Stimulation of thyroid function by several pituitary hormones results in an increase in plasma thyroxine and reverse triiodothyronine in tilapia (Tilapia nilotica)

Profiles of thyrotropin, thyroid hormones, follicular cells and type I deiodinase gene expression during ontogenetic development of tilapia larvae and juveniles

The aims of the present study are to determine whether triiodothyronine (T3) and/or thyroxine (T4) in tilapia larvae is gifted through the mother, and to investigate the change profiles of thyrotropin (TSH), thyroid follicular cells and type I deiodinase (D1) gene expression following larval development. T3 and T4 contents were measured using radioimmunoassay, thyrotropin was observed using immunocytochemistry, and the D1 gene was cloned and measured using real-time PCR. Results indicated that the β-TSH-immunoreactive cells (thyrotropin ICC) signals were detected at 9 dph (i.e., 9 days of post-hatching). Thyroid follicular cells were observed first at 3 dph, while the T3 contents of the whole body gradually decreased before 11 dph. T4 contents were detected until 13 dph, with higher secretion during 19-21 dph. In addition, the T3 synthesis was not inhibited by thiourea (TU) before 13 dph, but the TU response in the larvae appeared after 13 dph. Type I deiodinase (D1: GenBank accession number KC591724) was found to contain 2444 bases and encoded 248 amino acids. The D1 mRNA expression began to increase at 13 dph, with a higher expression during 15-19 dph. These results suggested that the T3 contents were maternally derived before 13 dph. Both thyroid hormonal changes and some parameters related to thyroid hormone synthesis in ontogenetic tilapia are discussed.

Thyrotropic activity of recombinant human glycoprotein hormone analogs and pituitary mammalian gonadotropins in goldfish (Carassius auratus): insights into the evolution of thyrotropin receptor specificity

Thyrotropin (TSH) is a pituitary glycoprotein hormone heterodimer that binds to its G-protein coupled receptor (TSH-R) at the thyroid to promote the synthesis and secretion of thyroid hormone. Very little is known about TSH-TSH-R interactions in teleost fish. Mammalian gonadotropins have been reported to have an intrinsic ability to activate teleost fish TSH-Rs, suggesting the TSH-R in teleost fish is more promiscuous than in other vertebrates. In this study we utilized the goldfish T(4)-release response and recombinant human TSH analogs as in vivo tools to evaluate the structural constraints on hormone-receptor interactions. We found that four positively charged lysines substituted for neutral or negatively charged amino acids within positions 11-20 of the glycoprotein hormone subunit α (GSUα) significantly increased biological activity of hTSH in fish, as it does in mammals. We further found that bovine follicle stimulating hormone but not luteinizing hormone, whose GSUα subunits also contain four lysine or arginine amino acid residues in the N-terminal portion of GSUα, was thyrotropic in goldfish, suggesting gonadotropin β subunit contributes to the heterothyrotropic activity. Though recombinant human FSH did not produce a dose-dependent increase in T(4), thyrotropic activity could be acquired with the addition of positively charged amino acids at the N-terminal portion of its GSUα, confirming the importance of the charge on those amino acids for activation of the goldfish TSH-R. These studies demonstrate that mammalian glycoprotein hormone analogs can be utilized to evaluate the conservation of receptor binding and activation mechanisms between fish and mammals.

The influence of stress on thyroid hormone production and peripheral deiodination in the Nile tilapia (Oreochromis niloticus)

The existence of an interaction between the adrenal/interrenal axis and the thyroidal axis has since long been established in vertebrates, including fish. However, in contrast to mammals, birds and amphibians, no effort was made in fish to expand these studies beyond the level of measuring plasma thyroid hormones. We therefore set out to examine the acute effects of a single dose of dexamethasone (DEX) on plasma thyroxine (T(4)) and 3,5,3'-triiodothyronine (T(3)) levels, as well as on the activity and mRNA expression of the different iodothyronine deiodinases in liver, gills, kidney and brain in Nile tilapia. To take into account the effect of handling stress, this treatment was compared both to a non-treated and to a saline injected group. In general, the observed changes were acute (3 and 6h) while values had returned to control levels by 24h post-injection. Only DEX administration caused an acute drop in circulating T(3) levels compared to non-treated animals, while none of the treatments affected plasma T(4) levels. This indicates that the DEX induced decrease in plasma T(3) levels was not due to a lowered thyroidal hormone production and secretion. DEX injection provoked a decrease in peripheral T(3) production capacity via a decrease in hepatic outer ring deiodination activity (both D1 and D2), whereas T(3) clearance increased by induction of the inner ring deiodinating D3 pathway in liver and in gills. Deiodination activities in kidney and brain were not affected. Effects of saline injection were only observed in liver, where D1 activity decreased and D3 activity increased as in the DEX group, but to a lesser extent. Real-time PCR showed that the changes in hepatic D3 were clearly regulated at the pretranslational level, while this was not confirmed for the other changes. Our results show that both handling stress and DEX injection acutely disturb peripheral deiodination activity in Nile tilapia. However, the effects of the long acting glucocorticoid analogue are more pronounced and result in a decrease in circulating T(3) availability.

