Differential responses to UCP1 ablation in classical brown versus beige fat, despite a parallel increase in sympathetic innervation

In the cold, the absence of the mitochondrial uncoupling protein 1 (UCP1) results in hyper-recruitment of beige fat, but classical brown fat becomes atrophied. Here we examine possible mechanisms underlying this phenomenon. We confirm that in brown fat from UCP1-knockout (UCP1-KO) mice acclimated to the cold, the levels of mitochondrial respiratory chain proteins were diminished; however, in beige fat, the mitochondria seemed to be unaffected. The macrophages that accumulated massively not only in brown fat but also in beige fat of the UCP1-KO mice acclimated to cold did not express tyrosine hydroxylase, the norepinephrine transporter (NET) and monoamine oxidase-A (MAO-A). Consequently, they could not influence the tissues through the synthesis or degradation of norepinephrine. Unexpectedly, in the cold, both brown and beige adipocytes from UCP1-KO mice acquired an ability to express MAO-A. Adipose tissue norepinephrine was exclusively of sympathetic origin, and sympathetic innervation significantly increased in both tissues of UCP1-KO mice. Importantly, the magnitude of sympathetic innervation and the expression levels of genes induced by adrenergic stimulation were much higher in brown fat. Therefore, we conclude that no qualitative differences in innervation or macrophage character could explain the contrasting reactions of brown versus beige adipose tissues to UCP1-ablation. Instead, these contrasting responses may be explained by quantitative differences in sympathetic innervation: the beige adipose depot from the UCP1-KO mice responded to cold acclimation in a canonical manner and displayed enhanced recruitment, while the atrophy of brown fat lacking UCP1 may be seen as a consequence of supraphysiological adrenergic stimulation in this tissue.

Triptolide enhances lipolysis of adipocytes by enhancing ATGL transcription via upregulation of p53

Lipolysis is an essential physiological activity of adipocytes. The Patatin Like Phospholipase Domain Containing 2 (PNPLA2) gene encodes the enzyme adipose triglyceride lipase (ATGL) responsible for triglyceride hydrolysis, the first step in lipolysis. In this study, we investigated the potential of triptolide (TP), a natural plant extract, to induce weight loss by examining its effect on ATGL expression. We found that long- and short-term TP administration reduced body weight and fat weight and increased heat production in brown adipose tissue in wild-type C57BL/6 mice. In 3T3-L1 fibroblasts and porcine adipocytes, TP treatment reduced the number of lipid droplets as determined by Oil Red O and BODIPY staining, with concomitant increases in free fatty acid and triglyceride levels in the culture medium. Combined treatment with TP and p53 inhibitor reversed these lipolytic effects. We next amplified the ATGL promoter region and identified conserved p53 binding sites in the sequence by in silico analysis. The results of the dual-luciferase reporter assay using a construct containing the ATGL promoter harboring the p53 binding site showed that p53 induces ATGL promoter activity and consequently, ATGL transcription. These results demonstrate that TP has therapeutic value as an anti-obesity agent and acts by promoting lipolysis via upregulation of p53 and ATGL transcription.