Primary culture of avian embryonic heart forming region cells to study the regulation of vertebrate early heart morphogenesis by vitamin A

Background

Important knowledge about the role of vitamin A in vertebrate heart development has been obtained using the vitamin A-deficient avian in ovo model which enables the in vivo examination of very early stages of vertebrate heart morphogenesis. These studies have revealed the critical role of the vitamin A-active form, retinoic acid (RA) in the regulation of several developmental genes, including the important growth regulatory factor, transforming growth factor-beta2 (TGFβ2), involved in early events of heart morphogenesis. However, this in ovo model is not readily available for elucidating details of molecular mechanisms determining RA activity, thus limiting further examination of RA-regulated early heart morphogenesis. In order to obtain insights into RA-regulated gene expression during these early events, a reliable in vitro model is needed. Here we describe a cell culture that closely reproduces the in ovo observed regulatory effects of RA on TGFβ2 and on several developmental genes linked to TGFβ signaling during heart morphogenesis.

Results

We have developed an avian heart forming region (HFR) cell based in vitro model that displays the characteristics associated with vertebrate early heart morphogenesis, i.e. the expression of Nkx2.5 and GATA4, the cardiogenesis genes, of vascular endothelial growth factor (VEGF-A), the vasculogenesis gene and of fibronectin (FN1), an essential component in building the heart, and the expression of the multifunctional genes TGFβ2 and neogenin (NEO). Importantly, we established that the HFR cell culture is a valid model to study RA-regulated molecular events during heart morphogenesis and that the expression of TGFβ2 as well as the expression of several TGFβ2-linked developmental genes is regulated by RA.

Conclusions

Our findings reported here offer a biologically relevant experimental in vitro system for the elucidation of RA-regulated expression of TGFβ2 and other genes involved in vertebrate early cardiovascular morphogenesis.

The expression of Stra6 and Rdh10 in the avian embryo and their contribution to the generation of retinoid signatures

Two new components of the retinoic acid (RA) synthetic pathway, the cell surface receptor for retinol, Stra6, and the enzyme converting retinol into retinal, Rdh10, have recently been described. To understand how different tissues of the chick embryo generate different retinoid signatures, we describe the expression patterns of these two genes and ask whether they are altered by RA levels. We performed wholemount in situ hybridisation and altered RA levels by applying RA soaked beads and used vitamin A-deficient quail embryos. In some areas of the embryo, these two genes co-localised with a retinaldehyde dehydrogenase (Raldh), as might be expected allowing retinol to be taken into the cell and converted into RA. In other areas of the embryo, the domain of expression of Rdh10 was much smaller than that of the corresponding Raldh, suggesting that retinal is transferred between cells. In yet other areas, only one of the cytochrome P450 enzymes co-localises with Stra6. In the case of co-localisation with Cyp1B1 in the hindbrain mesenchyme, this reveals that retinol is taken up into the cells for conversion to RA by Cyp1B1 and used in establishing ventral progenitor domains in the hindbrain. In the case of co-localisation with a Cyp26, it suggests that other retinol dehydrogenases (Rdhs) have yet to be discovered. We propose that in certain regions of the embryo, there are new Rdhs and Raldhs yet to be discovered and that RA is not a major regulator of its synthetic enzymes.

Transforming growth factor beta2 is negatively regulated by endogenous retinoic acid during early heart morphogenesis

Vitamin A-deficient (VAD) quail embryos lack the vitamin A-active form, retinoic acid (RA) and are characterized by a phenotype that includes a grossly abnormal cardiovascular system that can be rescued by RA. Here we report that the transforming growth factor, TGFbeta2 is involved in RA-regulated cardiovascular development. In VAD embryos TGFbeta2 mRNA and protein expression are greatly elevated. The expression of TGFbeta receptor II is also elevated in VAD embryos but is normalized by treatment with TGFbeta2-specific antisense oligonucleotides (AS). Administration of this AS or an antibody specific for TGFbeta2 to VAD embryos normalizes posterior heart development and vascularization, while the administration of exogenous active TGFbeta2 protein to normal quail embryos mimics the excessive TGFbeta2 status of VAD embryos and induces VAD cardiovascular phenotype. In VAD embryos pSmad2/3 and pErk1 are not activated, while pErk2 and pcRaf are elevated and pSmad1/5/8 is diminished. We conclude that in the early avian embryo TGFbeta2 has a major role in the retinoic acid-regulated posterior heart morphogenesis for which it does not use Smad2/3 pathways, but may use other signaling pathways. Importantly, we conclude that retinoic acid is a critical negative physiological regulator of the magnitude of TGFbeta2 signals during vertebrate heart formation.

Retinoid signalling is required for information transfer from mesoderm to neuroectoderm during gastrulation

The hindbrain region of the vertebrate central nervous system (CNS) presents a complex regionalisation. It consists of 7-8 distinct morphological segments called rhombomeres, each with a unique identity provided by combinations of transcription factors. One class of signalling molecules, retinoids, have been shown to be crucial for hindbrain patterning through direct trans-activation of Hox genes in the neuroectoderm. However, how this morphogen acts is not yet fully understood. Here, we show that the retinoid receptor antagonist AGN193109 causes a posterior hindbrain defect in Xenopus, comparable to that seen in other vertebrates. We show that this defect arises during gastrulation. Blocking endogenous retinoid activity during gastrulation causes downregulation of the most 3' Hox genes (paralogues 1-5) in gastrula neuroectoderm, but their initial activation in gastrula non-organiser mesoderm is unaffected. Similar results were obtained in avian embryos: Vitamin A-deficient quail embryos have defective expression of 3 Hox genes (i.e. Hoxb1, Hoxb4 ) in the neural tube, but their early expression in the primitive streak and emerging paraxial and lateral mesoderm is not affected. In Xenopus, depletion of retinoids from mesoderm by targeted injection of mRNAs for the retinoic acid catabolising enzyme xCYP26 and the cellular retinoic acid binding protein xCRABP blocks 3 Hox gene expression in the overlying neuroectoderm. We propose that the gastrula non-organiser mesoderm and its later derivative, the paraxial mesoderm, is the source of a retinoid, which acts as a transforming (caudalising) signal for the future posterior hindbrain.

Vitamin A-not for your eyes only: requirement for heart formation begins early in embryogenesis

Vitamin A insufficiency has profound adverse effects on embryonic development. Major advances in understanding the role of vitamin A in vertebrate heart formation have been made since the discovery that the vitamin A active form, all-trans-retinoic acid, regulates many genes, including developmental genes. Among the experimental models used, the vitamin A-deficient avian embryo has been an important tool to study the function of vitamin A during early heart formation. A cluster of retinoic acid-regulated developmental genes have been identified that participate in building the heart. In the absence of retinoic acid the embryonic heart develops abnormally leading to embryolethality.

Retinoic acid is a negative physiological regulator of N-cadherin during early avian heart morphogenesis

The vitamin A-deficient (VAD) early avian embryo has a grossly abnormal cardiovascular system that is rescued by treating the embryo with the vitamin A-active form, retinoic acid (RA). Here we examine the role of N-cadherin (N-cad) in RA-regulated early cardiovascular morphogenesis. N-cad mRNA and protein are expressed globally in the presomite through HH14 normal and VAD quail embryos. The expression in VAD embryos prior to HH10 is significantly higher than that in normal embryos. Functional analyses of the N-cad overproducing VAD embryos reveal N-cad involvement in the RA-regulated cardiovascular development and suggest that N-cad expression may be mediated by Msx1. We provide evidence that in the early avian embryo, endogenous RA is a negative physiological regulator of N-cad. We hypothesize that a critical endogenous level of N-cad is needed for normal early cardiovascular morphogenesis to occur and that this level is ensured by stage-specific, developmentally regulated RA signaling.

