Retinoid receptors in rat vaginal and uterine epithelia: changes with ovarian steroids

Physiological and pathological implications of retinoid action in the endometrium

Retinol (vitamin A) and its derivatives, collectively known as retinoids, are required for maintaining vision, immunity, barrier function, reproduction, embryogenesis and cell proliferation and differentiation. Despite the fact that most events in the endometrium are predominantly regulated by steroid hormones (estrogens and progesterone), accumulating evidence shows that retinoid signaling is also involved in the development and maintenance of the endometrium, stromal decidualization and blastocyst implantation. Moreover, aberrant retinoid metabolism seems to be a critical factor in the development of endometriosis, a common gynecological disease, which affects up to 10% of reproductive age women and is characterized by the ectopic localization of endometrial-like tissue in the pelvic cavity. This review summarizes recent advances in research on the mechanisms and molecular actions of retinoids in normal endometrial development and physiological function. The potential roles of abnormal retinoid signaling in endometriosis are also discussed. The objectives are to identify limitations in current knowledge regarding the molecular actions of retinoids in endometrial biology and to stimulate new investigations toward the development potential therapeutics to ameliorate or prevent endometriosis symptoms.

All-Trans Retinoic Acid Increases Aquaporin 3 Expression in Human Vaginal Epithelial Cells

Introduction

Water channel aquaporin 3 (AQP3) is an aquaglyceroporin that transports small neutral solutes and water. All-trans retinoic acid (ATRA), a member of the retinoid drug class, acts as a regulator in several biological processes.

Aim

To investigate the effect of ATRA on the expression of AQP3 in human vaginal epithelial cells.

Methods

Human vaginal mucosal epithelial cells (CRL2616) were treated with ATRA 0, 0.01, 0.1, and 1 μmol/L for 24 hours to examine the dose-dependent effects of ATRA and with ATRA 1 μmol/L for 0, 3, 6, 12, and 24 hours.

Main Outcome Measures

The expression of AQP3 and retinoic acid receptor (RAR) was determined by western blot analysis and reverse transcription polymerase chain reaction.

Results

AQP3 was detected in the cell membrane of human vaginal epithelial cells. ATRA increased the protein expression and mRNA levels of AQP3 in a dose-dependent manner (P < .05). ATRA also increased the protein expression of RARα (P < .05). Treatment of CRL2616 cells with an RAR antagonist (Ro 41-5253) significantly decreased AQP3 protein expression (P < .05).

Conclusion

ATRA mediated by RARα increased AQP3 gene and protein expression in human vaginal mucosal epithelial cells. These results imply that AQP3 regulated by ATRA could play an important role in the mechanism of vaginal lubrication.

Regulation of expression of the retinoic acid metabolizing enzyme CYP26A1 in uteri of ovariectomized mice after treatment with ovarian steroid hormones

The retinoic acid (RA) synthesizing enzymes, retinaldehyde dehydrogenases (RALDH), are expressed in specific spatial and temporal patterns in uterine tissues during estrous cycle and early pregnancy in mice. Expression of RALDH1 and 2 has been shown to be induced by estrogen treatment within the uterus. In this study, we determined the influence of progesterone and 17-ss-estradiol on the uterine expression of the RA-metabolizing enzyme CYP26A1 after specific time intervals (1, 4, 24, and 48 hr after treatment of ovariectomized mice). In a following experiment, we investigated the influence of gestagen (promegestone 0.3 mg/kg body weight), estrogen (estradiol 3 microg/kg), their combination, as well as the antagonizing anti-progesterone hormone (RU 486 10 mg/kg) on the uterine expression of CYP26A1. Expression of CYP26A1 was localized using in situ hybridization and quantified using RT-PCR. CYP26A1 mRNA expression was strongly--although transiently--induced in uterine endometrial epithelial and glandular cells after administration of gestagen or the combination of gestagen + estrogen, but not by estrogen alone. These observations were confirmed by semi-quantitative RT-PCR experiments on whole uteri. Thus, we show that the expression of CYP26A1 in endometrial epithelial cells is regulated by progesterone and not significantly influenced by co-administration of estrogen. These data indicate an additional level of hormonal control of endogenous RA levels in the mouse uterus, where its synthesis would rely on estrogen-dependent expression of RALDH enzymes, whereas its active metabolism would be triggered by progesterone-induced CYP26A1 expression.

