Insulin-like growth factors stimulate the release of insulin-like growth factor-binding protein-3 (IGFBP-3) and degradation of IGFBP-4 in nonsmall cell lung cancer cell lines

Insulin-like growth factors (IGFs) are potent mitogens for lung cancer cells. Their proliferative activity is influenced by their binding proteins (IGFBPs). We report here on the regulatory effects of IGF-I and IGF-II on the production and release of IGFBPs by nonsmall cell lung cancer cell lines (NSCLC). The nine NSCLC cell lines used in this study showed messenger ribonucleic acid (mRNA) expression of all six IGFBPs known, as determined by PCR, and protein secretion of IGFBP-1, -2, -3, -4, and -6, as analyzed by Western immunoblots. The addition of IGFs to a serum-free medium showed divergent effects on IGFBP-3 and IGFBP-4 levels in a conditioned medium (CM). IGF-I and IGF-II, but not insulin, led to a much higher concentration of IGFBP-3 in the CM: of all tested NSCLC cell lines, whereas the level of immunologically detected membrane-associated IGFBP-S was decreased. Furthermore, Northern analysis of mRNA isolated from A549 revealed that IGFBP-3 specific mRNA was not changed by IGF-I or IGF-II, suggesting that the IGF-induced effects on IGFBP-3 depend on the release of cell-associated IGFBP-3. In contrast, IGFBP-4 levels were diminished by increasing concentrations of IGFs in the CM of the NSCLCs A549, NCI-H157, and U1752, with no response to insulin or the IGF-I analog, whereas IGFBP-4-specific mRNA was not changed by IGF-I or IGF-II, as determined by Northern analysis. The same effects were seen in a cell-free system after incubation of the CM with IGFs. The decrease in IGFBP-4 concentrations was prevented by coincubation of the CM with the IGFs and either ethylenediamine tetraacetate or 1,10-phenanthrolene, but not with other protease inhibitors. We suggest that IGFs may either activate an IGFBP-4-specific metalloprotease present in NSCLC CM or that the binding of IGFs to IGFBP-4 may enhance the susceptibility of IGFBP-4 to proteolytic degradation. Based on these data, we present evidence that IGFs may regulate their own availability both by releasing IGFBP-3 from cell membranes and through proteolytic degradation of IGFBP-4.