TAMOXIFEN INTERFERES WITH THE INSULIN-LIKE-GROWTH-FACTOR-I RECEPTOR (IGF-IR) SIGNALING PATHWAY IN BREAST-CANCER CELLS

The insulin-like growth factor I receptor (IGF-IR) is involved in the control of breast cancer cell growth. The cytostatic activity of tamox ifen (Tam), a nonsteroidal antiestrogen, is partially mediated through interference with IGF-I-R-dependent proliferation, yet the effects of Tam on IGF-IR intracellular signaling have never been elucidated. Con sequently we investigated how Tam modifies the IGF-IR signaling pathwa y in estrogen receptor-positive MCF-7 breast cancer cells and in MCF-7 -derived clones overexpressing either the IGF-IR (MCF-7/IGF-IR cells) or its major substrate, IRS-I (MCF-7/IRS-1 cells). MCF-7/IGF-IR and MC F-7/IRS-1 cells exhibit greatly reduced estrogen growth requirements b ut retain estrogen receptors and express sensitivity to antiestrogens comparable to that in the parental cells. In all tested cell lines, re gardless of the amplification of IGF signaling, a 4-day treatment with 10 nM Tam produced a similar cytostatic effect. In MCF-7 and MCF-7/IG F-IR cells, growth inhibition by Tam was associated with the reduced t yrosine phosphorylation of the IGF-IR in the presence of IGF-I; howeve r, the basal level of the IGF-IR remained unaffected. Moreover, Tam in hibited both basal and IGF-I-induced tyrosine phosphorylation of IRS-I , which was accompanied by down-regulation of IRS-l-associated phospha tidylinositol 3'-kinase activity and reduced IRS-1/growth factor recep tor-bound protein 2 (GRB2) binding. In contrast, under the same treatm ent, tyrosine phosphorylation of Src-homology/collagen proteins (SHC; another substrate of the IGF-IR) and SHC/GRB2 binding were elevated. T he protein levels of the IGF-IR and IRS-1 were not modified by Tam, wh ereas SHC protein expression was either not affected or moderately dec reased by the treatment. In summary, this work provides the first evid ence that in MCF-7 cells, cytostatic effects of Tam are associated wit h the modulation of IGF-IR signaling, specifically with: (a) down-regu lation of IGF-I-induced tyrosine phosphorylation of the IGF-IR; (b) in hibition of IRS-1/phosphatidylinositol 3'-kinase signaling; and (c) up -regulation of SHC tyrosine phosphorylation and increased SHC/GRB2 bin ding. It is hypothesized that dephosphorylation of IRS-I could be a ma jor contributing factor in Tam cytostatic activity.