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Production of insulin-like growth factor binding protein (IGFBP), IGFBP-3 protease, and expression of IGF-I receptors by cells of the sheep immune system

Tonner E, Beattie J, Flint DJ. Production of insulin-like growth factor binding protein (IGFBP), IGFBP-3 protease, and expression of IGF-I receptors by cells of the sheep immune system. Eur J Endocrinol 1995;132:118–22. ISSN 0804–4643

Single-cell suspensions of sheep thymus cells were cultured in serum-free medium with or without polyclonal activators (phytohaemagglutinin or concanavalin A) and the resultant conditioned medium was assayed for insulin-like growth factor binding protein (IGFBP) activity by binding of [¹²⁵I]IGF-I, using charcoal to separate free from bound. All cultures produced IGFBP but mitogen stimulation significantly increased IGFBP concentrations, indicating production by lymphoid cells. Conditioned medium also degraded recombinant human [¹²⁵I]IGFBP-3, suggesting IGFBP-3 protease production within the thymus. This degradation was inhibited by several protease inhibitors (phenylmethylsulphonyl fluoride, aprotinin, N--p-tosyl-L-lysine chloromethyl ketone), suggesting the presence of a serine protease. Cell surface [¹²⁵I]IGF-I binding was demonstrated on cells from thymus, mesenteric lymph node, peripheral blood mononuclear cells and platelets. The [¹²⁵I]IGF-I binding to platelets could be inhibited by unlabelled peptides, with relative potencies IGF-I > IGF-II >> insulin. Scatchard analysis of IGF-I competitive binding revealed a K_d of 266 pmol/l and approximately 40 receptor sites per cell. The high-affinity binding of IGF-I and competition by insulin suggested that the [¹²⁵I]IGF-I binding was to an IGF-I receptor rather than to a membrane-associated IGFBP, to which insulin does not bind. These data provide further support for the role of the IGF-IGFBP axis in the immune system, particularly in relation to the thymus.

Elizabeth Tonner, Hannah Research Institute, Ayr, UK

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