Effects of short-term 17β-estradiol treatment on the properties of t3-binding proteins in the plasma of immature rainbow trout,salmo gairdneri

The relationship between ingested thyroid hormones, thyroid homeostasis and iodine metabolism in humans and teleost fish

Oral l-thyroxine (T4) therapy is used to treat human hypothyroidism but T4 fed to teleost fish does not raise plasma thyroid hormone (TH) levels nor induce growth, even though oral 3,5,3'-triiodo-l-thyronine (T3) is effective. This suggests a major difference in TH metabolism between teleosts and humans, often used as a starting thyroid model for lower vertebrates. To gain further insight on the proximate (mechanistic) and ultimate (survival value) factors underlying this difference, the several steps in TH homeostasis from intestinal TH uptake to hypothalamic-hypophyseal regulation were compared between humans and teleosts, and following dietary TH challenges. A major proximate factor limiting trout T4 uptake is a potent constitutive thiol-inhibited intestinal complete T4 deiodination that is ineffective for T3. At the hepatic level, T4 deiodination, conjugation and extensive biliary excretion with negligible T4 enterohepatic recycling can further block teleost T4 uptake to plasma. Such protection of plasma T4 from dietary T4 may be particularly critical for piscivorous fish consuming thyroid tissue, rich in T4 but not T3. It would prevent disruption by unregulated ingested T4 of the characteristic acute and transient changes in teleost plasma T4 due to diel rhythms, food intake and stress-related factors. These marked natural short-term fluctuations in teleost plasma T4 levels are enabled by the relatively small and rapidly-cleared plasma T4 pool, stemming largely from properties of the plasma T4-binding proteins. Humans, however, due mainly to plasma T4-binding globulin, have a relatively massive circulating pool of T4 and an extremely well-buffered free T4 level, consistent with the major TH role in regulating basal metabolic rate. Furthermore, this large well-buffered and slowly-cleared plasma T4 pool, in conjuction with enterohepatic recycling and relaxation of hypothalamic-hypophyseal negative feedback, allows humans to temporarily 'store' ingested T4 in plasma, thereby sparing endogenous TH secretion and conserving thyroidal iodine reserves. Indeed, iodine conservation is likely the key ultimate factor determining the divergent evolution of the human and teleost systems. For humans, ingested iodine in the form of I^-, or TH and their derivatives, is the sole iodine source and may be limiting in many environments. However, most freshwater teleosts, in addition to their ability to assimilate dietary I^-, can derive sufficient I^- from their copious gill irrigation, with no selective advantage in absorbing dietary T4 which would disrupt their natural acute and transient changes in plasma T4. Thus T4 may act also as a vitamin (vitamone) in humans but not in teleosts; in contrast, T3, naturally ingested at much lower levels, may act as a vitamone in both humans and teleosts.

Thyroid disruption in walleye (Sander vitreus) exposed to environmental contaminants: cloning and use of iodothyronine deiodinases as molecular biomarkers

Thyroid hormones play a role in the initiation of ovarian maturation in fish. Thus, reports of delayed sexual maturation in female walleye (Sander vitreus) exposed to contaminants in the Ottawa River suggest the presence of endocrine disrupting chemicals. The objectives of this study were to assess the effects of environmental contaminants in the Ottawa River on thyroid hormones of immature walleye and to develop a molecular biomarker of thyroid status. Walleye were sampled in the Ottawa River at Deep River (reference site), at Rivière Blanche (downstream from the Ottawa and Gatineau municipal wastewater treatment plants outflows), and at Plaisance (downstream from a pulp and paper mill). Plasma thyroid hormone levels were measured by radioimmunoassay. Walleye at Plaisance had two-fold elevated levels of thyroxine (T(4)) and 1.5-fold elevated levels triiodothyronine (T(3)), whereas the molar ratio of T(3):T(4) was reduced by over 50% compared to Deep River. Plasma T(3) levels were also elevated by approximately 1.5-fold at Rivière Blanche. Three iodothyronine deiodinases, a family of enzymes responsible for converting the prohormone T(4) to biologically active T(3), as well as for inactivating these two hormones, were partially cloned in walleye. A real-time PCR assay of deiodinase expression indicated that hepatic mRNA levels of type I and type III deiodinase were not modified between sites, whereas they were increased for type II deiodinase at Rivière Blanche as compared to the other sites. The response of this novel molecular transcript indicates a divergence with that expected based on the effects of experimentally induced hyperthyroidism on fish deiodinase expression; additional endpoints are therefore necessary to interpret changes in thyroid hormones levels in fish exposed to environmental contaminants.

