Follicle-stimulating hormone inhibits all-trans-retinoic acid-induced retinoic acid receptor alpha nuclear localization and transcriptional activation in mouse Sertoli cell lines

Effects of glucagon-like peptide-1 receptor agonists on spermatogenesis-related gene expression in mouse testis and testis-derived cell lines

Glucagon-like peptide-1 (GLP-1) is an incretin released into the gastrointestinal tract after food ingestion, and stimulates insulin secretion from the beta cells of the pancreatic islets. Incretins have recently been reported to have extrapancreatic actions, and they are anticipated to have potential efficacy for conditions such as male infertility as well as diabetes. However, the effects of incretins on male reproductive function remain unclear. In this study, GLP-1 receptor expression and the effects of GLP-1 on spermatogenesis-associated genes were investigated using mouse testes and testis-derived cultured cell lines. Glp1r mRNA and GLP-1 protein were expressed in mouse testes at levels comparable to or greater than those in positive control adipose tissue, and the liver and intestine, and also in a Sertoli cell line (TM4) and a Leydig cell line (MA-10) as well as the GC-1 spg and GC-2 spd (ts) germ cell lines. TM4 cells treated with the GLP-1 receptor agonist exenatide showed transiently and significantly upregulated Kitl, Pdgfa, and Glp1r mRNA expression. Furthermore, at 1 hr post-exenatide administration to male mice, Kitl and Glp1r mRNA expression levels were significantly increased, and Pdgfa mRNA expression level also showed a tendency toward increase. TM4 cells were treated with various cell-activating agents, and bucladesine elicited significantly increased Glp1r mRNA expression. We suggest that GLP-1 provides acute stimulation of Sertoli cells in the mouse testis and has a stimulatory effect on the expression of spermatogenesis-related genes.

FSH regulates RA signaling to commit spermatogonia into differentiation pathway and meiosis

BACKGROUND

Spermatogenesis is a complex process that is controlled by interactions between germ cells and somatic cells. The commitment of undifferentiated spermatogonia to differentiating spermatogonia and normal spermatogenesis requires the action of gonadotropins. Additionally, numerous studies revealed the role of retinoic acid signaling in induction of germ cell differentiation and meiosis entry.

MAIN TEXT

Recent studies have shown that expression of several RA signaling molecules including Rdh10, Aldh1a2, Crabp1/2 are influenced by changes in gonadotropin levels. Components of signaling pathways that are regulated by FSH signaling such as GDNF, Sohlh1/2, c-Kit, DMRT, BMP4 and NRGs along with transcription factors that are important for proliferation and differentiation of spermatogonia are also affected by retinoic acid signaling.

CONCLUSION

According to all studies that demonstrate the interface between FSH and RA signaling, we suggest that RA may trigger spermatogonia differentiation and initiation of meiosis through regulation by FSH signaling in testis. Therefore, to the best of our knowledge, this is the first time that the correlation between FSH and RA signaling in spermatogenesis is highlighted.

A calreticulin-dependent nuclear export signal is involved in the regulation of liver receptor homologue-1 protein folding

LRH-1 (liver receptor homologue-1) contained a calreticulin-dependent NES (nuclear export signal) that regulate shutting, protein folding and transactivity.

Retinol improves in vitro differentiation of pre-pubertal mouse spermatogonial stem cells into sperm during the first wave of spermatogenesis

Testicular tissue freezing has been proposed for fertility preservation in pre-pubertal boys. Thawed frozen testicular tissue must undergo a maturation process to restore sperm production. The purpose of the current study was to evaluate the ability of retinol to improve the in vitro differentiation of pre-pubertal mouse spermatogonial stem cells into sperm. Testes from pre-pubertal mice, aged 2.5 and 6.5 days post-partum, were cultured on agarose gel at a gas-liquid interphase for 34, 38 and 60 days (D) and for 16, 30 and 36 D respectively. Assessment of basal medium (BM) supplemented with retinol (RE) alone, FSH/LH alone or a combination of both, was performed. Stereological analyses and tissue lesion scoring were performed at the culture time points indicated above. Sperm production was quantified at D30 and D34 after mechanical dissection of the testicular tissues. FSH/LH significantly increased the percentage of round spermatids at D30 and D38, when compared to BM alone. However, RE significantly increased the percentages of round but also elongated spermatids at D30 and D34. Moreover, RE significantly increased the number of spermatozoa per milligram of tissue at D30 and D34 when compared to BM. Therefore, RE improved the in vitro production of spermatids and spermatozoa from pre-pubertal SSCs during the first wave of spermatogenesis. The use of RE could be a useful tool for in vitro spermatogenesis from pre-pubertal human testicular tissue.

