Binding and signalling properties of a GH-enhancing monoclonal antibody

Actions of monoclonal antibodies on the activity of human growth hormone (GH) in an in vitro bioassay

An in vitro bioassay for GH was established, based on the response of the 3T3-F442A mouse preadipocyte cell line, together with a parallel receptor-binding assay using the same cells. The effects of monoclonal antibodies on the biological activity of human GH in vitro were then explored. Antibodies that did not bind GH had no effect on the bioassay or on receptor binding. Antibodies EB1 and EB2, which strongly enhance growth-promoting actions in vivo, inhibited the actions of human GH in the in vitro bioassay, and blocked binding of human GH to receptors. Antibody NA71, which weakly enhances growth promotion by human GH in vivo, enhanced biological activity in vitro but did not affect receptor binding. Thus, enhancement of the biological activity of human GH has been shown in this in vitro system, but the effect does not correlate completely with the established enhancement effects in vivo. Of the various mechanisms that have been proposed to explain the enhancement effect these results support the 'restriction hypothesis'--the idea that monoclonal antibodies may enhance GH action in vivo by preventing binding of GH to receptors/binding sites that are not involved in growth promotion.