Multivalent cations depress ligand binding to cell-associated insulin-like growth factor binding protein-5 on human glioblastoma cells

Endocrinology
R L Sackett, R H McCusker

Abstract

The current studies quantified the effect of the multivalent cations zinc, cadmium, lanthanum, chromium, and gold (Zn2+, Cd2+, La3+, Cr3+, and Au3+) on [125I]-insulin-like growth factor ([125I]-IGF) binding to T98G human glioblastoma cells. The major binding site for the IGFs on T98G cells is IGF binding protein-5 (IGFBP-5), as determined by affinity labeling. Competitive binding studies, using either [125I]-IGF-I or [125I]-IGF-II, indicated that La3+ and Cr3+ did not affect [125I]-IGF-I or [125I]-IGF-II binding to cell-associated IGFBP-5. Zn2+, Au3+, and Cd2+ depressed binding of both [125I]-IGF-I and [125I]-IGF-II. [125I]-IGF-I and [125I]-IGF-II binding resulted in nonlinear concave-down Scatchard plots, indicating the presence of high- and low-affinity equilibrium constant of association (Ka) sites. Assuming a preexisting asymmetric model with independent high (KaHi) and low (KaLo) sites; Zn2+, Cd2+, and Au3+ eliminated KaHi and Zn2+, and Au3+ lowered KaLo, compared with control values. The same results were found, independent of whether [125I]-IGF-I or [125I]-IGF-II was used. Similarly, assuming a ligand-induced model of negative cooperativity, all three cations eliminated the initial affinity for the high affinity sites (K...Continue Reading

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