Endogenous ApoE Expression Modulates Adipocyte Triglyceride Content and Turnover

Abstract

Apolipoprotein E (apoE) is highly expressed in adipose tissue and adipocytes in which its expression is regulated by peroxisome proliferator-activated receptor (PPAR)-␥ agonists and tumor necrosis factor-␣. There is, however, no information regarding a role for endogenous apoE in differentiated adipocyte function. In this report, we define a novel role for apoE in modulating adipocyte lipid metabolism. ApoE ؊/؊ mice have less body fat and smaller adipocytes compared with wild-type controls. Freshly isolated adipose tissue from apoE ؊/؊ mice contains lower levels of triglyceride and free fatty acid, and these differences are maintained in cultured adipocytes derived from preadipocytes. Adenoviral expression of apoE in apoE ؊/؊ -cultured adipocytes increases triglyceride and fatty acid content. During incubation with apoE-containing triglyceride-rich lipoproteins, apoE ؊/؊ adipose tissue accumulates less triglyceride than wild type. The absence of apoE expression in primary cultured adipocytes also leads to changes in the expression of genes involved in the metabolism/turnover of fatty acids and the triglyceride droplet. Markers of adipocyte differentiation were lower in freshly isolated and cultured apoE ؊/؊ adipocytes. Importantly, PPAR-␥-mediated changes in lipid content and gene expression are markedly altered in cultured apoE ؊/؊ adipocytes. These results establish a novel role for endogenous apoE in adipocyte lipid metabolism and have implications for constructing an integrated model of adipocyte physiology in health and disease. Diabetes 55:3394 -3402, 2006 O besity and its consequent insulin resistance are major health problems in the U.S., imparting significant risk for diabetes and cardiovascular disease (1-4). The prevalence of obesity is predicted to substantially increase over the next several decades, and there is a need to better understand adipocyte and adipose tissue physiology. In the past several years, it has become apparent that adipocytes and adipose tissue actively modulate systemic substrate availability and produce a number of protein factors with endocrine, paracrine, and autocrine regulatory activity (5,6). Apolipoprotein E (apoE), which was first described as a product of hepatocytes and a surface component of lipoproteins, e.g., in humans, chylomicrons, VLDL, remnant lipoproteins, and HDL, has been shown to be highly expressed in adipocytes and adipose tissue (7). Interestingly, apoE has been shown to be highly expressed in a number of cell types that experience high lipid flux (8 -15). The physiologic role of apoE expression in other cell types has been intensively studied and characterized (8 -15). In macrophages and steroidogenic cells, for example, endogenous apoE expression plays an important role in cellular lipid balance. Adipocytes and adipose tissue, like macrophages and steroidogenic cells, also experience large lipid fluxes integral to their differentiated function, yet there is no information regarding a potential physiologic role of apoE expressed in the adipocyte. Our lab...

Peroxisome Proliferator-activated Receptor γ Stimulation of Adipocyte ApoE Gene Transcription Mediated by the Liver Receptor X Pathway

“…This defect in TG accumulation can be corrected by the adenoviral expression of apoE in EKO adipocytes. Furthermore, in EKO adipocytes, TG accumulation in response to treatment with PPAR␥ agonists is impaired compared with WT adipocytes (6). All of these observations support a role for apoE in adipocyte TG metabolism.…”

“…Reactive oxygen species produced by the stromovascular fraction of adipose tissue harvested from obese mice also reduce adipocyte apoE expression via the NF-B pathway (8). With respect to apoE function, adipocytes from EKO mice are smaller than adipocytes from WT mice, and cultured EKO adipocytes and adipose tissue accumulate less TG compared with WT cells after incubation with apoE-containing lipoproteins (6). This defect in TG accumulation can be corrected by the adenoviral expression of apoE in EKO adipocytes.…”

“…Experiments were performed on post differentiation days 8 -10. Triglyceride content of cells was measured using a kit from Wako Chemicals as previously described (6).…”

“…Zechner and colleagues first described expression of apoE by adipocytes in 1991 (5), but only recently has additional information been provided regarding regulation and function of adipocyte apoE (6 -10). Treating isolated adipocytes or intact humans with PPAR␥ agonists increases adipocyte and adipose tissue apoE gene expression (4,6). The proinflammatory cytokine, tumor necrosis factor-␣, reduces adipocyte apoE expression, an effect mediated by the NF-B transcriptional complex binding an element in the apoE gene proximal promoter (4,7).…”

The effect of inhibition of endoplasmic reticulum stress on lipolysis in white adipose tissue in a rat model of chronic kidney disease

“…We performed adipose tissue organ culture to evaluate ex vivo lipolysis as previously described [19] . Freshly isolated visceral white adipose tissue samples (epididymal, omental, and retroperitoneal adipose tissue) were washed in PBS and placed in 60-mm dishes at 100 mg total tissue (wet weight) per dish.…”