Chondrogenesis of limb-bud cells: improved culture method and the effect of the potent teratogen retinoic acid

Optimization of human nasal epithelium primary culture conditions for optimal proton oligopeptide and organic cation transporters expression in vitro

AIM

To investigate the effect of key tissue culture conditions on cell growth, gene expression and functional uptake of peptide and organic cation transporter substrates in the human nasal epithelium (HNE).

METHODS

HNE were cultured on different growth surfaces (polystyrene plastic, collagen film, and hydrated collagen gel) and were maintained with three popular nasal tissue culture media supplements [DMEM/F12 supplemented with Ultroser(®) G (2%), FBS (10%) and NuSerum(®) (10%)], respectively. The expression of gene transcripts for organic cation and peptide transporters were screened using qPCR and substrate uptake studies.

RESULTS

Cell growth surface (polystyrene plastic surface, dried collagen film and hydrated collagen gel) did not significantly alter gene expression levels. However, Ultroser(®) G and FBS caused significant increase in PEPT1, PEPT2, PHT1, OCT3, and OCTN1 levels (~/=2-5-fold for FBS and 2-8-fold for Ultroser(®) G). In terms of the degree to which the supplements affected gene expression, the following observations were made: effect on OCTN1>PEPT2>OCT3>PHT1>PEPT1. Functional uptake of organic cation (4-Di-1-ASP) and peptide [β-Ala-Lys (AMCA)] transporter substrates was significantly lower in cells cultured with NuSerum(®) compared to Ultroser(®) G and FBS cultured cells (p>0.05).

CONCLUSIONS

Tissue culture media had a major effect on SLC gene expression levels of the human nasal epithelium in primary culture. Ultroser(®) G was identified as the most efficient culture supplement in maintaining SLC transporter expression under most culture conditions, whereas FBS appears to be an economical choice. We do not recommend the use of NuSerum(®) as a supplement for growing HNE for transport studies involving SLC transporters.

The Comparative Testing of Eight Coded Chemicals in the Rat Limb Bud Micromass and Rat Embryo Culture Systems

When cultured at high density, mesenchymal cells from rat limb buds proliferate and differentiate into chondrocytes. Inhibition of this in vitro chondrogenic process has been used for the preliminary evaluation of teratogenic potential. Alternatively, intact post-implantation rat embryos, maintained in short-term culture, provide a system for the in vitro study of abnormal development not limited to the skeletal system. Both systems isolate the test agent from maternal metabolism and pharmacokinetic restraints. In this study, drug-associated selective inhibition of alcian blue uptake by cartilage proteoglycans, in micromass cultures of limb bud cells prepared from 13-day-old rat embryos, was used to assess teratogenic potential in vitro following exposure for 48 hours to eight coded compounds (acetylsalicylic acid, isoniazid. penicillin G, saccharine, vincristine sulphate, 6-aminonicotinamide, retinoic acid, and amaranth). Following drug exposure, cultures were incubated for another 96 hours, and the cells were then fixed and stained with 0.5% alcian blue. Bound dye was then extracted and quantitated. In parallel cultures, cell viability was measured by neutral red uptake, and protein content was assayed by using the bicinchoninic acid method. Except for retinoic acid and vincristine sulphate, the maximum test concentration was 1000μg/ml. Inhibition of alcian blue uptake (> 50%) was noted at 0.001μg/ml vincristine sulphate, 0.5/μg/ml retinoic acid and 5μg/ml 6-aminonicotinamide, demonstrating that strong teratogens inhibit differentiation in micromass cultures at lower concentrations than those which affect limb cell viability. When the same eight compounds were tested in a 24-hour embryo culture model, dysmorphogenesis was evident at 0.005μg/ml vincristine sulphate, 0.1μg/ml retinoic acid and 0.3μg/ml 6-aminonicotinamide. For the other five chemicals, little or no toxicity was noted up to the maximum test concentration in either model. We conclude that the two test systems, both based on the developing rat embryo, are consistent with each other, and that either of them would be useful for the preliminary screening of potential teratogens.

A retinoic acid receptor alpha antagonist counteracts retinoid teratogenicity in vitro and reduced incidence and/or severity of malformations in vivo

The role of retinoic acid receptors (RAR) in retinoid-induced teratogenesis is mainly unknown. The aim of the present studies was to demonstrate the effect of a RAR alpha antagonist on retinoid-induced teratogenic effects in vitro and in vivo. In micromass cultures of rat limb bud cells a RAR alpha antagonist was able to counteract differentiation inhibiting effects of a RAR alpha agonist. In mouse studies, the selective RAR alpha antagonist reduced frequency and/or severity of major malformations. Our observations indicate the potentiality of selective RAR agonists and antagonists in dissecting the function of nuclear receptors and in particular cases of retinoid teratogenesis, to assign to the different receptors a primary role in determining one or another of the multiple malformations.

