Relationship between cell size, plasma cholesterol and rat adipocyte cholesterol storage

Systemic MCPIP1 deficiency in mice impairs lipid homeostasis

Atherosclerosis involves interactions between inflammation system and dyslipidemia. MCPIP1 (Monocyte Chemotactic Protein induced Protein-1) is induced by proinflammatory molecules and serves as a negative feedback loop in regulating inflammatory responses. Our current study was designed to test the role of MCPIP1 in maintaining lipid homeostasis, the latter a pivotal factor that contributes to the pathogenesis of atherosclerosis. We found that MCPIP1 knockout mice displayed a decrease in levels of serum HDL-cholesterol and total triglycerides but an increase in serum LDL/VLDL-cholesterol levels when compared to wild-type mice. Additionally, ApoA-1 expression was reduced but LPL expression was upregulated in plasma from MCPIP1 knockout mice. The livers from the MCPIP1 knockout mice revealed a decrease in hepatocyte number and an increase in collagen deposition when compared to wild-type mice. These findings suggest that MCPIP1 deficiency can induce liver fibrosis, alter the expression of lipoproteins, and affect transportation and metabolism of lipids, indicating that MCPIP1 is involved in maintaining lipid homeostasis, possibly via negatively regulating inflammatory responses.

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Atherosclerosis is the result of interaction between inflammation and dyslipidemia.

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MCPIP1 is a negative regulator in inflammatory responses.

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MCPIP1 is upregulated in the atherosclerotic plaques.

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MCPIP1 deficiency induces dyslipidemia and hepatic remodeling.

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MCPIP1 deficiency may increase the risk of atherosclerosis.

Dietary cholesterol effects on adipose tissue inflammation

PURPOSE OF REVIEW

Adipose tissue is a critical endocrine and immunological organ that regulates systemic energy homeostasis. During the pathogenesis of obesity, adipocyte hypertrophy is accompanied by adipose tissue inflammation, impeding insulin sensitivity and endocrine function of adipose tissue and other tissues. Adipocyte cholesterol accumulates in proportion to triglyceride as adipocytes undergo hypertrophy. Recent studies suggest that dietary cholesterol contributes to increased adipocyte cholesterol. However, how dietary cholesterol accumulates in adipocytes and its metabolic consequences are poorly understood. This review summarizes recent advances in knowledge of adipocyte cholesterol balance and highlights the emerging role of dietary cholesterol in adipose tissue cholesterol balance, inflammation, and systemic energy metabolism.

RECENT FINDINGS

Perturbation of cholesterol balance in adipocytes alters intracellular cholesterol distribution and modulates adipocyte insulin and proinflammatory signaling. Adipocyte cholesterol levels are maintained by a balance between dietary cholesterol uptake from triglyceride-enriched lipoproteins and cellular cholesterol efflux to HDL. Recent animal studies established a critical role for dietary cholesterol in promoting adipose tissue inflammation, thereby worsening obesity-mediated metabolic complications.

SUMMARY

Recent studies identified high dietary cholesterol as a potentiator of adipose tissue inflammation and dysfunction. Reducing excessive dietary cholesterol intake is suggested as a simple, but novel, way to attenuate obesity-associated metabolic diseases.

Cholesterol suppresses adipocytic differentiation of mouse adipose-derived stromal cells via PPARγ2 signaling

The effects of cholesterol on cell proliferation and adipocytic differentiation have been evaluated for the first time in mouse adipose-derived stromal cells (mASCs). Cholesterol loading by using cholesterol:methyl- β-cyclodextrin (Chol:MβCD) complexes promoted cellular levels of free cholesterol (FC) and cholesteryl ester (CE), induced high cell proliferation of mASCs dose-dependently. Compared with control cells, cholesterol-treated mASCs showed an impaired differentiation process in both dose- and time-dependent manners, based on reduced oil red O-stained lipid droplets, SREBP-1, PPARγ2 and aP2 expression levels. The involvement of SREBP-1-mediated PPARγ2 in the effects of cholesterol on mASC adipogenesis was elucidated. These results point to cholesterol as a modulator of adipogenesis, which separate cholesterol itself from other components of modified lipoproteins.

The signal transducer and activator of transcription 5B gene polymorphism contributes to the cholesterol metabolism in Japanese children with growth hormone deficiency

BACKGROUND AND AIMS

GH plays a significant role in the lipid metabolism. In this study, we focused on the JAK2 - signal transducer and activator of the transcription 5 (STAT5) pathway, which transmit the signals from the GH receptor, and selected the STAT5A/B gene as a candidate for the regulation of lipid metabolism in GH deficiency (GHD).

