Coordination of cell-cell contact, cell structure, and transcriptional regulation to promote adipogenesis

The profound transcriptional repression of cell structure related genes in confluent preadipocytes during early adipogenesis suggest that coordination of cell-cell contact, cell structure, and transcriptional regulation can provide insight into the mechanisms driving commitment of precursor cells to the adipocytic fate. We have undertaken two independent strategies for studying the integration of cytostructure and transcription. First, we report that the transcriptional repressor TCF7L1 is induced by cell contact in confluent mouse 3T3-L1 preadipocytes. TCF7L1 is only present in early adipogenesis, where it represses cytostructural related genes. Ectopic expression of TCF7L1 alleviates the confluency requirement for preadipocyte differentiation, and commits non-adipogenic NIH-3T3 cells, which do not normally express TCF7L1, to an adipogenic fate. TCF7L1 indirectly induces the adipogenic transcription factor PPARgamma in a myosin-dependent manner, thus linking transcription-dependent cytostructural regulation to adipocyte differentiation. These results establish TCF7L1 as a transcriptional hub coordinating cell-cell contact with cell structure changes required for adipogenic competency. Next, we sought to study the regulation and role of TIMP2, the most profoundly repressed gene during adipogenesis. Unlike previous reports describing it as a housekeeping gene, we found TIMP2 is transcriptionally repressed during differentiation. TIMP2 repression during adipogenesis relies upon induction of C/EBPbeta and C/EBPdelta during adipogenesis and requires cAMP and dexamethasone for sustained down-regulation. TIMP2 levels are elevated in adipose from mice on a high fat diet. Unexpectedly, despite early down-regulation, TIMP2 is required for adipogenesis. Preliminary work suggests that the defect in adipogenesis is downstream of initial C/EBPbeta up-regulation in early adipogenesis, but upstream of PPARgamma activation.