Snail is a repressor of RKIP transcription in metastatic prostate cancer cells

Abstract

Diminished expression of the metastasis suppressor protein RKIP was previously reported in a number of cancers. The underlying mechanism remains unknown. Here, we show that the expression of RKIP negatively correlates with that of Snail zinc-transcriptional repressor, a key modulator of normal and neoplastic epithelial-mesenchymal transition (EMT) program. With a combination of loss-of-function and gain-of-function approaches, we showed that Snail repressed the expression of RKIP in metastatic prostate cancer cell lines. The effect of Snail on RKIP was on the level of transcriptional initiation and mediated by a proximal E-box on the RKIP promoter. (Chatterjee et al., 2004;Park et al., 2005). Consistent with its demonstrated inhibitory effect on Raf and NF-kB signaling, we and others have shown that the expression levels of RKIP are downregulated in a number of tumors, including highly metastatic prostate, breast and colon cancer, hepatocellular carcinoma, melanomas and insulinomas (Fu et al., 2003;Chatterjee et al., 2004;Schuierer et al., 2004Schuierer et al., , 2006Zhang et al., 2004;Hegan et al., 2005;Al-Mulla et al., 2006;Lee et al., 2006). The importance of RKIP in metastases was highlighted by the finding that restoration of RKIP expression inhibits prostate cancer metastasis in a murine model (Fu et al., 2003(Fu et al., , 2005. More recent studies have shown that RKIP is also a good prognostic marker of the pathogenesis of human prostate cancer (Fu et al., 2005) and a prognostic indicator for overall survival and disease-free survival in colorectal cancer (Al-Mulla et al., 2006). Collectively, these studies suggest that RKIP is a novel cancer metastasis suppressor and an effector of signal transduction pathways leading to apoptosis. In spite of the abundance of experimental evidence on the deleterious consequences of reduced RKIP expression in tumors, the mechanisms responsible for the downregulation of RKIP in cancer are not completely understood.To set up a system to study the transcription regulation of RKIP, we examined RKIP expression levels in cancer cell lines with different metastatic capacity. In accordance with clinical tumor studies, we observed that expression levels of RKIP proteins progressively decreased in breast and prostate cancer cell lines of increasing metastatic potential. The expression of RKIP is low in invasive and metastatic breast (MB231, MB435 and 578T) and prostate (DU145 and PC3) cell lines and high in noninvasive cell lines like MCF7, BT20 and LNCaP (Figure 1a). Notably, RKIP protein levels correlated well with those of the intercellular adhesion protein E-cadherin (E-cad). E-cad is a well-documented tumor metastasis suppressor protein that is regulated by the Snail and closely related Slug transcription factors (Peinado et al., 2007). Quantitation of RKIP transcript levels in the different cancer cell lines by qRT-PCR demonstrated that they correlated with the levels of the protein, (Figure 1b), suggesting that RKIP expression is downregulated at the RNA level, via changes...

Mechanisms of nitric oxide-mediated inhibition of EMT in cancer

“…[28][29][30][31] Snail and RKIP expression levels are inversely correlated in prostate cancer cell lines and patient's samples. 25 Snail and RKIP have also shown opposite effects in the regulation of tumor cell resistance to apoptotic stimuli. and activity.…”

“…23,24 Besides E-cadherin, Snail was recently shown to repress the transcription of another tumor suppressor gene product, namely Raf-1 kinase inhibitor protein (RKIP). 25 RKIP is a member of the phosphatidylethanolamine-binding proteins (PEBP) family and among its main functions RKIP inhibits both the NFκB and MAPK signaling pathways. RKIP mediates its inhibitory activity on NFκB and MAPK through physical association with Raf-1 and TAK/NIK and IKK kinases, respectively, leading to inhibition of their activities as kinases.…”

“…is directly repressed by Snail; 25 thus, we hypothesized that DETANONOate-mediated inhibition of Snail expression could result downstream in RKIP induction. Snail involvement in RKIP regulation was confirmed in DU145 cells transfected either with a wild-type RKIP-Luc reporter or an RKIP-Luc reporter carrying mutations in E-boxes where Snail is bound.…”

Raf kinase inhibitory protein: a signal transduction modulator and metastasis suppressor

“…Interestingly, a Merkel carcinoma line was identified that overexpressed RKIP and lost ERK signaling; however, further studies revealed that RKIP depletion did not rescue the ERK signaling defect consistent with RKIP's role as a modulator rather than suppressor of the MAPK pathway [34]. Finally, a recent study showed that Snail, a mediator of the epithelial-mesenchymal transition (EMT), can inhibit RKIP transcription and negatively correlates with RKIP levels in tumors, consistent with a role for RKIP in metastasis [35]. Since RKIP expression is decreased during progression in a variety of cancers, it is likely that RKIP affects a fundamental step in the process rather than targeting tumor-specific mechanisms of invasion or colonization.…”

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

“…Mechanisms including gene methylation and transcriptional repression by the epithelial-mesenchymal transition (EMT)-promoting factor, SNAIL, have been implicated in rkip gene regulation in other types of cancers. 80,81 Another important issue is to identify the molecular basis of RKIP's effects on tumor malignancy, invasion, and metastasis. Are the signaling pathways that are known to be influenced by RKIP involved?…”