A thyrotropin-like molecule from the pituitary of an Indian freshwater murrel: comparison of its biological activity with other thyrotropins

Murrel pituitary thyrotropin-like molecule (mTSH) was purified to homogeneity with the help of a convenient and sensitive in vitro assay system where addition of this material to the thyroid follicle incubation stimulated thyroxine (T(4)) secretion into the medium. Pituitary extract of a freshwater murrel, Channa punctatus, was solvent extracted to obtain glycoprotein enriched fraction. This was subjected to Sephadex G-100 gel filtration and eluate of void volume (peak I) showed strong TSH activity (as reflected from T(4) secretion) which was further purified by using concanavalin A-Sepharose, FPLC Mono Q and immunoaffinity chromatography. Purified mTSH gave a single band in PAGE, and SDS PAGE revealed two dissimilar subunits, alpha and beta. Addition of increasing concentrations of mTSH, Indian carp TSH (cTSH) and bovine TSH (bTSH) to in vitro murrel thyroid follicle incubations caused a linear increase in thyroxine (T(4)) release into the medium, effect was highest with mTSH and lowest with bTSH. However, in in vivo experiments, injections of increasing doses of mTSH to murrel elevated plasma T(4) level in a linear manner while bTSH gave a biphasic response. Addition of mTSH and bTSH to rat or goat thyroid epithelial cell incubations equally stimulated T(4) release into the medium, while cTSH had significantly less effect. Binding affinity (K(a)) and receptor occupancy (B(max)) of mTSH to murrel thyroid follicular membrane preparation was considerably higher in comparison to cTSH or bTSH whereas both mTSH and bTSH had nearly similar K(a) and B(max) with rat thyroid epithelial cell membrane preparation. Findings indicate that mTSH is a more potent TSH as compared to carp and bovine TSH in murrel and has equipotent biological activity as bTSH on rat and goat thyroid.

3,5,3'-Triiodothyronine (T3) clearance and T3-glucuronide (T3G) appearance kinetics in plasma of freshwater-reared male tilapia, Oreochromis mossambicus

Distribution and metabolism of the thyroid hormone 3,5, 3'-l-triiodothyronine (T3) were studied in several ways to gain insights into these processes in the warm water fish tilapia Oreochromis mossambicus. Trace doses of 125I-labeled T3 (T*3)1 were injected intraarterially, extraarterially, or intraperitoneally in freshwater-reared male tilapia to explore plasma clearance kinetic responses to these different input modalities. Multicompartmental analysis of the plasma clearance data indicated a kinetic distribution of T*3 much like that reported for the rat and human, with about 2% of total body T*3 in plasma, 5% in rapidly exchanging tissues such as kidney and liver, and 93% in slowly exchanging tissues such as muscle. However, plasma clearance rates (PCR, 5.37 mL/h . 100 g body wt) and plasma appearance rates (PAR3 = PCR x [T3] plasma = 36.3 ng/h . 100 g body wt) were quite different than these indices in rat and human and 5 to 50 times larger than values reported for rainbow trout. On a whole-body basis, normalized for body weight, the tilapia we studied produced and accumulated much more T3 than rat, human, or rainbow trout. Enzymatic and chromatographic analyses of the plasma clearance data samples indicated substantial production of labeled glucuronide, but not sulfate, conjugates of iodothyronines (TiG) of unknown origin appearing in plasma. The TiG appeared beginning a few hours postinjection, peaked at 6 hours, and yielded a predicted steady-state TiG level of 8.3% of the T3 level in plasma. In contrast, in published studies, no conjugates were detected in rainbow trout plasma from 2 to 24 h after iv injection of T*3, T*4, or reverse-T*3, although conjugates of all were present in bile. To our knowledge, although T3 and T4 sulfate conjugates are present in the sera of several mammals, this is the first quantification of iodothyronine glucuronides reported in blood of any species under normal conditions. This might have physiological significance for the tilapia, with T3G providing a reversible storage form of T3 in blood, as has been suggested for sulfate conjugates of T3 and T4 in blood of several mammals.

