Hypoxia-inducible factor induction by tumour necrosis factor in normoxic cells requires receptor-interacting protein-dependent nuclear factor kappaB activation

Tumour necrosis factor α (TNF-α) binds to its receptor (TNFR1) and activates both death- and inflammation/survival-related signalling pathways. The inflammation and survival-related signalling cascade results in the activation of the transcription factor, nuclear factor κB (NF-κB) and requires recruitment of receptor-interacting protein (RIP) to TNFR1. The indispensable role of RIP in TNF-induced NF-κB activation has been demonstrated in RIP^-/- mice and in cell lines derived from such mice. In the present study, we show that the TNF-α-induced accumulation of hypoxia-inducible factor 1α (HIF-1α) protein in normoxic cells is RIP-dependent. Exposing fibroblasts derived from RIP^-/- mice to either cobalt or PMA resulted in an equivalent HIF-1α induction to that seen in RIP^+/+ fibroblasts. In contrast, RIP^-/- cells were unable to induce HIF-1α in response to TNF-α. Further, transient transfection of NIH 3T3 cells with an NF-κB super-repressor plasmid (an inhibitor of NF-κB activation) also prevented HIF-1α induction by TNF-α. Surprisingly, although HIF-1α mRNA levels remained unchanged after induction by TNF, induction of HIF-1α protein by the cytokine was completely blocked by pretreatment with the transcription inhibitors actinomycin D and 5,6-dichlorobenzimidazole riboside. Finally, TNF failed to induce both HIF-1α, made resistant to von Hippel—Lindau (VHL), and wild-type HIF-1α transfected into VHL^-/- cells. These results indicate that HIF-1α induction by TNF-α in normoxic cells is mediated by protein stabilization but is nonetheless uniquely dependent on NF-κB-driven transcription. Thus the results describe a novel mechanism of HIF-1α up-regulation and they identify HIF-1α as a unique component of the NF-κB-mediated inflammatory/survival response.