Direct promoter induction of p19Arf by Pit-1 explains the dependence receptor RET/Pit-1/p53-induced apoptosis in the pituitary somatotroph cells

Somatotrophs produce growth hormone (GH) and are the most abundant secretory cells of the pituitary. Somatotrophs express the transcription factor Pit-1 and the dependence receptor RET, its co-receptor GFRa1 and ligand GDNF. Pit-1 is a transcription factor essential for somatotroph proliferation and differentiation and for GH expression. GDNF represses excess Pit-1 expression preventing excess GH. In the absence of GDNF, RET behaves as a dependence receptor, becomes intracellularly processed and induces strong Pit-1 expression leading to p53 accumulation and apoptosis. How accumulation of Pit-1 leads to p53 expression is unknown. We have unveiled the relationship of Pit-1 with the p19Arf gene. There is a parallel correlation of RET processing, Pit-1 increase and ARF protein and mRNA expression. Interfering the pathway with RET, Pit-1 or p19Arf siRNA blocked apoptosis. We have found a Pit-1 DNA-binding element within the ARF promoter. Pit-1 directly regulates the CDKN2A locus and binds to the p19Arft promoter inducing p19Arf gene expression. The Pit-1-binding element is conserved in rodents and humans. RET/Pit-1 induces p19Arf/p53 and apoptosis not only in a somatotroph cell line but also in primary cultures of pituitary somatotrophs, where ARF siRNA interference also blocks p53 and apoptosis. Analyses of the somatotrophs in whole pituitaries supported the above findings. Thus Pit-1, a differentiation factor, activates the oncogene-induced apoptosis (OIA) pathway as oncogenes exerting a tight control in somatotrophs to prevent the disease due to excess of GH (insulin-resistance, metabolic disease, acromegaly).

Identification of a paired box gene 8-peroxisome proliferator-activated receptor gamma (PAX8-PPARgamma) rearrangement mosaicism in a patient with an autonomous functioning follicular thyroid carcinoma bearing an activating mutation in the TSH receptor

Our main objective was to search for mutations in candidate genes and for paired box gene 8-peroxisome proliferator-activated receptor gamma (PAX8-PPARgamma) rearrangement in a well-differentiated angioinvasive follicular thyroid carcinoma (FTC) causing hyperthyroidism. DNA and RNA were extracted from the patient's thyroid tumor, as well as 'normal' thyroid tissue, and from peripheral blood lymphocytes (PBLs) of the patient, her daughter, and two siblings. Nuclear isolation was extracted from the patient's tumor, 'normal' thyroid tissue, PBLs, and uterine leiomyoma tissue. TSH receptor (TSHR), RAS, and BRAF genes were sequenced. We searched for PAX8-PPARgamma in thyroid, PBL, and uterine leiomyoma samples from the patient and family members. Proliferative effects of detected mutants on non-transformed human thyrocytes cultures. An activating TSHR mutation, M453T, was detected in the tumor. PAX8 (exons 1-8+10)-PPARgamma was found in all tested patient's tissues. A second rearrangement, PAX8 (exons 1-8)-PPARgamma, was detected in the patient's normal thyroid tissue. Under deprived medium condition, co-transfection of PAX8-PPARgamma and TSHR-M453T dramatically increased the number of thyrocytes, an effect that it was not observed with TSHR wild-type (WT); under complete medium conditions, co-transfection of PAX8-PPARgamma with either TSHR-M453T or TSHR-WT inhibited cell proliferation. We report a patient with hyperthyroidism due to a FTC bearing an activating TSHR mutation and PAX8-PPARgamma rearrangements. PAX8-PPARgamma was present as a mosaicism affecting tissues from endodermal and mesodermal origin. PAX8-PPARgamma and TSHR-M453T inhibited or promoted thyrocyte proliferation depending on medium conditions. The activating TSHR mutation could promote in vivo FTC development in PAX8-PPARgamma-positive thyrocytes under poor blood supply with deprivation of growth factors but restraint the tumor growth when growth factors are supplied.

New insights into thyroglobulin pathophysiology revealed by the study of a family with congenital goiter

CONTEXT

Thyroglobulin (TG) gene mutations cause congenital hypothyroidism (CH) with goiter. A founder effect has been proposed for some frequent mutations. Mutated proteins have a defect in intracellular transport causing intracellular retention with ultrastructural changes that resemble an endoplasmic reticulum storage disease.

OBJECTIVE

To reveal new aspects of thyroglobulin pathophysiology through clinical, cellular, molecular, and genetic studies in a family presenting with CH due to TG mutations from Galicia, an iodine-deficient area of Spain.

DESIGN

The included clinical evaluation of family members, DNA sequencing for TG gene mutation and haplotyping analysis, ultrastructural analysis of thyroid tissue specimens from affected subjects, analysis of effects of mutations found on TG gene transcription, and in vitro studies of cellular production and secretion of mutated proteins.

SETTING

Locations included primary care and university hospitals.

RESULTS

Family members with CH, mental retardation, and goiter were compound heterozygous for c.886C-->T (p.R277X) and g.IVS35+1delG. For c.886C-->T, a founder effect cannot be excluded, and its transcription was hardly detectable. g.IVS35+1delG caused an in-frame deletion in exon 35 and produced a protein that, although synthesized, could not be secreted. Ultrastructural analyses showed morphological changes consistent with an endoplasmic reticulum storage disease.

CONCLUSION

The shorter thyroglobulin resulting from the novel g.IVS35+1delG was retained within the endoplasmic reticulum of thyrocytes, and together with p.R227X caused severe hypothyroidism with goiter. p.R277X, the most commonly described TG mutation, is caused by a TG exon-7 highly mutation-prone region, and the possibility that some cases were introduced to South America from Galicia cannot be excluded.

