Abstract
Insulin-like growth factor-I receptor (IGF-IR) represents one of the major targets by which dietary or chemically induced energy restriction mediates chemopreventive effects in animal tumor models. However, the mechanism underlying this cellular response remains unclear. In the course of investigating the suppressive effect of the energy restriction-mimetic agent CG-5 on IGF-IR expression in prostate cancer cells, we identified a novel posttranscriptional mechanism by which the RNA-binding protein human antigen R (HuR) regulates IGF-IR expression through messenger RNA (mRNA) stabilization. Previously, we demonstrated that Sp1 and HuR proteins were concomitantly targeted for ubiquitin-dependent degradation by β-transducin repeat-containing protein in response to CG-5. Although this loss of Sp1 expression contributed to CG-5-mediated IGF-IR downregulation, enforced specific protein 1 (Sp1) expression could only partially protect cells from the drug effect. The small interfering RNA-mediated silencing of HuR suppressed IGF-IR expression by reducing mRNA stability, whereas ectopic HuR expression increased IGF-IR mRNA stability and protein expression and, when coexpressed with Sp1, blocked CG-5-mediated IGF-IR ablation. RNA pull-down and immunoprecipitation analyses indicated that HuR selectively bound to the distal region of the IGF-IR 3' untranslated region (UTR), whereas no interaction with the 5'UTR was noted. Evaluation of a series of truncated HuR mutants revealed that the RNA recognition motifs (RRM2 and RRM3) were involved in IGF-IR 3'UTR binding and the consequent increase in IGF-IR mRNA stability. Although these data contrast with a previous report that HuR acted as a translation repressor of IGF-IR mRNA through 5'UTR binding, our finding is consistent with the reported oncogenic role of HuR in conferring stability to target mRNAs encoding tumor-promoting proteins.