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 o f 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 trans location. Here, we provide the first identification and characterization of NFAT in native smooth muscle, showing that NFAT4 is the predominant isofor m detected by reverse transcriptase-polymerase chain reaction and Western b lot analysis. PDGF induces NFAT4 translocation in smooth muscle, leading to an increase in NFAT transcriptional activity. NFAT4 activation by PDGF dep ends on Ca2+ entry through voltage-dependent Ca2+ channels, because its nuc lear accumulation is prevented by the Ca2+ channel blocker nisoldipine and the K+ channel opener pinacidil. Interestingly, elevation of [Ca2+](i) by m embrane depolarization or ionomycin treatment are not effective stimuli for NFAT4 nuclear accumulation, indicating that Ca2+ influx is necessary but n ot sufficient for NFAT4 activation. In contrast, membrane depolarization re adily activates the Ca2+-dependent transcription factor CREB (cAMP-responsi ve element-binding protein). The calcineurin blockers CsA and FK506 also pr evented 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 tis sue.