Blueberry Peel Extracts Inhibit Adipogenesis in 3T3-L1 Cells and Reduce High-Fat Diet-Induced Obesity

This study examined the anti-obesity effect and mechanism of action of blueberry peel extracts (BPE) in 3T3-L1 cells and high-fat diet (HFD)-induced obese rats. The levels of lipid accumulation were measured, along with the changes in the expression of genes and proteins associated with adipocyte differentiation in 3T3-L1 cells. Evidenced by Oil-red O staining and triglyceride assay, BPE dose-dependently inhibited lipid accumulation at concentrations of 0, 50, and 200 µg/ml. BPE decreased the expression of the key adipocyte differentiation regulator C/EBPβ, as well as the C/EBPα and PPARγ genes, during the differentiation of preadipocytes into adipocytes. Moreover, BPE down-regulated adipocyte-specific genes such as aP2 and FAS compared with control adipocytes. The specific mechanism mediating the effects of BP revealed that insulin-stimulated phosphorylation of Akt was strongly decreased, and its downstream substrate, phospho-GSK3β, was downregulated by BPE treatment in 3T3-L1 cells. Together, these data indicated that BP exerted anti-adipogenic activity by inhibiting the expression of PPARγ and C/EBPβ and the Akt signaling pathway in 3T3-L1 adipocytes. Next, we investigated whether BP extracts attenuated HFD-induced obesity in rats. Oral administration of BPE reduced HFD-induced body weight gain significantly without affecting food intake. The epididymal or perirenal adipose tissue weights were lower in rats on an HFD plus BPE compared with the tissue weights of HFD-induced obese rats. Total cholesterol and triglyceride levels in the rats fed BPE were modestly reduced, and the HDL-cholesterol level was significantly increased in HFD plus BP-fed rats compared with those of HFD-fed rats. Taken together, these results demonstrated an inhibitory effect of BP on adipogenesis through the down-regulation of C/EBPβ, C/EBPα, and PPARγ and the reduction of the phospho-Akt adipogenic factor in 3T3-L1 cells. Moreover, BPE reduced body weight gain and inhibited fat accumulation in an HFD-induced animal model of obesity.     

Protein Inhibitor of Activated STAT 1 (PIAS1) Is Identified as the SUMO E3 Ligase of CCAAT/Enhancer-Binding Protein β (C/EBPβ) during Adipogenesis

It is well recognized that PIAS1, a SUMO (small ubiquitin-like modifier) E3 ligase, modulates such cellular processes as cell proliferation, DNA damage responses, and inflammation responses. Recent studies have shown that PIAS1 also plays a part in cell differentiation. However, the role of PIAS1 in adipocyte differentiation remains unknown. CCAAT/enhancer-binding protein β (C/EBPβ), a major regulator of adipogenesis, is a target of SUMOylation, but the E3 ligase responsible for the SUMOylation of C/EBPβ has not been identified. The present study showed that PIAS1 functions as a SUMO E3 ligase of C/EBPβ to regulate adipogenesis. PIAS1 expression was significantly and transiently induced on day 4 of 3T3-L1 adipocyte differentiation, when C/EBPβ began to decline. PIAS1 was found to interact with C/EBPβ through the SAP (scaffold attachment factor A/B/acinus/PIAS) domain and SUMOylate it, leading to increased ubiquitination and degradation of C/EBPβ. C/EBPβ became more stable when PIAS1 was silenced by RNA interference (RNAi). Moreover, adipogenesis was inhibited by overexpression of wild-type PIAS1 and promoted by knockdown of PIAS1. The mutational study indicated that the catalytic activity of SUMO E3 ligase was required for PIAS1 to restrain adipogenesis. Importantly, the inhibitory effect of PIAS1 overexpression on adipogenesis was rescued by overexpressed C/EBPβ. Thus, PIAS1 could play a dynamic role in adipogenesis by promoting the SUMOylation of C/EBPβ.     

Transactivation of Atg4b by C/EBPβ Promotes Autophagy To Facilitate Adipogenesis

Autophagy is a highly conserved self-digestion pathway involved in various physiological and pathophysiological processes. Recent studies have implicated a pivotal role of autophagy in adipocyte differentiation, but the molecular mechanism for its role and how it is regulated during this process are not clear. Here, we show that CCAAT /enhancer-binding protein β (C/EBPβ), an important adipogenic factor, is required for the activation of autophagy during 3T3-L1 adipocyte differentiation. An autophagy-related gene, Atg4b, is identified as a de novo target gene of C/EBPβ and is shown to play an important role in 3T3-L1 adipocyte differentiation. Furthermore, autophagy is required for the degradation of Klf2 and Klf3, two negative regulators of adipocyte differentiation, which is mediated by the adaptor protein p62/SQSTM1. Importantly, the regulation of autophagy by C/EBPβ and the role of autophagy in Klf2/3 degradation and in adipogenesis are further confirmed in mouse models. Our data describe a novel function of C/EBPβ in regulating autophagy and reveal the mechanism of autophagy during adipocyte differentiation. These new insights into the molecular mechanism of adipose tissue development provide a functional pathway with therapeutic potential against obesity and its related metabolic disorders.