Raf Kinase Inhibitory
Protein or RKIP was initially identified as a Raf-1
binding protein using the yeast 2-hybrid screen. RKIP inhibits the activation phosphorylation of
MEK by Raf-1 by competitively inhibiting the binding of
MEK to Raf-1 and thus exerting an inhibitory effect on the Raf-MEK-Erk pathway. RKIP has been identified as a metastasis suppressor gene. Expression of RKIP is low in
cancer metastases. Although primary
tumor growth remains unaffected, re- expression of RKIP inhibits
cancer metastasis. Mechanistically, RKIP constrains
metastasis by inhibiting angiogenesis, local invasion, intravasation, and colonization. The molecular mechanism of how RKIP inhibits these individual steps remains undefined. In our present study, using an unbiased PCR based screening and by analyzing
DNA microarray expression datasets we observe that the expression of multiple
metalloproteases (
MMPs) including MMP1, MMP3, MMP10 and MMP13 are negatively correlated with RKIP expression in
breast cancer cell lines and clinical samples. Since expression of
MMPs by
cancer cells is important for
cancer metastasis, we hypothesize that RKIP may mediate suppression of
breast cancer metastasis by inhibiting multiple
MMPs. We show that the expression signature of RKIP and
MMPs is better at predicting high metastatic risk than the individual gene. Using a combination of loss- and gain-of-function approaches, we find that MMP13 is the cause of RKIP-mediated inhibition of local
cancer invasion. Interestingly expression of MMP13 alone is not sufficient to reverse the inhibition of
breast cancer cell
metastasis to the lung due to the expression of RKIP. We find that RKIP negatively regulates MMP13 through the Erk2 signaling pathway and the repression of MMP13 by RKIP is
transcription factor AP-1 independent. Together, our findings indicate that RKIP inhibits
cancer cell invasion, in part, via MMP13 inhibition. These data also implicate RKIP in the regulation of
MMP transcription, suggesting a potential mechanism by which RKIP inhibits
tumor progression and
metastasis.