Plate tectonics on the Earth triggered by plume-induced subduction initiation.

Scientific theories of how subduction and plate tectonics began on Earth--and what the tectonic structure of Earth was before this--remain enigmatic and contentious. Understanding viable scenarios for the onset of subduction and plate tectonics is hampered by the fact that subduction initiation processes must have been markedly different before the onset of global plate tectonics because most present-day subduction initiation mechanisms require acting plate forces and existing zones of lithospheric weakness, which are both consequences of plate tectonics. However, plume-induced subduction initiation could have started the first subduction zone without the help of plate tectonics. Here, we test this mechanism using high-resolution three-dimensional numerical thermomechanical modelling. We demonstrate that three key physical factors combine to trigger self-sustained subduction: (1) a strong, negatively buoyant oceanic lithosphere; (2) focused magmatic weakening and thinning of lithosphere above the plume; and (3) lubrication of the slab interface by hydrated crust. We also show that plume-induced subduction could only have been feasible in the hotter early Earth for old oceanic plates. In contrast, younger plates favoured episodic lithospheric drips rather than self-sustained subduction and global plate tectonics.

Specific suppression of microgliosis cannot circumvent the severe neuropathology in peroxisomal ß-oxidation-deficient mice.

An important hallmark of various neurodegenerative disorders is the proliferation and activation of microglial cells, the resident immune cells of the central nervous system (CNS). Mice that lack multifunctional protein-2 (MFP2), the key enzyme in peroxisomal ß-oxidation, develop excessive microgliosis that positively correlates with behavioral deficits whereas no neuronal loss occurs. However, the precise contribution of neuroinflammation to the fatal neuropathology of MFP2 deficiency remains largely unknown. Here, we first attempted to suppress the inflammatory response by administering various anti-inflammatory drugs but they failed to reduce microgliosis. Subsequently, Mfp2-/- mice were treated with the selective colony-stimulating factor 1 receptor (CSF1R) inhibitor PLX5622 as microglial proliferation and survival is dependent on CSF1R signaling. This resulted in the elimination of >95% of microglia from control mice but only 70% of the expanded microglial population from Mfp2-/- mice. Despite microglial diminution in Mfp2-/- brain, inflammatory markers remained unaltered and residual microglia persisted in a reactive state. CSF1R inhibition did not prevent neuronal dysfunction, cognitive decline and clinical deterioration of Mfp2-/- mice. Collectively, the unaltered inflammatory profile despite suppressed microgliosis concurrent with persevering clinical decline strengthens our hypothesis that neuroinflammation importantly contributes to the Mfp2-/- phenotype.

Urokinase-generated plasmin activates matrix metalloproteinases during aneurysm formation.

The molecular mechanisms predisposing to atherosclerotic aneurysm formation remain undefined. Nevertheless, rupture of aortic aneurysms is a major cause of death in Western societies, with few available treatments and poor long-term prognosis. Indirect evidence suggests that matrix metalloproteinases (MMPs) and plasminogen activators (PAs) are involved in its pathogenesis. MMPs are secreted as inactive zymogens (pro-MMPs), requiring activation in the extracellular compartment. Plasmin, generated from the zymogen plasminogen by tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA; refs 14,15), has been proposed as a possible activator in vitro, but evidence for such a role in vivo is lacking. Analysis of atherosclerotic aorta in mice with a deficiency of apoliprotein E (Apoe-/-; ref. 18), singly or combined with a deficiency of t-PA (Apoe-/-:Plat-/-) or of u-PA (Apoe-/-:Plau-/-; ref. 19), indicated that deficiency of u-PA protected against media destruction and aneurysm formation, probably by means of reduced plasmin-dependent activation of pro-MMPs. This genetic evidence suggests that plasmin is a pathophysiologically significant activator of pro-MMPs in vivo and may have implications for the design of therapeutic strategies to prevent aortic-wall destruction by controlling Plau gene function.

A mouse model for Zellweger syndrome.

