The remarkable increase in the number of cases involving obesity and related disorders, such as diabetes and heart diseases driven by ready access to high-calorie food and an increasingly sedentary way of life, left an urgent need to try discovering pharmacotherapies targeting adipose tissue. How could that be effectively done? The main idea came from the existence and the potential conversion of fat-storing cells into metabolically active thermogenic cells.
Mammals possess two different kinds of adipose tissue:
- White adipose tissue (WAT), which stores excess energy and also has a role in regulating satiety through leptin secretion.
- Brown adipose tissue (BAT), which releases energy in the form of heat by uncoupling the respiratory chain (UCP1), so that body temperature may be maintained through non-shivering thermogenesis. Its activation has been shown to ameliorate insulin resistance and to protect against obesity.
A basic, but often misunderstood, concept is that weight gain is caused by a fundamental energy imbalance, when energy intake from food chronically exceeds energy expended by physical activity and metabolic processes. Humans have evolved efficient biological mechanisms to acquire and defend their energy stores. A therapy for weight loss must, therefore, involve a decrease in food intake and/or an increase in energy expenditure, in this case, provided by converting fat-storing cells into metabolically active thermogenic cells.
RATIONALE, METHODS AND RESULTS
Browning is the phenomenon referred as the white-to-brown metabolic conversion in human adipocytes, in which white adipocytes are conferred brown-like metabolic activity: elevated UCP1 expression and increased mitochondrial activity, therefore being a huge therapeutic target to treat metabolic disorders.
A screening platform for adipocyte browning was set to identify the induction of UCP1 activation, using a human pluripotent stem cell (PSC)-derived adipocyte model. As a browning index, UCP1 expression was monitored by UCP1 mRNA capture plates followed by branched DNA (bDNA) amplification, and expression of fatty acid binding protein 4 (FABP4), an adipocyte-specific gene served as an internal control to eliminate anti- and pro-adipogenic compounds not specific to UCP1.
As a result, two inhibitors of JAK3 and SYK showed highest UCP1/FABP4 ratio and lipid droplet morphology changes, typical of brown-like adipocytes:
- Tofacitinib – JAK3 inhibitor
- R406 – SYK inhibitor
Both were also shown to block tyrosine phosphorylation of the JAK-STAT1/3 pathway during adipocyte browning, mediators of pro-inflammatory pathways. Noticeably, STAT1 protein level decreased after treatment with tofacitinib and R406. In addition, it was observed that they also had the ability to antagonize Tumor necrosis factor alpha (TNF-alpha), previously observed to repress UCP1.
Cytokine Signaling by the JAK/STAT Pathway
Progressive and stable conversion of adipocytes by JAK inhibition
If a stable white to brown-like conversion is achieved through JAK inhibition, the brown-like phenotype should persist on removal of tofacitinib and R406 and be stable. Results indicate that JAK inhibition leads to stable acquisition of brown-like metabolic properties (high mitochondrial content as well as high metabolic activity) through functionally remodelling in human adipocytes, since all cells pre-treated with JAK inhibitors exhibited high levels of UCP1 mRNA and reduced lipid droplet size.
Down-regulation of IFN and activation of hedgehog signaling contribute to metabolic browning downstream of JAK inhibition.
In addition to the use of Tofacitinib and R406 to inhibit the JAK–STAT pathway in human adipocytes – leading to downregulation of the interferon alpha, beta and gamma responses -, the persistent repression of IFN signaling relieves inhibition of the SHH pathway, contributes to the up regulation of UCP1 and promotes the metabolic browning of adipocytes, therefore being an extra browning indutor.
Meanwhile, by using Hedgehog activator Smoothened Agonist (SAG), previous studies show that activation of hedgehog blocks white, but not brown, adipocyte differentiation, promoting the acquisition of a brown-like metabolic phenotype.
In conclusion, the central question addressed is whether BAT function significantly impacts energy balance and human obesity, and how that can be reached through developmental biology and targeted molecular pathways. Although the idea of stimulating BAT activity to combat obesity is a rational approach, it is also conceivable that this would trigger counterregulatory mechanisms such as increased appetite to maintain energy homeostasis and preserve fuel reserves. (1)
The new human data, generated from research projects such the one presented above, have invigorated interest and excitement in the function and physiological relevance of BAT. Hopefully, these findings can be, in the time to come, translated into:
1) a better understanding of the mechanisms that work together to regulate body weight and;
2) a novel therapeutic interventions to reduce the burden of obesity in our society.
BROWN FAT IS GOOD!
Moisan et al, “White-to-brown metabolic conversion of human adipocytes by JAK inhibition” Nature Cell biology 17, 57–67 (2015) DOI: 10.1038/ncb3075
Figure 1: http://www.vivo.colostate.edu/hbooks/pathphys/misc_topics/brownfat.html
Figure 2: http://www.scielo.br/img/revistas/abem/v56n4/01f01.jpg
Figure 3: http://diabetes.diabetesjournals.org/content/58/7/1482/F1.expansion.html
Figure 4-10: http://www.nature.com/ncb/journal/v17/n1/full/ncb3075.html
Video 1: https://www.youtube.com/watch?v=G4K6IQZGHJc
Video 2: https://www.youtube.com/watch?v=JYZkJkHRsRY
Video 3: https://www.youtube.com/watch?v=KMbhBL3rhz0