Glucocorticoid receptors in murine embryonic facial mesenchyme cells

Bone brittleness varies with genetic background in A/J and C57BL/6J inbred mice

The contribution of genetic and environmental factors to variations in bone quality are understood poorly. We tested whether bone brittleness varies with genetic background using the A/J and C57BL/6J inbred mouse strains. Whole bone four-point bending tests revealed a 70% decrease in postyield deflection of A/J femurs compared with C57BL/6J, indicating that A/J femurs failed in a significantly more brittle manner. Cyclic loading studies indicated that A/J femurs accumulated damage differently than C57BL/6J femurs, consistent with their increased brittleness. Differences in matrix composition also were observed between the two mouse strains. A/J femurs had a 4.5% increase in ash content and an 11.8% decrease in collagen content. Interestingly, a reciprocal relationship was observed between femoral geometry and material stiffness; this relationship may have contributed to the brittle phenotype of A/J femurs. A/J femurs are more slender than those of C57BL/6J femurs; however, their 47% smaller moment of inertia appeared to be compensated by an increased tissue stiffness at the expense of altered tissue damageability. Importantly, these differences in whole bone mechanical properties between A/J and C57BL/6J femurs could not have been predicted from bone mass or density measures alone. The results indicated that bone brittleness is a genetically influenced trait and that it is associated with genetically determined differences in whole bone architecture, bone matrix composition, and mechanisms of cyclical damage accumulation.

Glucocorticoids have pleiotropic effects on small intestinal crypt cells

Glucocorticoids have long been known to accelerate maturation of the intestinal tract, but the molecular mechanisms that account for their physiological function in the epithelium remain poorly characterized. Using rat intestinal epithelial cell lines (IEC-6, IEC-17, and IEC-18) as models, we have characterized glucocorticoid receptors in crypt cells and documented striking morphological, ultrastructural, and functional alterations induced by these hormones in intestinal cells. They include arrest of growth, formation of tight junctions, appearance of long, slender microvilli, reorganization of the endoplasmic reticulum and trans-Golgi network, and downregulation of the cell cycle regulatory proteins cyclin-dependent kinase 6 and p27(Kip1). These effects are consistent with the activation or modulation of multiple genes important in the physiological function of absorptive villous cells but are probably not directly involved in the induction of cell differentiation.

Significant association of HLA-B and HLA-DRB1 alleles with cleft lip with or without cleft palate

Nucleotide sequence level typing of HLA-B, -DRB1, and -DPB1 alleles was performed on Japanese patients with cleft lip with or without cleft palate (CL/P). Two HLA-B alleles, B*1501 and B*5101, showed a significant positive association with CL/P. The increase of B*1501 was evident in female patients (OR=3.6, Pc=0.003), whereas the increase of B*5101 was evident in male patients (OR=3.7, Pc < 0.001). One HLA-DRB1 allele, HLA-DRB1*0802 also showed an increase in CL/P patients. Conversely, HLA-B*4403 and DRB1*1302 were not observed in the patient group (Pc=0.01 and Pc=0.02, respectively). No HLA-DPB1 alleles showed significant association with CL/P. Thus, the present study indicates that HLA alleles, or closely linked loci, may be involved in the pathogenesis of CL/P.

Review of the interaction between TCDD and glucocorticoids in embryonic palate

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that produces adverse biological effects including developmental toxicity and teratogenesis. In the mouse embryo, TCDD induces cleft palate and hydronephrosis. The synthetic glucocorticoid, hydrocortisone (HC), induces cleft palate and a potent, synergistic interaction has been observed between TCDD and HC in C57BL/6N embryonic mice. The morphology and etiology of TCDD- and HC-induced clefts are distinctly different with formation of small palatal shelves following HC exposure and failure of normally-sized shelves to fuse after TCDD treatment. Each exposure also alters expression of several growth factors. When EGF, TGF alpha, EGF receptor, and the TGF beta's are considered as a combinatorial, interacting set of regulators, TCDD and HC each produce a unique pattern of increased and/or decreased expression across the set. The interaction of HC and TCDD results in a cleft palate whose etiology most closely resembles that observed after HC exposure, i.e. small palatal shelves. HC+TCDD-exposure also produces a pattern of growth factor expression which closely resembles that seen after HC. Both TCDD and HC act through receptor-mediated mechanisms and each compound has its own receptor. The Ah receptor (AhR) binds TCDD and the glucocorticoid receptor (GR) binds HC. On gestation day (GD) 14, in the embryonic palate exposed to TCDD, the AhR was downregulated and the GR expression increased. Conversely, following HC exposure, the GR was downregulated and AhR levels were elevated. HC+TCDD produced increased expression of both receptors and this pattern would be predicted to produce HC-like clefts as the GR-mediated responses would result in small palatal shelves. The observed cross-regulation of the receptors is believed to be important in the synergistic interaction between TCDD and HC for the induction of cleft palate.