The cerebro-hepato-renal syndrome of Zellweger is a fatal inherited disease caused by deficient import of peroxisomal matrix proteins. The pathogenic mechanisms leading to extreme hypotonia, severe mental retardation and early death are unknown. We generated a Zellweger animal model through inactivation of the murine Pxr1 gene (formally known as Pex5) that encodes the import receptor for most peroxisomal matrix proteins. Pxr1-/- mice lacked morphologically identifiable peroxisomes and exhibited the typical biochemical abnormalities of Zellweger patients. They displayed intrauterine growth retardation, were severely hypotonic at birth and died within 72 hours. Analysis of the neocortex revealed impaired neuronal migration and maturation and extensive apoptotic death of neurons.

Inhibition of plasminogen activators or matrix metalloproteinases prevents cardiac rupture but impairs therapeutic angiogenesis and causes cardiac failure.

Cardiac rupture is a fatal complication of acute myocardial infarction lacking treatment. Here, acute myocardial infarction resulted in rupture in wild-type mice and in mice lacking tissue-type plasminogen activator, urokinase receptor, matrix metalloproteinase stromelysin-1 or metalloelastase. Instead, deficiency of urokinase-type plasminogen activator (u-PA-/-) completely protected against rupture, whereas lack of gelatinase-B partially protected against rupture. However, u-PA-/- mice showed impaired scar formation and infarct revascularization, even after treatment with vascular endothelial growth factor, and died of cardiac failure due to depressed contractility, arrhythmias and ischemia. Temporary administration of PA inhibitor-1 or the matrix metalloproteinase-inhibitor TIMP-1 completely protected wild-type mice against rupture but did not abort infarct healing, thus constituting a new approach to prevent cardiac rupture after acute myocardial infarction.

A new orphan member of the nuclear hormone receptor superfamily that interacts with a subset of retinoic acid response elements.

We have identified and characterized a new orphan member of the nuclear hormone receptor superfamily, called MB67, which is predominantly expressed in liver. MB67 binds and transactivates the retinoic acid response elements that control expression of the retinoic acid receptor beta 2 and alcohol dehydrogenase 3 genes, both of which consist of a direct repeat hexamers related to the consensus AGGTCA, separated by 5 bp. MB67 binds these elements as a heterodimer with the 9-cis-retinoic acid receptor, RXR. However, MB67 does not bind or activate other retinoic acid response elements with alternative hexamer arrangements or any of several other wild-type and synthetic hormone response elements examined. The transactivation of retinoic acid response elements by MB67 is weaker than that conferred by the retinoic acid receptors but does not require the presence of all-trans retinoic acid, 9-cis-retinoic acid, or any exogenously added ligand. We propose that MB67 plays an important role in the complex network of proteins that govern response to retinoic acid and its metabolites.

Addition of isosorbide mononitrate to misoprostol for cervical ripening in post-term pregnancy: A randomized controlled trial

ObjectiveThe purpose of this paper is to compare the effect of vaginal isosorbide mononitrate added to misoprostol versus misoprostol alone in cervical ripening and labor induction in post-term pregnancy.MethodsIn this double-blind controlled trial study, 150 pregnant women in post-term pregnancy who were candidates for labor induction were selected. The participants were assigned randomly to receive either vaginal isosorbide mononitrate (IMN) (40mg) or placebo. Misoprostol (25mg) was added to both groups as needed. Time to full cervical ripening, time to delivery, and the amount of misoprostol used in each group were assessed.ResultsThe time interval from the administration of IMN to full cervical ripening was shown to be significantly lower in the IMN+ misoprostol groups versus the comparison group (p=.032). The adjusted analysis of this time interval after controlling for age, BMI, gravidity, and Bishop score on administration remained significantly less (p=.045),the mean difference being −4.85h, CI 95% −9.58 to −.12. Isosorbide treatment resulted in significantly less misoprostol used versus misoprostol alone (2.37±1.02 versus 3.08±1.29), adjusted p-value=.001, CI 95% −1.09 to −.32. We found no significant increase in maternal–fetal outcomes or side effects of the IMN+ misoprostol group compared with the misoprostol group.ConclusionThis study found that intravaginal IMN added to misoprostol is more effective in reducing time to full cervical ripening versus misoprostol alone in post-term pregnancy. It also reduces the need for more misoprostol.

