The polypyrimidine tract-binding protein stimulates HIF-1 alpha IRES-mediated translation during hypoxia

When oxygen supply is restricted, protein synthesis is rapidly abrogated owing to inhibition of global translation. However, HIF-1 alpha protein expression can persist during hypoxia, owing to an internal ribosome entry site (IRES) in the 5'-untranslated region of its mRNA. Here, we report on the molecular mechanism of HIF-1 alpha IRES-mediated translation during oxygen deprivation. Using RNA affinity chromatography and UV-crosslinking experiments, we show that the polypyrimidine tract binding protein (PTB) can specifically interact with the HIF-1 alpha IRES, and that this interaction is enhanced in hypoxic conditions. Overexpression of PTB enhanced HIF-1 alpha IRES activity, whereas RNA interference-mediated downregulation of PTB protein expression inhibited HIF-1 alpha IRES activity. Furthermore, hypoxia-induced stimulation of the HIF-1 alpha IRES was reduced in cells in which PTB function was downregulated. In agreement with these results, the IRES activity of HIF-1 alpha IRES deletion mutants that are deficient in PTB-binding could not be stimulated by oxygen deprivation. All together, our data suggest that PTB plays a stimulatory role in the IRES-mediated translation of HIF-1 alpha when oxygen supply is limited.