Calcium Homeostasis Modulator (CALHM1/2) and Pulmonary Arterial Hypertension

RATIONALE Concentric pulmonary vascular remodeling is a major cause for the elevated pulmonary vascular resistance (PVR) and pulmonary arterial pressure (PAP) in patients with pulmonary arterial hypertension (PAH). Excessive PASMC proliferation and inhibited PASMC apoptosis have been implicated in the development and progression of pulmonary vascular wall thickening in PAH. Transition from the contractile phenotype to the synthetic or proliferative phenotype of PASMC is thus an important pathogenic process that promotes vascular remodeling. An increase in cytosolic free Ca 2+ concentration ([Ca 2+ ] cyt ) in PASMC is not only a trigger for PASMC contraction (and pulmonary vasoconstriction), but also an important stimulus for PASMC proliferation. Calcium homeostasis modulators (CALHM), CALHM1/2, are membrane Ca 2+ channels that are allosterically regulated by voltage and extracellular Ca 2+ . CALHM1/2 are closed at the resting membrane potential but can be opened by strong membrane depolarization. Reduction of extracellular [Ca 2+ ] increases CALHM open probability, which allows the channel to be activated at negative potential. We aimed to study whether CALHM1/2 are involved in the phenotypical transition of PASMC and whether CALHM1/2 are upregulated in highly proliferative PASMC from patients with PAH. METHODS Freshly isolated pulmonary artery with the endothelium removed (as contractile PASMC) and primary cultured PASMC (as synthetic or proliferative PASMC) from rats were used for this study. Western blot analysis was used to compare protein expression levels of CALHM1 and CALHM2 in fresh PA tissue and cultured PASMC derived from the same rats. In addition, we compared protein expression levels of CALHM1/2 in PASMC from normal subjects and PAH patients. RESULTS The protein expression level of CALHM2 in proliferative PASMC (the primary cultured PASMC in growth factor‐containing medium) was significantly higher than in contractile PASMC (the freshly isolated PA tissue). The increased CALHM2 in proliferative PASMC was indeed associated with a significantly increased expression of PCNA, a cell proliferation marker, and a significantly decreased expression of MHC and TGLN4, markers for contractile phenotype of SMC. Furthermore, we also observed that mRNA expression level of CALHM2 was significantly higher in PASMC isolated from PAH patients than in PASMC from normal subjects. CONCLUSION Upregulation of CALHM2 may play an important role in the phenotypical transition of PASMC from the contractile phenotype to the synthetic or proliferative phenotype. Ca 2+ influx through voltage‐ and extracellular Ca 2+ ‐regulated CALHM2 in the plasma membrane of PASMC is an important trigger for PASMC proliferation and may play an important role in stimulating PASMC proliferation in patients with PAH.