Anti-diabetic drugs inhibit obesity-linked phosphorylation of PPARgamma by Cdk5

Abstract

Obesity induced in mice by high-fat feeding activates the protein kinase Cdk5 (cyclin-dependent kinase 5) in adipose tissues. This results in phosphorylation of the nuclear receptor PPARgamma (peroxisome proliferator-activated receptor gamma), a dominant regulator of adipogenesis and fat cell gene expression, at serine 273. This modification of PPARgamma does not alter its adipogenic capacity, but leads to dysregulation of a large number of genes whose expression is altered in obesity, including a reduction in the expression of the insulin-sensitizing adipokine, adiponectin. The phosphorylation of PPARgamma by Cdk5 is blocked by anti-diabetic PPARgamma ligands, such as rosiglitazone and MRL24. This inhibition works both in vivo and in vitro, and is completely independent of classical receptor transcriptional agonism. Similarly, inhibition of PPARgamma phosphorylation in obese patients by rosiglitazone is very tightly associated with the anti-diabetic effects of this drug. All these findings strongly suggest that Cdk5-mediated phosphorylation of PPARgamma may be involved in the pathogenesis of insulin-resistance, and present an opportunity for development of an improved generation of anti-diabetic drugs through PPARgamma.

Figure 1. Specific fat cell gene dysregulation by the cdk5-mediated S273 phosphorylation of PPARγ

a, In vitro CDK assays performed using cdk5/p35 with either wild type (WT) or S273A mutated PPARγ. b, Phosphorylation of PPARγ in differentiated 3T3-L1 adipocytes stimulated with TNF-α for the indicated times. c, Phosphorylation of PPARγ in cells expressing scrambled or CDK5 shRNA stimulated with indicated cytokines. NT, no treatment. d, Staining of PPARγ-null fibroblasts expressing WT or S273A mutant PPARγ with Oil-Red-O. e, Gene expression in these cells was analyzed by real-time quantitative PCR (qPCR) for expression of various genes (n=3). f, mRNA expression in transplanted fat pads was analyzed by qPCR (n=5) (Error bars are s.e.m.; *p<0.05, **p<0.01).

Figure 2. CDK5-mediated phosphorylation of PPARγ is increased in fat tissues of high fat diet fed mice (HFD)

a, White adipose tissue (epididymal) from mice on HFD for the indicated time was analyzed with phospho-S273 PPARγ and PPARγ antibodies. b, Epididymal (Epi.) or inguinal (Ing.) fat tissue from 13 weeks HFD mice was analyzed with phospho-S273 antibody.

Figure 3. Anti-diabetic PPARγ ligands block CDK5-mediated phosphorylation of PPARγ

a, TNF-α-induced phosphorylation of PPARγ in 3T3-L1 adipocytes expressing either WT or Q286P mutant of PPARγ treated with rosiglitazone and/or GW9662. b and c, In vitro CDK5 assay with either rosiglitazone or MRL24. d, Transcriptional activity of a PPAR-derived reporter gene in response to rosiglitazone or MRL24 (n=3). e, Microarray analysis of differentiated PPARγ-null fibroblasts expressing WT (NT, rosiglitazone or MRL24 treated) or S273A mutant PPARγ. f, mRNA expression of genes regulated by the phosphorylation of PPARγ in epididymal fat tissue of mice on either chow or HFD for 13 weeks (n=5). Error bars are s.e.m.; *p<0.05, **p<0.01, ***p<0.001.

Figure 4. Differential HDX MS data for PPARγ-LBD ± rosiglitazone and MRL24

a, Histograms showing the percent reduction in HDX for Helix 3 (IRIFQGCQF), the β-sheet region (ISEGQGFMTRE), Helix 12 (QEIYKDLY) and the Helix 2-2’ link region containing the site of CDK5 phosphorylation (KTTDKSPFVIYDM). Values are calculated relative to the measured %D value for apo PPARγ-LBD (n=4; error bars are s.e.m.; **p<0.01, ***p<0.001). b, HDX data for the four peptides of interest are plotted over the structures of PPARγ-LBD bound with rosiglitazone (left, PDB:2PRG) and MRL24 (right, PDB:2Q5P). Percent reduction in HDX relative to unliganded receptor is colored according to the key. Red circle indicates Ser273 residue of PPARγ.

Figure 5. Correlation between the inhibition of phosphorylation and improvement of insulin sensitivity by anti-diabetic PPARγ ligands

a, Glucose-tolerance tests in 16-week HFD mice treated with vehicle, rosiglitazone or MRL24 (n=10). b, phosphorylation of PPARγ in WAT. c, The expression of gene sets regulated by PPARγ phosphorylation in WAT (Error bars are s.e.m.; *p<0.05, **p<0.01, ***p<0.001). d, Changes of phosphorylation of PPARγ in human fat biopsies which were treated with rosiglitazone for 6 months. before, non-treated; after, 6 months treatment. e, Correlation between the changes of PPARγ phosphorylation normalized to total PPARγ protein and the changes of glucose infusion rate measured by clamp. The data is presented as percent change after 6 months of rosiglitazone treatment.