DESIGN AND PARTICIPANTS

The study population comprised 83 children with idiopathic GHD. The serum total cholesterol (TC), high-density lipoprotein cholesterol (HDL-C) and the non-HDL cholesterol (non-HDL-C) levels were monitored before and at 3, 6, 9 and 12 months after starting GH treatment. The height, weight, body mass index, and serum insulin-like growth factor-I (IGF-I) level were also measured before and 12 months after starting the GH treatment. For the genetic analysis of the STAT5A/B gene, five tag SNPs were selected using the tag SNP picker programme on the homepage of the HapMap project. The evaluation of promoter activity of the -44816A/G SNP in the STAT5B gene was performed by a luciferase assay in vitro.

RESULTS

The TC and non-HDL-C levels were gradually decreased during the GH treatment. Five tag SNPs (rs4029774, rs6503691, rs9900213, rs16967637 and rs2272087) were picked up for the STAT5A/B gene, and the genetic study demonstrated that the paediatric GHD patients who were heterozygotes or homozygotes of minor alleles of the analysed SNPs in the same block of the STAT5B gene showed significantly higher serum TC or non-HDL-C levels both before and after GH treatment for 12 months. Most of the SNPs also demonstrated significant differences among genotypes in the decreases in serum TC or non-HDL-C levels during the 12 months of GH treatment. A luciferase assay showed that the -44816A/G SNP (rs4029774) in the STAT5B gene functionally affected the expression level in vitro.

CONCLUSION

These results indicate that STAT5B may therefore play a role in regulating the cholesterol metabolism in children with GHD.

The role of fat tissue in the cholesterol lowering and the pleiotropic effects of statins – statins activate the generation of metabolically more capable adipocytes

Statins have very important influences on adipocyte physiology, particularly via their effects on sterol regulatory element binding proteins (SREBPs), which are extremely important transcription factors for the regulation of adipocyte physiology. SREBP-1c activation in fat cell by the cytosolic cholesterol deprivation through HMG-reductase inhibition is responsible for most of the beneficial effects of statins. Glucose and lipid excess, and oxidative overload coming with energy dense nutrition, which might be deleterious, can be removed from the circulation by adipo-lipogenetic enhancement through the activated SREBP-1c. SREBP-1c causes formation of new small and competent adipocytes, which are insulin and leptin sensitive. Thus, adipocytes gain the ability of fat storing, and much more importantly, fat-burning machines. Statins already have a large and growing number of indications in the treatment of atherovascular diseases. General mechanism for the action of statins is their inhibitory effects on 3-hydroxy 3-methyl glutaryl CoA reductase, which is the rate-limiting enzyme in cholesterol biosynthesis, particularly in the liver. Recently, the beneficial effects of statins beyond their cholesterol lowering, which are called pleiotropic effects, have been the focus of great attention. Certain characteristics of the statin molecule itself have been proposed to explain these effects. Adipocytes are in the very heart of metabolic regulations. Therefore, the important elements of the pleiotropic effects of statins occur through executive improvements on impaired adipocyte functions by the activation of SREBP-1c.

Intracellular distribution and mobilization of unesterified cholesterol in adipocytes: triglyceride droplets are surrounded by cholesterol-rich ER-like surface layer structures

In addition to their central role in triglyceride storage, fat cells are a primary depot of unesterified cholesterol (FC) in the body. In comparison, peripheral cells contain very little FC. This difference in adipocytes versus peripheral tissues is inconsistent with the current theory of cholesterol homeostasis. Attempting to resolve this discrepancy, we examined intracellular storage sites of FC in murine 3T3-F442A adipocytes. Using the cholesterol-binding antibiotic, filipin, in combination with high resolution fluorescence microscopy, intense fluorescent staining characteristically decorated the periphery of triglyceride droplets (TGD) as well as the plasma membrane (PM) of fat cells. Filipin-staining was not visible inside the lipid droplets. Purification of TGD by subcellular fractionation demonstrated that the rise in total FC content of adipocytes upon differentiation was attributable to an increase in TGD-FC, which contributed up to one third of the total cellular FC. The protein component of purified TGD from cultured adipocytes as well as from murine adipocytes obtained from fresh tissues contained the lumenal endoplasmic reticulum (ER) immunoglobulin binding protein (BiP) and the integral ER membrane protein calnexin. Efflux experiments using the extracellular FC acceptors (β)-cyclodextrin or apolipoprotein A-I demonstrated that TGD-associated FC was releasable from TGD. Whereas FC efflux from adipocytes was unaffected in the presence of brefeldin A or monensin, the secretion of a control protein, lipoprotein lipase, was effectively reduced. In summary, our findings identify the TGD surface layer as primary intracellular storage site for FC within adipocytes. We suggest that the structural role of ER-resident proteins in this adipocyte TGD envelope has been previously neglected. Our findings support the suggestion that an ER-like structure, albeit of modified lipid composition, constitutes the lipid droplets' surface layer. Finally, the efflux process of FC from adipocytes upon extracellular stimulation with (beta)-cyclodextrin provides evidence for an energy-dependent intracellular trafficking route between the TGD-FC pool and the PM-FC sites which is distinct from the secretory pathway of proteins.