The application of an in vitro perfused liver preparation to examine the effects of epinephrine and bovine thyroid-stimulating hormone on triiodo-L-thyronine release from the liver of rainbow trout (Oncorhynchus mykiss)

An isolated, perfused rainbow trout liver preparation was developed to investigate the action of nonthyroidal hormones on hepatic thyroid hormone metabolism. Several assessments were made of the stability and viability of the preparations under a range of conditions, including measures of lactate dehydrogenase flux and tissue ATP and glycogen content, all of which indicated that the perfused liver was stable for the 60-min perfusion period. Moreover, the liver preparations were responsive to an epinephrine challenge and, throughout the series of experiments, sustained hepatic glucose release. Triiodo-L-thyronine (T3) flux from the liver preparation was significantly increased by the provision of thyroxine (T4) substrate. Epinephrine and bovine thyroid stimulating hormone (TSH) were perfused alone and in combination with T4 to evaluate the effect of these hormones on T3 flux from the liver. Both epinephrine and TSH significantly enhanced hepatic T3 flux in the absence of T3 substrate, but neither had an additional effect on T3 flux when perfused in combination with T4. The results of the study suggest that a relationship exists between the circulating levels of nonthyroid hormones and peripheral thyroid hormone metabolism that may be receptor-mediated.

Hypothalamic regulation of the pituitary-thyroid axis in the tilapia Oreochromis mossambicus

Electrolytic lesioning of the preoptic area resulted in an increase in plasma thyroxine (T4) and reverse triiodothyronine (rT3) 10 days later; plasma triiodothyronine (T3) levels were not affected, so that there was also a significant decrease in the T3:T4, but not rT3:T4, ratios. No significant changes in T4, T3, or rT3 levels were observed in fish with lesions in either the anterior or posterior portions of the lateral tuberal nucleus. The pituitary contents of growth hormone and the two prolactins were not affected by any lesion. This indicates that the preoptic area may play a role in the inhibitory regulation of the pituitary-thyroid axis in Oreochromis mossambicus, presumably by way of effects on thyrotropin secretion.

Transition from a hatchery to a laboratory environment induces inner-ring monodeiodination pathways for thyroid hormones in liver of rainbow trout, Oncorhynchus mykiss

In laboratory-acclimated rainbow trout (Oncorhynchus mykiss) the main hepatic deiodination pathway for thyroid hormones is L-thyroxine (T4) outer-ring deiodination (T4ORD), which produces biologically active 3,5,3′-triiodo-L-thyronine (T3); T4 inner-ring deiodination (T4IRD) as well as T3ORD and T3IRD activities are low or undetectable. Surprisingly, trout transported 48 h previously from a local hatchery to the laboratory demonstrated not only low T4ORD activity but also significant T4IRD and T3IRD activities. To test if the transition from hatchery to laboratory environment had induced the unexpected inner-ring deiodinations, we measured hepatic deiodinase activities over the same time frame in trout recently transported to the laboratory and also in trout retained undisturbed at the hatchery. Undisturbed hatchery trout showed typical hepatic deiodinase function: T4ORD activity predominated, while T3IRD, T4IRD, and T3ORD activities were basal. However, after 1–3 days in the laboratory, hepatic T4ORD activity was reduced and T4IRD and T3IRD activities were increased. By 5 days, deiodinase activities of laboratory trout reverted to the levels of hatchery trout. We conclude that physical disturbance can temporarily depress thyroidal status by simultaneously decreasing hepatic production of biologically active T3 and inducing degradation of T4 and T3.

One-step immunoaffinity purification and partial characterization of hypophyseal growth hormone from the African catfish, Clarias gariepinus (Burchell)

Growth hormone (GH) was purified from African catfish (Clarias gariepinus) pituitary extracts in a single step by use of immunoaffinity chromatography. A monoclonal antibody to chicken GH, which labels the catfish hypophyseal somatotropes in immunocytochemistry, was coupled to CNBr-activated Sepharose, and crude alkaline pituitary extracts were run over the immunoadsorbent. Reversed-phase high-performance liquid chromatography analysis of the eluted material suggested heterogeneity, whereas silver staining upon SDS-polyacrylamide gel electrophoresis showed one single band with an estimated molecular weight between 22,000 and 23,000 Da. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis of the same preparation revealed the presence of several components with molecular weights ranging from 20,170 to 20,900 Da. The amino terminus of the protein was homogeneous, and the first 50 residues matched the proposed sequence of GH from two other siluran species (Ictalurus punctatus and Pangasius pangasius), except for one substitution at position 3. These data unequivocally confirm the identity of the purified molecule as suggested by immunochemical evidence. The bioactivity of the GH preparation was demonstrated by the short-term effect of GH on T3 plasma levels in juvenile catfish.