GHRH proliferative action on somatotrophs is cell-type specific and dependent on Pit-1/GHF-1 expression

To investigate the mechanisms by which the hypothalamic peptide GHRH influences cell division, we analyzed its effects on the proliferation of two different cell lines: CHO-4, an ovary-derived cell line, and GH3, a pituitary-derived cell line. We found that GHRH induces the proliferation of pituitary-derived cells but inhibits the proliferation of ovary-derived cells. We further characterized this dual effect of GHRH to find that the cytoplasmic signals induced by this hormone are similar in both cell lines. Moreover, in CHO-4 cells GHRH stimulates two well-known positive cell cycle regulators, c-myc and cyclin D1, but is unable to induce the degradation of the negative cell cycle regulator p27(Kip1). Significantly, when the Pit-1/GHF-1 gene is exogenously expressed in CHO-4 cells, the negative effect of GHRH on the proliferation of these cells is attenuated. Furthermore, when the levels of Pit-1 are downregulated by siRNA in GH3-GHRHR cells, the positive effects of GHRH on the proliferation of these cells are diminished. These findings add to our understanding of the molecules involved in the regulation of cell proliferation by GHRH, as we demonstrate for the first time that Pit-1 is not only required to drive the expression of the GHRH receptor, as previously described, but is also needed for the downstream effects that occur after its activation to modulate cell proliferation. These data suggest that the regulation of cell proliferation in response to a specific growth factor depends in certain cell populations on the presence of a tissue-specific transcription factor.

Ghrelin localization in the medulla of rat and human adrenal gland and in pheochromocytomas

OBJECTIVE

Ghrelin is predominantly produced by neuroendocrine cells of stomach and has been expressed in several normal and tumour endocrine tissues. It has been reported that the localization of ghrelin is exclusively in the cortex of human and rat adrenal gland and in adrenocortical tumours. This prompted us to analyze the expression of this peptide in medulla of human and rat adrenal glands and in human pheochromocytomas.

DESIGN AND METHODS

Analysis of ghrelin mRNA expression in rat adrenal gland was conducted by means of semi-quantitative RT-PCR. Ghrelin localization was studied in medulla of human and rat adrenal gland by immunohistochemistry. In addition, we have carried out a double immunofluorescence with chromogranin A to determine the specific cell type expressing ghrelin immunoreactivity. Ghrelin expression was also analyzed in five cases of pheochromocytoma by immunohistochemistry. Finally, Western blotting analysis was performed with goat ghrelin antibody in the cortex and in the medulla of rat adrenal gland.

RESULTS

RT-PCR demonstrated expression of ghrelin mRNA in rat adrenal gland. We also detected ghrelin expression in virtually all rat pheochromocytes by immunohistochemistry and double immunofluorescence. Furthermore, we showed ghrelin immunoreactivity in the medulla of human adrenal gland and in pheochromocytomas. By Western blotting, we found the expression of ghrelin precursor, proghrelin and mature ghrelin in the medulla of rat adrenals. However, the cortex of rat adrenal gland only expressed ghrelin precursor.

CONCLUSIONS

Our study is the first to demonstrate a medullar expression of ghrelin in human and rat adrenal gland; we also showed ghrelin expression in pheochromocytomas.

Cellular distribution and regulation of ghrelin messenger ribonucleic acid in the rat pituitary gland

Ghrelin, a 28-amino-acid acylated peptide, strongly stimulates GH release and food intake. In the present study, we found that ghrelin is expressed in somatotrophs, lactotrophs, and thyrotrophs but not in corticotrophs or gonadotrophs of rat pituitary. Persistent expression of the ghrelin gene is found during postnatal development in male and female rats, although the levels significantly decrease in both cases from pituitaries of 20-d-old rats onward, but at 60 d old, the levels were higher in male than female rats. This sexually dimorphic pattern appears to be mediated by estrogens because ovariectomy, but not orchidectomy, increases pituitary ghrelin mRNA levels. Taking into account that somatotroph cell function is markedly influenced by thyroid hormones, glucocorticoids, GH, and metabolic status, we also assessed such influence. We found that ghrelin mRNA levels decrease in hypothyroid- and glucocorticoid-treated rats, increase in GH-deficient rats (dwarf rats), and remain unaffected by food deprivation. In conclusion, we have defined the specific cell types that express ghrelin in the rat anterior pituitary gland. These data provide direct morphological evidence that ghrelin may well be acting in a paracrine-like fashion in the regulation of anterior pituitary cell function. In addition, we clearly demonstrate that pituitary ghrelin mRNA levels are age and gender dependent. Finally, we show that pituitary ghrelin mRNA levels are influenced by alteration on thyroid hormone, glucocorticoids, and GH levels but not by fasting, which indicates that the regulation of ghrelin gene expression is tissue specific.

Localization of growth hormone receptors in rat and human thyroid cells

Growth hormone (GH) exerts its multiple actions by binding to a specific receptor (GHR) widely distributed in the organism. It is well established that, in acromegaly, the thyroid gland is larger than normal and that GH increases triiodothyronin concentrations and decreases those of tetraiodothyronin (thyroxine). The aim of the present study was to analyze the presence of GHR and its mRNA in rat and human thyroid gland by Western blot, in situ hybridization techniques, and immunohistochemistry. A band of the expected size for GHR was shown in rat and human thyroid by Western blot. GHR immunoreactivity was found in virtually all follicles. The signal was mainly localized in the cytoplasm, although a nuclear positivity was also found. In situ hybridization techniques demonstrated the presence of GHR messenger RNA in the thyroid gland (cytoplasm of the follicular cells). These results provide direct morphological evidence that GHR is localized in the thyroid gland of mammals and opens up the possibility that GH regulates thyroid cell function directly or via local autocrine or paracrine production of insulin-like growth factor I.

