title: Insulin-like growth factor-I receptor signaling in tamoxifen-resistant breast cancer: a supporting role to the epidermal growth factor receptor.
abstract: There is considerable evidence that the epidermal growth factor receptor (EGFR) and IGF-I receptor (IGF-IR) cross-talk in breast cancer cells. In the present study, we have examined whether EGFR/IGF-IR cross-talk exists in EGFR-positive tamoxifen-resistant variants of MCF-7 (Tam-R) and T47D (T47D-R) breast cancer cell lines. Although Tam-R cells expressed reduced IGF-IR protein levels compared with their wild-type MCF-7 counterparts, phosphorylated IGF-IR protein levels were equivalent in the two cell lines under basal growth conditions, possibly as a consequence of increased IGF-II expression in Tam-R cells. IGF-II activated both IGF-IR and EGFR in Tam-R cells, whereas only activation of IGF-IR was observed in wild-type cells. In contrast, epidermal growth factor rapidly induced EGFR, but not IGF-IR, phosphorylation in Tam-R cells. IGF-II promoted direct association of c-SRC with IGF-IR, phosphorylated c-SRC, and increased EGFR phosphorylation at tyrosine 845, a c-SRC-dependent phosphorylation site. Pretreatment with either AG1024 (IGF-IR-specific inhibitor) or an IGF-II neutralizing antibody inhibited basal IGF-IR, c-SRC, and EGFR phosphorylation, and AG1024 significantly reduced Tam-R basal cell growth. The c-SRC inhibitor SU6656 also inhibited growth, reduced basal and IGF-II-induced c-SRC and EGFR phosphorylation, and blocked EGFR activation by TGFalpha. Similarly, in T47D-R cells, AG1024 and SU6656 inhibited basal and IGF-II-induced phosphorylation of c-SRC and EGFR, and SU6656 reduced TGFalpha-induced EGFR activity. These results suggest the existence of a unidirectional IGF-IR/EGFR cross-talk mechanism whereby IGF-II, acting through the IGF-IR, regulates basal and ligand-activated EGFR signaling and cell proliferation in a c-SRC-dependent manner in Tam-R cells. This cross-talk between IGF-IR and EGFR is not unique to Tam-R cells because this mechanism is also active in a tamoxifen-resistant T47D-R cell line.
authors.affiliation: Tenovus Centre for Cancer Research, Welsh School of Pharmacy, Cardiff University, Redwood Building, King Edward VII Avenue, Cardiff CF10 3XF, United Kingdom.
authors.forename: J M
authors.initials: JM
authors.lastname: Knowlden
authors.forename: I R
authors.initials: IR
authors.lastname: Hutcheson
authors.forename: D
authors.initials: D
authors.lastname: Barrow
authors.forename: J M W
authors.initials: JM
authors.lastname: Gee
authors.forename: R I
authors.initials: RI
authors.lastname: Nicholson
mesh_terms: D018931:Antineoplastic Agents, Hormonal / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000503:physiopathology*; D045744:Cell Line, Tumor; D019008:Drug Resistance, Neoplasm; D066246:ErbB Receptors / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D006801:Humans; D007335:Insulin-Like Growth Factor II / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000378:metabolism; D020239:Receptor Cross-Talk; D017526:Receptor, IGF Type 1 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D015398:Signal Transduction; D013629:Tamoxifen / Q000494:pharmacology*; D016211:Transforming Growth Factor alpha / Q000494:pharmacology
chemical_list: D018931:Antineoplastic Agents, Hormonal; D016211:Transforming Growth Factor alpha; D013629:Tamoxifen; D007335:Insulin-Like Growth Factor II; D066246:ErbB Receptors; D017526:Receptor, IGF Type 1; D016392:Proto-Oncogene Proteins pp60(c-src)