In vitro assessment of teratogenic potential of organotin compounds using rat embryo limb bud cell cultures

Assessment of the relative teratogenic potential of bis(tri-n-butyltin)oxide (TBTO), tri-n-butyltin chloride (TBT), and its metabolites, i.e., (3-OH)hydroxybutyl dibutyltin chloride ((3-OH-Bu)DBT), di-n-butyltin dichloride (DBT), and butyltin trichloride (MBT) have been conducted using rat embryo limb bud cell cultures (LBC) to gain some knowledge of TBT embryotoxicity and DBT teratogenicity. Triphenyltin chloride (TPT), trimethyltin chloride (TMT), and triethyltin bromide (TET) have also been tested to obtain data for validation of LBC as a teratogen prescreening for organotin compounds. Fifty percent inhibition concentration for cell proliferation (IP50), and for cell differentiation (ID50), and the ratio of the former to the latter (P/D ratio) were obtained. The ID50 values in increasing order were as follows; TPT, DBT < TBT, TBTO < (3-OH-Bu)DBT < TET < TMT << MBT. With the exception of MBT, organotin compounds tested were very strong inhibitors of cell differentiation (ID50; 0.13-1.71 microM) and cell proliferation (IP50; 0.12-2.81 microM). P/D ratios for TBT, (3-OH-Bu)DBT, DBT and MBT were 1.0, 1.43, 1.32 and 1.08, respectively. These results suggest that the proximal toxin of DBT teratogenicity is DBT itself, and TBT is rather embryocidal than teratogenic so that TBT might mask the teratogenic and/or fetotoxic effects of its metabolites.

Altered differentiation of limb bud cells by transforming growth factors-beta isolated from bone matrix and from platelets

A crude extract of demineralized bone matrix caused an altered differentiation of limb bud cells which was seen within 5 days in culture. Using this bioassay system we purified two factors to homogeneity and found that according to their N-terminal sequences they corresponded to TGF-beta 1 and TGF-beta 2 isolated from platelets. Biochemical analyses and biological studies (molecular mass determination, inactivation by reducing agents and proteases, antibody neutralization, competitive binding to TGF-beta receptors and influence on protein expression) provided additional evidence that the two proteins isolated from demineralized bone matrix were apparently identical to TGF-beta 1 and TGF-beta 2. Proteoglycan content, alkaline phosphatase activity and response of the cells to PTH stimulated adenylate cyclase were quantitatively changed by the factors. Culturing limb bud cells on polycarbonate membranes resulted in a rapid and extensive growth and differentiation of the cells to palpable tissue pieces. Relative to controls distinct cell and tissue morphology was observed macroscopically and in histological sections of these tissue pieces.

Embryolethality of new herbicides is not detected by the micromass teratogen tests

New herbicidal compounds (11 pyrimidine-diones, 3 benzoates and 1 sulfonamide) were found to be embryolethal but not teratogenic in rats. The range of the embryolethal dose varied from 0.2 to greater than 200 mg/kg. This broad range enabled us to validate whether proposed in vitro teratogen tests can detect the embryolethality of these herbicides. The IC50 values (inhibition concentration 50%) for both differentiation and proliferation of midbrain and limb bud cells of rat embryos were determined and found to be above 50 micrograms/ml in all cases, confirming that the herbicides were not teratogenic. No correlation, however, was observed between the embryolethality in vivo and the activities in these cells. In order to test whether the potential to cause embryolethality could be predicted and detected as a general cytotoxic effect, the inhibition of colony forming ability in V79 cells was determined. The results indicated that cytotoxicity in V79 cells may be useful for preliminary testing of the embryolethal effect of herbicides.

Teratogenicity of arotinoids (retinoids) in vivo and in vitro

The effect of structural modifications on the arotinoid molecule, a new class of retinoids, on their teratogenicity in mice was studied. Animals were treated on days 8 and 9 of gestation, the most susceptible stages to retinoid-induced malformations in rodents. The teratogenic potency of the 13 arotinoids tested varied over a dose range of more than five orders of magnitude. Next, we tested whether the quantitative differences in the teratogenicity of these arotinoids correlates with their activity in high density (micromass) cultures of rat embryonic limb bud and midbrain cells. There was a good quantitative correlation between the in vivo teratogenicity and the in vitro activity in limb bud cells but no correlation was found in midbrain cells. Thus, the limb bud cell culture system may be useful for a preliminary testing to select non-teratogenic retinoids. For the risk assessment in humans, however, the in vitro results should be verified in animals studies.

