Parathyroid hormone-related protein (
PTHrP) contains a classical bipartite
nuclear localization signal. Nuclear
PTHrP induces proliferation of arterial vascular smooth muscle cells (VSMC). In the arterial wall,
PTHrP is markedly up-regulated in response to angioplasty and promotes arterial restenosis.
PTHrP overexpression exacerbates arterial restenosis, and knockout of the
PTHrP gene results in decreased VSMC proliferation in vivo. In arterial VSMC, expression of the cell cycle inhibitor, p27, rapidly decreases after angioplasty, and replacement of p27 markedly reduces
neointima development. We have shown that
PTHrP overexpression in VSMC leads to p27 down-regulation, mostly through increased proteosomal degradation. Here, we determined the molecular mechanisms through which
PTHrP targets p27 for degradation.
S-phase kinase-associated protein 2 (skp2) and c-myc, two critical regulators of p27 expression and stability, and
neointima formation were up-regulated in
PTHrP overexpression in VSMC. Normalization of skp2 or c-myc using
small interfering RNA restores normal cell cycle and p27 expression in
PTHrP overexpression in VSMC. These data indicate that skp2 and c-myc mediate p27 loss and proliferation induced by
PTHrP. c-myc promoter activity was increased, and c-myc target genes involved in p27 stability were up-regulated in
PTHrP overexpression in VSMC. In primary VSMC,
PTHrP overexpression led to increased c-myc and decreased p27. Conversely, knockdown of
PTHrP in primary VSMC from
PTHrP(flox/flox) mice led to cell cycle arrest, p27 up-regulation, with c-myc and skp2 down-regulation. Collectively, these data describe for the first time the role of
PTHrP in the regulation of skp2 and c-myc in VSMC. This novel PTHrP-c-myc-skp2 pathway is a potential target for therapeutic manipulation of the arterial response to injury.