Cell lines: Valuable tools or useless artifacts

Cell lines are often used in place of primary cells to study biological processes. However, care must be taken when interpreting the results as cell lines do not always accurately replicate the primary cells. In this article, we will briefly talk about advantages and disadvantages of cell lines and then discuss results using the mouse Sertoli cell line, MSC-1, compared with primary mouse Sertoli cells. MSC-1 cells resemble Sertoli cells morphologically and possess several biochemical markers associated with Sertoli cells. Studies have demonstrated that the function and regulation of retinoic acid receptor α (RARα) is similar between MSC-1 and rat Sertoli cells. However, MSC-1 cells lack some of the immune privilege properties associated with primary Sertoli cells, including survival in animals with a fully functional immune system. Therefore, it has to be kept in mind that cell lines do not behave identically with primary cells and should not be used to replace primary cells. In order to strengthen the findings, key control experiments using primary cells should always be performed.

Immunolocalization of retinoic acid receptor-alpha, -beta, and -gamma, in bovine and canine sperm

Retinoic acid is an important regulator of cellular proliferation and differentiation. The action of retinoic acid is mediated by retinoic acid receptors (RARs) and the retinoid X receptors. The objective was to elucidate the protein localization and expression of RARα, RARβ, and RARγ in bull and dog sperm. Bull and dog sperm were subjected to an immunostaining procedure to determine presence of RARα, RARβ, and RARγ. We concluded that all three receptors were present in different regions of bull and dog sperm at varying levels. Protein expression in bull and dog sperm lysates was investigated using protein dot-blot analyses. The protein levels of RARα and RARγ were higher than the protein level of RARβ in bull and dog sperm. Protein sequences of RARα, RARβ, and RARγ for bull and dog were 98%, 89%, and 98%, respectively, on similarity alignment. In conclusion, the presence of RARα, RARβ, and RARγ receptors supported their role in sperm structure and function.

Lycopene induces retinoic acid receptor transcriptional activation in mice

SCOPE

Lycopene is a lipophilic carotenoid and provides the red colour to tomatoes and tomato product. Various studies indicated that lycopene and tomatoes/tomato products are able to positively influence various diseases associated with a chronic inflammation. The mechanism of action of lycopene to elicit these effects is partly unknown. A possible mechanism is that biological metabolites of lycopene may activate nuclear hormone receptors in mammalian cells. The aim of this study was to investigate the potential of orally administered lycopene and all-trans retinoic acid (ATRA) for the induction of the retinoic acid receptor (RAR) in a transgenic retinoic acid response-element (RARE)-reporter mouse system.

METHODS AND RESULTS

Orally administered lycopene (100 mg/kg bw in beadlets, n = 6) and ATRA as an endogenous RAR ligand (50 mg/kg bw, n = 6) for the induction of the retinoic acid receptor in male mice using a transgenic RARE-reporter mouse system.

CONCLUSION

Lycopene treatments induced RARE-mediated cell signalling indicated by quantified bioimaging, increased luciferase activity and up-regulated the retinoid target genes in selected organs of the mice. We conclude that lycopene can induce RAR-transcriptional activation in mice and lycopene might be a precursor of still non-identified biologically active metabolites.

Is the adult Sertoli cell terminally differentiated?

New data have challenged the convention that the adult Sertoli cell population is fixed and unmodifiable. The Sertoli cell has two distinct functions: 1) formation of the seminiferous cords and 2) provision of nutritional and structural support to developing germ cells. For these to occur successfully, Sertoli cells must undergo many maturational changes between fetal and adult life, the main switches occurring around puberty, including the loss of proliferative activity and the formation of the blood-testis barrier. Follicle-stimulating hormone plays a key role in promoting Sertoli cell proliferation, while thyroid hormone inhibits proliferative activity in early postnatal life. Together these regulate the Sertoli-germ cell complement and sperm output in adulthood. By puberty, the Sertoli cell population is considered to be stable and unmodifiable by hormones. But there is mounting evidence that the size of the adult Sertoli cell population and its maturational status is modifiable by hormones and that Sertoli cells can gain proliferative ability in the spermatogenically disrupted hamster and human model. This new information demonstrates that the adult Sertoli cell population, at least in the settings of testicular regression in the hamster and impaired fertility in humans in vivo and from mice and men in vitro, is not a terminally differentiated population. Data from the hamster now show that the adult Sertoli cell population size is regulated by hormones. This creates exciting prospects for basic and clinical research in testis biology. The potential to replenish an adult Sertoli-germ cell complement to normal in a setting of infertility may now be realized.

A novel cytoplasmic adaptor for retinoic acid receptor (RAR) and thyroid receptor functions as a Derepressor of RAR in the absence of retinoic acid

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testis-specific derepressor of RAR.

