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

Protein kinase C and chemical-induced abnormal palate development

The protein kinase C (PKC) family of proteins mediates the action of growth factors and other ligands by activating a network of transcription factors that bind to TRE sequences in the promoters of many genes that regulate cell proliferation, differentiation, extracellular matrix synthesis, apoptosis and others in a cell type-, isozymeand context-specific manner. The critical role of PKC in embryonic development is indicated by early death of embryos in which one or more of these isozymes are inactivated. Our studies together with others show that palatal PKC signalling is functional and may be essential for normal palate development. Although single gene knockouts have failed to exhibit the cleft palate (CP) phenotype, owing to compensation by other kinases, many chemicals including the mycotoxin, secalonic acid D, disrupt palatal PKC signalling leading to altered palatal mesenchymal gene expression. The potential relevance of such effects to chemical-induced CP is discussed.

Sustained-release fluocinolone acetonide intravitreal insert for macular edema: clinical pharmacology and safety evaluation

INTRODUCTION

Inflammation plays a key role in the pathological processes leading to macular edema. Sustained release, low-dose intraocular corticosteroid delivery devices provide long-term anti-inflammatory therapy. Recently, a novel fluocinolone acetonide intravitreal insert (FAi, Iluvien), has been introduced with promising long-term results in the treatment of macular edema.

AREAS COVERED

An extensive review of the literature in the English language was performed to provide comprehensive information on the pharmacological properties of FAi and its safety and efficacy data from various multi-center randomized clinical trials.

EXPERT OPINION

The FAc, Retisert is a sustained-release device that is surgically implanted in the vitreous and has been approved by the US FDA for the treatment of non-infectious intermediate, posterior or panuveitis. FAi was developed after FAc and is an intravitreal corticosteroid delivery system that allows controlled release of therapeutic levels of fluocinolone acetonide (FA). Initial efficacy and safety data suggest that this delivery system maintains clinical effectiveness for up to 3 years after a single delivery of the device. This second-generation fluocinolone delivery device has shown superior safety results in clinical trials compared to the previous version of the higher dose FAc (0.59 mg). Sustained delivery preparations may help to reduce the treatment burden and its associated risks by decreasing the frequency of intravitreal injections. However, much needs to be learnt from additional clinical trials, post-marketing surveillance and results of extension studies. Concerns of intravitreal corticosteroids, such as cataract and increase in intraocular pressure, remain major challenges for this therapeutic strategy.

Alterations in protein kinase A signalling and cleft palate: a review

Cyclic AMP is an important second messenger mediating the actions of many hormones and other ligands in a variety of cells. Cells of the developing organism are no exception. Once generated, it releases the catalytic subunit of protein kinase A (PKA) from the inhibitory influence of its regulatory subunit, which then migrates into the nucleus to phosphorylate and enhance the binding of relevant transcription factors to the promoter element CRE of genes involved in above cellular responses. This review summarizes the available data on the essential role of this pathway in embryonic development as well as the functionality, ontogeny and consequences of genetic and chemical disruption of this pathway in the developing orofacial structures, especially the secondary palate as influenced by the mycotoxin, secalonic acid D.

Inhibition of human embryonic palatal mesenchymal cell cycle by secalonic acid D: a probable mechanism of its cleft palate induction

OBJECTIVE

To assess the mechanism(s) of cleft palate induction by secalonic acid D (SAD) in human embryonic palatal mesenchymal (HEPM) cells and compare them with those evaluated in the murine embryonic palate.

DESIGN

Effect of SAD on HEPM cell proliferation was studied by obtaining dose response curves for cell numbers, uptake of 3H-thymidine and the expression of proliferating cell nuclear antigen (PCNA). Effects of SAD on cell cycle were assessed by flowcytometry. Cell-labeling with 3H-glucosamine and immunoblot analysis were conducted to study SAD effects on the synthesis of glycosaminogycans (GAG) and the expression of fibronectin and tenascin, respectively.