Hormonal control of growth hormone secretion

Growth hormone secretion by the somatotroph cells depends upon the interaction between hypothalamic regulatory peptides, target gland hormones and a variety of growth factors acting in a paracrine or autocrine fashion. This review will be focused on recent data regarding the mechanism by which growth hormone-releasing hormone (GHRH) influences somatotroph cell function and the physiological role played by Ghrelin and leptin in the regulation of growth hormone (GH) secretion. It is well established that binding of GHRH to its receptor leads to activation of protein kinase A (PKA). More recently, it was found that GHRH can also activate mitogen-activated protein (MAP) kinase both in pituitary cells and in a cell line overexpressing the GHRH receptor. Whether somatotroph adenomas, either with or without a GS-alpha mutation, have alterations in some of the components of the activation of the MAP kinase pathway remains to be known. The recent isolation of Ghrelin, the endogenous ligand of the growth hormone secretagogue receptor, can be considered a landmark in the GH field, which opens up the possibility of gaining greater insight into our understanding of the mechanisms involved in the regulation of GH secretion and somatic growth. Indeed, preliminary evidences indicate that this peptide exerts a marked stimulatory effect on plasma GH levels in both rats and humans. Finally, it is well known that GH secretion is markedly influenced by nutritional status. Leptin has emerged as an important adipose tissue-generated signal that is involved in the regulation of GH secretion, thus providing an integrated regulatory system of growth and metabolism. Although the effects of leptin on GH secretion in humans remain to be clarified, indirect evidences indicate that it may play an inhibitory role.

Regulation of Pit-1 expression by ghrelin and GHRP-6 through the GH secretagogue receptor

GH secretagogues are an expanding class of synthetic peptide and nonpeptide molecules that stimulate the pituitary gland to secrete GH through their own specific receptor, the GH-secretagogue receptor. The cloning of the receptor for these nonclassical GH releasing molecules, together with the more recent characterization of an endogenous ligand, named ghrelin, have unambiguously demonstrated the existence of a physiological system that regulates GH secretion. Somatotroph cell-specific expression of the GH gene is dependent on a pituitary-specific transcription factor (Pit-1). This factor is transcribed in a highly restricted manner in the anterior pituitary gland. The present experiments sought to determine whether the synthetic hexapeptide GHRP-6, a reference GH secretagogue compound, as well as an endogenous ligand, ghrelin, regulate pit-1 expression. By a combination of Northern and Western blot analysis we found that GHRP-6 elicits a time- and dose-dependent activation of pit-1 expression in monolayer cultures of infant rat anterior pituitary cells. This effect was blocked by pretreatment with actinomycin D, but not by cycloheximide, suggesting that this action was due to direct transcriptional activation of pit-1. Using an established cell line (HEK293-GHS-R) that overexpresses the GH secretagogue receptor, we showed a marked stimulatory effect of GHRP-6 on the pit-1 -2,500 bp 5'-region driving luciferase expression. We truncated the responsive region to -231 bp, a sequence that contains two CREs, and found that both CREs are needed for GHRP-6-induced transcriptional activation in both HEK293-GHS-R cells and infant rat anterior pituitary primary cultures. The effect was dependent on PKC, MAPK kinase, and PKA activation. Increasing Pit-1 by coexpression of pCMV-pit-1 potentiated the GHRP-6 effect on the pit-1 promoter. Similarly, we showed that the endogenous GH secretagogue receptor ligand ghrelin exerts a similar effect on the pit-1 promoter. These data provide the first evidence that ghrelin, in addition to its previously reported GH-releasing activities, is also capable of regulating pit-1 transcription through the GH secretagogue receptor in the pituitary, thus giving new insights into the physiological role of the GH secretagogue receptor on somatotroph cell differentiation and function.

GDNF and RET-gene expression in anterior pituitary-cell types

Glial cell line-derived neurotrophic factor (GDNF) is expressed in many neuronal and non-neuronal tissues during development as well as in adult animals. GDNF signaling is mediated through a two-component system consisting of the so called GDNF receptor-alfa (GFRalpha1) which binds to GDNF. Thereafter this complex binds to and activates the tyrosine kinase receptor RET. In this work, for the first time, we have characterized the expression of both GDNF and RET in the anterior pituitary. First of all, RT-PCR analysis, Western blot and immunohistochemistry of the whole anterior pituitary showed that GDNF, GFRalpha1 and RET are expressed in this gland. Following double-immunofluorescence of consecutive sections we found GDNF immunoreactivity in most cell types, and it was most abundant in corticotrophs (55%), LH (59%) and FSH-producing cells (81%). In contrast, while the majority of somatotrophs (87%) were stained for RET, no positive immunostaining could be detected in other cell types. Taken together, this data indicate that gonadotrophs and corticotrophs are the main source of GDNF synthesized in the anterior pituitary and that the somatotrophs appears to be their target cell. This study provides direct morphological evidences that GDNF may well be acting in a paracrine-like fashion in the regulation of somatotroph cell growth and/or cell function.

