Background: 

G protein coupled receptor signaling pathways are important components of the complex biological processes controlling cellular proliferation, which plays an essential role in vascular diseases such as atherosclerosis and restenosis. Regulator of G-protein signaling (RGS) proteins potently inhibit G-protein signaling by enhancing GTP hydrolysis of Ga subunits. In the present study we sought to determine the role of the "regulator of G-protein signaling 5" (RGS5) in VSMC proliferation and vascular lesion formation.

Methods and Results: 

Strong expression of RGS5 was detected in VSMC of native mouse and human arteries. However, during the development of atherosclerosis (ApoE/LDLr KO mice) as well as restenosis (dilation of the mouse femoral artery), RGS5 was downregulated as assessed by qPCR. Moreover, RGS5 was found to be downregulated in proliferating-, as compared to quiescent VSMC. To further assess the role of RGS5 for VSMC proliferation, human VSMC were transduced with Ad-RGS5-WT or Ad-C2A-RGS5, a non-degradable mutant. Overexpession of RGS5-WT and even more of C2A-RGS5 significantly prevented VSMC proliferation as assessed by BrdU incorporation. Conclusively, FACS analysis of propidium iodide stained cells indicated a cell cycle arrest in G0/G1 phase. In contrast, siRNA-mediated knock down of RGS5 significantly augmented VSMC proliferation. In addition, we were able to show, that reconstitution/overexpression of RGS5 is able to prevent ERK phosphorylation and MAPK downstream signalling following angiotensin II stimulation, indicatin that inhibition of MEK/ERK signaling pathway might be a key mechanism mediating RGS5's anti-proliferative effect. For in vivo studies, the femoral artery of C57BL/6J mice was dilated and adenoviral vectors (5 X 108 pfu of Ad-RGS5-WT or Ad-C2A-RGS5 or a control vector) were delivered extraluminally to the denuded segment in a self-degrading pluronic gel. Morphological analysis of femoral arteries 21 days after dilation revealed that the neointima/media ratio was significantly reduced (1.6 0.4 and 1.7 0.3 vs. 2.6 0.4; P<0.005).

Conclusion: 

RGS5 is strongly expressed in native arterial VSMC and downregulated during vascular remodeling in atherosclerotic and neointimal lesions. Reconstitution of the RGS5-WT form and even more the non-degradable mutant prevents MEK/ERK activation, VSMC cell cycle entry and proliferation and significantly reduces neointima formation in vivo. These data add substantially to the understanding of RGS5's role in vascular remodeling processes and identifies RGS5 as an interesting target for future strategies to prevent vascular proliferative disease.