APOLIPOPROTEIN E MODULATION OF VASCULAR SMOOTH MUSCLE CELL RESPONSE TO INJURY
Apolipoprotein E (apoE) has been shown to exert anti-proliferative and anti-migratory effects on smooth muscle cells in vitro and protection against neointimal hyperplasia in vivo. In this study, apoE was detected in the medial layers of the carotid artery following vascular injury. The presence of a liver-specific human apoE transgene confirmed the recruitment from circulation, although detection of native apoE message by in situ hybridization indicated that local synthesis also occurs. Although the neointima-resistant C57BL/6 strain showed a transient apoE presence, the susceptible FVB/N strain showed a continuous presence. Interestingly, detection of iNOS expression was positive only in the C57BL/6 wildtype strain. Upon addition of the overexpressed apoE transgene, the FVB/N strain recovered expression of iNOS, while loss of the apoE gene in the C57BL/6 strain resulted in a concurrent loss of iNOS expression. ApoE and iNOS colocalization was demonstrated in the medial layer of mice that are resistant to injury-induced neointimal hyperplasia. NOS2-/- deficient C57BL/6 mice showed no difference in neointimal hyperplasia compared to wildtype, but a significant increase in medial thickness and area was observed. These data documented that injury-induced activation of iNOS requires apoE recruitment, both apoE and iNOS are necessary for suppression of cell proliferation, and apoE recruitment without iNOS expression resulted in medial smooth muscle hyperplasia without their migration to the intima. In addition, the apoE-dependent inhibition of smooth muscle cell migration is dependent on LRP-1 function in the smooth muscle cell. Interestingly, apoE accumulation in apoE transgenic mice followed a layer-specific pattern, and was inversely associated with smooth muscle alpha-actin expression. The vascular accumulation of apoE after endothelial denudation, and its association with alpha-actin-depleted smooth muscle cells, suggests that apoE inhibition of injury-induced neointimal hyperplasia is not due to the inhibition of injury-induced smooth muscle cell de-differentiation, but is likely a direct effect of apoE on smooth muscle cell migration and proliferation. Examination of A7R5 cells in culture indicated that the addition of apoE prior to PDGF mitogenic stimulation resulted in a loss of alpha-actin signal as seen my immunocytofluorescence. Specific inhibition of kinase mediators of the alpha-actin polymerization pathway showed that both PKC and PKA mediate the apoE/PDGF effect. Examination of the RhoA activation state showed that apoE pretreatment lowers active levels of RhoA, causing a depolymerization of the alpha-actin cytoskeleton. We conclude there to be a significant apoE-dependent effect on the smooth muscle cell component of the vessel wall in response to vascular injury, including direct recruitment, iNOS-dependent inhibition of proliferation, LRP-dependent inhibition of migration, and modulation of the cytoskeleton.
School:University of Cincinnati
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:apolipoprotein e lrp 1 inos rhoa vascular injury pdgf mouse
Date of Publication:01/01/2005