Ubiquitination is a central regulatory mechanism controlling protein stability and signaling in eukaryotic cells. The precise control of this machinery is crucial to avoid the development of diseases. Here, we identify a previously unrecognized interaction between the E3 ligase RNF8 and HIF1alpha, the oxygen-sensitive subunit of the hypoxia-inducible transcription factor HIF1. RNF8 antagonizes TRAF6-mediated stabilization of HIF1alpha under hypoxic conditions. Importantly, this regulatory effect is independent of the RNF8 E3 ligase activity but requires its forkhead-associated (FHA) domain. Yeast two-hybrid assays reveal an interaction between RNF8 FHA domain and the C-terminal transactivation domain (TAD) of HIF1alpha, despite the absence of a canonical FHA-binding motif in HIF1alpha. This interaction is maintained in a hydroxylation-deficient HIF1alpha mutant, indicating that prolyl hydroxylation is not required. Our findings suggest a non-canonical mode of FHA-dependent association by which RNF8 modulates HIF1alpha stability and downstream transcriptional control, with potential implications for hypoxia-driven signaling in triple-negative breast cancer.