Pharmacological concepts and developmental toxicology

This communication provides evidence to support the concept that developmental toxicants (teratogens) produce their effect by either interfering with or enhancing the time-dependent signal-response mechanisms within the embryo. Essential to this hypothesis is the need to show that an observed effect is a function of the administered dose, that there is a positive correlation between the observed effect and pharmacokinetic parameters and that there is evidence for the existence of a specific receptor for the toxicant. While extensive effort is required for ultimate validation of this concept, it serves to emphasize the value of applying known pharmacological principles in defining a mechanistic framework for the biological activity of developmental toxicants.

Limb bud cell culture for in vitro teratogen screening: validation of an improved assessment method using 51 compounds

Rat embryo limb bud cells multiply and undergo chondrogenesis in micromass culture. Teratogenic agents are identified from their inhibition of chondrogenesis, which is quantified by determination of cartilaginous foci number or proteoglycan production. In other in vitro systems, the detection is based on their ability to affect cell proliferation. So far, these methods have failed to distinguish among true inhibition of differentiation, inhibition of cell proliferation, and nonspecific cytotoxicity. The improved technique involves simultaneous measurement of cartilage synthesis and cell multiplication. Differentiation was evaluated by measurement, using an Artek Counter, of nodule areas after Alcian blue staining and proliferation by spectrophotometric quantification of Crystal-Violet bound to micromass cells. Using this method, retinoic acid was shown to inhibit chondrogenesis without affecting cell multiplication, whereas 6-aminonicotinamide preferentially inhibited cell multiplication without affecting nodule size. Doxylamine (succinate), a known nonteratogen, induced inhibition of chondrogenesis, but with a parallel inhibition of cell multiplication, reflecting a nonspecific toxic effect. This improvement increases the specificity of the micromass culture test. Validation was performed using 51 compounds. Compounds were classified according to their inhibitory activity and their active concentration. The sensitivity of the test was 61%; the specificity, 100%; and the final accuracy, 75%. The method is fully miniaturised, automated, and computerised, allowing numerous compounds to be rapidly tested at very low cost.

Selective stimulation of in vitro limb-bud chondrogenesis by retinoic acid

Embryonic exposure to pharmacologic doses of vitamin A analogs (retinoids) is a well-known cause of limb-skeletal deletions, limb truncation and other skeletal malformations. The exclusively inhibitory effect of retinoic acid (RA) on chondrogenesis in standard serum-containing cultures of limb-bud mesenchymal cells is equally well known and has provided a means to explore the cellular basis for RA-mediated skeletal teratogenesis. Recent studies showing that lower RA concentrations can cause skeletal duplication when applied directly to the anterior border of a developing limb, suggest that RA may have a role in normal limb development as a diffusible morphogen capable of regulating skeletal pattern. While RA treatment causes both, skeletal deletions and duplications are clearly different (if not opposing) effects, the latter of which is difficult to reconcile with RA's heretofore exclusively inhibitory effect on in vitro chondrogenesis. In the present study. RA's effects on chondrogenesis and myogenesis were examined in serum-free cultures of chick limb-bud mesenchymal cells and compared with its effects on similar cultures grown in serum-containing medium. When added to serum-free medium, concentrations of RA known to cause skeletal duplication in vivo dramatically enhanced in vitro chondrogenesis (to over 200% of control values) as judged by both Alcian-blue staining and [35S]sulfate incorporation, while having little effect on myogenesis. Higher concentrations inhibited both chondrogenesis and myogenesis. The results indicate that at physiological concentrations. RA can selectively modulate chondrogenic expression and suggest that at higher concentrations, RA's inhibitory effects are less specific.(ABSTRACT TRUNCATED AT 250 WORDS)

Limb bud cell cultures for estimating the teratogenic potential of compounds. Validation of the test system with retinoids

Mesenchyme cells, derived from embryonic limb buds, cultured at high cell density, multiply and differentiate into chondrocytes. Using alcian blue, a stain specific for cartilage proteoglycans, the degree of chondrogenesis can be visualized in the micromass cultures as well as quantified by extraction of the stain and spectrophotometric determination of its absorbance. In the presence of active retinoids chondrogenesis is concentration-dependently inhibited. For comparison of the activity of the various retinoids the concentration needed to reduce alcian blue staining by 50% was estimated. In order to validate whether the activity in limb bud cells can predict the teratogenic potential in vivo, the in vitro activity of 25 retinoids was compared with their in vivo teratogenicity observed mainly in rodents. For retinoids which were already in the biologically active form like those with a free carboxylic acid endgroup, there was a good quantitative correlation between the in vitro and in vivo activity. In contrast, the ethylester analog etretinate was slightly active and the ethylamine analog motretinide inactive in vitro but both were teratogenic in vivo. This finding may indicate that these retinoids were not metabolized to the active form in vitro. In conclusion, these results suggest that the limb bud cell culture system may be useful for a preliminary testing to select non-teratogenic retinoids. For the risk assessment in humans, however, the in vitro result should be verified in animal studies.