RESULTS

SAD induced a concentration-dependent decrease in HEPM cell number and 3H-thymidine uptake beginning at 0.1 microg of SAD/ml. Expression of PCNA and progression of cell cycle from G1 to S phase were inhibited following SAD exposure. Cell viability was significantly reduced only at 7.5 microg/ml of SAD or higher indicating that the reduction in cell numbers by SAD at lower concentrations is likely due to reduced proliferation and at higher concentrations due to both reduced proliferation and cell death. Synthesis of extra cellular matrix components (GAGs, fibronectin or tenascin) by HEPM cells, however, was not inhibited by SAD.

CONCLUSION

The results of these studies confirmed those of our previous studies with mice and the MEPM cells that SAD may induce cleft palate by reducing numbers of palatal mesenchymal cells by inhibition of their proliferation thereby leading to a reduction in the size of the developing palate shelves.

Divergence of epidermal growth factor - transforming growth factor beta signaling in embryonic orofacial tissue

The epidermal growth factor (EGF) and transforming growth factor beta (TGFbeta) families of signaling molecules play a major role in growth and development of embryos. Abrogation of either signaling pathway results in defects in embryogenesis, including cleft palate. In the developing palate, both EGF and TGFbeta regulate cellular proliferation, extracellular matrix synthesis, and cellular differentiation but often in an opposing manner. Evidence from various adult cell types suggests the existence of cross talk between the EGF and TGFbeta signaling pathways, although it is unclear whether such cross talk exists in murine embryonic maxillary mesenchymal cells, from which the developing palate is derived. In this study, embryonic maxillary mesenchymal cells in culture were treated with EGF and TGFbeta, either singly or in combination, and the cells were subsequently examined for signaling interactions between these two pathways. Immunoblot analyses of nuclear extracts of embryonic maxillary mesenchymal cells revealed that TGFbeta-induced nuclear translocation of Smad 2 and Smad 3 proteins was not affected by EGF. Conversely, immunoblot analyses of whole-cell extracts of these cells indicated that EGF-induced phosphorylation of extracellular signal-regulated kinase proteins, ERK1 and ERK2, was not affected by TGFbeta. Expression of a transfected luciferase reporter gene driven by a promoter with Smad binding elements was induced by TGFbeta in these cells but was not affected by EGF. Last, TGFbeta was found to induce expression of the endogenous gelatinase B gene in embryonic maxillary mesenchymal cells; however, this effect was independent of any interaction of EGF. Collectively, data from this study suggest that the EGF and TGFbeta signal transduction pathways do not converge in murine embryonic maxillary mesenchymal cells.

The blockade of mineralocorticoid hormone signaling provokes dramatic teratogenesis in cultured rat embryos

Although the administration of adrenocortical hormones to pregnant rats provokes only limited effect on the growth and development of the fetus, the direct influence of these steroids on cultured embryos has never been studied. The disruption of cell signaling by ZK 91587, which specifically occupies the mineralocorticoid receptor, resulted within 2 days in significant and pronounced adverse effects on the total length, the somite number, the embryo curvature, the communication between vitelline and umbilical blood vessels in the allantoid, and the vascularization of the vitelline sac, in 244-hour Wistar rat embryos in culture. The average score of 16 organs declined in a dose-dependent manner, following exposure to ZK 91587, and this was totally reversed by 10 microM aldosterone which, by itself, did not at all influence the embryonic development. The organogenesis was inhibited in the order: hind limb > fore limb > optic stalk > brain > olfactory pit > otic vesicle. ZK 91587 was completely ineffective in embryos that had attained the age of 260 hours. Similar, but less dramatic, results were obtained with the mineralocorticoid antagonist RU 26752, and with the antiglucocorticoid RU 38486. Sprague-Dawley rat embryos responded in a manner similar to the Wistar conceptuses. Thus, steroid receptor-mediated cell signaling is of critical importance to the growth and development of cultured rat embryos, which form a new model system to unravel adrenocortical hormone action.

