Thyroid hormone and retinoic acid induce the synthesis of insulin-like growth factor-binding protein-4 in prepubertal pig sertoli cells

E4 Transcription Factor 1 (E4F1) Regulates Sertoli Cell Proliferation and Fertility in Mice

Simple Summary

Male fertility relies on the generation of functional sperm in seminiferous tubules of the testis. In mammals, Sertoli cells are the only somatic cells that directly interact with spermatogenic cells. Compelling evidences suggest that the number of Sertoli cells determines testis size and sperm output, however, molecular mechanisms regulating Sertoli cell proliferation and maturation are not well-understood. Using a Sertoli cell specific loss-of-function approach, here we showed that transcription factor E4F1 played an important role in murine Sertoli cell proliferation. Compared with their littermate control, E4f1 conditional knockout male mice sired a significantly low number of pups. E4f1 deletion resulted in reduced Sertoli cell number and testis size. Further analyses revealed that E4f1 deletion affected Sertoli cell proliferation in the neonatal testis and caused an increase in apoptosis of spermatogenic cells without affecting normal development of spermatogonia, meiotic and post-meiotic germ cells. These findings have shed new light on molecular controlling of spermatogenesis in mice and a similar mechanism likely exists in other animals.

Abstract

In the mammalian testes, Sertoli cells are the only somatic cells in the seminiferous tubules that provide structural, nutritional and regulatory support for developing spermatogenic cells. Sertoli cells only proliferate during the fetal and neonatal periods and enter a quiescent state after puberty. Functional evidences suggest that the size of Sertoli cell population determines sperm production and fertility. However, factors that direct Sertoli cell proliferation and maturation are not fully understood. Transcription factor E4F1 is a multifunctional protein that serves essential roles in cell fate decisions and because it interacts with pRB, a master regulator of Sertoli cell function, we hypothesized that E4F1 may have a functional role in Sertoli cells. E4f1 mRNA was present in murine testis and immunohistochemical staining confirmed that E4F1 was enriched in mature Sertoli cells. We generated a conditional knockout mouse model using Amh-cre and E4f1^flox/flox lines to study E4F1 fucntion in Sertoli cells and the results showed that E4f1 deletion caused a significant reduction in testis size and fertility. Further analyses revealed that meiosis progression and spermiogenesis were normal, however, Sertoli cell proliferation was impaired and germ cell apoptosis was elevated in the testis of E4f1 conditional knockout mice. On the basis of these findings, we concluded that E4F1 was expressed in murine Sertoli cells and served important functions in regulating Sertoli cell proliferation and fertility.

Igf Binding Proteins Protect Undifferentiated Spermatogonia in the Zebrafish Testis Against Excessive Differentiation

IGF binding proteins (IGFBPs) modulate the availability of IGFs for their cognate receptors. In zebrafish testes, IGF3 promotes the proliferation and differentiation of type A undifferentiated (A_und) spermatogonia, and igf3 expression is strongly elevated by FSH but also responds to T₃. Here we report the effects of FSH and T₃ on igfbp transcript levels in adult zebrafish testis. We then examined T₃ and FSH effects on zebrafish spermatogenesis and explored the relevance of IGFBPs in modulating these T₃ or FSH effects, using a primary tissue culture system for adult zebrafish testis. T₃ up-regulated igfbp1a and igfbp3 expression, whereas FSH reduced igfbp1a transcript levels. To quantify effects on spermatogenesis, we determined the mitotic index and relative section areas occupied by A_und, type A differentiating, or type B spermatogonia. In general, T₃ and FSH stimulated spermatogonial proliferation and increased the areas occupied by spermatogonia, suggesting that both self-renewal and differentiating divisions were stimulated. Preventing IGF/IGFBP interaction by NBI-31772 further increased T₃- or FSH-induced spermatogonial proliferation. However, under these conditions the more differentiated type A differentiating and B spermatogonia occupied larger surface areas at the expense of the area held by A_und spermatogonia. Clearly decreased nanos2 transcript levels are in agreement with this finding, and reduced amh expression may have facilitated spermatogonial differentiation. We conclude that elevating IGF3 bioactivity by blocking IGFBPs shifted T₃- or FSH-induced signaling from stimulating spermatogonial self-renewal as well as differentiation toward predominantly stimulating spermatogonial differentiation, which leads to a depletion of type A_und spermatogonia.

The Effects of Triiodothyronine on Rat Testis: A Morphometric and Immunohistochemical Study

The aim of present study was to investigate the effects of 3,3',5-triiodothyronine (T(3)) on rat testis both morphometrically and immunohistochemically with determining of insulin-like growth factor I (IGF-I) expression. Adult male Wistar-albino rats used in the study were divided into two groups; control and T(3)-treated groups. After T(3) treatment there was observed to be a decrease in testicular weights, diameters of seminiferous tubules and the number of sertoli cells, and an increase in the number of leydig cells (P<0.05). Some of the seminiferous tubule lumens of T(3) administrated rats had cellular debris. IGF-I was localized in sertoli cells, late spermatids and leydig cells of all groups. IGF-I immunoreactivity in T(3) treated rats was higher than in controls in all stages of the cycle of rat seminiferous epithelium, but the staining intensity of leydig cells were similar in both groups. In conclusion, the present results suggest that T(3) may modulate the testicular function by affecting IGF-I activity at the gonadal level.

