Kalirin Promotes Neointimal Hyperplasia by Activating Rac in Smooth Muscle Cells

Abstract

Objective Kalirin is a multifunctional protein that contains two guanine nucleotide exchange factor (GEF) domains for the GTPases Rac1 and RhoA. Variants of KALRN have been associated with atherosclerosis in humans, but Kalirin’s activity has been characterized almost exclusively in the CNS. We therefore tested the hypothesis that Kalirin functions as a RhoGEF in arterial smooth muscle cells (SMCs). Methods and Results Kalirin-9 protein is expressed abundantly in aorta and bone marrow, as well as in cultured SMCs, endothelial cells, and macrophages. Moreover, arterial Kalirin was up-regulated during early atherogenesis in apolipoprotein E-deficient mice. In cultured SMCs, signaling was affected similarly in three models of Kalirin loss-of-function: heterozygous Kalrn deletion, Kalirin RNAi, and treatment with the Kalirin RhoGEF-1 inhibitor 1-(3-nitrophenyl)-1H-pyrrole-2,5-dione. With reduced Kalirin function, SMCs showed normal RhoA activation but diminished Rac1 activation, assessed as reduced Rac-GTP levels, p21-activated kinase autophosphorylation, and SMC migration. Kalrn−/+ SMCs proliferated 30% less rapidly than WT SMCs. Neointimal hyperplasia engendered by carotid endothelial denudation was ~60% less in Kalrn−/+ and SMC-specific Kalrn−/+ mice than in control mice. Conclusions Kalirin functions as a GEF for Rac1 in SMCs, and promotes SMC migration and proliferation both in vitro and in vivo.

RACing up a New Regulatory Mechanism for Vascular Smooth Muscle Cell Migration

“…The RhoGEF1 domain and RhoGEF2 domain activate Rac1 and RhoA, respectively. 17 Wu et al 16 demonstrated Kalirin-9 expression in the aorta, and that Kalirin expression was increased in a model of atherosclerosis, consistent with clinical findings. 18 They also established that Kalirin-9 is expressed by multiple cell types found within the vascular wall, including endothelial cells, VSMCs, and macrophages.…”

“…The authors set out to test whether Kalirin-9 functions in VSMCs as a dual Rho-GEF in vitro and in vivo. 16 convincingly demonstrated that in VSMCs, despite the presence of a GEF domain specific for Rac1 and a GEF domain specific for RhoA, 17 Kalirin primarily mediates Rac1 activation via its RhoGEF1 domain, but does not alter RhoA activation, to promote VSMC migration.…”

“…In this issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Wu et al 16 describe the presence of the novel GEF for Rac1 and RhoA, Kalirin-9, in VSMCs. Kalirin was described previously in the central nervous system, where it regulates neuronal shape, growth, and plasticity, in part, through effects on the actin cytoskeleton.…”

Genetic Dissection of the Vav2-Rac1 Signaling Axis in Vascular Smooth Muscle Cells

“…5H). Possible candidates include ArhGEF7 and Kalirin, since these exchange factors promote the proliferation and migration of vSMC in vitro and, in the case of Kalirin, neointima formation in vivo (44,45). Our results suggest that Vav3 also can represent a good candidate for this regulatory step (Fig.…”

The Ins and Outs of Small GTPase Rac1 in the Vasculature

“…Neointimal hyperplasia, induced by carotid endothelial denudation, was significantly reduced in VSMC-specific Kalrn -/1 mice compared with control mice. Reduced Kalrn function gave normal RhoA activation but diminished Rac1 activation in serum-, endothelin-1-, or PDGF-stimulated VSMC, as assessed by reduced Rac1-GTP levels, PAK1 activation, and reduced VSMC proliferation and migration (Wu et al, 2013a). These data suggest that the Rac-GEF domain is dominant over the Rho-GEF domain in Kalrn under these conditions.…”