Sustained stimulation of G-protein-coupled receptors (GPCRs) typically causes receptor desensitisation, which is mediated by phosphorylation, often within the C-terminal tail of the receptor. The consequent binding of beta-arrestin not only prevents the receptor from activating its G protein (causing desensitisation), but can also target it for internalisation via clathrin-coated vesicles and can mediate signalling to proteins regulating endocytosis and mitogen-activated protein kinase (MAPK) cascades. GnRH acts via phospholipase C (PLC)-coupled GPCRs on pituitary gonadotrophs to stimulate a Ca(2+)-mediated increase in gonadotrophin secretion. The type I GnRH receptors (GnRH-Rs), found only in mammals, are unique in that they lack C-terminal tails and apparently do not undergo agonist-induced phosphorylation or bind beta-arrestin; they are therefore resistant to receptor desensitisation and internalise slowly. In contrast, the type II GnRH-Rs, found in numerous vertebrates, possess such tails and show rapid desensitisation and internalisation, with concomitant receptor phosphorylation (within the C-terminal tails) or binding of beta-arrestin, or both. The association with beta-arrestin may also be important for regulation of dynamin, a GTPase that controls separation of endosomes from the plasma membrane. Using recombinant adenovirus to express GnRH-Rs in Hela cells conditionally expressing a dominant negative mutant of dynamin (K44A), we have found that blockade of dynamin-dependent endocytosis inhibits internalisation of type II (xenopus) GnRH-Rs but not type I (human) GnRH-Rs. In these cells, blockade of dynamin-dependent internalisation also inhibited GnRH-R-mediated MAPK activation, but this effect was not receptor specific and therefore not dependent upon dynamin-regulated GnRH-R internalisation. Although type I GnRH-Rs do not desensitise, sustained activation of GnRH-Rs causes desensitisation of gonadotrophin secretion, and we have found that GnRH can cause down-regulation of inositol (1,4,5) trisphosphate receptors and desensitisation of Ca(2+) mobilisation in pituitary cells. The atypical resistance of the GnRH-R to desensitisation may underlie its atypical efficiency at provoking this downstream adaptive response. GnRH-Rs are also expressed in several extrapituitary sites, and these may mediate direct inhibition of proliferation of hormone-dependent cancer cells. Infection with type I GnRH-R-expressing adenovirus facilitated expression of high-affinity, PLC-coupled GnRH-R in mammary and prostate cancer cells, and these mediated pronounced antiproliferative effects of receptor agonists. No such effect was seen in cells transfected with a type II GnRH-R, implying that it is mediated most efficiently by a non-desensitising receptor. Thus it appears that the mammalian GnRH-Rs have undergone a period of rapidly accelerated molecular evolution that is of functional relevance to GnRH-Rs in pituitary and extrapituitary sites.
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CA McArdle, J Franklin, L Green, and JN Hislop
A. P. Weetman, C. Green, and L. K. Borysiewicz
We have used the continuously growing FRTL-5 rat thyroid cell line to examine the regulation of major histocompatibility complex (MHC) class II (or la) antigen expression. Of the various stimuli investigated, only the supernatant from activated T cells or recombinant γ-interferon induced Ia expression. All Ia-inducing activity was removed from the T cell supernatant by acid dialysis, suggesting that γ-interferon is the single critical mediator for class II antigen expression. Its action was not TSH dependent but expression of class II antigens increased from the G0-G1 to the S and G2 phases of the cell cycle, so that TSH enhanced Ia expression by its action on cell division. Other agents including lectins, hormones, epidermal growth factor, a calcium ionophore and a phorbol ester did not induce Ia expression. Substances known to inhibit murine macrophage Ia expression (cortisol, prostaglandin E2 and 5-hydroxytryptamine) had no effect on FRTL-5 Ia expression. The use of this thyroid cell line has permitted direct examination of modulators in the absence of any possible effects from contaminating non-thyroid cells present in primary cultures and the results suggest that, of the agents tested, only γ-interferon has significance in the context of Ia antigen expression by the thyroid.
