Avian hepatic T3 generation by 5'-monodeiodination: characterization of two enzymatic pathways and the effects of goitrogens

The enzymatic nature of 5'-monodeiodination (5'-D) in avian liver homogenates was demonstrated by abolishment of activity by iopanoic acid (IOP). T3 production from T4 was dependent on enzyme and substrate concentrations, incubation time, incubation temperature, and pH. Two pathways of 5'-D activity were present in avian liver and exhibited characteristics similar to those described in mammalian tissues. Type II activity was identified as propylthiouracil (PTU)-insensitive activity. Type I (PTU-sensitive) was determined by difference between Total and Type II. Km values were 1.58 microM T4 for Total activity and 0.90 nM T4 for Type II, corresponding to the characteristics of the mammalian pathways. The effects of goitrogens on avian hepatic 5'-D were equivalent to those reported for the mammalian enzyme.     

Hepatic 5'-deiodinase activity of Japanese quail using reverse-T3 as substrate: assay validation, characterization, and developmental studies

Using rT3 as substrate, an in vitro 5'D assay was validated for use with liver tissue from adult Japanese quail, by defining conditions under which activity is proportional to enzyme (protein) concentration and is linear with incubation time. Activity was measured as the release of 125I from labeled rT3. Using validated assay conditions we found the following 5'D characteristics: maximal activity from 10 to 50 mM dithiothreitol (cofactor), an apparent Km of 0.52 microM rT3, pH optimum of 7.6-8.5, complete inhibition by 1 mM propylthiouracil and by 1.0 mM iopanoic acid, and substrate "preference" of rT3 greater than T4 greater than T3. Based on these characterizations the quail hepatic 5'D activity is like the Type I 5'D activity found in mammalian liver and kidney and embryonic chicken liver. To determine how previous unvalidated assays, that used high tissue and relatively low substrate (T4) concentrations, influenced 5'D studies we reevaluated 5'D development using an assay validated for each developmental stage with rT3 as substrate. We found extreme quantitative differences in the activities measured and in the proportional relationships between stages, and only limited qualitative similarity in the pattern of 5'D development when unvalidated T4 assay results were compared with validated rT3 assay results. Our data in this paper show good correspondence between whole liver 5'D activity per unit body weight and plasma T3/T4 ratios for the developmental stages sampled.     

Gene activation of alcohol dehydrogenase in Japanese quail and chicken-quail hybrid embryos

No preferential activation of the maternally derived alcohol dehydrogenase (ADH) allele was found in any of the chicken male x Japanese quail female hybrids examined. ADH activity in the liver was, in fact, found to exist in two different cathodal zonal regions on starch gel electropherograms; the zone II bands appeared at day 5 of incubation in the quail embryo (day 6 in the hybrid embryo) and the zone I bands appeared in 9-day quail embryos (10-day hybrid embryos). By day 13 of incubation, only the faster-migrating zone I bands could be detected in both quail and hybrid embryos.     

Serotonin effect on deiodinating activity in the rat

Serotonin (5-HT) and thyroid hormones are part of a complex system modulating eating behaviour and energy expenditure. 5-Deiodinase (5-D) converts the relatively inactive thyroxine (T4) to triiodothyronine (T3), and its activity is an indirect measure of T3 production in peripheral tissues, particularly in the brain, intrascapular brown adipose tissue (IBAT), heart, liver, and kidney. We evaluated the effect of 5-HT on 5'-D activity during basal conditions and after short (30 min) cold exposure (thyroid stimulating hormone stimulation test, TST). 5'-D activity was assessed in the liver, heart, brain, kidney, and IBAT. TST increases 5'-D activity in the brain, heart, and IBAT and decreases it in kidney, leaving it unchanged in the liver. 5-HT alone did not modify 5'-D activity in the organs under study but decreased it in the IBAT, heart, and brain when injected before the TST was administered. Our results confirm the important role of 5-HT in thermoregulation, given its peripheral site of action, in modulating heat production controlling intracellular T3 production. These effects are more evident when heat production is upregulated during cold exposure in organs containing type II 5'-D, such as the brain, heart, and IBAT, which are able to modify their function during conditions that alter energy balance. In conclusion, 5-HT may also act peripherally directly on the thyroid and organs containing type II 5'-D, thus controlling energy expenditure through heat production.     

