Biomedical subjects

A Enjalbert

Publications and source records attributed to A Enjalbert.

At least 55 records · Page 3Linked to original sources

PubMed1997

Measles virus modulates human T-cell somatostatin receptors and their coupling to adenylyl cyclase.

The possible role of immunomodulatory peptide somatostatin (SRIF) in measles virus (MV)-induced immunopathology was addressed by analysis of SRIF receptors and their coupling to adenylyl cyclase in mitogen-stimulated Jurkat T cells and human peripheral blood mononuclear cells (PBMC). SRIF-specific receptors were assayed in semipurified membrane preparations by using SRIF14 containing iodinated tyrosine at the first position in the amino acid chain ([125I]Tyr1) as a radioligand. A determination of receptor number by saturation of radioligand binding at equilibrium showed that in Jurkat cells, MV infection led to a dramatic decrease in the total receptor number. The virus-associated disappearance of one (Ki2 = 12 +/- 4 nM [mean +/- standard error of the mean [SEM]]; n = 4) of two somatostatin binding sites identified in control Jurkat cells (Ki1 = 78 +/- 3 pM and Ki2 = 12 +/- 4 nM [mean +/- SEM]; n = 4) was also observed. Almost identical results were obtained for phytohemagglutinin-activated human PBMC. In the absence of MV infection, two somatostatin binding sites were present (Ki1 = 111 +/- 31 pM and Ki2 = 17 +/- 2 nM [mean +/- SEM]; n = 2), whereas in MV-infected cells, only the high-affinity (Ki1 = 48 +/- 15 pM [mean +/- SEM]; n = 2) binding site remained. In addition, MV infection reinforced the inhibitory effects of SRIF on adenylyl cyclase activity, since maximal inhibition at 1 microM peptide was 11% +/- 4% in control cells versus 25% +/- 3% (P < 0.05) in infected Jurkat cells. Moreover, MV infection severely impaired the capacity of adenylyl cyclase to be activated directly (by forskolin) or indirectly (via Gs protein-coupled vasoactive intestinal peptide receptor). An assessment of [methyl-3H]thymidine incorporation showed that SRIF increased proliferative responses to mitogens only in control cells, not in MV-infected cells. Altogether, our data emphasize that MV-associated alteration of SRIF transduction appears to be related to the loss of SRIF-dependent increase of mitogen-induced proliferation.

Adenylyl Cyclases↗

PubMed1997

Expression of angiotensin II receptor subtypes AT(1A) and AT(1B) in enriched fractions of dispersed rat pituitary cells.

Differential evaluation of angiotensin II (Ang II) receptors (AT1A, AT1B and AT2) expression was performed in dispersed adenohypophyseal cells fractionated by unit gravity sedimentation. Binding of [125I-Sar1-Ile8]-Ang II and its displacement by specific nonpeptidic AT1 (DuP753) and AT2 (PD123319) antagonists was monitored throughout the gradient. Quantification of mRNA levels corresponding to both AT1 receptor subtypes (AT1A and AT1B) was achieved by reverse transcriptase polymerase chain reaction (RT-PCR) amplification in the presence of an AT1 receptor mutant cRNA as internal standard. Fractions were characterised by radioimmunoassay for the five major anterior pituitary hormones and by counting immunocytochemically labelled cells. Quantification of AT1 receptor subtype mRNA levels was also performed in four hypophyseal cell lines secreting prolactin, growth hormone, corticotropin and a gonadotropin subunit. As already described for the whole pituitary, AT1B receptor mRNA is predominantly expressed (80% of total AT1A + AT1B receptor mRNA content), whereas AT1A is expressed at lower level (20%) in dispersed pituitary cells. Most AT1 receptor mRNA and binding co-elute with fractions enriched in lactotropes and corticotropes. In contrast to AT1B, AT1A receptor mRNA is not present in heavier populations of lactotropes or in somatomammotropes. Low AT1B mRNA levels are detected in GH4C1 and in GC cells, two clones which secrete respectively prolactin and growth hormone. In contrast, no AT1 receptor mRNA expression was found in two other cell lines, AtT20 and alphaT3-1, which produce pro-opiomelanocortin and gonadotropin. It is concluded that expression of AT1 receptor subtypes is heterogeneous in different populations of lactotropes and corticotropes.

