Microinjections of cultured rat conceptuses: studies with 4-oxo-all-trans-retinoic acid, 4-oxo-13-cis-retinoic acid and all-trans-retinoyl-beta-glucuronide

Comparative studies on effects of all-trans-retinoic acid and all-trans-retinoyl-beta-d-glucuronide on the development of foetal mouse thymus in an organ culture system

The thymus is a target organ of retinoid teratogens. Retinoids with a generally reduced teratogenic potency should therefore also exert reduced adverse effects on thymus development. The effects of all-trans-retinoic acid (a-tRA) and all-trans-retinoyl-beta-glucuronide (a-tRAG) on the in vitro development of thymic lobes of 15-day-old mouse foetuses were compared in an organ culture system. Both compounds were added to the medium at concentrations ranging from 10(-7) to 10(-5)m. The culture period was 6 days. The investigations showed a concentration-dependent effect of both substances on the proliferation of the lymphatic cells. At 10(-5)m the number of thymocytes was significantly reduced to values of about 70% of the controls by either of the retinoids (P 0.05). Results of flow cytometry showed significant differences concerning the differentiation markers CD4 and CD8 after the culture period. The presence of 10(-6)m a-tRA induced a significant increase in the percentages of CD4(+)CD8(-) cells and a significant decrease of CD4(+)CD8(+) cells. At 10(-5)m a-tRA an additional significant increase in the percentages of CD4(-)CD8(-) cells was found. In contrast, after treatment with a-tRAG, percentages of these populations were in the same range as the controls. Light and electron microscopic investigations revealed a depletion of lymphatic cells and an increase of intracytoplasmic vacuoles in the thymic epithelial cells at 10(-6) and 10(-5)m of either retinoid. HPLC analyses revealed a remarkable degree of retinoid isomerization and (in the case of a-tRAG) of hydrolysis. Compared with the culture medium, retinoids were accumulated in the thymic lobes. Possibly a-tRAG acts by way of limited hydrolysis to retinoic acid.

Histological observations of palatal malformations in rat embryos induced by retinoic acid treatment

Malformations of the palate were induced in white rat embryos following maternal exposure to retinoic acid (tretinoin). Five experimental groups and the controls were treated by the following protocol: Group 1: pregnant rats received 100 mg retinoic acid (RA)/kg b.w. suspended in corn oil on gestational day (GD) 11.5; Group 2: 20 mg RA/kg b.w. from GD 8-12; Group 3: 20 mg RA/kg b.w. from GD 7.5-11.5; Group 4: 100 mg RA/kg b.w. on GD 10-11; Group 5: 100 mg RA/kg b.w. on GD 10 and 12; Group 6 received corn oil vehicle from GD 7-14.5; and Group 6: served as non-injected controls. In all retinoic acid treated groups, varying degrees of clefts with occasional attempts of fusion were noted. The severity and frequency of the malformations were dependent on dosage or gestational day of drug treatment. Our results indicate that RA, even at the lowest dose tested (20 mg/kg b.w.) severely affects the various tissues constituting the embryonic palatal shelves by altering cell interaction and possibly programmed cell death. These events would then result in lack of or inadequate differentiation with subsequent formation of aberrant craniofacial architecture.

Retinoyl beta-glucuronide: a biologically active form of vitamin A

Retinoyl beta-glucuronide is a naturally occurring, biologically active metabolite of vitamin A. Although retinoyl beta-glucuronide is regarded as a detoxification product of retinoic acid, it plays several roles in the functions of vitamin A. It can serve as a source of retinoic acid, and it may be a vehicle for transport of retinoic acid to target tissues. Topically applied retinoyl beta-glucuronide is comparable in efficacy to retinoic acid in the treatment of acne in humans, without the same side effects. Retinoyl beta-glucuronide may or may not be teratogenic, depending on the mode of administration and the species in which it is used. It may be a valuable therapeutic compound for the treatment of skin disorders and certain types of cancers.

Endogenous distribution of retinoids during normal development and teratogenesis in the mouse embryo

We have analysed the endogenous retinoids present in whole mouse embryos from day 9 to day 14 of development and in individual components of the embryo at two stages, day 10.5 and day 13, by HPLC. We can only detect two retinoids, all-trans-RA (tRA) and all-trans-retinol (t-retinol), and t-retinol is 5-10-fold in excess over tRA. We cannot detect 9-cis-RA or any didehydroretinoids; thus mammalian embryos seem to differ in their retinoid content from other embryos such as chick, Xenopus, and fish. The levels of tRA do not change significantly over the 6 days of development analysed, whereas t-retinol rises sharply as the liver develops. Within the embryo, tRA is present at high levels in the developing spinal cord and at very low levels in the forebrain; indeed there is a gradient of endogenous tRA from the forebrain to the spinal cord. Other parts of the embryo had intermediate levels of tRA. When a teratogenic dose of RA was administered to day 10.5 embryos, the levels of tRA present in individual tissues of the embryo rose dramatically--from 175-fold to 1,400-fold--and the levels rose in all tissues not in any exclusive areas. We then determined which areas of the embryo were malformed by such a teratogenic dose. The lower jaw, palate, vertebrae, tail, and limbs were consistently abnormal, and since these areas received a dose of tRA no higher than any other it was concluded that cell-specific factors must determine the teratogenic response of these tissues. We then considered whether cellular retinoic acid-binding protein I or II (CRABP I or II) played any role in this response by determining their relative levels in each of the tissues analysed. There was no correlation between the presence of CRABP I and II and the distribution of administered RA. Neither was there a clear correlation in detail between the presence of CRABP I and II and the sites of teratogenesis. We therefore conclude that other factors, for example, nuclear factors, must be responsible for the teratogenic response to RA.