Co-ordinated induction of beta-carotene cleavage enzyme and retinal reductase in the duodenum of the developing chicks

The developmental patterns of expression of beta-carotene cleavage enzyme activity were compared with those of retinal reductase and NAD-dependent retinol dehydrogenase activities in chick duodenum during the perinatal period. The beta-carotene cleavage enzyme activity was not detected in the duodenum before hatching, but it increased rapidly during 24 h after hatching. On the other hand, a considerable level of beta-carotene cleavage enzyme activity was observed in the liver of embryonic stages and its activity gradually rose during the perinatal period. Comparison of kinetic constants for the beta-carotene cleavage enzyme activities in the duodenum and the liver indicated that the enzyme in the duodenum possessed a lower affinity for beta-carotene than that in the liver. The retinal reductase activity was detected in the microsomes of the duodenum at the earliest time examined, i.e. day 16 of embryogenesis and its activity began to rise on the last day of embryogenesis, which was followed by a gradual increase until 1 day of age. The NAD-dependent retinol dehydrogenase activity was also seen in the microsomes of the duodenum in embryonic stages and its activity increased in parallel with the retinal reductase activity around the hatching period. These developmental inductions of beta-carotene cleavage enzyme and retinal reductase activities in the duodenum coincided with those of cellular retinol-binding protein, type II (CRBPII) and lecithin: retinol acyltransferase (LRAT). These results suggest that a co-ordinated induction mechanism should be operative for beta-carotene cleavage enzyme and retinal reductase, both of which are inevitable in the process of beta-carotene absorption and metabolism.     

Effect of the sex-linked dwarf gene on thyrotrophic and somatotrophic axes in the chick embryo

Plasma concentrations of thyroxine (T4), triiodothyronine (T3), reversed triiodothyronine (rT3), and insulin-like growth factors I and II (IGF-I, IGF-II) together with peripheral 5'-monodeiodination activity were measured in both normal and sex-linked dwarf embryos between day 14 of incubation and day 1 posthatch. Plasma T4 levels increased gradually during embryonic development while T3 concentrations remained low until day 20, when a sharp increase was observed. rT3 levels also increased from day 14 and dropped on day 20 when T3 levels started to increase. 5'-monodeiodination activity was high on day 14 of incubation, decreased thereafter, and showed an increase at the time of air sac penetration together with increased T3 levels. At this stage, differences between normal and dwarf embryos were observed; the latter had lower nonsignificant 5'-Monodeiodination activity and lower (P less than 0.01) plasma T3 levels. Plasma IGF-II levels were high during the whole embryonic period studied. Dwarf embryos had lower (P less than 0.05) IGF-II levels at the time of hatching. IGF-I levels were high on days 14 and 16, declined afterwards, and started to increase again around hatching. With the exception of T3 and IGF-II levels, introduction of the dwarf gene did not cause major changes in the hormonal parameters studied. This may explain the identical body weight at hatching.     

Food intake after hatching inhibits the growth hormone induced stimulation of the thyroxine to triiodothyronine conversion in the chicken.

The effect of a single injection of 10 micrograms chicken GH on circulating thyroid hormones as well as in vitro liver 5'-monodeiodination (5'-D) activity was studied in posthatch chicks submitted to different feeding conditions. One group was normally fed after hatching, a second group was only fed after three days and a third group was food deprived after 2 days of feeding. Combination of all results indicates that the start of food intake abolishes the stimulatory effect of a GH injection on circulating T3 and liver 5'-D activity. Food deprivation after a period of food intake restores the GH effect on plasma T3 but not on liver 5'-D.     

Ontogeny of insulin-like growth factor I and insulin receptor kinase activity in rat liver.