A ryanodine receptor-dependent Ca(i)(2+) asymmetry at Hensen's node mediates avian lateral identity

In mouse, the establishment of left-right (LR) asymmetry requires intracellular calcium (Ca(i)(2+)) enrichment on the left of the node. The use of Ca(i)(2+) asymmetry by other vertebrates, and its origins and relationship to other laterality effectors are largely unknown. Additionally, the architecture of Hensen's node raises doubts as to whether Ca(i)(2+) asymmetry is a broadly conserved mechanism to achieve laterality. We report here that the avian embryo uses a left-side enriched Ca(i)(2+) asymmetry across Hensen's node to govern its lateral identity. Elevated Ca(i)(2+) was first detected along the anterior node at early HH4, and its emergence and left-side enrichment by HH5 required both ryanodine receptor (RyR) activity and extracellular calcium, implicating calcium-induced calcium release (CICR) as the novel source of the Ca(i)(2+). Targeted manipulation of node Ca(i)(2+) randomized heart laterality and affected nodal expression. Bifurcation of the Ca(i)(2+) field by the emerging prechordal plate may permit the independent regulation of LR Ca(i)(2+) levels. To the left of the node, RyR/CICR and H(+)V-ATPase activity sustained elevated Ca(i)(2+). On the right, Ca(i)(2+) levels were actively repressed through the activities of H(+)K(+) ATPase and serotonin-dependent signaling, thus identifying a novel mechanism for the known effects of serotonin on laterality. Vitamin A-deficient quail have a high incidence of situs inversus hearts and had a reversed calcium asymmetry. Thus, Ca(i)(2+) asymmetry across the node represents a more broadly conserved mechanism for laterality among amniotes than had been previously believed.

Retinoids control anterior and dorsal properties in the developing forebrain

We have previously shown that retinoic acid (RA) synthesized by the retinaldehyde dehydrogenase 2 (RALDH2) is required in forebrain development. Deficiency in RA due to inactivation of the mouse Raldh2 gene or to complete absence of retinoids in vitamin-A-deficient (VAD) quails, leads to abnormal morphogenesis of various forebrain derivatives. In this study we show that double Raldh2/Raldh3 mouse mutants have a more severe phenotype in the craniofacial region than single null mutants. In particular, the nasal processes are truncated and the eye abnormalities are exacerbated. It has been previously shown that retinoids act mainly on cell proliferation and survival in the ventral forebrain by regulating SHH and FGF8 signaling. Using the VAD quail model, which survives longer than the Raldh-deficient mouse embryos, we found that retinoids act in maintaining the correct position of anterior and dorsal boundaries in the forebrain by modulating FGF8 anteriorly and WNT signaling dorsally. Furthermore, BMP4 and FGF8 signaling are affected in the nasal region and BMP4 is ventrally expanded in the optic vesicle. At the optic cup stage, Pax6, Tbx5 and Bmp4 are ectopically expressed in the presumptive retinal pigmented epithelium (RPE), while Otx2 and Mitf are not induced, leading to a dorsal transdifferentiation of RPE to neural retina. Therefore, besides being required for survival of ventral structures, retinoids are involved in restricting anterior identity in the telencephalon and dorsal identity in the diencephalon and the retina.

Distinct roles for hindbrain and paraxial mesoderm in the induction and patterning of the inner ear revealed by a study of vitamin-A-deficient quail

The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.

Critical role for retinol in the generation/differentiation of angioblasts required for embryonic blood vessel formation

Numerous studies demonstrate that vitamin A (retinol) deficiency causes abnormal cardiovascular morphogenesis. We evaluated the impact of retinol deficiency on the regulation of the numbers of endothelial cells and angioblasts (endothelial progenitors) produced during embryonic quail development. At the one-somite stage, there were no discernible differences in the mean number of endothelial cells or angioblasts in normal and retinol-deficient embryos. However, retinol-deficient embryos at the three-somite stage had an increase in the mean number of endothelial cells but no difference in the mean number of angioblasts. By contrast, retinol-deficient embryos at the five-somite stage have 61% of the normal number of endothelial cells and 12% of the normal number of angioblasts. Similarly, retinol-deficient embryos at the 10-somite stage had 71% and 60% of normal numbers of endothelial cells in capillary-like networks and the sinuses venosus, respectively. Furthermore, we show that retinol deficiency did not elicit a global reduction in mesodermal cell numbers but was specific to cells of the endothelial lineage. Taken together, our findings suggest that vascular abnormalities observed under conditions of retinol deficiency are due to reduction in the number of angioblasts and consequently an insufficiency in the number of endothelial cells required to build complex vascular networks.

Vitamin a requirement for early cardiovascular morphogenesis specification in the vertebrate embryo: insights from the avian embryo

Vitamin A is required throughout the life cycle, including crucial stages of embryonic and fetal development. With the identification of retinoic acid-specific nuclear transcription factors, the retinoid receptors, considerable advances have been made in understanding the molecular function of vitamin A. The requirement for vitamin A during early embryogenesis has successfully been examined in the vitamin A-deficient avian embryo during neurulation, when in the vertebrates crucial developmental decisions take place. These studies revealed that retinoic acid is essential during these early stages of embryogenesis for the initiation of organogenesis (i.e., formation of the heart). If retinoic acid is not present at this time, abnormal development ensues, leading to early embryonic death. Though the initial insult of the absence of vitamin A appears to be on the specification of cardiovascular tissues, subsequently all development is adversely affected and the embryo dies. Molecular and functional studies revealed that retinoic acid regulates the expression of the cardiogenic transcription factor GATA-4 and several heart asymmetry genes, which explains why the heart position is random in vitamin A-deficient quail embryos. During the crucial retinoic acid-requiring developmental window, retinoic acid transduces its signals to genes for heart morphogenesis via the receptors RARalpha2, RARgamma, and RXRalpha. Elucidation of the function of vitamin A during early embryonic development may lead to a better understanding of the cardiovascular birth defects prevalent in the Western world.

Function of RARgamma and RARalpha2 at the initiation of retinoid signaling is essential for avian embryo survival and for distinct events in cardiac morphogenesis

Avian embryogenesis requires retinoid receptor activation by the vitamin A active form, retinoic acid (RA), during neurulation. We conducted loss-of-function analysis in quail embryos by nutritional deprivation of RA and by blocking generation of retinoid receptors. Here we identify a distinct role for RARalpha2 in cardiac inflow tract morphogenesis and for RARgamma in cardiac left/right orientation and looping morphogenesis. Blocking normal embryos with antisense oligonucleotides to RARalpha2 or RXRalpha diminishes GATA-4 transcripts, while blocking RARgamma or RXRalpha diminishes nodal and Pitx2 transcripts; the expression of these genes in the heart forming region resembles that of the vitamin A-deficient embryo. Blocking the function of RARgamma, RARalpha2, and RXRalpha recapitulates the complete vitamin A-deficient phenotype. RARgamma is the most potent mediator of the retinoid signal at this time of development. Our studies provide strong evidence that critical RA-requiring developmental events in the early avian embryo are regulated by means of distinct retinoid receptor signaling pathways.