Liarozole fumarate inhibits the metabolism of 4-keto-all-trans-retinoic acid

The metabolism of 4-keto-all-trans-retinoic-acid (4-keto-RA), a biologically active oxygenated metabolite of all-trans-retinoic (RA), has been examined. In vitro, incubation of [14C]4-keto-RA with hamster liver microsomes in the presence of NADPH produced two major radioactive metabolites which were more polar than the parent compound. Following isolation, appropriate derivatization and analysis by GC-MS, these compounds were tentatively identified as 2-hydroxy- and 3-hydroxy-4-ketoretinoic acid. Formation of both hydroxy-keto derivatives was suppressed by the imidazole-containing P450 inhibitor liarozole fumarate (IC50, 1.3 microM). In vitro, an i.v. injection of 4-keto-RA (20 micrograms) into rats was followed by rapid disappearance of the retinoid from plasma with a half-life of 7 min. Pretreatment with liarozole fumarate (40 mg/kg, -60 min) reduced the elimination rate of 4-keto-RA: it prolonged the plasma half-life of the retinoid to 12 min, without affecting its distribution volume. These results indicate the important role of the P450 enzyme system in the metabolism of 4-keto-RA both in vitro and in vivo. The inhibitory effect of liarozole fumarate on this metabolic process may contribute to the reported retinoid-mimetic activity of this drug.

In vitro embryotoxicity of N-methyl-N-(7-propoxynaphthalene-2-ethyl)hydroxylamine (QAB): evidence for N-dehydroxylated metabolite as a proximate dysmorphogen

The rat conceptus biotransforms N-methyl-N-(7-propoxynaphthalene-2-ethyl)hydroxylamine (QAB) in vitro to 7-propoxynaphthalen-2-ylacetic acid (QAA) and six more (M1 to M6) metabolites. Thus far, M4 has been identified as N-demethyl-QAB and M6 as N-desoxy-QAB. We investigated which of these two metabolites might be involved in QAB-embryotoxicity in vitro. Conceptuses were cultured from day 9.5 to 11.5 post-coitum, and were exposed to N-demethyl-QAB or N-desoxy-QAB either in the culture medium or by microinjection directly into the amniotic cavity. When added to the culture medium, N-demethyl-QAB (No Observed Adverse Effect Level, NOAEL, for growth 122 microM and for differentiation 41 microM) was less active than QAB itself (NOAEL for growth and differentiation 12 microM). N-desoxy-QAB caused severe growth retardation and an impairment of differentiation at a concentration of 11 microM (NOAEL 3.6 microM). As regards causing anomalies, the NOAEL of N-demethyl-QAB (41 microM) was 10-fold higher than that of QAB (NOAEL 3.9 microM) and that of N-desoxy-QAB (NOAEL 3.6 microM). At an intraamniotic concentration of 0.7 mM, N-demethyl-QAB caused no effects on growth and differentiation and no increase of anomalies was observed, whereas QAB and N-desoxy-QAB each elicited an increase in dysmorphogenic embryos at equimolar concentrations without affecting growth and differentiation. It is, therefore, concluded that N-desoxy-QAB, but not N-demethyl-QAB, could be a proximate dysmorphogen responsible for the embryotoxicity/teratogenicity of QAB in vitro.

9-cis-retinoic acid: a direct-acting dysmorphogen

Experiments in vitro with cultured rat conceptuses demonstrated that 9-cis-retinoic acid (9-cis-RA) (300 ng/mL amniotic fluid) produced branchial arch and somite defects similar to those elicited by equal concentrations of all-trans-retinoic acid (all-trans-RA), but with an increase in cephalic defects that included missing optic vesicles. After conceptuses were intraamniotically microinjected with 600 ng 9-cis-RA/mL amniotic fluid on day 10 of gestation, an unusual heart defect was also observed. HPLC analyses indicated that 9-cis-RA readily underwent conversion to the less active metabolite, 13-cis-retinoic acid (13-cis-RA), in cultured conceptuses during the first 4 hr after treatment but only after 6 hr could elevated levels of the potent dysmorphogen all-trans-RA be detected. In separate experiments, conversion of 13-cis-RA or of all-trans-RA to 9-cis-RA could not be detected during a 6-hr embryo culture period. Endogenous levels of 9-cis-RA in whole rat embryos also were below limits of detection but small quantities of this isomer could be detected in neonatal rat eye and human embryonic brain. Our present study strongly suggests that 9-cis-RA is a direct-acting dysmorphogen with probable specific target sites of action.

