The PED/PEA-15 diabetogene as a potential thiazolidinedione target in type 2 diabetes treatment

The phosphoprotein enriched in diabetes/phosphoprotein enriched in astrocytes (PED/PEA-15) gene is over-expressed in tissues from individuals with type 2 diabetes. In cultured cells and in vivo, ped/pea-15 over-expression impairs insulin activation of protein kinase C zeta (PKCζ) and glucose disposal. Indeed, transgenic mice ubiquitously over-expressing ped/pea-15 feature impaired glucose tolerance and insulin resistance. However, it is still unknown how PED/PEA-15 gene expression is regulated. Recently, we have obtained evidence that Thiazolidinediones (TZDs), a class of peroxisome proliferator-activated receptor gamma (PPARγ) ligands, repress ped/pea-15 expression both at mRNA and protein levels. PPARγ is a member of the nuclear hormone receptor super-family that modulates gene expression upon ligand binding. Ligand-mediated activation of PPARγ has been linked to glucose homeostasis, cellular differentiation, apoptosis and anti-inflammatory responses. The aim of this work was to study whether and how TZD-activated PPARγ may exert its action on ped/pea-15 expression. In L6 skeletal muscle cells, rosiglitazone (RGTZ) decreased both ped/pea-15 mRNA and protein levels. This was paralleled by the RGTZ-dependent reduction of PKCα activation and a consequent increase of PKCζ activity. Consistent with the in vitro data, muscle tissue from control mice showed a 40% decrease of ped/pea-15 protein levels upon 10 days of treatment with RGTZ. HeLa cells treated with TPA, the prototypical AP1 activator, showed an increase in PED/PEA-15 mRNA levels. TPA induced also the activation of PED/PEA-15 promoter activity measured by luciferase assays. Interestingly, TPA effect was suppressed by RGTZ. EMSA and ChIP assays revealed that TPA caused an increased binding to the CRE-like site (a putative AP1 binding site) on the PED/PEA-15 promoter, and this binding was reduced by RGTZ. Since PPARγ activation by TZDs blocks AP1-mediated gene transcription, we hypothesized that PPARγ transrepression of the PED/PEA-15 gene could be due to a competition for limiting amounts of co-activators present in the cell. Indeed, the suppressive effect of rosiglitazone on PED/PEA-15 gene expression was blocked by over-expressing a dominant negative form of PPARγ, which lacks the ability to recruit the coactivator CBP/p300 (CREB-binding protein). The over-expression of p300 did not rescue the cells from the suppressive effect of PPARγ on PED/PEA-15 promoter activity, suggesting that PPARγ-mediated repression of AP1 transcriptional activity occurs in a CBP/p300-independent manner. Taken together, these results indicate that regulation through the CRE-like site on PED/PEA-15 promoter induces PED/PEA-15 transcription. This effect was blocked by treatment with TZDs. Finally, the results of this study identified PED/PEA-15 gene as a potential target for TZDs therapeutic action.