Animals↗

PubMed1997

Abnormal transduction mechanisms in pituitary adenomas.

Pituitary adenomas are differentiated tumors expressing their appropriate mature hormone. Tumoral cells sometimes present with a defective physiological inhibitory or stimulatory control, resulting in paradoxical responses or nonresponsiveness to regulatory neurohormones. These abnormalities can be explained by defects at the intracellular transduction mechanism level. Knowledge of these defective pathways has made progress in the understanding of the pathogenesis of pituitary adenomas possible. The discovery of mutations of Gs alpha named gsp oncogenes in 40% of human somatotropinomas represents one of the most important advances in this field. Other molecular alterations were identified but are rare and sporadic and the pathogenesis of pituitary adenomas remains largely unknown. Abnormal transduction mechanisms may also result in a variable sensitivity of tumors to pharmacological therapy. The dopamine agonist, bromocriptine, is able to normalize blood PRL levels and to reduce tumor size in the majority of patients with prolactinoma, but is ineffective in 8-15% of them. Under physiological conditions, PRL secretion is under the tonic inhibitory control of dopamine which binds D2 receptors negatively coupled to adenylyl cyclase. Several defects in the dopaminergic transduction pathways participate in this bromocriptine resistance. The mean D2-binding site density is decreased to 50% as compared to responsive tumors. This loss of D2 receptors can account for a lower transcription level of its gene and is accompanied by modifications in the messenger alternative splicing; the D2 short isoform receptor expression decreases preferentially. A reduction in Gi2 alpha protein expression is also observed and is correlated to that of the D2 receptor. Finally, the pituitary-specific transcription factor Pit-1 expression is affected. A highly significant correlation was seen between the D2 receptor mRNA and Pit-1 mRNA levels. These defects observed on many levels of the dopaminergic transduction cascade may be the first steps in the loss of the functional features of differentiated tumors toward more proliferative tumors.

Adenoma↗

PubMed1997

Pit-1 gene expression in human pituitary adenomas.

The anterior pituitary-specific transcription factor Pit-1 (also known as GHF-1) was initially identified and cloned as a transactivator of the GH and PRL genes, and later as a regulator of the TSH beta gene. Analysis of Pit-1 expression during mouse embryogenesis revealed that initiation of its expression correlates both temporally and spatially with activation of its target genes. Immunocytochemical studies revealed a high expression of Pit-1 protein in the nuclei of only three cell types: somatotropes, lactotropes and thyrotropes. The importance of Pit-1 as a regulator of the anterior pituitary development has been further demonstrated by the absence of somatotropes, lactotropes and thyrotropes in the pituitary glands of Pit-1-defective mice and humans. Since Pit-1 is required for both cell phenotype and proliferation, one may ask if this transcription factor might be associated with development of pituitary tumors. Several investigators have reported Pit-1 gene expression in human pituitary adenomas. These studies, while not in total agreement, show that pituitary tumorigenesis does not seem to be associated with a gross alteration of Pit-1 gene expression in humans. Pit-1 transcripts, identical in size and sequence to those observed in normal pituitary, were described in human GH-, PRL- and TSH-secreting pituitary adenomas and in most cases the presence of Pit-1 transcripts correlated with the localization of Pit-1 protein. The biological relevance of Pit-1 expression reported in some nonfunctioning adenomas remains to be clarified. As expression of the PRL and GH genes is ultimately confined to distinct lactotropic and somatotropic populations despite the presence of Pit-1 protein in both cell types, there must be additional mechanisms that control the cell-specific activation of the PRL and GH promoters. The Pit-1 beta isoform, raised through alternative splicing of exon 2 of the Pit-1 gene, is a more potent inducer of GH transcription than the major Pit-1 form. This form could, at least in part, account for the cell-specific activation of the PRL and GH genes. Pit-1 beta was invariably found present in all the tumors expressing the Pit-1 major form, no significant difference in the Pit-1 beta/Pit-1 expression ratio being observed between tumors identified as pure GH- or PRL-producing tumors. This lack of selectivity together with its low level of expression is therefore not in favor of a key role for the beta-isoform in the cell type-specific expression of the GH and PRL genes in humans. The failure of somatotropes, lactotropes and thyrotropes to proliferate in Pit-1-defective mice and humans indicates that Pit-1 might be competent to activate genes required for cell proliferation or survival of the three cell types. Recent data indeed suggest that Pit-1 may directly or indirectly regulate somatotropes and lactotropes through activation of the receptors for GRF and SRIF on the one hand, and dopamine on the other hand. Such regulatory mechanisms could contribute to the differentiation of the somatomammotropic lineage in fully differentiated somatotropic and lactotropic cells.