ObjetivoEl objetivo de esta investigación es determinar si el mononitrato de isosorbida vaginal, agregado al misoprostol, acorta el tiempo hasta la maduración cervical completa en el embarazo postérmino.MétodosEn este estudio de prueba controlado doble ciego, se seleccionaron 150 mujeres embarazadas en embarazo postérmino candidatas para la inducción del trabajo de parto. Los participantes fueron asignados al azar para recibir mononitrato de isosorbida vaginal (NMI) (40mg) o placebo. Se añadió misoprostol (25mg) a ambos grupos según fuera necesario. Se evaluaron el tiempo hasta la maduración cervical completa, el tiempo hasta el parto y la cantidad de misoprostol utilizado en cada grupo.ResultadosEl intervalo de tiempo desde la administración de la NMI hasta la maduración cervical completa se mostró significativamente más bajo en los grupos de NMI versus el grupo de comparación (P=0,032). El análisis ajustado de este intervalo de tiempo después de controlar la edad, el IMC, la gravidez y la puntuación de Bishop en la administración se mantuvo significativamente menor (P=0,045) con la diferencia media -4,85h, IC 95% -9,58 a -0,12. El tratamiento con isosorbida dio como resultado una menor cantidad de misoprostol usado significativamente en comparación con el misoprostol solo (2,37±1,02 versus 3,08±1,29), valor de P ajustado=0,001, IC 95% -1,09 a -0,32. No se encontró un aumento significativo en los resultados materno-fetales y los efectos secundarios del grupo de NMI en comparación con el grupo de misoprostol.

Isoprenoid biosynthesis is not compromised in a Zellweger syndrome mouse model.

Because several studies indicated that peroxisomes are important for the biosynthesis of isoprenoids, we wanted to investigate whether a reduced availability of isoprenoids could be one of the pathogenic factors contributing to the severe phenotype of the Pex5(-/-) mouse, a model for Zellweger syndrome. Total cholesterol was determined in plasma, brain and liver of newborn mice. In none of these tissues a significant difference was observed between Pex5(-/-) and wild type or heterozygous mice. The hepatic ubiquinone content was found to be even higher in Pex5(-/-) mice as compared to wild type or heterozygous littermates. To investigate whether the Pex5(-/-) fetuses are able to synthesise their own isoprenoids, fibroblasts derived from these mice were incubated with radiolabeled mevalonolactone as a substrate for isoprenoid synthesis. No significant difference was observed between the cholesterol production rates of Pex5(-/-) and normal fibroblasts. Our results show that there is no deficiency of isoprenoids in newborn Pex5(-/-) mice, excluding the possibility that a lack of these compounds is a determinant factor in the development of the disease state before birth.

Metabolism of dolichol, dolichoic acid and nordolichoic acid in cultured cells.

The uptake and metabolism of [1-(14)C]-labelled dolichol, dolichoic acid and nordolichoic acid were investigated in MDCK and HepG2 cells. Each of the three isoprenoids, bound to human serum albumin, was taken up effectively. None of the compounds was broken down in HepG2 cells, although these converted dolichol into fatty acid esters. In MDCK cells dolichoic acid gave rise to the formation of [14C]CO2 and radiolabelled formic acid, indicating that dolichoic acid can be broken down by alpha-oxidation. Dolichoic acid was also converted to a mixture of polar compounds, possibly polyols. MDCK cells generated radiolabelled CO2 from nordolichoic acid, presumably through beta-oxidation, although we could not find any labelled propionic acid. No oxidative breakdown of dolichol was found, apparently due to the lack of or very low conversion to dolichoic acid.

Do sphingoid bases interact with the peroxisome proliferator activated receptor alpha (PPAR-alpha)?