Thyroid hormone profile during annual reproductive cycle of diploid and triploid catfish, Heteropneustes fossilis (Bloch)

Triploid fishes generally show sterility along with retarded gonadal development and aneuploid gametes. In teleosts, thyroid hormones influence seasonal adaptations and annual events such as reproduction. In addition, thyroid hormone deposition in matured ova is important for reproductive success as the role of thyroid hormones in early development and metamorphosis is well established. The present study deals with measurements of free and total thyroxine (T4) and triiodothyronine (T3) in the plasma of triploid and diploid catfish Heteropneustes fossilis (Bloch) in a complete reproductive cycle. Accumulations of total T4 and T3 within the oocytes have also been measured during the spawning period from fishes of both ploidy groups. No difference of plasma free hormones was noticed between the diploids and triploids of both the sexes in any period of reproductive cycle, although, seasonal variations were noted in both the groups. A significant decrease in the total thyroid hormone levels was noticed in plasma of the diploids in the spawning period compared to triploid fish. During the same period, accumulation of THs was significantly higher in the oocytes of diploids than that of the triploids. Thyroid gland structure also revealed a higher state of activity in the female diploids than the triploids during spawning period. Lower activity of thyroid tissue, higher levels of THs in plasma, and lower accumulation of maternally derived hormones in the oocytes of triploid females during spawning period may be associated with sterility of triploids.

The effects of 17β-estradiol injections on thyroid hormone deiodination pathways in liver and other tissues of female and male rainbow trout (Oncorhynchus mykiss) at different stages of sexual maturity

The effects of two intraperitoneal injections of 17β-estradiol (E2) over 7 days were studied on thyroid hormone plasma levels and on activities of thyroxine (T4), 3,5,3'-triiodothyronine (T3), and 3,3',5'-triiodothyronine (rT3) outer-ring deiodination (ORD) and inner-ring deiodination (IRD) pathways in various tissues of female and male rainbow trout (Oncorhynchus mykiss (Walbaum, 1792)). E2 administered to adult females and males at different stages of sexual maturity increased liver mass, depressed plasma T3 levels with no change in plasma T4 levels, and severely decreased liver T4ORD activity. E2 also modestly depressed hepatic rT3ORD activity, but only at low substrate levels, and had no consistent effect on the hepatic IRD pathways. There were no E2-induced changes in brain, gill, or heart deiodination, but E2 increased kidney T3IRD activity. In contrast, an all-female stock of trout with rudimentary ovaries responded to E2 with an increase in liver mass but without change in plasma T4 and T3 levels or liver and brain deiodination activities. In conclusion, the decrease in plasma T3 levels in both male and female adult E2-injected trout may be due to both decreased hepatic T3 production and increased renal T3 degradation. However, thyroidal responses to E2 depend on physiological/developmental state and were absent in a highly immature all-female trout stock.

Vitellogenin association and oocytic accumulation of thyroxine and 3, 5,3'-triiodothyronine in gravid Fundulus heteroclitus

The association of the thyroid hormone (TH) thyroxine (T(4)) and its metabolically active metabolite 3,5,3'-triiodothyronine (T(3)) with serum vitellogenin (VTG) in gravid female and estrogenized male (E2+) Fundulus heteroclitus was investigated. In in vivo, time-course experiments, sera from gravid female fish exposed to [(125)I]T(4) showed time- and dose-dependent increases in total [(125)I]TH content. The [(125)I]T(4):[(125)I]T(3) ratio was also affected by dose and time. In analysis of sera from female fish, >80% of detected radioactivity was associated with VTG (approximately 35%) and a second chromatographic peak (approximately 45%), a lipoprotein fraction possibly consisting of high-density lipoproteins. In experiments comparing estrogenized versus control male fish, the presence of VTG significantly increased the overall quantity and altered the profile of serum protein-associated [(125)I]TH. When serum VTG was present in the very large quantities typical of male fish treated with high doses of E2, the majority (59-70%) of detected radioactivity was associated with VTG. Both [(125)I]T(4) and [(125)I]T(3) were detected in extracts from oocytes collected during the in vivo female study. The total TH content and [(125)I]T(4):[(125)I]T(3) ratios in these extracts presented an accumulation profile that mirrored, in a delayed manner, the profiles observed in sera data. Furthermore, this accumulation was related to oocyte maturational state (i.e., size) and, correspondingly, VTG uptake. Together, these data suggest an important role for VTG as a vector of maternal transfer for both T(4) and T(3).

