GRP78/BiP is a novel downstream target of IGF‐1 receptor mediated signaling

Abstract

Glucose regulated protein 78/immunoglobulin binding protein (GRP78/BiP) is an ER chaperone protein and master regulator of the unfolded protein response (UPR). The response of GRP78 to overt pharmacologically induced ER stress is well established, whereas the modulation of GRP78 to physiologic changes is less characterized. In this study, we examined the regulation of GRP78 in response to reduced IGF-1 growth factor signaling, a common consequence of calorie restriction (CR). ER chaperone protein expression was quantified in cell lysates prepared from the livers of calorie restricted (CR) and ad libitum fed mice, as well as MEFs grown in normal medium or serum starved. The requirement of IGF-1 signaling on GRP78 expression was studied using MEFs with IGF-1 receptor overexpression (R+) or deletion (R−), and the regulatory mechanism was examined using mTORC1 and PI3K inhibitors, as well as R− cells with knockdown of transcription factor FOXO1 compared to shRNA control. We observed a 40% reduction in GRP78 protein expression in CR mice and in serum-starved MEF cells. R− cells had drastically reduced AKT phosphorylation and exhibited lower levels of ER chaperones, in particular 80% less GRP78. Despite an 80% reduction in GRP78 expression, R− cells were not under chronic ER stress, but were fully capable of activating the unfolded protein response (UPR). Neither forced expression of FOXO1-AAA nor knockdown of FOXO1 in R− cells affected GRP78 expression. In conclusion, we report that IGF-1 receptor signaling regulates GRP78 expression via the PI3K/AKT/mTORC1 axis independent of the canonical UPR and FOXO1.

Restriction on an Energy-Dense Diet Improves Markers of Metabolic Health and Cellular Aging in Mice Through Decreasing Hepatic mTOR Activity

“…The UPR may be either a consequence of accumulation of damaged proteins provoked by stress (e.g., pharmacological agents) or of accumulation of nascent proteins through increased synthesis under physiological conditions. In the latter case, mTOR activation 45,46 and growth factor signaling 47 are mainly responsible for induction of UPR in liver cells. In our HFDR mice, expression of the UPR members Bip/Grp78 and Pdia3 was lower compared to AL mice, whereas proteasome activity and LC3-related autophagy were not different.…”

Glucose-regulated proteins in cancer: molecular mechanisms and therapeutic potential

“…Although cell surface GRP78 has been shown to regulate PI3K signaling, further studies are required to determine whether GRP78 in the ER or other cellular locations might also regulate PI3K-AKT signaling. Recent studies revealed GRP78 is a downstream target of the IGF-1R-PI3K signaling pathway in mouse embryo fibroblasts, as well as in cancer cell lines 18,19 , and this could represent a feedback regulatory mechanism that balances GRP78 expression and cancer cell proliferation.…”

“…ER stress, as well as development of therapeutic resistance, actively promotes cell surface expression of GRP78, which serves as an upstream regulator of the PI3K-AKT oncogenic signaling pathway 15-17 . GRP78 is also a downstream target of AKT activation 18,19 . At the cell surface, GRP94 and GRP170 function in antigen presentation, and their secreted forms have the ability to elicit innate and adaptive immune responses, which could be useful in the development of cancer vaccines 1,2,20 .…”

Ovine subclinical mastitis: Proteomic analysis of whey and milk fat globules unveils putative diagnostic biomarkers in milk

“…Their concentration changes may be associated with eventual physiological/metabolic changes inducing a reduction in the mammary function as a result of alterations in alveolar cell integrity [69,70]. In this context, it is tempting to speculate that the increased levels observed for H2B are associated with neutrophil extracellular traps involved in tissue defense and injury, actively released by neutrophils or other cells included within milk somatic cells [71].…”

“…Alternatively, high H2B levels could be related to chromatin remodeling [72] as well as to the passive release of nuclear proteins by apoptotic or necrotic cells [73]. On the other hand, GRP78 takes part in protection against noxious agents and protein misfolding, which occur under various stresses [70,74,75]. It is also involved in cell communication pathways associated with unfolded protein response, apoptosis and autophagy [74,76].…”