Aberrant distribution of junctional complex components in retinoic acid receptor alpha-deficient mice

Retinoic acid receptor alpha (RARalpha)-deficient mice are sterile, with abnormalities in the progression of spermatogenesis and spermiogenesis. In this study, we investigated whether defective retinoid signaling involved at least in part, disrupted cell-cell interactions. Hypertonic fixation approaches revealed defects in the integrity of the Sertoli-cell barrier in the tubules of RARalpha-deficient testes. Dye transfer experiments further revealed that coupling between cells from the basal to adluminal compartments was aberrant. There were also differences in the expression of several known retinoic acid (RA)-responsive genes encoding structural components of tight junctions and gap junctions. Immunostaining demonstrated a delay in the incorporation of zonula occludens (ZO-1), a peripheral component protein of tight junctions, into the Sertoli cell tight junctions. Markedly reduced expression of connexin-40 in mutant pachytene spermatocytes and round spermatids was found by in situ hybridization. An ectopic distribution of vimentin and disrupted cyclic expression of vimentin, which is usually tightly regulated during spermiogenesis, was found in RARalpha-deficient testes at all ages examined. Thus, the specific defects in spermiogenesis in RARalpha-deficient testes may correlate with a disrupted cyclic expression of RA-responsive structural components, including vimentin, a downregulation of connexin-40 in spermatogenic cells, and delayed assembly of ZO-1 into Sertoli cell tight junctions. Interestingly, bioinformatic analysis revealed that many genes that are components of tight junctions and gap junctions contained potential retinoic acid response element binding sites.

Activity of retinoic acid receptor-alpha is directly regulated at its protein kinase A sites in response to follicle-stimulating hormone signaling

Retinoic acid receptor-alpha (RARA) is crucial for germ cell development in the testis, as shown by the degenerated testis in Rara gene knockout mice, which are sterile. Similarly, FSH is known to regulate Sertoli cell proliferation and differentiation, indirectly controlling the quantity of the spermatogenic output. Interestingly, FSH inhibited, via activation of FSH receptor, cAMP, and protein kinase A (PKA), the nuclear localization and transcriptional activity of RARA. Given that retinoic acid, the ligand for RARA, is known to regulate cell proliferation and differentiation, we investigated whether FSH regulates RARA by a direct posttranslational phosphorylation mechanism. Mutagenesis of serine 219 (S219) and S369 at the PKA sites on RARA to either double alanines or double glutamic acids showed that both PKA sites are important for RARA activity. The negative charges at the PKA sites, whether they are from glutamic acids or phosphorylation of serines, decreased the nuclear localization of RARA, heterodimerization with retinoid X receptor-alpha, and the transcriptional activity of the receptor. On the other hand, the double-alanine mutant that cannot be phosphorylated at the 219 and 369 amino acid positions did not respond to cAMP and PKA activation. Wild-type and double-mutant RARA interacted with PKA, but only in the presence of cAMP or FSH. These results together suggest that FSH may regulate cell proliferation and differentiation of Sertoli cells, at least partially, by directly affecting the PKA sites of RARA and controlling the transcriptional function of the receptor.

Disulfide isomerase glucose-regulated protein 58 is required for the nuclear localization and degradation of retinoic acid receptor α

Retinoic acid receptor α (RARA) is critical for spermatogenesis, as shown by a sterility phenotype observed inRaraknockout mice. RARA is important in both Sertoli and germ cells of the testis. Here, we demonstrate that a disulfide isomerase glucose-regulated protein 58 (GRp58) participates in the nuclear import and degradation of RARA in Sertoli cells. GRp58 interacted with RARA in the presence of all-transretinoic acid (ATRA) ligand and, as a complex, it was translocated from the cytoplasm to the nucleus and, then with time, GRp58 dissociated from RARA and was found in the cytoplasm. The GRp58 RNAi treatment disrupted ATRA-dependent RARA nuclear localization, indicating the requirement of GRp58 for RARA nuclear localization. Moreover, treatment with sulfhydryl-modifying agents that oxidize SH-groups of cysteine residues to disulfide bonds abolished ATRA-mediated RARA nuclear localization, suggesting that the thiol oxidoreductase activity of GRp58 may be required for RARA nuclear import. Additionally, the proteasome inhibitor treatment resulted in the co-localization of GRp58 and RARA at the endoplasmic reticulum (ER), suggesting that GRp58 may bring RARA to the ER for the ER-associated degradation (ERAD) of RARA before it is de-coupled from RARA for recycling. In this regard, proteasome inhibitor treatment also increased the interaction of RARA with UBE2J2, an ERAD-associated ubiquitin E2 enzyme. Collectively, the results indicate that GRp58 may act as a molecular chaperone that alters the protein conformation of RARA for its delivery to the nucleus and, then with time, accompanies RARA to the ER for RARA ubiquitination and proteasome-mediated ERAD.