Retinoid receptors and vitamin A deficiency: differential patterns of transcription during early avian development and the rapid induction of RARs by retinoic acid

The functional links of specific retinoid receptors to early developmental events in the avian embryo are not known. Before such studies are undertaken, knowledge is required of the spatiotemporal expression patterns of the receptor genes and their regulation by endogenous retinoic acid levels during the early stages of development. Here, we report the expression patterns of mRNAs for RARalpha, RARalpha2, RARbeta2, RARgamma, RARgamma2, RXRalpha, and RARgamma from neurulation to HH10 in the normal and vitamin A-deficient (VAD) quail embryo. The transcripts for all retinoid receptors are detectable at HH5, except for RXRgamma, which is detected at the beginning of HH6. At the 4/5 somite stage of HH8, when retinoid signaling is initiated in the avian embryo, mRNAs of all receptors are present, with very strong and ubiquitous expression patterns for RARalpha, RARalpha2, RARgamma, RARgamma2, and RXRalpha, a persistent expression of RARgamma in the neural tissues, a strong expression of RARbeta2 in lateral plate mesoderm and somites, and an anterior expression of RXRgamma. All retinoid receptors are expressed in the heart primordia. In the VAD quail embryo, the general pattern of retinoid receptor transcript localization is similar to that of the normal, except that the expression of RARalpha2 and RARbeta2 is severely diminished. Administration of retinol or retinoic acid to VAD embryos at or before the 4/5 somite stage rescues the expression of RARalpha2 and RARbeta2 within approximately 45 min and restores normal development. RARbeta2 expression requires the expression of RARalpha2. After neurulation, the expression of all retinoid receptors in the VAD quail embryo becomes independent of vitamin A status and is similar to that of the normal. The mRNA levels and sites of expression of the key enzyme for retinoic acid biosynthesis, Raldh-2, are not affected by vitamin A status; the expression pattern is restricted and does not correspond to that of retinoid receptors at all sites. The general patterns and intensity of retinoid receptor gene expression during early quail development are comparable to those of the mammalian and thus validate the application of results from retinoid-regulated avian development studies to those of the mammalian.

A novel role for retinoids in patterning the avian forebrain during presomite stages

Retinoids, and in particular retinoic acid (RA), are known to induce posterior fates in neural tissue. However, alterations in retinoid signalling dramatically affect anterior development. Previous reports have demonstrated a late role for retinoids in patterning craniofacial and forebrain structures, but an earlier role in anterior patterning is not well understood. We show that enzymes involved in synthesizing retinoids are expressed in the avian hypoblast and in tissues directly involved in head patterning, such as anterior definitive endoderm and prechordal mesendoderm. We found that in the vitamin A-deficient (VAD) quail model, which lacks biologically active RA from the first stages of development, anterior endodermal markers such as Bmp2, Bmp7, Hex and the Wnt antagonist crescent are affected during early gastrulation. Furthermore, prechordal mesendodermal and prospective ventral telencephalic markers are expanded posteriorly, Shh expression in the axial mesoderm is reduced, and Bmp2 and Bmp7 are abnormally expressed in the ventral midline of the neural tube. At early somite stages, VAD embryos have increased cell death in ventral neuroectoderm and foregut endoderm, but normal cranial neural crest production, whereas at later stages extensive apoptosis occurs in head mesenchyme and ventral neuroectoderm. As a result, VAD embryos end up with a single and reduced telencephalic vesicle and an abnormally patterned diencephalon. Therefore, we propose that retinoids have a dual role in patterning the anterior forebrain during development. During early gastrulation, RA acts in anterior endodermal cells to modulate the anteroposterior (AP) positional identity of prechordal mesendodermal inductive signals to the overlying neuroectoderm. Later on, at neural pore closure, RA is required for patterning of the mesenchyme of the frontonasal process and the forebrain by modulating signalling molecules involved in craniofacial morphogenesis.

Retinoid signaling regulates primitive (yolk sac) hematopoiesis

It is known from nutritional studies that vitamin A is an important factor for normal hematopoiesis, though it has been difficult to define its precise role. The vitamin A-deficient (VAD) quail embryo provides an effective ligand "knockout" model for investigating the function of retinoids during development. The VAD embryo develops with a significant reduction in erythroid cells, which has not been noted previously. Activation of the primitive erythroid program and early expression of the erythroid marker GATA-1 occurs, though GATA-1 levels eventually decline, consistent with the erythropoietic and hemoglobin deficits. However, from its early stages, the GATA-2 gene fails to be expressed normally in VAD embryos. The bone morphogenetic protein (BMP)-signaling pathway regulates GATA-2, and BMP4 expression becomes reduced in the caudal embryonic region of VAD embryos. Adding BMP4 to cultured VAD-derived explants rescues the production of erythroid cells, whereas normal embryos cultured in the presence of the BMP antagonist noggin are defective in primitive hematopoiesis. We find that cell clusters of primitive blood islands undergo an inappropriate program of apoptosis in the VAD embryo, which can explain the deficit in differentiated primitive blood cells. We propose that vitamin A-derived retinoids are required for normal yolk sac hematopoiesis and that an embryonic retinoid-BMP-GATA-2 signaling pathway controls progenitor cell survival relevant to primitive hematopoiesis.

Abnormal β-catenin gene expression with invasiveness of primary hepatocellular carcinoma in China

AIM: To study the abnormal expression of β-catenin gene and its relationship with invasiveness of primary hepatocellular carcinoma among Chinese people.

METHODS: Thirty-four hepatocellular carcinoma (HCC) specimens and adjacent paracancerous tissues, 4 normal liver tissues were immunohistochemically stained to study subcellular distribution of β-catenin. Semiquantitive analysis of expression of β-catenin gene exon 3 mRNA was examined by RT-PCR and in situ hybridization. The relationship between expressions of both β-catenin protein, mRNA and clinicopathological characteristics of HCC was also analyzed.

RESULTS: Immunohistochemistry showed that all normal liver tissues and para-cancerous tissues examined displayed membranous type staining for β-catenin protein, occasionally with weak expression in the cytoplasm. While 21 cases (61.8%) of HCC examined showed accumulated type in cytoplasms or nuclei. The accumuled type Labling Index (LI) of cancer tissue and para-cancarous tissue was (59.9 ± 26.3) and (18.3 ± 9.7) respectively (P < 0.01). Higher accumulated type LI was closely related with invasiveness of HCC. Results of RT-PCR showed the β-catenin gene exon 3 mRNA Expression Index (EI) of 34 HCCs was higher than that of para-cancerous tissue and normal liver tissue. Using in situ hybridization, the signal corresponding to β-catenin gene exon 3 mRNA was particularly strong in cytoplasm of HCC when compared with those of para-cancerous and normal liver tissues. Over expression of β-catenin exon 3 was also found to be correlated with high metastatic potential of HCC.

CONCLUSION: Abnormal expression of β-catenin gene may contribute importantly to the invasiveness of HCC among Chinese people.

Chick CFC controls Lefty1 expression in the embryonic midline and nodal expression in the lateral plate

Members of the EGF-CFC family of proteins have recently been implicated as essential cofactors for Nodal signaling. Here we report the isolation of chick CFC and describe its expression pattern, which appears to be similar to Cfc1 in mouse. During early gastrulation, chick CFC was asymmetrically expressed on the left side of Hensen's node as well as in the emerging notochord, prechordal plate, and lateral plate mesoderm. Subsequently, its expression became confined to the heart fields, notochord, and posterior mesoderm. Implantation experiments suggest that chick CFC expression in the lateral plate mesoderm is dependent on BMP signaling, while in the midline its expression depends on an Activin-like signal. The asymmetric expression domain within Hensen's node was not affected by application of FGF8, Noggin, or Shh antibody. Implantation of cells expressing human or mouse CFC2, or chick CFC on the right side of Hensen's node randomized heart looping without affecting expression of genes involved in left-right axis formation, including SnR, Nodal, Car, or Pitx2. Application of antisense oligodeoxynucleotides to the midline of Hamburger-Hamilton stage 4-5 embryos also randomized heart looping, but in contrast to the overexpression experiments, antisense oligodeoxynucleotide treatment resulted in bilateral expression of Nodal, Car, Pitx2, and NKX3.2, whereas Lefty1 expression in the midline was transiently lost. Application of the antisense oligodeoxynucleotides to the lateral plate mesoderm abolished Nodal expression. Thus, chick CFC seems to have a dual function in left-right axis formation by maintaining Nodal expression in the lateral plate mesoderm and controlling expression of Lefty1 expression in the midline territory.