Effects of TGF-beta1 on prolactin synthesis and secretion: an in-vitro study

The hypothalamus exerts a predominantly inhibitory influence on prolactin secretion through dopamine. In addition, the expression of anterior pituitary hormone-gene products are regulated by intrapituitary growth factors. In particular, TGF-beta1 produced in the pituitary regulates lactotroph cell proliferation and prolactin gene-expression. This study characterized the regulation of in-vitro prolactin synthesis and secretion by TGF-beta1 using rat anterior pituitary cells in monolayer culture. Furthermore, we studied the interaction of TGF-beta1 with other signals involved in the neuroregulation of prolactin secretion, such as dopamine and TRH, as well as the importance of different signal transduction pathways in this response. TGF-beta1 inhibited prolactin secretion in a time- and concentration-dependent manner, with half-maximal inhibition occurring at the range of 15-30 pM. The inhibitory effect was observed after 4 h, being maximal after 4 days of exposure of the cells to the peptide. This inhibitory effect was mimicked by TGF-beta2 but not by inhibin, and was not influenced by oestrogens, being similar in male, normal female or oestradiol-treated rats. Prolonged pretreatment of the cells with TGF-beta1(4 days) did not modify GH or TSH secretion nor dopamine-induced inhibition of prolactin secretion, and blunted prolactin responses to TRH, Forskolin, But2-cAMP and to the calcium ionophore A23187. The effect observed after long-term treatment (24 h to 4 days) is essentially caused by a decrease in prolactin synthesis, since TGF-beta1 inhibited prolactin mRNA levels and de novo prolactin protein synthesis. However, in the short term (up to 12 h) TGF-beta1 inhibition of prolactin secretion was associated with an increase in intracellular prolactin content, dissecting a dual mechanism of action of TGF-beta1. The short-term TGF-beta1 effect did not modify Erk-2 phosphorylation, basal or TRH-induced increase in intracellular calcium concentration, but blunted basal and forskolin stimulated cAMP levels. But2-cAMP replacement did not revert the inhibition of prolactin secretion. However, pertussis toxin was able to recover a large percentage of TGF-beta1-induced inhibition of prolactin secretion. This study indicates that TGF-beta1 plays a crucial role as a modulator of lactotroph function, inhibiting prolactin biosynthesis after long-term treatment, as well as, after short-term exposure prolactin secretion at the level of the secretory process, through a mechanism pertussis toxin sensitive but independent of Erk-2 phosphorylation, calcium concentrations or intracellular cAMP.

Alkali metal ions protect mitochondrial rhodanese against thermal inactivation

Incubation of bovine liver mitochondrial rhodanese in dilute, reducing solutions at temperatures ranging between 30 and 45 degreesC conduced to a rapid loss of enzymatic activity. This inactivation was substantially reduced in the presence of millimolar concentrations of alkali metal ions, divalent cations (including Mg2+, Ca2+, and Ba2+) were ineffective. The extent of protection afforded by monovalent cations was highly dependent on their ionic radii, with K+ and Na+ ions being the most effective protective agents. The protection afforded by a number of anions, including thiosulfate, could be totally ascribed to the presence of the accompanying monovalent cation. The overall results indicate that K+ and Na+, at concentrations and temperatures within the physiological range, substantially contribute to the stabilization of the functional structure of rhodanese.

TGF-beta1 actions on FRTL-5 cells provide a model for the physiological regulation of thyroid growth

Little is known about the TGF-beta1 mechanism that promotes thyroid cell growth arrest. We assessed TGF-beta1 effects on Fisher rat thyroid cell line (FRTL-5). This allowed us to study TGF-beta1 action on thyroid cells in various physiological situations such as actively proliferating cells, resting cells stimulated to proliferate by the action of various mitogens, and resting cells. TGF-beta1 arrested proliferating FRTL-5 cells, increasing c-myc mRNA levels and reducing p27-free cyclin D1 protein levels, without affecting either the cellular content of p27 or the cyclin D1-p27 complexes. Moreover, TGF-beta1 treatment reduced the activity of cyclin E-CDK2 complexes and, consequently, pRB was found to be hypophosphorylated. TGF-beta1 prevented resting cells to enter in the cell cycle when stimulated with growing medium (newborn calf serum plus a mixture of five hormones) but not when TSH (thyroid stimulating hormone) plus IGF-1 (Insulin-like growth factor I) were used as mitogens. Both stimuli increased the levels of cyclins D1, D3 and E but TGF-beta1 had a greater effect in decreasing these cyclin levels in growing-medium stimulated cells than in TSH + IGF-1. This suggests that for FRTL-5 cells, the content of these cyclins must exceed a threshold to progress through the cell cycle. TGF-beta1 induced apoptosis in quiescent cells, accompanied by a reduction in p27 protein levels and an increase in c-myc expression. Interestingly, TGF-beta1-induced variations in prothymosin alpha and c-myc mRNA levels were not correlated. TGF-beta1 always promoted an increase of p15 mRNA levels. In summary, our results point to the fact that TGF-beta1 could play a physiological role in the control of thyroid growth through the modification of cell cycle regulatory proteins.

