ADIPOGENESIS CDK4 at the Promoter

Cyclin-dependent kinase 4 (CDK4) stimulates reentry to the cell cycle, and this is a key step in adipogenesis. Abella et al. present evidence that suggests that, in addition to this cell cycle role, CDK4 also directly interacts with the peroxisome proliferator-activated receptor (PPAR) to regulate adipocyte differentiation and biology. CDK4 was detectable by immunohistochemistry in mature mouse adipose tissue and was catalytically active when isolated from differentiated 3T3-L1 adipocytes or primary mouse adipocytes. Whereas 3T3-L1 preadipocytes expressed CDK4 and rapidly divided, the differentiated 3T3-L1 adipocytes expressed CDK4 but were relatively quiescent. Addition of the CDK4 inhibitor 2-bromo-12, 12-dihydro-5H-indolo(2,3-a)pyrrolo (3,4) carbazole (IDCX), either before clonal expansion of 3T3-L1 preadipocytes (the stage of rapid cell proliferation) or after clonal expansion, blocked adipocyte differentiation, which is further support for a cell cycle-independent role for CDK4. Treatment of fully differentiated 3T3-L1 adipocytes with IDCX inhibited uptake of glucose in response to insulin despite causing a higher basal level of glucose uptake, which appeared to be due to increased abundance of Glut-1 transporter mRNA. Expression of several genes involved in glucose transport and homeostasis, as well as genes involved in lipogenesis, was inhibited by IDCX treatment. As detected in a reporter gene assay in transfected COS cells, CDK4 stimulated PPAR-regulated gene expression through a mechanism that required the kinase activity of CDK4. PPAR and CDK4 were found to directly interact in coimmunoprecipitation experiments in transfected cells and in differentiated 3T3-L1 adipocytes. In addition, chromatin immunoprecipitation (ChIP) experiments showed that in differentiated 3T3-L1 adipocytes, but not undifferentiated 3T3-L1 preadipocytes, CDK4 and PPAR were bound to the promoter of the PPAR target gene aP2. Thus, CDK4 appears to be involved in directly regulating gene expression through interactions with transcription factors at the promoter.

A. Abella, P. Dubus, M. Malumbres, S. G. Rane, H. Kiyokawa, A. Sicard, F. Vignon, D. Langin, M. Barbacid, L. Fajas, Cdk4 promotes adipogenesis through PPAR activation. Cell Metabolism 2, 239-249 (2005). [PubMed]