Function of vitamin A in vertebrate embryonic development

Advances in molecular biology and retinoic acid receptor research have significantly contributed to the understanding of the role of vitamin A during vertebrate development. Examination of the function of this vitamin during very early developmental stages using the completely vitamin A-depleted avian embryo has revealed that the vitamin A requirement begins at the time of formation of the primitive heart, circulation and specification of hindbrain. The lack of vitamin A at this critical time results in gross abnormalities and early embryonic death. In rodent models, vitamin A deficiency can be targeted to later gestational windows and documents the need for vitamin A for more advanced stages of development. Major target tissues of vitamin A deficiency include the heart, central nervous system and structures derived from it, the circulatory, urogenital and respiratory systems, and the development of skull, skeleton and limbs. These abnormalities are also evident in mice mutants from retinoid receptor knockouts; they have revealed both morphological and molecular aspects of vitamin A function during development. Retinoic acid receptors (RAR) in partnership with retinoid X receptor (RXR)alpha appear to be the important retinoid receptor transcription factors regulating vitamin A function at the gene level during development via the physiologic ligand all-trans-retinoic acid. Homeostasis of retinoic acid is maintained by developmentally regulated vitamin A metabolism enzyme systems. Inadequate vitamin A nutrition during early pregnancy may account for some pediatric congenital abnormalities.

Retinoic acid, midkine, and defects of secondary neurulation

BACKGROUND

Retinoic acid (RA) is necessary for normal differentiation of the tail bud into the secondary neural tube. Excess RA, however, is teratogenic and causes neural tube defects (NTDs). The way in which RA modulates secondary neurulation is unclear but probably involves RA-regulated downstream genes such midkine (MK), which encodes a growth factor implicated in tail bud mesenchymal-neuroepithelial conversion. Our objective was to determine whether RA-deficiency would produce similar defects and if MK is involved.

METHODS

Citral, a drug that blocks endogenous RA formation, as well as a neutralizing antibody, were used to block RA activity in chick embryos. Immunohistochemistry and in situ hybridization were used to localize RA and MK in the tail bud. Competitive RT-PCR was used to examine the effects of excess RA and RA deficiency due to citral on the expression of MK mRNA.

RESULTS

Citral-induced NTDs displayed a morphological resemblance to those caused by excess RA. However, citral treatment did not significantly increase embryonic mortality, and RA rescue of citral-treated embryos proved unsuccessful. MK mRNA was detected in the differentiating tail bud by in situ hybridization. Competitive RT-PCR showed that excess RA decreased MK expression by 60%. Doses of citral that caused a comparable incidence of defects, however, caused only a 25% decrease.

CONCLUSIONS

The results show that excess RA and RA deficiency both cause defects of secondary neurulation. While excess RA decreased MK expression, RA deficiency had minimal effects. However, whether or not MK is an intermediary in the developmental phenomena regulated physiologically or pathologically by RA remains to be elucidated.

Retinoid signaling is required to complete the vertebrate cardiac left/right asymmetry pathway

Vitamin A-deficient (VAD) quail embryos have severe abnormalities, including a high incidence of reversed cardiac situs. Using this model we examined in vivo the physiological function of vitamin A in the left/right (L/R) cardiac asymmetry pathway. Molecular analysis reveals the expression of early asymmetry genes activin receptor IIa, sonic hedgehog, Caronte, Lefty-1, and Fgf8 to be unaffected by the lack of retinoids, while expression of the downstream genes nodal-related, snail-related (cSnR), and Pitx2 is altered. In VAD embryos nodal expression in left lateral plate mesoderm (LPM) is severely downregulated and the expression domain altered during neurulation. Similarly, the expression of cSnR in the right LPM and of Pitx2 in the left side posterior heart-forming region (HFR) is downregulated in the VAD embryos. The lack of retinoids does not cause randomization or ectopic expression of nodal, cSnR, or Pitx2. At the six- to eight-somite stage nodal is expressed transiently in the left posterior HFR of normal quail embryos; this expression is missing in VAD embryos and may be linked to the loss of Pitx2 expression in this region of VAD quail embryos. Administration of retinoids to VAD embryos prior to the six-somite stage rescues the expression of nodal, cSnR, and Pitx2 as well as the randomized VAD cardiac phenotype. There is an absolute requirement for retinoids at the four- to five-somite developmental window for cardiogenesis and cardiac L/R specification to proceed normally. We conclude that retinoids do not regulate the left/right-specific sidedness assignments for expression of genes on the vertebrate cardiac asymmetry pathway, but are required during neurulation for the maintenance of adequate levels of their expression and for the development of the posterior heart tube and a loopable heart. Cardiac asymmetry may be but one of several critical events regulated by retinoid signaling in the retinoid-sensitive developmental window.

Retinoic acid is a potent regulator of growth plate chondrogenesis

Vitamin A deficiency and excess both cause abnormalities in mammalian longitudinal bone growth. Because all-trans retinoic acid (RA) is synthesized from vitamin A, we hypothesized that RA regulates growth plate chondrogenesis. Consistent with this hypothesis, a single oral dose of RA reduced the height of the rat proximal tibial growth plate. To determine whether RA acts directly on growth plate, fetal rat metatarsal bones were cultured in the presence of RA. In this system, RA inhibited longitudinal bone growth by three mechanisms: 1) decreased chondrocyte proliferation, (assessed by 3H-thymidine incorporation), particularly in the proliferative zone of the growth plate; 2) decreased matrix synthesis (assessed by 35SO4 incorporation into glycosaminoglycans); and 3) decreased cell hypertrophy (determined histologically). The growth-inhibiting effects of RA were completely reversed by a retinoic acid receptor (RAR) antagonist. In the absence of exogenous RA, this antagonist accelerated bone growth, as did an RA-specific neutralizing antibody, suggesting that endogenous RA negatively regulates growth plate chondrogenesis. We conclude that RA, acting through RARs, negatively regulates longitudinal bone growth by inhibiting growth plate chondrocyte proliferation, chondrocyte hypertrophy, and matrix synthesis.

Vitamin A and embryonic development: an overview

Vitamin A is an essential micronutrient throughout the life cycle. Its active form, retinoic acid via retinoid receptors, is involved in signal transduction pathways regulating development. Both the lack and excess of vitamin A during embryonic development result in congenital malformations. Approaches to examine the function of vitamin A in embryonic development have included treatment with excess retinoids and the use of retinoid receptor knock-out mice, which have provided important insights into the complexity of the retinoid signaling system. A recently explored model is the retinoid ligand knock-out, i.e., the vitamin A-deficient embryo. Early development can be successfully examined in the vitamin A-deficient avian embryo, in which bioactive retinoids can rescue the deficient genotype as well as phenotype. In this model it has been possible to unequivocally link the physiological function of vitamin A to development of heart, embryonal circulatory and central nervous systems and the regulation of heart asymmetry. Several developmental genes regulated by endogenous vitamin A during early embryogenesis have been identified. Retinoid receptors and their endogenous ligands, the vitamin A-active forms, are present in the early embryo. It is the developmentally regulated biogeneration of the vitamin A-active forms via distinct spatio-temporal metabolic pathways that is critically linked to the initiation of retinoid signal transduction during embryonic development.