Purification and characterization of thyroid-hormone-binding protein from masu salmon serum. A homolog of higher-vertebrate transthyretin

We purified a thyroid-hormone-binding protein (THBP) from serum of masu salmon at the stage of smoltification when the concentrations of endogenous thyroid hormones in plasma reach the highest levels. All steps of sequential column chromatography suggest that this THBP is responsible for most L-3,5,3'-triiodothyronine-binding activity in serum at this stage. The molecular mass of this protein was estimated to be 60 kDa by gel filtration but only 15 kDa by SDS/PAGE, which suggests that it is comprised of four identical subunits. The amino acid sequence of its N-terminal portion was highly similar to those of vertebrate transthyretins. These molecular features indicate that masu salmon THBP is a homolog of transthyretins from tetrapods. However, in contrast with mammalian transthyretins, the affinity of masu salmon transthyretin for L-3,5,3'-triiodothyronine was three times greater than for L-thyroxine. This rank order affinity is similar to that of avian and frog transthyretins. Scatchard analysis revealed that masu salmon transthyretin possesses a single class of binding site for L-3,5,3'-triiodothyronine, with a Kd of 13.8 nM at 0 degrees C. Taken together with the data reported by Chang et al. [Eur. J. Biochem. (1999) 259, 534-542], these results suggest that transthyretin has changed from a L-3,5, 3'-triiodothyronine-carrier protein to a L-thyroxine-carrier protein during mammalian evolution.

Thyroid hormone concentrations in the gonads of wild chum salmon during maturation

Changes in gonadal and plasma concentrations of thyroid hormones were examined at various stages of maturation in chum salmon (Oncorhynchus keta) caught in the Bering Sea and the Bay of Alaska. Plasma concentrations of thyroxine (T4) were less than 5 ng ml(-1), and those of 3,5,3'-triiodo-L-thyroxine (T3) were less than 2 ng ml(-1) I in both males and females, regardless of the degree of sexual maturity or the gonadosomatic index (GSI).There was no clear relationships between circulating thyroid hormone levels and tissue levels. The ovarian T4 concentrations were undetectable (less than 0.2 ng g(-1)) or less than 2 ng g(-1) when GSI was lower than 1%, but increased thereafter and reached a plateau of 8-10 ng g(-1) when GSI became 2%. The ovarian T3 concentrations were about 5 ng g(-1) when GSI was 1%, increased to a maximum level (20 ng g(-1)) when GSI was about 2%, and decreased to a constant level of 10 ng g(-1) thereafter. The T4 and T3 content in single oocyte increased proportionally to the oocyte volume, indicating a constant incorporation of the hormones into the oocyte.The T4 concentrations in the testis were 1 ng g(-1) or less regardless of the GS1. On the other hand, the T3 concentrations were highest (15 ng g(-1)) when the GSI was less than 1%, decreased thereafter when spermatocytes appeared in the testis, and became about 5 ng g(-1) I in testes containing spermatozoa, raising the possibility of a role for T3 during early gamete and/or gonad maturation of testes.

Effects of short-term 17 beta-estradiol treatment on the properties of T4-binding proteins in the plasma of immature rainbow trout, Oncorhynchus mykiss

To determine the effects of 17 beta-estradiol (E2) on the properties of the plasma proteins that bind L-thyroxine (T4) immature rainbow trout, Oncorhynchus mykiss, were injected intraperitoneally on days 0 and 3 with 0.5 mg E2-3-benzoate/100 g body weight, and plasma was sampled on days 4, 7, or 12. Control trout received peanut oil alone. E2 caused a small but significant decrease in the free T4 index. Saturation analysis on miniature G-25 Sephadex columns revealed at least two major T4-binding sites. Filtration on agarose Bio-gel A 1.5 also indicated two major T4-binding protein fractions with molecular weights of 150 and 55 kDa with a small proportion of T4 binding to a 1,500-kDa site presumed to be lipoprotein. Addition of unlabeled T4 displaced [125I]T4 from the 55-kDa site and unmasked an adjacent site of higher molecular weight. E2 increased the proportion of T4 bound to the low-affinity (150 kDa) site relative to that bound to the high-affinity (55 kDa) site, increased the level of protein associated with the 1,500-kDa site and its T4 binding, and also initiated the production of presumed vitellogenin (VTG), which bound a small amount of T4. It is concluded that the E2-induced depression in FT4 is caused by a shift in T4 binding between high-affinity and low-affinity sites, and also by binding of small amounts of T4 to presumed lipoprotein and VTG.