Adenoma↗

PubMed1997

[Homeoproteins and pituitary adenoma].

Several transactivating factors specifically involved in the differentiation and proliferation of anterior pituitary cell types have been recently identified. Among them Pit-1 a member of the POU-domain transcription factors family is specific of anterior pituitary cells, and was initially identified and cloned as a transactivator of the GH and PRL genes and as a regulator of the TSH beta gene. Pit-1 play a key role during embryogenesis in the differentiation and proliferation of somatotrophs, lactotrophs and thyreotrophs. The importance of Pit-1 as a regulator in the anterior pituitary development has been further demonstrated by naturally occurring mutations or delections in dwarf mouse strains. In the Snell and Jackson dwarf mice, the levels of Pit-1 gene expression are low or undetectable, GH, PRL and TSH beta gene expression are absent and lactotrophs, somatotrophs, and threotrophs fail to proliferate. Furthermore Pit-1 carries out similar functions in humans. This is supported by the fact that children with mutations of the Pit-1 gene present with a congenital combined GH, PRL and TSH deficiency analogous to the phenotype of the Snell and Jackson dwarf mice. In children who were born to healthy consanguinous parents and present such combined deficiencies we recently reported a Pit-1 mutation causing a transition from a Phe to a Cys in a region of the protein known to be involved in DNA binding. Pit-1 transcripts identical in size and sequence to those observed in normal pituitary were described in human GH, PRL and TSH secreting pituitary adenomas. The Pit-1 beta isoform, raised through alternative splicing of exon 2 of the Pit-1 gene, is a more potent inducer of GH transcription than the major Pit-1 form. However no difference in the level of expression of the different Pit-1 isoforms was observed between tumors identified as pure GH or PRL producing tumors. The results support the existence of other transcription factors interacting with Pit-1 to coordinately regulate the activity of the GH and PRL promotors in a cell specific manner. In contrast, variable Pit-1 expression was observed in prolactinomas, according to their sensitivity to bromocriptine treatment. A highly significant correlation was indeed evidenced between the D2 receptors mRNA and the Pit-1 mRNA levels. These results raise the possibility that Pit-1 may either directly or indirectly affect the transcription of the D2 dopaminergic receptor gene. In fact, receptors for other hypothalamic neurohormones such a GHRH and somatostatin are known to be potential Pit-1 target genes. Such mechanisms could be implicated in the differentiation and proliferation of lactotrophs and somatotrophs.

Adenoma↗

PubMed1997

[Congenital multiple anterior pituitary hormone deficiencies. An approach of pituitary ontogenesis].

A number of transcription factors evidenced in the pituitary may play a part in the development of this gland, namely Pit-1, Prop-1, P-Lim, Ptx1, Rpx, Dax-1, SF-1. Several of these factors are involved in animal models of hypopituitarism, while up to now only Pit-1 gene alterations have been shown to be responsible for hypopituitarism in man. These factors are briefly presented, and current data on genetically determined uni- or multi-hormonal pituitary deficiencies are reviewed. In particular, data on combined somatotroph, lactotroph and thyrotroph deficiencies due to Pit-1 gene alterations are detailed. Analysis of phenotype-genotype relationships in this syndrome and in other pathological models of multiple pituitary hormone deficiencies will provide useful information on the complex sequence of events that contribute to the development of the pituitary gland and to differentiation and regulation of the different pituitary cell lines.

