Induction by leptin of uncoupling protein-2 and enzymes of fatty acid oxidation

Abstract

We have studied mechanisms by which leptin overexpression, which reduces body weight via anorexic and thermogenic actions, induces triglyceride depletion in adipocytes and nonadipocytes. Here we show that leptin alters in pancreatic islets the mRNA of the genes encoding enzymes of free fatty acid metabolism and uncoupling protein-2 (UCP-2). In animals infused with a recombinant adenovirus containing the leptin cDNA, the levels of mRNAs encoding enzymes of mitochondrial and peroxisomal oxidation rose 2- to 3-fold, whereas mRNA encoding an enzyme of esterification declined in islets from hyperleptinemic rats. Islet UCP-2 mRNA rose 6-fold. All in vivo changes occurred in vitro in normal islets cultured with recombinant leptin, indicating direct extraneural effects. Leptin overexpression increased UCP-2 mRNA by more than 10-fold in epididymal, retroperitoneal, and subcutaneous fat tissue of normal, but not of leptin-receptor-defective obese rats. By directly regulating the expression of enzymes of free fatty acid metabolism and of UCP-2, leptin controls intracellular triglyceride content of certain nonadipocytes, as well as adipocytes.

Figure 1

(A) Ratios of mRNA of ACO, CPT-I, ACC, and GPAT to mRNA of β-actin in islets isolated from hyperleptinemic rats infused with AdCMV–leptin, pairfed controls, free-feeding AdCMV–β-gal controls, and saline-infused controls. Numbers in parentheses indicate the number of rats studied. *, P < 0.001 vs. AdCMV–β-gal-infused rats. ^†, P < 0.001 vs. pairfed rats. (B) Ratios of mRNA of ACO, CPT-I, ACC, and GPAT to mRNA of β-actin in normal rat islets cultured for 48 hr in either 0 or 20 ng/ml of recombinant leptin. *, P < 0.001 vs. 0 leptin. Values are the mean ± SEM of three individual experiments.

Figure 2

Representative Southern blot of RT-PCR products for enzymes of fatty acid metabolism, ACO, CPT-I, ACC, and GPAT in islets isolated from rats made hyperleptinemic by AdCMV–leptin infusion and pairfed controls and AdCMV–β-gal and saline-infused free-feeding controls.

Figure 3

Representative Southern blot of RT-PCR products for UCP-2 in islets isolated from leptin-overexpressing rats, pairfed controls, and AdCMV–β-gal- and saline-infused free-feeding controls. In vitro effects of 20 ng/ml leptin on UCP-2 mRNA in cultured islets are also shown.

Figure 4

Representative Southern blot of the UCP-2 RT-PCR products in white adipose tissue from subcutaneous, retroperitoneal, and epididymal fat depots of hyperleptinemic rats and pairfed controls and free-feeding AdCMV–β-gal- and saline-infused controls.