Acipimox-mediated plasma free fatty acid depression per se stimulates growth hormone (GH) secretion in normal subjects and potentiates the response to other GH-releasing stimuli

Increases in plasma free fatty acids (FFA) inhibit the GH response to a variety of stimuli; however, the role of FFA depression in GH control is far from understood. In the present work, FFA reduction was obtained by the administration to normal subjects of acipimox, a lipid-lowering drug devoid of side-effects. Each subject tested underwent two paired tests. In one, acipimox was administered orally at a dose of 250 mg at -270 min and at a dose of 250 mg at -60 min; in the matched test, placebo was given at similar intervals. To induce GH release, four stimuli acting through different mechanisms were used: pyridostigmine (120 mg, orally) at -60 min, GHRH (1 microgram/kg, iv) at 0 min, GH-releasing peptide (GHRP-6; His-D-Trp-Ala-Trp-D-Phe-Lys-NH2; 1 microgram/kg, iv) at 0 min, and finally, GHRH plus GHRP-6 at the same doses at 0 min. GH secretion was analyzed as the area under the secretory curve (AUC; mean +/- SE, micrograms per L/120 min). Acipimox pretreatment alone (n = 6) induced a reduction in FFA levels compared with placebo treatment. The FFA reduction led to a sustained GH secretion that increased from 2.4 +/- 1.8 micrograms/L at -120 min to 14.2 +/- 4.0 at 120 min. The GH AUC for placebo was 266 +/- 100, and that for acipimox was 1781 +/- 408 (P < 0.05). In the pyridostigmine-treated group (n = 6), the acipimox-pyridostigmine AUC (2046 +/- 323) was higher (P < 0.05) than the placebo-pyridostigmine AUC (764 +/- 101), but was not different from the AUC of acipimox alone. Previous FFA reduction nearly doubled the GHRH-mediated GH secretion (n = 6; placebo-GHRH AUC, 1817 +/- 365; acipimox-GHRH test, 3228 +/- 876; P < 0.05). A similar enhancement was observed when the stimulus employed was GHRP-6 (n = 6; placebo-GHRP-6 AUC, 2034 +/- 295; acipimox-GHRP-6, 4827 +/- 703; P < 0.05). Furthermore, even the most potent GH stimulus known to date, i.e. GHRH plus GHRP-6, was enhanced by the FFA suppression (placebo-GHRH-GHRP-6 AUC, 2034 +/- 277; acipimox-GHRH-GHRP-6, 5809 +/- 758; P < 0.05). The enhancing effect of lowering FFA levels was additive regardless of the stimulus employed. These results indicate that 1) FFA reduction per se stimulates GH secretion with a delayed time of action; 2) FFA reduction enhanced in an additive manner the GH secretion elicited by such different stimuli as pyridostigmine, GHRH, and GHRP-6; and 3) the observation that FFA reduction enhanced the response to the most potent GH stimulus, GHRH plus GHRP-6, suggests that FFA suppression acts by a separate mechanism. FFA reduction may have value in the clinical setting for assessing GH reserve.

Impaired growth hormone secretion in obese subjects is partially reversed by acipimox-mediated plasma free fatty acid depression

GH secretion in response to provocative stimuli is blunted in obese patients. On the other hand, increases in plasma free fatty acids (FFA) inhibit the GH response to a variety of stimuli, and FFA levels in plasma are increased with obesity. To ascertain whether FFA might be responsible for the GH secretory alterations of obesity, we studied spontaneous and stimulated GH secretion in 31 obese patients after FFA reduction by acipimox, a lipid-lowering drug devoid of serious side-effects. Each subject underwent two paired tests. In one, acipimox was administered orally at a dose of 250 mg at -270 min and at a dose of 250 mg at -60 min; in the matched test, placebo was given at similar intervals. To induce GH release, three stimuli acting through different mechanisms were used: pyridostigmine (60 mg, orally, at -60 min), GHRH (100 micrograms, iv, at 0 min), and GHRH plus GH-releasing peptide (GHRP-6; His-D-Trp-Ala-Trp-D-Phe-Lys-NH2; both at a dose of 100 micrograms, iv, at 0 min). GH secretion was analyzed as the area under the secretory curve (AUC; mean +/- SE; micrograms per L/60 min). Acipimox pretreatment alone (n = 13) induced a large reduction in FFA levels compared with placebo treatment. The FFA reduction led to a slight GH rise (AUC, 123 +/- 47), not different from that in the placebo group (61 +/- 15). In the pyridostigmine-treated group (n = 6), the acipimox-pyridostigmine AUC (408 +/- 107) was significantly higher (P < 0.05) than that in the placebo-pyridostigmine group (191 +/- 25). Furthermore, the GHRH-mediated (n = 6) AUC of GH secretion in the placebo test (221 +/- 55) was tripled by FFA reduction due to acipimox, with an AUC of (691 +/- 134; P < 0.05). Even the most potent GH stimulus known to date, i.e. GHRH plus GHRP-6, was enhanced by FFA suppression. In fact, the placebo-GHRH-GHRP-6 AUC was 1591 +/- 349, lower (P < 0.05) than that in the acipimox-GHRH-GHRP-6 test (2373 +/- 242). The enhancing effects of FFA lowering on GHRH-mediated and GHRH- plus GHRP-6-mediated GH release were synergistic. These results indicate that in obese subjects, unlike normal weight subjects. FFA reduction per se does not stimulate GH secretion. A reduction in FFA with acipimox, however, increased pyridostigmine-. GHRH-, and even GHRH- plus GHRP-6-mediated GH release, suggesting that FFA reduction operates through a different mechanism from that of these three stimuli. The abnormally high FFA levels may be a contributing factor for the disrupted GH secretory mechanisms in obesity.