IGF-I and insulin receptors possess tyrosine-kinase enzymatic activity considered to be essential for signal transduction and thereby mediating the putative effects of these hormones on fetal growth and development. We investigated the ontogeny of IGF-I and insulin receptor tyrosine-kinase activity in at least 3 separate membrane preparations from liver of rats at 21 day of embryonic life (21ED), 1 and 5 day of postnatal life (1PD and 5PD respectively) and adult. Receptors purified by wheat germ agglutinin chromatography (WGA) were exposed to graded concentrations of IGF-I or insulin, and tyrosine-kinase activity was measured by quantifying incorporation of 32P into the exogenous substrate poly[Glu,Tyr; 4:1]. IGF-I stimulated tyrosine-kinase solely at 1 PD as documented by a maximal increase of 346 +/- 167% over basal kinase activity with 6.6 nmol/L IGF-I. While the lack of response in adult animals could be explained by a striking decrease in receptors at that age, 125I-IGF-I binding and affinity labelling of the WGA preparations indicated substantial IGF-I receptors were present in the liver at each of the perinatal ages. Furthermore, this dissociation between IGF-I binding and the tyrosine-kinase activity of these IGF-I receptors could not be attributed to the presence/absence of IGF-I binding proteins as judged by affinity labelling. In contrast, insulin-stimulated tyrosine-kinase activity was observed at all ages tested although it appeared greatest at 1PD. We conclude that (i) expression of IGF-I tyrosine-kinase activity is linked to developmental events and differs from that found for the insulin receptor tyrosine-kinase activity, (ii) during the perinatal period there is an apparent dissociation between ligand binding by the IGF-I receptor and receptor tyrosine-kinase activity. These observations suggest modulation of IGF-I receptor tyrosine-kinase activity may be an important regulator of IGF-I action during the perinatal period.     

Differential responses of rat pineal thyroxine type II 5′-deiodinase andN-acetyltransferase activities to either light exposure,isoproterenol, phenylephrine,or propranolol

1. Compared to pineal N-acetyl transferase (NAT) activity, which exhibited a dramatic drop following acute light exposure at night, nocturnal rat pineal thyroxine type II 5'-deiodinase (5'-D) activity was minimally influenced by the same light exposure. The injection of cycloheximide, a potent inhibitor of protein synthesis, although it did curtail the rise in NAT activity for at least 2 hr, did not elicit decreases in the activities of either 5'-D or NAT enzymes. Propranolol, a beta-adrenergic blocker, either delayed the continued nocturnal rise in 5'-D activity when injected at 0000 hr or slightly enhanced the fall in 5'-D activity when injected at 0200 hr. These results suggest that interruption of the synthesis of proteins is responsible for the slow deterioration of 5'-D activity induced by either light or propranolol. 2. The slight fall in 5'-D activity induced by light at night was prevented by isoproterenol; phenylephrine, however, did not prevent the fall and the effect of isoproterenol + phenylephrine was similar to that obtained with isoproterenol alone. On the other hand, the light-inhibited NAT activity recovered after the injection of isoproterenol; phenylephrine did not elicit any effect, but the injection of both isoproterenol and phenylephrine simultaneously caused a greater NAT response than that induced by isoproterenol alone. 3. When injected during the day, phenylephrine had no effect on either pineal 5'-D or NAT activities; however, the injection of either isoproterenol alone or isoproterenol + phenylephrine elicited 5-fold and 10-fold increases in nocturnal, light-suppressed 5'-D and NAT activities, respectively. During the day, phenylephrine did not potentiate the effects of isoproterenol on NAT activity as it did at night. When the effects of isoproterenol on the 5'-D activity were compared to rats exposed to light during the day and at night, the activity of 5'-D reached a higher level at night than during the day.     

Up-Regulation of Type 2 Iodothyronine Deiodinase mRNA in Reactive Astrocytes Following Traumatic Brain Injury in the Rat

