Thyroid hormone induces the generation of a novel putative protein in piscine ovarian follicle that stimulates the conversion of pregnenolone to progesterone

Influence of triiodothyronine (T(3)) on secretion of steroids and thyroid hormone receptor expression in chicken ovarian follicles

The present study was designed to (1) assess the role of triiodothyronine (T(3)) with regard to in vitro steroid hormone secretion by chicken ovarian follicles; (2) determine whether T(3) influences the in vivo function of the pituitary-ovarian axis in the hen; and (3) detect expression of thyroid hormone receptor (TR) mRNA in chicken ovarian follicles. In the first experiment, laying hens were decapitated 22.5h before ovulation. White prehierarchical follicles (1-8mm) and fragments of theca and granulosa layers of the 3 largest yellow preovulatory follicles F3-F1 (22-35mm) were incubated in a medium supplemented with T(3) (0, 0.1, 1, 10, 100, or 1000ng/mL) or ovine luteinizing hormone (LH) (10ng/mL) in combination with doses of T(3) (1, 10, and 100ng/mL). Triiodothyronine decreased basal and LH-stimulated estradiol secretion by white follicles and the theca layer of all preovulatory follicles. On the other hand, it increased progesterone secretion by F2 and F1 follicles. In the second experiment, hens were injected 1h after ovulation with saline (control) or T(3) (10microg/100g body weight, intraperitoneally). Results indicated that exogenous T(3) decreased plasma concentrations of LH and estradiol and increased plasma concentrations of progesterone. In the third experiment, using reverse transcription polymerase chain reaction (RT-PCR) analysis, expression of thyroid hormone receptor (TRalpha and TRbeta0), mRNA was detected in all of the ovarian compartments. The expression of TRalpha mRNA was relatively greater in comparison with TRbeta0. There were no differences between white ovarian follicles in the expression of TRalpha and TRbeta0 mRNA. A considerably higher TRalpha and lower TRbeta0 expression was detected in the granulosa layer of preovulatory follicles in comparison with the theca layer. In conclusion, the data indicate that thyroid hormones acting via nuclear receptors are involved in regulation of the pituitary-ovarian axis and processes associated with follicle growth and maturation.

Novel 3beta-hydroxysteroid dehydrogenases from gonads of the Nile tilapia: phylogenetic significance and expression during reproductive cycle

Multiple forms of 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD) and their differential tissue expression pattern have not been shown in any lower vertebrates. In the present study, we report cloning of two novel 3beta-HSDs and two variants from gonads of the Nile tilapia. 3beta-HSD cDNAs encode two peptides of 375 (3beta-HSD type-I/variant 1) and 367 (3beta-HSD type-II/variant 1) amino acid residues that share 31.9% homology. 3beta-HSD type-I/variant 1 shared high homology with other piscine counterparts while 3beta-HSD type-II/variant 1 exhibited homology to mammalian DeltaC27-3beta-HSD and multifunctional viral 3beta-HSD. The latter seems to be ancient form among vertebrates. Transiently transfected 3beta-HSDs' open reading frames in COS-7 cells converted exogenous pregnenolone/androsta-5-ene-3beta-17beta-diol to progesterone/testosterone. Tissue distribution pattern of 3beta-HSDs by RT-PCR revealed varied expression pattern. Northern blot analysis of 3beta-HSDs demonstrated steady or gradual rise in transcripts level at different gonadal stages. These data revealed the importance of novel 3beta-HSDs in teleosts and also provided phylogenetic significance.

Molecular biology of the 3beta-hydroxysteroid dehydrogenase/delta5-delta4 isomerase gene family

