Role of retinoic acid in the differentiation of embryonal carcinoma and embryonic stem cells

Retinoic acid availability drives the asynchronous initiation of spermatogonial differentiation in the mouse

Throughout the reproductive lifespan of most male mammals, sperm production is constant because of the regulated differentiation of spermatogonia. Retinoic acid (RA) and a downstream target, Stra8, are required for complete spermatogenesis. To examine the role of RA in initiating spermatogonial differentiation, a transgenic mouse model expressing beta-galactosidase under the control of an RA response element was used. Cells in the neonatal testis undergoing active RA signaling were visualized by beta-galactosidase activity, the relationship between RA and differentiation determined, and the role of RA-degrading enzymes in regulating RA demonstrated. Beta-galactosidase activity was found to be predominantly associated with differentiating, premeiotic germ cells and to be distributed nonuniformly throughout the seminiferous tubules. Additionally, beta-galactosidase activity in premeiotic germ cells colocalized with STRA8 protein and was induced in germ cells with exogenous RA treatment. The RA-degrading enzyme, CYP26B1, was found to have germ cell localization and nonuniform distribution between tubules via immunohistochemistry. Treatment with a CYP26 enzyme inhibitor resulted in an increased number of germ cells with both beta-galactosidase activity and STRA8 protein and an increase in the expression of genes associated with differentiation and reduced expression of a gene associated with undifferentiated germ cells. These results show the action of RA in a subset of spermatogonia leads to nonuniform initiation of differentiation throughout the neonatal testis, potentially mediated through the action of CYP26 enzymes. Thus, the presence of RA is a likely driving factor in the initiation of spermatogonial differentiation and may result in asynchronous spermatogenesis.

Retinoic acid induces expression of the thyroid hormone transporter, monocarboxylate transporter 8 (Mct8)

Retinoic acid (RA) and thyroid hormone are critical for differentiation and organogenesis in the embryo. Mct8 (monocarboxylate transporter 8), expressed predominantly in the brain and placenta, mediates thyroid hormone uptake from the circulation and is required for normal neural development. RA induces differentiation of F9 mouse teratocarcinoma cells toward neurons as well as extraembryonal endoderm. We hypothesized that Mct8 is functionally expressed in F9 cells and induced by RA. All-trans-RA (tRA) and other RA receptor (RAR) agonists dramatically (>300-fold) induced Mct8. tRA treatment significantly increased uptake of triiodothyronine and thyroxine (4.1- and 4.3-fold, respectively), which was abolished by a selective Mct8 inhibitor, bromosulfophthalein. Sequence inspection of the Mct8 promoter region and 5'-rapid amplification of cDNA ends PCR analysis in F9 cells identified 11 transcription start sites and a proximal Sp1 site but no TATA box. tRA significantly enhanced Mct8 promoter activity through a consensus RA-responsive element located 6.6 kilobases upstream of the coding region. A chromatin immunoprecipitation assay demonstrated binding of RAR and retinoid X receptor to the RA response element. The promotion of thyroid hormone uptake through the transcriptional up-regulation of Mct8 by RAR is likely to be important for extraembryonic endoderm development and neural differentiation. This finding demonstrates cross-talk between RA signaling and thyroid hormone signaling in early development at the level of the thyroid hormone transporter.

The X-linked mental retardation gene PHF8 is a histone demethylase involved in neuronal differentiation

Recent studies have identified mutations in PHF8, an X-linked gene encoding a JmjC domain-containing protein, as a causal factor for X-linked mental retardation (XLMR) and cleft lip/cleft palate. However, the underlying mechanism is unknown. Here we show that PHF8 is a histone demethylase and coactivator for retinoic acid receptor (RAR). Although activities for both H3K4me3/2/1 and H3K9me2/1 demethylation were detected in cellular-based assays, recombinant PHF8 exhibited only H3K9me2/1 demethylase activity in vitro, suggesting that PHF8 is an H3K9me2/1 demethylase whose specificity may be modulated in vivo. Importantly, a mutant PHF8 (phenylalanine at position 279 to serine) identified in the XLMR patients is defective in enzymatic activity, indicating that the loss of histone demethylase activity is causally linked with the onset of disease. In addition, we show that PHF8 binds specifically to H3K4me3/2 peptides via an N-terminal PHD finger domain. Consistent with a role for PHF8 in neuronal differentiation, knockdown of PHF8 in mouse embryonic carcinoma P19 cells impairs RA-induced neuronal differentiation, whereas overexpression of the wild-type but not the F279S mutant PHF8 drives P19 cells toward neuronal differentiation. Furthermore, we show that PHF8 interacts with RARalpha and functions as a coactivator for RARalpha. Taken together, our results suggest that histone methylation modulated by PHF8 plays a critical role in neuronal differentiation.

