Regulation of GH Binding to Specific Cellular Receptors In Vitro - A New Model of Growth Regulation In Vivo

Summary

A new theory on the complex growth hormone (GH) mediated promotion of tissue growth has been developed on the basis of in vitro studies of growth hormone receptors on human peripheral mononuclear cells (PMC). It is hypothesized that GH acts through its tissue receptors and regulates its own receptors through two different pathways: first GH directly downregulates GH binding, second a partially GH-dependent serum factor (the so-called SM-B) enhances GH binding. Some experimental evidence for this hypothesis is presented using a new method to investigate GH receptors in circulating human blood cells: The effect of trypsin, antitrypsin, growth hormone, somatomedin-B (SM-B) and anti-SM-B-antiserum on GH binding to PMC was studied. Trypsinization of cells leads to a decrease both of specific binding and of binding affinity (affinity constant after 60 minutes of trypsinization 0.5 × 10⁶ M^-1 versus 1.5 × 10⁶ M^-1 in untreated control cells). Exposure of PMC to antitrypsin activities was followed by an increase of binding affinity and specific binding (affinity constants with 10 KIU 1.9 × 10⁶ M^-1, with 100 KIU 2.4 × 10⁶ M^-1, with 1000 KIU 3.6 × 10⁶ M^-1). This antitrypsin effect exceeds the binding values expected after blocking trypsin activities possibly being present in the incubation medium. In a subset of experiments the partially GH-dependent serum factor SM-B was used as the antitrypsin moiety and was shown to increase specific GH binding to PMC in a similar manner as did antitrypsin (with 1000 ng SM-B affinity constant 12.0 × 10⁶ M^-1, specific binding 9.7 %). Anti-SM-B-antiserum completely abolished this effect. In contrast, preincubation of PMC with hGH lead to a decrease of specific GH-binding (1.0-1.3% with GH versus 3.3% without GH preincubation). This increase was mainly caused by a decrease of binding affinity (0.5-0.6 × 10⁶ M^-1 with GH versus 1.3 × 10⁶, M^-1 without GH). It is concluded that enzymatic factors and their inhibitors in serum modulate GH-induced action by regulation of GH receptors in GH target tissues. Since one of such factors is partially GH dependent (the so-called SM-B) an indirect short loop positive feedback regulation of GH-binding is hypothesized. This GH-induced enhancement of GH binding may well counteract the GH-dependent down-regulation of the GH receptor in PMC.

Key-Words

Growth Hormone - Growth Regulation - Growth Hormone Receptors - Lymphocytes