Exchange protein activated by cyclic AMP is involved in the regulation of adipogenic genes during 3T3‐L1 fibroblasts differentiation

Adipogenesis is stimulated in 3T3‐L1 fibroblasts by a combination of insulin, dexamethasone and isobutylmethylxanthine, IBMX, (I+D+M). Two transcription factors are important for the acquisition of the adipocyte phenotype, C/EBP beta (CCAT enhancer‐binding protein beta) and PPAR gamma (peroxisome proliferator‐activated receptor gamma). IBMX increases cAMP content, which can activate protein kinase A (PKA) and/or EPAC (exchange protein activated by cAMP). To investigate the importance of IBMX in the differentiation mixture, we first evaluated the effect of the addition of IBMX on the increase of C/EBP beta and PPAR gamma and found an enhancement of the amount of both proteins. IBMX addition (I+D+M) or its replacement with a cAMP analogue, dibutyryl‐cAMP or 8‐(4‐chlorophenylthio)‐2‐O′‐methyl‐cAMP (8CPT‐2‐Me‐cAMP), the latter activates EPAC and not PKA, remarkably increased PPAR gamma mRNA. However, neither I+D nor any of the inducers alone, increased PPAR gamma mRNA to a similar extent, suggesting the importance of the presence of both IBMX and I+D. It was also found that the addition of IBMX or 8CPT‐2‐Me‐cAMP was able to increase the content of C/EBP beta with respect to I+D. In agreement with these findings, a microarray analysis showed that the presence of either 8CPT‐2‐Me‐cAMP or IBMX in the differentiation mixture was able to upregulate PPAR gamma and PPAR gamma‐activated genes as well as other genes involved in lipid metabolism. Our results prove the involvement of IBMX‐cAMP‐EPAC in the regulation of adipogenic genes during differentiation of 3T3‐L1 fibroblasts and therfore contributes to elucidate the role of cyclic AMP in this process.