Activation of retinoic acid receptor signaling coordinates lineage commitment of spontaneously differentiating mouse embryonic stem cells in embryoid bodies

Retinoid signaling has been implicated in embryonic stem cell differentiation. Here we present a systematic analysis of gene expression changes in mouse embryonic stem cells (mESCs), during their spontaneous differentiation into embryoid bodies and the effect of all-trans retinoic acid (ATRA) on this process. We show that retinoic acid is present in the serum and is sufficient to activate retinoid signaling at a basal level in undifferentiated mESCs. This signal disappears during embryoid body formation. However exogenously added ATRA resets the spontaneous differentiation programs in embryoid bodies and initiates a distinct genetic program. These data suggest that retinoid signaling not only promotes a particular pathway but also acts as a context dependent general coordinator of the differentiation states in embryonic stem cells.

Sex determination in mammalian germ cells: extrinsic versus intrinsic factors

Mammalian germ cells do not determine their sexual fate based on their XX or XY chromosomal constitution. Instead, sexual fate is dependent on the gonadal environment in which they develop. In a fetal testis, germ cells commit to the spermatogenic programme of development during fetal life, although they do not enter meiosis until puberty. In a fetal ovary, germ cells commit to oogenesis by entering prophase of meiosis I. Although it was believed previously that germ cells are pre-programmed to enter meiosis unless they are actively prevented from doing so, recent results indicate that meiosis is triggered by a signaling molecule, retinoic acid (RA). Meiosis is avoided in the fetal testis because a male-specifically expressed enzyme actively degrades RA during the critical time period. Additional extrinsic factors are likely to influence sexual fate of the germ cells, and in particular, we postulate that an additional male-specific fate-determining factor or factors is involved. The full complement of intrinsic factors that underlie the competence of gonadal germ cells to respond to RA and other extrinsic factors is yet to be defined.

Retinoic acid orchestrates fibroblast growth factor signalling to drive embryonic stem cell differentiation

Embryonic stem (ES) cells fluctuate between self-renewal and the threshold of differentiation. Signalling via the fibroblast growth factor (Fgf)/Erk pathway is required to progress from this dynamic state and promote mouse ES cell differentiation. Retinoic acid also induces differentiation in many cellular contexts, but its mechanism of action in relation to Fgf/Erk signalling in ES cells is poorly understood. Here, we show for the first time that endogenous retinoid signalling is required for the timely acquisition of somatic cell fate in mouse ES cells and that exposure to retinoic acid advances differentiation by a dual mechanism: first increasing, but in the long-term decreasing, Fgf signalling. Rapid retinoid induction of Fgf8 and downstream Erk activity on day 1 in differentiation conditions may serve to ensure loss of self-renewal. However, more gradual repression of Fgf4 by retinoic acid is accompanied by an overall reduction in Erk activity on day 2, and the acquisition of neural and non-neural fates is now advanced by inhibition of Fgf signalling. So, although blocking Fgf/Erk activity is known to promote ES cell self-renewal, once cells have experienced a period of such signals, subsequent inhibition of Fgf signalling has the opposite effect and drives differentiation. We further show in the embryo that retinoid repression of Fgf signalling promotes neural differentiation onset in an analogous step in the extending embryonic body axis and so identify attenuation of Fgf signalling by retinoic acid as a conserved fundamental mechanism driving differentiation towards somatic cell fates.