Comparison on collagen gene expression in the developing chick embryo tendon and heart. Tissue and development time-dependent action of dexamethasone

Glucocorticoids modulate various cellular functions such as proliferation, energy metabolism and the synthesis of proteins. In the present study, the response of collagen genes to dexamethasone in different stages of chick embryo development was studied in tendon and heart using Northern blot analysis and specific cDNA probes. The changes in collagen gene expression were compared to alterations in two reference mRNAs: actin and glyceraldehyde-3-phosphate dehydrogenase (GAPDH). The levels of specific mRNAs measured per ribosomal RNA in tendon and heart varied markedly during normal development. In tendon the relative levels of alpha 1(I), alpha 2(I) and alpha 1(III) collagen mRNAs were highest between days 14-16 when also the synthesis of matrix proteins is most active. In heart the levels of these mRNAs peaked at day 12. In addition, qualitative differences were observed in the expression of actin genes between tendon and heart. Dexamethasone in high dose decreased collagen mRNA levels in tendons, while in heart a stimulatory effect was noted. Dexamethasone also decreased GAPDH mRNA levels in tendons. The alterations in gene expression after dexamethasone treatment in tendon and heart did not correlate with the level of specific glucocorticoid receptors, which varied markedly during the development of chick embryos. The cDNA for pro alpha 1(I) collagen hybridized to two transcripts corresponding to 6.2 and 5.1 kb in tendon and heart. During normal development of chick embryos the ratio of 6.2/5.1 kb mRNAs decreased markedly in heart, but no such change was observed in tendons. Dexamethasone, however, decreased the ratio of 6.2/5.1 kb transcripts in tendons. There was a significant correlation between the ratio 6.2/5.1 kb transcripts and total alpha 1(I) mRNA both in tendon and heart, suggesting that the 6.2 kb transcript may be associated with the rate of synthesis of type I collagen.

Inhibition of palatal fusion in vitro by indomethacin in two strains of mice with different H-2 backgrounds

Susceptibility to glucocorticoid-induced cleft palate in mice has been related to the H-2 histocompatibility complex on chromosome 17. Indomethacin administered in vitro to palatal processes from 13.5-day-old mouse embryos inhibited palatal fusion. Strains with the A background and the H-2a haplotype had significantly higher rates of inhibition than their partners with the H-2b haplotype. The inhibition was prevented in both strains by the addition to the media of prostaglandin E2, but this corrective effect was greater in the A strain with the H-2a haplotype. Thus, blockade of palatal fusion involves prostaglandins, suggesting a similar genetic and biochemical pathway for the different susceptibilities to cleft palate induced by both indomethacin and glucocorticoids.

Corticosterone induction of cleft palate in mice dosed with orciprenaline sulfate

Orciprenaline sulfate is a beta-adrenoceptor stimulant chemically described as 1-(3,5-dihydroxyphenyl)-1-hydroxy-2-isopropylaminoethane sulfate (Alupent). The drug has broncho-dilating activity and has been developed in numerous countries since 1961. The purpose of these studies was to investigate the teratogenic potential of orciprenaline and its mode of action in pregnant Jcl:ICR mice, when administered during the period of organogenesis and, more systematically, during the critical period of palate formation. Daily doses of 5, 50, and 500 mg/kg were given orally by gavage to mice on days 6-15, 11-13, or on day 12 of gestation. Additional studies were done to evaluate the maternal cardiotoxic action of orciprenaline and its effects on adrenal cortex and endogenous serum corticosterone. Five mg/kg triamcin-olone acetonide, a glucocorticoid, were given subcutaneously as a positive control causing 100% cleft palate. Myocardial necroses occurred in pregnant mice only after 500 mg/kg orciprenaline had been given, and a significant increase in cleft palate occurred if exposure took place during days 11-13 or day 12 of gestation. This increase in cleft palate can be explained by the teratogenic effect of an elevated maternal serum corticosterone level 1 hr after orciprenaline treatment, about three times the control value.

Murine glucocorticoid receptors: new evidence for a discrete receptor influenced by H-2

