Regulation of non-AU-rich element containing c-fms proto-oncogene expression by HuR in breast cancer

The role of RNA-binding proteins in cancer biology is recognized increasingly. The nucleocytoplasmic shuttling and AU-rich RNA-binding protein HuR stabilizes several cancer-related target mRNAs. The proto-oncogene c-fms, whose 3'untranslated region (3'UTR) is not AU-rich, is associated with poor prognosis in breast cancer. Using a large breast-cancer tissue array (N=670), we found nuclear HuR expression to be associated with nodal metastasis and independently with poor survival (P=0.03, RR 1.45), as well as to be co-expressed with c-fms in the breast tumors (P=0.0007). We described c-fms mRNA as a direct target of HuR in vivo, and that HuR bound specifically to a 69-nt region containing 'CUU' motifs in 3'UTR c-fms RNA. Overexpressing or silencing HuR significantly up- or down-regulated c-fms RNA expression, respectively. We also found that known glucocorticoid stimulation of c-fms RNA and protein is largely dependent on the presence of HuR. HuR, by binding to the 69-nt wild type, but not mutant, c-fms sequence can regulate reporter gene expression post-transcriptionally. We are the first to describe that HuR can regulate gene expression by binding non-AU-rich sequences in 3'UTR c-fms RNA. Collectively, our findings suggest that HuR plays a supportive role for c-fms in breast cancer progression by binding a 69-nt element in its 3'UTR, thus regulating its expression.