C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family, has been found at its highest tissue concentrations in the anterior pituitary, where it is localised in gonadotrophs. Its specific guanylyl cyclase-containing receptor, GC-B, is also expressed on several anterior pituitary cell types, and CNP potently stimulates cGMP accumulation in rat pituitary cell cultures and pituitary cell lines. The mouse gonadotroph-derived alpha T3-1 cell line has been shown to express CNP as well as GC-B (but not GC-A) receptors, suggesting that CNP may well be an autocrine regulator of gonadotrophs. Comparing effects of three natriuretic peptides (atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP) and CNP) on cGMP accumulation in four pituitary cell lines (alpha T3-1, TtT-GF, AtT-20 and GH(3)) we find that CNP is most potent and effective in alpha T3-1 cells. In these cells, CNP-stimulated cGMP accumulation was found to desensitise during a 30 min exposure to CNP. Pretreatment with CNP for up to 6 h also caused a significant reduction in the ability of CNP to subsequently stimulate cGMP accumulation. This effect was receptor specific, because pretreatment with sodium nitroprusside (an activator of nitric oxide-sensitive guanylyl cyclase), or with ANP or BNP, did not cause desensitisation of CNP-stimulated cGMP accumulation. Protein kinase C activation with phorbol esters also inhibited CNP-stimulated cGMP accumulation and such inhibition was also seen in cells desensitised by pretreatment with CNP. Thus it appears that the endogenous GC-B receptors of alpha T3-1 cells are subject to both homologous and heterologous desensitisation, that the mechanisms underlying these forms of desensitisation are distinct, and that cGMP elevation alone is insufficient to desensitise GC-B receptors.
J Franklin, J Hislop, A Flynn and CA McArdle
Gonadotrophin-releasing hormone receptors (GnRH-Rs) are found in cancers of reproductive tissues, including those of the prostate, and gonadotrophin-releasing hormone (GnRH) can inhibit growth of cell lines derived from such cancers. Although pituitary and extra-pituitary GnRH-R transcripts appear identical, their functional characteristics may differ. Most extra-pituitary GnRH-Rs have low affinity for GnRH analogues and may not activate phospholipase C or discriminate between agonists and antagonists in the same way as do pituitary GnRH-Rs. Here we have assessed whether GnRH-Rs expressed exogenously in prostate cancer cells differ functionally from those of gonadotrophs. We found no evidence for endogenous GnRH-Rs in PC3 cells, but after infection with adenovirus expressing the GnRH-R (Ad GnRH-R) at 10 plaque forming units (p.f.u.)/cell or greater, at least 80% of the cells expressed GnRH-Rs. These sites had high affinity (K(d )for [(125)I]Buserelin 1.1+/-0.4 nM) and specificity (rank order of potency: Buserelin> GnRH>>chicken (c) GnRH-II), and mediated stimulation of [(3)H]inositol phosphate (IP) accumulation. Increasing viral titre from 3 to 300 p.f.u./cell increased receptor number (2000 to 275 000 sites/cell respectively) and [(3)H]IP responses. GnRH also caused a biphasic increase in the cytoplasmic Ca(2+) concentration in Ad GnRH-R-infected cells but not in control cells. Mobilization of Ca(2+) from intracellular stores contributed to the spike phase of this response whereas the plateau phase was dependent upon Ca(2+) entry across the plasma membrane. This effect on Ca(2+) and stimulation of [(3)H]IP accumulation were both blocked by the GnRH-R antagonist, Cetrorelix. In addition, GnRH reduced cell number (as measured in MTT activity assays) and DNA synthesis (as measured by [(3)H]thymidine incorporation) in Ad GnRH-R-infected cells (but not in control cells). This effect was mimicked by agonist analogues and inhibited by two antagonists. Thus, when exogenous GnRH-Rs are expressed at a density comparable to that in gonadotrophs, they are functionally indistinguishable from the endogenous GnRH-Rs in gonadotrophs. Moreover, expression of high affinity GnRH-Rs can facilitate a direct anti-proliferative effect of GnRH agonists on prostate cancer cells.
CA McArdle, J Franklin, L Green and JN Hislop
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.
B Williams, AN Brooks, TC Aldridge, WD Pennie, R Stephenson and CA McArdle
GnRH acts via phospholipase C (PLC) activating G-protein coupled receptors to stimulate secretion of gonadotrophins from gonadotrophs. These cells are also regulated by gonadal steroids, which act centrally to influence GnRH secretion, and peripherally to modulate GnRH action. We have shown that oestradiol can stimulate proliferation and modulate GnRH-stimulated [(3)H]inositol phosphate ([(3)H]IP(x)) accumulation (used as a measure of PLC activity) in a gonadotroph-derived cell line (alphaT3-1). Here we show that when alphaT3-1 cells were incubated in medium with 2% foetal calf serum (FCS), [(3)H]thymidine incorporation was not stimulated by oestradiol but was reduced to <2% of control by the oestrogen antagonist, raloxifene. The inhibitory effect of 10 or 1000 nM raloxifene was reversed competitively by oestradiol. A similar pattern of effects was seen when effects of oestradiol and raloxifene on the proportion of cells in the S-phase of the cell cycle (as measured by flow cytometry of propidium iodide-labelled cells) and on oestrogen receptor activity (as measured by trans-activation of the oestrogen-response elements in the vitellogenin promoter) were quantified. In addition, RT-PCR revealed expression of alpha and beta (but not beta2) subtypes of oestrogen receptors. Thus, oestrogen is an essential mitogen for alphaT3-1 cells, its mitogenic effect is oestrogen receptor mediated and is associated with a marked alteration of cell cycle distribution, and the full extent of these effects are best revealed in the presence of raloxifene. Using this strategy, we found that cells cultured for 4 days with 10 nM raloxifene expressed GnRH receptors (K(d) for (125)I-buserelin 4.33 nM) and that their activation by GnRH caused a concentration-dependent increase in [(3)H]IP(x) (in cells labelled with [(3)H]inositol) and inositol 1,4,5 trisphophate (in unlabelled cells). Addition of 10 nM oestradiol (to overcome receptor blockade by raloxifene) reduced GnRH receptor number by 31% but increased maximal effects on [(3)H]IP(x) and Ins(1,4,5)P(3) approximately 4-fold. The effects of oestradiol on GnRH receptor number and signalling were not, however, mimicked by culture for 2 days in medium with 10% FCS and the S-phase blocker, thymidine (15 mM). This treatment increased the proportion of cells in the S-phase 2- to 3-fold but did not alter GnRH receptor number or signalling. Other treatments which altered cell cycle transition (hydroxyurea, colcemid, methotrexate) also failed to alter GnRH receptor number or signalling and no correlation was seen between GnRH receptor number or GnRH-stimulated [(3)H]IP(x) accumulation and the proportion of cells in the S-phase or G2/M-phases of the cell cycle. Thus, oestradiol has pronounced effects on GnRH signalling, proliferation and cell cycle distribution in alphaT3-1 cells, but these trophic effects do not underlie the modulation of GnRH signalling.