A GROWTH-HORMONE (GH) ANALOG CAN ANTAGONIZE THE ABILITY OF NATIVE GH TO PROMOTE DIFFERENTIATION OF 3T3-F442A PREADIPOCYTES AND STIMULATE INSULIN-LIKE AND LIPOLYTIC-ACTIVITIES IN PRIMARY RAT ADIPOCYTES

The effect of amino acid substitutions introduced to the third alpha-h elix in bovine GH (bGH) was investigated. A GH analog (bGH-M8), in whi ch three amino acids were substituted to form an idealized amphiphilic alpha-helix, possessed the same specific binding affinity as wild-typ e bGH to cell membranes prepared from 3T3-F442A cells or rat adipocyte s. However, bGH-M8 failed to stimulate preadipocyte differentiation, a s measured by the level of glycerol-3-phosphate dehydrogenase activity . An equimolar concentration of bGH-M8 was inhibitory for this adipoge nic effect caused by bGH at a concentration of 30 pM. bGH-M8 also fail ed to induce an insulin-like resposne and reduced lipolytic potency in rat primary adipocytes. A 10-fold excess of bGH-M8 abolished the effe ct of wild-type bGH in the insulin-like and lipolytic assays. Thus, bG H-M8 inhibited these actions of wild-type bGH and, therefore, appears to be a competitive antagonist. These results suggest that a major bio logically active domain resides in the third alpha-helix of bGH, which is independent of amino acids important in the initial interaction of GH with its receptor.