Iodothyronine 5'-deiodinating activity in the pineal gland

GABAergic signaling in the rat pineal gland

Pinealocytes secrete melatonin at night in response to norepinephrine released from sympathetic nerve terminals in the pineal gland. The gland also contains many other neurotransmitters whose cellular disposition, activity, and relevance to pineal function are not understood. Here, we clarify sources and demonstrate cellular actions of the neurotransmitter γ-aminobutyric acid (GABA) using Western blotting and immunohistochemistry of the gland and electrical recording from pinealocytes. GABAergic cells and nerve fibers, defined as containing GABA and the synthetic GAD67, were identified. The cells represent a subset of interstitial cells while the nerve fibers were distinct from the sympathetic innervation. The GABAA receptor subunit α1 was visualized in close proximity of both GABAergic and sympathetic nerve fibers as well as fine extensions among pinealocytes and blood vessels. The GABAB 1 receptor subunit was localized in the interstitial compartment but not in pinealocytes. Electrophysiology of isolated pinealocytes revealed that GABA and muscimol elicit strong inward chloride currents sensitive to bicuculline and picrotoxin, clear evidence for functional GABAA receptors on the surface membrane. Applications of elevated potassium solution or the neurotransmitter acetylcholine depolarized the pinealocyte membrane potential enough to open voltage-gated Ca(2+) channels leading to intracellular calcium elevations. GABA repolarized the membrane and shut off such calcium rises. In 48-72-h cultured intact glands, GABA application neither triggered melatonin secretion by itself nor affected norepinephrine-induced secretion. Thus, strong elements of GABA signaling are present in pineal glands that make large electrical responses in pinealocytes, but physiological roles need to be found.

DNA microarray analysis and functional profile of pituitary transcriptome under core-clock protein BMAL1 control

Although it is known to contain five cell types that synthesize and release hormones with a circadian pattern, the pituitary gland is poorly characterized as a circadian oscillator. By a differential microarray analysis, 252 genes were found to be differentially expressed in pituitaries from Bmal1(-/-) knockout versus wild-type mice. By integrative analyses of the data set with the Annotation, Visualization, and Integrated Discovery (DAVID) Bioinformatics Resources annotation analysis system, pituitary genes with altered expression in Bmal1(-/-) mice were dispatched among functional categories. Clusters of genes related to signaling and rhythmic processes as well as transcription regulators, in general, were found enriched in the data set, as were pathways such as circadian rhythm, transforming growth factor β (TGFβ) signaling, valine, leucine, and isoleucine degradation, and peroxisome proliferator-activated receptor (PPAR) signaling pathways. Gene Ontology term overrepresentation analyses revealed significant enrichment for genes involved in 10 key biological processes. To determine whether genes with altered expression in Bmal1(-/-) mice were actually circadian genes, we further characterized in the mouse pituitary gland the daily pattern of some of these genes, including core-clock genes. Core-clock genes and genes selected from three identified overrepresented biological processes, namely, hormone metabolic process, regulation of transcription from RNA polymerase II promoter, and cell adhesion, displayed a rhythmic pattern. Given the enrichment in genes dedicated to cell adhesion and their daily changes in the pituitary, it is hypothesized that cell-cell interactions could be involved in the transmission of information between endocrine cells, allowing rhythmic hormone outputs to be controlled in a temporally precise manner.

Histone modifications on the adrenergic induction of type II deiodinase in rat pinealocytes

Histone modifications have been shown to play an important role in regulating gene expression. In this study, we investigated the impact of histone modifications on the adrenergic-regulated transcription of type 2 deiodinase (Dio2), a CREB-target gene in the rat pinealocyte. Treatment of pinealocytes with inhibitors of aurora C, a histone kinase, resulted in an inhibitory effect on the adrenergic-stimulated histone H3 Ser10 phosphorylation and Dio2 transcription. Given the established link between histone phosphorylation and acetylation, the role of histone acetylation on the adrenergic-induced Dio2 transcription was investigated. Treatment of pinealocytes with histone deacetylase inhibitors inhibited the adrenergic-induced Dio2 transcription. Chromatin immunoprecipitation with antibodies against acetylated Lys14 of H3 showed an increase in DNA recovery of the promoter region of Dio2 following treatment with trichostatin A. Together, our results indicate that, beside activation of CREB, epigenetic factors such as histone modifications also play an important role in regulating Dio2 transcription.