Patterns of thyroxine and triiodothyronine in serum and follicle-bound oocytes of the tilapia, Oreochromis mossambicus, during oogenesis

This study describes simultaneous measurements of thyroid hormones, thyroxine (T4) and triiodothyronine (T3), in the oocytes and serum of a female teleost fish over a complete reproductive cycle. We have identified patterns in circulating T4 and T3 levels as well as their accumulation into oocytes during the reproductive cycle of the tilapia (Oreochromis mossambicus). This is the first description of the patterns with which thyroid hormones accumulate in teleost oocytes. The sampling strategy used in the study eliminated the possible influences of covarying environmental factors that may affect thyroid hormone levels independently of reproductive events. Hormones in serum and oocytes were measured by radioimmunoassay utilizing miniature Sephadex columns. The total content of both thyroid hormones in the oocytes increased throughout most of the ovarian cycle as the oocytes increased in size from less than 2 mg to approximately 6.5 mg by ovulation. By contrast, concentrations of thyroid hormones in the oocytes rose only during the first third of post-spawning oocyte growth (up to approximately 2 mg) before attaining plateaus at approximately 6 ng/g for T4 and 13 ng/g for T3. Serum concentrations of T4 and T3 varied in cyclical patterns during oogenesis, dropping to lows of 3.4 ng/ml (T4) and 2.7 ng/ml (T3) when the oocytes were 1.5 and 2 mg, respectively, and then increasing to 6.5 ng/ml (T4) and 4.8 ng/ml (T3) when the oocytes reach approximately 6 mg. The concentrations of both hormones decreased shortly before spawning. Maximum concentrations of thyroid hormones in the oocytes were reached approximately 10 days prior to those in the serum. Although the serum levels of T4 were greater than those of T3, the reverse was found in the oocytes. Triiodothyronine appears to be accumulated selectively over T4 and the patterns with which both thyroid hormones accumulate in the oocytes of the tilapia do not appear to be tied to serum levels.

Identification and purification of a high-affinity thyroxine binding protein that is distinct from albumin and prealbumin in the blood of a turtle, Trachemys scripta

Fractionation of plasma proteins in the turtle, Trachemys scripta, confirmed the presence of a high-affinity thyroxine (T4) binding protein (TBP) that was distinct from albumin (ALB) and prealbumin (PA). The TBP was isolated by adsorption on a T4-affinity column and a high degree of purification was achieved by gel filtration and preparative electrophoresis. Analysis by reversed-phase HPLC showed a single peak of protein with T4 binding activity. The electrophoretic mobility of the TBP, based on staining and binding to [125I]T4 on nondenaturing polyacrylamide gels (PAGE), corresponded to that of the major T4 binding activity previously identified in plasma (ca. 60 kDa). PA was fractionated as a complex with retinol binding protein (PA-RBP) based on retinol associated fluorescence using ion exchange chromatography on DEAE-Sephacel and gel filtration. This complex behaved as a larger and more highly charged molecule than TBP; it was partially dissociated in low ionic strength basic solution. SDS-PAGE of the PA-RBP-enriched fraction revealed a major component of about 48 kDa (possibly free PA), with smaller components corresponding to those expected for free RBP (ca. 22 kDa) and subunits of PA (e.g., 14 and 28 kDa). ALB was purified by ion exchange chromatography on DEAE and gel filtration; it behaved as less basic than PA with MW approximately 67 kDa. TBP accounted for virtually all the T4 binding activity of whole plasma: TBP was about 100 times as active and PA and ALB were less than 1% as active as plasma. The binding affinity of purified TBP was similar to that of whole plasma from turtle and human (e.g., approx. 10(9) M-1 on Sephadex G-25).