In a search for possible endogenous ligands of nuclear receptors that are activated by peroxisome proliferators (PPARs), a solid phase binding assay was developed employing recombinant mouse PPAR-alpha, containing a myc-epitope, a histidine repeat and a kinase A domain. After in vitro labelling with 32P-gamma-ATP, the binding of purified 32P-PPAR-alpha to a panel of different natural and synthetic lipids, immobilized on silica layers, was evaluated. Autoradiographs of the silica layers revealed binding to two main classes of lipophilic compounds. A first class comprised (poly)unsaturated fatty acids. Compounds belonging to a second class were characterized by the presence of an overall positive charge such as long chain amines, sphingoid bases (sphingenine), and lysoglycosphingolipids (psychosine). PPAR-alpha did not bind to N-acylated sphingoid bases (ceramides) or to sphingenine phosphorylated at the primary hydroxy group (sphingenine-1-phosphate). The binding of PPAR-alpha to sphingoid bases might be of interest given the role of PPAR-alpha and sphingolipids in various cellular processes.

[New treatment strategies for hypertension. Which guidelines and how to apply them]. / Nouvelles stratégies thérapeutiques dans l'hypertension artérielle. Quelles recommandations et comment les appliquer?

The diagnosis of hypertension by blood pressure measurements taken in the physician's office has been called into question by several studies. The onset of cardiovascular events appears to correlate better with ambulatory blood pressure measurements than with those taken during consultation (either "white coat" or masked hypertension). While the US, WHO, French and European guidelines diverge as to the specific antihypertensive drug among the seven classes available should be chosen for first-line treatment, there is a consensus for specific choices as a function of the type of hypertension. In any case, most treatment trials show that more than two antihypertensive drugs are often necessary. Treatment can thus begin with two drugs. The optimal target blood pressure is defined by the US JNC7 according to whether the patient also has diabetes or a nephropathy. When hypertension is uncomplicated, the target level is 140/90 mmHg. In the case of diabetes or nephropathy, it is 130/80 mmHg. In all cases, diet and exercise changes are also necessary and it is essential that patients understand them if they are to comply with them. Diastolic blood pressure remains the most important figure for those younger than 50 years, but afterwards, systolic pressure is more relevant. Aortic pressure may be more closely associated with cardiovascular risk than the blood pressure measured at the brachial artery. The concept of comprehensive management is radically modifying our behavior : the hypertensive patient is now above all a patient at high cardiovascular risk and the treatments to consider must not be limited to antihypertensive drugs but must also include treatment of other cardiovascular risk factors (aspirin, statins, smoking cessation, etc.).

The neuronal migration defect in mice with Zellweger syndrome (Pex5 knockout) is not caused by the inactivity of peroxisomal beta-oxidation.

The purpose of this study was to investigate whether deficient peroxisomal beta-oxidation is causally involved in the neuronal migration defect observed in Pex5 knockout mice. These mice are models for Zellweger syndrome, a peroxisome biogenesis disorder. Neocortical development was evaluated in mice carrying a partial or complete defect of peroxisomal beta-oxidation at the level of the second enzyme of the pathway, namely, the hydratase-dehydrogenase multifunctional/bifunctional enzymes MFP1/L-PBE and MFP2/D-PBE. In contrast to patients with multifunctional protein 2 deficiency who present with neocortical dysgenesis, impairment of neuronal migration was not observed in the single MFP2 or in the double MFP1/MFP2 knockout mice. At birth, the double knockout pups displayed variable growth retardation and about one half of them were severely hypotonic, whereas the single MFP2 knockout animals were all normal in the perinatal period. These results indicate that in the mouse, defective peroxisomal beta-oxidation does not cause neuronal migration defects by itself. This does not exclude that the inactivity of this metabolic pathway contributes to the brain pathology in mice and patients with complete absence of functional peroxisomes.

Evidence that stimulation of growth hormone release by epinephrine and vasoactive intestinal peptide is based on cell-to-cell communication in the pituitary.

