The rate of obesity among adults and children is rising, which underlines the need for effective anti-obesity strategies. Multiple promising pharmacotherapies have recently been developed and approved for human use. These focus primarily on reducing energy intake by interfering with satiety and nutrient uptake. Strategies to simultaneously decrease energy intake and increase energy expenditure are an interesting focus for future research. 

Since the (re)discovery of brown adipose tissue (BAT) in adult humans, activation of its unique uncoupling protein 1 (UCP1) has been suggested to enhance energy dissipation to increase energy expenditure. However, the UCP1 knockout (KO) mouse model paradoxically remains lean on a high-fat diet (HFD) in mild cold. This suggests the presence of alternative thermogenic mechanisms, which are of high translational value, as obese individuals typically have little to no BAT and UCP1. It was previously suggested that fibroblast growth factor 21 (FGF21) plays a crucial role in the lean phenotype of UCP1 KO mice by remodeling and transforming the inguinal white adipose tissue (iWAT) towards a more beige phenotype. While BAT is studied intensively, the influence of beige fat on obesity remains unclear, particularly in the context of UCP1. Moreover, the underlying molecular mechanisms by which FGF21 regulates body weight are not yet fully understood.

In Chapter I, we investigate the ability of the over-the-counter drug Ibuprofen to activate UCP1 in UCP1-expressing human embryonic kidney 293 cells and isolated brown fat mitochondria. We show that Ibuprofen is a viable activator, but different variants are needed for improved delivery to activate brown adipocytes in vivo. In Chapter II, we show that overexpression of one key enzyme of the futile lipid cycle, glycerol kinase, did not induce major changes in adipocyte lipid metabolism. However, beta-adrenergic stimulation of these adipocytes may induce increased lipid cycling. Chapter III shows that FGF21 treatment results in immediate weight loss in UCP1 KO mice on HFD, whereas wildtype (WT) mice have a delayed response. We find that the iWAT has increased levels of FGF21-responsive genes only in UCP1 KO mice, which suggests that FGF21 sensitivity in iWAT underlies the body weight loss. In Chapter IV, we show that male UCP1 KO mice are susceptible to diet-induced obesity resistance, whereas female UCP1 KO mice have a similar weight to female WT mice, with a higher food intake. Interestingly, the levels of FGF21 and iWAT browning are similar between sexes. Overall, our results suggest that hyperphagia may counterbalance the beneficial effects of FGF21 on metabolism in female mice. 

Collectively, our results highlight new ways to activate the metabolic capacity of beige adipocytes within iWAT. Furthermore, we demonstrate the importance of iWAT in sustained FGF21 sensitivity and show sex-specific differences in response to high endogenous FGF21 levels.

Increasing energy expenditure through uncoupling protein 1 (UCP1) activity in thermogenic adipose tissue is widely investigated to correct diet-induced obesity (DIO). Paradoxically, UCP1-deficient male mice are resistant to DIO at room temperature. Recently, we uncovered a key role for fibroblast growth factor 21 (FGF21), a promising drug target for treatment of metabolic disease, in this phenomenon. As the metabolic action of FGF21 is so far understudied in females, we aim to investigate potential sexual dimorphisms. Here, we confirm that male UCP1 KO mice display resistance to DIO in mild cold, without significant changes in metabolic parameters. Surprisingly, females gained the same amount of body fat as WT controls. Molecular regulation was similar between UCP1 KO males and females, with an upregulation of serum FGF21, coinciding with beiging of inguinal white adipose tissue and induced lipid metabolism. While energy expenditure did not display significant differences, UCP1 KO females significantly increased their food intake. Altogether, our results indicate that hyperphagia is likely counteracting the beneficial effects of FGF21 in female mice. This underlines the importance of sex-specific studies in (pre)clinical research for personalized drug development.

Objective: Uncoupling protein 1 (UCP1) catalyses mitochondrial proton leak in brown adipose tissue to facilitate nutrient oxidation for heat production, and may combat metabolic disease if activated in humans. During the adrenergic stimulation of brown adipocytes, free fatty acids generated from lipolysis activate UCP1 via an unclear interaction. Here, we set out to characterise activator binding to purified UCP1 to clarify the activation process, discern novel activators and the potential to target UCP1.

Methods: We assessed ligand binding to purified UCP1 by protein thermostability shift analysis, which unlike many conventional approaches can inform on the binding of hydrophobic ligands to membrane proteins. A detailed activator interaction analysis and screening approach was carried out, supported by investigations of UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1 expression-controlled cell lines.

Results: We reveal that fatty acids and other activators influence UCP1 through a specific destabilising interaction, behaving as transport substrates that shift the protein to a less stable conformation of a transport cycle. Through the detection of specific stability shifts in screens, we identify novel activators, including the over-the-counter drug ibuprofen, where ligand analysis indicates that UCP1 has a relatively wide structural specificity for interacting molecules. Ibuprofen successfully induced UCP1 activity in liposomes, isolated brown fat mitochondria and UCP1-expressing HEK293 cells but not in cultured brown adipocytes, suggesting drug delivery differs in each cell type.

Conclusions: These findings clarify the nature of the activator-UCP1 interaction and demonstrate that the targeting of UCP1 in cells by approved drugs is in principle achievable as a therapeutic avenue, but requires variants with more effective delivery in brown adipocytes.

Metabolic diseases represent the major health burden of our modern society. With the need of novel therapeutic approaches, fibroblast growth factor 21 (FGF21) is a promising target, based on metabolic improvements upon FGF21 administration in mice and humans. Endogenous FGF21 serum levels, however, are increased during obesity-related diseases, suggesting the development of FGF21 resistance during obesity and thereby lowering FGF21 efficacy. In uncoupling protein 1 knockout (UCP1 KO) mice, however, elevated endogenous FGF21 levels mediate resistance against diet-induced obesity. Here, we show that after long-term high fat diet feeding (HFD), circulating FGF21 levels become similarly high in obese wildtype and obesity-resistant UCP1 KO mice, suggesting improved FGF21 sensitivity in UCP1 KO mice. To test this hypothesis, we injected FGF21 after long-term HFD and assessed the metabolic and molecular effects. The UCP1 KO mice lost weight directly upon FGF21 administration, whereas body weights of WT mice resisted weight loss in the initial phase of the treatment. The FGF21 treatment induced expression of liver Pck1, a typical FGF21-responsive gene, in both genotypes. In iWAT, FGF21-responsive genes were selectively induced in UCP1 KO mice, strongly associating FGF21-sensitivity in iWAT with healthy body weights. Thus, these data support the concept that FGF21-sensitivity in adipose tissue is key for metabolic improvements during obesogenic diets.

Fibroblast growth factor 21 (FGF21) is in biomedical focus as a treatment option for metabolic diseases, given that administration improves metabolism in mice and humans. The metabolic effects of exogenous FGF21 administration are well-characterized, but the physiological role of endogenous FGF21 has not been fully understood yet. Despite cold-induced FGF21 expression and increased circulating levels in some studies, which co-occur with brown fat thermogenesis, recent studies in cold-acclimated mice demonstrate the dispensability of FGF21 for maintenance of body temperature, thereby questioning FGF21's role for thermogenesis. Here we discuss the evidence either supporting or opposing the role of endogenous FGF21 for thermogenesis based on the current literature. FGF21, secreted by brown fat or liver, is likely not required for energy homeostasis in the cold, but the nutritional conditions could modulate the interaction between FGF21, energy metabolism, and thermoregulation.