Differential insulin-like growth factor I receptor signaling and function in estrogen receptor (ER)-positive MCF-7 and ER-negative MDA-MB-231 breast cancer cells

The insulin-like growth factor I receptor (IGF-JR) is a ubiquitous and mult ifunctional tyrosine kinase that has been implicated in breast cancer devel opment. In estrogen receptor (ER)-positive breast tumors, the levels of the IGF-IR and its substrate, insulin-receptor substrate I (IRS-1), are often elevated, and these characteristics have been linked with increased radiore sistance and cancer recurrence. lit vitro, activation of the lGF-IR/ IRS-1 pathway in ER-positive cells improves growth and counteracts apoptosis indu ced by anticancer treatments. The function of the IGF-IR in hormone-indepen dent breast cancer is not clear. ER-negative breast cancer cells often expr ess low levels of the IGF-IR and fail to respond to IGF-I with mitogenesis. On the other hand, anti-IGF-IR strategies effectively reduced metastatic p otential of different ER-negative cell lines, suggesting a role of this rec eptor in late stages of the disease. Here we examined IGF-IR signaling and function in ER-negative MDA-MB-231 breast cancer cells and their IGF-IR-ove rexpressing derivatives. We demonstrated that IGF-1 acts as a chemoattracta nt for these cells. The extent of IGF-1-induced migration reflected IGF-IR levels and required the activation of phosphatidylinositol 3-kinase (PI-3K) and p38 kinases. The same pathways promoted IGF-1-dependent motility in ER positive MCF-7 cells. In contrast with the positive effects on cell migrati on, IGF-I was unable to stimulate growth or improve survival in MDAMB-231 c ells, whereas it induced mitogenic and antiapoptotic effects in MCF-7 cells . Moreover, IGF-I partially restored growth in ER-positive cells treated wi th PI-3K and ERKI/ERK2 inhibitors, whereas it had no protective effects in ER-negative cells. The impaired IGF-I growth response of ER-negative cells was not caused by a low IGF-IR expression, defective IGF-IR tyrosine phosph orylation, or improper tyrosine phosphorylation of IRS-1. Also, the acute ( 15-min) IGF-I activation of PI-3 and Akt kinases was similar in ER-negative and ER-positive cells. However, a chronic (2-day) IGF-I exposure induced t he PI-3K/Akt pathway only in MCF-7 cells. The reactivation of this pathway in ER-negative cells by overexpression of constitutively active Akt mutants was not sufficient to significantly improve proliferation or survival (wit h or without IGF-1), which indicated that other pathways are also required to support these functions. Our results suggest that in breast cancer cells , IGF-IR can control nonmitogenic processes regardless of the ER status, wh ereas IGF-IR growth-related functions may depend on ER expression.