DIFFERENTIAL IN-VIVO ACTIVITIES OF BOVINE GROWTH-HORMONE ANALOGS

In rodents, bovine (b) growth hormone (GH) binds only to GH receptors, while human (h) GH binds to both GH and PRL receptors. The phenotypic consequences of expression of bGH and hGH in transgenic mice are diff erent and, in some cases, opposite. In the present study, site-directe d in vitro mutagenesis of the bGH gene was used systematically to elim inate its differences from hGH at one, two, three or four sites suspec ted of conferring lactogenic activity: D-11, H-18, S-57 and T-60, resp ectively (corresponding to sites 12, 19, 57 and 60 of the bGH molecule ). The resulting bGH analogues were expressed in cell lines and in tra nsgenic mice. All of the seven bGH analogues produced retained their a bility to bind to GH receptors and exhibited somatogenic activity in v itro and in vivo. However, none of them were able to bind to PRL recep tors or to elicit detectable lactogenic response in vitro. Transgenic animals expressing any of the generated analogues were characterized b y gigantism and splanchnomegaly. The effects of expression of each of the double, triple or quadruple mutants on the seminal vesicle weight resembled the effects of wild-type hGH and differed from the effects o f expression of wild-type bGH. There were differences between the effe cts of the expression of different bGH analogues on plasma PRL levels and on the PRL response to pharmacological blockade of catecholamine s ynthesis. Plasma LH levels in ovariectomized females were suppressed b y several of the analogues tested, an effect not seen in animals expre ssing wild-type bGH or hGH. Dopamine turnover in the median eminence o f male mice was also altered in animals expressing different bGH analo gues but not in those expressing wild-type bGH or hGH. In ovariectomiz ed females, the effects of different bGH analogs on the turnover of do pamine and norepinephrine in the median eminence included changes rese mbling those detected in animals expressing hGH, as well as alteration s differing from the effects of both bGH and hGH. The results indicate that biological actions of these bGH analogues cannot be characterize d simply in terms of enhanced or reduced somatogenic or lactogenic act ivity and raise a possibility that different sites, domains or feature s of the tri-dimensional structure of GH are involved in its actions o n different cellular targets.