Glucocorticoids inhibit growth and antagonize growth hormone (GH) action, presumably by acting upon peripheral tissues. This study tested the hypothesis that glucocorticoids antagonize GH action at the cellular level and investigated the mechanism of this antagonism. Pretreatment of 3T3-F442A fibroblasts with the synthetic glucocorticoid dexamethasone (DEX) antagonized the ability of GH to induce tyrosyl phosphorylation of the GH receptor (GHR), the GHR-associated kinase JAK2, the MAP kinases designated ERK1 and 2, and the transcription factors Stats 1 and 3, but did not decrease expression of these proteins. DEX also decreased $\sp{125}$I-GH binding with a time course (t$\sb{1/2}$ = 6h) and magnitude (50%) consistent with decreases in GH-induced phosphorylation. DEX decreased GHR number but not affinity. The finding that DEX does not decrease cellular GHR protein levels suggests that DEX sequesters GHR or inactivates them on the plasma membrane. 4$\beta$-Phorbol 12-myristate 13-acetate (PMA), previously shown to internalize GHR in IM-9 cells, also rapidly (t$\sb{1/2}$ = 15 min) decreases $\sp{125}$I-GH binding and GH-induced tyrosyl phosphorylation in 3T3-F442A fibroblasts. DEX and PMA also decrease GH binding in H35 hepatoma cells and CHO cells expressing a rat liver GHR. Using mutated GHRs expressed in CHO cells, the DEX induced decrease in GH binding was found to require amino acids between residues 456-506 and tyrosines 333 and 338 of GHR. These amino acids are not required for GH-induced internalization. Phe 346, required for GH-induced internalization, was not required for the effects of DEX, suggesting DEX may redistribute GHR by a mechanism independent of GH-induced internalization. PMA decreased GH binding to all tested mutants, but appeared to require residues 506-638 for a maximal effect. In conclusion, glucocorticoids can antagonize GH action by desensitizing cells to GH, apparently by sequestering GHR. Specific amino acids of the GHR are required for this sequestration. They may represent phosphorylation or binding sites for glucocorticoid induced kinase(s) or binding proteins(s) which can mediate redistribution of GHRs away from the plasma membrane.Ph.D.PhysiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104969/1/9624651.pdfDescription of 9624651.pdf : Restricted to UM users only.