Effects of follicle-stimulating hormone and vitamin A upon purinergic secretion by rat Sertoli cells

Follicle-stimulating hormone (FSH) and vitamin A (retinol) are two of the main regulators of the male reproductive system. Recently, it has been described that extracellular purines can affect some important reproductive-related functions in Sertoli cells and germinative cells, by activating specific purinergic receptors. In this work, we report that both FSH and retinol are able to induce changes in the levels of extracellular purines of cultured rat Sertoli cells. FSH induced an increase in adenosine, mainly caused by enhanced ecto-ATPase activity, while retinol increased xanthine and hypoxanthine levels, and decreased uric acid concentration by an unknown mechanism. These data indicate that purinergic signaling may be involved in the control and/or regulation of some of the reproductive-related actions of these hormones.

Thyroid hormone, glucocorticoids, and prolactin at the nexus of physiology, reproduction, and toxicology

A symposium at the 2003 Annual Meeting of the Society of Toxicology brought together an expert group of endocrinologists to review how non-reproductive hormones can affect the endocrine system. This publication captures the essence of those presentations. Paul Cooke and Denise Holsberger recapitulate the evidence of how thyroid hormones affect male and female reproduction, and reproductive development. Ray Witorsch summarizes the many effects of glucocorticoids on the reproductive system. Finally, Paul Sylvester reviews the mechanism of action of prolactin, and reminds us that this ancient hormone has many functions beyond lactation.

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.

Insulin-like growth factor binding protein-4 expression is dependent on the carbohydrate in the media in HT-29 cells

HT-29 are colonic carcinoma cells that follow a unique pattern of differentiation dependent on the medium's carbohydrate source. This study compared levels of insulin-like growth factor (IGF)-II and IGF binding protein (IGFBP)-4 when HT-29 cells were grown with standard glucose-containing medium versus galactose-containing (glucose-free) medium. Serum-free media conditioned for 24h were collected at low density, pre-confluence, confluence, and 48-h post-confluence. Ligand blotting of the conditioned galactose medium demonstrated low IGFBP-4 levels until the cells approached confluence, when the levels increased significantly. In standard medium, IGFBP-4 levels increased with increasing cell numbers except for a transient decrease at confluence. Radioimmunoassay showed little change in IGF-II concentrations, although HT-29 cells grown with galactose had lower IGF-II concentrations. HT-29 cells treated with retinoic acid had dose-dependent increases in IGFBP-4 and reduced IGF-II expression. These studies suggest that HT-29 cell differentiation correlates with an increase in IGFBP-4 levels.

Regulation of cell proliferation by insulin-like growth factor 1 in hyperoxia-exposed neonatal rat lung

Hyperoxic exposure of the developing lung leads to characteristic peribronchial and mesenchymal fibroproliferative changes. We hypothesize that O2-induced changes in the neonatal lung are mediated by Insulin-like growth factor 1 (IGF-1) and IGF-1 receptor (IGF-1R). Lung explant cultures were prepared from 3-day-old neonatal rat pups and exposed to room air or 95% O2 for 72 h. Western blots and immunohistochemistry were used to determine if hyperoxia stimulated IGF-1 and IGF-1R, and to identify the cell types involved. Retinoic acid was used to learn if this would inhibit oxygen-induced cell proliferation. Hyperoxia induced a significant increase in thymidine incorporation (control, 54 +/- 9; hyperoxia, 254 +/- 24 dpm/nM DNA; mean +/- SEM; N = 3; P < 0.05). This was inhibited by 5 x 10(-5) M RA (149 +/- 18 dpm/nM DNA; P < 0.05) and by anti-IGF-1 antibody (115 +/- 25 dpm/nM DNA; P < 0.05; N = 3). BrdU labeling in the mesenchymal cells was significantly increased in mesenchymal cells after exposure to oxygen (91% higher than the room air control) but not in epithelial cells. This increase was inhibited in the presence of retinoic acid. Western blots showed IGF-1 protein was increased after 72 h of O2 exposure compared to room air exposure (57 +/- 7 compared to 32 +/- 5 densitometric units; P < 0.05; N = 3). The increase was inhibited when the cultures were exposed to 95% O2 in the presence of anti-IGF-1 antibody (28 +/- 4; P < 0.05; N = 3). IGF-1 protein decreased in the presence of retinoic acid after oxygen exposure but not in room air. Immunostaining of O2-exposed lung showed IGF-1 was most abundant in airway and alveolar epithelial cells. We conclude that hyperoxia increases cell proliferation by stimulating IGF-1 in the neonatal rat lung. Interaction of IGF-1 and IGF-1R is an important cell-cell communication mechanism in the developmental and repair processes of hyperoxic neonatal lung injury.