The Caenorhabditis elegans rhy-1 gene inhibits HIF-1 hypoxia-inducible factor activity in a negative feedback loop that does not include vhl-1.

Hypoxia-inducible factor (HIF) transcription factors implement essential changes in gene expression that enable animals to adapt to low oxygen (hypoxia). The stability of the C. elegans HIF-1 protein is controlled by the evolutionarily conserved EGL-9/VHL-1 pathway for oxygen-dependent degradation. Here, we describe vhl-1-independent pathways that attenuate HIF-1 transcriptional activity in C. elegans. First, the expression of HIF-1 target genes is markedly higher in egl-9 mutants than in vhl-1 mutants. We show that HIF-1 protein levels are similar in animals carrying strong loss-of-function mutations in either egl-9 or vhl-1. We conclude that EGL-9 inhibits HIF-1 activity, as well as HIF-1 stability. Second, we identify the rhy-1 gene and show that it acts in a novel negative feedback loop to inhibit expression of HIF-1 target genes. rhy-1 encodes a multi-pass transmembrane protein. Although loss-of-function mutations in rhy-1 cause relatively modest increases in hif-1 mRNA and HIF-1 protein expression, some HIF-1 target genes are expressed at higher levels in rhy-1 mutants than in vhl-1 mutants. Animals lacking both vhl-1 and rhy-1 function have a more severe phenotype than either single mutant. Collectively, these data support models in which RHY-1 and EGL-9 function in VHL-1-independent pathway(s) to repress HIF-1 transcriptional activity.