Foxa1 functions as a pioneer transcription factor at transposable elements to activate Afp during differentiation of embryonic stem cells

Epigenetic control of genes that are silent in embryonic stem cells, but destined for expression during differentiation, includes distinctive hallmarks, such as simultaneous activating/repressing (bivalent) modifications of chromatin and DNA hypomethylation at enhancers of gene expression. Although alpha-fetoprotein (Afp) falls into this class of genes, as it is silent in pluripotent stem cells and activated during differentiation of endoderm, we find that Afp chromatin lacks bivalent histone modifications. However, critical regulatory sites for Afp activation, overlapping Foxa1/p53/Smad-binding elements, are located within a 300-bp region lacking DNA methylation, due to transposed elements underrepresented in CpG sequences: a short interspersed transposable element and a medium reiterated sequence 1 element. Forkhead family member Foxa1 is activated by retinoic acid treatment of embryonic stem cells, binds its DNA consensus site within the short interspersed transposable/medium reiterated sequence 1 elements, and displaces linker histone H1 from silent Afp chromatin. Small interfering RNA depletion of Foxa1 showed that Foxa1 is essential in providing chromatin access to transforming growth factor beta-activated Smad2 and Smad4 and their subsequent DNA binding. Together these transcription factors establish highly acetylated chromatin and promote expression of Afp. Foxa1 acts as a pioneer transcription factor in de novo activation of Afp, by exploiting a lack of methylation at juxtaposed transposed elements, to bind and poise chromatin for intersection with transforming growth factor beta signaling during differentiation of embryonic stem cells.

A novel signaling by vitamin A/retinol promotes self renewal of mouse embryonic stem cells by activating PI3K/Akt signaling pathway via insulin-like growth factor-1 receptor

Pluripotent embryonic stem (ES) cells are a potential source of all types of cells for regenerative medicine. ES cells maintain pluripotency through a complex interplay of different signaling pathways and transcription factors, including leukemia inhibitory factor (LIF), Nanog, Sox2, and Oct3/4. Nanog, however, plays a key role in maintaining the pluripotency of mouse and human ES cells. Phosphoinositde 3-kinase (PI3K) signaling pathway which is activated in response to growth factors and cytokines also plays a critical role in promoting the survival and proliferation of ES cells. Our earlier studies revealed that retinol, the alcohol form of vitamin A, enhances the expression of Nanog and prevents differentiation of ES cells in long-term cultures. Normally vitamin A/retinol is associated with cell differentiation via its potent metabolite, retinoic acid. Thus far, no direct function has been ascribed to retinol itself. In this study, we demonstrate for the first time that retinol directly activates phosphoinositide three (PI3) kinase signaling pathway through IGF-1 receptor/insulin receptor substrate one (IRS-1) by engaging Akt/PKB-mTORC1 mammalian target of rapamycin-2 (mammalian target of rapamycin complex 2), indicating a growth factor-like function of vitamin A. Furthermore, ES cells do not express enzymes to metabolize retinol into retinoic acid and lack receptors for retinol transport into the cytoplasm, indicating that retinol signaling is independent of retinoic acid. This study presents a novel system to investigate how extracellular signals control the self renewal of ES cells which will be important for high-quality ES cells for regenerative medicine.

LIM protein Ajuba functions as a nuclear receptor corepressor and negatively regulates retinoic acid signaling

Corepressors play an essential role in nuclear receptor-mediated transcriptional repression. In general, corepressors directly bind to nuclear receptors via CoRNR boxes (L/I-X-X-I/V-I) in the absence of ligand and appear to act as scaffolds to further recruit chromatin remodeling complexes to specific target genes. Here, we describe the identification of the multiple LIM domain protein Ajuba as a unique corepressor for a subset of nuclear hormone receptors. Ajuba contains functional nuclear-receptor interacting motifs and selectively interacts with retinoic acid receptors (RARs) and rexinoid receptor (RXRs) subtypes in a ligand-dependent manner. Simultaneous mutation of these motifs abolishes RAR binding and concomitantly leads to loss of repression on RARE reporter genes. P19 cells depleted of Ajuba are highly sensitized to all-trans retinoic acid (atRA)-induced transcription and differentiation. In the absence of atRA, Ajuba can be readily found at the RARE control elements of RAR endogenous target genes. Stimulation of cells with atRA results in the dissociation of Ajuba from these regions. Moreover, we observed that coexpression of the known Ajuba binding partner Prmt5 (protein arginine methyltransferase-5) inhibited the Ajuba/RAR interaction. The high-affinity Ajuba-RAR/RXR interaction site overlaps the region responsible for Ajuba/Prmt5 binding, and thus binding appears to be mutually exclusive, providing a potential mechanism for these observations. Identification of Ajuba as a unique corepressor for nuclear receptors sheds new light on mechanisms for nuclear receptor-mediated repression and provides a unique target for developing more effective therapeutics to modulate this important pathway.

