HSF1 regulation of β-catenin in mammary cancer cells through control of HuR/elavL1 expression

There is now compelling evidence to indicate a place for heat shock factor 1 (HSF1) in mammary carcinogenesis, tumour progression and metastasis. Here we have investigated a role for HSF1 in regulating the expression of the stem cell renewal factor beta-catenin in immortalized human mammary epithelial and carcinoma cells. We found HSF1 to be involved in regulating the translation of beta-catenin, by investigating effects of gain and loss of HSF1 on this protein. Interestingly, although HSF1 is a potent transcription factor, it was not directly involved in regulating levels of beta-catenin mRNA. Instead, our data suggest a complex role in translational regulation. HSF1 was shown to regulate levels of the RNA-binding protein HuR that controlled beta-catenin translation. An extra complexity was added to this scenario when it was shown that the long non-coding RNA molecule lincRNA-p21, known to be involved in beta-catenin mRNA (CTNNB1) translational regulation, was controlled by HSF1 repression. We have shown previously that HSF1 was positively regulated through phosphorylation by mammalian target of rapamycin (mTOR) kinase on a key residue, serine 326, essential for transcriptional activity. In this study, we found that mTOR knockdown not only decreased HSF1-S326 phosphorylation in mammary cells, but also decreased beta-catenin expression through a mechanism requiring HuR. Our data point to a complex role for HSF1 in the regulation of HuR and beta-catenin expression that may be significant in mammary carcinogenesis.