Insulin-like growth factors (IGF-I and IGF-II) are potent mitogens for breast cancer cells. IGF bioactivity is influenced by IGF binding proteins (IGFBPs). In vitro, most breast cancer cell lines express and secrete IGFBP-4. Sense and antisense complementary DMA of human IGFBP-4 were ligated into a mammalian expression vector and transfected into MCF-7 cells in order to investigate the role of IGFBP-4 on breast cancer cell proliferation in vitro and in vivo. IGFBP-4 concentrations were 4-8-fold higher in conditioned media (CM) of sense IGFBP-4 transfected cells compared to control cells, while IGFBP-4 concentrations in the CM of antisense IGFBP-4 transfected cells were lower than those present in the CM of control cells. Basal growth of sense IGFBP-4 transfected cells (S11) in media supplemented with 0.5% fetal bovine serum was significantly lower (P<0.01) than that of a vector-transfected control (C13) and antisense IGFBP-4 transfected cells (AS4). Loss of IGF but not EGF responsiveness in S11 cells was observed 48 h after exposure to these mitogens. Equal response to Des (1-3) IGF-I (an IGF-I analogue with reduced affinity for IGF binding proteins) was observed for C13, S11 and AS4 cells, suggesting that loss of responsiveness to IGFs by S11 cells is a consequence of IGFBP-4 expression. The in viva proliferation of S11 was significantly (P<0.01) less than that of control C13 and AS4 cells in both lit/lit (IGF-I deficient) or lit/+ (IGF-I replete) hosts. These data demonstrate that IGFBP-4 expression influences breast cancer behaviour.

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