Exogenous administration of protease-resistant, non-matrix-binding IGFBP-2 inhibits tumour growth in a murine model of breast cancer
Cells are known to undergo apoptosis when cultured in high serum concentrations. However, the serum factors responsible for this induction of apoptosis have not been identified. The IGF-binding protein-3 (IGFBP-3), a negative growth regulator, is found at concentrations of 5 microgram/ml in serum. We have recently demonstrated that IGFBP-3 induces apoptosis in PC-3 cells, a prostate cancer cell line, at a concentration of 500 ng/ml. In this communication, we demonstrate the role of IGFBP-3 as one of the apoptosis-inducing agents in high serum concentrations. Treatment of PC-3 cells with increasing concentrations (40% to 90%) of intact human serum (HS) resulted in a dose-dependent decrease in cell growth. Valinomycin, an ionophore, was used as a positive control to measure the induction of apoptosis by serum treatment in PC-3 cells. Treatment with 90% serum showed significant suppression of growth (P<0.001) compared with the effect of 10% serum. Treatment with increasing concentrations of HS (40% to 90%) resulted in a dose-dependent increase in apoptosis. Treatment with 90% HS showed a 10-fold increase in apoptotic index compared with cells treated with 10% HS. Treatment of PC-3 cells with IGFs and IGFBP-3-depleted 90% human sera (depleted serum=DS) demonstrated significantly lower levels of apoptosis (50% reduction in the effect of 90% HS) suggesting a role of IGFBP-3 in inducing apoptosis in high serum concentration. Furthermore, treatment with DS supplemented with recombinant IGFBP-3 (500 ng/ml) brought the apoptotic index down close to the level of apoptosis induced by 90% intact serum treatment (P<0.001). However, DS supplemented with physiological concentrations of IGFs (500 ng/ml) showed only partial recovery of cell survival demonstrated by 90% DS. This data indicates that IGFBP-3 is one of the factors in serum that is responsible for high-serum-induced apoptosis.