Abstract 535: IGF-I stimulates amino acid transporter xC- function to reduce intracellular ROS level and promote proliferation in human breast cancer cells

Our laboratory identified SLC7A11/xCT as an IGF-I-induced gene specifically regulated through the adaptor protein IRS-1 in breast cancer cells. xCT encodes the functional subunit of the heterodimeric plasma membrane transport system xC-, which is critical for the cellular uptake of cystine in exchange for intracellular glutamate. xC- transporter is involved in the regulation of proliferation, metastasis, and treatment resistance in cancer. To determine the role for xC- expression in IGF-mediated breast cancer cell biology, two classes of breast cancer cell lines were studied: estrogen receptor (ER) positive cell lines (IRS-1 activated) MCF-7, ZR-75-1, and T47D; basal-like cell lines (IRS-2 activated) MDA-MB-231, BT549, and HS578T. Significantly increased xCT mRNA expression was observed only in ER positive cell lines, which was eliminated by IRS-1 specific siRNA (25 nM). IGF-I increased xCT protein expression measured by immunblots and flow cytometry only in ER positive cells in an IRS-1 dependent manner. To measure the function of upregulated xC-, extracellular levels of glutamic acid and intracellular level of total glutathione were measured in MCF-7 and MDA-MB-231. IGF-I regulated xC- transporter function only in IRS-1 activated MCF-7 cells. In MCF-7 cells, IGF-I-stimulated monolayer and anchorage-independent growth were suppressed by treating cells with xCT shRNA or the xCT chemical inhibitor sulfasalazine (SASP, 0.1 mM). Direct measurement of reactive oxygen species (ROS) and levels of phospho-p38MAPkinase showed that IGF-I reduced cellular ROS level (induced by 10 Gy of irradiation or 1 ug/ml of mitomycin C), SASP (0.1 mM) reversed this IGF-I effect. Anchorage-independent growth assays suggested that inhibiting xC- function by 0.1 mM SASP sensitized the response of MCF-7 cells to anti-IGF-IR monoclonal antibody humanized EM164 (ImmunoGen, Inc.) and tyrosine kinase inhibitor NVP-AEW-541. These data suggest that IGF-I promotes the proliferation of ER positive breast cancer cells by regulating xC- transporter function in an IRS-1 dependent manner. Furthermore, IGF-I protects cells from ROS via upregulation of xC-. Our findings also imply that combination of xC- transporter inhibitor with anti-IGF-IR agents may have synergic therapeutic effects. Citation Format: Yuzhe Yang, Marc A. Becker, Douglas Yee. IGF-I stimulates amino acid transporter xC- function to reduce intracellular ROS level and promote proliferation in human breast cancer cells. [abstract].