The glucocorticoid receptor contents in the lungs of females of two congenic strains of mice, B10.A (H-2a) and B10 (H-2b), differing only in the H-2 histocompatibility region of chromosome 17, have been measured by the dextran-charcoal method and by our previously described methods of molecular sieving and ion exchange chromatography [M. Katsumata, C. Gupta, and A. S. Goldman (1985) Arch. Biochem. Biophys. 243, 385-395]. As reported, two receptors, II and IB, are demonstrable by each column chromatographic method, and 5,5-diphenylhydantoin binds to receptor IB but not to receptor II. Receptor IB cannot be detected unless molybdate is added in cytosols prepared with hypotonic buffer [10 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid and 10 mM dithiothreitol, pH 7.35) according to S. L. Liu, J. F. Grippo, R. P. Erickson, and W. B. Pratt (1984) J. Steroid Biochem. 21, 633-637], a method which has been reported to give maximal receptor levels. Using hypotonic buffer containing 10 mM molybdate we observed a small but significantly higher content of receptor IB in B10.A mice than that in B10 mice, but no significant difference in receptor II or total receptor content. On the other hand, cytosols prepared with isotonic buffer (50 mM Tris-HCl, 120 mM NaCl, 1 mM EDTA, 10 mM dithiothreitol, and 10 mM molybdate, a modification of the buffer used in our previous report) contained significantly higher levels of receptor IB and of total binding in pulmonary cytosols of B10.A as compared to those of B10. There was no difference in receptor II content. Molybdate stabilizes receptor IB in both buffers. These results explain the apparent contradiction between our results and those of Liu et al. by showing that the hypotonic buffer used by them allows for determination of maximal levels of receptor II, but permits selective destruction of receptor IB. However, the use of isotonic buffer gives maximal values of both receptors II and IB. With isotonic buffer, it is demonstrated that only the level of receptor IB is influenced by H-2-linked genes.

Glucocorticoid receptor-mediated teratogenesis and cell proliferation in the limbs and face of the chick embryo

The susceptibility of the face region of the chick embryo to the teratogenic action of intraamniotically injected hydrocortisone contrasts with the resistance of the limbs to its action while at the same time their dysmorphogenesis may be induced by other agents. Since glucocorticoid receptors were shown to mediate face teratogenesis, their development was investigated in freshly dissected limb buds of 3-, 3.5-, and 4-day-old chick embryos in comparison with the face region. The specific binding of 3H-dexamethasone to molybdate-stabilized glucocorticoid receptors was estimated by the dextran-coated charcoal method and complemented by cytologic analysis of mitotic activity in control and hydrocortisone-treated embryos. The glucocorticoid receptors were found in both organ anlagen already on day 3 when their concentration in femtomoles per microgram DNA was significantly higher in the face region. Accordingly, on day 3 intraamniotic hydrocortisone inhibited the mitotic activity in the face without affecting the developing limbs. On days 3.5 and 4 the concentration of glucocorticoid receptors was similar in both organ anlagen. Administration of hydrocortisone on day 4 induced mitotic depression in the face as well as in the limbs. However, the degree of inhibition appeared to be dependent upon the actual mitotic rate. In the face region where the mitotic activity culminated at that time, the inhibition was much deeper and longer-lasting than in the developing limbs characterized by continuous decrease of proliferation rate in controls. These findings are consistent with a view that glucocorticoid receptors are a prerequisite, but not the only factor in receptor-mediated teratogenesis.

Effects of dexamethasone on phospholipase activities in palate mesenchyme cells in vitro

Treatment of primary cultures of palate mesenchyme cells from AJAX strain embryos with dexamethasone inhibited only phospholipase activity expressed at pH 7.5. A similar treatment did not have such an effect on palate mesenchyme cells from C57BL/6J strain embryos. Since the AJAX strain embryo is sensitive to the induction of cleft palate by exogenous glucocorticoids and the C57BL/6J strain is less so, these data allow consideration of phospholipase activity as a site of regulation for development of the palate.

Murine glucocorticoid receptors and the H-2 locus--a reappraisal

It has been demonstrated that susceptibility to glucocorticoid-induced formation of cleft palate is regulated by the mouse histocompatibility complex (H-2). This has encouraged us to examine H-2 effects on glucocorticoid binding in tissues of adult animals which would provide sufficient material with which to study the biochemical mechanism of the H-2 effect. Although it has been reported that cytosol prepared from lungs of adult mice with a high susceptibility to steroid-induced cleft palate formation have a higher level of glucocorticoid binding than lung cytosol prepared from a low-susceptibility strain, we are unable to demonstrate any influence of H-2 on binding capacity in this tissue from adult animals when glucocorticoid receptors are assayed in the presence of receptor reducing and stabilizing agents that maximize binding capacity. Cytosol prepared from rat liver contains an endogenous receptor-reducing system composed of NADPH and thioredoxin. It has also been reported that the murine H-2 complex contains a gene(s) that regulates the level of a modifier(s) in fetal hepatic cytosol that affects the binding of glucocorticoids to the receptor. Of two known low molecular weight modifiers that could account for this effect, we have previously established that the heat-stable, steroid receptor "modulator" is not regulated by the H-2 complex. In the present work we have assayed thioredoxin, a second potential modifier, in liver cytosols prepared from adults of two pairs of two H-2 congenic mouse strains. Our results show that the amount of thioredoxin is the same in all four mouse strains and that it is not regulated by the H-2 locus. At this time, we are unable to identify a system in adult mice in which the widely reported regulation of glucocorticoid binding by the mouse histocompatibility locus can be submitted to definitive biochemical study.

