Binding of thyroid hormone to the goat testicular Leydig cell induces the generation of a proteinaceous factor which stimulates androgen release

Triiodothyronine stimulates steroid and VEGF production in murine Leydig cells via cAMP-PKA pathway

Thyroid hormones affect testicular development as well as functions like spermatogenesis and steroidogenesis, thereby influencing male fertility. Our group earlier showed that the stimulatory role of the thyroid hormone, T₃ , on the production of vascular endothelial growth factor (VEGF) by murine Leydig cells is mediated by steroids and hypoxia-inducible factor-1 (HIF-1α). The current study further defines the signalling pathway(s) utilised by T₃ to stimulate the production of steroids, VEGF and HIF-1α in mouse Leydig tumour cell line (MLTC-1). Specific inhibitors for different signalling molecules were used to study the role of cyclic AMP (cAMP), and its downstream mediators. Expression of VEGF and HIF-1α mRNA were measured by quantitative RT-PCR; VEGF secretion by ELISA; steroid secretion by radioimmunoassay and HIF-1α protein levels by western blotting. Inhibitors of adenylate cyclase (AC), protein kinase A (PKA), sarcoma kinase (SrcK), phosphoinositide 3-kinase (PI3K) and MAP kinase kinase (MEK1/2) abolished the T₃ -induced increase in VEGF mRNA and protein levels. The same signalling molecules also mediated the increased production of steroids and HIF-1α protein in response to T₃ . Therefore, it was concluded that T₃ stimulates steroid secretion and HIF-1α protein in MLTC-1 cells through the AC-cAMP-PKA-PI3K-MEK pathway, which in turn stimulate VEGF production.

Disruption of deiodinase type 2 in zebrafish disturbs male and female reproduction

Thyroid hormones are crucial mediators of many aspects of vertebrate life, including reproduction. The key player is the biologically active 3,5,3'-triiodothyronine (T3), whose local bio-availability is strictly regulated by deiodinase enzymes. Deiodinase type 2 (Dio2) is present in many tissues and is the main enzyme for local T3 production. To unravel its role in different physiological processes, we generated a mutant zebrafish line, completely lacking Dio2 activity. Here we focus on the reproductive phenotype studied at the level of offspring production, gametogenesis, functioning of the hypothalamic-pituitary-gonadal axis and sex steroid production. Homozygous Dio2-deficient zebrafish were hypothyroid, displayed a delay in sexual maturity, and the duration of their reproductive period was substantially shortened. Fecundity and fertilization were also severely reduced. Gamete counts pointed to a delay in oogenesis at onset of sexual maturity and later on to an accumulation of oocytes in mutant ovaries due to inhibition of ovulation. Analysis of spermatogenesis showed a strongly decreased number of spermatogonia A at onset of sexual maturity. Investigation of the hypothalamic-pituitary-gonadal axis revealed that dysregulation was largely confined to the gonads with significant upregulation of igf3, and a strong decrease in sex steroid production concomitant with alterations in gene expression in steroidogenesis/steroid signaling pathways. Rescue of the phenotype by T3 supplementation starting at 4 weeks resulted in normalization of reproductive activity in both sexes. The combined results show that reproductive function in mutants is severely hampered in both sexes, thereby linking the loss of Dio2 activity and the resulting hypothyroidism to reproductive dysfunction.

Triiodothyronine stimulates VEGF expression and secretion via steroids and HIF-1α in murine Leydig cells