Strain dependency of TGFbeta1 function during embryogenesis

There is incomplete penetrance to Tgfb1 knockout phenotypes. About 50% of Tgfb1 homozygous mutant (Tgfb1-/-) and 25% of Tgfb1 heterozygous (Tgfb1+/-) embryos die during embryogenesis. In a mixed NIH/Ola x C57BL/6J/Ola x 129 background partial embryonic lethality of the Tgfb1-/-embryos occurs due to defective yolk sac vasculopoiesis and/or hematopoiesis. We show here that on a predominantly CF-1 genetic background, lack of TGFbeta1 causes a pre-morula lethality in about 50% of the null embryos. This partial lethality is not reversed by transfer of Tgfb1-/- embryos to Tgfb1-/+ hosts. The extent of embryonic lethality in Tgfb1-/- embryos ranges in a background dependent manner from 20% to 100%. Based on these and other studies it is clear that TGFbeta1 acts at two distinct phases of embryogenesis: pre-implantation development and yolk sac vasculogenesis/hematopoiesis. The susceptibility for the pre-implantation lethality depends on a small number of genetic modifiers since a small number of backcrosses onto the high susceptibility strain C57BL/6 leads to complete penetrance of the lethality.

Cross-talk between signaling pathways in murine embryonic palate cells: effect of TGF beta and cAMP on EGF-induced DNA synthesis

Signaling pathways utilized by EGF, cAMP, and TGF beta have been demonstrated to play critical roles in normal palate development. Stimulation of these pathways has been shown in palate cells and numerous other systems to affect cell growth. Because proper regulation of cell growth is critical to palate development, we speculate that fine regulation of palatal cell growth may be accomplished through crosstalk between these signaling pathways. We therefore set out to determine the effects of cAMP and TGF beta on EGF-induced cell proliferation in murine embryonic palate cells. We found that both TGF beta and cAMP inhibited the proliferative response of cells to treatment with EGF, whereas H89, a serine/ threonine protein kinase inhibitor with selectivity towards cAMP-dependent protein kinase, increased the cells' proliferative response to EGF. Genestein, a selective inhibitor of tyrosine kinases, at high doses abrogated the cells' proliferative response to EGF, confirming that EGF's ability to induce cell proliferation is critically dependent upon tyrosine kinase activity. Lower doses of genestein, however, actually enhanced cellular response to EGF. The data suggest that both the TGF beta- and cAMP-mediated signaling pathways may be involved in modulation of the effects of EGF on palate cell growth in vivo.

Epidermal growth factor inhibits morphogenesis of the embryonic quail uropygial gland cultured in vitro

Formation of the uropygial papilla and glandular lumena was inhibited when the uropygial rudiment of a day 8 1/3 quail embryo was cultured for 2 days in a chemically defined medium in the presence of 50 ng/mL of epidermal growth factor (EGF). The epithelium of EGF-treated explants remained at the placode stage, or underwent minor invagination into the mesenchyme and became stratified like that of a 12- or 13-day-old embryo. EGF promoted cellular proliferation in the uropygial epithelium and the epidermis adjacent to the gland and it shortened the lag phase of proliferation and markedly stimulated epithelial DNA synthesis, detected immunocytochemically by labeling explants with 5-bromodeoxyuridine (BrdU). The maximal labeling index in EGF-treated uropygial epithelium was 55% higher than in the control. Electron microscopic observation revealed that the basal lamina had become irregular in the EGF-treated explants and that epithelial cytoplasmic processes penetrated through the basal lamina toward the mesenchyme. These same phenomena are observed in vivo when the glandular buds are formed during day 12-13. Some precocious changes occurred in the uropygial epithelium when the rudiment was cultured in the presence of EGF.

Characterization of the fate of midline epithelial cells during the fusion of mandibular prominences in vivo

The fusion of the mandibular prominences along the midline is achieved with the absence of medial epithelial cells at the fusion site. Failure of fusion of the mandibular prominences results in median cleft of the lower lip and mandible. Cellular and molecular events controlling mandibular fusion were examined during the fusion process in mouse embryogenesis. Cell lineage analyses at the fusion site revealed that epithelial cells migrated to the surface and oral epithelia. DiI-labeled epithelial cells were not observed within the mandibular mesenchyme at any state of fusion. Examination of the midline region did not reveal cells with ultrastructural changes characteristic of apoptotic cell death. An increase in lysosomal enzymes in the midline epithelial cells, which would be correlated with programmed cell death, was not observed. Mice lacking TGF-beta 3 did not have cleft mandible, but had clefting of the secondary palate as a feature of null mutation phenotype. We interpret our comparisons between wild type and homozygous TGF-beta 3 (-/-) mice to suggest that different developmental processes control palatal vs. mandibular fusion. We hypothesize that medical epithelial cells at the fusion site of mandibular prominences migrate to the surface epithelium during the fusion process and neither transdifferentiate into mesenchyme nor express apoptosis.

