Hypoxia-inducible factor-1α in pulmonary artery smooth muscle cells lowers vascular tone by decreasing myosin light chain phosphorylation.

RATIONALE Hypoxia-inducible factor-1α (HIF-1α), an oxygen (O2)-sensitive transcription factor, mediates transcriptional responses to low-O2 tension states. Although acute hypoxia causes pulmonary vasoconstriction and chronic hypoxia can cause vascular remodeling and pulmonary hypertension, conflicting data exist on the role of HIF-1α in modulating pulmonary vascular tone. OBJECTIVE To investigate the role of smooth muscle cell (SMC)-specific HIF-1α in regulating pulmonary vascular tone. METHODS AND RESULTS Mice with an SMC-specific deletion of HIF-1α (SM22α-HIF-1α(-/-)) were created to test the hypothesis that pulmonary artery SMC (PASMC) HIF-1α modulates pulmonary vascular tone and the response to hypoxia. SM22α-HIF-1α(-/-) mice exhibited significantly higher right ventricular systolic pressure compared with wild-type littermates under normoxia and with exposure to either acute or chronic hypoxia in the absence of histological evidence of accentuated vascular remodeling. Moreover, myosin light chain phosphorylation, a determinant of SMC tone, was higher in PASMCs isolated from SM22α-HIF-1α(-/-) mice compared with wild-type PASMCs, during both normoxia and after acute hypoxia. Further, overexpression of HIF-1α decreased myosin light chain phosphorylation in HIF-1α-null SMCs. CONCLUSIONS In both normoxia and hypoxia, PASMC HIF-1α maintains low pulmonary vascular tone by decreasing myosin light chain phosphorylation. Compromised PASMC HIF-1α expression may contribute to the heightened vasoconstriction that characterizes pulmonary hypertension.