Leydig cells are the principal steroidogenic cells of the testis. Leydig cells also secrete a number of growth factors including vascular endothelial growth factor (VEGF) which has been shown to regulate both testicular steroidogenesis and spermatogenesis. The thyroid hormone, T_3, is known to stimulate steroidogenesis in Leydig cells. T₃ has also been shown to stimulate VEGF production in a variety of cell lines. However, studies regarding the effect of T₃ on VEGF synthesis and secretion by the Leydig cells were lacking. Therefore, we investigated the effect of T₃ on VEGF synthesis and secretion in a mouse Leydig tumour cell line, MLTC-1. The effect of T₃ was compared with that of LH/cAMP and hypoxia, two known stimulators of Leydig cell functions. The cells were treated with T₃, 8-Br-cAMP (a cAMP analogue), or CoCl₂ (a hypoxia mimetic) and VEGF secreted in the cell supernatant was measured using ELISA. The mRNA levels of VEGF were measured by quantitative RT-PCR. In the MLTC-1 cells, T₃, 8-Br-cAMP, and CoCl₂ stimulated VEGF mRNA levels and the protein secretion. T₃ also increased steroid secretion as well as HIF-1α protein levels, two well-established upstream regulators of VEGF. Inhibitors of steroidogenesis as well as HIF-1α resulted in inhibition of T₃-stimulated VEGF secretion by the MLTC-1 cells. This suggested a mediatory role of steroids and HIF-1α protein in T₃-stimulated VEGF secretion by MLTC-1 cells. The mediation by steroids and HIF-1α were independent of each other.

ABBREVIATIONS

8-Br-cAMP: 8-bromo - 3', 5' cyclic adenosine monophosphate; CoCl₂: cobalt chloride; HIF-1α: hypoxia inducible factor -1α; LH: luteinizing hormone; T₃: 3, 5, 3'-L-triiodothyronine; VEGF: vascular endothelial growth factor.

Thyroid and male reproduction

Male reproduction is governed by the classical hypothalamo-hypophyseal testicular axis: Hypothalamic gonadotropin releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle stimulating hormone (FSH) and the gonadal steroid, principally, testosterone. Thyroid hormones have been shown to exert a modulatory influence on this axis and consequently the sexual and spermatogenic function of man. This review will examine the modulatory influence of thyroid hormones on male reproduction.

Abolition of endocrine dimorphism in hyperthyroid males? An argument for the positive feedback effect of hyperoestrogenaemia on LH secretion

Our aim was (i) to investigate the hypothalamo-hypophyseal-gonadal axis in hyperthyroid Indian males, (ii) to rule out the modulatory role of adrenal steroids on it and (iii) to determine if the simultaneous rise in oestradiol and luteinising hormone (LH) in hyperthyroid males is due to a positive feedback action of oestradiol on pituitary LH release. Age- and BMI-matched men were divided into two groups, I, euthyroid subjects (n = 17) and II, hyperthyroid patients (n = 12) on the basis of their thyroid hormone levels. Serum levels of thyroid-stimulating hormone, triiodothyronine, thyroxine, LH, follicle-stimulating hormone (FSH), prolactin, E(2), T, P(4), sex hormone binding globulin and dehydroepiandrosterone sulphate (DHEAS) were assayed. Mean levels of T and E(2) were approximately two times higher in group II in comparison with group I. DHEAS levels were similar in both groups ruling out any adrenal involvement. Mean serum LH level was 2.6 folds higher in group II in comparison with group I. Mean serum levels of FSH were higher in group II, it was marginally nonsignificant. On the basis of these and previous observations, we hypothesise that endocrinological dimorphism in human male and female is not rigid; a sustained rise in serum oestradiol probably induces a positive feedback action on pituitary leading to elevated gonadotrophin levels.

Clinical implications of altered thyroid status in male testicular function

Thyroid hormones are involved in the development and maintenance of virtually all tissues. Although for many years the testis was thought to be a thyroid-hormone unresponsive organ, studies of the last decades have demonstrated that thyroid dysfunction is associated not only with abnormalities in morphology and function of testes, but also with decreased fertility and alterations of sexual activity in men. Nowadays, the participation of triiodothyronine (T3) in the control of Sertoli and Leydig cell proliferation, testicular maturation, and steroidogenesis is widely accepted, as well as the presence of thyroid hormone transporters and receptors in testicular cells throughout the development process and in adulthood. But even with data suggesting that T3 may act directly on these cells to bring about its effects, there is still controversy regarding the impact of thyroid diseases on human spermatogenesis and fertility, which can be in part due to the lack of well-controlled clinical studies. The current review aims at presenting an updated picture of recent clinical data about the role of thyroid hormones in male gonadal function.

