Differential signaling pathways of HO-1 gene expression in pulmonary and systemic vascular cells

Heme oxygenase-1 (HO-1) is induced by oxidative stress and plays an importa nt role in cellular protection against oxidant injury. Increasing evidence also suggests that HO-1 is markedly modulated by hypoxia in vitro and in vi vo. Our group has previously demonstrated that the transcription factor hyp oxia-inducible factor (HIF)-1 mediates hypoxia-induced HO-1 gene transcript ion and expression in systemic (aortic) vascular smooth muscle (AoVSM) cell s (P. J. Lee, B.-H. Jiang, B. Y. Chin, N. V. Iyer, J. Alam, G. L. Semenza, and A. M. K. Choi. J. Biol. Chem. 272: 5375-5381, 1997). Because the pulmon ary circulation is an important target of hypoxia, this study investigated whether HO-1 gene expression in pulmonary arterial vascular smooth muscle w as differentially regulated by hypoxia in comparison to AoVSM cells. Intere stingly, hypoxia neither induced HO-1 gene expression nor increased HIF-1 D NA binding activity in pulmonary arterial vascular smooth muscle cells. Con versely, pulmonary arterial endothelial cells (PAECs) demonstrated a marked induction of HO-1 gene expression after hypoxia. Electrophoretic mobility shift assays detected an increase in activator protein-1 rather than in HIF -1 DNA binding activity in nuclear extracts of hypoxic PAECs. Analyses of t he promoter and 5'-flanking regions of the HO-1 gene were performed by tran siently transfecting PAECs with either the hypoxia response element (HIF-1 binding site) or the HO-1 gene distal enhancer element (AB1) linked to a ch loramphenicol acetyltransferase reporter gene. Increased chloramphenicol ac etyltransferase activity was observed only in transfectants containing the AB1 distal enhancer, and mutational analysis of this enhancer suggested tha t the activator protein-1 regulatory element was critical for hypoxia-induc ed HO-1 gene transcription. Collectively, our data demonstrate that the mol ecular regulation of HO-1 gene transcription during hypoxia differs between the systemic and pulmonary circulations and also provide evidence that hyp oxia-induced HO-1 gene expression in PAECs and AoVSM cells is regulated thr ough two discrete signaling pathways.