Small ubiquitin-like modifier-2 modification of retinoic acid receptor-alpha regulates its subcellular localization and transcriptional activity

The retinoic acid receptor-alpha (Rara) gene is critical for germ cell development in the testis, as demonstrated by infertile Rara knockout male mice. The encoded protein for Rara (RARA) is expressed in both Sertoli cells and germ cells, but it is not always in the nucleus. Previously, all-trans retinoic acid (ATRA) was shown to increase the nuclear localization and transcriptional activity of RARA in Sertoli cells. Here, we identified a small ubiquitin-like modifier-2 (SUMO-2) modification as a novel posttranslational regulatory mechanism controlling the ATRA-dependent RARA subcellular localization and transcription. ATRA increased the SUMO-2 modification of RARA. In the presence of ATRA, lysine 166 (K166) and K171 of RARA were modified at a physiological concentration of SUMO-2, whereas in the absence of ATRA, K399 was the only site that was modified, but at a higher SUMO-2 concentration. However, K399 was critical for ATRA-controlled nuclear trafficking of RARA. In the presence of ATRA, a K399 mutation to arginine resulted in the cytoplasmic localization of K399R mutant, indicating that K166 and K171 sumoylations were inhibitory to nuclear localization. This may be due to SUMO/sentrin-specific peptidase 6 (SENP6) not being able to bind K399R mutant to desumoylate K166 and K171 in Sertoli cells, whereas it can bind RARA with intact K399. On the other hand, functional K166 and K171 sites for sumoylation were required for a full transcriptional activity, when K399 was intact. These results together suggest that both K166 and K171 sumoylation and desumoylation are critical for optimal RARA function.

Immunolocalization of enzymes, binding proteins, and receptors sufficient for retinoic acid synthesis and signaling during the hair cycle

Retinoic acid (RA) is essential for maintenance of most epithelial tissues. One RA biosynthesis pathway consists of cellular retinol-binding protein (Crbp), retinol dehydrogenase (Dhrs9/eRoldh), retinal dehydrogenase 1-3 (Aldh1a1-3), and cellular RA-binding protein 2 (Crabp2). Previously, we localized Aldh1a2 and Aldh1a3 to both epithelial and mesenchymal cells within the hair follicle throughout the hair cycle. This study expands that observation by examining the complete pathway of RA biosynthesis and signaling via RA receptors alpha, beta, and gamma by immunohistochemistry in C57BL/6J mice wax-stripped to initiate and synchronize the cycle. This pathway of RA biosynthesis and signaling localized to the majority of layers of the hair follicle, sebaceous gland, and interfollicular epidermis in a hair cycle-dependent manner, suggesting that RA biosynthesis within the hair follicle is regulated in both a spatial and temporal manner. This localization pattern also revealed insights into epithelial-mesenchymal interactions and differentiation state differences within the RA biosynthesis and signaling pathway, as well as novel observations on nuclear versus cytoplasmic localization of Crabp2 and RA receptors. This complex pattern of RA biosynthesis and signaling identified by immunolocalization suggests that endogenous RA regulates specific aspects of hair follicle growth, differentiation, and cycling.

Effects of inflammatory cytokines IL-1beta, IL-6, and TNFalpha on the intracellular localization of retinoid receptors in Schwann cells

It was investigated whether retinoic acid (RA) and the proinflammatory cytokines IL-1beta, IL-6, and TNFalpha influence the intracellular distribution of retinoic acid receptors (RAR) and retinoid X receptors (RXR) in Schwann cells. This question arose because nuclear translocation of RARalpha, RXRalpha, and RXRbeta was observed after nerve injury, and because mutual interactions exist between the signal transduction pathways of RA and proinflammatory cytokines. Schwann cell primary cultures from the rat sciatic nerve were incubated with IL-1beta, IL-6, and TNFalpha, with all-trans RA and with a combination of IL-1beta and RA. After incubation periods ranging from 5 min to 5 h, the intracellular distributions of RARalpha, RARbeta, RXRalpha, and RXRbeta were analyzed. All three cytokines caused a shift of RARalpha from the cytosolic compartments into the cell nuclei. This was also observed with RA, and combining RA with IL-1beta produced an additive effect. IL-1beta and IL-6 also affected the distribution of RARbeta, although immunoreactivity of this receptor always remained stronger in the cytosol. No effect of the cytokines on RXRalpha or RXRbeta was observed, whereas RA treatment caused a stronger nuclear signal of both receptors. Effects on the subcellular localization of retinoid receptors may provide a link in a feedback loop between RA/RAR and cytokines.

Macrophages and neurons are targets of retinoic acid signaling after spinal cord contusion injury

The physiological reactions after spinal cord injury are accompanied by local synthesis of the transcriptional activator retinoic acid (RA). RA exerts its effects by binding to retinoic acid receptors (RAR) which heterodimerize with retinoid X receptors (RXR) and then act as ligand-activated transcription factors. To identify possible cellular targets of RA we investigated protein levels and cellular distribution of retinoid receptors in the rat spinal cord at 4, 7, 14 and 21 days after a contusion injury. In the nonlesioned spinal cord, immunoreactivity for RARalpha, RXRalpha, RXRbeta and RXRgamma was localized in the cytosol of neurons, that of RXRalpha and RXRbeta in astrocytes and that of RARalpha, RXRalpha and RXRgamma in some oligodendrocytes. After contusion injury RARalpha and all RXRs appeared in the cell nuclei of reactive microglia and macrophages. This nuclear staining began at 4 days, was most prominent at 7 and 14 days and had decreased at 21 days after injury. A similar nuclear translocation was also observed for the RARalpha, RXRalpha and RXRbeta staining in neurons situated around the border of the contusion. These observations suggest that RA participates as a signal for the physiological responses of microglia and neurons after CNS injury.