Hormonal profile of growing male and female diploids and triploids of the blue tilapia,Oreochromis aureus, reared in intensive culture

Triploidy as a result of thermal shock exposure of fertilized eggs decreases the growth rate ofOreochromis aureus as compared to their diploid controls, but this is due to the higher female ratio present in triploids (86%) and the lower growth rate of females. When females and males are considered separately, the growth rate is not significantly different in diploids and triploids. Since triploidy results in a malfunctioning steroidogenesis in females (mainly testosterone (T) and 17β-estradiol (E2)), but does not affect the growth rate, it is concluded that female gonadal steroids do not influence growth unless in pharmacological concentrations. These low levels of gonadal steroids are generally accompanied by higher levels of gonadotropin (GtH), but the difference is not always significant.Despite their lower growth rate diploid females have higher plasma concentrations of growth hormone (GH) during several months compared to the triploid females and diploid males. 3,5,3'-triiodo-L-thyronine (T3) levels, however, are comparable between diploid and triploid females (except for 1 month), but higher in diploid males in 4 of the 5 months studied. 11-ketotestosterone (11kT) is always higher in males. These results indicate that the higher growth rate of males may be related to the high circulating levels of T3 and 11kT.

Triiodothyronine is necessary for the action of growth hormone in acclimation to seawater of brown (Salmo trutta) and rainbow trout (Oncorhynchus mykiss)

Brown (BT) and rainbow trout (RT) in freshwater (FW) were treated with ovine growth hormone (GH), GH + iopanoic acid (IOP), and GH + IOP plus triiodothyronine (T3) for RT only. After 1 week of treatment, trout were transferred to 30 o/oo SW and treatment continued. In FW, GH treatment increased significantly plasma T3 level (BT) and T3/T4 ratio (BT and RT) by stimulating T4 to T3 deiodination. In the GH + IOP group, the plasma T3 levels and T3/T4 ratio fell significantly as T4 to T3 deiodination was inhibited. In GH + IOP + T3-treated RT, plasma T3 and T3/T4 ratios increased significantly relative to other groups. No mortality occurred and plasma osmolarity (PO) was not altered by any treatment in FW. After transfer to SW, all IOP + GH trout died within 2 (BT) or 3 days (RT). All GH-treated or control BT survived to the end of the experiment (6 days). RT survival rates tended to be improved in GH and GH + IOP + T3 groups relative to controls. Correlatively on day 1 the PO increase was significantly higher in IOP + GH groups (BT and RT) than in the other groups and significantly lower in GH and GH + IOP + T3 treated RT than in controls from days 1 to 6. These data confirm the requirement of T3 and deiodination of T4 to T3 for the development of hypoosmoregulatory mechanisms in SW as previously shown (Lebel and Leloup 1992). Furthermore, the suppression of the hypoosmoregulatory effect of GH, when conversion of T4 to T3 was inhibited by IOP and the reversal when T3 was added to IOP + GH treatment suggests that GH osmoregulatory action in SW acts via the simulation of T4-5' monodeiodination which increases T3 production.

Thyroid hormone deiodinase systems in salmonids, and their involvement in the regulation of thyroidal status

The trout thyroid secretes L-thyroxine (T4) which undergoes enzymatic deiodination in liver and other tissues. Based on mammalian studies, T4 outer-ring deiodination (ORD) or T4 inner-ring deiodination (IRD) could generate respectively 3,5,3'-triiodo-L-thyronine (T3) or 3,3',5'-T3(rT3), while subsequent T3ORD or T3IRD could generate respectively 3,5-diiodo-L-thyronine (T2) or 3,3'-T2, and rT3ORD or rT3IRD could generate respectively 3,3'-T2 or 3',5'-T2. In practice, T4 in trout undergoes hepatic ORD to produce T3 but negligible IRD to produce rT3, and T3 in turn undergoes negligible ORD but modest IRD to produce 3,3'-T2. T4ORD, which is particularly important in converting T4 to the biologically more potent T3, also occurs in gill, muscle and kidney. At least two isozymes are involved: i) a high-affinity, propylthiouracil (PTU)-sensitive T4ORD which displays ping-pong kinetics, requires thiol as a cofactor, and is present in liver, gill and muscle, and ii) a low-affinity, PTU-insensitive T4ORD with sequential kinetics with a thiol cofactor, and is present in liver and kidney. Receptor-bound T3 is derived primarily from the plasma for kidney, mainly from intracellular sources for gill and about equally from both plasma and intracellular sources for liver. Thus, the high-affinity T4ORD may produce T3 for local intracellular use while the low-affinity 5'-monodeiodinase may produce T3 for systemic use. T4ORD activity responds to nutritional factors and the physiologic state of the fish. Furthermore, T3 administered orally for either 6 weeks or 24h reduces the functional level (Vmax) of hepatic T4ORD, and T3 added to isolated hepatocytes also reduces activity, indicating direct T3 autoregulation of T4ORD to maintain hepatocyte T3 homeostasis. However, T3 administration also induces T4IRD to produce biologically inactive rT3 and induces T3IRD to produce 3,3'-T2. Thus, the trout liver has several iodothyronine deiodinase systems which in a coordinated manner regulate tissue T3 homeostasis in the face of a T3 challenge. It does this by decreasing formation of T3 itself, by diverting T4 substrate to biologically inactive rT3 and by increasing the degradation of T3. These deiodinases differ in many respects from any mammalian counterparts.