Effect of wheat bran fiber on the development of mammary tumors in female intact and ovariectomized rats treated with 7,12-dimethylbenz(a)anthracene and in mice with spontaneously developing mammary tumors

We examined the effect of consumption of graded increases of dietary fiber (soft white wheat bran) on the development of mammary gland carcinomas in intact female Sprague-Dawley rats during the promotion stage of carcinogenesis, induced with 7,12-dimethylbenz(a)anthracene (DMBA). The percent of rats with mammary carcinomas, the total number of mammary carcinomas and the mean number of mammary carcinomas per rat were reduced significantly at all fiber levels examined compared to rats fed a control diet. Inclusion of 9.6% fiber in the diets of ovariectomized rats that had been treated with a single i.v. dose of 2.5 mg DMBA/100 g body weight 2 weeks prior to removal of the ovaries resulted in a significant decrease of carcinomatous and benign mammary tumors compared to ovariectomized rats fed a control diet. Development of spontaneous mammary carcinomas in virgin C3H/HeOuJ female mice and growth of a transplantable mammary gland tumor in such mice were reduced by inclusion of 9.6% fiber in the diet, a reduction that was significant or just barely missed significance, depending on the source of the fiber. Our observations provide evidence that inclusion of soft white wheat bran in the diet is effective in the suppression of mammary gland tumorigenesis in an array of experimental animal models.

Retinoids in eggs and embryos of birds fed fish from the Great Lakes

Retinoids were analyzed in 11-day chick embryos and eggs from white Leghorn hens (Gallus domesticus) fed environmentally-derived polychlorinated biphenyls (PCB) in carp (Cyprinus carpio) from Saginaw Bay. The yolks and the embryos contained all-trans-retinol, 3,4-didehydroretinol and retinyl esters. There was no significant difference in the total retinoid content in the yolks of 11-day incubated eggs among hens fed for 7 weeks diets containing 0.5-6.6 mg PCB/kg diet. However, the proportional amount of retinols in the high (6.6 mg) PCB group was significantly less than that in low (0.5 mg) PCB controls, while the amount of retinyl palmitate in the high PCB group was significantly greater than that in the controls. Retinoids in the embryos were not affected by any of the PCB levels fed to hens for 7 weeks prior to laying the eggs. The 50% reduction in the molar ratio of retinols to retinyl palmitate in the yolks of eggs as the result of the high PCB level fed to hens for 7 weeks can serve as an indicator for chronic exposure to PCB contamination at the level of 6.6 mg or higher PCB/kg diet.

Retinoic acid reverses ethanol-induced cardiovascular abnormalities in quail embryos

The effect of ethanol on early avian cardiovascular development was investigated in stage 8 quail embryos grown in culture for 24 hr. When the culture medium contained 1% ethanol, 50% of the embryos developed abnormalities of the cardiovascular system, some of which resembled vitamin A deficiency. Only 15% of the embryos grown in control media developed abnormalities attributed to the manipulation of the embryo. When all-trans-retinoic acid, the active form of vitamin A, was added at 10(-8) M to the ethanol-containing medium, the cardiovascular development was similar to that of untreated controls. Inclusion of 4-methylpyrazole and citral, enzyme inhibitors for the conversion of retinol to retinoic acid, produced cardiovascular abnormalities in embryos similar to those observed in vitamin A deficiency. These abnormalities were partially prevented by the presence of 10(-8) M all-trans-retinoic acid in the medium. Immunohistochemical studies using antibodies specific for the heart muscle myosin heavy chain (MF-20) and quail endothelial cells (QH-1) revealed that looping of the heart of ethanol-treated embryos was prevented, and the embryonal circulation had no or minimal vascular connections to the extraembryonic circulation. Our studies provide indirect evidence that ethanol is producing vitamin A deficiency during embryonic cardiovascular development and that these effects are specifically prevented by the presence of retinoic acid. These findings may explain some of the symptoms of fetal alcohol syndrome.

Hensen's node from vitamin A-deficient quail embryo induces chick limb bud duplication and retains its normal asymmetric expression of Sonic hedgehog (Shh)

Both Hensen's node, the organizer center in chick embryo, and exogenous retinoic acid are known to induce limb duplication when grafted or applied to the host chick limb bud. Retinoic acid is known to be present in the node and has been proposed as the putative morphogen for chick limb development. Here, we report that Hensen's node from vitamin A-deficient quail embryo induces limb duplication in the host chick embryo similar to that induced by the node from vitamin A-sufficient control embryos. We also demonstrate that the expression of Sonic hedgehog (Shh), recently shown to be the mediator of polarizing activity in the chick limb bud, is not affected by the endogenous vitamin A status of the embryo. Furthermore, whole-mount in situ hybridization revealed asymmetry of Shh expression in the Hensen's node of both vitamin A-sufficient and -deficient quail embryos. Retinoids were not detectable in the eggs from which vitamin A-deficient embryos were obtained. Extracts from normal embryos induced a level of expression of reporter gene equivalent to the presence of 3.4 pg of active retinoids per embryo, while those from vitamin A-deficient embryos induced a baseline level of reporter gene expression similar to that of the controls. Our studies suggest that endogenous retinoic acid is not involved in Shh expression nor in regulating its asymmetry during normal early avian embryogenesis and support the current view that endogenous retinoic acid may not be a direct morphogen for limb bud duplication.

Left-right asymmetric localization of flectin in the extracellular matrix during heart looping

The early embryo is initially bilaterally symmetrical. One of the first distinct indications of asymmetry in the embryo occurs during heart looping. The midline tubular heart begins to bend to the right to form a C-shaped structure around 30 hr of development in the avian model. A molecular basis for heart asymmetry and direction of looping is not known, although factors inherent to the myocardium are believed to underlie looping. A left-right asymmetric localization of a specific molecule in the bilateral heart forming regions has not been reported previously. One molecule that we are calling flectin (flectere, in L., to bend or to loop) shows a bilateral asymmetric localization early in the heart forming mesoderm and continues to be expressed asymmetrically in a highly organized manner in the cardiac jelly during heart looping. This large extracellular matrix molecule has been identified using a monoclonal antibody F-22 (Mieziewska et al., 1994a,b). Flectin shows a discrete spatiotemporal pattern of extracellular matrix expression during avian heart development. An asymmetric expression of flectin is observed during heart development at stage 7+/8- (approximately at 24 hr of development around the 3-somite stage). It is predominantly expressed in the left precardiac mesoderm at this developmental period. Between stages 12 and 14, flectin continues to be asymmetrically expressed in the myocardium and is localized at high levels on the basal side of the myocardium and within the cardiac jelly extending to the endocardial cell surfaces. In the same plane of the looping part of the heart it is differentially organized within the cardiac jelly on the convex side and in the outer loop areas. A reduced expression is apparent anteriorly and posteriorly along the tubular heart. The initial asymmetry of localization is maintained throughout the tubular heart. At stage 22 (Embryonic Day 3.5), intensity of immunolocalization of flectin is significantly decreased, with left-right asymmetry becoming less discernible or absent. It again is expressed in Day 10 embryonic hearts. Flectin expression appears to be modulated by retinoids. In vitamin A-deficient quail embryonic hearts that do not loop (Dersch and Zile, 1993; Twal et al., 1995), flectin protein expression is decreased and disorganized, as are other extracellular matrix components comprising the cardiac jelly.