Abstract: Type 2 5'-deiodinase (5'-D2), which converts thyroxine to the more active thyroid hormone 3,5,3'-triiodothyronine (T3), is believed to be an important source of intracellular T3 in the brain. The activity of this enzyme is increased in hypothyroidism and decreased in hyperthyroidism, and as such, it serves an important role to protect the brain from wide fluctuations in T3 during changes in thyroidal state. Although it has been hypothesized that T3 may facilitate neuronal regeneration after CNS injury, the 5'-D2 response to brain injury is unknown. To assess the 5'-D2 mRNA response to injury, we performed in situ hybridization following traumatic brain injury. In unlesioned animals, 5'-D2 mRNA was undetectable. At 3 days posttrauma, 5'-D2 mRNA was detected in ipsilateral cortex near the contusion. A significant further increase of 5'-D2 mRNA was noted 7 days posttrauma in both hippocampus and cortex. Similar response was also observed on the contralateral side. Colocalization of 5'-D2 mRNA with glial fibrillary acidic protein indicates that reactive astrocytes were the major cellular source for the trauma-induced 5'-D2 expression. These data demonstrate, for the first time, a trauma-induced, astrocytic up-regulation of 5'-D2 mRNA, suggesting a potential role for T3 action in adult brain's response to injury and recovery.     

Developmental changes in rat liver branched-chain 2-oxo acid dehydrogenase.

Branched-chain 2-oxo acid dehydrogenase catalyses the first irreversible step in the degradation of the branched-chain amino acids leucine, isoleucine and valine. With specifically labelled 4-methyl-2-oxo[1-14C]pentanoate as substrate, the enzyme's activity was measured in rat liver homogenates. Activity (per g wet wL of liver or per mg of protein) increased most rapidly during the perinatal period (2 days before to 1 day after birth), reaching approximately adult values by the time of weaning. The apparent Vmax, of the enzyme increased with age, but its Km appeared unchanged. The data suggest that hepatic branched-chain 2-oxo acid dehydrogenase is induced or activated during the perinatal period. The enzyme's activity at birth was unaffected by maternal diabetes, or by treating the mother with pharmacological doses of corticosterone or 3,3',5-tri-iodothyronine, during the last 5 days of pregnancy.     

Temporal and tissue-specific expression of myosin heavy chain isoforms in developing and adult avian muscle

We have raised monoclonal antibodies (Mabs) to myosin heavy chain isoforms (MHCs) that have specific patterns of temporal expression during the development of quail pectoral muscle and that are expressed in very restricted, tissue-specific patterns in adult birds. We find that an early embryonic, a perinatal, and an adult-specific, fast myosin heavy chain are co-expressed at different levels in the pectoral muscle of 8-12 day quail embryos. The early embryonic MHC disappears from the pectoral muscle at approximately 14 days in ovo, whereas the perinatal MHC persists until 26 days post-hatching. The adult-specific MHC accumulates preferentially and eventually completely replaces the other isoforms. These Mabs cross-react with the homologous isoforms of the chick and detect a similar pattern of MHC expression in the pectoral muscle of developing chicks. Although the early embryonic and perinatal MHC isoforms recognized by our Mabs are expressed in the pectoral muscle only during distinct developmental stages, our Mabs also recognize MHC isoforms present in the heart and extraocular muscle of adult quail. Immunofingerprinting using Staphylococcus aureus protease V8 suggests that the early embryonic and perinatal MHC isoforms that we see are strongly homologous with the adult ventricular and extraocular muscle isoforms, respectively. These observations suggest that at least three distinct MHC isoforms, which are normally expressed in adult muscles, are co-expressed during the early development of the pectoral muscle in birds. In this respect, the pattern of expression of the MHCs recognized by our Mabs in developing, fast muscle is very similar to the patterns described for other muscle contractile proteins.     

Evidence for chicken GH as the only hypophyseal factor responsible for the stimulation of hepatic 5'-monodeiodination activity in the chick embryo

The influence of an intravenous injection of chicken growth hormone (cGH), a total chicken pars distalis (PD) extract, and a PD extract depleted of cGH by immunoadsorption was studied in the 18-d-old chick embryo. Plasma concentrations of triiodothyronine (T3), thyroxine (T4), and hepatic 5'-monodeiodination (5'-D) activity were measured. An injection of total PD extract raised plasma T3, T4, and 5'-D activity, whereas a PD extract depleted of GH only increased plasma T4. The amount of cGH present in the PD extracts, as measured by homologous cGH radioimmunoassay, increased T3 and raised liver 5'-D, but had no effect on plasma T4. The effect on liver 5'-D was more pronounced with cGH than with a total PD extract, whereas the effect on plasma T3 was somewhat less pronounced. It was concluded that cGH increased the peripheral conversion of T4 into T3 in the chick embryo, whereas a PD extract depleted of cGH was purely thyrotropic. The PD extract also seemed to have 5'-D-suppressing activity.     

