Arterial expression of PTH-related protein is markedly induced by angioplasty. PTH-related protein contains a nuclear localization signal (NLS). PTH-related protein mutants lacking the NLS (ΔNLS-PTH-related protein) are potent inhibitors of arterial vascular smooth muscle cell (VSMC) proliferation in vitro. This is of clinical relevance because adenoviral delivery of ΔNLS-PTH-related protein at angioplasty completely inhibits arterial restenosis in rats. In this study we explored the cellular mechanisms through which ΔNLS-PTH-related protein arrests the cell cycle. In vivo, adenoviral delivery of ΔNLS-PTH-related protein at angioplasty markedly inhibited VSMC proliferation as compared with angioplastied carotids infected with control adenovirus (Ad.LacZ). In vitro, ΔNLS-PTH-related protein overexpression was associated with a decrease in phospho-pRb, and a G0/G1 arrest. This pRb underphosphorylation was associated with stable levels of cdks 2, 4, and 6, the D and E cyclins, p16, p18, p19, and p21, but was associated with a dramatic decrease in cdk-2 and cdk4 kinase activities. Cyclin A was reduced, but restoring cyclin A adenovirally to normal did not promote cell cycle progression in ΔNLS-PTH-related protein VSMC. More importantly, p15INK4 and p27kip1, two critical inhibitors of the G1/S progression, were markedly increased. Normalization of both p15INK4b and p27kip1 by small interfering RNA knockdown normalized cell cycle progression. These data indicate that the changes in p15INK4b and p27kip1 fully account for the marked cell cycle slowing induced by ΔNLS-PTH-related protein in VSMCs. Finally, ΔNLS-PTH-related protein is able to induce p15INK4 and p27kip1 expression when delivered adenovirally to primary murine VSMCs. These studies provide a mechanistic understanding of ΔNLS-PTH-related protein actions, and suggest that ΔNLS-PTH-related protein may have particular efficacy for the prevention of arterial restenosis. This study provides the mechanistic underpinnings for understanding how Δ-NLS-PTHrP functions, and suggests that Δ-NLS-PTHrP may have particular efficacy for the prevention of arterial re-stenosis.