The 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) isoenzymes are responsible for the oxidation and isomerization of Delta(5)-3beta-hydroxysteroid precursors into Delta(4)-ketosteroids, thus catalyzing an essential step in the formation of all classes of active steroid hormones. In humans, expression of the type I isoenzyme accounts for the 3beta-HSD activity found in placenta and peripheral tissues, whereas the type II 3beta-HSD isoenzyme is predominantly expressed in the adrenal gland, ovary, and testis, and its deficiency is responsible for a rare form of congenital adrenal hyperplasia. Phylogeny analyses of the 3beta-HSD gene family strongly suggest that the need for different 3beta-HSD genes occurred very late in mammals, with subsequent evolution in a similar manner in other lineages. Therefore, to a large extent, the 3beta-HSD gene family should have evolved to facilitate differential patterns of tissue- and cell-specific expression and regulation involving multiple signal transduction pathways, which are activated by several growth factors, steroids, and cytokines. Recent studies indicate that HSD3B2 gene regulation involves the orphan nuclear receptors steroidogenic factor-1 and dosage-sensitive sex reversal adrenal hypoplasia congenita critical region on the X chromosome gene 1 (DAX-1). Other findings suggest a potential regulatory role for STAT5 and STAT6 in transcriptional activation of HSD3B2 promoter. It was shown that epidermal growth factor (EGF) requires intact STAT5; on the other hand IL-4 induces HSD3B1 gene expression, along with IL-13, through STAT 6 activation. However, evidence suggests that multiple signal transduction pathways are involved in IL-4 mediated HSD3B1 gene expression. Indeed, a better understanding of the transcriptional factors responsible for the fine control of 3beta-HSD gene expression may provide insight into mechanisms involved in the functional cooperation between STATs and nuclear receptors as well as their potential interaction with other signaling transduction pathways such as GATA proteins. Finally, the elucidation of the molecular basis of 3beta-HSD deficiency has highlighted the fact that mutations in the HSD3B2 gene can result in a wide spectrum of molecular repercussions, which are associated with the different phenotypic manifestations of classical 3beta-HSD deficiency and also provide valuable information concerning the structure-function relationships of the 3beta-HSD superfamily. Furthermore, several recent studies using type I and type II purified enzymes have elegantly further characterized structure-function relationships responsible for kinetic differences and coenzyme specificity.

Molecular cloning and characterization of 3beta-hydroxysteroid dehydrogenase/Delta5-Delta4 isomerase cDNAs from Japanese eel ovary

3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD) is a crucial steroidogenic enzyme which catalyzes an essential step in the biosynthesis of all classes of steroid hormones. Two closely related cDNAs, encoding Japanese eel ovarian types I and II 3beta-HSD, were cloned and characterized. Both cDNAs putatively encoded 375 amino acid residues sharing high sequence homology with those of rainbow trout (71%) and mammalian (approximately 45-50%) 3beta-HSD. Transient expression of types I and II 3beta-HSD in COS-7 cells revealed that both proteins possess 3beta-hydroxysteroid dehydrogenase as well as Delta(5)-Delta(4) isomerase activity for both pregnenolone and dehydroepiandrosterone, with the preference of pregnenolone over dehydroepiandrosterone as substrate, although the type I protein is more active than the type II. By northern blot analysis, a single band of the 3beta-HSD transcript of approximately 1.5kb in length was observed in ovarian tissue and the total transcript abundance of both 3beta-HSDs remained constant throughout ovarian development artificially induced by gonadotropin-rich salmon pituitary homogenate. This lack of change in 3beta-HSD transcript abundance during ovarian development did not correlate with the fluctuation of its enzymatic activity reported previously, which may suggest that changes in 3beta-HSD activity during ovarian development may be, in part, post-transcriptionally regulated in the Japanese eel ovary.

Thyroid hormone-induced protein (TIP) gene expression by 3,5,3(')-triiodothyronine in the ovarian follicle of perch (Anabas testudineus, Bloch): modulation of 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4)-isomerase enzyme by TIP