Low-density lipoprotein receptors in rat adipocytes: regulation with fasting

Adipose tissue metabolism is exquisitely sensitive to caloric intake. With increasing adiposity more triglyceride and cholesterol are stored within increasingly large adipocytes, whereas less triglyceride and cholesterol are stored as the size of the fat cell decreases. A portion of the uptake of cholesterol by adipocytes is mediated by low-density lipoprotein (LDL) receptors. The present studies addressed whether LDL receptors are differentially regulated in adipose tissue and the liver during fasting in the rat. Two days of fasting caused a reduction in body weight with an approximately 40% decrease in the epididymal fat depot and fat cell size. No changes in serum cholesterol were noted, but serum triglycerides fell approximately 55% with fasting. LDL receptors detected by immunoblotting decreased progressively with fasting to levels that were 95% below controls in adipocytes isolated from epididymal fat pads by 2-3 days. In contrast, hepatic LDL receptor expression was unaltered by fasting. After 2 days of fasting, the rate of synthesis of LDL receptors in isolated adipose cells was decreased approximately 35%, whereas levels of LDL receptor mRNA were diminished approximately 55%. It is concluded that the expression of LDL receptors in rat adipocytes is markedly downregulated during fasting through transcriptional and posttranscriptional mechanisms. Furthermore, LDL receptor expression is differentially regulated in adipose tissue and liver during fasting in the rat.

Accumulation of adipocyte cholesterol during hypolipidemic drug treatment in cholesterol-fed rats

An attempt was made to mobilize adipose tissue cholesterol independently of triacylglycerol by feeding cholesterol to intact Fischer 344 rats to 'load' adipocytes followed by hypolipidemic drug treatment in order to lower plasma cholesterol and, hence, adipocyte cholesterol. In this strain of rat, body weight and adipocyte sizes remain relatively constant after 1 year of age. Therefore, alterations in adipocyte cholesterol can be ascribed to factors other than cell size. Both oxandrolone and combined cholestyramine/clofibrate treatment caused significant reductions in plasma cholesterol in cholesterol-fed rats, but cholesterol concentrations in liver were reduced only by cholestyramine/clofibrate treatment. Oxandrolone enhanced the development of liver fatty liver in the cholesterol-fed rats, but cholestyramine/clofibrate significantly reduced liver triacylglycerol concentrations. Adipocyte cholesterol in the epididymal depot was significantly elevated, not lowered, in both concentrations. Adipocyte cholesterol in the epididymal depot was significantly elevated, not lowered, in both groups of drug-treated animals. Subcutaneous adipocytes from rats receiving drug treatment also contained more cholesterol, especially in rats given oxandrolone. Increments in adipocyte cholesterol were associated with decreases in the absolute amounts of apolipoproteins, A-I and A-IV, as measured by densitometric scanning of electrophoretic gels. Under the present experimental conditions, changes in plasma cholesterol scanning of electrophoretic gels. Under the present experimental conditions, changes in plasma cholesterol concentration did not adequately reflect the cholesterol content of either liver or adipose tissue.

Effects of weight reduction on squalene, methyl sterols and cholesterol and on their synthesis in human adipose tissue

Quantitation of cholesterol and its precursors from human adipose tissue biopsies revealed very high squalene and moderately high methyl sterol concentrations. The squalene and cholesterol values were correlated with each other. Weight reduction in obese subjects following a jejuno-ileal bypass resulted in a significant but transient increase in adipose tissue cholesterol. The squalene concentration was also increased postoperatively, the maximum being reached about 6 months later than that of cholesterol as if the mobilization of squalene from shrunken adipocytes had been slow. Weight reduction with a 2--14 day total fast significantly reduced the adipocyte size but had no consistent effect on adipose tissue squalene, methyl sterol and cholesterol concentrations or on their adipocyte contents. Incubation of adipose tissue with labelled acetate and mevalonate revealed that the bulk of the labels in non-saponifiable lipids stayed in the large intermediate pools of methyl sterols and squalene in particular, fairly little being found in the cholesterol fraction itself. The total fast inhibited the incorporation of both 14C-acetate and 3H-mevalonate to squalene, methyl sterols and cholesterol, suggesting that cholesterol synthesis was inhibited before and after the mevalonate step.