0021-972X/91/7301-0030$03.00/0 Journal of Clinical Endocrinology and Metabolism Copyright © 1991 by The Endocrine Society
Vol. 73, No. 1 Printed in U.S.A.
Plasma Growth Hormone (GH)-Binding Proteins: Effect on GH Binding to Receptors and GH Action* DANA A. MANNOR, LORI M. WINER, MELISSA A. SHAW, AND GERHARD BAUMANN Center for Endocrinology, Metabolism, and Nutrition, Department of Medicine, Northwestern University Medical School, Chicago, Illinois 60611
ABSTRACT. GH-binding proteins (GH-BP) have recently been discovered in human plasma, but their biological function is unknown. The high affinity GH-BP is related to the GH receptor and may modulate the interaction of GH with tissue receptors, whereas the low affinity GH-BP should be inert in this regard. Modulation of receptor binding probably results in modulation of GH bioactivity. To address these issues, we examined the effects of purified GH-BPs as well as whole plasma on GH binding to receptors in human, rabbit, and female rat liver and in rat adipocytes. High affinity BP inhibited binding of GH to receptors in a dose-dependent fashion. Substantial inhibition was observed within the physiological range of BP concentrations; human liver and rat adipocytes were the most sensitive in this regard. In contrast, the low affinity BP had no
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effect on receptor binding of GH. Whole human plasma also inhibited GH binding to receptors. This effect could be attributed to its content of GH-BP, since removal or primary absence of the BP from plasma abolished its inhibitory effect. Purified high affinity BP also inhibited GH-dependent insulin-like growth factor-I production by cultured human fibroblasts to a degree comparable to receptor inhibition. We conclude that the circulating high affinity GH-BP, by sequestering GH, substantially interferes with binding of GH to its receptor, and that the well known inhibitory effect of plasma in RRAs for GH is largely due to this BP. GH action in human fibroblasts in vitro is similarly inhibited by this BP. (J Clin Endocrinol Metab 73: 30-34, 1991)
preliminary data on the effects of GH-BPs on GH action in vitro.
PECIFIC binding proteins for human GH (GH-BPs) have recently been described in human plasma ( 1 3). The high affinity BP is related to the GH receptor (4-6) and has an affinity for GH similar to that of the receptor (1, 2, 4, 6). Therefore, it appears likely that this BP competes with tissue GH receptors for GH binding. The low affinity GH-BP has an association constant at least 1000-fold lower than the receptor (3), a property that should render it impotent as a competitor with GH receptors. Since a substantial proportion of GH is complexed with the BPs (7, 8), it is important to know 1) how GH is partitioned between BPs and receptors, 2) to what extent complexed GH may interact with receptors, and 3) how GH bioactivity is affected by the presence of BPs. To address these questions, we examined the effects of isolated BPs and whole plasma on the interaction of GH with various GH receptors. In addition, we obtained
Materials and Methods Materials Human GH (hGH; 22K form; lot 306-13-3) was a gift from Dr. U. J. Lewis (La Jolla, CA). It was radioiodinated by a lactoperoxidase method, as previously described (SA, 65-79 Ci/ g) (1). Normal heparinized basal human plasma (endogenous GH,
Vol. 73, No. 1 Printed in U.S.A.
Plasma Growth Hormone (GH)-Binding Proteins: Effect on GH Binding to Receptors and GH Action* DANA A. MANNOR, LORI M. WINER, MELISSA A. SHAW, AND GERHARD BAUMANN Center for Endocrinology, Metabolism, and Nutrition, Department of Medicine, Northwestern University Medical School, Chicago, Illinois 60611
ABSTRACT. GH-binding proteins (GH-BP) have recently been discovered in human plasma, but their biological function is unknown. The high affinity GH-BP is related to the GH receptor and may modulate the interaction of GH with tissue receptors, whereas the low affinity GH-BP should be inert in this regard. Modulation of receptor binding probably results in modulation of GH bioactivity. To address these issues, we examined the effects of purified GH-BPs as well as whole plasma on GH binding to receptors in human, rabbit, and female rat liver and in rat adipocytes. High affinity BP inhibited binding of GH to receptors in a dose-dependent fashion. Substantial inhibition was observed within the physiological range of BP concentrations; human liver and rat adipocytes were the most sensitive in this regard. In contrast, the low affinity BP had no
S
effect on receptor binding of GH. Whole human plasma also inhibited GH binding to receptors. This effect could be attributed to its content of GH-BP, since removal or primary absence of the BP from plasma abolished its inhibitory effect. Purified high affinity BP also inhibited GH-dependent insulin-like growth factor-I production by cultured human fibroblasts to a degree comparable to receptor inhibition. We conclude that the circulating high affinity GH-BP, by sequestering GH, substantially interferes with binding of GH to its receptor, and that the well known inhibitory effect of plasma in RRAs for GH is largely due to this BP. GH action in human fibroblasts in vitro is similarly inhibited by this BP. (J Clin Endocrinol Metab 73: 30-34, 1991)
preliminary data on the effects of GH-BPs on GH action in vitro.
PECIFIC binding proteins for human GH (GH-BPs) have recently been described in human plasma ( 1 3). The high affinity BP is related to the GH receptor (4-6) and has an affinity for GH similar to that of the receptor (1, 2, 4, 6). Therefore, it appears likely that this BP competes with tissue GH receptors for GH binding. The low affinity GH-BP has an association constant at least 1000-fold lower than the receptor (3), a property that should render it impotent as a competitor with GH receptors. Since a substantial proportion of GH is complexed with the BPs (7, 8), it is important to know 1) how GH is partitioned between BPs and receptors, 2) to what extent complexed GH may interact with receptors, and 3) how GH bioactivity is affected by the presence of BPs. To address these questions, we examined the effects of isolated BPs and whole plasma on the interaction of GH with various GH receptors. In addition, we obtained
Materials and Methods Materials Human GH (hGH; 22K form; lot 306-13-3) was a gift from Dr. U. J. Lewis (La Jolla, CA). It was radioiodinated by a lactoperoxidase method, as previously described (SA, 65-79 Ci/ g) (1). Normal heparinized basal human plasma (endogenous GH,
