Heat Shock Protein 47 Maintains Cancer Cell Growth by Inhibiting the Unfolded Protein Response Transducer IRE1α

Abstract

HSP47 is a collagen-specific protein chaperone expressed in fibroblasts, myofibroblasts, and stromal cells. HSP47 is also expressed in and involved in growth of cancer cells in which collagen levels are extremely low. However, its role in cancer remains largely unclear. Here, we showed that HSP47 maintains cancer cell growth via the unfolded protein response (UPR), the activation of which is well known to be induced by endoplasmic reticulum (ER) stress. We observed that HSP47 forms a complex with both the UPR transducer inositol-requiring enzyme 1α (IRE1α) and ER chaperone BiP in cancer cells. Moreover, HSP47 silencing triggered dissociation of BiP from IRE1α and IRE1α activation, followed by an increase in the intracellular level of reactive oxygen species (ROS). Increase in ROS induced accumulation of 4-hydroxy-2-nonenal-protein adducts and activated two UPR transducers, PKR-like ER kinase (PERK) and activating transcription factor 6α (ATF6α), resulting in impaired cancer cell growth. Our work indicates that HSP47 expressed in cancer cells relieves the ER stress arising from protein synthesis overload within these cells and tumor environments, such as stress induced by hypoxia, low glucose, and pH. We also propose that HSP47 has a biological role that is distinct from its normal function as a collagen-specific chaperone. IMPLICATIONS: HSP47 maintains cancer cell growth by inhibiting IRE1α.