Entry Site-Dependent Translation of Hypoxia-Inducible -Evoked Bcl-2 Expression to Provoke Internal Ribosome α Protein Kinase Signaling Allows Tumor Necrosis Factor

Hypoxia-inducible factor (HIF)-1, a heterodimeric transcription factor composed of HIF-1 and HIF-1 subunits coordinates pathophysiologic responses toward decreased oxygen availability. It is now appreciated that enhanced protein translation of HIF-1 under normoxia accounts for an alternative regulatory circuit to activate HIF-1 by hormones, growth factors, or cytokines such as tumor necrosis factor (TNF). Here, we aimed at understanding molecular details of HIF-1 translation in response to TNF. In tubular LLC-PK1 cells, activation of nuclear factor B (NF B) by TNFresulted in HIF-1 protein synthesis as determined by [S]methionine pulse experiments. Protein synthesis was attenuated by blocking NF B, phosphatidylinositol 3 -kinase (PI3k), and mitogen-activated protein kinase (MAPK). Use of a dicistronic reporter with the HIF-1 5 -untranslated region (5 UTR) between two coding regions indicated that TNFpromoted an internal ribosome entry site (IRES) rather than a cap-dependent translation. IRES-mediated translation required the functional integrity of the NF B, PI3k, and MAPK signaling pathways. Although no signal cross-talk was noticed between NF B, PI3k, and MAPK signaling, these pathways are needed to up-regulate the antiapoptotic target protein Bcl-2 by TNF. Expression of Bcl-2 provoked not only IRES-dependent translation but also HIF-1 protein synthesis. We conclude that Bcl-2 functions as an important determinant in facilitating HIF-1 protein expression by TNFvia an IRES-dependent translational mechanism. These observations suggest a link between Bcl-2 and HIF-1 expression, a situation with potential relevance to cancer biology.