Overexpression of insulin-like growth factor-binding protein-2 in C6 glioma cells results in conditional alteration of cellular growth

To examine the relationship between the expression of insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) and cell growth in a cell type with a defined IGF/IGFBP system, an ovine IGFBP-2 complementary DNA was overexpressed in C6 glioma cells. C6 cells produce IGFBP-3, IGFBP-4, a negligible amount of IGFBP-2, and IGF-I. An ovine IGFBP-2 complementary DNA was transfected into C6 cells, and nine colonies that stably expressed variable levels of IGFBP-2 messenger RNA were selected. Synthesis of corresponding levels of IGFBP-2 was confirmed by ligand blot and immunoblot analyses of conditioned media. Three clones exhibited significantly reduced growth rates, and the remainder showed growth rates similar to those of the wild-type C6 cells. The clones, which overexpressed high levels of IGFBP-2 and IGF-I, had growth rates similar to the wild-type cells, whereas the three clones that overexpressed IGFBP-2 without a concomitant increase in IGF-I had reduced growth rates. In addition, a cell-associated IGFBP was identified in the slow growing clones, but not in the wild-type or the fast growing clones. This cell-associated IGFBP was deduced to be IGFBP-5 based on its molecular size, detection of IGFBP-5 messenger RNA only in slow growing clones, and competition of its binding by heparin. Growth of the slow growing clone, C6BP2-1, could not be overcome by the addition of exogenous IGF-I, suggesting that the cell-associated IGFBP-5 was the dominant regulator of IGF action. These observations suggested that 1) in C6 glioma cells cellular growth is altered by a disturbance in the equilibrium between IGF-I and IGFBPs and/or the functional properties of the IGFBPs; and 2) C6 cells may have a limited capacity to modulate IGF/IGFBP expression in response to changes in endogenous expression of IGFBPs. Endogenous regulation of the balance between IGFs and IGFBPs may be a model of regulation of cellular growth in tumor cells.