New therapeutic target for CNS injury? The role of retinoic acid signaling after nerve lesions

Experiments with sciatic nerve lesions and spinal cord contusion injury demonstrate that the retinoic acid (RA) signaling cascade is activated by these traumatic events. In both cases the RA-synthesizing enzyme is RALDH-2. In the PNS, lesions cause RA-induced gene transcription, intracellular translocation of retinoid receptors, and increased transcription of CRBP-I, CRABP-II, and retinoid receptors. The activation of RARbeta appears to be responsible for neurotrophic and neuritogenic effects of RA on dorsal root ganglia and embryonic spinal cord. While the physiological role of RA in the injured nervous system is still under investigation three domains of functions are suggested: (1) neuroprotection and support of axonal growth, (2) modulation of the inflammatory reaction by microglia/macrophages, and (3) regulation of glial differentiation. Few studies have been performed to support nerve regeneration with RA signals in vivo, but a large number of experiments with neuronal and glial cell cultures and spinal cord explants point to beneficial effects of RA, so that future therapeutic approaches will likely focus on the activation of RA signaling.

Differential Effects of Phthalates on the Testis and the Liver1

Phthalates have been shown to elicit contrasting effects on the testis and the liver, causing testicular degeneration and promoting abnormal hepatocyte proliferation and carcinogenesis. In the present study, we compared the effects of phthalates on testicular and liver cells to better understand the mechanisms by which phthalates cause testicular degeneration. In vivo treatment of rats with di-(2-ethylhexyl) phthalate (DEHP) caused a threefold increase of germ cell apoptosis in the testis, whereas apoptosis was not changed significantly in livers from the same animals. Western blot analyses revealed that peroxisome proliferator-activated receptor (PPAR) alpha is equally abundant in the liver and the testis, whereas PPAR gamma and retinoic acid receptor (RAR) alpha are expressed more in the testis. To determine whether the principal metabolite of DEHP, mono-(2-ethylhexyl) phthalate (MEHP), or a strong peroxisome proliferator, 4-chloro-6(2,3-xylindino)-2-pyrimidinylthioacetic acid (Wy-14,643), have a differential effect in Sertoli and liver cells by altering the function of RAR alpha and PPARs, their nuclear trafficking patterns were compared in Sertoli and liver cells after treatment. Both MEHP and Wy-14,643 increased the nuclear localization of PPAR alpha and PPAR gamma in Sertoli cells, but they decreased the nuclear localization of RAR alpha, as previously shown. Both PPAR alpha and PPAR gamma were in the nucleus and cytoplasm of liver cells, but RAR alpha was predominant in the cytoplasm, regardless of the treatment. At the molecular level, MEHP and Wy-14,643 reduced the amount of phosphorylated mitogen-activated protein kinase (activated MAPK) in Sertoli cells. In comparison, both MEHP and Wy-14,643 increased phosphorylated MAPK in liver cells. These results suggest that phthalates may cause contrasting effects on the testis and the liver by differential activation of the MAPK pathway, RAR alpha, PPAR alpha, and PPAR gamma in these organs.

Effects of ethanol on embryonic and neonatal rat testes in organ cultures

Ethanol exposure in adult animals and humans has shown to elicit significant inhibitory effects on the function of male reproduction, but consequences of ethanol exposure on the embryonic and early postnatal testis development are not known. The current study investigated the effect of ethanol on embryonic and neonatal testis development using an organ culture technique. In embryonic day 13 (E13) testis organ cultures, ethanol had no effect on the testicular cord formation, the expression of Müllerian-inhibiting substance (MIS) in Sertoli cells or the number of gonocytes. Similarly, in the ethanol-treated embryonic day 18 (E18) testes, both the number of gonocytes and the expression of GATA-4 and MIS were similar to those from the control testes. In contrast, in postnatal day 3 (P3) testes, ethanol at concentrations of 150 and 200 mM significantly decreased the number of gonocytes without affecting the expression of GATA-4 and MIS in Sertoli cells. This effect was shown to be resulting from the enhanced apoptosis of gonocytes. In addition, ethanol abnormally activated retinoic acid receptor alpha (RARalpha), as indicated by increased nuclear localization of RARalpha with increasing doses of ethanol treatment. These observations suggest that the effect of ethanol on testis varies at different stages during embryonic and neonatal testis development. Furthermore, germ cells may be the main target for the action of ethanol on the early postnatal testis.