Epinephrine (Epi) evoked a strong concentration-dependent (1-1000 nM) rise of GH release from perifused rat anterior pituitary cells cultured as aggregates in a serum-free defined culture medium. Dexamethasone (Dex), added to the culture medium, enhanced the secretory response to Epi. Aggregates of pituitary cells separated by gradient sedimentation at unit gravity widely differed in responsiveness to Epi, provided Dex was added to the culture medium. The poorest response was seen in aggregates composed of a population highly enriched in large somatotrophs from adult male rats even when cultured in the presence of 80 nM Dex. However, when these large somatotrophs were coaggregated with various somatotroph-poor cell populations, all of which were enriched in lactotrophs, the GH response to Epi increased by a factor of 3-4. The latter populations also enhanced GH secretion stimulated by vasoactive intestinal peptide (1-10 nM). In contrast, the GH response to rat GH-releasing factor (GRF, 0.01-0.1 nM) was not significantly potentiated in the coaggregates. The facilitation of the GH response to Epi was not seen when Dex was omitted from the culture medium. All of the lactotroph-enriched populations enhancing the response to Epi also contained corticotrophs, but none were highly enriched in the latter cell type. The magnitude of the Epi effect on GH release was not affected when the large somatotrophs were coaggregated with enriched populations of gonadotrophs, thyrotrophs, or folliculostellate cells. However, coaggregation with GH3 tumor cells provoked some stimulation. The present data suggest that GH release stimulated by Epi is modulated by facilitatory interactions of somatotrophs with other cells, the latter being most likely lactotrophs, although participation of corticotrophs in this interactions cannot be unequivocally excluded. Facilitatory interactions also modulate GH secretion in response to vasoactive intestinal peptide, but the GH response to GRF weakly, if at all.

Beta-adrenergic stimulation of prolactin release from superfused pituitary cell aggregates.

l-Isoproterenol (l-ISO), a specific agonist of beta-adrenergic receptors, evoked a prompt rise of prolactin (PRL) release from superfused anterior pituitary cell aggregates established in culture for 5 days. The effect was concentration-dependent between 1 and 100 nM. d-Isoproterenol was more than 2 orders of magnitude weaker than the l-isomer. When dopamine receptors were blocked with domperidone, PRL secretion was also stimulated by l-epinephrine (E) and l-norepinephrine (NE), the rank order of potency being l-ISO greater than E much greater than NE. Under the latter conditions dopamine and the alpha-adrenergic agonists, clonidine and phenylephrine, had no stimulatory effect at 1 microM. Stimulation of PRL release by l-ISO and E was blocked by the beta-receptor antagonist, propranolol, but not by the alpha-receptor blocker, prazosin.

The glucocorticoid hormone dexamethasone reverses the growth hormone-releasing properties of the cholinomimetic carbachol.

We recently reported that dexamethasone (DEX) enhances acetylcholine (ACh) release from pituitary cell aggregates. In the present study, the effect of DEX on the GH-releasing properties of the cholinergic agonist carbachol (CCh) was investigated. Perifusion of hemipituitaries from 14-day-old rats with CCh stimulated basal GH release. CCh also increased basal GH release from organ-cultured pituitaries and from pituitary cells cultured as reaggregates, but only when the thyroid hormone T3 was supplemented to the culture medium. Pretreatment of the animals in vivo with DEX abolished the CCh-induced increase in basal GH release from hemipituitaries tested in vitro. Treatment of pituitary organ cultures and reaggregate cell cultures with DEX reversed the stimulation of basal GH release by CCh into an inhibition. CCh also inhibited isoproterenol- and GRF-stimulated GH release from DEX-treated pituitary cell reaggregates. In contrast, the responsiveness of tumoral GH3 cell aggregates to CCh was not dependent on T3 or DEX during culture. The half-maximal concentration of CCh for inhibition was significantly lower than that for stimulation (1 and 10 microM, respectively). Perifusion with CCh of DEX-treated cell reaggregates consisting of a highly enriched somatotroph population (greater than 90% GH immunoreactive cells), obtained by sequential velocity and buoyant density sedimentation of dispersed cells, also inhibited basal GH release. Pretreatment of pituitary cell reaggregates cultured in DEX-supplemented medium with pertussis toxin completely abolished the inhibition by CCh. The inhibition of GH release by CCh was not affected by the Na+ conductance blocker tetrodotoxin, the Cl- channel blocker picrotoxin, or the K+ channel blocker caesium, but was abolished by the Ca2+ channel blockers cadmium and verapamil. In conclusion, CCh is capable of both stimulating and inhibiting GH release in different pituitary in vitro assay systems; the inhibition is dependent on glucocorticoids and the stimulation on the thyroid hormone T3. The mechanism of action of the inhibition seems to involve a GTP-binding protein and most probably a decrease in calcium conductance in the somatotroph.

