Soluble forms of the rabbit adipose tissue and liver growth hormone receptors are antigenically identical, but the integral membrane forms differ

The role of 'adipotropins' and the clinical importance of a potential hypothalamic–pituitary–adipose axis

Since adipocytes express specific receptors for pituitary hormones and hypothalamic releasing factors, adipose tissue has to be regarded as a fast-acting endocrine gland under the control of the brain. Expanding on this suggestion, the existence and clinical impact of a hypothalamic-pituitary-adipose axis is reviewed. The term 'adipotropins' is introduced in order to describe pituitary and hypothalamic hormones or releasing factors that directly target adipocytes by their specific receptors.

GH induced lipolysis stimulation in 3T3-L1 adipocytes stably expressing hGHR: analysis on signaling pathway and activity of 20K hGH

We have constructed a cell line of 3T3-L1 which can efficiently express human GHR (3T3-L1-hGHR) after differentiation to adipocytes. The expressed hGHR was detected as two bands with approximate molecular sizes of 120K by Western analysis using hGHR specific monoclonal antibody. Maximum lipolytic activity induced by hGH in the 3T3-L1-hGHR was enhanced 10-fold as compared to that in 3T3-L1, suggesting that expressed hGHR is functionally active. Comparative analysis using bGH and hGH revealed that 70% of lipolysis stimulation by 1-10 ng/ml hGH could be attributed to hGHR-mediated response. Analyses on inhibition and phosphorylation of signaling molecules suggested that GH-induced lipolysis stimulation is dependent on gene expression and not mediated through PKA-, PKC-, PLA-, PLC-, nor MAPK-pathway but possibly through JAK-STATs pathway. Duration of STAT5 activation by hGH continued up to 48 h. We also revealed that 22 K hGH isoform, 20K hGH which has been reported as a weaker agonist for GH-induced lipolysis stimulation, possesses equipotent activity and shows stronger action in the presence of hGHBP as compared to 22 K hGH. Taken together we conclude that the hGH-induced lipolysis was not mediated through MAP-, PKA-, PKC-, nor PLA-pathway but might be mediated through STAT pathway and that 20K hGH might show higher lipolytic activity than 22 K hGH in adipose tissue that produces a large amount of GHBP.

Evidence for involvement of the carboxy terminus of helix 1 of growth hormone in receptor binding: use of charge reversal mutagenesis to account for calcium dependence of binding and for design of higher affinity analogues

In this study we have demonstrated that the C-terminus of helix 1 of porcine GH (pGH) is a receptor-interactive region, thus extending the current binding site model of GH. This was achieved by introducing charge reversal mutations into this region of pGH, which influenced receptor affinity and Ca2+ dependence of binding. The first mutant (R34E pGH, conversion of Arg 34 to Glu) introduced a putative Ca2+ binding site which is present in human GH (hGH) [Barnard et al. (1989) J. Theor. Biol. 140, 355-367] and sits opposite E220 of receptor subunit 1. This mutant exhibited increased Ca2+ dependence of receptor binding but even at optimal Ca2+ did not display higher than wild-type affinity. Introduction of a second Ca2+ binding site adjacent to the first by a second charge reversal (K30E R34E pGH) further increased Ca2+ dependence of binding and also increased affinity for the rabbit GH receptor (2.4 +/- 0.4)-fold relative to wild-type pGH at optimal Ca2+. Equilibrium dialysis and Scatchard analysis of binding of 45Ca2+ to pGH and K30E R34E pGH revealed two Ca2+ binding sites on wild-type pGH and an additional two Ca2+ binding sites on the K30E R34E pGH mutant (Kd 0.5-0.8 mM), as predicted. A third partial charge reversal mutant in the fourth helix (H170D) also led to enhanced Ca2+ dependence of binding, supporting our proposal that E34 and D170 are responsible for the Ca2+ dependence of hGH binding to the rabbit GH receptor. Examination of the crystal structure shows that E34 and D170 are in close proximity and would interact repulsively with a cluster of acidic residues on the receptor consisting of E126, E127, and E220 unless neutralized by Ca2+ or an introduced basic residue. Accordingly, charge reversal at the adjacent pGH residue E33 (E33K pGH) led to a Ca2+ independent (3.0 +/- 0.4)-fold increase in affinity of binding. As well as extending the binding site model of GH, these studies provide a mechanistic explanation for the unique Ca2+ dependence of hGH binding to the rabbit GH receptor. They also indicate that charge reversal can be used to design higher affinity GH analogues and could assist in the mapping of interactive regions in ligand-receptor complexes generally.

Growth hormone binding proteins in pig adipose tissue: number, size and effects of pGH treatment on pGH and bGH binding

The present study was undertaken to determine the number and size of growth hormone binding proteins present in pig adipose tissue, determine if there were differences in binding of pGH and bGH to adipose tissue membranes and establish the effects of pGH treatment on GH binding. Administration of pGH (0, 25, 50 or 100 micrograms pGH/kg BW/d) for 7 d did not affect binding of [125I]bGH to adipose tissue microsomes. Maximum binding of bGH was approximately 8-fold higher than that observed for pGH. Half-maximal inhibition of [125I]bGH binding was observed at 11 ng/ml of bGH. In contrast, a more than 10-fold greater concentration of pGH was required to half-maximally inhibit [125I]pGH binding. bGH and pGH both bound to the same GH binding proteins (Mr of 92,000, 73,000 and 53,000). The GH binding proteins appear to be produced by post-translational modification of a single GH receptor transcript rather than alternative splicing of a primary transcript since only one GH receptor mRNA transcript (4.2 kb) was detected on Northern analysis. Our findings indicate that: 1) bGH is the preferred ligand to use to study GH binding in pig adipose tissue membranes (or adipocytes); 2) exogenous pGH does not alter GH binding; and 3) only one GH receptor mRNA transcript is present in pig adipose tissue.