Biochemical mechanism of glucocorticoid-and phenytoin-induced cleft palate

The production of cleft palate by glucocorticoids and phenytoin is a complicated interference in a complex developmental program involving many genetic and biochemical processes. The H-2 histocompatibility region includes genes which affect (1) susceptibility to glucocorticoid- and phenytoin-induced cleft palate; (2) glucocorticoid receptor level in a variety of tissues including maternal and embryonic palates, adult thymuses, and lungs; and (3) the degree of inhibition of prostaglandin and thromboxane production by glucocorticoids and phenytoin in thymocytes. A gene linked to a minor histocompatibility locus (H-3) on the second chromosome also influences susceptibility to glucocorticoid- and phenytoin-induced cleft palate. Phenytoin is an alternate ligand for the glucocorticoid receptor affecting prostaglandin and/or thromboxane production. The capacity of glucocorticoids to induce cleft palate is correlated with their anti-inflammatory potency. At least some of the anti-inflammatory effects of glucocorticoids can be explained by the inhibition of prostaglandin and/or thromboxane release, which in turn could be caused by inhibition of arachidonic acid release from phospholipids. Similar mechanisms may be involved in cleft palate induction, as exogenous arachidonic acid injected into pregnant rats and mice at the same time as glucocorticoids reduces the teratogenic potency of the steroids, and indomethacin, an inhibitor of cyclooxygenase, blocks the corrective action of arachidonic acid. Glucocorticoids and phenytoin cause a delay in shelf elevation, and this delay is promoted by fetal membranes and the tongue. However, the cells of the medial edge epithelium are programmed to die whether contact is made with the apposing shelf or not. Glucocorticoids and phenytoin interfere with this programmed cell death, and this interference by both drugs seems to be glucocorticoid receptor mediated, to require protein synthesis, and to be related to arachidonic acid release.

Role of glucocorticoids and epidermal growth factor in normal and abnormal palatal development

The purpose of this chapter has been to discuss glucocorticoid and EGF involvement in normal and abnormal palatal development. It is to be hoped that we have made clear the important point that these hormone/growth factors and their receptors are present during normal embryonic palatal development to provide for regulation of growth and cellular differentiation. When these hormone/growth factors are administered in pharmacological or large doses that result in teratogenesis, these potent chemicals and their receptors then become inducers of cleft palate. The primary reason for this is that the hormone/growth factor receptors have unique and special areas of localizations in target (embryonic and fetal) tissues, e.g., glucocorticoids in the palate. Therefore, large amounts of these chemicals are specifically bound to receptors in these target tissues and these high levels of hormone/growth factor-receptor complexes result in aberrant development, e.g., glucocorticoids cause inhibition of palatal mesenchymal cell growth. These effects are distinct from the interactions of physiological levels of these hormone/growth factors with their receptors in these target tissues during development, e.g., glucocorticoids cause induction of key enzymes and modulation of EGF receptor levels. The exact molecular mechanism(s) by which high levels of hormone/growth factors--receptor complexes exert harmful effects on embryos or fetuses is (are) unknown and remain(s) a challenge for the future. Interaction of hormone/growth factors and their receptors certainly cannot provide an explanation for the mechanism of all types of craniofacial teratogenesis, but this concept certainly appears capable of providing important information relating to the mechanisms of many animal and human teratogens. The fact that these chemicals and their receptors are involved in normal development makes them all the more important since subtle alterations in their levels or activities could result in teratogenesis without an exposure to pharmacological levels of these hormone/growth factors. It seems that progress in this area will develop quickly since the techniques of recombinant DNA research are available in conjunction with responsive in vitro cell systems such as the established line of human embryonic palatal mesenchymal cells. Clearly, the future looks very exciting for understanding the role that these hormone/growth factors and their receptors play in normal and abnormal palate development.

Glucocorticoid receptor-mediated teratogenesis in the chick embryo

The susceptibility of chick embryos to the teratogenic action of intraamniotically injected hydrocortisone increases by several orders during the first four days of incubation. An attempt was made to correlate this phenomenon with the appearance of specific intracellular binding proteins for glucocorticoids. The binding of [3H] corticosterone to the soluble cytoplasmic proteins of the chick embryo was investigated on days 1.5, 2, 3, and 4 of incubation using a gel filtration method. No evidence of high affinity binding was found in embryos on day 1.5. High affinity binding of [3H] corticosterone to the cytosol proteins was first observed in embryos on day 2, but the binding capacity was four times lower than that found in embryos on days 3 and 4. A correlation was obtained between the increasing sensitivity of the chick embryo to hydrocortisone and the appearance of the intracellular binding protein for glucocorticoids. The causal relationship between these two phenomena is further supported by the finding that administration of a nonteratogenic dose of cortexolone completely prevents the teratogenic "cleft beak" action of hydrocortisone, presumably on the basis of competition for binding sites to the glucocorticoid receptor. These findings are consistent with the hypothesis that the teratogenic action of glucocorticoids is mediated by specific cytoplasmic receptors in the chick embryo.

