MARCKS Signaling Differentially Regulates Vascular Smooth Muscle and Endothelial Cell Proliferation through a KIS-, p27(kip1)- Dependent Mechanism

BACKGROUND: Overexpression of the myristolated alanine-rich C kinase substrate (MARCKS) occurs in vascular proliferative diseases such as restenosis after bypass surgery. MARCKS knockdown results in arrest of vascular smooth muscle cell (VSMC) proliferation with little effect on endothelial cell (EC) proliferation. We sought to identify the mechanism of differential regulation by MARCKS of VSMC and EC proliferation in vitro and in vivo. METHODS AND RESULTS: siRNA-mediated MARCKS knockdown in VSMCs inhibited proliferation and prevented progression from phase G(0)/G(1) to S. Protein expression of the cyclin-dependent kinase inhibitor p27(kip1), but not p21(cip1) was increased by MARCKS knockdown. MARCKS knockdown did not affect proliferation in VSMCs derived from p27(kip1)-/- mice indicating that the effect of MARCKS is p27(kip1)-dependent. MARCKS knockdown resulted in decreased phosphorylation of p27(kip1) at threonine 187 and serine 10 as well as, kinase interacting with stathmin (KIS), cyclin D1, and Skp2 expression. Phosphorylation of p27(kip1) at serine 10 by KIS is required for nuclear export and degradation of p27(kip1). MARCKS knockdown caused nuclear trapping of p27(kip1). Both p27(kip1) nuclear trapping and cell cycle arrest were released by overexpression of KIS, but not catalytically inactive KIS. In ECs, MARCKS knockdown paradoxically increased KIS expression and cell proliferation. MARCKS knockdown in a murine aortic injury model resulted in decreased VSMC proliferation determined by bromodeoxyuridine (BrdU) integration assay, and inhibition of vascular wall thickening. MARCKS knockdown increased the rate of re-endothelialization. CONCLUSIONS: MARCKS knockdown arrested VSMC cell cycle by decreasing KIS expression. Decreased KIS expression resulted in nuclear trapping of p27(kip1) in VSMCs. MARCKS knockdown paradoxically increased KIS expression in ECs resulting in increased EC proliferation. MARCKS knockdown significantly attenuated the VSMC proliferative response to vascular injury, but accelerated reestablishment of an intact endothelium. MARCKS is a novel translational target with beneficial cell type-specific effects on both ECs and VSMCs.