HuR contributes to cyclin E1 deregulation in MCF-7 breast cancer cells

Many cancers overexpress cyclin El and its tumor-specific low molecular weight (LMW) isoforms. However, the mechanism of cyclin El deregulation in cancers is still not well understood. We show here that the mRNA-binding protein HuR increases cyclin El mRNA stability in MCF-7 breast carcinoma cells. Thus, mRNA stabilization may be a key event in the deregulation of cyclin El in MCF-7 cells. Compared with MCF10A immortalized breast epithelial cells, MCF-7 cells overexpress full-length cyclin El and its LMW isoforms and exhibit increased cyclin El mRNA stability. Increased mRNA stability is associated with a stable adenylation state and an increased ratio of cytoplasmic versus nuclear HuR. UV crosslink competition and UV cross-link immunoprecipitation assays verified that HuR specifically bound to the cyclin El 3'-untranslated region. Knockdown of HuR with small interfering RNA (siRNA) in MCF-7 cells decreased cyclin El mRNA half-life (t(1/2)) and its protein level: a 22% decrease for the full-length isoforms and 80% decrease for the LMW isoforms. Hall siRNA also delayed G(1)-S phase transition and inhibited MCF-7 cell proliferation, which was partially recovered by overexpression of a LMW isoform of cyclin E1. Overexpression of HuR in MCF10A cells increased cyclin E1 mRNA t(1/2) and its protein level. Taken together, our data show that Hall critically contributes to cyclin El overexpression and its growth-promoting function, at least in part by increasing cyclin El mRNA stability, which provides a new mechanism of cyclin El deregulation in breast cancer.