Distribution and metabolism of triamcinolone acetonide in the rat embryomaternal unit during a teratogenically sensitive period

The distribution and metabolism of triamcinolone acetonide (TAC) in the rat embryomaternal unit were investigated during a teratogenically sensitive period. Pregnant rats (Day 12 of gestation) were injected im with 0.125 or 0.5 mg/kg [3H]TAC. Maternal plasma and embryos were collected at selected time points and analyzed by HPLC and liquid scintillation counting. No significant differences in the percentage of total radioactivity representing unchanged TAC, concentration of TAC, or its elimination half-life were detected in either plasma or embryos of the two dose groups. These results provide evidence that the metabolism and distribution of TAC in the rat embryomaternal unit are dose independent over this known teratogenic dose range. To determine whether multiple administration of TAC resulted in any alterations in maternal or embryonal exposure, the same parameters were evaluated following one (Day 12), two (Days 12 and 13), or three (Days 12, 13, and 14) injections of [3H]TAC (0.5 mg/kg, im). The only alterations detected were an increase in the percentage of total radioactivity in maternal plasma representing unchanged TAC at 1 hr following the second or third injection and an increase in the embryonal concentration of TAC at the same time points.

Vitamin B6 reduces cortisone-induced cleft palate in the mouse

Administration of Vitamin B6 during gestation to mice on a Vitamin B6-containing diet resulted in a substantial reduction in cortisone-induced cleft palate. Mice maintained on a Vitamin B6-deficient diet demonstrated an increase in the frequency of cortisone-induced cleft palate; this effect was prevented by administration of Vitamin B6. Vitamin B6 inhibited the specific binding of a labeled glucocorticoid to cytosolic receptors from cultured palatal mesenchyme cells. These results indicate that Vitamin B6 reduces the incidence of cortisone-induced cleft palate by altering the binding of glucocorticoids to their cytoplasmic receptors and subsequently nuclear acceptors.

Susceptibility to cleft palate and the major histocompatibility complex (H-2) in the mouse

The congenic mouse strains B10 and B10.A differ genetically only at the H-2 locus, B10 having the H-2b and B10.A the H-2a haplotype. The two strains also differ in susceptibility to cortisone-induced cleft palate, B10 being more resistant than B10.A. Since stage of palate closure is also associated with susceptibility to cortisone-induced cleft palate it was postulated that the H-2 haplotype may affect the cortisone-induced cleft palate susceptibility by altering the stage of palate closure. The present study shows that palate closure occurs at the same stage in the two strains, so this hypothesis must be rejected.

Cortisone-induced cleft palate in A/J mice: failure of palatal shelf contact

Although cortisone treatment for induction of cleft palate in mice has been shown to delay the time of palatal shelf elevation, the effects of delayed elevation of shelf contact have not been critically evaluated in a cortisone-sensitive mouse strain. The objective of this study was to evaluated palatal development in cortisone-treated A/J mice in order to determine whether the shelves make contact upon elevation. Morphometric analysis of frozen sections revealed that cortisone-treated shelves were smaller than control shelves with apparent reductions in both the content of extracellular matrix and the number of cells. At a light microscopic level, thinning of medial epithelium in cortisone-treated palates appeared similar to that in untreated palates with spontaneous cleft lip and palate. Shelf elevation was delayed by approximately 12 hours and only half of the cortisone-treated palates achieved complete horizontal positioning of the shelves in all regions of the palate. Immediately after elevation, all control palates had extensive vertical contact along the complete length of the palate. In contrast, approximately 20% of the cortisone-treated fetuses had contact between the shelves in the middle palate region only, with the mean area of contact only 20% as large as in control fetuses. As result, the net shelf contact in all the cortisone-treated fetuses was only 4% of the potential contact shown in control fetuses. Therefore, failure of the palatal shelves to elevate and make extensive contact appeared to be the major factor contributing to cortisone-induced cleft palate in A/J mice.

