Peroxisome proliferator-activated receptor γ-independent activation of p38 MAPK by thiazolidinediones involves calcium/calmodulin-dependent Protein kinase II and protein kinase R: Correlation with endoplasmic reticulum stress

Abstract

The thiazolidinediones (TZDs) are synthetic peroxisome proliferator- activated receptor γ (PPARγ) ligands that promote increased insulin sensitivity in type II diabetic patients. In addition to their ability to improve glucose homeostasis, TZDs also exert anti-proliferative effects by a mechanism that is unclear. Our laboratory has shown that two TZDs, ciglitazone and troglitazone, rapidly induce calcium-dependent p38 mitogen-activated protein kinase (MAPK) phosphorylation in liver epithelial cells. Here, we further characterize the mechanism responsible for p38 MAPK activation by PPARγ ligands and correlate this with the induction of endoplasmic reticulum (ER) stress. Specifically, we show that TZDs rapidly activate the ER stress-responsive pancreatic eukaryotic initiation factor 2α (eIF2α) kinase or PKR (double-stranded RNA-activated protein kinase)-like endoplasmic reticulum kinase/pancreatic eIF2α kinase, and that activation of these kinases is correlated with subsequent eIF2α phosphorylation. Interestingly, PPARγ ligands not only activated calcium/calmodulin- dependent kinase II (CaMKII) 2-fold over control, but the selective CaMKII inhibitor, KN-93, attenuated MKK3/6 and p38 as well as PKR and eIF2α phosphorylation. Although CaMKII was not affected by inhibition of PKR with 2-aminopurine, phosphorylation of MKK3/6 and p38 as well as eIF2α were significantly reduced. Collectively, these data provide evidence that CaMKII is a regulator of PKR-dependent p38 and eIF2α phosphorylation in response to ER calcium depletion by TZDs. Furthermore, using structural derivatives of TZDs that lack PPARγ ligand-binding activity as well as a PPARγ antagonist, we show that activation of these kinase signaling pathways is PPARγ-independent. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.