Programmed cell death and cell transformation in craniofacial development

Fusion of branchial arch derivatives is an essential component in the development of craniofacial structures. Bilaterally symmetric branchial arch processes fuse in the midline to form the mandible, lips, and palate. The mechanism for fusion requires several different morphologic and molecular events prior to the completion of the mesenchymal continuity between opposing tissue processes. The ectodermal covering of the branchial arches is one of the cell types that has an important role during craniofacial development. The surface epithelia provide the initial adherence between the processes; however, this population of cells is ultimately absent from the fusion zone. The medial edge epithelium of the secondary palatal shelves is one example of such an epithelium that must disappear from the fusion zone of the secondary palate during development in order to complete palatal fusion. The mechanisms for removal of the epithelial cells from the fusion zone could include either programmed cell death, epithelial-mesenchymal transformation, or migration to adjacent epithelia. All three of these fates have been hypothesized as a mechanism for the removal of the palatal medial edge epithelia. The processes of programmed cell death, epithelial-mesenchymal transformation, and epithelial migration are reviewed with respect to both palatal fusion and results reported in other model systems.

The influence of earthquake-induced stress on human facial clefting and its simulation in mice

A large earthquake (8-9 on the Richter scale) and a series of aftershocks took place on 2 March 1985 in Santiago, Chile. The characteristics of over 22,000 births registered in three public hospitals in the same year were reviewed. A significant increase in the rate of facial clefts was found; 2.01 per 1000 births in contrast to 1.6 per 1000 births in previous years. The increase was greater in those born in September: 3.8 per 1000 births. This increase in clefting could be related to the effects of stress in mothers induced by the earthquake, and to test this hypothesis 13.5-day-old embryos from two inbred mouse strains, A/Sn and C57BL/10, were subjected to a similar stress using a vibrator cage to imitate the main shock and the first five replicas of the earthquake. The same intensity and duration of shock as in the original earthquake were applied. The results were 19.8% cleft palates in stressed A/Sn mice and no clefting in C57BL/10. This was highly significant in A/Sn mice (chi 2 = 19.9; P < 0.001) but not in C57BL/10. No clefting was found in controls in both strains. A surprising finding was the proportion of resorbed embryos in the stressed groups, which increased from 8.3 to 49.3% in A/Sn and from 5.8 to 48.3% in C57BL/10. It is known that A/Sn mice are genetically sensitive to cleft palate induction by cortisone, while C57BL/10 are not. These findings in mice support the stress hypothesis for the increase in cleft palate observed in humans. The increase in resorbed embryos in both strains also suggests an effect on stress.

