Molecular Characterization Of White And Brown Adipocytes Reveals Complex Phenotypes

Over the last two decades the prevalence of obesity has reached epidemic levels worldwide. Secondary major health risks, such as hypertension, insulin resistance, type 2 diabetes and cardiovascular diseases, can be summarized as the metabolic syndrome and are highly associated with obesity. In cases in which energy intake e.g. through high caloric food intake exceeds energy expenditure the overall energy homeostasis is imbalanced. White adipose tissue depots mainly act as the body's main energy storage and additionally act as an endocrine organ by releasing adipokines and cytokines into the body. Additionally, functional brown adipose tissue (BAT) has been discovered through radiological detection in the last years in substantial amount in adults. Formerly overlooked BAT, which was thought to be absent in the human adult, has therefore recently become an interesting anti‐obesity target due to its ability to dissipate energy in form of heat. We extensively characterized human brown PAZ6 adipocytes in comparison with a white adipose cell line SW872. In addition, human SGBS adipocytes were included in the analysis. Brown and white adipocyte markers were tested by quantitative Real-time PCR. Fluorescent and Oil‐Red staining assessed the quantity and quality of the differentiation process. Next generation RNA sequencing of undifferentiated and mature SGBS and PAZ6 cells was performed in order to elucidate pathways distinctly activated in white vs. brown human adipocytes. Functional assessment of oxidative rates of each cell line was conducted using the Seahorse technology. Whereas PAZ6 and SW872 cells showed classical molecular and phenotypic markers of brown and white adipocytes, respectively, SGBS cells presented a versatile phenotype of adipocyte. 14 days after initiating the differentiation process the expression of classical brown marker such as UCP‐1 and PPARγ peaked and declined until day 28. The white adipocyte marker Tcf21 however, showed reciprocal behavior. Interestingly, Leptin levels peaked at day 28 whereas the highest adiponectin mRNA levels were monitored at day 14. Phenotypic analysis of the abundance and shape of lipid droplets were consistent with the molecular patterns. On day 14, SGBS cells showed multiple small droplets, however the number of droplets decreased and the size increased until day 28 as expected for a white adipocyte phenotype. Lastly, functional metabolic analysis showed the highest oxidative rate of mature SGBS cells on Day 14, which remained consistent or slightly decreased until day 28. SGBS cells are widely used as a model for white adipocytes. Our data suggest that the cell line harbors a versatile phenotype, which changes throughout their mature stage. Many reports suggest recently that the traditional classification of adipocytes is not exclusive and the existence of beige/brite adipocytes has been shown. We are presenting data derived from a human cell line model, which harbors characteristics of both distinct phenotypes. This knowledge will be of importance for studies aimed to increase brown fat depots in order to increase energy expenditure in obese subjects with the ultimate goal of weight reduction.