J. Endocr. (1987) 115, 481–487
I. C. GREEN, S. EL SEIFI, D. PERRIN, and S. L. HOWELL
It was possible to vary the replication rate of cells in the islets of Langerhans of adult rats. The rate of incorporation of [3H]thymidine into islet DNA was increased at 12 days of pregnancy to 2·3-fold and at 19 days of pregnancy to 1·3-fold that in control rats. Ovariectomy, which leads to lowered plasma levels of ovarian steroids, induced a significant and unexpected increase in the rate of thymidine incorporation into islets; treatment of ovariectomized rats with 2 μg oestradiol/rat per day for 3 days reversed this upward trend. When islets from normal rats were cultured with certain combinations of steroid hormones including progesterone and oestradiol or with insulin secretagogues, with the exception of glucose, a decreased rate of DNA synthesis was usually found compared with that in control rats.
Since treatment with steroid hormones inhibited incorporation of [3H]thymidine into islets from ovariectomized rats and directly reduced incorporation into tissue-cultured islets from normal rats in vitro, it was concluded that increased levels of steroid hormones were not responsible for the higher rate of regeneration of islet cells in pregnant rats. However, a striking correlation between levels of blood glucose in vivo and DNA synthesis in islets in vitro has been observed.
R. W. Kuhn, A. L. Green, W. J. Raymoure, and P. K. Siiteri
Previous studies utilizing steroid-binding assays have suggested that corticosteroid-binding globulin (CBG)-like glucocorticoid binding sites are present in various tissues of the rat. It is not known, however, whether such binding reflects the intracellular presence of CBG derived from serum or a special class (type III) of receptors. In order to elucidate this problem, immunocytochemical localization of rat CBG was carried out using a specific antiserum prepared against rat serum CBG and the peroxidase-antiperoxidase technique. Positive staining was found in certain cells of the liver, the distal and/or convoluted tubules of the kidney, the uterus, the follicular cells of the thyroid, and some cells of the anterior pituitary. Other tissues including heart, muscle, thymus, hypothalamus, supraoptic and paraventricular nuclei, and diaphragm were negative. The presence of immunoreactive CBG in specific cells of some glucocorticoid-responsive tissues and not others raises interesting questions concerning the transport of glucocorticoids and their mechanism of action.
J. Endocr. (1986) 108, 31–36
Joke Delbaere, Pieter Vancamp, Stijn L J Van Herck, Nele M A Bourgeois, Mary J Green, Richard J T Wingate, and Veerle M Darras
Inactivating mutations in the human SLC16A2 gene encoding the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in the Allan–Herndon–Dudley syndrome accompanied by severe locomotor deficits. The underlying mechanisms of the associated cerebellar maldevelopment were studied using the chicken as a model. Electroporation of an MCT8-RNAi vector into the cerebellar anlage of a 3-day-old embryo allowed knockdown of MCT8 in Purkinje cell precursors. This resulted in the downregulation of the thyroid hormone-responsive gene RORα and the Purkinje cell-specific differentiation marker LHX1/5 at day 6. MCT8 knockdown also results in a smaller and less complex dendritic tree at day 18 suggesting a pivotal role of MCT8 for cell-autonomous Purkinje cell maturation. Early administration of the thyroid hormone analogue 3,5,3′-triiodothyroacetic acid partially rescued early Purkinje cell differentiation. MCT8-deficient Purkinje cells also induced non-autonomous effects as they led to a reduced granule cell precursor proliferation, a thinner external germinal layer and a loss of PAX6 expression. By contrast, at day 18, the external germinal layer thickness was increased, with an increase in presence of Axonin-1-positive post-mitotic granule cells in the initial stage of radial migration. The concomitant accumulation of presumptive migrating granule cells in the molecular layer, suggests that inward radial migration to the internal granular layer is stalled. In conclusion, early MCT8 deficiency in Purkinje cells results in both cell-autonomous and non-autonomous effects on cerebellar development and indicates that MCT8 expression is essential from very early stages of development, providing a novel insight into the ontogenesis of the Allan–Herndon–Dudley syndrome.