STRA8 shuttles between nucleus and cytoplasm and displays transcriptional activity

Stra8 (stimulated by retinoic acid 8) encodes a protein crucial for mammalian germ cells entering into premeiotic stages. Here, to elucidate the still unknown STRA8 molecular functions, we studied the cellular localization of the protein in several cell types, including premeiotic mouse germ cells and stem cell lines. We reported distinct STRA8 localization in germ and stem cell types and a heterogeneous protein distribution in the cytoplasm and nucleus of such cells suggesting that the protein can shuttle between these two compartments. Moreover, we identified specific protein motifs determining its nuclear import/export. Furthermore, we demonstrated that in transfected cell lines the nuclear import of STRA8 is an active process depending on an N-terminal basic nuclear localization signal. Moreover, its nuclear export is mainly mediated by the Exportin1 (XPO1) recognition of a nuclear export signal. Significantly, we also demonstrated that STRA8 associates with DNA and possesses transcriptional activity. These observations strongly suggest that STRA8 can exert important functions in the nucleus rather than in the cytoplasm as believed previously, likely depending on the cell type and regulated by its nuclear-cytoplasmic shuttling.

C38, equivalent to BM88, is developmentally expressed in maturing retinal neurons and enhances neuronal maturation

C38 antigen is specifically expressed in neuronal cells of the retina. The purpose of this study was to isolate C38 cDNA and determine its molecular functions. Sequence analysis of C38 cDNA revealed that C38 is equivalent to rat BM88, which has been reported to induce cell-cycle arrest and neuronal differentiation in Neuro2a cells. C38 and Ki67, a marker of proliferating cells, were not colocalized during retinal development. C38 was first detected in the retinal ganglion cells at embryonic day 16, much later than the expression of doublecortin, a marker of immature neurons. Although all the horizontal cells were post-mitotic at this stage, C38 was not detected in horizontal cells until the postnatal period. In addition, C38 over-expression did not induce neuronal differentiation or cell-cycle arrest of pluripotent P19 embryonal carcinoma cells. Instead, C38 promoted maturation during neuronal differentiation of P19 embryonal carcinoma cells by down-regulating Oct-3, a pluripotent cell marker and enhancing the expressions of positive regulators of neurogenesis. In conclusion, during retinal development, C38 is first expressed in post-mitotic retinal neurons and is up-regulated during their maturation. C38 does not induce neuronal competence in pluripotent cells, but does promote maturation in already committed neuronal cells.

Mbd3, a component of NuRD/Mi-2 complex, helps maintain pluripotency of mouse embryonic stem cells by repressing trophectoderm differentiation

Embryonic stem cells (ES cells) can differentiate into cells derived from all three germ layers and extraembryonic tissues. While transcription factors such as, Oct4 and Nanog are well known for their requirements for undifferentiated ES cell growth, mechanisms of epigenetic repression of germ layer specific differentiation in ES cells are not well understood. Here, we investigate functions of Mbd3, a component of nucleosome remodeling and histone deacetylation complex (NuRD/Mi-2) in mouse ES cells. We find that compared to wild type ES cells, Mbd3 knockdown cells show elevated RNA expression of trophectoderm markers, including Cdx2, Eomesodermin, and Hand1. In parallel, these cells show an increased acetylation level of histone 3 in promoters of the respective genes, suggesting Mbd3 plays a role in repression of these genes in undifferentiated ES cells. However, these changes are not sufficient for definitive differentiation to trophectoderm (TE) in chimeric embryos. When further cultured in ES medium without LIF or in trophoblast stem (TS) cell medium, Mbd3 knockdown cells differentiate into TE cells, which express Cdx2 and, at later stages, trophoblast lineage specific marker Cadherin 3. These results suggest that Mbd3 helps restrict ES cells from differentiating towards the trophectoderm lineage and is an important epigenetic player in maintaining full pluripotency of mouse ES cells.

