RNA-Binding Proteins HuR and PTB Promote the Translation of Hypoxia-Inducible Factor 1α

Abstract

The levels of hypoxia-inducible factor 1α (HIF-1α) are tightly controlled. Here, we investigated the posttranscriptional regulation of HIF-1α expression in human cervical carcinoma HeLa cells responding to the hypoxia mimetic CoCl₂. Undetectable in untreated cells, HIF-1α levels increased dramatically in CoCl₂-treated cells, while HIF-1α mRNA levels were unchanged. HIF-1α translation was potently elevated by CoCl₂ treatment, as determined by de novo translation analysis and by monitoring the polysomal association of HIF-1α mRNA. An internal ribosome entry site in the HIF-1α 5′ untranslated region (UTR) was found to enhance translation constitutively, but it did not further induce translation in response to CoCl₂ treatment. Instead, we postulated that RNA-binding proteins HuR and PTB, previously shown to bind HIF-1α mRNA, participated in its translational upregulation after CoCl₂ treatment. Indeed, both RNA-binding proteins were found to bind HIF-1α mRNA in a CoCl₂-inducible manner as assessed by immunoprecipitation of endogenous ribonucleoprotein complexes. Using a chimeric reporter, polypyrimidine tract-binding protein (PTB) was found to bind the HIF-1α 3′UTR, while HuR associated principally with the 5′UTR. Lowering PTB expression or HuR expression using RNA interference reduced HIF-1α translation and expression levels but not HIF-1α mRNA abundance. Conversely, HIF-1α expression and translation in response to CoCl₂ were markedly elevated after HuR overexpression. We propose that HuR and PTB jointly upregulate HIF-1α translation in response to CoCl₂.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/ .

ACKNOWLEDGMENTS

We thank S. D. Baird and B. Frank for their assistance with this study.

This research was supported by the Intramural Research Program of the National Institute on Aging, National Institutes of Health.