Do insulin-like growth factor binding proteins (IGFBPs) modulate the IGF-I growth promoting and differentiating effects in human neuroblastoma cells?

The insulin-like growth factors (IGFs) are known to stimulate both the proliferation and differentiation of neuroblastoma cells, but the role of the IGF binding proteins (IGFBPs) has not yet been established. In this study, human neuroblastoma SH-SY5Y cells have been treated with IGF-1 and its potent analogue des (1-3) IGF-I alone or following preincubation with a differentiating agent such as 12-o-tetradecanoylphorbol-13-acetate (TPA). Cell proliferation and differentiation were evaluated. Conditioned medium was tested for the presence of IGFBPs by ligand blotting. The SH-SY5Y cell proliferation was maximally stimulated by des (1-3) IGF-I. The TPA-induced differentiation of SH-SY5Y, evaluated by assessment of cell morphology and GAP-43 expression as a biochemical marker of differentiation, was potentiated by nanomolar concentrations of des (1-3) IGF-I and, to a smaller extent, IGF-I. Conditioned medium showed the presence of a major IGFBP band with an approximate molecular weight of 32.5 kD and a very faint band of approximately 24 kD. The IGFBP immunoblotting results suggest that the predominant band might represent IGFBP-2. Our data represent a first demonstration of the presence of IGFBPs in conditioned medium of human neuroblastoma SH-SY5Y cells. The finding that the potent IGF-I analogue des (1-3) IGF-I with reduced affinity for IGFBPs induce major effects on cell growth and differentiation suggests that the IGFBPs may play an active role in the neuronal response to the proliferative and differentiative effects of IGF-I.