NFAT4 movement in native smooth muscle -: A role for differential Ca2+ signaling

The transcription factor NFAT (nuclear factor of activated T-cells) plays a central role in mediating Ca2+-dependent gene transcription in a variety of cell types. Sustained increases in intracellular calcium concentration ([Ca2+](i)) are presumed to be required for NFAT dephosphorylation by the Ca2+/calmodulin-dependent protein calcineurin and its subsequent nuclear translocation. Here, we provide the first identification and characterization of NFAT in native smooth muscle, showing that NFAT4 is the predominant isoform detected by reverse transcriptase-polymerase chain reaction and Western blot analysis. PDGF induces NFAT4 translocation in smooth muscle, leading to an increase in NFAT transcriptional activity. NFAT4 activation by PDGF depends on Ca2+ entry through voltage-dependent Ca2+ channels, because its nuclear accumulation is prevented by the Ca2+ channel blocker nisoldipine and the K+ channel opener pinacidil. Interestingly, elevation of [Ca2+](i) by membrane depolarization or ionomycin treatment are not effective stimuli for NFAT4 nuclear accumulation, indicating that Ca2+ influx is necessary but not sufficient for NFAT4 activation. In contrast, membrane depolarization readily activates the Ca2+-dependent transcription factor CREB (cAMP-responsive element-binding protein). The calcineurin blockers CsA and FK506 also prevented the PDGF-induced NFAT4 nuclear localization. These results indicate that both the nature of the calcium signal and PDGF-induced modulation of nuclear import-export of NFAT are critical for NFAT4 activation in this tissue.