Receptor dimerization determines the effects of growth hormone in primary rat adipocytes and cultured human IM-9 lymphocytes.
Binding of GH to its cell surface receptors is thought to result in the formation of a complex comprised of one molecule of hormone per two molecules of receptor. It has been proposed that this hormone-induced receptor dimerization is important for the mechanism of signal transduction. We have developed a mathematical model for quantitative evaluation of the biological responses associated with sequential receptor dimerization. Based on these predictions, we have investigated whether GH-induced receptor dimerization plays a role in two classical effects of GH, i.e. stimulation of lipogenesis in primary rat adipocytes and GH receptor down-regulation in cultured human IM-9 lymphocytes. Model predictions of biological responses linked to dimer formation yielded a bell-shaped pattern, with self-antagonism at high GH concentrations when monomeric GH-receptor complexes become predominant. The GH lipogenic bioactivity curve was indeed biphasic and first increased in a concentration-dependent manner between 10(-10)-10(-8) M GH (ED50, 0.5 nM), up to a maximum of 1.7-fold stimulation above basal. Then, the response decreased continuously above 5 x 10(-8) M GH, returning to basal levels around 10(-5) M GH. Incubation of IM-9 cells with wild-type human GH resulted in a dose-dependent loss of their surface receptors. In contrast, a human GH analog (G120R), mutated in the second binding surface of the hormone and, therefore, unable to induce GH receptor dimerization, failed to induce receptor down-regulation in the IM-9 cells. Furthermore, when added together with wild-type human GH, human GH(G120R) inhibited, in a concentration-dependent manner, the down-regulation induced by wild-type human GH. Taken together, these data support the hypothesis that receptor dimerization is critical for the stimulation by GH of both lipogenesis in primary rat adipocytes and receptor down-regulation in cultured human IM-9 lymphocytes.
Ilondo
M M
MM
Hagedorn Research Institute, Gentofte, Denmark.
Damholt
A B
AB
Cunningham
B A
BA
Wells
J A
JA
De Meyts
P
P
Shymko
R M
RM
D017667:Adipocytes / Q000187:drug effects* / Q000378:metabolism; D000818:Animals; D002460:Cell Line; D003198:Computer Simulation; D015536:Down-Regulation; D013006:Growth Hormone / Q000378:metabolism / Q000494:pharmacology*; D006801:Humans; D008055:Lipids / Q000096:biosynthesis; D008214:Lymphocytes / Q000187:drug effects* / Q000378:metabolism; D046911:Macromolecular Substances; D008297:Male; D008433:Mathematics; D008954:Models, Biological; D051381:Rats; D017208:Rats, Wistar; D011986:Receptors, Somatotropin / Q000737:chemistry* / Q000378:metabolism
D008055:Lipids; D046911:Macromolecular Substances; D011986:Receptors, Somatotropin; D013006:Growth Hormone