The lipogenic enzymes DGAT1, FAS, and LPL in adipose tissue: effects of obesity, insulin resistance, and TZD treatment

Abstract

Acyl-coenzyme A:diacylglycerol transferase (DGAT), fatty acid synthetase (FAS), and LPL are three enzymes important in adipose tissue triglyceride accumulation. To study the relationship of DGAT1, FAS, and LPL with insulin, we examined adipose mRNA expression of these genes in subjects with a wide range of insulin sensitivity (SI). DGAT1 and FAS (but not LPL) expression were strongly correlated with SI. In addition, the expression of DGAT1 and FAS (but not LPL) were higher in normal glucose-tolerant subjects compared with subjects with impaired glucose tolerance (IGT) (P < 0.005). To study the effects of insulin sensitizers, subjects with IGT were treated with pioglitazone or metformin for 10 weeks, and lipogenic enzymes were measured in adipose tissue. After pioglitazone treatment, DGAT1 expression was increased by 33 +/- 10% (P < 0.05) and FAS expression increased by 63 +/- 8% (P < 0.05); however, LPL expression was not altered. DGAT1, FAS, and LPL mRNA expression were not significantly changed after metformin treatment. The treatment of mice with rosiglitazone also resulted in an increase in adipose expression of DGAT1 by 2- to 3-fold, as did the treatment of 3T3 F442A adipocytes in vitro with thiazolidinediones. These data support a more global concept suggesting that adipose lipid storage functions to prevent peripheral lipotoxicity.

Figures

Fig. 1

Relationship between insulin sensitivity (SI) and acyl-coenzyme A:diacylglycerol transferase (DGAT1) or fatty acid synthetase (FAS) expression. DGAT1 and FAS mRNA levels in normal glucose tolerance (NGT; triangles) and impaired glucose tolerance (IGT; circles) subjects were determined by real-time RT-PCR analysis and expressed in relation to endogenous 18S RNA, as described in Methods. SI was determined using the frequently sampled intravenous glucose tolerance test for each subject.

Fig. 2

Expression of DGAT1, FAS, and LPL in IGT and NGT subjects. DGAT1, FAS, and LPL mRNA levels were measured as described in Methods; data presented are relative values expressed in relation to endogenous 18S RNA (means ± SEM). The NGT and IGT subjects are matched for body mass index (BMI). Subject characteristics are as shown in Table 1. * P < 0.01, ** P < 0.001.

Fig. 3

A: Effect of pioglitazone (Pio; n = 17) or metformin (Met; n = 20) treatment on DGAT1, FAS, and LPL mRNA in IGT subjects. DGAT1, FAS, and LPL mRNA were measured in adipose tissue as described in Methods and normalized to endogenous 18S RNA; the values represent percentage change from baseline after drug treatment (means ± SEM) * P < 0.05 versus baseline. B: DGAT1 activity in IGT subjects after pioglitazone or metformin treatment. DGAT1 activity was measured in subcutaneous adipose tissue before and after treatment. DGAT1 activity is expressed as nmol/106 cells (means ± SEM). * P < 0.03 versus baseline (pretreatment). Subject characteristics are as shown in Table 2.

Fig. 4

DGAT1 expression in mouse white adipose tissue (WAT). DGAT1 mRNA and protein were measured in mouse WAT after rosiglitazone (Rosi) treatment. DGAT1 mRNA measurements were done by real-time PCR of six samples per experiment performed in duplicate. DGAT1 protein was measured as described in Methods, by Western blot analysis followed by densitometric analysis of the band. Error bars show SD. The change in DGAT1 expression in rosiglitazone-treated animals was significant (DGAT1 mRNA, P < 0.03 vs. untreated control; DGAT1 protein, P < 0.05 vs. untreated control). The inset shows a representative blot of WAT from control or rosiglitazone-treated WAT, probed with DGAT antibody.

Fig. 5

DGAT1 expression in 3T3 F442A adipocytes. DGAT1 mRNA and protein in 3T3 F442A adipocytes differentiated with insulin or rosiglitazone (Rosi). DGAT1 mRNA quantitations were done by real-time PCR using triplicate measurements in three separate experiments. The change in DGAT1 mRNA expression in rosiglitazone-treated adipocytes was significant (P < 0.03). Error bars show SD. DGAT1 activity was measured as described in Methods using duplicate measurements in at least six different experiments. The change in DGAT1 activity versus insulin-differentiated cells was significant (P < 0.05). Error bars represent SD. The inset shows a representative Western blot of adipocytes differentiated with insulin or rosiglitazone probed with DGAT1 antibody.