Differential response of cardiac aquaporins to hyperosmotic stress; salutary role of AQP1 against the induced apoptosis

OBJECTIVE Multiple pathophysiological conditions are associated with disturbance of myocardial osmotic equilibrium, exerting detrimental effects on cardiac performance. Cardiac myocytes may encounter hyperosmotic stress during hyperglycemia, ischemia/reperfusion injury, myocardial infarction, diabetes mellitus, severe dehydration, hypoxia or heat stress. Aquaporins (AQPs) constitute transmembrane channels that facilitate water transport in response to osmotic gradients. Therefore, the present study aimed at probing into AQPs mode of response and potential role as effector molecules and sensors, under hyperosmotic stress. MATERIALS AND METHODS H9c2 cardiac myoblasts were left untreated (control) or were exposed to 0.5 M sorbitol so as to induce hyperosmotic stress conditions. After the experimental treatments, MTT assay was performed to assess cell viability. Endogenous mRNA levels of AQP1 and AQP7 were assessed by ratiometric RT-PCR. Their subcellular localization pattern was revealed by immunofluorescence microscopy. Protein levels of AQP1 and AQP7, as well as of apoptotic markers (cleaved caspase-3 and PARP), were detected by immunoblot analysis. RESULTS Hyperosmotic stress (0.5 M sorbitol) induced a time-dependent upregulation of AQP7 (but not of AQP1) mRNA in H9c2 cells. Of note, biochemical and immunocytochemical analyses revealed the increased membrane-associated protein expression of AQP1, under these conditions, while AQP7 respective levels remained unchanged. Interestingly, inhibition of AQP1 by HgCl2, aggravated the sorbitol-induced apoptosis in H9c2 cells, as evidenced by chromatin condensation and fragmentation of caspase-3 and PARP. CONCLUSIONS AQP1 and AQP7 are differentially regulated under hyperosmotic stress conditions in H9c2 cells. AQP1, acting as an osmotic stress sensor and response factor, exerts a beneficial effect against the sorbitol-induced apoptosis, potentially favoring preservation of cardiac function.