Our previous reports had shown that 3,5,3'-triiodothyronine (T(3)) induced the generation of a 52-kDa monomer protein, i.e., TIP (thyroid hormone-induced protein) in the perch ovarian follicle. TIP, in turn, increased progesterone formation by stimulating Delta(5)-3beta-HSD activity (3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase) [Eur. J. Endocrinol. 134 (1996) 128-135; Gen. Comp. Endocrinol. 113 (1999) 212-220]. In the present investigation, perch ovarian follicles were incubated in the absence (control) or the presence of T(3) or gonadotropin (GTH) or human chorionic gonadotropin (hCG). RNAs were isolated and allowed to hybridize with a radiolabeled TIP oligonucleotide probe prepared on the basis of the N-terminal 17-amino-acid sequence of TIP. Only RNA from T(3)-incubated follicles hybridized with the probe, while RNA from control or GTH- or hCG-incubated follicles did not hybridize with the probe. The transcript size of TIP mRNA was approximately 1.8 kb. mRNA isolated from T(3)-incubated ovarian follicles subjected to in vitro translation and Western blot analysis clearly identified a 52-kDa protein which was not found with the mRNA from the control follicles. However, both TIP and GTH stimulated progesterone secretion from perch ovarian follicles in vitro. GTH stimulation of Delta(5)-3beta-HSD was due to the stimulation of enzyme protein synthesis as a more than twofold increase in Delta(5)-3beta-HSD occurred in response to GTH. But TIP did not stimulate synthesis of Delta(5)-3beta-HSD protein. However, in vitro incubation of Delta(5)-3beta-HSD enzyme with TIP in the presence of NAD and substrate (pregnenolone) greatly stimulated enzyme activity, while incubation with GTH had no effect, indicating a modulation of Delta(5)-3beta-HSD protein from a less active to a more active state by TIP. This has been supported by another observation, in which TIP (52 kDa) and Delta(5)-3beta-HSD (45 kDa) incubation resulted in a complex of 99 kDa. This suggests a protein-protein interaction in the process of Delta(5)-3beta-HSD activation by TIP. The present work, therefore, shows some new and interesting aspects of thyroid hormone regulation of the reproductive control mechanism.

The hypothalamic-pituitary-thyroid axis and the female reproductive system

Increasing evidence derived from experimental and clinical studies suggests that the hypothalamic-pituitary-thyroid axis (HPT) and the hypothalamic-pituitary-ovarian axis (HPO) are physiologically related and act together as a unified system in a number of pathological conditions. The suggestion that specific thyroid hormone receptors at the ovarian level might regulate reproductive function, as well as the suggested influence of estrogens at the higher levels of the HPT axis, seems to integrate the reciprocal relationship of these two major endocrine axes. Both hyper- and hypothyroidism may result in menstrual disturbances. In hyperthyroidism the most common manifestation is simple oligomenorrhea. Anovulatory cycles are very common. Increased bleeding may also occur, but it is rare. Hypothyroidism in girls can cause alterations in the pubertal process; this is usually a delay, but occasionally it can result in pseudo-precocious puberty. In mature women hypothyroidism usually is associated with abnormal menstrual cycles characterized mainly by polymenorrhea, especially anovulatory cycles, and an increase in fetal wastage.

Gonadotropin-releasing hormone stimulates thyroid activity in a freshwater murrel, Channa gachua (ham.), and Carps, Catla catla (ham.) and Cirrhinus mrigala (ham.)

Injections of mammalian GnRH (mGnRH), salmon GnRH (sGnRH), and a homologous murrel, Channa punctatus, GnRH (cGnRH) to a murrel, Channa gachua, and a carp, Catla catla, at a dose of 1 microg/250 g body wt significantly increased plasma thyroxine (T4) levels above control. Piscine GnRHs (sGnRH and cGnRH) had significantly greater stimulatory effects compared with mGnRH. To observe whether this stimulatory effect by GnRHs is direct or indirect, thyroid follicles were isolated from hypobranchial muscles of freshwater murrel, C. gachua and incubated (1 x 10(6) follicles/well) in vitro at 30 degrees C for 2 h without hormone and for 3 h with hormones. Addition of these three GnRHs separately at a concentration of 1 microg/well stimulated T4 secretion; sGnRH and cGnRH caused greater secretion of T4 into the medium compared with mGnRH. Specificity of GnRH action in vitro was assessed by using anti-GnRH antibody which significantly (P < 0.01) inhibited GnRH-augmented T4 secretion. To gain further insight, 125I uptake by thyroid follicles and formation of [125I]T4 from this radioiodine pool was monitored in the presence or absence of sGnRH. sGnRH greatly augmented 1251 uptake by the follicles which resulted a fourfold increase in [125I]T4 formation out of this pool of 125I compared with the control. The results indicate GnRH stimulation of thyroid hormone formation and release in these teleosts suggesting a possible different mode of regulation of thyroid hormone secretion in teleosts.

