Up-regulation of GH-binding protein by mouse GH in transgenic mice overexpressing GH-releasing hormone

To study the effects of homologous mouse GH (mGH) on the presence and chara cteristics of serum GH-binding protein (GHBP) we have used transgenic mice expressing GH-releasing hormone (GHRH) as a model. Chromatographic techniqu es allowed the characterization of GHBP bioactivity, and immunological tech niques were used to determine its concentration and molecular components. C hromatographic separation of labeled human GH or mGH cross-linked to serum GHBPs showed two GH-binding serum fractions in normal as well as in transge nic mice serum. SDS-PAGE of this material revealed a specific band of 66 kD a and another higher molecular weight broad band, which, in the presence of a-mercapto-ethanol, is converted into the 66 kDa fraction. Since normal mice serum has an mGH concentration of 0.40 +/- 0.06 nM and a GHBP concentration of 5.7 +/- 1.1 nM, while the high-affinity site for mGH has a K-d of 159 +/- 27 nM, only a small percentage (2.9%) of total serum m GH is bound to GHBP in the sera of these mice. In transgenic mice serum, wh ere the mGH concentration is 60 times higher (23 +/- 2.7 nM), 22.5% of tota l serum mGH is bound to serum GHBP, These values agree with the experimenta l data (4 +/- 2% and 17 +/- 4% for normal and transgenic mice serum respect ively). The concentration of GHBP in GHRH transgenic mice was found to be increased four- to tenfold, depending on the technique used. This increment closely resembles the increased concentration of GHBP in the serum of transgenic bo vine GH (bGH) mice, in which peripheral bGH levels are grossly elevated. Ou r results support the idea that the circulating levels of mGH in normal mou se serum are capable of influencing the levels of GHBP in peripheral circul ation in a way similar to that of bGH, in spite of the different affinities of these two hormones, The fact that the: up-regulation of GHBP occurs, ev en though a small percentage of mGH is bound in these animals, strongly sug gests the existence of a physiological function for GHBP. These results als o question some of the assigned or attributed physiological roles of GHBP, at least in the mouse, since only a negligible percentage of total mGH woul d be prevented from degradation and/or renal filtration by binding to GHBP, This small percentage of bound mGH also invalidates its role as a reservoi r or a buffer of mGH concentration during pulses of GH release or rapid cha nges of mGH levels. Our results also demonstrate the presence of high molec ular weight forms of GH-GHBP complexes that could be dissociated by dilutio n or in the presence of 2-mercapto-ethanol.