Liver X receptors (LXR) are deemed as potential drug targets for atherosclerosis, whereas a role in adipose tissue expansion and its relation to insulin sensitivity remains unclear. To assess the metabolic effects of LXR activation by the dual LXRα/β agonist T0901317, C57BL/6 mice fed a high-fat diet (HFD) were treated with T0901317 (30 mg/kg once daily by intraperitoneal injection) for 3 weeks. Differentiated 3T3-L1 adipocytes were used for analysing the effect of T0901317 on glucose uptake. The following results were obtained from this study. T0901317 reduced fat mass, accompanied by a massive fatty liver and lower serum adipokine levels in HFD mice. Increased adipocyte apoptosis was found in epididymal fat of T0901317-treated HFD mice. In addition, T0901317 treatment promoted basal lipolysis, but blunted the anti-lipolytic action of insulin. Furthermore, LXR activation antagonised PPARγ target genes in epididymal fat and PPARγ-PPRE-binding activity in 3T3-L1 adipocytes. Although the glucose tolerance was comparable to that in HFD mice, the insulin response during IPGTT was significantly higher and the insulin tolerance was significantly impaired in T0901317-treated HFD mice, indicating decreased insulin sensitivity by T0901317 administration, and which was further supported by impaired insulin signalling found in epididymal fat and decreased insulin-induced glucose uptake in 3T3-L1 adipocytes by T0901317 administration. In conclusion, these findings reveal that LXR activation impairs adipose expansion by increasing adipocyte apoptosis, lipolysis and antagonising PPARγ-mediated transcriptional activity, which contributes to decreased insulin sensitivity in whole body. The potential of LXR activation being a therapeutic target for atherosclerosis might be limited by the possibility of exacerbating insulin resistance.
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Yueting Dong, Zhiye Xu, Ziyi Zhang, Xueyao Yin, Xihua Lin, Hong Li, and Fenping Zheng
Beibei Wu, Ruojun Qiu, Shuo Wang, Yingzi He, Jing Wang, Zhiye Xu, Xihua Lin, Hong Li, and Fenping Zheng
Liver transthyretin (TTR) synthesis and release are exacerbated in insulin-resistant states but are decreased by exercise training, in relation to the insulin-sensitizing effects of exercise. We hypothesized that TTR knockdown (TTR-KD) may mimic this exercise-induced metabolic improvement and skeletal muscle remodeling. Adeno-associated virus-mediated TTR-KD and control mice were trained for 8 weeks on treadmills. Their metabolism status and exercise capacity were investigated and then compared with sedentary controls. After treadmill training, the mice showed improved glucose and insulin tolerance, hepatic steatosis, and exercise endurance. Sedentary TTR-KD mice displayed metabolic improvements comparable to the improvements in trained mice. Both exercise training and TTR-KD promoted the oxidative myofiber compositions of MyHC I and MyHC IIa in the quadriceps and gastrocnemius skeletal muscles. Furthermore, training and TTR-KD had an additive effect on running performance, accompanied by substantial increases in oxidative myofiber composition, Ca2+-dependent Ca2+/calmodulin-dependent protein kinase II (CaMKII) activity, and the downstream expression of PGC1α as well as the unfolded protein response (UPR) segment of PERK-p-eIF2a pathway activity. Consistent with these findings, electrical pulse stimulation of an in vitro model of chronic exercise (with differentiated C2C12 myoblasts) showed that exogenous TTR protein was internalized and localized in the endoplasmic reticulum, where it disrupted Ca2+ dynamics; this led to decreases in intracellular Ca2+ concentration and downstream pathway activity. TTR-KD may function as an exercise/Ca2+-dependent CaMKII-PGC1α-UPR regulator that upregulates the oxidative myofiber composition of fast-type muscles; it appears to mimic the effect of exercise training on insulin sensitivity-related metabolic improvement and endurance capacity.