Hormonal Regulation of Growth Hormone Receptors in Primary Cultured Rat Hepatocytes

Abstract

The hormonal regulation of GH receptors was studied by measuring specific binding of [125I]human GH to primary cultured rat hepatocytes. The binding of labeled GH to primary cultured hepatocytes decreased during culture, but addition of dexamethasone (100 nM) compensated for this decrease and even increased GH binding. After addition of dexamethasone, the binding increased to a maximum after 10 h, and after 24 h was about 6 times that of control cells. Glucagon (100 nM) did not have any significant effect on GH binding by itself, but enhanced the increased binding caused by dexamethasone about 1.5-fold. For this effect, glucagon could be replaced by (Bu)2cAMP. Insulin (10 nM) and epidermal growth factor (20 ng/ml) reduced the increase by dexamethasone plus glucagon by about half. Scatchard plot analysis showed that the changes of GH binding induced by various hormones were due to changes in the number of binding sites without significant changes in their affinity. The GH bound to dexamethasone or dexamethasone plus glucagon-treated cells was not replaced by unlabeled ovine PRL. This strongly suggests that the number of somatogenic (GH) receptors may be subject to hormonal regulation: dexamethasone alone or with glucagon may induce GH receptors, whereas insulin and EGF may suppress the induction of GH receptors. These patterns of hormonal regulations were almost the same as those of proteins whose expressions were known to be differentiated functions of liver. On the other hand, the increase of GH binding by dexamethasone was inhibited by cycloheximide and actinomycin D, though the GH binding was inhibited by cycloheximide, but not by actinomycin D in the cells cultured without dexamethasone. This result suggests that the increased binding induced by dexamethasone is dependent on the synthesis of new protein and is probably regulated at a pretranslational level.