ISL1 regulates PPARg activation and early adipogenesis via BMP4-dependent and -independent mechanisms

While adipogenesis is controlled by a cascade of transcription factors, the global gene expression profiles in the early phase of adipogenesis are not well defined. Using microarray analysis of gene expression in 3T3-L1 cells we have identified evidence for the activity of 2568 genes during the early phase of adipocyte differentiation. One of these, ISL1, was of interest since its expression was markedly upregulated at 1 h after initiation of differentiation with a subsequent rapid decline. Overexpression of ISL1 at early times during adipocyte differentiation, but not at later times, was found to profoundly inhibit differentiation. This was accompanied by moderate down-regulation of PPARg levels, substantial down-regulation of PPARg downstream genes and down-regulation of BMP4 levels in preadipocytes. Readdition of BMP4 overcame the inhibitory effect of ISL1 on PPARg but not aP2 expression, a downstream gene of PPARg; and BMP4 also partially rescued ISL1 inhibition of adipogenesis, an effect which is additive with rosiglitazone. These results suggest that ISL1 is intimately involved in early regulation of adipogenesis, modulating PPARg expression and activity via BMP4-dependent and -independent mechanisms. Our time course gene expression survey sets the stage for further studies to explore other early and immediate regulators. To explore global gene expression profiles during early adipocyte differentiation, total cellular RNA was isolated from 3T3-L1 cells at 0h, 0.5h, 1h, 2h, 4h, 8h and 48h following treatment with differentiation cocktail. At the same time, to check ISL1-regulated genes in 3T3-L1 cells, total cellular RNA was isolated from 3T3-L1 cells overexpressing Flag-ISL1 at 0h and 48h. Individual RNA from biological triplicates was used for microarray analysis.