Peroxisome proliferators disrupt retinoic acid receptor alpha signaling in the testis

Peroxisome proliferators include a diverse group of chemicals, some of which have been demonstrated to be testicular toxicants. However, the mechanism by which peroxisome proliferators, such as phthalates, cause testicular damage is not clear. It is known that retinoic acid receptor alpha (RARalpha) and its retinoic acid ligand, the acid form of vitamin A, are required for spermatogenesis. It has been demonstrated that the absence of RARalpha gene or vitamin A in the animal leads to testis degeneration and sterility. Therefore, any compound that disrupts the action of vitamin A in the testis could potentially be damaging to male fertility. The current investigation examined a novel hypothesis that a mechanism of degeneration by peroxisome proliferators in the testis is due, in part, to disruption of the critical RARalpha signaling pathway. We show that peroxisome proliferators were able to disrupt the retinoic acid-induced nuclear localization of RARalpha and the retinoic acid-stimulated increase in transcriptional activity of a retinoic acid-responsive reporter gene in Sertoli cells. Concomitantly, peroxisome proliferators increased the nuclear localization of PPARalpha and the transcriptional activity of a peroxisome proliferator-responsive reporter gene in these cells. These results indicate that peroxisome proliferators can indeed shift the balance of nuclear localization for RARalpha and PPARalpha, resulting in deactivation of the critical RARalpha transcriptional activity in Sertoli cells.

Positive regulation of retinoic acid receptor alpha by protein kinase C and mitogen-activated protein kinase in sertoli cells

Retinoic acid receptor alpha (RARalpha) is required for normal testis function. Similar to other steroid hormone receptors, RARalpha appears to undergo an activation process by which it translocates from the cytoplasm to the nucleus where it acts as a transcription factor. In this report, we demonstrate that RARalpha nuclear trafficking in Sertoli cells is positively regulated by phorbol-12-myristate-13-acetate-activated protein kinase C without the requirement of ligand, retinoic acid. Protein kinase C then stimulates the downstream mitogen-activated protein kinase, and the nuclear localization of RARalpha is dependent on activation of both kinases. The increase in RARalpha nuclear translocation is also coupled with enhanced transcriptional activity of RARalpha. This mechanism of RARalpha positive regulation is unique, different from that of its negative regulation, that has previously been shown to be dependent on cAMP-dependent protein kinase A and more importantly, dependent on its ligand. However, the mechanism by which retinoic acid positively influences the nuclear localization of RARalpha is not due to retinoic acid directly increasing protein kinase C or mitogen-activated protein kinase activities. Nonetheless, the positive influence of retinoic acid is also dependent on these two kinases as determined by inhibitor studies. These results suggest two mechanisms for RARalpha activation in Sertoli cells: one involving only the two kinases, the other involving both the ligand and the two kinases. These regulatory mechanisms for RARalpha activation, both positive and negative, may be critical for the proper function of RARalpha in the testis.

Retinoic acid mediates transcriptional repression of ovine follicle-stimulating hormone receptor gene via a pleiotropic nuclear receptor response element

The FSH receptor (FSHR) and retinoid receptors are critical regulators of gonadal function. Unlike the latter, the FSH receptors are expressed exclusively in ovarian granulosa and testicular Sertoli cells in a developmental fashion. Toward understanding the nature of various transcription factors that direct a tissue- and stage-specific expression of the FSHR gene, we have studied FP4, one of the two footprinting regions (FP3 and FP4) mapped at -241 to -269 and -284 to -303, respectively, upstream of the transcription start site of the ovine FSHR gene. Gel mobility shift assays with FP4 probe revealed two sequence-specific DNA-protein complexes in the presence of nuclear extracts from two immortal gonadal cell lines. Antibody supershift assays demonstrated that retinoic acid receptor (RAR) was involved in the complex 1 whereas steroidogenic factor-1 (SF-1) was present in the complex 2. Mutation studies revealed that DNA binding sites for RAR and SF-1 were overlapping each other within a 19-base pair length of nucleotide sequence of FP4, and a mutation in the half RAR binding site seriously affected SF-1 binding. Reporter assays showed that FP4 conferred SF-1 transactivation as well as RAR-mediated, ligand-dependent repression. Overexpression of SF-1 in a transformed Sertoli cell line partially overcame RAR-mediated suppression. For the first time, our studies reveal a direct retinoid modulation of the gonadotropin receptor promoter and suggest a mechanism by which activators and repressors compete for composite elements providing antagonistic pathways that could modulate the expression of FSHR.