Animals↗

PubMed1996

Differential involvement of calcium channels and protein kinase-C activity in GnRH-induced phospholipase-C, -A2 and -D activation in a gonadotrope cell line (alpha T3-1).

The mode of action of GnRH on pituitary gonadotropes involves metabolism of phospholipids, protein kinase-C (PKC) and voltage sensitive Ca2+ channels (VSCC) activation. We have studied the differential role of PKC and VSCC on the coupling of the GnRH receptor with phospholipases-C (PLC), -A2 (PLA2) and -D (PLD) activities in a gonadotrope cell line (alpha T3-1), by measuring the production of inositol phosphates (IPs), arachidonic acid (AA) and phosphatidylethanol (PEt) respectively. We demonstrated that in these cells GnRH stimulated through a specific receptor, IPs formation, a rapid and sustained diacylglycerol generation, consequently AA release and a delayed PEt production in a dose-dependent manner. In contrast to GnRH-induced PLC activity, the PLA2 and PLD stimulation by the neuropeptide involved Ca2+ mobilization via VSCC activation. BAY-K8644 a VSCC agonist significantly potentiated, while the VSCC antagonist nitrendipine markedly inhibited GnRH-induced AA release and PEt production. TPA, a phorbol ester which induced a rapid and important redistribution of PKC, although unable to elicit PLC or PLA2 stimulation, specifically provoked PLD activation in a PKC-dependent but Ca(2+)-independent manner. The PKC stimulation by TPA significantly inhibited the GnRH-stimulated IPs and AA formation, while it potentiated the GnRH-evoked PEt production. This negative feed-back of PKC on GnRH-Induced PLC and PLA2 activities was reversed when PKC was either down regulated after long TPA treatments or inhibited by the PKC inhibitors, staurosporine or GF109203X. The GnRH-induced PEt formation was markedly diminished in PKC depleted cells or after PKC inhibition. Under such conditions, both agonist and antagonist of VSCC became less effective in modulating the remaining GnRH-evoked PEt formation. These results suggest that PKC, in coordination with Ca2+, plays a key role in regulating the cross-talk between the multiple phospholipases implicated in the GnRH signal transduction.

Animals↗

PubMed1996

Rho proteins are localized with different membrane compartments involved in vesicular trafficking in anterior pituitary cells.

In order to explore the role of certain GTP binding proteins in the rat anterior pituitary, we have analyzed the subcellular distribution of the proteins rho and rab. They were found in both membrane and cytosolic fractions. Rab1 and rab2 were localized in both Golgi and endoplasmic reticulum (ER) membranes, while rab4 and rab6 were found in fractions enriched with Golgi and plasma membranes, implicating these proteins in the control of vesicular intracellular trafficking as described in other systems. Rab3 was localized like a fraction of synaptophysin, suggesting a role for rab3 in the targeting of "synaptic-like' microvesicles. We have identified three substrates of C. botulinum exoenzyme C3. A 26-kDa substrate with an isoelectric point (pI) of 5.2, probably rhoB, was localized in the lightest fractions such as rab3 and synaptophysin proteins. Two other 23-24 kDa substrates with pI of 5.5-5.8, probably rhoA and/or rhoC, were found in both fractions enriched with ER and secretory granules. Rho proteins have been implicated in the control of actin polymerization. Their localization in anterior pituitary suggests that rhoB could control the association of synaptic-like microvesicles and plasma membrane, and that rhoA/rhoC could play a role in secretory granule exocytosis; these two pathways being involved in cytoskeleton protein reorganisation in response to extracellular signals.

ADP Ribose Transferases↗

PubMed1996

Alteration of G alpha subunits mRNA levels in bromocriptine resistant prolactinomas.