Glutamine synthetase activity in the developing secondary palate and induction by dexamethasone

Glutamine synthetase (EC 6.3.1.2) (GS) and glutamyltransferase (EC 2.3.2.1) (GT) specific activity were examined in developing A/Jax and C57BL/6J (C57) mouse fetal secondary palates. In addition, the induction of palatal GS was also examined after maternal injection of dexamethasone. Palatal GT activity was uniformly higher in A/J than C57 palates with both strains showing highest activity late on day 13 of gestation and a drop in activity by early day 14. In contrast, A/J palatal GS activity peaked transiently late on day 13, dropped by early day 14 and remained lower throughout the remaining period of palatal development. Palatal GS activity in C57 mouse fetuses, although failing to show a discrete transient peak of activity, remained at a constant elevated level from early day 13 to late day 14 and did not decrease until day 15 of gestation. These elevated levels of palatal GS and GT activity correspond to the gestation period of maximal palatal glycoconjugate biosynthesis. Thus, palatal GS activity may play an important regulatory role in the synthesis of these macromolecules. A/J and C57BL/6J mice exhibit different susceptibilities to glucocorticoid-induced cleft palate. However, maternal administration of a non-teratogenic dose of dexamethasone on either late day 12 or late day 13 resulted in a dramatic stimulation of both A/J and C57 fetal palatal GS but not GT activity when assay 18 h later. A/J palatal tissue responded to dexamethasone with greater induction of palatal GS activity than enzyme activity in C57 palates. Palatal GS, sensitive to glucocorticoid stimulation, may thus be an important link in expressing hormonal control of normal palatal differentiation.

Palate morphogenesis. VI. Identification of stellate cells in culture

Mesenchymal cells from the palate of mouse embryos at day 14.5 of gestation produce a minor population of stellate cells in culture. These cells are often bipolar and spindle-shaped with long cytoplasmic processes similar to neural-crest cells. Culturing of explants of palatal mesenchyme enriched for this type of cell. Stellate cells were the first to migrate from explants, followed by fibroblast-like cells and then by squamous cells. The majority of the cells in the explant were fibroblast-like. Squamous cells were present mostly in the anterior and mid-palate and least frequently in those from the posterior palate. They may represent tooth-germ epithelium. When pieces of palate were dissected out and cultured for enrichment of non-muscle contractile systems, most of the migrating cells were stellate. These may represent the highly migratory cells that are, in part, responsible for elevation of the palate shelf. Serotonin was measured in cultured mesenchymal cells from the palate. Its occurrence is consistent with regulation of movement of palate cells.

Cortisone-induced cleft palate in the brachymorphic mouse

Previous studies have shown that the autosomal recessive gene brachymorphic (bm/bm), which is maintained on a C57BL/6J (C57) background, reduces limb growth and sulfation of cartilage proteoglycans. Hydrocortisone administered on gestational days 11-14 resulted in 20% CP in the C57 mouse, but 95% CP in the bm/bm mouse. The bm/bm mouse had a median effective dose for CP of 45 mg/kg, compared to 325 mg/kg for C57 and 40 mg/kg for A/J. Morphometric analysis indicated that the time of palatal elevation was delayed in the bm/bm relative to the C57 mouse both with and without hydrocortisone treatment. The amount of cytoplasmic glucocorticoid receptor protein present in the bm/bm palate on day 14 was the same as the amount found in the C57 palate, and was not elevated as it is in the A/J palate. The levels of cyclic AMP in the bm/bm palate on day 14 were 30-70% higher than that found in the C57 palate with or without hydrocortisone. These results suggest that both bm/bm and A/J exhibit a delay in palatal shelf rotation and elevated levels of cyclic AMP, which appear to be predisposing factors for cortisone-induced cleft palate. These strains differ in that elevated levels of steroid receptors are present in A/J palate, whereas lower levels are found in the C57 and bm/bm mice.

Influence of H-2-linked genes on glucocorticoid receptors in the fetal mouse palate

The binding of 3H-dexamethasone to cytosolic receptors in fetal jaws and in cytosols and nuclei of primary cell cultures of fetal palates was studied in various congenic strains of mice. The amount of specific binding was greater in palatal tissues from B10.A and B10.A(2R) mice than in B10 or B10.A(5R) preparations. These differences were not observed in the liver. Since the strains with higher levels of glucocorticoid receptor are known to be more susceptible to cortisone-induced cleft palate than the strains with low receptor levels, it is suggested that quantitative variation in receptor levels may be involved in determining H-2-linked differences in cleft-palate susceptibility. Whether or not this is the case, it appears that an H-2-linked gene affects the quantity of a cytosolic glucocorticoid-binding protein which translocates to the nucleus.

A glucocorticoid receptor in fetal mouse: its relationship to cleft palate formation

Fetal mouse tissue was investigated for a glucocorticoid binding receptor which might be responsible for cleft palate formation. Fetal mouse heads contain a soluble component which binds the glucocorticoid triamcinolone acetonide in vitro with high affinity. This binding component is present in small finite amounts. Other glucocorticoids compete with triamcinolone acetonide for the binding site in a manner consistent with their potency ranking as cleft palate teratogens. Several mineralocorticoids and progestins also compete when administered in vitro but not when administered in vivo. Triamcinolone acetonide binding was determined in three mouse strains, A/J, C3H, and C57BL, which are listed in decreasing order of cleft palate susceptibility to cortisone. No positive correlation was found between cortisone cleft palate susceptibility and either triamcinolone acetonide binding affinity or binding amount in fetuses from these strains. Cleft palate dose response curves for triamcinolone acetonide were determined in these strains, but they were not parallel to each other as they were for cortisone. This suggests that triamcinolone acetonide may cause cleft palate by different mechanisms in these strains. Thus, fetal mouse tissue contains an apparent glucocorticoid receptors, but its relationship to cleft palate formation in mice is not clear.

