Calcineurin and Jak2 Signaling Induce Expression of Cardioprotective αB-crystallin Gene in Response to Cardiac Hypertrophic Signals.

Abstract

Among the stress proteins that are up-regulated in the heart due to imposed biomechanical stress, αB-crystallin (CryAB) is the most abundant and pivotal in rendering protection against stress-induced cell damage. Heightened interest in understanding the mechanism of CryAB expression in the heart emerged from studies that demonstrated the requirement of the αBE4 element in the CryAB promoter for maximal expression of CryAB in cardiomyocytes. However, there is no evidence, to date, on the identity of regulatory proteins involved in expression of the CryAB gene, nor have the stress-associated signaling molecules that trigger CryAB expression in cardiomyocytes been identified. We observed that in response to various hypertrophic stimuli the transcription factors NFAT, STAT3 and Nished form a dynamic ternary complex that interacts with αBE4. Chromatin immunoprecipitation and gel shift assays confirmed that interaction of the NFAT/Nished complex with αBE4 DNA is enhanced in cultured cardiomyocytes exposed to endothelin (En-1), a potent hypertrophic agonist. Likewise, hypertrophic cardiomyocytes from mice subjected to trans-aortic constriction and from heart-targeted calcineurin transgenic animals show a similar requirement. Our data demonstrate that in addition to NFAT/calcineurin signaling, there is an interactive role of Jak/STAT signaling in up-regulation of the CryAB gene. The heart-targeted calcineurin transgenic mice exhibit a marked increase in STAT3 phosphorylation and consequently an increase of CryAB gene expression. AG490, a potent inhibitor of Jak2 phosphorylation, not only blocked NFAT translocation into the nucleus, but also the expression of the CryAB gene. We conclude that the stimulation of CryAB gene expression can be attributed to a functional linkage between the two signaling pathways, Jak/STAT and calcineurin/NFAT, each previously documented to be involved in promotion of the hypertrophic response in cardiomyocytes subjected to stress.