Gonadotropins, via cAMP, negatively regulate GATA-1 gene expression in testicular cells

We and others demonstrated that the mRNAs encoding GATA-binding proteins, GATA-1 and GATA-4, were detected in mouse and rat testis, and in isolated rat Sertoli cells and testicular tumor cell lines derived from Leydig and Sertoli cells. In this study, we investigated the possible effects of gonadotropins and cAMP on the expression of GATA-binding protein genes in testicular cells. Unexpectedly, FSH negatively regulated GATA-1 (but not GATA-4) mRNA in a dose-dependent manner in primary cultures of rat Sertoli cells isolated from 21-d-old animals. GATA-1 mRNA was also negatively regulated by cAMP in a dose- and time-dependent manner in MA-10, a mouse Leydig tumor cell line. When 0.3 mM cAMP was administered to MA-10 cell cultures for 4 h, more than 95% of the GATA-1 mRNA and protein was abolished. The reduction of GATA-1 mRNA by cAMP can be mimicked by treatment with forskolin, which elevates intracellular cAMP levels. The inhibitory effect of cAMP was specific to the GATA-1 gene, given that GATA-4 and alpha-tubulin mRNA levels were not changed by any of the cAMP treatments. Inhibin alpha-subunit mRNA, on the other hand, was evidently increased by cAMP treatment in both MA-10 and Sertoli cells. However, inhibin alpha-subunit mRNA levels were elevated at 60-90 min before the suppression of GATA-1 mRNA detected. The inhibitory effect of cAMP on GATA-1 mRNA and protein was shown to be specific to testicular cells. The GATA-1 mRNA expressed in MEL, a mouse erythroid leukemia cell line, was not affected by cAMP. The reduction of GATA-1 mRNA by cAMP can be prevented when a translational inhibitor, cycloheximide, is added. In summary, we demonstrated that gonadotropins via cAMP negatively regulate the mRNA and protein levels of GATA-1, but not GATA-4, in testicular cells. The inhibitory effect on GATA-1 gene expression was specific to testicular cells and was not observed in erythroid cells.

Identification of a novel retrovirus expressed in rat Sertoli cells and granulosa cells

Differential display reverse transcriptase-polymerase chain reaction (DDRT-PCR) was used to identify a novel retrovirus, designated SC1, that is expressed at high levels in rat granulosa cells and prepubertal Sertoli cells. The initial DDRT-PCR screen was performed using RNA from cultured prepubertal rat Sertoli cell, liver, and brain samples. SC1 was detected in the prepubertal rat Sertoli cell samples but not in those from liver and brain. SC1 cDNA was 6 kilobases in length and contained regions encoding for the gag, pol, and env retroviral proteins. Northern blot analysis failed to detect expression of the SC1 gene in total RNA isolated from adult brain, heart, spleen, lung, liver, skeletal muscle, kidney, prostate, and epididymis. Similarly, Northern blot analysis of testes from rats at various ages of development showed that high-level expression of the SC1 gene was limited to prepubertal testis samples. In situ hybridization analysis localized the SC1 mRNA to the seminiferous tubules of prepubertal testes and at a much lower level in Sertoli cells of adult testes. Northern blot analysis of total RNA isolated from Sertoli cells from 20-, 27-, and 35-day-old rat Sertoli cells and type A spermatogonia, pachytene spermatocytes, and round spermatids showed expression of the SC1 gene to be restricted to 20- and 27-day-old Sertoli cells, with no expression detected in germ cells. Furthermore, Northern blot analysis also showed expression of the SC1 gene in rat ovaries, and the level of expression was affected during eCG/hCG-induced ovulation. Expression of SC1 mRNA was localized by in situ hybridization of eCG-treated ovaries to the granulosa cell layer in developing follicles. Southern blot analysis showed SC1 to be endogenous in the rat and absent in mouse and human cell genomes. Transient transfection assays using the SC1 promoter region showed high promoter activity in MSC-1 and cultured prepubertal rat Sertoli cells, and no activity in 3T3 or MCF-7 cell lines.

Subcellular steroid/nuclear receptor dynamics

Steroid hormones, thyroid hormones, retinoic acids, and vitamin D bind to their receptors, which are now called steroid/nuclear receptors, and liganded receptors translocate either intracellularly or intranuclearly and form large protein complexes with cofactors to induce or repress gene transcription. Therefore, steroid/nuclear receptors are ligand-dependent transcription factors. With the advent of green fluorescent protein (GFP) and its color variants, the subcellular distribution of many steroid/nuclear receptors has been found to be much more dynamic than previously thought, with some of the receptors shuttling between the cytoplasm and nucleus. Steroid/nuclear receptors can be divided into three categories based on their unliganded distribution: those that are primarily in the nucleus, those in the cytoplasm, and those with mixed cytoplasmic and nuclear distributions. However, in all cases, the addition of a ligand leads to almost complete nuclear translocation of the receptors. Hormonal stimulation induces intranuclear receptor distribution from a homogeneous pattern to a heterogeneous dot-like image. Ligand binding to steroid/nuclear receptors leads to the recruitment of many proteins including cofactors to provoke the redistribution of receptor complexes in the nucleus. This focal organization could involve more complex events than simple DNA binding sites for transcription. Protein activities and interactions of steroid/nuclear receptors can be imaged and localized in a single cell.