Regulation of expression of the retinoic acid-synthesising enzymes retinaldehyde dehydrogenases in the uteri of ovariectomised mice after treatment with oestrogen, gestagen and their combination

The active metabolite of vitamin A, retinoic acid (RA), plays an important role in the female reproductive system. The synthesis of RA is tightly regulated by the activity of retinaldehyde dehydrogenases (Raldh). Among these, Raldh1 and Raldh2 exhibit specific temporal and spatial expression patterns in the mouse uterus, both during the oestrous cycle and early pregnancy. In the present study, we have assessed whether oestradiol and progesterone directly influence the uterine expression of Raldh1 and Raldh2 in ovariectomised mice. We investigated the effect of gestagen (promegestone 0.3 mg kg(-1) bodyweight), oestrogen (oestradiol 3 microg kg(-1) bodyweight) and their combination on the uterine expression of Raldh2. Expression was analysed using in situ hybridisation and quantified using real-time detection reverse transcription-polymerase chain reaction. The results show that the expression of Raldh2 is rapidly (within 1-4 h) induced in stromal cells by oestrogen, but not by gestagen, treatment, whereas combined oestrogen + gestagen treatment leads to a more prolonged (48 h) response. In contrast, oestrogen, but not progesterone, treatment downregulates (within 4-24 h) Raldh1 expression in the uterine glandular epithelium. We conclude that the uterine RA concentrations are regulated by oestrogens via an effect on the expression of the Raldh synthesising enzymes. Such a regulation is consistent with the natural fluctuations of Raldh expression during the oestrous cycle, early pregnancy and blastocyst implantation.

Immunohistochemical detection of 1,25-dihydroxyvitamin D receptor in rat vaginal epithelium

OBJECTIVE

To determine the distribution of 1,25-dihydroxyvitamin D receptor (VDR) in rat vaginal epithelium during the estrus cycle and in ovariectomized rats.

DESIGN

Animal study performed in two groups of rats. The expression of VDR was examined in the first group during the estrous cycle and in the second group after ovariectomy.

SETTING

University animal laboratory.

ANIMAL(S)

Balb/c female rats.

INTERVENTION(S)

Vaginas were removed and processed for immunohistochemical analysis.

MAIN OUTCOME MEASURE(S)

We recorded the localization, distribution, and expression of VDR in vaginal epithelium during the rat estrus cycle and in ovariectomized rats.

RESULT(S)

In cyclic rats, VDR was detected in basal and suprabasal cells during all of the cycle periods. In apical cells, VDR was positive in diestrus and estrus but negative in proestrous. In ovariectomized rats, VDR was not detected in any layers of vaginal epithelium.

CONCLUSION(S)

In vaginal epithelium, the presence of VDR was shown by using immunohistochemical techniques. During the estrous cycle, VDR has an important role in the proliferation and differentiation of vaginal squamous epithelium that is similar to the effects of estrogen.

Dexamethasone blocks the rapid biological effects of 17beta-estradiol in the rat uterus without antagonizing its global genomic actions

Estrogens and glucocorticoids have opposing effects on the female reproductive tract, but the molecular basis for this antagonism is poorly understood. We therefore examined the biological and transcriptional programs induced by estrogens and glucocorticoids in the uterus of immature female rats. Estradiol 17beta (E2) rapidly induced morphological changes reminiscent of an acute inflammatory response, including infiltration of eosinophils, edema in the stroma and myometrium, and a decrease in the height of luminal epithelial cells, whereas dexamethasone (Dex) only altered stromal cell morphology. When coadministered with E2, Dex completely blocked the proinflammatory effects of E2. Surprisingly, examination of E2 and Dex effects on gene expression using cDNA microarrays and real-time PCR revealed that these hormones had similar effects on the expression of many genes and that very few genes displayed antagonistic regulation. Together, these results indicate strong discord between the early biologic and genomic actions of estrogens and glucocorticoids and highlight a complex regulatory role for glucocorticoids and GR in the mammalian uterus.

The role of vitamin A in mammalian reproduction and embryonic development

Since the late 1980s, there has been an explosion of information on the molecular mechanisms and functions of vitamin A. This review focuses on the essential role of vitamin A in female reproduction and embryonic development and the metabolism of vitamin A (retinol) that results in these functions. Evidence strongly supports that in situ-generated all-trans retinoic acid (atRA) is the functional form of vitamin A in female reproduction and embryonic development. This is supported by the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency with atRA, the block in embryonic development that occurs in retinaldehyde dehydrogenase type 2 null mutant mice, and the essential roles of the retinoic acid receptors, at least in embryogenesis. Early studies of embryos from marginally vitamin A-deficient (VAD) pregnant rats revealed a collection of defects called the vitamin A-deficiency syndrome. The manipulation of all-trans retinoic acid (atRA) levels in the diet of VAD female rats undergoing a reproduction cycle has proved to be an important new tool in deciphering the points of atRA function in early embryos and has provided a means to generate large numbers of embryos at later stages of development with the vitamin A-deficiency syndrome. The essentiality of the retinoid receptors in mediating the activity of atRA is exemplified by the many compound null mutant embryos that now recapitulate both the original vitamin A-deficiency syndrome and exhibit a host of new defects, many of which can also be observed in the VAD-atRA-supported rat embryo model and in retinaldehyde dehydrogenase type 2 (RALDH2) mutant mice. A major task for the future is to elucidate the atRA-dependent pathways that are normally operational in vitamin A-sufficient animals and that are perturbed in deficiency, thus leading to the characteristic VAD phenotypes described above.