H-2 and non-H-2 determined strain variation in palatal shelf and tongue adenosine 3':5' cyclic monophosphate: a possible role in the etiology of steroid-induced cleft palate

The concentration of cAMP was measured in palatal shelves and tongues of 14.5-day old foetuses, 14.5-day old foetuses from steroid treated mothers, and 15.5-day old foetuses from four inbred lines of mice which represent the four possible combinations of two H-2 alleles and two residual genetic backgrounds. The incidence of spontaneous and steroid-induced cleft palate in these four strains was also determined. Analyses of variance of the cAMP data reveal that both the H-2 region and residual genetic background determine cAMP concentrations in both tissues and on both days of development. Similar analyses of cAMP concentrations after steroid treatments of the mother indicate that the interaction between H-2 and residual genetic background is significantly different in the injected than in the uninjected mice in both palatal shelves and tongues. The incidence of steroid-induced cleft palate parallels the palatal shelf concentration of cAMP before steroid treatment of the mother with one exception. These data suggest that a portion of the H-2 controlled component of susceptibility to steroid-induced cleft palate is mediated through alterations in the metabolism of cAMP.

Involvement of glucocorticoids in the development of the secondary palate

Genetic differences between various inbred strains of mice in the levels of glucocorticoid receptors embryonic in maxillary mesenchyme cells appear to be reflected in the magnitude of the responses to steroids in these cells. High levels of glucocorticoids cause significant growth inhibition in maxillary mesenchyme cells with subsequent alterations in the production of extracellular matrix components. The presence of higher levels of cytoplasmic glucocorticoid receptor proteins may be one factor which could predispose those strains such as A/J to a greater inhibition of craniofacial growth in vivo by glucocorticoids and therefore increase the frequency of cleft palate production. Furthermore, women with infertility treated with glucocorticoids to support pregnancy give birth to infants with a marked decrease in birth weight [98]. Pharmacologic doses of glucocorticoids can also cause a dramatic reduction in the growth of a number of fetal tissues in mice and humans. In fact, there is evidence that glucocorticoids may be a causative factor in the production of cleft palate in primates [52]. The nature of the molecular elements which determine the biochemical and physiologic responses to glucocorticoids in the palate still remains largely unknown. Although in the mouse there is some evidence to suggest that the major histocompatibility locus (H-2) might be involved, the level(s) at which this control is exerted is unknown. It is possible that this locus may regulate in some manner the level of glucocorticoid receptors and the response to glucocorticoids in the secondary palate. Moreover, there is evidence to suggest that other genes distinct from, but closely linked to the H-2 locus may be important in determining both the strain-dependent differences in susceptibility to glucocorticoid-induced cleft palate and the intracellular levels of cyclic AMP in the secondary palate. It is also apparent that glucocorticoids in conjunction with other hormones or growth factors such as epidermal growth factor and agents which regulate cyclic nucleotide metabolism are essential for the normal development of the secondary palate. Excesses or deficiencies in either the level of these growth regulators and/or in their receptors in specific fetal tissues at defined periods in development are likely to lead to certain fetal malformations. Definition and integration of the genetic, biochemical, and endocrine factors which are involved in the control of cellular growth as influenced by alterations in the composition of cell surface and extracellular matrix components should provide some insights into the events associated with normal palatogenesis.

