Peroxisome proliferator-activated receptor gene expression in human tissues: Effects of obesity, weight loss, and regulation by insulin and glucocorticoids

Abstract

The peroxisome proliferator activated receptor (PPAR γ) plays a key role in adipogenesis and adipocyte gene expression and is the receptor for the thiazolidinedione class of insulin-sensitizing drugs. The tissue expression and potential for regulation of human PPAR γ gene expression in vivo are unknown. We have cloned a partial human PPAR γ cDNA, and established an RNase protection assay that permits simultaneous measurements of both PPAR γ1 and PPAR γ2 splice variants. Both γ1 and γ2 mRNAs were abundantly expressed in adipose tissue. PPAR γ1 was detected at lower levels in liver and heart, whereas both γ1 and γ2 mRNAs were expressed at low levels in skeletal muscle. To examine the hypothesis that obesity is associated with abnormal adipose tissue expression of PPAR γ, we quantitated PPARγ mRNA splice variants in subcutaneous adipose tissue of 14 lean and 24 obese subjects. Adipose expression of PPARγ 2 mRNA was increased in human obesity (14.25 attomol PPAR γ2/18S in obese females vs 9.9 in lean, P = 0.003). This increase was observed in both male and females. In contrast, no differences were observed in PPAR γ1/18S mRNA expression. There was a strong positive correlation (r = 0.70, P < 0.001) between the ratio of PPAR γ2/γ1 and the body mass index of these patients. We also observed sexually dimorphic expression with increased expression of both PPAR γ1 and PPAR γ2 mRNAs in the subcutaneous adipose tissue of women compared with men. To determine the effect of weight loss on PPAR γ mRNA expression, seven additional obese subjects were fed a low calorie diet (800 Kcal) until 10% weight loss was achieved. Mean expression of adipose PPAR γ2 mRNA fell 25% (P = 0.0250 after a 10% reduction in body weight), but then increased to pretreatment levels after 4 wk of weight maintenance. Nutritional regulation of PPAR γ1 was not seen. In vitro experiments revealed a synergistic effect of insulin and corticosteroids to induce PPAR γ expression in isolated human adipocytes in culture. We conclude that: (a) human PPAR γ mRNA expression is most abundant in adipose tissue, but lower level expression of both splice variants is seen in skeletal muscle; to an extent that is unlikely to be due to adipose contamination. (b) RNA derived from adipose tissue of obese humans has increased expression of PPAR γ2 mRNA, as well as an increased ratio of PPAR γ2/γ1 splice variants that is proportional to the BMI; (c) a low calorie diet specifically down-regulates the expression of PPAR γ2 mRNA in adipose tissue of obese humans; (d) insulin and corticosteroids synergistically induce PPAR γ mRNA after in vitro exposure to isolated human adipocytes; and (e) the in vivo modulation of PPAR γ2 mRNA levels is an additional level of regulation for the control of adipocyte development and function, and could provide a molecular mechanism for alterations in adipocyte number and function in obesity.