Development of a retinoic acid receptor-binding assay with rainbow trout tissue: characterization of retinoic acid binding, receptor tissue distribution, and developmental changes

Retinoic acid (RA) regulates the transcription of various genes required for several essential functions in vertebrates through binding to two classes of nuclear receptors, the retinoic acid receptors (RAR) and retinoid X receptors (RXR). We investigated nuclear RA binding in tissues from rainbow trout using the radiolabeled all-trans and 9-cis isomers of RA. Specific binding (indicative of receptor binding) of both all-trans- and 9-cis-RA was found in all tissues tested, including the adult trout ovary, testis, gill, liver, kidney, blood, white muscle, and heart. The kinetics and absolute amount of RA binding were dependent on both the tissue and the isomer of RA used. All-trans-RA bound with high affinity (K(d) approximately 1.0-3.9 nM), and low capacity (B(max) approximately 75-484 fmol RA/mg protein), while 9-cis-RA bound with lower affinity (K(d) approximately 7-56 nM), but with a greater capacity (B(max) approximately 214-1076 fmol RA/mg protein). The B(max) results were used to estimate RAR and RXR levels and revealed that the gill possesses primarily RARs while the liver possesses primarily RXRs. The RAR-specific competitor TTNPB was able to effectively displace all-trans-[3H]RA in most tissues, and the RXR-specific competitor AGN 194204 was able to effectively displace 9-cis-[3H]RA. However, TTNPB and AGN 194204 could not displace all of the RA in the kidney and testis, suggesting the existence of another nuclear RA binding protein. Binding of all-trans- and 9-cis-RA was also found in developing trout embryos and fry. Kinetic analysis revealed that RAR levels predominated at the eyed-embryo stage, but decreased 87% by the swim-up fry stage, while RXR levels remained relatively constant over the same time period. These findings suggest that RA and its receptors may play a key role in early trout development. This study has provided a simple and rapid radioligand binding assay that can identify RAR and RXRs in trout tissues.

Lessons from pregnancy and parturition: uterine leiomyomas result from discordant differentiation and dedifferentiation responses in smooth muscle cells

Leiomyomas, benign smooth muscle tumors of the uterus, are the most common gynecological neoplasm in women. Studies with human tissues and primary cultures have revealed little about the development of leiomyomas, although several genes have been shown to be differentially expressed in leiomyomas compared to matched normal myometrium. We propose that uterine smooth muscle tumor cells mimic a differentiated myometrial cell of pregnancy, and are associated with a hypersensitivity to sex steroid hormones, preventing the cells from responding to normal apoptotic or dedifferentiation signals which would return the cells to a nongravid phenotype. Support of this hypothesis is derived from experimental studies in female Eker rats which develop uterine leiomyoma with many similarities to the human disease. Members of the steroid receptor superfamily as well as the binding partners and co-regulators necessary for transactivation and gene transcription, may be involved in the altered pathway of cellular differentiation and regulation observed in uterine leiomyomas.

Retinoic acid suppresses the osteogenic differentiation capacity of murine osteoblast-like 3/A/1D-1M cell cultures

Bone is a target tissue for action of retinoids though their precise role remains unclear. This study investigated the effects of retinoic acid (RA) on the marrow stromal 3/A/1D-1M osteoblast-like cells, derived from the in vivo transplantation of 3/A/1D-1 chondroprogenitor cells. Long-term treatment with 1 microM RA for 7 weeks induced a marked decrease in bone-like nodule number and ultrastructural alterations in the striation and the size of the collagen fibres. RA at concentrations varying from 10 nM to 3.16 microM had a dose-dependent inhibition effect on alkaline phosphatase (AP) activity with an IC50 of 0.7 microM. Treatment with 1 microM RA for up to 17 days induced a time-dependent inhibition of AP activity, while the beginning of RA treatment (4 or 52 h of culture) produced a differential magnitude of inhibition. These variations were unrelated to modifications of the expression of RAR receptor at the protein level, as assessed by Western blot analysis. Exposure to 1 microM RA for 6 or 24 h administered at day 14 produced an inhibition of AP activity, which reached a maximum after 48 h, with a recovery time of 8 days in both cases. Long-term treatment with RA at 1 microM completely abolished the level of osteocalcin mRNA on both days 12 and 16, as revealed by Northern blot analysis. However, such RA-treated cells retained the constitutive expression of type II procollagen transcripts. These results suggest that RA inhibits several aspects of osteogenic differentiation capacity, a loss of phenotype, which, in association with the maintenance of type II procollagen cartilage-related characteristic, could be a prerequisite step for cellular plasticity.