The type VI adenylyl cyclase protects cardiomyocytes from β-adrenergic stress by a PKA/STAT3-dependent pathway

Background: The type VI adenylyl cyclase (AC6) is a main contributor of cAMP production in the heart. The amino acid (aa) sequence of AC6 is highly homologous to that of another major cardiac adenylyl cyclase, AC5, except for its N-terminus (AC6-N, aa 1-86). Activation of AC6, rather than AC5, produces cardioprotective effects against heart failure, while the underlying mechanism remains to be unveiled. Using an AC6-null (AC6(-/-)) mouse and a knockin mouse with AC6-N deletion (AC6(Delta N/Delta N)), we aimed to investigate the cardioprotective mechanism of AC6 in the heart. Methods: Western blot analysis and immunofluorescence staining were performed to determine the intracellular distribution of AC6, AC6-Delta N (a truncated AC6 lacking the first 86 amino acids), and STAT3 activation. Activities of AC6 and AC6-Delta N in the heart were assessed by cAMP assay. Apoptosis of cardiomyocytes were evaluated by the TUNEL assay and a propidium iodine-based survival assay. Fibrosis was examined by collagen staining. Results: Immunofluorescence staining revealed that cardiac AC6 was mainly anchored on the sarcolemmal membranes, while AC6-Delta N was redistributed to the sarcoplasmic reticulum. AC6(Delta N/Delta N) and AC6(-/-) mice had more apoptotic myocytes and cardiac remodeling than WT mice in experimental models of isoproterenol (ISO)-induced myocardial injury. Adult cardiomyocytes isolated from AC6(Delta N/Delta N) or AC6(-/-) mice survived poorly after exposure to ISO, which produced no effect on WT cardiomyocytes under the condition tested. Importantly, ISO treatment induced cardiac STAT3 phosphorylation/activation in WT mice, but not in AC6(Delta N/Delta N) and AC6(-/-) mice. Pharmacological blockage of PKA-, Src-, or STAT3-pathway markedly reduced the survival of WT myocytes in the presence of ISO, but did not affect those of AC6(Delta N/Delta N) and AC6(-/-) myocytes, suggesting an important role of AC6 in mediating cardioprotective action through the activation of PKA-Src-STAT3-signaling. Conclusions: Collectively, AC6-N controls the anchorage of cardiac AC6 on the sarcolemmal membrane, which enables the coupling of AC6 with the pro-survival PKA-STAT3 pathway. Our findings may facilitate the development of novel therapies for heart failure.