Adipocytic differentiation and liver x receptor pathways regulate the accumulation of triacylglycerols in human vascular smooth muscle cells

Oxysterol nuclear receptor LXRbeta regulates cholesterol homeostasis and contractile function in mouse uterus

The uterus is an organ where lipid distribution plays a critical role for its function. Here we show that nuclear receptor for oxysterols LXRbeta prevents accumulation of cholesteryl esters in mouse myometrium by controlling expression of genes involved in cholesterol efflux and storage (abca1 and abcg1). Upon treatment with an LXR agonist that mimics activation by oxysterols, expression of these target genes was increased in wild-type mice, whereas under basal conditions, lxralpha;beta(-/-) mice exhibited a marked decrease in abcg1 accumulation. This change resulted in a phenotype of cholesteryl ester accumulation. Besides, a defect of contractile activity induced by oxytocin or PGF2alpha was observed in mice lacking LXRbeta. These results imply that LXRbeta provides a safety valve to limit cholesteryl ester levels as a basal protective mechanism in the uterus against cholesterol accumulation and is necessary for a correct induction of contractions.

Vascular smooth muscle cell phenotypic plasticity and the regulation of vascular calcification

Vascular smooth muscle cells (VSMCs) exhibit an extraordinary capacity to undergo phenotypic change during development, in vitro and in association with disease. Unlike other muscle cells they do not terminally differentiate. Development and maintenance of the mature contractile phenotype is regulated by a number of interacting transcription factors. In response to injury contractile VSMCs can be induced to change phenotype, proliferate and migrate to effect repair. On completion of the repair process VSMCs return to a nonproliferating contractile phenotype. In this way, in the context of atherosclerosis, a protective fibrous cap is formed and maintained at sites of injury. However in disease, when modulatory signals are perturbed, this phenotypic transition is dysregulated and VSMCs are induced to undergo inappropriate differentiation into cells with features of other mesenchymal lineages such as osteoblasts, chondrocytes and adipocytes. Moreover, evidence is accumulating that these aberrant phenotypic transitions contribute to the pathogenesis of vascular diseases such as atherosclerosis and Monckeberg's Sclerosis. Indeed, the osteo/chondrocytic conversion of VSMCs and the association of this phenotype with vascular calcification is a paradigm for how inappropriate differentiation can influence disease processes. Understanding of the mechanisms and signalling pathways involved in this particular phenotype change is well advanced offering the possibility for the design of successful therapeutic interventions in the future.

Angiogenesis and pericytes in the initiation of ectopic calcification

Ectopic calcification of blood vessels, heart valves, and skeletal muscle is a major clinical problem. There is now good evidence that angiogenesis is associated with ectopic calcification in these tissues and that it is necessary, but not sufficient, for calcification to occur. Angiogenesis may regulate ectopic calcification in several ways. First, many angiogenic factors are now known to exert both direct and indirect effects on bone and cartilage formation. Second, cytokines released by endothelial cells can induce the differentiation of osteoprogenitor cells. Third, the new blood vessels provide oxygen and nutrients to support the growing bone. Finally, the new blood vessels can serve as a conduit for osteoprogenitor cells. These osteoprogenitor cells may be derived from the circulation or from pericytes that are present in the neovessels themselves. Indeed, there is now compelling evidence that pericytes can differentiate into osteoblasts and chondrocytes both in vitro and in vivo. Other vascular cells, including adventitial myofibroblasts, calcifying vascular cells, smooth muscle cells, and valvular interstitial cells, have also been shown to exhibit multilineage potential in vitro. Although these cells share many properties with pericytes, the precise relationship between them is not known. Furthermore, it still remains to be determined whether all or some of these cells contribute to the ectopic calcification observed in vivo. A better understanding of the underlying mechanisms that link angiogenesis, pericytes, and ectopic calcification should provide a basis for development of therapeutic strategies to treat or arrest this clinically significant condition.

IGF-1 induces rat glomerular mesangial cells to accumulate triglyceride

Rat glomerular mesangial cells (MC) become lipid-laden foam cells when they are exposed to IGF-1. IGF-1 increased accumulation of triglyceride (TG) 2.5-fold in MC after 7 days. TG accumulation resulted from enhanced macropinocytosis and decreased efflux secondary to a 40-50% reduction in peroxisome proliferator-activated receptor (PPAR)-delta (PPARdelta). There was no evidence of primary or secondary changes in cholesterol or TG synthesis, increased uptake by LDL or scavenger receptors, or reduced efflux via ATP-binding cassette A-1. Although the lipid moiety taken up can be influenced by the concentration of cholesterol or TG in the medium, in standard medium MC preferentially accumulate TG. TG-rich MC foam cells fail to contract in response to angiotensin II (Berfield AK, Andress DL, and Abrass CK. Kidney Int 62: 1229-1237, 2002); however, their migratory response to IGF binding protein-5 is unaffected. This differs from cholesterol loading, which impairs both phagocytosis and migration. These findings have important implications for understanding the mechanisms that contribute to lipid accumulation in MC and the functional consequences of different forms of foam cells. These observations are relevant to understanding vascular disease and progressive renal diseases that are accelerated by abnormalities in lipid metabolism.

Genes of cholesterol metabolism in human atheroma: overexpression of perilipin and genes promoting cholesterol storage and repression of ABCA1 expression

OBJECTIVE

Accumulation of cholesterol in foam cells of atheroma plaques depends on the balance between uptake and efflux of cholesterol. It may also depend on proteins surrounding lipid droplets, adipophilin, and perilipins. They favor triglyceride storage in adipocytes and could play a similar role for cholesterol in atheroma.

METHODS AND RESULTS

We measured in human atheroma and nearby macroscopically intact tissue (MIT) the expression of perilipin, adipophilin, and regulatory factors of cholesterol metabolism. We identified perilipin A in human arterial wall. Its expression was largely increased in atheroma compared with MIT, and perilipin was present in macrophages and vascular smooth muscle cells. Adipophilin, ACAT1, and CD36 were also overexpressed in atheroma. mRNA levels of low-density lipoprotein receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and SREBP-2 were unchanged. With respect to efflux of cholesterol, the mRNA levels of NCEH and ABCA-1 were unchanged, whereas CLA-1 mRNA was slightly higher in atheroma. Importantly, immunoblotting of ABCA-1 showed a dramatic decrease of ABCA1 protein, the key molecule of cholesterol efflux, in atheroma compared with MIT.

CONCLUSIONS

We show the presence and induction of perilipin in atheroma. This overexpression and the coordinated modifications of expression of key regulatory factors for cholesterol metabolism could favor cholesterol accumulation.