A Phosphorylation Target for Diabetes Therapy

Science Signaling  27 Jul 2010:
Vol. 3, Issue 132, pp. ec227
DOI: 10.1126/scisignal.3132ec227

Stimulation of the nuclear receptor PPARγ (peroxisome proliferator–activated receptor γ) leads to the transcription of genes that promote adipogenesis and glucose uptake. PPARγ is targeted by the thiazolidinedione class of antidiabetic drugs, which includes rosiglitazone. However, efficacy of thiazolidinediones and other antidiabetic drugs cannot always be predicted from the degree of PPARγ agonism. Choi et al. found that cyclin-dependent kinase 5 (Cdk5)–mediated phosphorylation of Ser273 in PPARγ increased in 3T3-L1 adipocytes exposed to free fatty acids or inflammatory cytokines such as TNF-α and IL-6. Wild-type PPARγ (PPARγWT) and a mutant form of PPARγ that could not be phosphorylated (Ser273→Ala; PPARγS273A) showed similar activity in transcriptional and adipogenesis assays. However, cells expressing PPARγS273A showed increased abundance of the transcripts encoding the fatty acid transporter CD36 and the adipokines adiponectin, adipsin, and leptin, and fat pads expressing PPARγS273A showed higher abundance of the transcripts for adiponectin and adipsin. Mice on a high-fat and high-sugar diet showed increased phosphorylation of PPARγ at Ser273 in fat and increased activation of Cdk5 compared with mice on a normal diet. Rosiglitazone inhibited TNF-α–induced Ser273 phosphorylation of PPARγ, an effect that was blocked by the PPARγ antagonist GW9662. Analysis of several antidiabetic drugs indicated that antidiabetic activity appeared to correlate better with the ability to reduce Cdk5-mediated phosphorylation of PPARγ than with PPARγ agonism. Treatment with rosiglitazone or MRL24 (which is a more potent antidiabetic drug but a poorer PPARγ agonist than rosiglitazone) reduced expression of the same set of genes as did expression of PPARγS273A. Hydrogen/deuterium exchange and mass spectrometry revealed that rosiglitazone and MRL24 induced a conformational change in PPARγ that the authors speculated was less favorable for phosphorylation by Cdk5. Treating mice on a high-fat diet with rosiglitazone or MRL24 improved glucose tolerance, reduced fasting insulin concentrations, decreased Cdk5-mediated phosphorylation of PPARγ, and altered gene expression patterns. Furthermore, phosphorylation of PPARγ at Ser273 in subcutaneous fat was reduced in individuals receiving rosiglitazone therapy. In the associated commentary, Houtkooper and Auwerx note that the results of Choi et al. may help in the design of antidiabetic drugs targeting PPARγ that are more effective and have fewer side effects.

J. H. Choi, A. S. Banks, J. L. Estall, S. Kajimura, P. Boström, D. Laznik, J. L. Ruas, M. J. Chalmers, T. M. Kamenecka, M. Blüher, P. R. Griffin, B. M. Spiegelman, Anti-diabetic drugs inhibit obesity-linked phosphorylation of PPARγ by Cdk5. Nature 466, 451–456 (2010). [PubMedl]

R. H. Houtkooper, J. Auwerx, New life for antidiabetic drugs. Nature 466, 443–444 (2010). [PubMed]