GRP78 regulates clusterin stability, retrotranslocation and mitochondrial localization under ER stress in prostate cancer

Expression of clusterin (CLU) closely correlates with the regulation of apoptosis in cancer. Although endoplasmic reticulum (ER) stress-induced upregulation and retrotranslocation of cytoplasmic CLU (presecretory (psCLU) and secreted (sCLU) forms) has been linked to its anti-apoptotic properties, mechanisms mediating these processes remain undefined. Here, we show using human prostate cancer cells that GRP78 (Bip) associates with CLU under ER stress conditions to facilitate its retrotranslocation and redistribution to the mitochondria. Many ER stress inducers, including thapsigargin, MG132 or paclitaxel, increased expression levels of GRP78 and CLU, as well as post-translationally modified hypoglycosylated CLU forms. ER stress increased association between GRP78 and CLU, which led to increased cytoplasmic CLU levels, while reducing sCLU levels secreted into the culture media. GRP78 stabilized CLU protein and its hypoglycosylated forms, in particular after paclitaxel treatment. Moreover, subcellular fractionation and confocal microscopy with CLUGFP indicated that GRP78 increased stress-induced CLU retrotranslocation from the ER with co-localized redistribution to the mitochondria, thereby reducing stress-induced apoptosis by cooperatively stabilizing mitochondrial membrane integrity. GRP78 silencing reduced CLU protein, but not mRNA levels, and enhanced paclitaxel-induced cell apoptosis. Taken together, these findings reveal novel dynamic interactions between GRP78 and CLU under ER stress conditions that govern CLU trafficking and redistribution to the mitochondria, elucidating how GRP78 and CLU cooperatively promote survival during treatment stress in prostate cancer.