of G protein-coupled receptors. Additionally, RKIP1 has been implicated as a negative regulator of nuclear factor κB (NF-κB)-signalling pathways and an activator of glycogen synthase kinase 3β (GSK3β)-dependent signalling pathways [8,9]. Therefore, RKIP1 mediates the crosstalk between various important cellular signalling pathways and, consequently, can influence a wide array of cellular functions including cell proliferation, differentiation and apoptosis.…”

Raf Kinase Inhibitory Protein (RKIP): Functional Pleiotropy in the Mammalian Brain

“…16 In a departure from RKIP's recognized role as a signaling inhibitor, Al-Mulla et al recently discovered that RKIP served as an activator of GSK3β signaling. 17 RKIP physically associated with GSK3β, and overexpression of RKIP suppressed the phosphorylation of GSK3β at residue 390, which would otherwise lead to GSK3β inactivation. Knockdown of RKIP had the opposite effect by promoting phosphorylation at this inhibitory site (a target site for p38 kinase), and also led to stabilization of GSK3β protein (Fig.…”