Endogenous retinoids in the early avian embryo

Vitamin A is required to rescue the vitamin A-deficient quail embryo from early death, but the endogenous presence of bioactive retinoids has not been documented in these embryos. The analysis of 2000 pooled stage 5-8 normal quail embryos described here provides for the first time direct evidence for the presence of endogenous all-trans-retinoic acid (5 nM) and 3,4-didehydroretinoic acid (4 nM), signaling molecules known to be potent ligands for nuclear retinoic acid receptors. The demonstration of all-trans-retinal (80 nM), all-trans-retinol (100 nM), 3,4-didehydroretinol (200 nM), and retinyl esters (100 nM) suggests the capability of the early avian embryo to generate in situ the vitamin A-bioactive molecules required for development. Analysis of 2100 pooled stage 5-8 quail embryos from vitamin A-deficient eggs revealed no vitamin A-like molecules, supporting the evidence that links vitamin A deficiency in these embryos to abnormalities and early death.

Anti-retinoic acid (RA) antibody binding to human premalignant oral lesions, which occurs less frequently than binding to normal tissue, increases after 13-cis-RA treatment in vivo and is related to RA receptor beta expression

Nuclear retinoic acid receptor beta (RAR-beta) expression decreases in human premalignant oral lesions (POLs). RAR-beta suppression could result from a decrease in the cellular level of retinoids because RAR-beta gene transcription is enhanced by retinoids. To explore this hypothesis, we compared the binding of a monoclonal antibody (mAb) against all-transretinoic acid (RA; anti-RA mAbs) to normal oral tissue and POLs. All 7 normal specimens stained positive with the antibody compared to only 20 of 43 POLs; similarly, 7 of 7 normal specimens contained RAR-beta mRNA compared to only 14 of 43 POLs. Twenty-four specimens were available before and after a 3-month treatment with 13-cis-RA in vivo. Anti-RA mAb binding to these specimens increased from 10 of 24 before to 22 of 24 after treatment, and the expression of RAR-beta mRNA increased from 7 of 24 before to 21 of 24 after treatment, respectively. There was a strong agreement between the binding of anti-RA mAbs and the expression of RAR-beta. Thus, we propose that the binding of anti-RA mAbs reflects the level of retinoids in the tissues and that this level is related strongly to RAR-beta expression.

Comparative study of Msx-1 expression in early normal and vitamin A-deficient avian embryos

Homeobox-containing genes may play an important role in establishing embryonic patterns during development of vertebrates. Retinoic acid is able to induce expression of Hox genes in cells in culture and to alter expression patterns in the developing vertebrate embryos. Using wholemount in situ hybridization, we have examined and compared the expression patterns of a homeobox-containing gene, Msx-1, in early normal and vitamin A-deficient quail embryos. At gastrulation stage, Msx-1 is primarily expressed in the posterior half of both normal and vitamin A-deficient embryos. However, the gene is expressed wider and stronger in the vitamin A-deficient embryos. At neurulation stages, Msx-1 is continuously expressed in the posterior region up to Hensen's node and in the edge of the neural fold in both normal and vitamin A-deficient embryos. Notably, in the vitamin A-deficient embryos, Msx-1 is expressed more strongly and is also expressed ectopically in the anterior and precardiac regions. These results provide evidence that endogenous retinoids are involved in the normal expression of Msx-1 in avian embryo and that the expression of Msx-1 is downregulated by endogenous and physiological retinoids in vivo during early avian embryogenesis.

Anti-retinoic acid monoclonal antibody localizes all-trans-retinoic acid in target cells and blocks normal development in early quail embryo

Avian cardiovascular development is vitamin A-dependent, and retinoic acid has been suggested to be active in this important developmental event. We report here that a monoclonal antibody against all-trans-retinoic acid blocks normal embryonic development in the quail causing cardiovascular abnormalities typical of avian vitamin A deficiency. In whole-mount preparations of stage 5 normal quail embryos the fluorescence associated with the antiretinoic acid monoclonal antibody localizes in Hensen's node and in caudal area. In stage 7-8 embryos fluorescence localizes in heart-forming areas as well as in head mesenchyme, in Hensen's node, in nephrotome, and in caudal area. These studies are the first to localize endogenous all-trans-retinoic acid during very early stages of normal avian development. We propose that all-trans-retinoic acid is biosynthesized in its target cells during early avian embryo-genesis and that the availability of this signal molecule is spatiotemporally regulated. We conclude that all-trans-retinoic acid or a closely related metabolite is the physiological form of vitamin A required for normal cardiovascular development and for other very early developmental events in the quail embryo.

c-erbA alpha/T3R and RARs control commitment of hematopoietic self-renewing progenitor cells to apoptosis or differentiation and are antagonized by the v-erbA oncogene

In AEV-transformed erythroleukemic cells the v-erbA gene product is likely to antagonize the function of triiodothyronine (T3) and retinoic acid (RA) receptors and thereby to block cell differentiation. We have thus investigated the effects of T3 and RA on normal early erythrocytic progenitor cells. Here we show: (1) that either RA or T3 play an essential role during the early commitment to erythrocytic differentiation, (2) that both T3 and RA induce death by apoptosis and a strong inhibition of self-renewal in progenitor cells grown in the absence of differentiation-inducing agents and (3) that the v-erbA oncogene renders erythrocytic progenitor cells insensitive to apoptosis and to self-renewal inhibition induced by RA or T3. The behaviour of a non-transforming mutant of v-erbA suggests that this v-erbA-induced protection is related to its transforming potential.

Induction of normal cardiovascular development in the vitamin A-deprived quail embryo by natural retinoids

The biological activity of various natural retinoids and the time "window" when vitamin A activity is required for normal cardiovascular development were examined in vitamin A-deprived Japanese quail embryos. The administration of 1 microgram of retinol at the beginning of incubation resulted in normal cardiovascular development in 97% of embryos; retinoic acid was toxic at this dose level. Treatment of embryos with 0.1 microgram of all-trans-retinol or 13-cis-retinoic acid at the beginning of incubation resulted in normal cardiovascular development in 47 and 12% of embryos, respectively; administration of these retinoids at other time points attenuated the percentage of embryos with normal cardiovascular development. Didehydroretinol, 0.1 microgram, and 9-cis-retinoic acid, 0.1 microgram, were inactive at all time points examined; 9-cis-retinoic acid did not enhance the biological activity of all-trans-retinoic acid. All-trans-retinoic acid, 0.1 microgram, administered during 22-28 hr of incubation induced normal cardiovascular development in 20-34% of embryos; biological activity was optimal when it was administered at 24 hr. All retinoids tested were inactive in establishing normal cardiovascular development when administered at 36 hr of incubation or later. The studies suggest that all-trans-retinoic acid is the biologically active form of vitamin A required for normal cardiovascular development in the avian embryo. There is a critical time point within the first 22-28 hr of quail embryogenesis when all-trans-retinoic acid initiates events that lead to normal cardiovascular development.