Inhibition of adenylate cyclase in perfusion mouse palate by secalonic acid D

Glucocorticoids (GC) induce cleft palate (CP) in the offspring of exposed pregnant mice. Glucocorticoids induce prostaglandin (PG) synthesis inhibition via reduced arachidonic acid (AA) release from membranes, and this results from inhibition of phospholipase A2. This metabolic event is associated with reduced palatal cAMP levels in the prefusion stages (d 13 of gestation). The mycotoxin secalonic acid D (SAD) induces CP in the offspring born to treated mothers, elevates maternal plasma corticosterone levels, and reduces prefusion palatal cAMP levels. In addition, an increase in cAMP was noted in the postfusion period (d 15 of gestation). Since exogenous AA given simultaneously to GC-exposed mothers may protect against GC-induced CP in the offspring, such a possibility was tested for SAD. Pregnant CD1 mice given a teratogenic dose of SAD (30 mg/kg, ip, on gestational d 11) were simultaneously treated with maximal tolerated doses of AA (200 mg/kg, sc, on gestational d 11, 12, and 13). At term, no significant reduction in SAD-induced CP was seen as a result of AA treatment. To evaluate if SAD-induced alterations in palatal cAMP are due to reduced palatal membrane-associated adenylate cyclase (AC) activity during pre- and postfusion periods, SAD-treated mothers were sacrificed at 12-h intervals between gestational d 13.5 and 15.5, palate shelves were collected from the fetuses, and AC activity (cAMP formed/mg protein/min) was assayed in the presence or absence of the enzyme stimulator, sodium fluoride (NaF). Although SAD did not alter unstimulated AC activity, it significantly reduced the NaF-induced stimulation of enzyme activity in the prefusion period. This inhibition could not be reversed by excess GTP in the incubation mixture. Since NaF stimulation of AC indicates post-receptor-site function involving GTP-binding and catalytic units, and since addition of GTP failed to correct SAD-induced alteration of NaF stimulation of the enzyme, it is suggested that SAD may inhibit the AC sensitivity to stimuli by its effect on the catalytic unit in a manner that does not affect enzymic basal activity.

Epidermal growth factor and transforming growth factor alpha regulate extracellular matrix production by embryonic mouse palatal mesenchymal cells cultured on a variety of substrata

Mouse embryonic palatal mesenchymal (MEPM) cells were cultured either on plastic tissue culture dishes or on the surface of three-dimensional collagen gels or within collagen gel matrices in DMEM/F12 medium containing 2.5% donor calf serum. MEPM cells proliferated exponentially when cultured on collagen or on plastic. Cells cultured within collagen gels did not proliferate but remained viable. Addition of 10 ng/ml epidermal growth factor (EGF) or transforming growth factor alpha (TGFα) stimulated the proliferation of those cells cultured on plastic or on collagen but not those cultured within collagen gels. Immunocytochemical analysis revealed that MEPM cells synthesise collagen types I, III, IV, V, VI and IX; fibronectin, heparan sulphate proteoglycan, laminin and tenascin in vitro. These molecules are all present in the developing palate in vivo. EGF and TGFα produced a generalised stimulation of extracellular matrix (ECM) synthesis by MEPM cells in vitro. Biochemical analysis indicated that cells cultured within collagen gels had the highest intrinsic rate of protein synthesis. On all substrata neither EGF nor TGFα markedly altered the types of ECM molecules synthesised but rather caused a general increase in the total amount produced. This stimulation was most marked where the cells were cultured within collagen gels. The lack of stimulation of proliferation of MEPM cells cultured within collagen gels (i.e. in a physiologically-relevant environment) by EGF or TGFα together with the marked stimulation of ECM synthesis suggests that these factors may act as differentiation signals via their effects on ECM production.

Medial edge epithelium fate traced by cell lineage analysis during epithelial-mesenchymal transformation in vivo

Vital cell labeling techniques were used to trace the fate of the medial edge epithelial (MEE) cells during palatal fusion in vivo. Mouse palatal tissues were labeled in utero with DiI. The fetuses continued to develop in utero and tissues of the secondary palate were examined at several later stages of palatal ontogeny. The presence and distribution of DiI was correlated with the presence of cell phenotype-specific markers. During the initial stages of palatal fusion the DiI-labeled MEE were present in the midline position. These cells were attached to an intact laminin-containing basement membrane and contained keratin intermediate filaments. At later stages of palatogenesis the DiI-labeled MEE were not separated from the mesenchyme by an intact basement membrane and did not contain keratin. In late fetal development, DiI-labeled cells without an epithelial morphology were present in the mesenchyme. The transition of the DiI-labeled cells from an epithelial phenotype to a mesenchymal phenotype is consistent with a fate of epithelial-mesenchymal transformation rather than programmed cell death.

