BACKGROUND AND AIMS: Sustained adipose activation of the transcriptional activators cAMP response binding proteins (CREB) in obesity leads to impaired expression of the glucose transporter GLUT4 and adiponectin (adipoq) in mice model of obesity. Diminution of GLUT4 and adipoq caused by CREB is indirect and relies on the increased repressive activity of the CREB target gene activating transcription factor 3 (ATF3). Specific inactivation of CREB in adipocytes decreases ATF3 production and improves whole-body insulin sensitivity of mice in the context of diet-induced obesity. Thus, elevation of CREB activity is a key mechanism responsible for adipocyte dysfunction and systemic insulin resistance. The inducible cAMP early repressor (ICER) is a negative regulator of the CREB activity. In fact, ICER antagonizes the CREB factor by competing for the regulation of similar target genes. The goal of the study was to investigate whether loss of ICER expression in adipocytes could be responsible for increased CREB activity in obesity.
MATERIALS AND METHODS: Mice C57bl6 were fed with a high fat diet (HFD) for 12 weeks to increase body weight and generate insulin resistance. Biopsies of visceral adipose tissues (VAT) were prepared from human lean (BMI=24}0.5 Kg/m2) or obese subjects (BMI>35 Kg/m2). Total RNA and protein were prepared from white adipose tissues (WAT) of chow- or HFD-fed mice and VAT of lean and obese subjects. Activities of CREBs and ICER were monitored by electromobility shift assays (EMSA). The role of ICER on CREB activity was confirmed in 3T3-L1 adipocytes cells. Briefly after differentiation, the cells were electroporated with the plasmid coding for ICER cDNA. Gene expression was quantified by quantitative real-time PCR and western Blotting experiments.
RESULTS: The expression of ICER is reduced in WAT of HFD-induced obese mice when compared to chow mice as measured by real-time PCR and EMSA. Similar result was found in human tissues. Reduction in ICER expression was associated with increased ATF3 expression and decreased adipoq and GLUT4 contents. Diminution in ICER levels was observed in adipocytes fraction whereas its expression was unchanged in stroma vascular fraction of WAT. Overexpression of ICER in 3T3-L1 adipocytes silenced the expression of ATF3, confirming the regulation of the factor by ICER. The expression of ICER is regulated by histone deacetylases activity (HDAC). Inhibition of HDACs in 3T3-L1 adipocytes cells using trichostatin inhibited the production of ICER. The whole activity of HDAC was reduced in WAT and VAT of obese mice and human obese subjects.
CONCLUSION: Impaired adipose expression of ICER is responsible of increased CREB activity in adipocytes in obesity. This mechanism relies on reduction of the HDAC activity.