Summary
Vascular smooth muscle cell (VSMC) proliferation occurs in vascular obstructive events
such as atherosclerosis and restenosis. We previously showed that Notch receptors
are induced in smooth muscle cells during vascular remodeling. Our goal was to determine
the mechanisms employed by Notch signaling to regulate proliferation. Activation of
Notch1 and Notch4 induced the VSMC-selective target genes HRT1 and HRT2, promoted
cell cycle transit in smooth muscle cells, and led to loss of density-dependent growth
inhibition. This was associated with a reduction in levels of the cyclin-dependent
kinase inhibitor (cdk) p27kip1.Over-expression of p27 kip1 resulted in a dose-dependent
rescue of the Notch-induced phenotype and exit from the cell cycle. In addition, HRT2
expression was sufficient to promote S-phase entry, and we demonstrate that HRT2 interacts
directly with the p27 kip1 promoter to repress transcription. Transcriptional repression
occurred within the ∼774bp minimal p27kip1 promoter region and mutational analysis
demonstrated that repression is largely dependent on a conserved class-C domain. Our
data show that Notch signaling acts to promote a proliferative phenotype in VSMC by
modulation of the G1/S-phase checkpoint. In addition, we define a novel mechanism
by which the Notch effector, HRT2, interacts directly with the class-C domain of the
p27kip1 promoter, repressing its expression. These studies identify a novel transcriptional
target of HRT2, and show that Notch effectors directly control cell cycle regulatory
components. We suggest that this mechanism is relevant to hyperproliferative states
in VSMC seen during vascular remodeling and repair.
Keywords
Notch signaling - smooth muscle - HRT2 - p27kip1 - proliferation