Being obese or overweight is a major risk factor for many human diseases, including type 2 diabetes, metabolic syndrome, heart disease, stroke, hypertension, and certain cancers. With roughly 2 of 3 adults in the U.S. falling in these two categories, obesity is now the most important public health issue in the U.S., with many other countries not far behind. Fundamentally, obesity is a disorder of energy balance that occurs when energy intake is consistently greater than energy expenditure. Excess calories are stored in white adipocytes, cells that are highly specialized for absorbing and releasing triglycerides in counterbalance to variations in systemic energy supply and demand. Mammals also possess brown and beige adipose, distinct subtypes of fat cells that function to combust nutrients and release heat. Brown and beige fat can therefore counteract obesity by burning off excess chemical energy.
Our lab has a broad interest in characterizing the key genetic pathways that control the development and function of the three types of adipose tissues. We employ a wide range of basic molecular biology techniques combined with genetic and metabolic analyses in mice. By understanding the normal process of white, brown, and beige adipose development, we hope to define novel therapeutic targets for obesity, insulin resistance, and metabolic diseases.
1) Genetic Control of Brown Adipocyte Development
Brown fat cells are packed with mitochondria that express Uncoupling Protein-1 (UCP1) in their inner membrane. Brown fat tissue is also highly vascular, enabling it to efficiently distribute heat via the circulation. The molecular pathways that control the process of adipogenic differentiation from preadipocytes have been extensively studied in animal models and cultured cells. Preadipocytes are specialized fibroblast-like cells contained in white and brown fat tissues that differentiate into mature fat storing adipocytes in response to hormonal cues. PPARg and members of the c/EBP family of transcription factors have been shown to orchestrate the terminal adipocyte differentiation process. However, these factors are expressed in both brown and white adipose cells and are thus presumed not to control white vs. brown adipose cell fate.
Several factors have been shown to influence the white versus brown adipose cell phenotype including: PGC-1a; FoxC2; pRb; p107 and RIP140. In a global expression screen of all known mouse transcriptional components, we identified PRDM16 as a gene expressed selectively in brown adipose cells. Functional and genetic analyses have shown that PRDM16 is necessary and sufficient for the appropriate differentiation of brown adipocytes. Specifically, expression of PRDM16 in white fat or skeletal muscle progenitors activates a near-complete program of brown adipogenesis including induction of most brown adipose-selective genes, suppression of white adipose-selective genes and increased mitochondrial biogenesis. Reduction of PRDM16 in brown adipocytes causes a complete loss of the brown adipose phenotype. Most strikingly, loss of PRDM16 from brown adipogenic cells promotes the induction of skeletal muscle genes and differentiation. These data are consistent with a function for PRDM16 in the control of brown adipocyte versus skeletal muscle cell fate.
More recently, we identified EBF2 (Early B-Cell Factor 2) as a potential upstream regulator of Prdm16. EBF2 is also a powerful activator of the brown adipogenic program
We are now investigating the mechanistic basis by which PRDM16 influences brown adipogenic cell fate using both cell based and mouse genetic approaches.
2) Identification of regulatory genes and pathways in white adipogenic precursors
A major unresolved and central issue in the field of obesity and metabolism research is the identity of the adipogenic precursor cell(s) and pathways that regulate the proliferation and self-renewal of these cells in vivo. Very recent reports by the Friedman and Graf labs have revealed new methods to isolate putative preadipocyte populations from fat tissue by fluorescence activated cell sorting (FACS) and by virtue of their physical association with blood vessels. These cell fractions likely contain functionally distinct subpopulations of progenitors with unknown molecular make-up. We are interested in developing approaches to characterize these precursors in normal development and in obesity. We seek to identify key regulatory components that control the activation, expansion and differentiation of these cells in response to developmental or metabolic cues.