The role of thyroid hormone in testicular development and function

Thyroid hormone is a critical regulator of growth, development, and metabolism in virtually all tissues, and altered thyroid status affects many organs and systems. Although for many years testis has been regarded as a thyroid hormone unresponsive organ, it is now evident that thyroid hormone plays an important role in testicular development and function. A considerable amount of data show that thyroid hormone influences steroidogenesis as well as spermatogenesis. The involvement of tri-iodothyronine (T(3)) in the control of Sertoli cell proliferation and functional maturation is widely accepted, as well as its role in postnatal Leydig cell differentiation and steroidogenesis. The presence of thyroid hormone receptors in testicular cells throughout development and in adulthood implies that T(3) may act directly on these cells to bring about its effects. Several recent studies have employed different methodologies and techniques in an attempt to understand the mechanisms underlying thyroid hormone effects on testicular cells. The current review aims at presenting an updated picture of the recent advances made regarding the role of thyroid hormones in male gonadal function.

Secretion of testicular steroids and gonadotrophins in hypothyroidism

Inconsistent alterations in gonadal steroidogenesis and pituitary functions have been reported in hypothyroid males. We have compared the lipid and endocrine profiles of the euthyroid and hypothyroid [thyroid-stimulating hormone (TSH) >100 mIU l(-1)] males. Hypothyroidism was found to be associated with an increase in the circulating level of total cholesterol and LDL-cholesterol (LDL-C) and a reduction in the levels of progesterone and testosterone, without any change in the serum levels of oestradiol and gonadotrophins. The failure of gonadotrophins to rise could be accounted by a normal level of serum oestradiol in the hypothyroid male. A mild hyperprolactinaemia was also noted in the hypothyroid patients. The reduction in serum testosterone level could be explained by (i) a reduced uptake of LDL-C by the Leydig cells and thereby a reduction in the synthesis of progesterone and consequentially testosterone, (ii) a further reduction in the rate of conversion of progesterone to testosterone, (iii) a higher rate of conversion of testosterone to oestradiol, (iv) a decrease in serum triiodothyronine and (v) hyperprolactinaemia. Rise in TSH needs to be investigated as a cause of the suppression of gonadal steroidogenesis.

Effects of Propylthiouracil on Testicular Tissue in Undescended Testes of Newborn Rats

AIM

The most important goal in the treatment of cryptorchidism is to preserve the potential for fertility. This experimental study was performed to investigate the effect of propylthiouracil (PTU) on the undescended testes (UTs) of newborn rats.

MATERIALS AND METHODS

The experimental cryptorchidism model in newborn male rats consisted of 4 groups. The groups A (control) and B (PTU) underwent no surgical intervention, whereas in groups C (UT only) and D (treatment) UTs were produced by dissecting and suturing the future right scrotal area. In groups D and B, 0.1% (w/v) PTU was added to the drinking water of mother rats between 2 and 24 days. At the end of the 90th day rat body weights, testicular weights, Johnsen tubular biopsy scores (JTBSs), seminiferous tubule diameters (STDs), testosterone, and thyroid hormone levels were measured. Mann-Whitney U test was used for statistical analysis.

RESULTS

Mean testicular weight was similar between groups A, B and D, and statistically lowest in group C. Mean body weight was statistically higher in groups A and C compared with groups B and D. Mean testosterone levels showed no statistical difference between the groups. Mean JTBSs were statistically higher in groups A and B compared with groups C and D. The value in treatment group D was statistically higher compared to group C (p<0.05). Mean STDs were statistically lowest in group C compared to other groups (p<0.05). No difference was found between groups A, B, and D (p>0.05). Both the mean free triiodothyronine and free thyroxine values between groups A and C and between groups B and D were similar. The values in groups A and C were statistically higher than those of groups B and D (p<0.05).

CONCLUSION

PTU-induced transient hypothyroidism in the newborn rat UT model shows protective effects on testicular growth parameters.

