INSULIN AND INSULIN-LIKE GROWTH-FACTOR SIGNALING ARE DEFECTIVE IN THEMDA MB-468 HUMAN BREAST-CANCER CELL-LINE

Polypeptide growth factors including the insulin-like growth factors ( IGFs), insulin, and transforming growth factor-alpha are mitogens for many breast cancer cell lines and may act as regulators of cancer cell growth. In a human breast cancer cell line MCF-7, which expresses ICF -I receptor (ICF-IR), stimulation with insulin or IGFs resulted in aut ophosphorylation of the IGF-IR in an increased proportion of ras bound to CTP and in the association of phosphatidylinositol 3'-kinase (PI3K ) activity and of p85-PI3K with M(r) 185,000 phosphotyrosinylated prot eins corresponding in size to insulin/lGF-IR substrates. These events were associated with enhanced proliferation. MDA MB-468 is a human bre ast cancer cell line which expresses insulin receptor and high levels of epidermal growth factor/transforming growth factor alpha receptor b ut low levels of IGF-IR. In this cell line, insulin stimulated autopho sphorylation of IR at physiological concentrations and promoted the as sociation of PI3K activity and of p85 with phosphotyrosine-containing proteins. Insulin did not, however, induce increased ras.GTP, and the cells exhibited minimal proliferation in response to insulin. Unlike i nsulin treatment, epidermal growth factor stimulation of MDA MB-468 ce lls is mitogenic and resulted in increased ras GTP content, suggesting that the failure of insulin to induce these changes is not due to alt erations in these signaling molecules. We conclude that there is a pos treceptor defect in insulin signaling in MDA MB-468 which prevents the activation of ras and the induction of mitogenesis. Activation of PI3 K by insulin is not sufficient to mediate mitogenesis. Alterations in the insulin/lCF pathway in MDA MB-468 may result from overexpression o f epidermal growth factor receptor that occurs during tumor developmen t and progression.