Patients with prolactinoma are commonly treated with the D2 dopamine agonist bromocriptine, which in most cases, normalizes prolactin (PRL) levels. However, resistance to bromocriptine has been observed in 5 to 18% of tested prolactinomas and is associated to a decrease in both D2 receptor density and mRNA levels. In this study, we used quantitative RT-PCR to investigate whether expression of G alpha proteins could be also modified in bromocriptine resistant prolactinomas. No difference in G alpha o mRNA levels or in the relative expression of G alpha s between bromocriptine sensitive and bromocriptine resistant prolactinomas was observed. In contrast, the relative expression of G alpha i2 was found to be decreased in bromocriptine resistant prolactinomas when compared to that of bromocriptine sensitive prolactinomas. Interestingly, the relative G alpha i2 expression was correlated to both bromocriptine inhibition of in vitro PRL secretion and D2 receptor mRNA levels. Bromocriptine resistance could thus result from a decrease in D2 dopamine receptors associated with a decrease in G alpha i2 expression.

Adenoma↗

PubMed1996

A new mutation of the gene encoding the transcription factor Pit-1 is responsible for combined pituitary hormone deficiency.

The pituitary-specific transcription factor Pit-1/GHF1 regulates the expression of PRL, GH, and TSH beta genes through binding to specific regions of the promoters of these genes. Mutations of the Pit-1 gene have been shown to be responsible for a syndrome of combined pituitary hormone deficiency (CPHD), including complete GH and PRL deficiencies and central hypothyroidism. We studied four siblings presenting with CPHD born to healthy consanguinous parents. All four affected children had complete GH deficiency diagnosed in early childhood. They later developed hypothyroidism and were found to have undetectable PRL levels. The pituitary gland was hypoplastic at magnetic resonance examination in one of the patients. Amplification of genomic DNA and subsequent sequencing of the six exons of the Pit-1 gene allowed identification in the four patients with CPHD of an as yet undescribed mutation in exon 3. A substitution of T go G induced a change from a Phe to a Cys residue at position 135 within the hydrophobic core of the POU-specific DNA-binding domain of the Pit-1 protein. All affected children were homozygous for the mutation, whereas the mother was heterozygous, suggesting a recessive mode of inheritance. Molecular studies in other affected families will allow instructive genotype-phenotype correlations concerning the Pit-1 gene.

Adolescent↗

PubMed1996

Pit-1 gene expression in human lactotroph and somatotroph pituitary adenomas is correlated to D2 receptor gene expression.

The expression of the pituitary-specific transcription factor Pit-1 gene was analyzed in a series of 30 human lactotroph and somatotroph pituitary tumors. Northern blot analysis failed to reveal any quantitative differences in Pit-1 gene expression between somatotroph and lactotroph tumors, and reverse transcription-PCR analysis showed similar patterns of Pit-1 isoforms expression in both populations of tumors. The expression of the D2 receptor gene was subsequently analyzed in the same adenomas. In the prolactinomas, which presented with a variable sensitivity to dopamine agonist treatment, the intensity of the D2 receptor transcripts (2.8 kilobases) was variable and was related to the sensitivity to the dopamine agonist treatment. Notably, the individual D2 receptor messenger ribonucleic acid (mRNA) levels were highly correlated to the Pit-1 mRNA levels measured in the same tumors (r = 0.90; P < 0.0001). In the GH-secreting tumors, a significant expression of the D2 receptor gene was evidenced by Northern blot in all mixed somato-lactotroph adenomas and in some of the pure somatotroph adenomas; again, a positive correlation was found between D2 mRNA and Pit-1 mRNA levels (r = 0.68; P < 0.01). These results suggest the existence of mechanisms responsible for a coordinate control of Pit-1 and D2 receptor genes that remain to be determined.

Adenoma↗

PubMed1996

Prolactinomas resistant to bromocriptine: long-term efficacy of quinagolide and outcome of pregnancy.