Hypoandrogenaemia is associated with subclinical hypothyroidism in men

Hypothyroidism has been shown to be associated with a reduction in serum testosterone level in males. This reduction in testosterone is reversible by thyroxine replacement therapy. However, to the best of our knowledge, it is not yet known, whether a similar reduction in serum testosterone level is observed in subclinically hypothyroid males [thyroid-stimulating hormone (TSH) < 10 mIU/L] in whom the benefits of thyroxine replacement therapy are still controversial. Our goal was to investigate the putative connections between subclinical hypothyroidism and the circulating levels of gonadotrophins and gonadal steroids in males (mean age +/- SEM, 34.67 +/- 1.52 years; ranging from 20 to 54 years). The serum samples from patients showing normal euthyroid and subclinical hypothyroid profiles (TSH < 10 mIU/L) were further analysed for the levels of luteinizing hormone, follicle-stimulating hormone, prolactin, testosterone, sex hormone-binding globulin, progesterone and oestradiol. Subclinical hypothyroidism was associated with a decrease in the levels of serum testosterone and its precursor progesterone. The data suggest that serum testosterone declines because of the non-availability of its precursor progesterone. The level of oestradiol was similar in both the groups, suggesting a greater conversion rate of testosterone to oestradiol in subclinically hypothyroid males, in order to maintain the oestradiol levels. Prolactin levels were slightly but significantly increased in subclinical hypothyroidism. To the best of our information this is a novel report, which shows a direct association between subclinical hypothyroidism and hypoandrogenaemia. Testosterone deficiency and its symptoms should be kept in view while managing subclinical hypothyroidism in male patients. Further studies are needed in order to reveal the physiological and molecular mechanisms leading to hypoandrogenaemia in subclinical hypothyroidism (TSH < 10 mIU/L).

Differential expression of procollagen lysine 2-oxoglutarate 5-deoxygenase and matrix metalloproteinase isoforms in hypothyroid rat ovary and disintegration of extracellular matrix

Hypothyroid-induced reproductive malfunction in both the sexes is a common phenomenon of global concern. In an attempt to characterize the differentially expressed genes that might be responsible for these disorders, we have identified a number of clones in hypothyroid rat ovary by subtractive hybridization. One such clone is procollagen lysyl hydroxylase2 (Plod-2), the key enzyme for the first step of collagen biosynthetic pathway, which was down-regulated in hypothyroid condition. We have also demonstrated the reduced expression of other isoforms of Plods, namely Plod-1 and -3 in hypothyroid rat ovary. The current studies are the first of their kind to report that thyroid hormone regulates the Plod gene in rat ovary. Moreover, we have shown the up-regulation of matrix-degrading enzyme(s), matrix metalloproteinase(s) in the hypothyroid rat ovary, whereas the tissue-inhibitory metalloproteinase is down-regulated. Finally, the results of the present studies indicate that in hypothyroid condition, collagen biosynthesis in ovary seems to be disturbed with concomitant enhancement in collagen degradation, resulting in disintegration of overall ovarian structure.

Thyroid hormones: their role in testicular steroidogenesis

Thyroid hormones are important for growth and development of many tissues. Altered thyroid hormone status causes testicular abnormalities. For instance, juvenile hypothyroidism/neonatal transient hypothyroidism induces macroorchidism, increases testicular cell number (Sertoli, Leydig, and germ cells) and daily sperm production. Triiodothyronine (T3) receptors have been identified in sperm, developing germ cells, Sertoli, Leydig, and peritubular cells. T3 stimulates Sertoli cell lactate secretion as well as mRNA expression of inhibin-alpha, androgen receptor, IGF-I, and IGFBP-4. It also inhibits Sertoli cell mRNA expression of Müllerian inhibiting substance (MIS), aromatase, estradiol receptor, and androgen binding protein (ABP) and ABP secretion. T3 directly increases Leydig cell LH receptor numbers and mRNA levels of steroidogenic enzymes and steroidogenic acute regulatory protein. It stimulates basal and LH-induced secretion of progesterone, testosterone, and estradiol by Leydig cells. Steroidogenic factor-1 acts as a mediator for T3-induced Leydig cell steroidogenesis. Although the role of T3 on sperm, germ, and peritubular cells has not yet been completely studied, it is clear that T3 directly regulates Sertoli and Leydig cell functions. Further studies are required to elucidate the direct effect of T3 on sperm, germ, and peritubular cells.