Structural requirements of the epidermal growth factor receptor for tyrosine phosphorylation of eps8 and eps15, substrates lacking Src SH2 homology domains

Phosphorylation of two newly identified epidermal growth factor (EGF) receptor substrates, eps8 and eps15, which do not possess Src homology (SH2) domains, was investigated using EGF receptor mutants of the autophosphorylation sites and deletion mutants of the carboxyl-terminal region. Two mutants, F5, in which all five tyrosine autophosphorylation sites substituted by phenylalanine, and Dc 123F, in which four tyrosines were removed by deletion and the fifth (Tyr-992) was mutated into phenylalanine, phosphorylated eps8 and eps15 as efficiently as the wild-type receptor. In contrast, SH2-containing substrates, phospholipase C gamma, the GTPase-activating protein of Ras, the p85 subunit of phosphatidylinositol 3 kinase, and the Src and collagen homology protein, are not phosphorylated by the F5 and Dc 123F mutants. A longer EGF receptor deletion mutant, Dc 214, lacking all five autophosphorylation sites, was unable to phosphorylate eps15 but phosphorylated eps8 13-fold more than the wild-type receptor. To determine the EGF receptor region important for phosphorylation of eps8 and eps15, progressive deletion mutants lacking the final 123, 165, 196, and 214 COOH-terminal residues were used. eps8 phosphorylation was progressively increased in Dc 165, Dc 196, and Dc 214 EGF receptor mutants, indicating that removal of the final 214 COOH-terminal residues increases the phosphorylation of this substrate by the EGF receptor. In contrast, eps15 was phosphorylated by Dc 123 and Dc 165 EGF receptor mutants but not by Dc 196 and Dc 214 mutants. This indicates that a region of 30 residues located between Dc 165 and Dc 196 is essential for eps15 phosphorylation. This is the first demonstration of structural requirements in the EGF receptor COOH terminus for efficient phosphorylation of non-SH2-containing substrates. In addition, enhanced eps8 phosphorylation correlates well with the increased transforming potential of EGF receptor deletion mutants Dc 196 and Dc 214, suggesting that this substrate may be involved in mitogenic signaling.

Regulation of thymosin beta 4 mRNA levels during cell proliferation

The levels of thymosin beta 4 mRNA were studied throughout the cell cycle of NIH 3T3 cells. In serum deprived, quiescent cells, the levels of thymosin beta 4 were undetectable; after serum restoration, the cells were induced to proliferate and we found a pronounced increase in thymosin beta 4 mRNA levels at the G1/S transition. Thymosin beta 4 mRNA was induced even in the presence of cycloheximide. On the other hand, cycling cells that were synchronized at different stages of the cycle by means of mitotic shake-off after nocodazole arrest or a double thymidine block did not show any variation in the levels of thymosin beta 4 mRNA when they progressed synchronously through the cycle. In conclusion, the present data indicate that the thymosin beta 4 gene is regulated by cell proliferation but it is not a cell cycle-regulated gene. Finally, we studied thymosin beta 4 mRNA stability by inhibiting thymosin beta 4 gene transcription with actinomycin D. Our results suggest that thymosin beta 4 mRNA has a pronounced stability, a fact that might be relevant to account for the presence of thymosin beta 4 in enucleated cells like platelets.

Potent SHC tyrosine phosphorylation by epidermal growth factor at low receptor density or in the absence of receptor autophosphorylation sites

The importance of epidermal growth factor (EGF) receptor expression level and autophosphorylation sites in src homology and collagen protein (SHC) tyrosine phosphorylation has been studied. In contrast to EGF-induced tyrosine phosphorylation of the GTPase-activating protein for ras (rasGAP) and phospholipase C-gamma 1 (PLC-gamma 1), SHC tyrosine phosphorylation occurs at a very low receptor density in parental NIH3T3 mouse fibroblasts expressing less than 1 x 10(4) EGF receptors per cell. In transfected NIH3T3 cells expressing human EGF receptors (approximately 4 x 10(5) receptors per cell), maximal levels of SHC and PLC-gamma 1 tyrosine phosphorylation occur when approximately 4 x 10(4) receptors or more are occupied by ligand. At lower levels of receptor occupancy only SHC phosphorylation was significant. Also, EGF treatment of mouse keratinocytes, which represent a physiological target of EGF, express a low number of EGF receptors (approximately 2 x 10(4) receptors per cell), and stringently require EGF to grow, results in intense SHC tyrosine phosphorylation, compared to rasGAP or PLC-gamma 1. SHC is also efficiently tyrosine phosphorylated by an EGF receptor deletion mutant (Dc214) that is devoid of autophosphorylation sites, but which remains mitogenically responsive to EGF. The EGF receptor mutant Dc214 is able to activate the ras guanine nucleotide exchanger and phosphorylate mitogen-activated protein kinase (MAPK), presumable as a result of complex formation between tyrosine phosphorylated SHC and GRB2. These results indicate that potent EGF-induced SHC tyrosine phosphorylation can be triggered in cells having relatively few receptors. Also, our data show that EGF receptors are able to phosphorylate SHC, activate the exchange of guanine nucleotide on ras and phosphorylate MAPK by a mechanism that does not require receptor autophosphorylation sites and, therefore, the src homology 2 (SH2):phosphotyrosine-dependent interaction of SHC or GRB2 with the EGF receptor.

Prothymosin alpha mRNA is expressed in competent and proliferating rat thyroid cells (FRTL-5) but is not sufficient to elicit cell progression through the cell cycle

Using flow cytometry we observed the effects that different hormonal treatments had on the progression of rat thyroid (FRTL-5) cells through the cell cycle. The absence of hormones or the addition of TSH (6 mU/ml) did not induce DNA synthesis; however, the addition of IGF-I (30 ng/ml) promoted cell proliferation. The number of cells recruited by IGF-I was lower than when IGF-I and TSH were used. We therefore concluded that we had a model with three different types of cells: (1) quiescent cells, cells cultured in the absence of hormones, considered to be G0-arrested cells, (2) competent cells, TSH-treated cells that did not proliferate (being arrested in a cycle phase different from G0) and (3) actively proliferating cells, cells treated with TSH plus IGF-I. Prothymosin alpha (PTA) mRNA levels were almost undetectable in cells cultured without hormones at all times studied, i.e. 8, 14 and 24 h. On the contrary, TSH and/or IGF-I greatly increased PTA mRNA. These data indicate that G0-arrested quiescent cells do not express PTA mRNA and that PTA mRNA is induced when FRTL-5 cells are committed to proliferate by the addition of TSH, in spite of being arrested by the lack of IGF-I. We therefore conclude that PTA mRNA expression may be an event that is necessary for cells to proliferate, but that it is not sufficient for the promotion of cell progression through the cell cycle.