Glucocorticoid receptors

Glucocorticoid receptors are found in most mammalian tissues and have been studied in detail in a number of tissue culture systems. With cells that have not been exposed to steroids, the receptors are found in the cytoplasmic fraction from which they can be isolated and studied. Methods for studying glucocorticoid receptors depend on their high-affinity specific binding of radioactive steroids. The reversible interaction is intracellular. It follows Michaelian kinetics, at least in cell-free cytosol, and involves a thermodynamically homogeneous population of about 10 000 sites per cell. The receptor is an asymmetric, slightly acidic protein of about 100 000 daltons. It is very labile, especially in the unbound form. Binding activity depends on the integrity of thiol groups and perhaps on phosphorylation of amino acid residues. Although indirect, the evidence is overwhelmingly convincing that this protein is the physiologic glucocorticoid receptor. The time-kinetics of binding and dissociation are consistent with the sequence of events in glucocorticoid action. Various steroid analogs display binding characteristics predictable from their glucocorticoid activity. Loss of the binding protein from certain cultured cell lines is accompanied by unresponsiveness to glucocorticoids. The extensive tissue distribution of receptors parallels the extensive role of glucocorticoids in regulation. Finally, there is a strong correlation between nuclear binding of receptors and nuclear effects of the steroid. The glucocorticoid receptor can be distinguished from other glucocorticoid-binding proteins, based on their steroid specificity and physicochemical properties. There is no clear-cut demonstration that the receptor differs from tissue to tissue, and it is in fact very similar in various species. Unlike in other systems, receptor concentration does not seem to be regulated by its ligand or by other hormones. However, certain cases of hypo- as well as hypersensitivity to glucocorticoids appear to result from changes at the receptor level. The data indicate that the receptor can exist in inactive and active forms. The former predominate in the absence of steroid or when an angatonist is bound. Glucocorticoid agonists bind the active form, allowing it to be "activated" and subsequently bound to the nucleus. All of the receptors in isolated cytosol do not appear to be available for immediate occupancy by an agonist and this may be due to the time required for conversion of the receptors from inactive to active forms. The correlations between receptor binding and the glucocorticoid response indicate that the receptor is a rate-limiting factor in the magnitude and kinetics of the response, and this finding has important implications regarding mechanisms.

Stage of palate closure as one indication of "liability" to cleft palate

A new inbred mouse strain, SW/Fr, developed from a random-bred SW stock has a 6% incidence of spontaneous cleft palate without cleft lip. SW/Fr mice close their palates comparatively late in development. After cortisone treatment, the mean of the distribution (mean time to reach palate stage 5) is shifted towards later gestational ages. There is no change in the variance of the distribution. These data lend further support to the hypothesis that cleft palate in mice may fit a model where a continuous distribution is separated into discontinuous parts by a developmental threshold, and that time of palate closure is an important component of liability to cleft palate.

Inhibition of growth in vitro by glucocorticoids in mouse embryonic facial mesenchyme cells

The growth of primary embryonic facial mesenchyme cells established from cleft palate sensitive A/J and resistant C57BL/6J (C57) mice is inhibited by glucocorticoid treatment. A reduction in cell number in both A/J and C57 culture is accompanied by a significant decrease in [3H] thymidine incorporation into both acid soluble and insoluble material. No significant changes in total cellular protein or [14C] leucine incorporation were observed in either cell type. A greater reduction in [3H] thymidine incorporation occurs in cells undergoing exponential growth following steroid exposure than in cells approaching stationary growth. In both A/J and C57 cultures the reduction in cell number exhibits a dose-dependent response to dexamethasone; is specific for glucocorticoids; and is dependent upon the concentration of serum in which the cells are maintained. A/J cells show a greater sensitivity to the inhibitory effect of dexamethasone on cell number and thymidine incorporation than comparably treated C57 cells. Specific, high affinity, saturable cytoplasmic receptors for [3H] dexamethasone are present in the maxillary cytosols from which the primary cultures were established. These receptors exhibit binding specificity for glucocorticoids, and have properties which are similar to glucocorticoid receptors identified in other systems. In both cell types, a correlation exists between the degree of growth inhibition or reduction of [3H] thymidine incorporation and the level of glucocorticoid receptors. These results provide evidence for a receptor-mediated set of responses to glucocorticoids in these cells.

Inhibitory effects of glucocorticoids on collagen synthesis by mouse sponge granulomas and granuloma fibroblasts in culture

The basis for the glucocorticoid-mediated decrease in tissue collagen was studied in mouse granulomas and in primary granuloma fibroblast cultures. Injection of mice for 12 days with dexamethasone (0.35 mg/kg body weight) resulted in a 50--70% inhibition of collagen synthesis and accumulation in polyvinyl sponge-induced granulomas whereas total protein synthesis was inhibited by only about 25%. The decreased collagen content of the granuloma was accounted for by both a reduced fibroblast number and diminished synthesis per cell. Growth rates, total protein synthesis and collagen synthesis were the same in granuloma fibroblast cultures derived from control or steroid-treated mice. However, addition of 3.10(-7) M hydrocortisone to the culture medium caused a 30--50% inhibition of both collagen and non-collagen protein synthesis in firbroblasts from either source. These inhibitory effects were dose- and time-dependent with a lag time of 12--24 h. Prolyl hydroxylase activity was reduced both in sponge granulomas from glucocorticoid-treated mice and in hydrocortisone-treated fibroblast cultures. However, protein synthesis was inhibited to the same extent as the inhibition of prolyl hydroxylase activity and there was no effect on peptidyl prolyl hydroxylation. These results indicate that the glucocorticoid-induced reduction of collagen synthesis and accumulation observed in mouse granulomas and primary granuloma fibroblast cultures is not specific for this protein. Furthermore, glucocorticoid-induced inhibition of collagen synthesis cannot be attributed to underhydroxylation of collagen prolyl residues.