Biphasic effect of citral, a flavoring and scenting agent, on spatial learning and memory in rats

Although some central effects of citral have been reported, cognitive effects on spatial memory have not been investigated. The evidence showed that citral can regulate the synthesis of retinoic acid (RA), which exerts a vital function in the development and maintenance of spatial memory. In this study, we applied Morris water maze to test the effect of citral on animals' spatial learning and memory. To elucidate the mechanism of this effect, we also measured the retinoic acid concentration in rats' hippocampus by high performance liquid chromatography (HPLC). Our data implied biphasic effects of citral. The low dose (0.1 mg/kg) of citral improved the spatial learning capability, and enhanced the spatial reference memory of rats, whereas the high dose (1.0 mg/kg) was like to produce the opposite effects. Meanwhile, the low dose of citral increased the hippocampal retinoic acid concentration, while the high dose decreased it. Due to the quick elimination and non-bioaccumulation in the body, effects of citral on spatial memory in this study seemed to be indirect actions. The change in hippocampal retinoic acid concentration induced by different doses of citral might be responsible for the biphasic effect of citral on spatial learning and memory.

Instructing an embryonic stem cell-derived oocyte fate: lessons from endogenous oogenesis

Female reproductive potential is limited in the majority of species due to oocyte depletion. Because functional human oocytes are restricted in number and accessibility, a robust system to differentiate oocytes from stem cells would enable a thorough investigation of the genetic, epigenetic, and environmental factors affecting human oocyte development. Also, the differentiation of functional oocytes from stem cells may permit the success of human somatic cell nuclear transfer for reprogramming studies and for the production of patient-specific embryonic stem cells (ESCs). Thus, ESC-derived oocytes could ultimately help to restore fertility in women. Here, we review endogenous and ESC-derived oocyte development, and we discuss the potential and challenges for differentiating functional oocytes from ESCs.

Roles of testicular orphan nuclear receptors 2 and 4 in early embryonic development and embryonic stem cells

The testicular orphan nuclear receptors (TRs) 2 and 4 act as either transcriptional activators or regulatory proteins of other nuclear receptor superfamily members. With no identified cognate ligands, their physiological roles remain unclear. Here we showed the phenotypes of TR2(-/-):TR4(-/-) mutant embryos, which reveal that the loss of TR2 and TR4 causes early embryonic lethality and increased cell death. We also found that TR2 and TR4 are expressed in blastocysts and embryonic stem (ES) cells, and can act as transcriptional activators in ES cells. The results on further investigating the roles of TR2 and TR4 in ES cells showed that TR2 and TR4 were differentially expressed when ES cells were induced into different specialized cell types, and their expression is regulated by retinoic acid. Knocking down TR2 and TR4 mRNAs decreased the expression of Oct-3/4 and Nanog genes. Mechanism dissection suggests that TR2 and TR4 may affect the Oct-3/4 gene by binding to a direct repeat-1 element located in its promoter region, which is influenced by retinoic acid. Together, our findings highlight possible roles for TR2 and TR4 in early embryonic development by regulating key genes involved in stem cell self-renewal, commitment, and differentiation.

Human embryonic stem cell differentiation on tissue engineering scaffolds: effects of NGF and retinoic acid induction

The indefinite proliferative capacity and ability to differentiate into all somatic cell types can make human embryonic stem cells (hESCs) useful in experimental and applied studies in embryonic development, tissue engineering, genetic engineering, pharmacokinetics, and the like. Cellular differentiation dynamics can be studied in monolayer cell cultures; however, it proceeds in three-dimensional (3D) organization in vivo. The aim of this study was to assess the effects of retinoic acid (RA) and nerve growth factor (NGF) on the differentiation patterns of hESCs in 3D culture environment and to compare it with the monolayer culture. Expanded hESCs (HUES-9) were differentiated in two experimental groups for 21 days: (i) two-dimensional (2D) monolayer cultures of hESC colonies, and (ii) 3D culture of hES single cells in poly(DL-lactic-co-glycolic acid) scaffolds. The media used were embryonic stem cell expansion medium (ES-EM), embryonic stem cell differentiation medium containing fetal calf serum (ES-DM), ES-EM containing either 10 ng/mL NGF or 10(-6) M RA, and their combination. Fixed specimens were analyzed with scanning electron microscopy, and expression of nestin, pan-cytokeratin, troponin, and alpha-fetoprotein at days 7, 14, and 21 was evaluated by immunohistomorphometry and reverse transcriptase--polymerase chain reaction. Results indicate different patterns of ectodermal, mesodermal, and endodermal marker expressions between groups, where NGF and RA preferentially favors the differentiation toward ectodermal and mesodermal lineages. While troponin and nestin expression is significantly elevated in 3D culture environment, pan-cytokeratin expression is favored by 2D culture instead. The effects of 3D scaffold culture imply the usefulness of testing in vitro differentiation properties of hESCs in various culture settings designed as models in prospective tissue engineering applications.