Cellular and molecular basis of deiodinase-regulated thyroid hormone signaling

The iodothyronine deiodinases initiate or terminate thyroid hormone action and therefore are critical for the biological effects mediated by thyroid hormone. Over the years, research has focused on their role in preserving serum levels of the biologically active molecule T(3) during iodine deficiency. More recently, a fascinating new role of these enzymes has been unveiled. The activating deiodinase (D2) and the inactivating deiodinase (D3) can locally increase or decrease thyroid hormone signaling in a tissue- and temporal-specific fashion, independent of changes in thyroid hormone serum concentrations. This mechanism is particularly relevant because deiodinase expression can be modulated by a wide variety of endogenous signaling molecules such as sonic hedgehog, nuclear factor-kappaB, growth factors, bile acids, hypoxia-inducible factor-1alpha, as well as a growing number of xenobiotic substances. In light of these findings, it seems clear that deiodinases play a much broader role than once thought, with great ramifications for the control of thyroid hormone signaling during vertebrate development and metamorphosis, as well as injury response, tissue repair, hypothalamic function, and energy homeostasis in adults.

The role of repressor proteins in the adrenergic induction of type II iodothyronine deiodinase in rat pinealocytes

In this study, we investigated the transcriptional regulation of the adrenergic induction of type II iodothyronine deiodinase (Dio2) in rat pinealocytes. Treatment of pinealocytes with norepinephrine (NE) caused an increase in the mRNA level of Dio2 that peaked around 2 h and declined over the next 5 h. Both beta- and alpha1-adrenergic receptors contributed to the NE induction of Dio2 expression through a cAMP/protein kinase A mechanism. In pinealocytes that had been stimulated by NE, inhibition of transcription by actinomycin had no discernible effect on Dio2 expression. In contrast, inhibition of protein synthesis by cycloheximide enhanced the NE induction of Dio2 expression, suggesting the involvement of a repressor protein. Transient transfection of pinealocytes with adenovirus expressing small interfering RNA against Fos-related antigen 2 (Fra2) enhanced the NE induction of Dio2 expression, whereas the effect of overexpression of the full-length transcript of Fra2 was inhibitory. Time-course study indicated that preventing the NE induction of Fra2 enhanced the NE induction of Dio2 after 3 h, and the enhancement persisted beyond 6 h after NE stimulation. In comparison, transient transfection of pinealocytes with small interfering RNA against inducible cAMP early repressor (Icer) had no effect on the NE induction of Dio2 expression, whereas overexpression of the full-length transcript of Icer caused a small reduction of the NE-stimulated Dio2 expression. Together, our results support Fra-2 as an important transcriptional repressor that helps shape the time profile of the adrenergic induction of Dio2 expression in the rat pineal gland.

Daily rhythm in pineal phosphodiesterase (PDE) activity reflects adrenergic/3',5'-cyclic adenosine 5'-monophosphate induction of the PDE4B2 variant

The pineal gland is a photoneuroendocrine transducer that influences circadian and circannual dynamics of many physiological functions via the daily rhythm in melatonin production and release. Melatonin synthesis is stimulated at night by a photoneural system through which pineal adenylate cyclase is adrenergically activated, resulting in an elevation of cAMP. cAMP enhances melatonin synthesis through actions on several elements of the biosynthetic pathway. cAMP degradation also appears to increase at night due to an increase in phosphodiesterase (PDE) activity, which peaks in the middle of the night. Here, it was found that this nocturnal increase in PDE activity results from an increase in the abundance of PDE4B2 mRNA (approximately 5-fold; doubling time, approximately 2 h). The resulting level is notably higher (>6-fold) than in all other tissues examined, none of which exhibit a robust daily rhythm. The increase in PDE4B2 mRNA is followed by increases in PDE4B2 protein and PDE4 enzyme activity. Results from in vivo and in vitro studies indicate that these changes are due to activation of adrenergic receptors and a cAMP-dependent protein kinase A mechanism. Inhibition of PDE4 activity during the late phase of adrenergic stimulation enhances cAMP and melatonin levels. The evidence that PDE4B2 plays a negative feedback role in adrenergic/cAMP signaling in the pineal gland provides the first proof that cAMP control of PDE4B2 is a physiologically relevant control mechanism in cAMP signaling.