Glucuronidation of all-trans-retinoic acid in liposomal membranes

Retinoyl beta-D-glucuronide is a biologically active metabolite of retinoic acid. The kinetics of UDP-glucuronosyltransferase-catalyzed biosynthesis of retinoyl beta-D-glucuronide was examined in rat liver and intestinal native microsomes incubated with [3H retinoic acid incorporated into liposomes. The product was identified by cochromatography with authentic all-trans retinoyl beta-D-glucuronide, by hydrolysis with beta-D-glucuronidase, and by mass spectrometry. In vitamin A-sufficient rats the apparent Km values for all-trans-retinoic acid were 173 microM and 125 microM, and the apparent Vmax, 62 and 41 pmol/min per mg, for small intestinal and liver microsomes, respectively. In vitamin A-deficient rats repleted with all-trans-retinyl acetate, the apparent Km (91 microM) and Vmax (53 pmol/min per mg) for intestinal microsomes were in range of those of vitamin A-sufficient rats. The similarities in the kinetic parameters for UDP-glucuronosyltransferase in small intestinal mucosa and liver suggest that the reactions are catalyzed by the same enzyme. In vitamin A-deficient rats given a large amount all-trans-retinoic acid (1.2 mmol/day for 3 days) the apparent Km was 105 microM and Vmax, 127 pmol/min per mg of intestinal microsomal protein. We conclude that the kinetics of intestinal retinoic acid glucuronidation are not characteristic of simple detoxification reactions. Retinoyl glucuronide may be important in mediating retinoic acid metabolism and function.

Vitamin A homeostasis endangered by environmental pollutants

Normal vitamin A function depends on adequate stores of the vitamin, a finely regulated supply of the vitamin to target tissues, and an ability of cells to generate functionally active forms of the vitamin. Both endogenous and exogenous factors can adversely affect vitamin A homeostasis. Polyhalogenated aromatic hydrocarbons are ubiquitous environmental pollutants and cause severe disturbances in vitamin A metabolism, manifested by an accelerated metabolism and breakdown of vitamin A and its metabolites and a depletion of vitamin A from the body; this sequence of events accounts for the vitamin A deficiency-like symptoms associated with PHAH intoxication. The mechanism(s) responsible for these events most likely includes altered activities of enzymes that are either directly or indirectly involved in critical vitamin A metabolic pathways. Human populations that continue to be exposed to environmental pollutants, may accumulate critical levels of polyhalogenated aromatic hydrocarbons and will be at risk for inadequate vitamin A function as well as for other health impairments that have been difficult to link to any specific causes. Therefore, it is important to seriously evaluate the similarities in physiological disturbances across species that have become apparent in studies with wildlife inhabiting polluted environments similar to ours; the relevance to human health is evident.

Production of a hybridoma cell line secreting retinoic acid-specific monoclonal antibody

A stabilized hybridoma cell line secreting anti-retinoic acid monoclonal antibodies of subclass IgG1 with kappa chains was produced by fusing NS-1 myeloma cells with the spleen cells from BALB/c female mice immunized with all-trans-4-oxoretinoic acid-oxime-chicken IgG conjugate. The antibody titer of mice ascitic fluid ranged from 1/12,800 to 1/25,600, as determined by competitive indirect enzyme-linked immunosorbent assay (ELISA). 50% inhibition dosage of all-trans-retinoic acid at a 1/20,000 dilution of mice ascitic fluid was 6.6 ng/ml, as determined by ELISA. The anti-retinoic acid monoclonal antibody was generated in mice ascitic fluid and purified by protein G affinity chromatography. Cross-reactivity of the monoclonal antibody was determined at 0.1 microgram/ml concentration of retinoids and indicated high specificity to both all-trans-retinoic acid (86% inhibition) and 13-cis-retinoic acid (87% inhibition), and strong cross-reactivity with 4-oxoretinoic acid (77%) and 4-oxoretinoic acid oxime (109%). Specificity was confirmed by the horseradish peroxidase-linked immunostaining method and immunoradioassay. The affinity constant of the monoclonal antibody, K, was determined to be 3.6 X 10(9) l/mol. A calibration curve for retinoic acid using the monoclonal antibody to retinoic acid was developed; the detection limit for all-trans-retinoic acid is 1 ng/ml in the competitive indirect ELISA. The antibody counteracts the effect of retinoic acid on growth inhibition and differentiation in HL-60 cells.

Metabolism and secretion of retinol transport complex in acute renal failure

Tissue uptake and distribution of retinol from circulatory vitamin A transport complex was studied in order to determine the origin of the increased serum retinol in rats with short-term acute renal failure. In rats with acute renal failure, serum retinol increased 37-70% within 2 h after surgery. After an injection of donor plasma containing 1.8 muCi of [3H]retinol in retinol transport complex, in rats with renal failure the ability to clear radioactivity was decreased 36% by 0.5 h and 57% by 2 h, as compared to sham-operated rats. The uptake and distribution of radioactivity by nonrenal tissues was similar in rats with acute renal failure and with intact kidneys. The lack of renal function did not alter hepatic cycling of [3H]retinol from the circulation and thus could not account for the increased serum retinol in renal failure. When hepatic release of retinol-retinol binding protein was blocked by colchicine, the up-regulation of serum retinol, normally observed in rats with acute renal failure, was abolished. Our studies provide strong evidence that kidney has an important role in maintaining serum retinol homeostasis by influencing the release of retinol-retinol binding protein from liver into circulation. Peripheral tissue uptake of circulatory retinol and hepatic cycling of nonutilized retinol are not directly influenced by the kidney.

Effect of retinoic acid and apo-RBP on serum retinol concentration in acute renal failure

We recently demonstrated a rapid up-regulation of serum retinol-retinol binding protein-transthyretin concentration in rats with short-term acute renal failure. We examine the effect of retinoic acid and apo-retinol binding protein (apo-RBP) on the up-regulation of serum retinol in renal failure. Injection of retinoic acid (10 micrograms) into rats with acute renal failure or sham-operated rats increased circulatory retinoic acid concentration 29-fold within 2 h but did not influence serum retinol concentration in either group. Injection of a large dose of retinoic acid (100 micrograms) decreased serum retinol concentration in rats with acute renal failure (19%) and sham-operated rats (29%). These results suggest that changes in serum retinoic acid concentration within the near-physiological range have no effect on regulation of hepatic retinol release. Injection of a large dose of retinoic acid may depress serum retinol indirectly via a retinol sparing effect in target tissues. In rats with renal failure the serum retinol concentration, elevated 44-52% above that of sham-operated controls, was also increased to 70-164% above controls by the injection of 52-63 micrograms of apo-RBP. This suggests that circulatory apo-RBP can up-regulate serum retinol. Circulatory apo-RBP may be a positive physiological feedback signal from peripheral tissues for hepatic release of retinol.

Upregulation of serum retinol in experimental acute renal failure

Serum vitamin A homeostasis was studied in rats with nonfiltering kidneys prepared by ligation of renal arteries. Within 1-2 h of acute renal failure, the serum retinol level increased by 11-73% and was maintained for at least 4 h. More than 90% of the increase in serum retinol was associated with retinol in the retinol binding protein-transthyretin (RBP-TTR) complex. The activities of acyl-CoA:retinol acyltransferase and retinyl-palmitate hydrolase were not altered by short-term acute renal failure. Oral administration of 3H-labeled retinol 3 h before surgery resulted in 350% more tritium in the serum retinol-RBP-TTR complex of rats with acute renal failure as compared to sham-operated rats; this increase represented the fraction of retinol in RBP-TTR contributed by hepatic retinol from newly absorbed 3H-labeled retinol. Total retinol in the retinol-RBP-TTR complex was increased by only 60%. We conclude that short-term acute renal failure causes rapid upregulation of serum retinol-RBP-TTR; the extent of the increase depends on the magnitude of hepatic vitamin A stores, particularly the retinol pools. We hypothesize that kidney modulates the regulation of hepatic release of retinol-RBP from the pool of newly acquired retinol.