Retinoid receptors involved in the effects of retinoic acid on rat testis development

We have previously shown that retinoic acid (RA) is able to act on the development of Leydig, Sertoli, and germ cells in the testis in culture (Livera et al., Biol Reprod 2000; 62:1303-1314). To identify which receptors mediate these effects, we have now added selective agonists and antagonists of retinoic acid receptors (RARs) or retinoid X receptors (RXRs) in the same organotypic culture system. The RAR alpha agonist mimicked most of the effects of RA on the cultured fetal or neonatal testis, whereas the RAR beta, gamma, and pan RXR agonists did not. The RAR alpha agonist decreased the testosterone production, the number of gonocytes, and the cAMP response to FSH of fetal testis explanted at 14.5 days postconception (dpc). The RAR alpha agonist disorganized the cords of the 14.5-dpc cultured testis and increased the cord diameter in cultured 3-days-postpartum (dpp) testis in the same way as RA. All these RA effects could be reversed by an RAR alpha antagonist and were unchanged by an RAR beta/gamma antagonist. The RAR beta agonist, however, increased Sertoli cell proliferation in the 3-dpp testis in the same way as RA, and this effect was blocked by an RAR beta antagonist. The RAR gamma and the pan RXR agonists had no selective effect. These results suggest that all the effects of RA on development of the fetal and neonatal testis are mediated via RAR alpha, except for its effect on Sertoli cell proliferation, which involves RAR beta.

Follicle-stimulating hormone and leukemia inhibitory factor regulate Sertoli cell retinol metabolism

Sertoli cells, the somatic epithelial cells of the seminiferous tubules, provide both structural and biochemical support for developing male germ cells. The Sertoli cells are targets of retinoid action in the testis. We have found that FSH, (Bu)(2)cAMP, and leukemia inhibitory factor elicit substantial changes in the metabolism of [(3)H]retinol (vitamin A) in primary cultures of purified rat Sertoli cells. Addition of (Bu)(2)cAMP for 2 h or FSH for 6 h results in a 3-fold increase in the metabolism of [(3)H]retinol to [(3)H]retinoic acid ([(3)H]RA); the esterification of [(3)H]retinol to [(3)H]retinyl esters, especially [(3)H]retinyl palmitate, is also increased by approximately 5-fold. The addition of 1 microM all-trans-RA also elicits changes in [(3)H]retinol metabolism, but in this case the metabolism of [(3)H]retinol to [(3)H]RA is inhibited, whereas the metabolism of [(3)H]retinol to [(3)H]retinyl esters is increased by over 50-fold. Leukemia inhibitory factor increases the esterification of [(3)H]retinol by 2- to 3-fold. FSH leads to a reduction in the level of cellular retinol binding protein I transcripts, whereas RA increases the cellular retinol binding protein I messenger RNA level by about 2-fold at approximately 24 h. Levels of AHD-2 (aldehyde dehydrogenase-2) and RALDH-2 (retinaldehyde dehydrogenase-2) messenger RNAs, which encode enzymes that convert [(3)H]retinaldehyde to [(3)H]RA, are increased by about 2-fold by FSH, whereas no change in CYP26 (RA hydroxylase) expression is seen. Our results suggest that one function of FSH (and/or (Bu)(2)cAMP) in Sertoli cells is to increase the metabolism of retinol to the biologically active metabolite RA and to retinyl esters.

Cloning of complementary deoxyribonucleic acids encoding quail (Coturnix coturnix japonica) retinoic acid receptor ss isoforms and changes in their gene expression during gonadotropic growth

Retinoids have important effects on the development of the reproductive system, where they act via their specific nuclear receptors: retinoic acid receptors (RARalpha, ss, gamma) and retinoid X receptors (RXRalpha, ss, gamma). The research reported here was conducted in an effort to clone quail RARbeta+ cDNA (qRARbeta) and to evaluate the expression of qRARbeta+ mRNAs in different tissues and during the development of gonadotropic organs. Two complete cDNAs of qRARbeta1 and qRARbeta2 were isolated by a combination of reverse transcription-polymerase chain reaction and 5'- and 3'-rapid amplification of cDNA ends techniques. An RNase protection assay revealed the widespread expression of qRARbeta1 and beta2 with large tissue-specific variations. The qRARbeta1 isoform was predominant in the testis, whereas qRARbeta2 was dominant in the other tissues examined with the exception of the brain, where both isoforms were almost equally expressed. In the developing testes, the qRARbeta1 mRNA level was high between 30 and 40 days of age, the period during which the testes grew rapidly. The level declined thereafter to its initial level. In contrast, qRARbeta2 mRNA did not exhibit obvious changes. In the developing oviducts, both qRARbeta1 and beta2 mRNAs reached their peak levels by 30 days of age, just before the rapid development of the oviduct occurred, and then decreased to almost undetectable levels when the oviduct developed to the laying stage (over 2.88 g in weight). Similar expression patterns of qRARbeta1 and beta2 were also observed in the developing follicles from the prehierarchical (<2-mm diameter) to the largest preovulatory follicle. In contrast, neither qRARbeta1 nor beta2 mRNA exhibited developmental changes in the brain. These results suggest that RARbeta+ may play an important role in the development of the reproductive systems of birds.