Hypoxia-induced migration in pulmonary arterial smooth muscle cells requires calcium-dependent upregulation of aquaporin 1

Pulmonary arterial smooth muscle cell (PASMC) migration is a key component of the vascular remodeling that occurs during the development of hypoxic pulmonary hypertension, although the mechanisms governing this phenomenon remain poorly understood. Aquaporin-1 (AQP1), an integral membrane water channel protein, has recently been shown to aid in migration of endothelial cells. Since AQP1 is expressed in certain types of vascular smooth muscle, we hypothesized that AQP1 would be expressed in PASMCs and would be required for migration in response to hypoxia. Using PCR and immunoblot techniques, we determined the expression of AQPs in pulmonary vascular smooth muscle and the effect of hypoxia on AQP levels, and we examined the role of AQP1 in hypoxia-induced migration in rat PASMCs using Transwell filter assays. Moreover, since the cytoplasmic tail of AQP1 contains a putative calcium binding site and an increase in intracellular calcium concentration ([Ca ²⁺] _i) is a hallmark of hypoxic exposure in PASMCs, we also determined whether the responses were Ca ²⁺ dependent. Results were compared with those obtained in aortic smooth muscle cells (AoSMCs). We found that although AQP1 was abundant in both PASMCs and AoSMCs, hypoxia selectively increased AQP1 protein levels, [Ca ²⁺] _i, and migration in PASMCs. Blockade of Ca ²⁺ entry through voltage-dependent Ca ²⁺ or nonselective cation channels prevented the hypoxia-induced increase in PASMC [Ca ²⁺] _i, AQP1 levels, and migration. Silencing AQP1 via siRNA also prevented hypoxia-induced migration of PASMCs. Our results suggest that hypoxia induces a PASMC-specific increase in [Ca ²⁺] _i that results in increased AQP1 protein levels and cell migration.