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

The thiazolidinediones (TZDs) are synthetic peroxisome proliferator-activated receptor gamma (PPAR gamma) 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 PPAR gamma 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 2 alpha (eIF2 alpha) kinase or PKR (double-stranded RNA-activated protein kinase)like endoplasmic reticulum kinase/pancreatic eIF2 alpha kinase, and that activation of these kinases is correlated with subsequent eIF2 alpha phosphorylation. Interestingly, PPAR gamma 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 eIF2 alpha phosphorylation. Although CaMKII was not affected by inhibition of PKR with 2-aminopurine, phosphorylation of MKK3/6 and p38 as well as eIF2 alpha were significantly reduced. Collectively, these data provide evidence that CaMKII is a regulator of PKR-dependent p38 and eIF2 alpha phosphorylation in response to ER calcium depletion by TZDs. Furthermore, using structural derivatives of TZDs that lack PPAR alpha ligand-binding activity as well as a PPAR gamma antagonist, we show that activation of these kinase signaling pathways is PPAR gamma-independent.