Signal transducer and activator of transcription 3 (STAT3) was reported to be involved in adipogenesis. However, the regulating mechanism of STAT3 remains unclear. The present results showed that STAT3 was activated within 2-h adipogenic induction, in which the phosphorylated STAT3 translocated from cytoplasm to the nucleus. In addition, we detected Janus kinase2 (JAK2) acted upstream of the STAT3 activation at the early stage of adipogenesis. Accordingly, the JAK2 inhibitor AG490 and siRNAs led to the partial inhibition of the STAT3 activation, and the inhibition of 3T3-L1 adipocyte differentiation. Furthermore, the results based on luciferase, chromatin immunoprecipitation, and gel shift approaches indicated that STAT3 could regulate the transcription of C/EBPβ by binding the distal region of C/EBPβ promoter at the early stage of adipogenesis. Collectively, our findings reveal that JAK2/STAT3 pathway is involved in the early stage of 3T3-L1 adipocyte differentiation though regulating the C/EBPβ transcription.

Coordination of cell differentiation and proliferation is a key issue in the development process of multi-cellular organisms and stem cells. Here we provide evidence that the establishment of adipocyte differentiation of 3T3-L1 cells requires two processes: the licensing of an adipogenesis gene-expression program within a particular growth-arrest stage, i.e., the contact-inhibition stage, and then the execution of this program in a cell-cycle-independent manner, by which the licensed progenitors are differentiated into adipocytes in the presence of inducing factors. Our results showed that differentiation licensing of 3T3-L1 cells during the contact-inhibition stage involved epigenetic modifications such as DNA methylation and histone modifications, whereas disturbing these epigenetic modifications by DNA methylation inhibitors or RNAi during the contact-inhibition stage significantly reduced adipogenesis efficiency. More importantly, when these licensed 3T3-L1 cells were re-cultured under non-differentiating conditions or treated only with insulin, this adipogenesis commitment could be maintained from one cell generation to the next, whereby the licensed program could be activated in a cell-cycle-independent manner once these cells were subjected to adipogenesis-inducing conditions. This result suggests that differentiation licensing and differentiation execution can be uncoupled and disparately linked to cell proliferation. Our findings deliver a new concept that cell-fate decision can be subdivided into at least two stages, licensing and execution, which might have different regulatory relationships with cell proliferation. In addition, this new concept may provide a clue for developing new strategies against obesity.

No abstract