Oncogenic ras causes resistance to the growth inhibitor insulin-like growth factor binding protein-3 (IGFBP-3) in breast cancer cells

Insulin-like growth factor binding protein-3 (IG-FBP-3) inhibits proliferation and promotes apoptosis in normal and malignant cells. In MCF-10A human mammary epithelial cells, 30 ng/ml human plasma-derived IGFBP-3 inhibited DNA synthesis to 70% of control. This inhibition appeared IGF-independent, since neither an IGF-receptor antibody nor IGFBP-6 inhibited DNA synthesis. Malignant transformation of MCF-10A cells by transfection with Ha-ras oncogene abolished the inhibitory effect of IGFBP-3, concomitant with an increase in IGFBP-3 secretion and cell association of approximately 60 and 300%, respectively. When mitogen-activated protein (MAP) kinase activation was partially inhibited using PD 98059, IGFBP-3 sensitivity in ras-transfected cells was restored, with a significant inhibitory effect at 10 ng/ml IGFBP-3. PD 98059 had no effect on IGFBP-3 secretion or cell association by ras-transfected or parent MCF-10A cells. Hs578T, a tumor-derived breast cancer cell line that expresses activated Ha-ras, similarly has a high level of secreted and cell-associated IGFBP-3. In the absence of PD 98059, DNA synthesis by Hs578T cells was reduced to 70% of control by 1000 ng/ml IGFBP-S. PD 98059 increased sensitivity to IGFBP-3, so that this level of inhibition was achieved with 100 ng/ml IGFBP-3. These results suggest that MAP kinase activation by oncogenic ras expression causes IGFBP-3 resistance, a possible factor in the dysregulation of breast cancer cell growth.