Evidence for functional communication between folliculo-stellate cells and hormone-secreting cells in perifused anterior pituitary cell aggregates.

Dispersed anterior pituitary cells from adult female rats were separated by gradient sedimentation at unit gravity. The small-sized cell population on top of the gradient consisted of 65.6 +/- (SE) 4.2% (n = 8) cells immunoreactive to antiserum against S-100 protein, a marker of folliculo-stellate (FS) cells in rat pituitary. The corresponding fraction derived from adult male or immature female rats were also enriched in S-100 positive cells but to a lower extent. Only small numbers of S-100 positive cells were found in medium- and large-sized cell populations. Coaggregating the S-100 cell-enriched populations from adult females with other pituitary cell populations resulted in a clear-cut inhibition of the GH response to rat GH-releasing factor and beta-adrenergic agents, of the PRL response to TRH and angiotensin II (AII) and the LH response to LHRH. The magnitude of inhibition increased with the number of FS cells put into the coaggregates. In perifused aggregates prepared from different gradient fractions from immature females, there was a negative correlation between the occurrence of FS cells and the magnitude of the PRL response to AII. The low responsiveness to AII in FS cell enriched aggregates was not abolished when these aggregates were redissociated into single cells. It is suggested that FS cells constitute an intercellular messenger system for local inhibitory control of pituitary hormone secretion which is not based on direct and intimate contact between the interacting cells.

Growth hormone-releasing factor secretion from fetal hypothalamic cell cultures is modulated by forskolin, phorbol esters, and muscimol.

The regulation of GRF secretion was studied using a fetal rat hypothalamic cell culture system. The cells were subjected to short term release experiments on days 10-18 after plating, and GRF secretion was assessed by RIA. The identity of GRF immunoreactivity in the incubation medium was confirmed by reverse phase liquid chromatographic analysis. Depolarization of the cells with 56 mM K+ evoked a 4-fold increase in basal GRF release. When cultures were pretreated for 6 days with the adenylate cyclase activator forskolin, basal GRF release was augmented in subsequent release experiments to levels 2-fold greater than those in the control cultures. In nonpretreated cultures, forskolin (1-100 microM) and the protein kinase C activator phorbol 12-myristate 13-acetate (10 nM-1 microM), stimulated basal GRF release in a dose-dependent fashion. The Ca2+ channel blocker verapamil (100 microM) significantly inhibited the GRF response to both forskolin and phorbol 12-myristate 13-acetate. The gamma-aminobutyric acid (GABA) agonist muscimol (0.1-10 microM) inhibited forskolin-stimulated, but not K+ stimulated, GRF release in a dose-dependent manner. This inhibition was reversed by the GABA antagonists bicuculline and picrotoxinin. Muscimol (10 microM) slightly suppressed basal GRF release. The present findings suggest that GRF secretion can be evoked by agents known to increase intracellular cAMP levels or activate protein kinase-C. They also support a role for GABA in the inhibitory control of GRF secretion.

Stimulation of prolactin secretion after short term or pulsatile exposure to dopamine in superfused anterior pituitary cell aggregates.