Resistance to bromocriptine, defined as the absence of normalization of prolactin (PRL) levels despite a 15-30 mg daily dose of bromocriptine during at least 6 months, has been observed in 5-17% of the prolactinomas according to the literature. The recent availability of a new potent dopamine agonist, quinagolide, prompted us to analyze its long-term therapeutic effects in 28 patients with prolactinomas resistant to bromocriptine. Before bromocriptine, their PRL levels were 520 +/- 185 micrograms/l (mean +/- SEM) and decreased to 291 +/- 154 micrograms/l after a 6-21 month period of bromocriptine treatment. All the women (N = 20) remained amenorrheic and hypogonadism was not improved in men (N = 8). Subsequently, after 1 year of 150-300 micrograms/day quinagolide, 12/28 patients of the present series recovered normal gonadal function and their initial mean baseline PRL value (404 +/- 180 micrograms/l) was 16 +/- 2 micrograms/l after 1 year of treatment. A significant tumor shrinkage was observed in 5/8 macroadenomas (62%). During the 3-year follow-up period under quinagolide, a similar good control was achieved in these patients, with the exception of one man presenting with a secondary rise of PRL under quinagolide. In contrast, 15 other patients (one patient interrupted quinagolide at 6 months because of poor tolerance) were not normalized under 150-450 micrograms/day quinagolide. Their initial PRL levels (606 +/- 298 micrograms/l) were reduced to 343 +/- 187 micrograms/l (versus 463 +/- 265 micrograms/l under bromocriptine after the same duration of treatment). Despite such a partial inhibitory effect of quinagolide, 7/12 women resumed menstrual cycles and three pregnancies occurred. In no case was any tumor shrinkage noticed during the 3-4-year follow-up. Three patients even presented, after 2 years of quinagolide treatment, with a secondary rise of PRL values associated with a further tumor growth in two patients. During the 3-year follow-up period, nine pregnancies occurred in seven women. In five women, after quinagolide withdrawal, the plasma PRL baseline values ranged from 52 to 158 micrograms/l and from 65 to 192 micrograms/l, respectively, at the first trimester and at the end of uneventful pregnancies. In contrast, in two women a rapid increase of PRL (240-400 micrograms/l) correlated with tumor growth during the first trimester. Such a tumor progression was blocked by quinagolide treatment but not by bromocriptine. These data, although observed in a limited series, justify the careful follow-up of pregnancies in this subclass of patients at risk. Finally, in the whole population, long-term control of hyperprolactinemia by quinagolide was obtained in 11/28 patients (39%) previously resistant to bromocriptine, and 15/20 women (75%) resumed normal gonadal function with a quinagolide daily dose of 300 micrograms in most of them.

Aminoquinolines↗

PubMed1995

Effect of essential fatty acid deficiency on membrane fatty acid content and growth hormone stimulation of rat pituitaries during postnatal development.

Fatty acid composition of anterior pituitary cell membranes of rats deprived of essential fatty acids (EFA) and of rats receiving a standard diet was determined during postnatal development and in adults. Pregnant rats were fed an EFA-deficient diet and the offspring were fed the same diet after weaning. In parallel, effects of the diet on growth and on growth hormone (GH) responsiveness to GHRH stimulation were determined in control animals. Membrane content of arachidonic acid (20:4n-6) and of its elongation product adrenic acid (22:4n-6) increased regularly from day 2 to day 12 after birth. EFA-deficiency resulted on day 2 in increased oleic acid and in substitution of arachidonic and adrenic acids by corresponding elongation-desaturation products of oleic acid: eicosatrienoic (20:3n-9) and docosatrienoic (22:3n-9) acids. At the age of 24 days, n-9 series fatty acid reached the same level as in adult animals. Two-day-old EFA-deficient rats paradoxically exhibited a higher level of 20:4n-6 as compared to control rats. EFA-deficiency also decreased growth rate and GH pituitary responses to GHRH during the prepubertal period. These results suggest that changes in the lipid structure and in pituitary secretion properties elicited by EFA-deficiency depend upon the stage of development.

Animals↗

PubMed1994

Angiotensin II effects on second messengers involved in prolactin secretion are mediated by AT1 receptor in anterior pituitary cells.