Differential regulation of Leydig cell 3beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase activity by gonadotropin and thyroid hormone in a freshwater perch, Anabas testudineus (Bloch)

Leydig cells were isolated from the perch testes belonging to the pre-spawning stage by collagenase treatment and mechanical separation followed by percoll gradient. They were incubated in vitro either for 5 h or at different times in the absence (control) or presence of piscine gonadotropin (GTH, 2 microg (1 x 10(6) cells)(-1)) or 3,5,3'-triiodothyronine (T3, 50 ng (1 x 10(6) cells(-1)) or T3-induced protein (TIP, 2 microg (1 x 10(6) cells)(-1)). 3Beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase (3beta-HSD) activity was determined by the conversion of [3H]delta5-dehydroepiandrosterone (DHEA) to [3H]delta4-androstenedione or [3H]delta5-pregnenolone to [3H]delta4-progesterone (P4) or by spectrophotometric estimation of NADH formation from NAD. T3 significantly increased (P < 0.01) both delta5-DHEA to delta4-androstenedione and delta5-pregnenolone to delta4-P4 conversion in Leydig cells indicating stimulation of 3beta-HSD activity. T3 stimulation of 3beta-HSD activity could be inhibited by cycloheximide (50 microg ml(-1)) suggesting the involvement of T3-induced protein (TIP) which was isolated and purified earlier in this laboratory from goat Leydig cells [15]. Addition of TIP or GTH significantly stimulated Leydig cell 3beta-HSD activity (P < 0.01). However, there was a difference between TIP and GTH stimulation in time kinetic study where TIP enhanced 3beta-HSD activity at 1 h (P < 0.05), reached its peak at 3 h (P < 0.01) and then plateaued till 8 h. GTH, on the other hand, did not show any stimulation of 3beta-HSD activity for 2 h, stimulation was marked only at 3 h (P < 0.05), reached a peak at 6 h (P < 0.01) and then leveled off. Determination of Km and Vmax of the enzyme showed an increase in the velocity of reaction by GTH with unaltered Km. TIP increased both velocity and affinity of the enzyme. GTH significantly increased the synthesis of 3beta-HSD protein at 3 h (P < 0.01) reaching maximal stimulation at 6 h which clearly coincided with the enzyme activity. In contrast, TIP had no effect on 3beta-HSD protein synthesis, but its direct addition to 3beta-HSD enzyme preparation in vitro caused significant augmentation of the enzyme activity (P < 0.01) suggesting thereby its modulatory effect on the enzyme. Results, therefore, show that although both T3 and GTH stimulated perch testicular Leydig cell 3beta-HSD activity, T3 effect was not direct but mediated via TIP and there is a clear distinction between GTH and TIP stimulation. GTH increased the enzyme activity by stimulating 3beta-HSD protein synthesis while TIP acts directly on the enzyme modulating it from less active to more active state.

Thyroid hormone regulation of perch ovarian 3beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase activity: involvement of a 52-kDa protein

Ovarian follicles were collected from perch belonging to the prespawning (vitellogenic) stage and incubated in vitro for 5 h in the absence (control) and presence of 3, 5, 3'-triiodothyronine (T3). Addition of increasing concentrations of T3 from 12.5 to 100 ng/ml caused a linear increase of 3beta-hydroxysteroid dehydrogenase/delta5-delta4-isomerase (3 beta-HSD) activity to 50 ng and then it leveled off indicating a saturation of enzyme activity with 50 ng T3. T3 stimulation of 3beta-HSD activity could be blocked by cycloheximide indicating the involvement of T3-induced protein (TIP) isolated and purified earlier from this laboratory. Addition of fish TIP purified from perch ovarian follicle (fTIP) or rat granulosa cell TIP to ovarian follicular incubation at a dose of 5 microg/ml significantly increased (P < 0.01) 3beta-HSD activity. To observe whether TIP acts directly on the enzyme or not, 3beta-HSD from perch ovarian follicle was purified to homogeneity by the following steps: (i) Sephadex G 75 gel filtration, (ii) DEAE-Sephacel chromatography, and (iii) NAD-affinity column chromatography. Purified 3beta-HSD gave a clear single band on an SDS gel and its molecular weight is 45 kDa. Addition of fTIP to an assay mixture containing purified 3beta-HSD resulted in a fourfold increase of the enzyme activity. fTIP alone did not show enzyme activity when incubated with the radiolabeled substrate. Addition of T3 (50 ng) to the 3beta-HSD assay mixture had no effect on the enzyme activity. Determination of Vmax and Km of the purified enzyme in the absence (control) and presence of fTIP demonstrated a considerable increase of 3beta-HSD affinity and rate of enzyme reaction.