Insulin-like growth factor 1-receptor signaling stimulates GRP78 expression through the PI3K/AKT/mTOR/ATF4 axis

Abstract

GRP78, a major molecular chaperone, is critical for the folding and maturation of membrane and secretory proteins and serves as the master regulator of the unfolded protein response. Thus, GRP78 is frequently upregulated in highly proliferative cells to cope with elevated protein synthesis and metabolic stress. IGF-1 is a potent regulator of cell growth, metabolism and survival. Previously we discovered that GRP78 is a novel downstream target of IGF-1 signaling by utilizing mouse embryonic fibroblast model systems where the IGF-1 receptor (IGF-1R) was either overexpressed (R+) or knockout (R-). Here we investigated the mechanisms whereby GRP78 is upregulated in the R+ cells. Our studies revealed that suppression of PI3K/AKT/mTOR downstream of IGF-1R signaling resulted in concurrent decrease in GRP78 and the transcription factor ATF4. Through knock-down and overexpression studies, we established ATF4 as the essential downstream nodal of the PI3K/AKT/mTOR signaling pathway critical for GRP78 transcriptional upregulation mediated by IGF-1R.