The two forms of angiotensin II (Ang II) receptors, AT1 and AT2 subtypes, have been demonstrated in many other cells beside the anterior pituitary cells. Attempting to investigate the subtype(s) of Ang II receptors implicated in the multiple transduction mechanisms involved in Ang II stimulation of prolactin (PRL) release by lactotropes, we studied the effect of selective nonpeptidergic Ang II antagonists on the PRL release, adenylate cyclase (AC), and phospholipase C activities. In intact cells, the AT1 antagonist DuP753 blocked Ang II-induced PRL release, reversed in a dose dependent manner Ang II-evoked inositol phosphates production, and inhibited completely the PLC and protein kinase C (PKC) dependent cAMP accumulation induced by Ang II. In membrane preparations, the Ang II receptors were negatively coupled to AC. The AT1 antagonist blocked in a dose dependent manner the inhibitory effect of Ang II on cAMP production. In intact cells, the negative coupling of Ang II receptor with AC was observed only when PKC was down regulated by long term 12-O-tetradecanolylphorbol-13-acetate pretreatment. Ang II was able to inhibit vasoactive intestinal peptide-induced cAMP accumulation, a response which was also prevented by DuP753. The different coupling of Ang II receptor described above implicated only the AT1 type receptor since the AT2 antagonists (PD123177 and PD123319) were ineffective at any doses tested (10(-8) to 10(-5) M). The obtained results indicate that the regulation of PRL secretion involves the AT1 receptor subtype and that this receptor might be coupled to multiple effectors.

Angiotensin I↗

PubMed1994

Epidermal growth factor treatment induces D2 dopamine receptors functionally coupled to delayed outward potassium current (IK) in GH4C1 clonal anterior pituitary cells.

GH4C1 cells, a clonal cell line from a rat pituitary tumor, have been widely used as a model to study the regulation of prolactin secretion. These cells, however, do not express dopamine D2 receptors and are therefore not suitable for exploring mechanisms involved in dopamine inhibition of prolactin secretion. The recent demonstration that epidermal growth factor (EGF) is able to induce functional expression of D2 receptors in GH3 cells, a parental clonal cell line, overcomes this difficulty. We have thus undertaken an electrophysiological study in order to check whether coupling of D2 receptors to K+ channels could be restored in that model. Effects of dopamine on the non-inactivating voltage-dependent outward K+ current (IK) were investigated both in control and in EGF-treated GH4C1 cells. The K+ current was not modified by EGF treatment alone. In control cells, IK measured before and during dopamine application was unchanged. In contrast, dopamine application markedly enhanced the K+ current in cells that had previously been exposed to EGF. The effect was mimicked by the specific D2 receptor agonist bromocriptine and blocked by sulpiride, a D2 receptor antagonist, thus indicating that the effect of dopamine was effectively due to the activation of D2 receptors. These results bring further evidence that EGF-induced D2 receptors in clonal strains from rat pituitary tumors are functional and are coupled to the delayed outward K+ current IK.

Animals↗

PubMed1994

Decreased expression of the two D2 dopamine receptor isoforms in bromocriptine-resistant prolactinomas.

Bromocriptine or other dopamine agonists are usually effective for the treatment of prolactin-secreting adenomas. Five to 18% of prolactinomas, however, do not respond to such therapy. We have shown previously that such resistance to bromocriptine correlates with reduced binding to the D2 receptor subtype of dopamine, the major PRL inhibiting factor. In the present work, we demonstrated that reduced binding actually corresponds to decreased expression of the gene coding for the D2 receptor in the pituitary from bromocriptine-resistant patients, as shown by 4-fold lower levels of the corresponding mRNAs compared to those coding for actin. The existence of two D2 receptor isoforms, D2S and D2L generated by alternative splicing, has been described in several tissues, including the pituitary. Both are negatively coupled to adenylyl cyclase and inhibit prolactin secretion, but, in addition, the shortest one (D2S) is more efficiently coupled to phospholipase C. Consequently, we also investigated whether expression of a particular D2 receptor isoform was preferentially affected in resistant adenomas. The proportion of messengers corresponding to the short receptor isoform (D2S) was lower in resistant compared to responsive adenomas: D2S/D2L = 0.74 +/- 0.08 and 1.00 +/- 0.07, respectively. In parallel, much lower levels of D2 receptor mRNAs were found in growth hormone-secreting adenomas, with a D2S/D2L ratio comparable to those of both normal human pituitary and bromocriptine-sensitive prolactinomas (1.05 +/- 0.11). Thus, resistance to bromocriptine therapy seems to involve defects in D2 dopamine receptor expression and possibly in posttranscriptional splicing.

