Chronic exposure of humans or rodents to high calorie diets leads to hypertriglyceridemia and ectopic lipid deposition throughout the body, resulting in metabolic disease. Cellular lipids are stored in organelles termed lipid droplets (LDs) that are regulated by tissue-specific LD proteins. These proteins are critical for lipid homeostasis, as humans with LD protein mutations manifest metabolic dysfunction. Identification of novel components of the LD machinery could shed light on human disease mechanisms and suggest potential therapeutics for Type 2 Diabetes. Microarray analyses pinpointed the largely unstudied Hypoxia-Inducible Gene 2 (Hig2) as a gene that was highly expressed in obese human adipocytes. Imaging studies demonstrated that Hig2 localized to LDs in mouse hepatocytes and the human SGBS adipocyte cell line. Thus, this work examined the role of Hig2 as a LD protein in liver and adipose tissue. Hig2 deficiency reduced triglyceride deposition in hepatocytes; conversely, ectopic Hig2 expression promoted lipid deposition. Furthermore, liver-specific Hig2-deficient mice displayed improved glucose tolerance and reduced liver triglyceride content. Hig2 deficiency increased lipolysis and -oxidation, accounting for the reduced triglyceride accumulation. Similarly, adipocyte-specific Hig2-deficient mice displayed improved glucose tolerance, reduced adipose tissue weight and brown adipose tissue that was largely cleared of lipids. These improvements were abrogated when the animals were placed in thermoneutral housing and brown adipocyte-specific Hig2-deficient mice also displayed improved glucose tolerance, suggesting that active brown fat largely mediates the metabolic phenotype of Hig2 deletion. Thus, this work demonstrates that Hig2 localizes to LDs in liver and adipose tissue and promotes glucose intolerance.

La thermogenèse adaptative est un mécanisme de production de chaleur médié par les adipocytes bruns. En réponse au froid, ou à un stimulus adrénergique, les adipocytes blancs peuvent être convertis en adipocytes beiges lors d’un processus que l’on nomme le « beiging ». Contrairement aux adipocytes blancs, les adipocytes beiges et bruns expriment des taux élevés de la protéine de découplage 1 (UCP1) et dissipent l'énergie sous forme de chaleur grâce à l'oxydation des lipides. Il a été démontré chez les rongeurs que l’activation des adipocytes bruns et beiges entraîne une réduction significative du poids corporel et l’activation de ces adipocytes chez l’humain semble être un traitement prometteur contre l’obésité et le diabète. Nous avons précédemment identifié un rôle essentiel de la protéine d’échafaudage 14-3-3ζ dans l'adipogenèse, mais son rôle dans d'autres processus adipocytaires reste incertain. Une des premières fonctions identifiées de la 14-3-3ζ est sa capacité à réguler l'activité enzymatique de la tyrosine hydroxylase, indispensable à la production de norépinephrine pour la thermogenèse. Notre étude vise donc à déterminer si la 14-3-3ζ influence le développement et la fonction des adipocytes beiges et bruns. Nos données montrent que la délétion d’un allèle du gène de la 14-3-3ζ n’affecte pas la tolérance au froid aiguë. Comparées aux souris de type sauvage (WT), les souris transgéniques mâles surexprimant la 14-3-3ζ (TAP) ont une meilleure tolérance au froid aiguë (3 heures, 4 °C) et chronique (3 jours, 4 °C). On observe chez les TAP une augmentation du beiging due à une élévation significative de l'ARNm et de la protéine UCP1 dans le tissu adipeux blanc inguinal (iWAT). Par ailleurs, les souris TAP présentent également une réduction significative de la conductance thermique lors d’exposition au froid leur permettant de mieux conserver la chaleur. Collectivement, nos résultats soulignent le rôle novateur de la 14-3-3ζ dans le beiging et nous permettent de mieux comprendre comment la thermogenèse adaptative est régulée. / Adaptive thermogenesis is a mechanism of heat production primarily mediated by brown fat. In some instances, cold exposure or adrenergic stimuli can convert white adipocytes into brown-like or beige adipocytes during a process termed “beiging”. Both beige and brown adipocytes express higher levels of uncoupling protein 1 (UCP1) and can release energy in the form of heat following lipid oxidation. The activation of these thermogenic adipocytes increases energy expenditure to reduce body weight in rodents, and it has been postulated to be a promising therapy for the treatment of obesity and diabetes. We previously identified an essential role of the molecular scaffold, 14-3-3ζ, in adipogenesis, but its roles in other adipocyte processes is uncertain. An early identified function of 14-3-3 was its ability to regulate the enzymatic activity of tyrosine hydroxylase, which is indispensable in the production of norepinephrine for thermogenesis. Thus, our study aims to investigate whether 14-3-3ζ influences the development and function of beige and brown adipocytes. We report here that one allele deletion of the gene of 14-3-3ζ did not affect acute cold tolerance. On the other hand, transgenic overexpression of 14-3-3ζ in male mice (TAP) improves cold tolerance due to enhanced beiging with a remarkable increase in Ucp1 mRNA and protein in inguinal white adipose tissue (iWAT). Interestingly, beiging is increased in the TAP mice without any changes in sensitivity to beta-adrenergic stimuli, sympathetic innervation, or norepinephrine content being detected between WT and TAP mice. TAP mice also displayed significantly lower thermal conductance decreasing heat loss during the chronic cold challenge. Collectively, our results point to a novel role of 14-3-3ζ in beiging and increases our understanding of how adaptive thermogenesis is regulated.

Recent studies suggest adipose tissue plays a critical role in regulating whole body energy homeostasis in both animals and humans. In particular, activating brown adipose tissue (BAT) activity is now appreciated as a potential therapeutic strategy against obesity and metabolic disease. However, the signaling circuits that coordinate nutrient uptake and BAT function are poorly understood. Here, I investigated the role of the nutrient-sensing mTOR signaling pathway in BAT by conditionally deleting Rictor, which encodes an essential component of mTOR Complex 2 (mTORC2) either in brown adipocyte precursors or mature brown adipocytes. In general, inhibiting BAT mTORC2 reduces glucose uptake and de novo lipogenesis pathways while increases lipid uptake and oxidation pathways indicating a switch in fuel utilization. Moreover, several key thermogenic factors (Ucp1, Pgc1α, and Irf4) are elevated in Rictor-deficient BAT, resulting in enhanced thermogenesis. Accordingly, mice with mTORC2 loss in BAT are protected from HFD-induced obesity and metabolic disease at thermoneutrality. In vitro culture experiments further suggest that mTORC2 cell-autonomously regulates the BAT thermogenic program, especially Ucp1 expression, which depends on FoxO1 activity. Mechanistically, mTORC2 appears to inhibit FoxO1 by facilitating its lysine-acetylation but not through the canonical AKT-mediated phosphorylation pathway. Finally, I also provide evidence that β-adrenergic signaling which normally triggers thermogenesis also induces FoxO1 deacetylation in BAT. Based on these data, I propose a model in which mTORC2 functions in BAT as a critical signaling hub for coordinating nutrient uptake, fuel utilization, and thermogenic gene expression. These data provide a foundation for future studies into the mTORC2-FoxO1 signaling axis in different metabolic tissues and physiological conditions.