Abstract 5897: PP1cγ1 Deficiency Attenuates and Its Overexpression Enhances Neointima Formation After Vascular Injury
We previously showed that protein phosphatase 1cγ1 (PP1cγ1) overexpression protects vascular smooth muscle cells (VSMC) against oxidative stress induced apoptosis by interfering with JNK1 and p53 phosphorylation cascades. It is suggested that early onset or acute apoptosis after vascular injury regulates neointima formation by either stimulating subsequent proliferation and migration of VSMC or decreasing viable VSMC population in the medial wall. The former increases whereas the latter attenuates neointima formation. To determine whether antiapoptotic PP1cγ1 overexpression modulates neointima formation, guidewire-injured mouse carotid arteries (CA) were treated with adenovirus encoding GFP or human PP1cγ1 (Ad PP1cγ1) (n=8 in each case). Overexpression of human PP1cγ1 and GFP in the vessel wall was confirmed by immunohistochemistry. Morphometric analysis 21 days after injury revealed a paradoxical and significant increase in neointima in CA treated with AdPP1cγ1 compared with the GFP controls (p<0.001). Adenoviral overexpression of PP1cγ1 significantly enhanced thrombin-induced proliferation of VSMC compared to wild-type cells treated with thrombin. Thrombin-induced thymidine incorporation was not inhibited in PP1cγ1−/− VSMC compared with wild-type cells. However, deficiency of PP1cγ1 resulted in a 46% decrease in cell number compared to wild-type VSMC (p<0.01) 48 hours after thrombin treatment. Rescue experiments involving overexpression of human PP1cγ1 partially but significantly restored thrombin-induced cell proliferation (p<0.05 vs thrombin-treated PP1cγ1−/− VSMC). Mitotic shake-off experiments and fluorescence-activated cell sorter analysis revealed that decrease in cell proliferation in thrombin-treated PP1cγ1−/− VSMC was associated with a failure of cell division in a late stage of cytokinesis and the formation of dikaryons. Further, delayed cytokinesis was associated with accumulation of p53. Arterial injury experiments showed decreased neointima formation in CA of PP1cγ1−/− mice compared with wild-type mice (p<0.01). These results indicate that PP1cγ1 regulates mitotic exit in a p53-dependent mechanism and deficiency of PP1cγ1 attenuates neointima formation by affecting VSMC proliferation.