title: 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 endoplasmic reticulum (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 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.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Southern California, Norris Comprehensive Cancer Center, Keck School of Medicine of USC, Los Angeles, California 90089-9176, USA.
authors.forename: Kyle T
authors.initials: KT
authors.lastname: Pfaffenbach
authors.forename: Michelle
authors.initials: M
authors.lastname: Pong
authors.forename: Todd E
authors.initials: TE
authors.lastname: Morgan
authors.forename: Hongjun
authors.initials: H
authors.lastname: Wang
authors.forename: Kate
authors.initials: K
authors.lastname: Ott
authors.forename: Beiyun
authors.initials: B
authors.lastname: Zhou
authors.forename: Valter D
authors.initials: VD
authors.lastname: Longo
authors.forename: Amy S
authors.initials: AS
authors.lastname: Lee
mesh_terms: D000818:Animals; D031204:Caloric Restriction; D002478:Cells, Cultured; D004721:Endoplasmic Reticulum; D000091342:Endoplasmic Reticulum Chaperone BiP; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; D005786:Gene Expression Regulation / Q000502:physiology*; D055785:Gene Knockdown Techniques; D006360:Heat-Shock Proteins / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008297:Male; D000076222:Mechanistic Target of Rapamycin Complex 1; D051379:Mice; D008810:Mice, Inbred C57BL; D046912:Multiprotein Complexes; D011506:Proteins / Q000235:genetics / Q000378:metabolism; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D015398:Signal Transduction / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases
chemical_list: D000091342:Endoplasmic Reticulum Chaperone BiP; D000071161:Forkhead Box Protein O1; D051858:Forkhead Transcription Factors; C467200:Foxo1 protein, mouse; C000717247:HSPA5 protein, human; D006360:Heat-Shock Proteins; C000717248:Hspa5 protein, mouse; D046912:Multiprotein Complexes; D011506:Proteins; D017526:Receptor, IGF Type 1; D000076222:Mechanistic Target of Rapamycin Complex 1; D051057:Proto-Oncogene Proteins c-akt; D058570:TOR Serine-Threonine Kinases