title: Activation of insulin-like growth factor signaling induces apoptotic cell death under prolonged hypoxia by enhancing endoplasmic reticulum stress response.
abstract: Malignant cells in solid tumors survive under prolonged hypoxia and can be a source of resistance to current cancer therapies. Mammalian target of rapamycin (mTOR), one of the downstream molecules of the insulin-like growth factor (IGF) pathway, is a key regulator of translation, integrating multiple environmental and nutritional cues. The activity of mTOR is known to be suppressed under hypoxic conditions in cancer cells, whereas the contribution of this suppression to cell survival has not yet been clarified. We show that stimulating IGF signaling provoked caspase-dependent apoptosis under low oxygen tension in two cancer cell lines, COLO 320 and AsPC-1. In concurrence with increased levels of BAD phosphorylation, cell death was not accompanied by cytochrome c release from mitochondria. The cells were rescued from apoptosis when phosphatidylinositol 3-kinase (PI3K) or mTOR activity was inhibited, suggesting that these signals are critical in the observed cell death. IGFs and insulin enhanced the endoplasmic reticulum (ER) stress response as monitored by induction of the CCAAT/enhancer binding protein homologous protein (CHOP) proteins and the X box protein-1 splicing under hypoxic conditions, and this response was suppressed by inhibiting PI3K and mTOR activity. IGF-induced cell death under hypoxic conditions was prevented by treatment with cycloheximide, suggesting that de novo protein synthesis is required. Indeed, suppression of CHOP protein levels with small hairpin RNA reduced cell death. Taken together, the data suggest that stimulating IGF signaling under hypoxic conditions provokes apoptosis by enhancing the ER stress response.
authors.affiliation: Department of Biochemistry, Osaka Medical Center for Cancer and Cardiovascular Diseases, Suita Municipal Hospital, Osaka, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Endo
authors.forename: Kohei
authors.initials: K
authors.lastname: Murata
authors.forename: Mutsuko
authors.initials: M
authors.lastname: Mukai
authors.forename: Osamu
authors.initials: O
authors.lastname: Ishikawa
authors.forename: Masahiro
authors.initials: M
authors.lastname: Inoue
mesh_terms: D017209:Apoptosis / Q000187:drug effects; D020169:Caspases / Q000378:metabolism / Q000502:physiology; D016923:Cell Death; D015687:Cell Hypoxia / Q000502:physiology*; D045304:Cytochromes c / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006801:Humans; D007328:Insulin / Q000494:pharmacology; D008928:Mitochondria / Q000378:metabolism; D008954:Models, Biological; D019869:Phosphatidylinositol 3-Kinases / Q000502:physiology; D014176:Protein Biosynthesis / Q000187:drug effects; D011494:Protein Kinases / Q000502:physiology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D013002:Somatomedins / Q000378:metabolism* / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases; D051742:Transcription Factor CHOP / Q000502:physiology; D014407:Tumor Cells, Cultured