GROWTH-HORMONE (GH) BINDING AND EFFECTS OF GH ANALOGS IN TRANSGENIC MICE

Overexpression of human (h) or bovine (b) growth hormone (GH) in trans genic mice is associated with marked (2- to 12-fold) and significant i ncrease in hepatic binding of GH and prolactin (PRL). This is due to a n increase in the number of GH and PRL receptors (GHR, PRLR) per mg of microsomal protein without changes in binding affinity. Comparison of results obtained in transgenic animals expressing bGH with a mouse me tallothionein (MT) or a rat phosphoenolpyruvate carboxykinase (PEPCK) promoter suggests that effects of bGH on hepatic GHR and PRLR do not r equire GH overexpression during fetal life and, within the dose range tested, the effects on PRLR are not dose dependent. The increase in he patic GHR was accompanied by significant increases in plasma GH-bindin g protein (GHBP) and in mean residence time of injected GH. Thus life long elevation of peripheral GH levels alters the availability of both free GH and GHR. Site-directed in vitro mutagenesis was used to produ ce hGH and bGH analogs mutated within one of the sites involved in bin ding to GHR and PRLR. Mutating hGH to produce amino acid identity with bGH at Position 11, 18 (within Helix 1), 57, or 60 (within the loop b etween Helix 1 and 2) did not affect binding to GHR in vitro, or somat otropic activity in transgenic mice in vivo but reduced lactogenic act ivity in Nb-2 cells by 22%-45%. Mutations of bGH designed to produce a mino acid identity with hGH at one to four of the corresponding positi ons in the bGH molecule did not interfere with binding to GHR or somat otropic activity in vivo, and failed to produce significant binding to PRLR but resulted in alterations in the effects on the hypothalamic a nd anterior pituitary function in transgenic mice. Apparently region(s ) outside the domains examined are essential for lactogenic activity o f hGH, and different portions of the GH molecule are responsible for i ts diverse actions in vivo.