Selenium, the thyroid, and the endocrine system

Recent identification of new selenocysteine-containing proteins has revealed relationships between the two trace elements selenium (Se) and iodine and the hormone network. Several selenoproteins participate in the protection of thyrocytes from damage by H(2)O(2) produced for thyroid hormone biosynthesis. Iodothyronine deiodinases are selenoproteins contributing to systemic or local thyroid hormone homeostasis. The Se content in endocrine tissues (thyroid, adrenals, pituitary, testes, ovary) is higher than in many other organs. Nutritional Se depletion results in retention, whereas Se repletion is followed by a rapid accumulation of Se in endocrine tissues, reproductive organs, and the brain. Selenoproteins such as thioredoxin reductases constitute the link between the Se metabolism and the regulation of transcription by redox sensitive ligand-modulated nuclear hormone receptors. Hormones and growth factors regulate the expression of selenoproteins and, conversely, Se supply modulates hormone actions. Selenoproteins are involved in bone metabolism as well as functions of the endocrine pancreas and adrenal glands. Furthermore, spermatogenesis depends on adequate Se supply, whereas Se excess may impair ovarian function. Comparative analysis of the genomes of several life forms reveals that higher mammals contain a limited number of identical genes encoding newly detected selenocysteine-containing proteins.

Stimulatory effect of melatonin on the 5'-monodeiodinase activity in the liver, kidney, and brown adipose tissue during the early neonatal period of the rabbit

The response of type I 5'-monodeiodinase activity (5'-MD) to a s.c. injection or oral administration of melatonin was studied in 3-, 5-, and 7-day-old rabbits. Melatonin-treated animals showed higher activity of the type I 5'-MD in the liver and kidney and of type II 5'-MD in brown adipose tissue (BAT). This respond to melatonin treatment was age dependent. The stimulatory effect of melatonin on renal 5'-MD activity was observed only in 3- and 5-day-old rabbits and in the liver and BAT during the first week of life. Oral melatonin administration tended to exert a more marked effect on enzyme activity than s.c. injection of the hormone. Changes in 5'-MD activities were accompanied by an increase in serum iodothyronine (T4, T3, and rT3) concentrations. The T3 and rT3 increases may result from the deiodinating processes by the type I 5'-MD and 5-MD, respectively, whereas the rise in the serum T4 was probably due to the stimulatory effect of melatonin on the secretory activity of the thyroid gland itself. These results are the first description of the effects evoked by melatonin treatment during the early neonatal period in newborns of the altricial type.

Beta- and alpha-adrenergic mechanisms are involved in regulating type II thyroxine 5'-deiodinase in rat thymus

The role of adrenergic receptors in regulation of rat thymus type II thyroxine 5'-deiodinase (5'-D) activity was investigated. Our results show that norepinephrine, an alpha- and beta-adrenergic agonist elicited an increase in thymus 5'-D activity. Isoproterenol, beta-adrenergic agonist, also increased the enzyme activity, being less effective than norepinephrine. Moreover, alpha-adrenergic agonists, i.e., methoxamine, an alpha1-agonist, and clonidine, an alpha2-agonist, did not increase 5'-D activity. The effect of isoproterenol was potentiated by methoxamine, but the potentiating effect was observed only at doses of isoproterenol which induce submaximal activation of the enzyme. Administration of propranolol, beta-adrenergic blocker, and prazosin, an alpha1-adrenergic blocker, inhibited significantly the activation of the enzyme induced by norepinephrine. However, yohimbine, an alpha2-adrenergic blocker, had small effect. These results show, in hypothyroid rats, a clear regulation by adrenergic mechanisms of 5'-D activity in the thymus, where alpha- and beta-adrenergic receptors might be involved.

