HMG-CoA reductase inhibitors activate the unfolded protein response and induce cytoprotective GRP78 expression

Since apoptosis of macrophages induced by stress to the endoplasmic reticulum (ER) contributes to advanced atherosclerotic lesions, we sought to understand the effects of statins on the unfolded protein response (UPR).We used pharmacological, biochemical, and siRNA (small interfering ribonucleic acid) approaches to determine the signalling cascades of statin-induced 78 kDa glucose-regulated protein (GRP78) gene transcription and its role in cytoprotection. Exposure of RAW264.7 macrophages to statins increased the expression of GRP78, activating transcription factor 6, X box protein-1, and phosphorylated eukaryotic translation initiation factor 2alpha, while it had no effect on CCAAT/enhancer binding protein-homologous protein. GRP78 induction was abolished by co-treatment with mevalonate and 1,2-bis(o-aminophenoxy)ethane-N, N, N',N'-tetraacetic acid, indicating the involvement of both 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase-dependent and -independent mechanisms. Studies on promoter activity measurements indicated that phosphoinositide turnover, cellular homologue of v-src (c-Src), protein kinase C (PKC), extracellular signal-regulated kinase (ERK), and p38 are involved in upregulating GRP78 gene transcription. We also observed that elevation of intracellular Ca(2+) and interruption of small G proteins are two bifurcated but cooperative signalling pathways for c-Src activation, leading to downstream activation of phospholipase C, PKC, ERK, and p38. Functionally we demonstrated that fluvastatin could protect macrophages from hypoxia-induced cell death through GRP78 induction.We demonstrate a novel action of statins of inducing a cytoprotective UPR, providing new insights into the clinical potential of statins for ameliorating ER stress-related diseases.