title: Silencing of the insulin receptor isoform A favors formation of type 1 insulin-like growth factor receptor (IGF-IR) homodimers and enhances ligand-induced IGF-IR activation and viability of human colon carcinoma cells.
abstract: Insulin receptor (IR) overexpression is common in cancers, with expression of the A isoform (IR-A, exon 11-) predominating over the B isoform. The IR-A signals a proliferative, antiapoptotic response to IGF-II, which itself can be secreted by tumors to establish an autocrine proliferative loop. Therefore, IGF-II signaling via the IR-A could mediate resistance to type 1 IGF receptor (IGF-IR) inhibitory drugs that are currently in development. This study addressed the role of the IR-A, using a small interfering RNA-based approach in SW480 human colon adenocarcinoma cells that coexpress the IGF-IR. Clonogenic survival was inhibited by depletion of the IGF-IR but not the IR-A, and dual receptor depletion had no greater effect than IGF-IR knockdown alone, suggesting that the IR-A could not compensate for IGF-IR loss. IGF-IR knockdown also resulted in a decrease in viability, whereas IR-A depletion resulted in increased viability. Consistent with this, upon IR-A depletion, we found a concomitant enhancement of IGF-IR activation by IGF-I and IGF-II, reduced formation of IGF-IR:IR-A hybrid receptors and increased IGF-IR homodimer formation. Together, these results suggest that IGF bioactivity is mediated more effectively by the IGF-IR than by the IR-A or receptor hybrids and that signaling via the IGF-IR is dominant to the IR-A in colon cancer cells that express both receptors.
authors.affiliation: CSIRO Molecular and Health Technologies, P.O. Box 10041, Adelaide Business Centre, Adelaide, South Australia 5000, Australia.
authors.forename: G V
authors.initials: GV
authors.lastname: Brierley
authors.forename: S L
authors.initials: SL
authors.lastname: Macaulay
authors.forename: B E
authors.initials: BE
authors.lastname: Forbes
authors.forename: J C
authors.initials: JC
authors.lastname: Wallace
authors.forename: L J
authors.initials: LJ
authors.lastname: Cosgrove
authors.forename: V M
authors.initials: VM
authors.lastname: Macaulay
mesh_terms: D015153:Blotting, Western; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects / Q000235:genetics; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005434:Flow Cytometry; D020868:Gene Silencing / Q000502:physiology*; D006801:Humans; D047468:Immunoprecipitation; D007189:Indans; D007328:Insulin / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000494:pharmacology; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D055503:Protein Multimerization / Q000235:genetics*; D034741:RNA, Small Interfering / Q000378:metabolism; D017526:Receptor, IGF Type 1 / Q000235:genetics / Q000378:metabolism; D011972:Receptor, Insulin / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014162:Transfection
chemical_list: C425384:HY 038; D007189:Indans; D007328:Insulin; D020033:Protein Isoforms; D034741:RNA, Small Interfering; D007334:Insulin-Like Growth Factor I; D007335:Insulin-Like Growth Factor II; D017526:Receptor, IGF Type 1; D011972:Receptor, Insulin