Regulation of murine embryonic epithelial cell differentiation by transforming growth factors beta

The expression of some members of the transforming growth factor beta (TGF beta) family of genes in embryonic craniofacial tissue suggests a functional role for these molecules in orofacial development. In an attempt to ascertain a role for the TGF beta s during palatal ontogeny, murine palatal shelves were excised on gestation day (GD) 12, prior to overt epithelial differentiation, grown in organ culture under serum-free conditions and exposed to TGF beta 1 and TGF beta 2 for 18 or 42 h. Shelves were labeled with [3H]-thymidine (20 microCi/ml) during the last 4 h in culture, fixed, dehydrated, embedded in paraffin and sections stained and examined by autoradiography. Treatment of GD12 palates with TGF beta 1 and TGF beta 2 resulted in precocious cessation of medial edge epithelium (MEE) DNA synthesis followed by elimination of the MEE. In addition, this response appeared to be dose-related with higher concentrations of growth factor eliciting a more marked biological response. TGF beta treatment of homologous shelves grown in apposition also resulted in precocious fusion of apposing MEE. Thus, members of the TGF beta family, known to be synthesized by palatal MEE, appear to act in an autocrine/paracrine fashion in this tissue and are capable of regulating differentiation of embryonic palatal medial edge epithelium.

Toward the origin of the secondary palate. A possible homologue in the embryo of fish, Onchorhynchus kisutch, with description of changes in the basement membrane area

The oral cavity of embryos and larvae of the teleost Onchorhynchus kisutch was examined. Tissues were obtained at different ages prior to and after hatching and processed for transmission and scanning electron microscopy. A bilaterally symmetrical bulge developed from the superolateral aspect of the oral cavity and projected toward its floor, along the sides of the tongue. The bulge extended from behind the primary palate to a position midway below the eye, anterior to the gill arches, and it is suggested to be the homologue of the secondary palate of higher vertebrates. Ultrastructurally, the epithelium differentiated as the stratified squamous type and it contained mucous cells. However, the features of programmed cell death seen during palatogenesis in mammals were absent in fish. The fish palate mesenchyme, unlike that of higher vertebrates, was chondrified. Also in contrast to higher vertebrates, alterations were seen in the fish palatal basement membrane. A transient appearance of adepidermal granules in the lamina lucida region was followed by organization of collagen fibrils, first into an orthogonal pattern and then into a herring-bone arrangement, in the lamina reticularis region. There was no further advancement in the morphogenesis of fish palate. It is suggested that the differences in the morphogenesis and structure of the secondary palates of various vertebrates may reflect environmentally enforced adaptation, resulting in different programming of cells.

New approaches and concepts in the study of differentiation of oral epithelia

Epithelial structural proteins, the keratins and keratin-associated proteins, are useful as markers of differentiation because their expression is both region-specific and differentiation-specific. In general, basal cells in all stratified oral epithelia express similar keratins, while the suprabasal cells express a specific set of markers indicating commitment to a distinct program of differentiation. Critical factors in the regulation of epithelial protein expression are now under investigation. The promoter regions of keratin genes are being characterized to determine what sequences within the genes are responsible for differential expression. One important extracellular factor that influences epithelial protein expression is retinol (vitamin A), which exerts its effects via a group of nuclear receptor proteins that may also be expressed in a region-specific manner. These molecular biological approaches enhance our understanding of the mechanisms regulating differentiation of oral epithelia and its regional complexity.

The influence of glucocorticoids on the growth of a human leiomyosarcoma cell line SK-LMS-1