Tissue levels of retinoids in human embryos/fetuses

In nonhuman vertebrate embryos, two endogenous retinoids with significant morphogenic activities have been identified thus far: all-trans retinoic acid and 3, 4-didehydroretinoic acid. To date, no information is available concerning endogenous retinoid levels in developing human embryos or fetuses. The purpose of the present study was to provide data relating to normal levels of retinoids in various human embryonic and fetal tissues at various stages of gestation measurable with HPLC techniques. Our investigations show that all-trans-retinoic acid, 13-cis-retinoic acid, retinol, all-trans-retinoyl-beta-glucuronide, and one unidentified metabolite were all present and quantifiable in several human embryos and fetuses investigated. Tissue levels of retinol were consistently much higher than those of the other three detected metabolites; 4-oxo metabolites were below the levels of detection in all samples studied.

Comparative teratogenicity and metabolism of all-trans retinoic acid, all-trans retinoyl beta-glucose, and all-trans retinoyl beta-glucuronide in pregnant Sprague-Dawley rats

When single large equimolar doses (0.38-0.41 mmol/kg BW) of all-trans retinoic acid (RA), all-trans retinoyl beta-glucose (RBGL), and all-trans retinoyl beta-glucuronide (RBG) are administered orally in oil on day 8.5 of pregnancy to Sprague-Dawley rats, RA and RBGL proved highly teratogenic, whereas RBG was not. Indeed, fetuses from RBG-treated dams were 16% heavier (P < 0.01) than control fetuses. After dosing with RA and RBGL, RA appeared in large amounts within 0.5 h in the maternal plasma and within 1.0 h in the embryo. In contrast, orally administered RBG seemed to be absorbed much more slowly, to be converted very slowly to RA, and not to accumulate either as RBG or as RA in the embryo. When incubated in vitro with embryos and attached membranes, however, both all-trans RBG and all-trans RA were partially converted to 13-cis RA. The nonteratogenicity of RBG, in contrast to RA, seems to be due to a much slower rate of GI absorption, a slow rate of hydrolysis to RA, a limited passage from the maternal circulation into the embryo, and a lower inherent toxicity.

Chemical teratogenesis

This review has briefly summarized what is currently known concerning the mechanisms whereby several groups of chemicals regarded as "recognized" human teratogens elicit their respective teratogenic effects. It is evident that the extent of our understanding of mechanisms for individual chemicals varies dramatically from that of a reasonably good understanding for methotrexate and other folic acid antagonists to that of virtually no understanding for the most widely recognized human teratogen, thalidomide. Even with methotrexate, however, much remains to be learned pertaining to mechanisms--i.e., the critical links in the chain of events between dihydrofolate reductase inhibition and the manifestation of specific abnormalities. Nevertheless, we can take some comfort in being able to say that we understand the primary causative mechanism. For thalidomide, as well as several others the chemical represents both a shame and a challenge--a challenge that should be addressed with our most serious efforts.

Conceptual biotransformation of 4-oxo-all-trans-retinoic acid, 4-oxo-13-cis-retinoic acid and all-trans-retinoyl-beta-glucuronide in rat whole embryo culture

In cultured rat conceptuses, intraamniotic microinjections of 2500 ng/mL of 4-oxo-13-cis-retinoic acid, 600 ng/mL 4-oxo-all-trans-retinoic acid or 4000 ng/mL all-trans-retinoyl-beta-glucuronide, produce qualitatively and quantitatively similar patterns of dysmorphogenesis as those reported after the intraamniotic microinjection of 250 ng/mL all-trans-retinoic acid [Lee et al., Teratology 44: 313-323, 1991; Creech Kraft et al., Teratology 45: 259-270, 1992]. In the present study, we utilized HPLC techniques to analyze retinoid levels in cultured rat conceptuses, 1.5 hr after intraamniotic microinjections of 4-oxo-13-cis-retinoic acid (2500 ng/mL), 4-oxo-all-trans-retinoic acid (600 ng/mL) or all-trans-retinoyl-beta-glucuronide (4000 ng/mL). Our findings show that, after the microinjections of 4-oxo-all-trans-retinoic acid or 4-oxo-13-cis-retinoic acid (at these selected concentrations), 4-oxo-all-trans-retinoic acid was predominant in the embryos proper at concentrations of about 200 nM. This was roughly equivalent to the levels of all-trans-retinoic acid assayed after microinjections of all-trans-retinoyl-beta-glucuronide (4000 ng/mL). We conclude from these studies that both 4-oxo-all-trans-retinoic acid and all-trans-retinoic acid behave as ultimate or proximate dysmorphogens.