Rho/Rho-associated coiled-coil forming kinase pathway as therapeutic targets for statins in atherosclerosis

Abstract

Significance: The 3-hydroxy-methylglutaryl coenzyme A reductase inhibitors or statins are important therapeutic agents for lowering serum cholesterol levels. However, recent studies suggest that statins may exert atheroprotective effects beyond cholesterol lowering. These so-called "pleiotropic effects" include effects of statins on vascular and inflammatory cells. Thus, it is important to understand whether other signaling pathways that are involved in atherosclerosis could be targets of statins, and if so, whether individuals with "overactivity" of these pathways could benefit from statin therapy, regardless of serum cholesterol level.

Recent advances: Statins inhibit the synthesis of isoprenoids, which are important for the function of the Rho/Rho-associated coiled-coil containing kinase (ROCK) pathway. Indeed, recent studies suggest that inhibition of the Rho/ROCK pathway by statins could lead to improved endothelial function and decreased vascular inflammation and atherosclerosis. Thus, the Rho/ROCK pathway has emerged as an important target of statin therapy for reducing atherosclerosis and possibly cardiovascular disease.

Critical issues: Because atherosclerosis is both a lipid and an inflammatory disease, it is important to understand how inhibition of Rho/ROCK pathway could contribute to statins' antiatherosclerotic effects.

Future directions: The role of ROCKs (ROCK1 and ROCK2) in endothelial, smooth muscle, and inflammatory cells needs to be determined in the context of atherogenesis. This could lead to the development of specific ROCK1 or ROCK2 inhibitors, which could have greater therapeutic benefits with less toxicity. Also, clinical trials will need to be performed to determine whether inhibition of ROCKs, with and without statins, could lead to further reduction in atherosclerosis and cardiovascular disease.

FIG. 1.

Cholesterol biosynthesis pathway and isoprenoids as modulators of Ras and Rho small GTP-binding proteins. Statins inhibit HMG-CoA reductase activity leading to a decrease in isoprenylation of small GTP-binding proteins. Ras-family proteins require FPP, while Rho-family proteins (such as RhoA, Rac1, and Cdc42) require GGPP for their anchoring to the cell membrane, where the small GTPases become activated by GTP loading and transduce signals to their effectors. FPP, farnesylpyrophosphate; GAP, GTPase-activating protein; GDI, GDP dissociation inhibitor; GEF, guanine nucleotide exchange factor; GGPP, geranyl-geranylpyrophosphate; HMG-CoA, 3-hydroxy-methylglutaryl coenzyme A.

FIG. 2.

Structure of ROCKs and their mode of activation. (A) Structure of ROCK isoforms. Both isoforms (ROCK1 and ROCK2) consist of an N-terminal kinase domain followed by a coiled-coil forming region containing a RBD and a CRD located within the PH domain. The isoforms share overall 65% homology in amino acid sequence and 92% homology in their kinase domains. (B) In the inactive form, the PH domain and RBD domain bind to the amino terminus of the enzyme, forming an autoinhibitory loop. Upon binding of RBD to active GTP-bound RhoA, the configuration becomes open, leading to the active kinase state of ROCK. CRD, carboxy-terminal cysteine-rich domain; PH, pleckstrin homology; RBD, Rho-binding domain; ROCK, Rho-associated coiled-coil forming kinase.

FIG. 3.

Rho/ROCK crosstalk with NO in the vascular wall. Endothelium-derived NO diffuses into VSMCs, stimulates synthesis of cGMP and leads to the activation of cGK. cGK blocks Rho/ROCK signaling by phosphorylating GTP and GDP-bound RhoA at Ser188, which increases Rho binding to Rho GDI and results in the sequestration of RhoA into the cytosol. On the other hand, Rho/ROCK in endothelial cells inhibits eNOS expression and activity leading to endothelial dysfunction that underlies vascular diseases, such as atherosclerosis. cGK, cGMP-dependent protein kinase; cGMP, cyclic GMP; eNOS, endothelial nitric oxide synthase; NO, nitric oxide; VSMCs, vascular smooth muscle cells.

FIG. 4.

Biological actions of ROCKs in the vasculature. In endothelial cells, Rho/ROCK decreases eNOS mRNA expression via reduction of its half-life, and also inhibits eNOS phosphorylation at Ser1177 through inactivation of Akt. In VSMCs, ROCK1 haploinsufficiency leads to decreased migration and reduced fibrotic gene expression. Pharmacological inhibition of ROCK prevents the development of atherosclerosis by inhibition of altered chemotaxis of macrophages and its transformation into foam cells. Genetic mouse models of ROCK1 or ROCK2 deletion revealed that ROCK1 promotes cholesterol uptake, while ROCK2 suppresses cholesterol efflux, in macrophages in the atherosclerotic lesion.

FIG. 5.

Role of Rho/ROCK in cardiac hypertrophy. Pharmacological studies using ROCK inhibitors have shown that Rho/ROCK pathway mediates pathological cardiac hypertrophy in response to angiotensin II infusion and thoracic aorta constriction. Recent genetic mouse models have revealed that ROCK2, but not ROCK1, mediates cardiomyocyte hypertrophy. ROCK2 activates SRF and ERK through inhibition of FHL2. Both ROCK1 and ROCK2 contribute to myocyte apoptosis and interstitial fibrosis in pathological cardiac hypertrophy. ERK, extracellular signal-regulated mitogen-activated protein kinase; FHL2, four-and-a-half LIM-only protein-2; SRF, serum response factor.

FIG. 6.

Role of Rho/ROCK in the etiology of insulin resistance. Experimental evidence in cultured cells and mice show conflicting results, between different models, for the effect of Rho/ROCK on insulin sensitivity. While ROCK phosphorylation of IRS-1 at Ser307 negatively impacts insulin signaling (left), ROCK phosphorylation of IRS-1 at Ser632/635 enhances insulin signaling (right). IRS-1, insulin receptor substrate 1.

FIG. 7.

Role of Rho/ROCK in adipocyte dysfunction and development. (Left) In the setting of obesity and metabolic syndrome, Rho/ROCK pathway is activated in adipose tissues, which promotes adipocyte hypertrophy, insulin resistance, upregulation of TNF-α and downregulation of adiponectin. (Right) In development and tissue regeneration, activated Rho/ROCK pathway positively regulates myogenesis, while it suppresses adipogenic differentiation. TNF-α, tumor necrosis factor.

FIG. 8.

Role of Rho/ROCK in diabetic vasculopathies. Experimental evidence for the role of ROCK in diabetic vascular complications. In endothelial cells, activation of the RAGE activates ROCK and mediates increased endothelial permeability of diabetes. HG promotes PAI-1 expression through ROCK and NF-κB. Also, high glucose upregulates arginase expression via Rho/ROCK and leads to decreased bioavailability of NO in diabetic endothelium. In VSMCs, HG upregulates Rho/ROCK through activation of PDGFR-beta. The activation of PDGFR-beta/Rho/ROCK results in the aberrantly, activated migration of diabetic VSMCs. HG, high glucose; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; PAI-1, plasminogen activator inhibitor-1; PDGFR, platelet-derived growth factor receptor; RAGE, receptor for advanced glycation end product.