Cell-specific Regulation of Hypoxia-inducible Factor (HIF)-1α and HIF-2α Stabilization and Transactivation in a Graded Oxygen Environment

The hypoxia-inducible factor (HIF)-1alpha and HIF-2alpha are closely related, key transcriptional regulators of the hypoxic response, countering a low oxygen situation with the up-regulation of target genes associated with numerous processes, including vascularization and glycolysis. This involves a dual mechanism of control through both stabilization and transactivation, regulated via prolyl and asparaginyl hydroxylation. Despite high similarity with respect to protein sequence and activation pathway, a growing number of physiological and mechanistic differences between HIF-1alpha and HIF-2alpha are being reported. To further characterize this nonredundancy, the stabilization of endogenous proteins and regulation of the transactivation domains were compared in a graded oxygen environment across a series of cell lines. Although generally similar results were found, interesting and specific differences between the HIF-alpha proteins were observed within certain cell lines, such as rat adrenal PC12s, emphasizing the cell-specific nature of HIF-alpha regulation. We characterize a conserved amino acid substitution between HIF-1alpha and HIF-2alpha that contributes to the intrinsically higher FIH-1-mediated asparaginyl hydroxylation of HIF-1alpha and, hence, lower HIF-1alpha activity. In addition, our data demonstrate that the different cell lines can be classified into two distinct groups: those in which stabilization and transactivation proceed in conjunction (HeLa, 293T, and COS-1) and those cells in which HIF-alpha is stabilized prior to transactivation (PC12, HepG2, and CACO2). Interestingly, the initial stabilization of HIF-alpha prior to transactivation up-regulation predicted from in vitro derived hydroxylation data is only true for a subset of cells.