A Growth Hormone (GH) Analog that Antagonizes the Lipolytic Effect but Retains Full Insulin-Like (Antilipolytic) Activity of GH

Abstract

An analog of bovine growth hormone (bGH–M8: [Leu¹¹⁷, Arg¹¹⁹, Asp¹²²]–bGH) with an idealized amphiphilic third αhelix has been proposed to be a functional antagonist of GH. In accordance with this proposition, bGH–M8 profoundly inhibited bGH–stimulated lipolysis by chicken adipose tissue <in vitro. bGH–M8 alone was a weak agonist in the lipolytic assay (1.9% the potency of bGH). The present evidence indicates that bGH–M8 is a competitive antagonist of the lipolytic action of GH based upon the following results: (i) increasing concentrations of bGH–M8 (antagonist) produce progressively greater inhibition of GH–stimulated lipolysis; (ii) increasing concentrations of bGH (agonist) are capable of overcoming this antagonism; and (iii) Schild plot analysis (slope = − 0.94) suggests a receptor antagonist with an equilibrium dissociation constant (K _B) of 4.54 nM. In contrast to the antagonistic effects of bGH–M8 on bGH–stimulated lipolysis, bGH–M8 retained full insulin–like (“antilipolytic”) activity (i.e., inhibition of glucagon–induced lipolysis). bGH–M8 and bGH were similarly potent in eliciting antilipolytic effects < in vitro. Moreover, the antilipolytic effects of bGH–M8 and bGH were additive. Therefore, the third α–helix (particularly residues 117, 119, and 122) of bGH contains major structural determinants for the lipolytic effects of GH. The ability of bGH–M8 to act as an antagonist for at least one action of GH (lipolysis) while being a full agonist for another (antilipolysis) suggests that different domains of GH are responsible for its various biologic activities, possibly involving different binding sites and/or signal transduction mechanisms.