Deepali Sachdev

The IGF system components have been implicated in breast cancer progression. IGF-I and IGF-II are mitogenic and anti-apoptotic peptides that influence the proliferation of various cell types including normal and transformed breast epithelial cells. The IGF system is a key growth regulatory pathway in breast cancer. As various components of the IGF system have been directly or indirectly implicated in breast cancer, it is essential to gain a better understanding of these in order to develop more specific therapeutic strategies for treating breast cancer.

The type I insulin-like growth factor receptor (IGF-IR) plays multiple roles in several cancers and increased circulating levels of insulin-like growth factor-I (IGF-I) are associated with increased risk of breast, colon, and prostate cancers. Because IGF-II and insulin signal via the insulin receptor (IR) to stimulate the growth of cancer cells, inhibition of IR might be necessary to totally disrupt the action of IGFs and their receptors. This review describes the well-recognized roles of IGF-IR in driving the malignant phenotype, examines the evidence that perhaps IR should also be targeted to inhibit the effects of the IGF ligands and insulin in cancer, describes the strategies to disrupt IGF signaling in cancer, and highlights some key issues that need to be considered as clinical trials targeting IGF-IR proceed.

The type I insulin-like growth factor receptor (IGF-IR) plays multiple roles in several cancers and increased circulating levels of insulin-like growth factor-I (IGF-I) are associated with increased risk of breast, colon, and prostate cancers. Because IGF-II and insulin signal via the insulin receptor (IR) to stimulate the growth of cancer cells, inhibition of IR might be necessary to totally disrupt the action of IGFs and their receptors. This review describes the well-recognized roles of IGF-IR in driving the malignant phenotype, examines the evidence that perhaps IR should also be targeted to inhibit the effects of the IGF ligands and insulin in cancer, describes the strategies to disrupt IGF signaling in cancer, and highlights some key issues that need to be considered as clinical trials targeting IGF-IR proceed.

Insulin-like growth factors (IGFs) stimulate breast cancer proliferation, motility, and survival. The type I IGF receptor (IGF1R) mediates the effects of IGF-I. Thus, inhibition of IGF1R activation could inhibit IGF action in breast cancer cells. A single-chain antibody directed against IGF1R (IGF1R scFv-Fc) has been shown to partially inhibit xenograft growth of MCF-7 cells in athymic mice. In this study, we have examined the effects of scFv-Fc on IGF1R signaling in the estrogen receptor-positive (ER+) MCF-7 breast cancer cells in vitro and in vivo. The antibody stimulated IGF1R activation in vitro in MCF-7 cells and was unable to block IGF-I effects. The antibody also stimulated proliferation of MCF-7 cells in monolayer growth assays. To determine how scFv-Fc could stimulate in vitro growth yet inhibit in vivo tumor growth, we examined the effect of scFv-Fc on IGF1R expression. In MCF-7 cells, scFv-Fc down-regulated IGF1R levels after 2 h, and the levels were greatly reduced after 24 h. In contrast, IGF-I treatment over the same time period did not affect IGF1R levels. Twenty-four-h pretreatment of cells with scFv-Fc blocked IGF-I mediated phosphorylation of insulin receptor substrate-1 and subsequent extracellular signal-regulated kinase 1/extracellular signal-regulated kinase 2 and phosphatidylinositol 3'-kinase activation. In contrast, cells treated with 5 nM IGF-I for 24 h still retained the ability to further activate downstream signaling pathways in response to IGF-I. Moreover, pretreatment of MCF-7 cells with scFv-Fc rendered them refractory to further proliferation induced by additional IGF-I. Twenty-four-h pretreatment of cells with scFv-Fc also inhibited IGF-I stimulated anchorage-independent growth. scFv-Fc did not enhance antibody-dependent cell-mediated cytotoxicity. In vivo, treatment of mice bearing MCF-7 xenograft tumors with scFv-Fc resulted in near complete down-regulation of IGF1R. Our data show that scFv-Fc stimulates biochemical activation of IGF1R, then causes receptor down-regulation, making MCF-7 cells refractory to additional IGF-I exposure. These results indicate that such chimeric single-chain antibodies against IGF1R have future potential in breast cancer therapy by causing down-regulation of receptor.

We have previously shown that LCC6 wild-type (WT) cells, a metastatic variant of MDA-MB-435 cancer cells originally derived from a breast cancer patient, exhibit enhanced motility in response to IGF-I compared with the parent MDA-MB-435 cells. To further understand the role of the type I insulin-like growth factor (IGF) receptor (IGF1R) in cancer metastasis we inhibited signaling via IGF1R using a C-terminal-truncated IGF1R. The truncated receptor retains the ligand binding domain but lacks the autophosphorylated tyrosine residues in the carboxyl terminus. Cells stably transfected with this truncated receptor (LCC6-DN cells) overexpressed the truncated IGF1R messenger RNA nearly 50-fold over endogenous receptor. The truncated receptor in the LCC6-DN cells behaved in a dominant negative manner to inhibit endogenous IGF1R activation by IGF-I. Compared with the LCC6-WT cells, LCC6-DN cells failed to phosphorylate the adaptor proteins insulin receptor substrate-1 and -2 in response to IGF-I and did not activate Akt after exposure to IGF-I. Unlike LCC6-WT cells, LCC6-DN cells did not show enhanced motility in response to IGF-I. To assay for metastasis, LCC6-WT and LCC6-DN cells were injected into the mammary fat pads of mice, and the primary xenograft tumors were removed after 21 days. Mice sacrificed 5 weeks later showed multiple lung metastases derived from LCC-WT xenografts, whereas mice harboring LCC6-DN xenografts showed no lung metastases. Our data show that IGF1R can regulate several aspects of the malignant phenotype. In these cells, metastasis but not proliferation requires IGF1R function.