A cell line derived from a human leiomyosarcoma, SK-LMS-I, has cystolic [Bmax 201.6 +/- 39.7, means +/- SD, fm/mg cytosol protein KD 14.6 +/- 7.8 nM (n = 8)] and nuclear [Bmax 189.2 +/- 87.3 fM/mg DNA, KD 4.8 +/- 0.26 nM (n = 5)] glucocorticoid receptor, by Scatchard analysis of tumors grown in male athymic mice. Tumor growth of SK-LMS-I cells in male athymic mice is inhibited by daily s.c. injection of DEX 5 micrograms, DEX 25 micrograms, DEX 5 micrograms with 5 mu RU-486 and 5 micrograms RU-486. In sharp contrast, in vitro, glucocorticoid markedly stimulates the growth (as determined by cell number) of SK-LMS-I cells, principally at higher cell densities (days 10-21 of growth carried on over a 21- to 23-day period), the greatest stimulation being seen with DEX 10(-6) to 10(-8) M, and no stimulation being seen with DEX 10(-9) and 10(-10) M. In vitro, glucocorticoids with higher affinity for the GR stimulate growth, steroids with lower affinity inhibit growth. No alterations in cell-cycle distribution (percent G0/G1, S, or G2/m) could be found by flow cytometric analysis of glucocorticoid-stimulated asynchronously growing cultures. Single, isolated, untreated SK-LMS-I cells form colonies in soft agar with an efficiency of 1.78 +/- 0.10%. Pre-treatment of cells with DEX 10(-7) M increases this to 3.24 +/- 0.17%, while cells pre-treated with both DEX 10(-7) M RU-486 10(7) M form colonies with the same efficiency as untreated cells. Glucocorticoids have inhibitory effects on in vivo growth and stimulatory effects on in vitro growth of a GR-positive human leiomyosarcoma cell line.

Growth factors, their receptors and development

The hypothesis is put forward that the localized production of growth factors and the regulation both of their receptors and their receptor-triggered transmembrane signaling reactions played an important role in organ development. The developing palate is used as an example to support this hypothesis. Data are summarized to demonstrate that during palatogenesis, there are variations in the amount of fetal epidermal growth factor-urogastrone (EGF-URO), in the amount of EGF-URO receptor, and that there is a developmentally regulated variation in cellular responsiveness to EGF-URO. Further, it is demonstrated that the growth factor EGF-URO itself can regulate the production by palate tissue of a second growth factor, insulin-like growth factor (IGF). The multiple actions of the many so-called growth factors, quite apart from their ability to regulate cell growth are pointed out; and the implications of the many distinct actions of a single growth factor on a single cell type are discussed in the context of a developmental process like palate formation. A model is developed to illustrate the kinds of interactive processes that may occur in a tissue microenvironment during a developmental process, involving cell-cell communication and an influence of the extracellular matrix.

Growth of the secondary palate in the hamster following hydrocortisone treatment: shelf area, cell number, and DNA synthesis

The contribution made by mesenchymal cells during the later stages of palatal development was examined in control and hydrocortisone-treated hamster embryos. Cross-sectional area of the palatal shelf was measured, and the numbers of both epithelial and mesenchymal cells were counted. DNA synthesis was measured by 3H-thymidine incorporation and was used as an index of growth by cell proliferation. The observations in controls indicated that, unlike development during the initial 24 hr, the later period of vertical palate development, followed by reorientation of shelves and their closure, was characterized by a steady level of mesenchymal cell number and palatal shelf area. An absence of corresponding growth in the epithelial cell number suggests that the cells may accommodate the growth either by increasing their size and/or by stretching along the basal lamina. Hydrocortisone treatment did not alter the growth pattern of cell numbers or shelf area. However, it prevented the fusion between the opposing shelves, perhaps by affecting the cytodifferentiation of the palatal tissues. Although a continuous increase in the number of mesenchymal cells during the latter half of vertical shelf development, i.e., between days 11:00 and 12:00 of gestation, is not required for reorientation and fusion of the shelves, it is not clear from the data from the present study whether a critical number of cells and/or cell density is essential for reorientation and fusion of the palate. It was suggested that, for normal palatal development, information on cell cycle and positioning of mesenchymal cells within the shelf during the vertical development may be crucial for further understanding of subsequent events of palatogenesis.