Circadian rhythm of type II 5'deiodinase activity in the rat hypothalamic-pituitary-adrenal axis

Type II 5' deiodinase (5'D-II) activity was assessed in the hypothalamus (HP), pituitary (PIT), and adrenal gland (AG) of adult male rats after two different photoperiodic regimes: (1) cyclic light-dark conditions (LD, 12∶12) or (2) 4 days in continuous darkness (DD). Under cyclic LD conditions HP, PIT and AG 5'D-11 activity showed a significant circadian rhythm (P<0.004-<0.0001). The acrophases obtained by cosinor analysis were: HP, 12∶37; PIT, 03∶04 and AG, 02∶30 h. In contrast, after 4 days in DD, HP 5'D-II rhythmicity was abolished and there was a significant shift in the acrophase of both PIT and AG enzyme activity. These results suggest that the 5'D-II oscillation in PIT and AG corresponds to an endogenous circadian rhythm and is not only a response to the presence of a L/D cycle.

Adrenal gland 5'deiodinase activity (AG-5'd). Kinetic characterization and fractional turnover rate (FTr)

We determined the kinetic parameters as well as the fractional turnover rate (FTr) and half-life (t(1/2)) of rat adrenal gland 5'deiodinase activity (AG-5'D). Adrenal glands from male euthyroid or surgically thyroidectomized (Tx) Wistar rats were homogenized (HEPES, 10MM: ; pH 7.5; sucrose, 0.25M: ; EDTA 1MM: ) and centrifuged at 10,000g for 15 min at 4°C. The resulting crude microsomal supernatants were used for ail measurements of 5'D activity. Using rT(3) (2-500NM: ) the true Km and the Vmax values were of 20.2NM: and 289 fmol of I(-) release/mg protein/h. With T(4) as substrate these values were 5.8NM: and 622 fmol/h/mg protein. Protein inhibitor (cycloheximide 6 mg/100 g wt) administration allowed to determine an FTr of 0.68 h(-1) and a t(1/2) of 1.01 h. Results demonstrate that the greatest 5'D activity in the rat adrenal gland corresponds to isotype II, because the reaction is GTG and PTU-resistant (70-80%), accepts T(4) as a far better substrate than rT(3) (17-fold) and the former thyronine has a 50-90% inhibitory concentration in the 4-100NM: range. Furthermore, rats thyroidectomized for 5 and 15 days showed a conspicuous increase in cerebral cortex and adrenal 5'D-II activity. These characteristics as well as the rapid FTr and short †(1/2) are shared by type II 5'D present in rat pineal, pituitary and brain.

Pineal control of aging: effect of melatonin and pineal grafting on aging mice

Dark-cycle, night administration of the pineal hormone melatonin in drinking water to aging mice (15 months of age) prolongs survival of BALB/c females from 23.8 to 28.1 months and preserves aspects of their youthful state. Similar results were seen in New Zealand Black females beginning at 5 months and C57BL/6 males beginning at 19 months. As melatonin is produced in circadian fashion from the pineal, we grafted pineals from young 3- to 4-month-old donors into the thymus of 20-month-old syngeneic C57BL/6 male recipients, and a 12% increase in survival was induced. Prolongation of survival was also seen on pineal transplant to the thymus in C57BL/6, BALB/cJ, and hybrid female mice at 16, 19, and 22 months. In all studies, the endogenous pineal of grafted mice was left in situ. Pineal grafted aged mice display a remarkable maintenance of thymic structure and cellularity. Preservation of T-cell-mediated function, despite age, as measured by response to oxazolone is seen. Other evidence suggests that melatonin and/or pineal-related factors could produce their effects through an influence on thyroid function. These data indicate that pineal influences have a place in the physiologic regulation of aging.

