Abstract
Hypoxia-inducible factor-1 (HIF-1) is a regulator of metabolic adaptation to hypoxia. It is now appreciated that HIF-1alpha accumulation is achieved under normoxic conditions by various factors, such as TNF-alpha. Here, it was our intention to gain insight into the signaling mechanisms used by TNF-alpha to stimulate HIF-1alpha. In tubular LLC-PK1 or human embryonic kidney cells, TNF-alpha induced accumulation of HIF-1alpha protein but not HIF-1alpha mRNA. Blocking nuclear factor (NF)-kappaB with sulfasalazine or expression of an IkappaB superrepressor attenuated HIF-1alpha accumulation, whereas transfection of active p50/p65-NF-kappaB subunits mimicked a TNF-alpha response. Experiments with actinomycin D and cycloheximide also pointed to a transcriptional and translational process in facilitating the TNF-alpha response. Interestingly, and in contrast to established hypoxic signaling concepts, TNF-alpha elicited HIF-1alpha accumulation in a ubiquitinated form that still bound the von Hippel-Lindau (pVHL) protein. These data indicate that HIF-1alpha accumulation by TNF-alpha demands the NF-kappaB pathway, preserves ubiquitination of HIF-1alpha, and allows the HIF-1alpha-pVHL interaction.