Adenoma↗

PubMed1994

Selective effect of a diet-induced decrease in the arachidonic acid membrane-phospholipid content on in vitro phospholipase C and adenylate cyclase-mediated pituitary response to angiotensin II.

Young rats were fed on an essential fatty acid (EFA)-deprived diet for 6 weeks after weaning. Their pituitary was removed and adenohypophyseal cells dispersed and maintained in culture. Membrane lipids were analyzed and basal and stimulated levels of hormone secretion were measured after 4-day incubation in a culture medium containing or not 160 microM arachidonic acid 20:4n-6 (AA) in order to obtain EFA-deficient or EFA-restored pituitary cells, respectively. In EFA-deficient cells membrane phosphoglycerides (PGL) were depleted in AA and adrenic acid 22:4n-6; the deficit was overcome by incubation in the presence of AA. Depletion diversely affected PGL classes. AA was highly depleted in choline phosphoglycerides (ChoPG), only moderately depleted in serine and ethanolamine phosphoglycerides (SerPG and EtnPG) and not depleted at all in inositol phosphoglycerides, suggesting preferential preservation of AA in that class of PGL. Restoration of AA by addition of the fatty acid to the culture medium was complete for ChoPG and EtnPG and only partial for SerPG. Depressed levels of AA and adrenic acid in PGL were compensated for by a concomitant increase in 20:3n-9 and 22:3n-9. Growth hormone and prolactin (PRL) secretion was assessed by radioimmunoassay and possible effects of a membrane AA deficit on hormone regulation were tested in cells challenged by either growth hormone-releasing hormone, thyrotropin-releasing hormone, angiotensin II (AII), vasoactive intestinal peptide (VIP) or dopamine. Neither basal nor stimulated growth hormone secretion was different from controls in EFA-deficient cells. PRL modulation by VIP or dopamine was not affected either in EFA-deficient cells. In contrast, the capacity of AII, but not of thyrotropin-releasing hormone, to release PRL was markedly decreased in EFA-deprived cells. It was restored by addition of AA to the incubation medium. Parallel depression of AII-induced inositol phosphates and cAMP accumulation was also observed after EFA deficiency. When tested on membranes, the paradoxical inhibition of adenylate cyclase by AII documented by previous observations was reinforced in EFA-deficient membranes. In contrast, binding of AII was not affected by EFA deficiency. It is concluded that under our experimental conditions EFA deficiency affects selectively coupling of the AII receptor to its effectors without alteration of binding. The effect could involve changes in receptor interactions with coupling proteins.

Adenylyl Cyclases↗

PubMed1994

Pit-1 gene expression in the human pituitary and pituitary adenomas.

The cellular basis for pituitary neoplasia is poorly understood. The POU domain protein Pit-1 is a pituitary-specific transcription factor involved in the generation, differentiation, and proliferation of three pituitary cell types: lactotrophs, somatotrophs, and thyrotrophs. In this study, we analyzed the expression of Pit-1 gene in a series of 15 different human pituitary tumors and compared it with that observed in normal tissue. Pit-1 transcripts, identical in size (2.4 and 4.5 kilobases) and sequence to those observed in normal tissue were evidenced in PRL-, GH-, and TSH-secreting tumors. Pit-1 is overexpressed (2.5- to 5-fold) in the PRL- and GH-secreting tumors, but to an extent consistent with the predominant cellular type of these adenomas. An isoform of Pit-1, with an insertion of 26 amino acids in the trans-activation domain as a result of alternative splicing, is also present in both normal and tumoral tissues. It is concluded that human pituitary tumorigenesis does not seem to be associated with a gross alteration of Pit-1 gene expression.

Adenoma↗