Bigger brains cycle faster before neurogenesis begins: a comparison of brain development between chickens and bobwhite quail

The chicken brain is more than twice as big as the bobwhite quail brain in adulthood. To determine how this species difference in brain size emerges during development, we examined whether differences in neurogenesis timing or cell cycle rates account for the disparity in brain size between chickens and quail. Specifically, we examined the timing of neural events (e.g. neurogenesis onset) from Nissl-stained sections of chicken and quail embryos. We estimated brain cell cycle rates using cumulative bromodeoxyuridine labelling in chickens and quail at embryonic day (ED) 2 and at ED5. We report that the timing of neural events is highly conserved between chickens and quail, once time is expressed as a percentage of overall incubation period. In absolute time, neurogenesis begins earlier in chickens than in quail. Therefore, neural event timing cannot account for the expansion of the chicken brain relative to the quail brain. Cell cycle rates are also similar between the two species at ED5. However, at ED2, before neurogenesis onset, brain cells cycle faster in chickens than in quail. These data indicate that chickens have a larger brain than bobwhite quail mainly because of species differences in cell cycle rates during early stages of embryonic development.     

Fas/FasL-mediated apoptosis in perinatal murine lungs

Postcanalicular lung development is characterized by a time-specific increase in alveolar epithelial type II cell apoptosis. We have previously demonstrated that, in fetal rabbits, developmental type II cell apoptosis coincides with transient upregulation of the cell death regulator Fas ligand (FasL). The aims of this study were 1) to determine the spatiotemporal patterns of pulmonary apoptosis and Fas/FasL gene expression in the murine model [embryonic day 17 (E17) through postnatal day 5 (P5)], and 2) to investigate the functional involvement of the Fas/FasL system by determining the effect of Fas activation and inhibition on perinatal pulmonary apoptosis. The apoptotic activity of alveolar epithelial type II cells, determined by combined TUNEL labeling and anti-surfactant protein B immunohistochemistry, showed a dramatic increase during the perinatal transition (type II cell apoptotic index <0.1% at E17, 1.5% at P1-P3, and 0.3% at P5). This timing of enhanced type II cell apoptosis coincided with a robust 14-fold increase in Fas mRNA and protein levels and a threefold increase in FasL protein levels; both Fas and FasL immunolocalized to type II and bronchial epithelial cells. In vitro and in vivo exposure of fetal and postnatal murine type II cells to anti-Fas antibody induced a fourfold increase in apoptotic activity that was prevented by administration of a broad-spectrum caspase inhibitor; the pulmonary apoptotic activity of Fas-deficient lpr mice remained unchanged. Conversely, administration of a caspase inhibitor to newborn mice (P1) resulted in marked diminution of pulmonary apoptotic activity. These combined findings strongly implicate the Fas/FasL system as a critical regulator of perinatal type II cell apoptosis. The developmental time dependence of apoptosis-related events in the murine model should facilitate investigations of the regulation of perinatal pulmonary apoptotic gene expression.     

Effect of in ovo leptin administration on the development of Japanese quail

Potential changes in the activity of endocrine axes related to growth as a result of leptin administration during embryonic development of birds were evaluated in the Japanese quail as a model bird with fast growth and development. On day 5 of incubation, 0.1 microg or 1 microg of recombinant mice leptin in 50 microl of phosphate buffered saline were injected into the albumen of eggs. Animals from each group were killed by decapitation on day 0, 2, 5, 7, 14, 21, 28, 35, 42, 49 and 56 of life. Plasma concentrations of triiodothyronine (T(3)), thyroxin (T(4)), corticosterone, testosterone, total lipids, triacylglycerols, cholesterol, glucose and alkaline phosphatase activity were measured. Quail treated by leptin hatched earlier (5-24 hours) and had a higher body weight than the control group (P<0.05-0.001). Mean body weight across the whole observed period was higher in both treated groups as compared to the control group (P<0.05). Leptin in ovo administration was accompanied by changes of endocrine and metabolic parameters during postembryonic development. The most prominent changes appeared immediately after hatching (T(3), T(4), total lipids, triacylglycerols) and before sexual maturity. It is suggested that leptin acts as a general signal of low energy status to neuroendocrine systems in birds which improves utilization of nutrients.     

