Thermogenesis in brown and beige adipocytes protects animals from hypothermia and obesity. The signaling cascades that promote adipose tissue thermogenesis have been extensively described, yet little is known about the inhibition of these pathways. Here, we show that sortilin acts as an inhibitor of adipose thermogenesis. The expression of sortilin is decreased upon cold exposure and increased after withdrawal of the cold stimulus. Sortilin directly targets ACSL1, the rate-limiting enzyme in mitochondrial FAO, leading to ACSL1 degradation by the lysosome. Thermogenic adipocytes have particularly large numbers of mitochondria; by restricting mitochondrial oxidative phosphorylation, sortilin prevents unexpected mitochondrial damage and cell death46. While non-shivering thermogenesis can be beneficial in pathological states such as obesity, it also can be a waste of caloric energy in normal healthy individuals. Sortilin potentially serves to fine tune energy homeostasis, which may confer a survival advantage to animals when food is scarce. Interestingly, in the case of abundant food supply and excessive energy intake, removing this restriction will increase heat production, thus greatly helping to cure diseases caused by excess energy, i.e., obesity and type 2 diabetes.
The severity of the obesity epidemic warrants further aggressive intervention. Currently, six major FDA-approved drugs have been used as anti-obesity medications. These drugs can be classified into two types: anorectics to suppress appetite and pancreatic lipase inhibitors to reduce intestinal fat absorption. Unfortunately, most of them have undesirable adverse effects 47. Previous studies have implemented cold stimuli or adrenergic signaling activation as a useful way to activate brown and beige fat for obesity treatment 27,48. However, these treatments have limited applications because of various health concerns and potential cardiovascular hazards 21,49,50. Thus, given the obesity epidemic, promising approaches to achieve safe and effective brown and beige fat activation are urgently needed. In the present study, we demonstrated that sortilin plays a critical role in HFD-induced obesity and systemic insulin resistance. We demonstrated that genetic depletion of sortilin or administration of the sortilin inhibitor AF38469 to dietary mouse models of obesity reduced weight and enhanced insulin resistance. Of clinical significance, we revealed the association of decreased Sort1 abundance in visceral adipose tissues with lower human BMI and improved metabolic traits. This study offers evidence of the positive effect of AF38469 in treating obesity and metabolic diseases and suggests that it would be worthwhile to utilize AF38469 as a therapeutic target for the treatment of obesity. Consistently, we found no severe adverse effects in mice orally treated with AF38469 upon long-term observation. Further studies are needed to optimize the molecular mechanism and evaluate the safety of AF38469.
Lipolysis is essential for adipose tissue thermogenesis, as the fatty acids released from NA-induced lipolysis serve as metabolic substrates fueling thermogenic respiration. The activation of fatty acids is regulated by a set of enzymes called acyl-CoA synthetase long-chain isoforms (ACSLs). As previously reported, the expression levels of ACSL family members in thermogenic tissues determined the substrate availability for β-oxidation and consequently the thermogenic capacity; ACSLs have been reported to be abundant in adipose, liver, and heart tissues 51. ACSL1 contributes to 80% of the total ACSL activity in adipose tissue. When localized to the outer mitochondrial membrane, ACSL1 generates long-chain acyl-CoA for β-oxidation in mitochondria, and fatty acid oxidation is reduced in adipocyte-specific Acsl1 KO mice 2,52,53. In the present study, sortilin was identified as a new regulator of ACSL1. Sort1-depleted adipocytes had greater ACSL1 expression than control beige adipocytes. Most of the ACSL1 was distributed on mitochondria, and due to the increased expression, the ACSL1 could channel more acyl-CoA to mitochondria for oxidation. Previous studies showed sortilin is involved in trans-Golgi trafficking to late endosomes 35,54. In this study, sortilin degrades ACSL1 exclusively in a lysosomal-dependent manner, not by autophagy and ubiquitination. This was consistent with previous studies and laterally highlighted the pivotal role of sortilin as an endosomal component target for metabolic homeostasis and diseases31.
Glucose transporter 4 (GLUT4) is expressed in adipocytes and muscle cells, where it regulates the uptake of glucose in an insulin-responsive manner. Previous studies showed that sortilin mediated the intracellular transport of GLUT4 in 3T3-L1 adipocytes 55,56. In this study, GLUT4 was also one of the extracted proteins that was immunoprecipitated by sortilin in beige adipocytes. Glucose uptake in thermogenic fat was stimulated in two metabolic states: sympathetically stimulated during active thermogenesis or stimulated by insulin during active anabolic processes. Insulin-stimulated glucose uptake in thermogenic fat is well characterized to occur via the PI3K/Akt pathway, resulting in the rapid translocation of GLUT4 from intracellular vesicles to the cell membrane31. In contrast, cold or β3-adrenoceptor activation increases glucose uptake through rapid de novo synthesis of GLUT1 by cAMP and subsequent translocation of GLUT1 to the plasma membrane57. In addition, the GLUT4 inhibitor indinavir does not inhibit isoproterenol-stimulated glucose uptake while inhibiting insulin-mediated glucose uptake57,58. This finding highlights a contrast between the mechanisms of insulin and β3-adrenoceptor-mediated glucose uptake in thermogenic fat 59,60. Consistently, we found depletion of sortilin increased glucose uptake in beige adipocytes and adipocyte-specific Sort1 depleted mice had improved systemic glucose clearance.
Thus, future research elucidating the role of endosomal trafficking in metabolic regulation are warranted; in particular, the dynamic processes regulating activity of the endosomal pathway control the distribution of more than 5,000 integral membrane proteins - many of which are metabolic transporters, signaling receptors and ligands61. It has been shown using organelle proteomics that the livers of mice fed a HFD undergo a redistribution of their subcellular structures, strongly suggesting changes intracellular transport in disease states such as obesity62. Furthermore, RNA sequencing datasets from rats fed a HFD revealed that genes encoding components of the endosomal membrane were differentially enriched63. By targeting the endosomal sorting receptor, sortilin, our work highlights this receptor as a critical negative regulator of mitochondrial β-oxidation, and offers preliminary evidence that modulating endosomal pathways may be a viable approach to treating obesity-related metabolic disorders.