Expression of epithelial phenotype is enhanced by v-Ha-ras in rat endometrial cells immortalized by SV40 T antigen

To study the interplay of steroid hormones and oncogenes in the control of endometrial cell proliferation and differentiation we have generated cell lines derived from rat endometrium by expressing the immortalizing oncogenes adeno E1A or SV40 large T antigen. These lines are positive for mesenchymal markers and contain very few characteristic epithelial proteins. Cell lines expressing a temperature-sensitive mutant of SV40 T antigen exhibit a temperature-dependent morphology and growth behavior, but do not manifest an epithelial phenotype at the non-permissive temperature. Cell lines additionally infected with retroviral vectors carrying the v-Ha-ras oncogene (p21rasArg-12) no longer express collagen type III and recover part of their epithelial potential by expressing cytokeratins and/or cadherin E. Some of these cells also express characteristic decidual marker proteins such as desmin, whereas others express glandular epithelial markers such as uteroglobin. Uteroglobin mRNA levels in these cells are increased by glucocorticoids. The parental temperature-sensitive cells do not contain progesterone receptor but become positive for progesterone receptor at the permissive temperature after infection with the v-Ha-ras-expressing retrovirus. Our results indicate that there is a fluent transition and overlapping between mesenchymal, glandular epithelial and decidual phenotypes of endometrial cells, suggesting that these three cell types are derived from the same stem/precursor cells. The v-Ha-ras oncogene product appears to act on the differentiation pathway at an early step prior to the distinction between decidual and glandular epithelial lineage.

Regulation of prothymosin alpha mRNA levels in rat pituitary tumor cells

Prothymosin alpha (PTA) mRNA and histone H4 (H4) mRNA levels were studied in various experimental conditions that affected GH1 pituitary tumor cell proliferation. Cell proliferation and progression through the cell cycle was assessed by counting cells, 3H-thymidine incorporation and flow cytometry. PTA mRNA levels were decreased in a time-dependent fashion following serum deprivation; when the cells were induced to grow by serum refeeding, PTA mRNA expression was greatly stimulated. Interestingly, after caprylic acid treatment (2.5 mM for 24 h) that arrested cells in the G0/G1 phase of the cell cycle, PTA mRNA and H4 mRNA levels were almost undetectable; conversely, following caprylic acid withdrawal, PTA mRNA and H4 mRNA expression were greatly stimulated. Furthermore, cells cultured in T3-deprived serum, which was found to decrease GH1 cell proliferation, had low levels of PTA and H4 mRNAs. This effect was reversed by the addition of nanomolar concentrations of T3 to the culture. On the other hand, IGF-1 addition to the culture did not substantially modify PTA mRNA levels. The present data clearly indicate that PTA mRNA expression is tied to the proliferating activity of GH1 cells and, thus, could be used as a marker of the action that various agents have on GH1 cell proliferation.

Regulation of His-dTrp-Ala-Trp-dPhe-Lys-NH2 (GHRP-6)-induced GH secretion in the rat

His-dTrp-Ala-Trp-dPhe,Lys-NH2(GHRP-6) is a synthetic compound that releases GH in a dose-response and specific manner in several species and that may well be related to an endogenous compound of similar structure. The aim of this study was to investigate the in vivo GH responses to GHRP-6 in pentobarbital anesthetized rats. Specifically and in order to avoid the influence of endogenous GHRH and somatostatin secretion we studied the GH responses to GHRP-6 in animals with surgical ablation of the hypothalamus, confirmed by histological assessment, as well as in hypophysectomyzed-transplanted rats bearing two hypophyses under the renal capsule. Since it has been previously reported that rats pretreated with GHRH (10 micrograms/kg i.p. every 12 h for 15 days) rather than saline-treated rats have greater GH responses to acutely administered GHRH, we compared the self-potentiating effect of chronic GH pretreatment with GHRP-6 (10 micrograms/kg i.p. every 12 h). Furthermore we also studied the influence of estrogens, glucocorticoids, free fatty acids (FFA) and bombesin on somatotroph responsiveness to GHRP-6 in intact rats. We found a greater GH response to GHRP-6 in rats that underwent a surgical ablation of the hypothalamus 36 h prior to the test than in sham-operated rats. A direct stimulatory effect of GHRP-6 on in vivo GH secretion was demonstrated by a clear GH response to GHRP-6 in hypophysectomyzed-transplanted rats. In addition, we found a similar response whether the animals were pretreated with GHRH or GHRP-6 over the previous 2 weeks. Finally, we found that both estrogen- and testosterone-treated rats have greater GH responses to GHRP-6 than untreated rats. On the other hand, chronic dexamethasone administration, acute elevation of circulating FFA levels and bombesin administration markedly inhibited GH responses to GHRP-6. In contrast to the effects exerted on GH responses to GHRP-6 estrogen administration led to a decrease in GH responses to GHRH while dexamethasone did not affect the GH responses to GHRH, highlighting a differential regulation of these hormones on somatotroph responsiveness to these peptides.

