Principal Investigator Harvey Lodish
Huangming Xie examined the role of miRNAs in adipogenesis using several adipocyte cell culture differentiation systems. He profiled miRNA expression during in vitro adipogenesis of the preadipocyte 3T3-L1 cells using miRNA microarrays and validated by RT-PCR eight miRNAs that are significantly upregulated and four that are downregulated. Similar changes in miRNA expression were observed by comparison of mature primary adipocytes and enriched primary preadipocytes. He also profiled miRNA expression in purified mature adipocytes and compared miRNA profiles in epididymal adipocytes from normal and leptin deficient or diet-induced obese mice. Importantly, miRNAs that were induced during adipogenesis were downregulated in adipocytes from both types of obese mice and vice versa. These changes are likely associated with the chronic inflammatory environment in obese adipose tissue as they were mimicked by TNFa treatment of differentiated adipocytes. Ectopic expression of two adipocyte-enriched miRNAs in preadipocytes accelerated adipogenesis, as measured both by the upregulation of many adipocyte-important genes including adiponectin and the key transcription factor PPARg, and by an increase in triglyceride accumulation at an early stage of adipogenesis.
Lei Sun, Huangming and Ryan Alexander are investigating the role of miRNAs in brown fat adipogenesis. Mammals have two principal types of fat: white adipose tissue (WAT) primarily serves to store extra energy as triglycerides, while brown adipose tissue (BAT) is specialized to burn lipids for heat generation and energy expenditure as a defense against cold and obesity. Recent studies demonstrate that brown adipocytes arise in vivo from a Myf5-positive, myoblastic progenitor by the action of the Prdm16 (PR domain containing 16) transcription factor. Lei and colleagues identified a brown fat-enriched miRNA cluster, miR-193b-365, as a key regulator of brown fat development. Blocking miR-193b and/or miR-365 in primary brown preadipocytes dramatically impaired brown adipocyte adipogenesis by enhancing Runx1t1 (runt-related transcription factor 1; translocated to 1) expression whereas myogenic markers were significantly induced. Forced expression of miR-193b and/or miR-365 in C2C12 myoblasts blocked the entire program of myogenesis, and, in adipogenic conditions, miR-193b induced myoblasts to differentiate into brown adipocytes. MiR-193b-365 was upregulated by Prdm16 partially through Pparα. Taken together, these results underlie the importance of tissue enriched miRNAs in regulating lineage specification between brown fat and muscle, and also suggest that certain miRNAs may have therapeutic potential in inducing expression of brown fat-specific genes.