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.