Post-hatching thyroid development and body growth in precocial vs altricial birds

Thyroid growth, thyroid function and body growth are markedly different in developing precocial Japanese quail and altricial Ring doves despite comparability in incubation period, hatchling size, adult body weight and adult serum thyroid hormone concentrations. In quail thyroid activity is high during the perinatal period, declines shortly after hatching, then gradually attains adult function. In contrast, in doves, there is no perinatal peak of thyroid activity. Thyroid function is low at hatching and increases steadily during the first week. Serum hormone concentrations vary around a mean similar to that of adults during the remainder of the nestling and fledgling periods.     

Induction of 5-Deiodinase Activity in Astroglial Cells by 12-O-Tetradecanoylphorbol 13-Acetate and Fibroblast Growth Factors

In the brain, 5'-deiodinase (5'-D) is responsible for the metabolic activation of thyroxine (T4) into 3,5,3'-triiodothyronine (T3) and 5-deiodinase (5-D) deiodinates T4 and T3 into inactive metabolites. This study examines the effects of factors known to induce astroglial 5'-D activity on the 5-D activity in cultured rat astroglial cells. The potencies of these factors were compared after 8 h of incubation, when stimulations by these factors near their maximal effects. 12-O-Tetradecanoylphorbol 13-acetate (TPA) at 10(-7) M was a potent inducer of 5-D activity, producing a 30- to 80-fold increase after 8 h. The maximal effect of TPA was observed after about 14 h. The TPA stimulation of 5-D activity was not dependent on glucocorticoids, unlike 5'-D activity. In comparison with TPA, 8-bromo-cyclic AMP (10(-3) M) was a poor inducer of 5-D activity whereas it is an excellent inducer of 5'-D activity. It produced a 2- to 20-fold increase in 5-D activity after 8 h. Natural acidic fibroblast growth factor (20 ng/ml) produced a degree of stimulation similar to that of TPA after 8 h. The maximal effect of acidic fibroblast growth factor was observed after about 16 h (until a 120-fold increase). Recombinant acidic fibroblast growth factor also induced 5-D activity. Basic fibroblast growth factor was less potent than acidic fibroblast growth factor for increasing 5-D activity (maximal increase by 40- to 50-fold after 8 h).(ABSTRACT TRUNCATED AT 250 WORDS)     

Thyrotropic and peripheral activities of thyrotrophin and thyrotrophin-releasing hormone in the chick embryo and adult chicken

The influence of an intravenous injection of thyrotrophin-releasing hormone (TRH) and bovine thyrotrophin (TSH) on circulating levels of thyroid hormones and the liver 5'-monodeiodination (5'-D) activity is studied in the chick embryo and the adult chicken. In the 18-day-old chick embryo, an injection of 1 microgram TRH and 0.01 I.U. TSH increase plasma concentrations of triiodothyronine (T3) and of thyroxine (T4). TRH, however, preferentially raises plasma levels of T3, resulting in an increased T3 to T4 ratio, whereas TSH preferentially increases T4, resulting in a decreased T3 to T4 ratio. The 5'-D-activity is also stimulated following TRH but not following TSH administration. The increase of reverse T3 (rT3) is much more pronounced following the administration of TSH. In adult chicken an injection of up to 20 micrograms of TRH never increased plasma concentrations of T4, but increases T3 at every dose used together with 5'-D at the 20 micrograms dose. TSH on the other hand never increased T3 or 5'-D, but elevates T4 consistently. It is concluded that TSH is mainly thyrotropic in the chick embryo or adult chicken whereas TRH is responsible for the peripheral conversion of T4 into T3 by stimulating the 5'-D-activity. The involvement of a TRH induced GH release in this peripheral activity is discussed.