Thyroxine type II 5'-deiodinase activity in pineal and Harderian gland is enhanced by hypothyroidism but is independent of serum thyroxine concentrations during hyperthyroidism

1. This paper studies the effect of thyroid status on 5'-D activity in pineal gland, Harderian gland, brown adipose tissue (BAT), pituitary gland, brain frontal cortex (BFC), and cerebellum. 2. Hypothyroidism clearly increased diurnal 5'-D activity in Harderian gland, BAT, pituitary gland, BFC, and cerebellum. In pineal gland, diurnal values of 5'-D activity were not affected by hypothyroidism. 3. Hypothyroidism in adult rats clearly enhanced nocturnal increase of 5'-D activity in pineal and Harderian gland. Congenital hypothyroidism also enhanced the nocturnal increase of 5'-D activity in pineal gland. 4. Hyperthyroidism inhibited 5'-D activity in pituitary gland, BFC, and cerebellum. A small inhibition, although significant, was found in BAT. 5. In pineal and Harderian gland, hyperthyroidism did not inhibit either the basal diurnal values of the enzyme or the nocturnal increase of its activity. 6. Results suggest that, in tissues where 5'D-activity is regulated by adrenergic mechanisms, mostly pineal gland and Harderian gland, the enzyme activity is independent of serum T4 concentrations during hyperthyroidism.

Potentiating effect of phenylephrine on isoproterenol activation of thyroxine type II deiodinase in the pineal gland of adult rats

In the present study we show, for the first time, that phenylephrine (PHE), an alpha-adrenergic receptor agonist, potentiates the effect of isoproterenol (ISO), a beta-adrenergic agonist, in activating pineal type II5'-deiodinase (5'-D) activity. The potentiating effect of PHE was observed only at doses of ISO which induce submaximal activation of the enzyme. However, at doses which lead to maximal activation of the enzyme, PHE was ineffective. The results suggest that not only beta-, but also alpha-adrenergic receptors, are involved in the sympathetic noradrenergic regulation of pineal 5'-D activity in the adult rat.

Effect of chronic ethanol administration on the rat pineal N-acetyltransferase and thyroxine type II 5'-deiodinase activities

Chronic ethanol intake resulted in a significant decrease in the rate of ponderal growth and an impaired nyctohemeral profile of pineal N-acetyltransferase (NAT) activity. In ethanol-treated animals, the onset of the nocturnal NAT increase is delayed by 2 hours when compared to control animals. Moreover, pineal NAT nocturnal peak was reached at 4 h (2 hours later than controls), while pineal type II thyroxine 5'-deiodinase (5'-D) nyctohemeral profile was not modified by ethanol administration. The effect of ethanol administration (12 weeks) on 5'-D activity in different tissues was also studied. Ethanol induced a 5'-D activity increase in hypothesis and brain frontal cortex, when compared to control animals. No change in 5'-D activity is observed in either pineal gland, Harderian gland, or brown adipose tissue. Since basal values of 5'-D activity in hypophysis or brain frontal cortex are particularly dependent on serum thyroxine (T4) concentration, the effect of chronic ethanol administration on thyroid hormone levels was studied. Serum T4 levels in ethanol-treated animals were significantly decreased when compared to controls at any time point studied. However, no change in serum 3',3,5-triiodothyronine (T3) levels were found.

Vasoactive intestinal peptide stimulates type II thyroxine 5'-deiodinase and N-acetyltransferase activities in dispersed pineal cells of euthyroid and hypothyroid rats

The effect of vasoactive intestinal peptide (VIP) on thyroxine type II 5'-deiodinase (5'-D) and N-acetyltransferase (NAT) activities were studied using pineal cells of euthyroid and hypothyroid rats. VIP activated 5'-D activity in a dose-dependent manner in both euthyroid and hypothyroid animals. However, basal and VIP stimulated activity was higher in pinealocytes from hypothyroid than in cells from euthyroid rats. VIP was also able to stimulate NAT activity but hypothyroidism did not induce modifications in its activity. Both 5'-D and NAT activities were stimulated not only by VIP, but also by isoproterenol, a beta-adrenergic receptor agonist, and forskolin, a potent activator of adenylate cyclase activity. The results suggest that VIP may be involved in the physiological regulation of pineal 5'-D activity.