Three HIF-alpha prolyl-4-hydroxylases (PHDs) (named PHD1, PHD2, and PHD3) effect the proteasome-mediated degradation of HIF by catalyzing the hydroxylation of key proline residues in the HIF-1alpha subunit under normoxic conditions. When oxygen tension is reduced, PHD-mediated hydroxylation cannot occur, HIF-1alpha accumulates in the nucleus, resulting in HIF-mediated gene transcription. In the present study, the expression and regulation of PHD mRNA and HIF protein expression was examined in human tissues and primary cells of cardiovascular origin. Treatment of human cardiac myocytes, smooth muscle cells, and endothelial cells with hypoxia or COCl2, a hypoxia mimic, resulted in a significant time-dependent increase in PHD3, but not PHD1 or PHD2, mRNA levels, which correlated with an increase in HIF-1alpha protein expression. Overexpression studies revealed that PHD3 levels influence HIF-1alpha stability in both normoxic and hypoxic conditions, suggesting that PHD3 may participate in a feedback loop controlling HIF activity. (C) 2003 Elsevier Science (USA). All rights reserved.