Evidence for the kidney as an important source of 5'-monodeiodination activity and stimulation by somatostatin in Oreochromis niloticus L

Radioimmunoassay of 3,5,3'-triiodothyronine (T3) and thyroxine (T4) in the thyroidal region of mature male Oreochromis niloticus revealed stores of T4 but negligible levels of T3, yielding a very low T3/T4 ratio (0.3%). 5'-Deiodination (5-D) of T4 into T3 was examined in liver and kidney homogenates in vitro by radioimmunoassay of T3 with T4 as substrate. In both organs, the 5'-D activity was temperature dependent: at 4 degrees C, T3 production was below the level of detection and maximal in both tissues at 37 degrees C; and at 45 degrees C, the enzymatic activity was reduced. T3 production seemed to reach a plateau after 60 min of incubation. The reaction required exogenous thiol cofactor (dithiothreitol) and was inhibited partially or completely by propylthiouracil depending on the concentrations used. Hepatic and renal 5'-D activities were stimulated by somatostatin (SRIF) within 4 hr, but a subsequent increase in plasma T3 was observed only when SRIF was injected together with T4, while the magnitude of rT3 production decreased. It is concluded that almost all the circulating T3 is provided by peripheral T4 to T3 conversion since T3 RIA in thyroidal follicles demonstrated insignificant T3 production. The kidney may contain the large part of the functional deiodinase which converts T4 into T3. As in mammals and unlike in other fishes, there is not only 5'-D activity, but also 5-D activity, and both may be influenced by SRIF.

Purification of tilapia thyrotropin from a crude pituitary homogenate by immunoaffinity chromatography using a matrix of antibodies against porcine follicle-stimulating hormone

An immunoadsorbent matrix using antibodies against porcine follicle-stimulating hormone (pFSH), a high heterothyrotropic stimulant in tilapia, was used to purify tilapia thyrotropic hormone (t-TSH) from crude pituitary extracts. A homologous bioassay monitored TSH bioactivity during the purification. Thyroid hormones (thyroxine, T4; triiodothyronine, T3; and reverse triiodothyronine, rT3) and testosterone were measured in vivo in Tilapia nilotica. TSH activity eluted as one major peak at pH 2.8 using a PBS-glycine buffer. The TSH fraction increased plasma T4 and plasma rT3. The potency of tTSH was comparable to that of pituitary extract or its Con A II fraction; however, pFSH was a stronger thyroid stimulant. tTSH had no effect on plasma T3 levels and was free of gonadotropic activity, as indicated by its failure to alter plasma testosterone concentrations. Chromatographic and electrophoretic analyses demonstrated a high degree of purity. Like other vertebrate TSHs, the tTSH appeared to have a subunit structure with a possible microgeneity in one subunit.

Somatostatin increases plasma T3 concentrations in Tilapia nilotica in the presence of increased plasma T4 levels

An injection of ovine growth hormone, porcine follicle stimulating hormone, and bovine thyrotropin stimulating hormone increased in Tilapia nilotica plasma concentrations of thyroxine (T4) and reverse triiodothyronine (rT3) after 4 and 8 hr, whereas plasma concentrations of T3 were unaffected. An injection of somatostatin (SRIF) alone did not influence thyroid hormone levels. If, however, SRIF was injected together with these hormones, which raised plasma T4, or together with T4 itself, an increase in plasma concentrations of T3 could be observed, whereas the increase in rT3 was less pronounced. It is concluded that SRIF may change the normal 5-deiodinase (5-D) activity and increased rT3 during hyperthyroxinemia into a 5'-D activity and a rise in T3, respectively, in T. nilotica.