Effect of neurotensin on growth hormone release in vivo

In order to investigate the mechanisms involved in the in vivo Growth Hormone (GH) response to Neurotensin (NT) we assessed the influence of estrogen status as well as the effect of passive immunization with antisomatostatin and anti-Growth Hormone-Releasing Hormone (GHRH) on NT-induced GH secretion in pentobarbital anesthetized rats. We found that, contrary to GH responses to GHRH, estrogen-treated rats (one single injection of 200 micrograms s.c. of estradiol valerate, 3 days before the experiment), exhibited markedly increased GH responses to different doses of NT (7.5, 15 and 30 micrograms/kg, i.v.). The stimulatory effect of NT (30 micrograms/kg) on estrogen-treated rats was similar in rats that received normal rabbit serum or passively immunized with antisomatostatin or anti-GHRH serum. In conclusion, estrogens play a facilitatory role on NT-induced GH release in the rat, which is exerted through a mechanism independent of hypothalamic GHRH or somatostatin release.

Prothymosin alpha mRNA levels are invariant throughout the cell cycle

Prothymosin alpha (PT-alpha) mRNA levels were evaluated at different stages during the cell cycle. NIH 3T3 cells were synchronized: (a) by serum deprivation, (b) by mitotic shake off after nocodazole arrest, and (c) by double thymidine block. Cell synchronism was estimated by flow cytometry. In cells grown in serum-free medium, PT-alpha mRNA levels were almost undetectable. 14 h after serum restoration PT-alpha mRNA was induced as had been described by others (Eschendfeldt, W. H., and Berger, S. L. (1986) Proc. Natl. Acad. Sci. U. S. A. 83, 9403-9407). PT-alpha mRNA induction seems to require the synthesis of proteic factor(s) since PT-alpha mRNA response to serum restoration was abolished in the presence of cycloheximide. Interestingly, cycling cells that were synchronized at different stages of the cycle by means of mitotic shake off after nocodazole arrest or double thymidine block did not show variations in the levels of PT-alpha mRNA when progressed synchronously through the cycle. On the contrary, histone H4 mRNA was expressed only during the S phase. These data indicate that PT-alpha mRNA was present in roughly the same amount through all phases of the cell cycle, arguing against the concept that PT-alpha is a cell cycle-regulated gene.

Evidence for a direct pituitary inhibition by free fatty acids of in vivo growth hormone responses to growth hormone-releasing hormone in the rat

The aim of this study was to determinate whether elevations in circulating free fatty acids (FFA) inhibit in vivo growth hormone (GH) responses to GH-releasing hormone (GHRH) by increasing hypothalamic somatostatin release or by acting directly on the pituitary. Thus, we have studied the effect of an Intralipid-heparin infusion on in vivo GH responses to GHRH in normal rats, normal rats passively immunized with antisomatostatin antiserum, rats with medial hypothalamic ablation, and hypophysectomized rats bearing two hypophyses under the renal capsule. Administration of 1 ml of Intralipid (500 microliters at -30 min and 500 microliters at -25 min) plus heparin (50 IU at -15 min) induced a marked decrease in GH responses to both 1 and 5 micrograms/kg of GHRH (p less than 0.01 at 5, 10 and 15 min for GHRH alone vs. GHRH plus Intralipid). A similar degree of inhibition was obtained after the administration of antisomatostatin antiserum (750 microliters i.v. at -60 min) previous to a challenge with 5 micrograms/kg of GHRH plus 1 ml of Intralipid (p less than 0.05 at 5 and 15 min, and p less than 0.01 at 10 min for GHRH plus normal rabbit serum vs. GHRH plus Intralipid plus antisomatostatin antiserum). Furthermore, administration of 1 ml of Intralipid also markedly reduced GH responses to GHRH in rats with medial hypothalamic ablation (p less than 0.01 at 5, 10, 15 and 30 min for GHRH alone vs. GHRH plus Intralipid) as well as in hypophysectomized rats bearing two hypophyses under the renal capsule (p less than 0.01 at 5, 10 and 15 min for GHRH alone vs. GHRH plus Intralipid).(ABSTRACT TRUNCATED AT 250 WORDS)

Estrogen-dependent effects of bombesin on in vivo growth hormone secretion in the rat

Previous studies carried out in normal male or ovariectomized female rats have shown that bombesin plays an inhibitory role on growth hormone (GH) secretion. Since estrogens play an important role in the neuroregulation of GH secretion, we have studied the effects of bombesin on basal GH secretion and GH responses to GH-releasing hormone (GHRH) in untreated and estrogen-treated male rats (200 micrograms estradiol valerate s.c., 1 single dose 3 days before the experiment or every 3 days for 2 weeks). All the experiments were carried out in rats anesthetized with pentobarbital. GH responses to GHRH (1 microgram/kg) were inhibited by bombesin (100 micrograms/kg) in untreated rats, but were markedly increased in rats treated with estrogens either 3 days before or for the previous 2 weeks. Similarly, bombesin administration (25 or 100 micrograms/kg) in estrogen-treated rats induced a clear-cut, dose-related increase in basal GH levels. This stimulatory effect of bombesin was not affected by passive immunization with antisomatostatin antiserum (750 microliters i.v., 60 min before) and only partially blocked by anti-rGHRH antiserum (750 microliters i.v., 1 h before). In conclusion, our data show that bombesin exerts an inhibitory effect in normal male rats but a stimulatory one in estrogenized rats. This latter effect is independent of somatostatin and only partially blocked by anti-rGHRH serum.