The dynamics of dopamine (DA) action on PRL release was studied in superfused rat anterior pituitary cell aggregates, cultured for for 5 days either in conventional or in serum-free defined medium. In aggregates cultured in conventional medium 0.1-1 nM DA applied for 20 min provoked a rapid and concentration-dependent inhibition of PRL release, lasting only a few minutes, after which there was a gradual rise in secretion up to near baseline levels. A sustained inhibition was obtained from DA concentrations more than or equal to 10 nM. When DA, used at the latter concentration, was withdrawn from the superfusion medium, a marked rebound secretion of PRL occurred, exceeding basal release for as long as 40-50 min. Rebound secretion was not followed by a compensatory fall in secretion rate. After a 10-min pulse of 10 or 30 nM DA, the amount of PRL released above baseline was considerably higher than the amount of PRL not released during the time DA was present. The latter stimulation of PRL release was not seen after a 40- or 90-min exposure time to DA. However, when DA was given for 40 min in 10 pulses of 4 min (4 min DA on 4 min DA off), a clear-cut stimulation of PRL release followed the termination of the pulses. When the serum used in the culture medium was extracted with dextran-coated charcoal, post-DA rebound secretion of PRL was markedly diminished. The latter secretion pattern partially reappeared when the extracted serum was supplemented with 10 nM dexamethasone. DA had similar effects on PRL release in aggregates cultured in serum-free defined medium. Dexamethasone did not affect DA-inhibition but strongly stimulated post-DA rebound, and this effect was potentiated by T3 present in the defined medium. There was three to four times more PRL secreted in excess of basal release than was inhibited during exposure to DA. The present data suggest a dual action of DA on PRL release: inhibition during tonic exposure to the catecholamine and inhibition-mediated stimulation after pulsatile exposure.

Regulation and cellular localization of the membrane-bound thyrotropin-releasing hormone-degrading enzyme in primary cultures of neuronal, glial and adenohypophyseal cells.

Using monolayer cultures from murine brain and reaggregate cell cultures of rat anterior pituitary we observed that TRH (pyroGlu-His-Pro-NH2) added to the culture medium was not taken up by these cells but hydrolyzed at the pyroGlu-His bond by an enzyme obviously located at the cell surface. This enzyme exhibited a high degree of substrate specificity and other characteristics of the membrane-bound TRH-degrading enzyme. Relatively high enzymatic activity was associated with cultured neuronal cells from embryonic rat brain while glial cells were almost devoid of this peptidase activity. Rather low, but significant activity was found on anterior pituitary cell aggregates. In agreement with previous in vivo studies we observed that the TRH-degrading ectoenzyme on adenohypophyseal cells was regulated by estradiol and stringently controlled by T3, but that the activity of the brain enzyme was not. When pituitary cells were separated according to their size and density and established in reaggregate cell culture, a close correlation was always observed between enzyme activity and the distribution of lactotrophic cells regardless of the animal models (eu- and hypothyroid adult male rats) used and the cell fractionation techniques (velocity sedimentation and sequential velocity/buoyant density sedimentation) employed. Such a close correlation was not observed with other cell types, such as the somatotrophic cells, the folliculo-stellate cells, the ACTH-producing AtT20 pituitary cells, or thyrotrophic cells. In conclusion, the high degree of substrate specificity, the tissue-specific regulation, and the very heterogeneous distribution of the TRH-degrading ectoenzyme on brain and pituitary cells strongly support the hypothesis that this enzyme serves very specialized functions in the transmission of TRH signals at specific target sites.

Peroxisome proliferators and T3 operate by way of distinct receptors.

Peroxisome proliferators and thyroid hormones have a number of common metabolic effects. The possibility that the signal transduction pathways of both groups of effectors converge at the receptor level was investigated. It was shown that T3, specifically bound to the rat thyroid beta-receptor, was not displaced to a significant extent by ciprofibrate or bezafibrate. No specific binding of T3 to the mouse peroxisome proliferator activated receptor could be demonstrated. In transactivation experiments peroxisome proliferators were unable to activate the thyroid receptor and T3 did not activate a chimeric receptor containing the ligand binding domain of the peroxisome proliferator activated receptor. It is concluded that peroxisome proliferators and thyroid hormone do not cross-react at the level of their nuclear receptors.