Vulnerability of HIF1α and HIF2α to damage by proteotoxic stressors

Transcription factors HIF1 and HIF2 are central regulators of physiological responses to hypoxia and important for normal functioning of tissue stem cells and maintenance of healthy microvasculature. Even modest decreases in HIF activity exert detrimental effects in tissues although it is unclear what factors can directly impair HIF functions. We hypothesized that the presence of functionally important, large intrinsically disordered regions in HIF alpha subunits of HIF1/2 could make them structurally vulnerable to protein-damaging conditions. We found that common protein-damaging agents such as endogenous/exogenous aldehydes (formaldehyde, acetaldehyde), moderate heat shock and the environmental toxicant cadmium cause inactivation of HIF1 and HIF2 due to structural damage to HIF alpha subunits. Aldehydes triggered a rapid and selective depletion of HIF1 alpha and HIF2 alpha, which occurred as a result of enhanced binding of Pro-hydroxylated/VHL-ubiquitinated HIF alpha by 26S proteasomes. In the absence of proteasomal degradation, aldehyde-damaged HIF1 and HIF2 were transactivation defective and HIF alpha subunits became insoluble/denatured when their VHL-mediated ubiquitination was blocked. Protein damage by heat shock and cadmium resulted in the insolubility of Pro-nonhydroxylated HIF alpha. Thus, VHL-dependent ubiquitination of damaged HIF alpha also acts as means to maintain their solubility, permitting capture by proteasomes. The observed control of HIF alpha stability at the point of proteasome binding may extend to several posttranslational modifications that occur in the conformationally flexible regions of these proteins. Our findings revealed vulnerability of HIF1 and HIF2 to direct inactivation by protein-damaging agents, which helps understand their tissue injury mechanisms and favorable responses of hypoxic tumors to hyperthermia.