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.

Impact of neonatal hypothyroidism on Leydig cell number, plasma, and testicular interstitial fluid sex steroids concentration

We have previously demonstrated that neonatal and transient neonatal hypothyroidism modulates Leydig, Sertoli, and germ cell numbers, sex steroids and androgen binding protein concentration. The present study was undertaken to study the effect of neonatal onset hypothyroidism on Leydig and peritubular myoid cell numbers, plasma and testicular interstitial fluid (TIF) sex steroid concentration at different age groups of Wistar rats. Hypothyroidism was induced by giving 0.05% methimazole (MMI) to lactating mothers or directly to the male pups from day 1 postpartum through days 10, 15, 30, 40 and 60 postpartum. To confirm hypothyroidism, plasma thyroid hormones and TSH were assayed. Plasma and TIF testosterone, progesterone, dihydrotestosterone (DHT) and estradiol were assayed by radioimmunoassay. Leydig cell number in hypothyroid rats were less than the age-matched controls. The diameter of Leydig cells in hypothyroid rats was smaller than the controls but 10 days old hypothyroids alone had larger than control rats. A significant decrease of peritubular myoid cell number was observed in 30, 40 and 60 days hypothyroid rats and increased in 10 and 15 days hypothyroidism. Hypothyroid rats had elevated level of plasma LH and decreased GH (except day 10 postpartum). Plasma PRL level was increased in 10 and 15 days hypothyroid rats and an opposite effect was observed in 40 and 60 days hypothyroidism. Plasma testosterone, DHT and estradiol were decreased in all hypothyroid rats. However, plasma progesterone level in hypothyroid rats was significantly higher at days 10, 30, and 40 postpartum and an opposite effect was seen in 15 and 60 days experimental groups. TIF testosterone and progesterone titre showed a consistent decrease in hypothyroid rats irrespective of the duration. In hypothyroid rats, TIF DHT levels decreased significantly in days 10, 40 and 60 postpartum. However, it increased in days 15 and 30 postpartum. Except at day 10 postpartum, the level of TIF estradiol in hypothyroid rats was significantly less than their age matched controls. Our data clearly indicate that neonatal onset hypothyroidism adversely affect Leydig cell proliferation along with impaired steroidogenesis.

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.

Molecular mechanisms of thyroid hormone-stimulated steroidogenesis in mouse leydig tumor cells. Involvement of the steroidogenic acute regulatory (StAR) protein

Using a mouse Leydig tumor cell line, we explored the mechanisms involved in thyroid hormone-induced steroidogenic acute regulatory (StAR) protein gene expression, and steroidogenesis. Triiodothyronine (T3) induced a approximately 3.6-fold increase in the steady-state level of StAR mRNA which paralleled with those of the acute steroid response ( approximately 4.0-fold), as monitored by quantitative reverse transcriptase-polymerase chain reaction assay and progesterone production, respectively. The T3-stimulated progesterone production was effectively inhibited by actinomycin-D or cycloheximide, indicating the requirement of on-going mRNA and protein synthesis. T3 displayed the highest affinity of [125I]iodo-T3 binding and was most potent in stimulating StAR mRNA expression. In accordance, T3 significantly increased testosterone production in primary cultures of adult mouse Leydig cells. The T3 and human chorionic gonadotropin (hCG) effects on StAR expression were similar in magnitude and additive. Cells expressing steroidogenic factor 1 (SF-1) showed marginal elevation of StAR expression, but coordinately increased T3-induced StAR mRNA expression and progesterone levels. In contrast, overexpression of DAX-1 markedly diminished the SF-1 mRNA expression, and concomitantly abolished T3-mediated responses. Noteworthy, T3 augmented the SF-1 mRNA expression while inhibition of the latter by DAX-1 strongly impaired T3 action. Northern hybridization analysis revealed four StAR transcripts which increased 3-6-fold following T3 stimulation. These observations clearly identified a regulatory cascade of thyroid hormone-stimulated StAR expression and steroidogenesis that provides novel insight into the importance of a thyroid-gonadal connection in the hormonal control of Leydig cell steroidogenesis.