Effect of tetrachlorodibenzo-p-dioxin (TCDD) on the glucuronidation of retinoic acid in the rat

Administration of a single oral dose (10 micrograms/kg) of tetrachlorodibenzo-p-dioxin (TCDD) caused a 33% decrease in retinyl esters in the livers of male rats, but a 13-fold increase in retinyl esters in the kidney and a 3-fold increase in serum retinol. Liver and kidney microsomal uridine diphosphoglucuronosyltransferase (UDPGT) activity toward all-trans-retinoic acid was increased 3.7- and 2.6-fold, respectively, ten days following exposure to TCDD. Verification of the in vitro formation of [3H]retinoyl beta-glucuronide (RG) was by cochromatography with authenic RG on reversed phase high pressure liquid chromatography (HPLC), identification of retinoic acid as the hydrolysis product after beta-glucuronidase treatment, and the characterization of the all-trans-retinoyl glucuronide by negative fragment mass spectroscopy, fast atom bobardment. We conclude that increased retinoic acid glucuronidation may be a contributing factor to the hepatic depletion of vitamin A and the increased excretion of vitamin A metabolites following TCDD exposure.

Alterations in vitamin A metabolism by polyhalogenated aromatic hydrocarbons

Adequate stores and adequate tissue levels of vitamin A are maintained by a balance of tissue demands and dietary intake of the vitamin and are modified by many factors, including xenobiotics. It is well established that exposure to polyhalogenated aromatic hydrocarbons (PHAH) decreases hepatic content of vitamin A. Recent findings indicate that hepatic depletion of vitamin A is accompanied by an increase in serum and renal vitamin A content and enhanced excretion of vitamin A metabolites in urine and feces. Examination of tissue retinoid profiles reveals that PHAH exposure causes the generation of increased amounts of polar retinoids. It is very likely that PHAH affect enzymes crucial for regulation of vitamin A storage as well as enhance activities of specific enzymes in vitamin A metabolic pathway.

Effect of hexachlorobiphenyl on vitamin A homeostasis in the rat

Vitamin A status and turnover were examined in rats that had been exposed to chronic dietary treatment of 3,4,5,3',4',5'-hexachlorobiphenyl (HCB), 1 mg/kg diet. HCB caused hepatic depletion and renal accumulation of vitamin A, and a 1.7-fold increase in the serum retinol concentration. Intravenously administered [3H]retinol bound to retinol binding protein-transthyretin complex (RBP-TTR complex) was used to study the dynamics of circulatory retinol in these rats. In HCB-treated rats, the plasma turnover rate of retinol was increased compared to vitamin A-adequate untreated controls. HCB caused a 50% reduction of total radioactivity in liver, and, except for 0.5 h after the [3H]retinol-RBP-TTR dose, the specific activity of the hepatic retinyl ester pool was greater compared to control rats. The kidneys of HCB-treated rats accumulated radioactivity in the retinyl ester fraction. HCB also caused a 50% reduction in adrenal radioactivity compared with control rats. Urinary and fecal excretion of radioactivity was 3-fold higher in HCB-treated rats as compared to controls. Our findings demonstrate that chronic HCB feeding results in expansion of plasma vitamin A mass, in changes of liver and kidney retinol and retinyl ester pool dynamics and in an increased metabolism of vitamin A.

Effect of dietary retinoic acid on circulatory vitamin A homeostasis in polybrominated biphenyl-treated rats

Retinoic acid (RA) supplementation is known to lower the amount of retinol in circulation. In contrast, the feeding of polyhalogenated aromatic hydrocarbons results in an elevated level of circulatory retinol. We investigated the effect of short-term dietary exposure to RA on the amount of serum retinol in female Sprague-Dawley rats fed either a basal diet (control rats) or the basal diet containing 100 mg of polybrominated biphenyls (PBBs)/kg diet (PBB-treated rats). After feeding of the above diets for 137 d, RA (12 mg/kg diet) was included in both the control and PBB-containing diets. The rats were fed the RA-containing diet for 3 d and then killed (d 140). Blood samples were obtained before and after RA treatment. Chronic PBB treatment of rats resulted in lower hepatic vitamin A and higher kidney vitamin A than in control rats. Serum retinol concentration was significantly higher in rats treated with PBB for 137 d than in controls; the subsequent treatment with RA lowered serum retinol to a level that was not different from that of control rats treated with RA. Our observations agree with earlier findings that 1) PBB treatment alters vitamin A homeostasis, and 2) dietary RA lowers the amount of circulatory retinol. An important new observation is that serum retinol homeostasis in PBB-treated rats appears to be regulated by a mechanism similar to that of normal rats. Polyhalogenated aromatic hydrocarbons may thus be useful tools to study the control mechanisms of vitamin A homeostasis.

Vitamin A metabolism in rats chronically treated with 3,3',4,4',5,5'-hexabromobiphenyl

Chronic dietary administration of 3,3',4,4',5,5'-hexabromobiphenyl (HBB), 1 mg/kg diet, caused a decrease in retinol (20-fold) and retinyl esters (23-fold) in the livers of female rats, but resulted in a 6.4-fold increase in retinol and 7.4-fold increase in retinyl esters in the kidneys. Liver acyl-CoA:retinol acyltransferase and retinyl palmitate hydrolase activities were reduced while serum concentration of retinol was unaffected by HBB feeding. Metabolism of a physiological dose of [11-3H]retinyl acetate (10 micrograms), was examined in rats fed either vitamin A-adequate diet, or marginal amounts of vitamin A, or vitamin A-adequate diet containing HBB. A 13-fold greater amount of the administered vitamin A was found in kidneys of HBB-treated rats. In rats fed adequate or low amounts of vitamin A, kidney radioactivity was primarily in the retinol fraction, while in HBB-fed rats the radioactivity was associated mostly with retinyl esters. Fecal and urinary excretion of radioactivity was greatly increased in HBB-treated rats. Chronic HBB feeding results in a loss of ability of liver to store vitamin A, and severely alters the uptake and metabolism of vitamin A in the kidneys. We conclude that HBB causes major disturbances in the regulation of vitamin A metabolism.

Induction of differentiation of human promyelocytic leukemia cell line HL-60 by retinoyl glucuronide, a biologically active metabolite of vitamin A

We examined the differentiation activity of retinoyl beta-D-glucuronide, a biologically active physiological metabolite of vitamin A, using the human promyelocytic leukemic cell line HL-60, which can be induced to differentiate with retinoic acid. Retinoyl beta-D-glucuronide (1 microM) inhibited HL-60 cell proliferation by 55-75%, inhibited tritiated thymidine incorporation into DNA by 63-80%, and induced 38-50% of the cells to differentiate into mature granulocytes. The potency of growth inhibition and induction of differentiation by retinoyl beta-D-glucuronide was similar to that of all-trans-retinoic acid. The continuous presence of either retinoyl beta-D-glucuronide or all-trans-retinoic acid was not required to obtain maximum growth arrest and differentiation: a 1-hr exposure of HL-60 cells to the retinoids gave the same response (measured after a total incubation time of 48 hr) as a 24-hr or 48-hr continuous treatment. Retinoyl beta-D-glucuronide (0.1-0.2 mM) was 50% less cytotoxic to HL-60 cells than all-trans-retinoic acid at an equimolar concentration. Retinoyl beta-D-glucuronide was not significantly metabolized to other retinoids; retinoic acid was not formed during incubation. We conclude that retinoyl beta-D-glucuronide can arrest HL-60 cell proliferation and induce their differentiation into mature granulocytes; it may act by itself or by being hydrolyzed to retinoic acid, which could be immediately utilized and metabolized. The therapeutic use of this retinoid as an antineoplastic agent is suggested.