Differentiation of pluripotent stem cells on multiwalled carbon nanotubes

This paper studies the adhesion, growth, and differentiation of stem cells on carbon nanotube matrices. Glass coverslips were coated with multiwalled carbon nanotube (MWNT) thin films using layer-by-layer self-assembling techniques. Pluripotent P19 mouse embryonal carcinoma stem cells were seeded onto uncoated or MWNT-coated coverslips, and either maintained in an undifferentiated state or induced to differentiate by the addition of retinoic acid. We found that cell adhesion was increased on the MWNT-coated surfaces, and that the expression patterns of some differentiation markers were altered in cells grown on MWNTs. The results suggest that MWNTs will be useful in directing pluripotent stem cell differentiation for tissue engineering purposes.

Up-regulation of the SOX3 gene expression by retinoic acid: characterization of the novel promoter-response element and the retinoid receptors involved

Sox3/SOX3 gene is considered to be one of the earliest neural markers in vertebrates and it is implicated in the genetic cascades that direct brain formation. We have previously shown that early phases of differentiation and neural induction of NT2/D1 embryonal carcinoma cells by retinoic acid (RA) involve up-regulation of the SOX3 gene expression. Here, we present identification of a novel positive regulatory promoter element involved in RA-dependent activation of the SOX3 gene expression in NT2/D1 cells. This element represents a direct repeat 3-like motif that directly interacts with retinoid X receptor (RXR) alpha in a sequence-specific manner. It is capable of independently mediating the RA effect in a heterologous promoter context and its disruption caused significant reduction of RA/RXR transactivation of the SOX3 promoter. Furthermore, by using synthetic antagonists of retinoid receptors, we have shown for the first time, that RA-induced SOX3 gene expression could be significantly down-regulated by the synthetic antagonist of RXR. Also, this data showed that RXRs, but not RA receptors, are mediators of RA effect on the SOX3 gene up-regulation in NT2/D1 cells. Presented data will be valuable for future investigation of SOX3 gene expression, not only in NT2/D1 model system, but also in diverse developmental, physiological and pathological settings.

Retinoic acid-stimulated sequential phosphorylation, PML recruitment, and SUMOylation of nuclear receptor TR2 to suppress Oct4 expression

We previously reported an intricate mechanism underlying the homeostasis of Oct4 expression in normally proliferating stem cell culture of P19, mediated by SUMOylation of orphan nuclear receptor TR2. In the present study, we identify a signaling pathway initiated from the nongenomic activity of all-trans retinoic acid (atRA) to stimulate complex formation of extracellular signal-regulated kinase 2 (ERK2) with its upstream kinase, mitogen-activated protein kinase kinase (MEK). The activated ERK2 phosphorylates threonine-210 (Thr-210) of TR2, stimulating its subsequent SUMOylation. Dephosphorylated TR2 recruits coactivator PCAF and functions as an activator for its target gene Oct4. Upon phosphorylation at Thr-210, TR2 increasingly associates with promyelocytic leukemia (PML) nuclear bodies, becomes SUMOylated, and recruits corepressor RIP140 to act as a repressor for its target, Oct4. To normally proliferating P19 stem cell culture, exposure to a physiological concentration of atRA triggers a rapid nongenomic signaling cascade to suppress Oct4 gene and regulate cell proliferation.

Effects of diethylstilboestrol on testicular function and luteinizing hormone receptors

Adult male Fisher-344 rats were implanted with DES-filled or empty Silastic capsules. After 14 weeks, capsules were removed and a second group of rats received DES capsules. Seven weeks later, all the rats were sacrificed. DES treatment decreased body, testes and seminal vesicle weights, and removal of the capsules partially restored the weight of these organs. The concentration of testicular LH receptors was increased by DES treatment. Circulating PRL levels were increased and gonadotropin levels were reduced in all animals having received DES at anytime. Plasma testosterone (T) levels were similar in all groups, but testicular T levels were reversibly decreased by DES. Similarly, whereas basal incubation media T levels were unchanged by DES treatment, the steroidogenic response in vitro to hCG was abolished by the presence of DES, and removal of the capsules restored this response. It appears that in this animal model DES and PRL exert opposing effects on testicular LH receptor.