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.

Development of the adult-type Leydig cell population in the rat is affected by neonatal thyroid hormone levels

We have investigated the effects of neonatal-prepubertal changes in thyroid hormone levels on the early phases of adult-type Leydig cell development in the rat testis. Hypothyroidism was induced by adding 6-propyl-2-thiouracil (PTU) to the drinking water, while hyperthyroidism was induced by daily injections with triiodothyronine (T3). The proliferative activity of the Leydig cells in PTU-treated animals was not different from that in age-matched controls through the age of 16 days. Nevertheless, the percentage of Leydig cells (i.e., the proportion of Leydig cells among the total interstitial cell population) was approximately 83% and 67% lower at the ages of 12 and 16 days, respectively. The proliferative activity of the Leydig cells in the T3-treated animals compared to the controls was increased approximately 3-fold at the ages of 12 and 16 days. The percentage of Leydig cells in T3-treated animals was also considerably increased at these two ages (400% and 725%, respectively). Concomitantly, the percentage of peritubular cells was decreased, suggesting that the increase in the percentage of Leydig cells may at least partially be the result of differentiation of peritubularly located precursor cells. Plasma testosterone levels fluctuated considerably at these ages. Hence, injection of T3 during the neonatal-prepubertal period not only affects Sertoli cell proliferation and differentiation but also directly or indirectly affects the onset of the formation of the adult-type Leydig cell population and its function.

Thyroid hormone induces a 52 kDa soluble protein in goat testis Leydig cell which stimulates androgen release

Incubation of goat testicular Leydig cells with 3,5,3'-triiodothyronine (T3) induces the generation of a proteinaceous factor (factors) which was located in the soluble supernatant fraction (100 000 x g supernatant, 100 k sup) of sonicated Leydig cells. Addition of this factor to Leydig cell incubation greatly stimulated androgen release. This factor(s) was purified based on its biological properties, i.e., its addition to Leydig cell incubation augmented the release of androgen. This was designated as TIP (T3-induced protein) activity. 100 k sup prepared from Leydig cells incubated in the absence (control) or presence of T3 was gel filtered through Sephadex G-100. 100 k sup from T3 incubated gave two protein peaks, P-I and P-II, control 100 k sup had similar nature of P-I, but P-II was not well marked. Incubation in the presence of [14C]leucine clearly showed TCA precipitable radioactivity only in the P-II region of T3 incubate. 5 microg of P-II protein stimulated androgen release from Leydig cells cells (1 x 10(6) cells/well) to more than 5-fold as compared to control. P-II protein was further purifies by FPLC Mono-Q column chromatography where one unadsorbed (MQ-I) and two adsorbed (MQ-II and MQ-III) protein peaks could be detected. MQ-II, which was eluted with 0.20 M NaCl gradient, demonstrated strong TIP activity (2 microg protein released 4.8-fold more androgen as compared to control). MQ-II was passed through FPLC Superose-6 column where it gave two peaks and Peak-I(SP-I) showed strong TIP activity (2 microg protein stimulated a 6-fold increase in androgen release as compared to control). Polyacrylamide gel electrophoresis (PAGE) indicated SP-I to be a homogeneous protein and SDS-PAGE demonstrated it to be a 52 kDa monomer protein. Results show that T3 induces the synthesis of a 52 kDa protein in testicular Leydig cells which in turn causes stimulation of androgen release suggesting this protein to be a novel mediator of T3 function in Leydig cells.