DISTINCT CYTOPLASMIC DOMAINS OF THE GROWTH-HORMONE RECEPTOR ARE REQUIRED FOR GLUCOCORTICOID-INDUCED AND PHORBOL ESTER-INDUCED DECREASES IN GROWTH-HORMONE (GH) BINDING - THESE DOMAINS ARE DIFFERENT FROM THAT REPORTED FOR GH-INDUCED RECEPTOR INTERNALIZATION
Apj. King et al., DISTINCT CYTOPLASMIC DOMAINS OF THE GROWTH-HORMONE RECEPTOR ARE REQUIRED FOR GLUCOCORTICOID-INDUCED AND PHORBOL ESTER-INDUCED DECREASES IN GROWTH-HORMONE (GH) BINDING - THESE DOMAINS ARE DIFFERENT FROM THAT REPORTED FOR GH-INDUCED RECEPTOR INTERNALIZATION, The Journal of biological chemistry, 271(30), 1996, pp. 18088-18094
Glucocorticoids inhibit growth in children and antagonize the growth-p
romoting action of GH in peripheral tissues, Recently, they have been
shown to decrease GH binding, In this study we examine the molecular m
echanisms by which the glucocorticoid dexamethasone (HEX) and the phor
bol ester phorbol myristate acetate (PMA) decrease cellular GH binding
, In 3T3-F442A fibroblasts, DEX and PMA decrease the number of GH rece
ptors (GHRs) capable of binding GH by 50% (t(1/2) = 6 h) and 70% (t(1/
2) = 15 min), respectively, Neither appear to de crease the total numb
er of cellular GHR, Rather, they appear to redistribute GHRs away from
the plasma membrane or inactivate GHRs on the membrane such that they
cannot bind GH, DEX and PIMA also decrease GH-induced tyrosyl phospho
rylation of GHR and JAK2 with a magnitude and time course correlating
with that of inhibition of GH binding, DEX- and PMA-induced reductions
of GH binding are also observed in a Chinese hamster ovary (CHO) cell
line stably transfected with a rat liver GHR cDNA, further arguing th
at DEX and PMA act post-translationally on GHR, Using mutant GHRs stab
ly expressed in CHO cells, amino acids 455-506 and tyrosines 333 and/o
r 338 of GHR were shown to be required for maximal DEX-induced inhibit
ion of GH binding, DEX decreased GH binding to a GHR mutant F346A, whi
ch is reported to be deficient in ligand-induced internalization, sugg
esting that HEX decreases GH binding by a mechanism distinct from that
of ligand-induced GHR internalization. PMA reduced GH binding to CHO
cells expressing all GHR mutants tested, However, deletion of the C te
rminal 132 amino acids decreased this effect, suggesting that at least
one component of PMA action on GHR requires amino acids 507-658. Thes
e data suggest that distinct pathways mediate the effects of GH, DEX,
and PMA on GHR number in the plasma membrane.