Genetic Deletion of FXR1 Reduces Intimal Hyperplasia and Induces Senescence in Vascular Smooth Muscle Cells.

Vascular smooth muscle cells (VSMC) play a critical role in the development and pathogenesis of intimal hyperplasia indicative of restenosis and other vascular diseases. Fragile-X related protein-1 (FXR1) is a muscle-enhanced RNA binding protein whose expression is increased in injured arteries. Previous studies suggest that FXR1 negatively regulates inflammation, but its causality in vascular disease is unknown. In the current study, RNA-sequencing of FXR1-depleted VSMC identified many transcripts with decreased abundance, most of which were associated with proliferation and cell division. mRNA abundance and stability of a number of these transcripts were decreased in FXR1-depleted hVSMC, as was proliferation (P < 0.05); however, increases in beta-galactosidase (P < 0.05) and γH2AX (P < 0.01), indicative of senescence, were noted. Further analysis showed increased abundance of senescence-associated genes with FXR1 depletion. A novel SMC-specific conditional knockout mouse (FXR1SMC/SMC) was developed for further analysis. In a carotid artery ligation model of intimal hyperplasia, FXR1SMC/SMC mice had significantly reduced neointima formation (P < 0.001) after ligation, as well as increases in senescence drivers p16, p21, and p53 compared with several controls. These results suggest that in addition to destabilization of inflammatory transcripts, FXR1 stabilized cell cycle-related genes in VSMC, and absence of FXR1 led to induction of a senescent phenotype, supporting the hypothesis that FXR1 may mediate vascular disease by regulating stability of proliferative mRNA in VSMC.

FXR1 regulates vascular smooth muscle cell cytoskeleton, VSMC contractility, and blood pressure by multiple mechanisms.

Appropriate cytoskeletal organization is essential for vascular smooth muscle cell (VSMC) conditions such as hypertension. This study identifies FXR1 as a key protein linking cytoskeletal dynamics with mRNA stability. RNA immunoprecipitation sequencing (RIP-seq) in human VSMCs identifies that FXR1 binds to mRNA associated with cytoskeletal dynamics, and FXR1 depletion decreases their mRNA stability. FXR1 binds and regulates actin polymerization. Mass spectrometry identifies that FXR1 interacts with cytoskeletal proteins, particularly Arp2, a protein crucial for VSMC contraction, and CYFIP1, a WASP family verprolin-homologous protein (WAVE) regulatory complex (WRC) protein that links mRNA processing with actin polymerization. Depletion of FXR1 decreases the cytoskeletal processes of adhesion, migration, contraction, and GTPase activation. Using telemetry, conditional FXR1SMC/SMC mice have decreased blood pressure and an abundance of cytoskeletal-associated transcripts. This indicates that FXR1 is a muscle-enhanced WRC modulatory protein that regulates VSMC cytoskeletal dynamics by regulation of cytoskeletal mRNA stability and actin polymerization and cytoskeletal protein-protein interactions, which can regulate blood pressure.

Deletion of LDLRAP1 Induces Atherosclerotic Plaque Formation, Insulin Resistance, and Dysregulated Insulin Response in Adipose Tissue.

Dyslipidemia, vascular inflammation, obesity, and insulin resistance often overlap and exacerbate each other. Mutations in low density lipoprotein receptor adaptor protein-1 (LDLRAP1) lead to LDLR malfunction and are associated with the autosomal recessive hypercholesterolemia disorder in humans. However, direct causality on atherogenesis in a defined preclinical model has not been reported. The objective of this study was to test the hypothesis that deletion of LDLRAP1 will lead to hypercholesteremia and atherosclerosis. LDLRAP1-/- mice fed a high-fat Western diet had significantly increased plasma cholesterol and triglyceride concentrations accompanied with significantly increased plaque burden compared with wild-type controls. Unexpectedly, LDLRAP1-/- mice gained significantly more weight compared with controls. Even on a chow diet, LDLRAP1-/- mice were insulin-resistant, and calorimetric studies suggested an altered metabolic profile. The study showed that LDLRAP1 is highly expressed in visceral adipose tissue, and LDLRAP1-/- adipocytes are significantly larger, have reduced glucose uptake and AKT phosphorylation, but have increased CD36 expression. Visceral adipose tissue from LDLRAP1-/- mice was hypoxic and had gene expression signatures of dysregulated lipid storage and energy homeostasis. These data are the first to indicate that lack of LDLRAP1 directly leads to atherosclerosis in mice and also plays an unanticipated metabolic regulatory role in adipose tissue. LDLRAP1 may link atherosclerosis and hypercholesterolemia with common comorbidities of obesity and insulin resistance.

Angiotensin II, Hypercholesterolemia, and Vascular Smooth Muscle Cells: A Perfect Trio for Vascular Pathology.

Cardiovascular disease is the leading cause of morbidity and mortality in the Western and developing world, and the incidence of cardiovascular disease is increasing with the longer lifespan afforded by our modern lifestyle. Vascular diseases including coronary heart disease, high blood pressure, and stroke comprise the majority of cardiovascular diseases, and therefore represent a significant medical and socioeconomic burden on our society. It may not be surprising that these conditions overlap and potentiate each other when we consider the many cellular and molecular similarities between them. These intersecting points are manifested in clinical studies in which lipid lowering therapies reduce blood pressure, and anti-hypertensive medications reduce atherosclerotic plaque. At the molecular level, the vascular smooth muscle cell (VSMC) is the target, integrator, and effector cell of both atherogenic and the major effector protein of the hypertensive signal Angiotensin II (Ang II). Together, these signals can potentiate each other and prime the artery and exacerbate hypertension and atherosclerosis. Therefore, VSMCs are the fulcrum in progression of these diseases and, therefore, understanding the effects of atherogenic stimuli and Ang II on the VSMC is key to understanding and treating atherosclerosis and hypertension. In this review, we will examine studies in which hypertension and atherosclerosis intersect on the VSMC, and illustrate common pathways between these two diseases and vascular aging.