Isoform- and tissue-specific regulation of the [Ca.sup.2+]-sensitive transcription factor NFAT in cardiac myocytes and heart failure

Nuclear factors of activated T cells (NFATs) are [Ca.sup.2+]-sensitive transcription factors that have been implicated in hypertrophy, heart failure (HF), and arrhythmias. Cytosolic NFAT is activated by dephosphorylation by the [Ca.sup.2+]-sensitive phosphatase calcineurin, resulting in translocation to the nucleus, which is opposed by kinase activity, rephosphorylation, and nuclear export. Four different NFAT isoforms are expressed in the heart. The activation and regulation of NFAT in adult cardiac myocytes, which may depend on the NFAT isoform and cell type, are not fully understood. This study compared basal localization, import, and export of NFATcl and NFATc3 in adult atrial and ventricular myocytes to identify isoform-and tissue-specific regulatory mechanisms of NFAT activation under physiological conditions and in HF. NFAT-green fluorescent protein fusion proteins and NFAT immunocytochemistry were used to analyze NFAT regulation in adult cat and rabbit myocytes. NFATcl displayed basal nuclear localization in atrial and ventricular myocytes, an effect that was attenuated by reducing intracellular [Ca.sup.2+] concentration and inhibiting calcineurin, and enhanced by the inhibition of nuclear export. In contrast, NFATc3 was localized to the cytoplasm but could be driven to the nucleus by angiotensin II and endothelin-1 stimulation in atrial, but not ventricular, cells. Inhibition of nuclear export (by leptomycin B) facilitated nuclear localization in both cell types. Ventricular myocytes from HF rabbits showed increased basal nuclear localization of endogenous NFATc3 and reduced responsiveness of NFAT translocation to phenylephrine stimulation. In control myocytes, [Ca.sup.2+] overload, leading to spontaneous [Ca.sup.2+] waves, induced substantial translocation of NFATc3 to the nucleus. We conclude that the activation of NFAT in adult cardiomyocytes is isoform and tissue specific and is tightly controlled by nuclear export. NFAT is activated in myocytes from HF animals and may be secondary to [Ca.sup.2+] overload. nuclear factor of activated T cells; intracellular [Ca.sup.2+] concentration; calcineurin; nuclear translocation doi: 10.1152/ajpheart.01072.2009.