Role of adipocyte-derived apoE in modulating adipocyte size, lipid metabolism, and gene expression in vivo

Abstract

Adipocytes isolated from apolipoprotein E (apoE)-knockout (EKO) mice display alterations in triglyceride (TG) metabolism and gene expression. The present studies were undertaken to evaluate the impact of endogenously produced adipocyte apoE on these adipocyte parameters in vivo, independent of the profoundly disturbed metabolic milieu of EKO mice. Adipose tissue from wild-type (WT) or EKO mice was transplanted into WT recipients, which were then fed chow or high-fat diet for 8-10 wk. After a chow diet, freshly isolated transplanted EKO adipocytes were significantly (P < 0.05) smaller (70%) than transplanted WT adipocytes and displayed significantly lower rates of TG synthesis and higher rates of TG hydrolysis. Transplanted EKO adipocytes also had higher mRNA levels for adiponectin, perilipin, and genes coding for enzymes in the fatty acid oxidation pathway and lower levels of caveolin. After a high-fat diet and consequent increase in circulating lipid and apoE levels, transplanted WT adipocyte size increased by 106 x 10(3) microm(3), whereas EKO adipocyte size increased only by 19 x 10(3) microm(3). Endogenous host adipose tissue harvested from WT recipients of transplanted WT or EKO adipose tissue did not demonstrate any difference in adipocyte size. Consistent with the in vivo observations, EKO adipocytes synthesized less TG when incubated with apoE-containing TG-rich lipoproteins than WT adipocytes. Our results establish a novel in vivo role for endogenously produced apoE, distinct from circulating apoE, in modulation of adipocyte TG metabolism and gene expression. They support a model in which endogenously produced adipocyte apoE facilitates adipocyte lipid acquisition from circulating TG-rich lipoproteins.

Fig. 1.

Adipocyte size and lipid content in transplanted adipose tissue. A: hematoxylin-eosin-stained sections of transplanted wild-type (WT) or apolipoprotein E (apoE)-knockout (EKO) white adipose tissue (WAT) harvested from WT recipients after 8 wk on a chow diet. Original magnification ×200. B and C: size of mature adipocytes from transplanted WT or EKO adipose tissue harvested from WT recipients. B: frequency distribution for adipocyte size in transplanted WT and EKO adipose tissue harvested from 5 WT recipients in each group. C: adipocyte size (mean ± SD) from transplanted WT and EKO adipose tissue harvested from 15 WT recipients in each group. *P < 0.05.

Fig. 2.

Triglyceride (TG) synthesis and hydrolysis in adipocytes isolated from transplanted adipose tissue. TG synthesis over 2 h (A) and TG hydrolysis over 90 min (B) were measured in adipocytes isolated from transplanted EKO or WT adipose tissue harvested from WT recipients. C: adipocytes were isolated from transplanted adipose tissue and incubated with or without 50 μM isoproterenol for 2 h for measurement of TG hydrolysis. Values are means ± SD of 5 mice in each experimental group. *P < 0.05.

Fig. 3.

Gene expression in adipocytes isolated from transplanted adipose tissue. Total RNA was extracted from adipocytes isolated from transplanted adipose tissue harvested from WT recipients for measurement of mRNA levels for adiponectin, peroxisome proliferator-activated receptor-γ (PPARγ), medium-chain acetyl-CoA dehydrogenase (ACADM), proliferator-activated receptor coactivator-1 (PGC-1), carnitine palmitoyltransferase (CPT) I, acyl-CoA oxidase (ACO), perilipin, caveolin-1, lipoprotein lipase (LPL), adipocyte TG lipase (ATGL), and hormone-sensitive lipase (HSL). Results are expressed as fold change for mRNA level in transplanted EKO adipocytes compared with transplanted WT adipocytes and are representative of 2 separate experiments, each performed using 5 recipient mice in each experimental group. *P < 0.05; ** P < 0.01 vs. WT.

Fig. 4.

Effect of high-fat diet (HFD) on adipocyte size distribution and mean adipocyte size in transplanted adipose tissue. A: hematoxylin-eosin-stained sections of transplanted WT and EKO adipose tissue harvested from HFD-fed WT recipients. Original magnification ×200. B and C: mature adipocytes from transplanted EKO or WT adipose tissues were harvested from chow- or HFD-fed WT recipients. B: adipocyte size distribution in transplanted adipose tissue from HFD-fed recipients. C: mean transplanted adipocyte size for chow- and HFD-fed groups. Values are means ± SD of 5 mice in each experimental group. *P < 0.01, EKO vs. WT adipocytes on the same diet. **P < 0.05. †P < 0.05 vs. chow.

Fig. 5.

Gene expression in adipocytes isolated from transplanted adipose tissue harvested from HFD-fed WT mice. WT and EKO transplanted adipose tissue was harvested from HFD-fed WT recipients, and total RNA was extracted for measurement of mRNA levels. Values (means ± SD from 5 mice in each experimental group) are expressed as fold change in mRNA level for transplanted EKO compared with transplanted WT adipocytes. *P < 0.05. **P < 0.01.

Fig. 6.

Cell size of adipocytes harvested from transplanted or endogenous adipose tissue of WT recipient mice. Adipose tissue from EKO or WT mice was transplanted into WT recipients. After 12 wk on an HFD, EKO and WT transplanted adipose tissue was harvested from WT recipients (top) for adipocyte sizing. Endogenous recipient visceral (Vis; middle) and subcutaneous (Sub; bottom) adipose tissue was also harvested from WT recipients for adipocyte sizing. *P < 0.05.

Fig. 7.

Endogenous adipocyte apoE expression facilitates adipocyte TG synthesis in response to incubation with apoE-containing TG-rich lipoprotein. Mature adipocytes freshly isolated from adipose tissue (A) or cultured adipocytes differentiated from preadipocytes (B) from WT or EKO mice were incubated with or without human apoE-containing VLDL (100 μg/ml) in 0.1% BSA-DMEM for 6 h at 37°C. Values are means ± SD of 3 separate experiments, each performed in triplicate. **P < 0.01.