Evidence for glomerular actions of epidermal growth factor in the rat

Epidermal growth factor (EGF), an endogenous mitogenic peptide, has recently been shown to be a potent vasoconstrictor of vascular smooth muscle. In view of its potential role in proliferative and inflammatory renal glomerular diseases, we examined the effects of EGF both on cultured rat mesangial cells and on in vivo glomerular hemodynamics. Mesangial cells possess specific, saturable EGF receptors of differing affinities, with Kd's of 0.1 and 1.7 nM, respectively. EGF produced a rapid increase in intracellular pH of 0.12 +/- 0.01 pH U, which was sodium dependent and amiloride inhibitable. The addition of EGF to mesangial cells cultured on either glass or dimethylpolysiloxane substratum induced reproducible cell contraction. Intrarenal EGF infusion did not affect systemic blood pressure or hematocrit but reversibly decreased GFR and renal blood flow from 4.19 +/- 0.33 to 3.33 +/- 0.26 and from 1.17 +/- 0.09 to 0.69 +/- 0.07 ml/min, respectively. Glomerular micropuncture confirmed decreases in single nephron plasma flow and in single nephron GFR (from 142 +/- 9 to 98 +/- 8 and from 51.6 +/- 11.7 to 28.5 +/- 3.5 nl/min, respectively) which were due to significant increases in both pre- and postglomerular arteriolar resistances (from 1.97 +/- 0.31 to 2.65 +/- 0.36 and from 1.19 +/- 0.11 to 2.00 +/- 0.15 10(10) dyn.s.cm-5 respectively) and to a significant decrease in the ultrafiltration coefficient, Kf, which fell from 0.100 +/- 0.019 to 0.031 +/- 0.007 nl/(s.mmHg). These studies demonstrate that mesangial cells possess specific receptors for EGF, and exposure of these cells to physiologic concentrations of EGF results in an in vitro functional response characterized by activation of Na+/H+ exchange and by resultant intracellular alkalinization, as well as by cell contraction. EGF administration in vivo significantly reduces the glomerular capillary ultrafiltration coefficient, Kf, which, in combination with EGF-induced constriction of both preglomerular and postglomerular arterioles, results in acute major reductions in the rates of glomerular filtration and perfusion.

The effects of epidermal growth factor on neural crest cells in tissue culture

Epidermal growth factor (EGF) stimulates the release of hyaluronic acid (HA) and chondroitin sulfate proteoglycan (CSPG) from quail trunk neural crest cultures in a dose-dependent fashion. It also promotes the expression of cell-associated heparan sulfate proteoglycan (HSPG) as detected by immunofluorescence and immunoprecipitation of the 3H-labeled proteoglycan. Furthermore, EGF stimulates [3H]thymidine incorporation into total cell DNA. These results raise the possibility that EGF or an analogous growth factor is involved in regulation of neural crest cell morphogenesis.

Glucocorticoid receptor IB: mediator of anti-inflammatory and teratogenic functions of both glucocorticoids and phenytoin

We studied the glucocorticoid receptor complexes of pulmonary and thymic cytosols of female A/J and CD-1 mice and of hepatoma G2 cells by two column-chromatographic systems, using both [3H]dexamethasone (DEX) and [3H]phenytoin (DPH) as ligands. Three DNA-cellulose adsorbable [3H]DEX-receptor complexes were separated in each system. Molecular sieving gave a 7-, a 5.4-, and a 3.5-nm complex (Stokes radii), and DEAE-Sephadex A-50 chromatography gave a complex eluting in the wash, one at 0.14 M KCl, and one at 0.20 M KCl by a KCl gradient. DPH blocked the binding of the 7- and 3.5-nm, wash, and 0.14 M KCl [3H]DEX complexes. Only two DNA-cellulose adsorbable [3H]DPH complexes, each blocked by DEX, were obtained in each system: a 7- and a 3.5-nm, a wash, and a 0.14 M KCl complex. Thus, there is a common receptor for both DPH and DEX. This receptor has two properties which distinguish it from the 5.4-nm DEX-specific receptor: (i) it binds with a variety of steroids other than glucocorticoids and DPH, and (ii) it rebinds new [3H]DEX or [3H]DPH after loss of ligand during chromatographic separation. These results indicate that DPH binds to receptor IB and not to receptor II of Litwack. [G. Litwack, 1976, in Glutathion: Metabolism and Function (Arias, I.M., and Jakoby, W.B., eds.), pp. 285-299, Raven Press, New York]. We have also found that hepatoma G2 cells have only receptor II. DPH affects neither the induction of tyrosine aminotransferase by DEX nor the basal level of this enzyme in these cells. Moreover, neither DEX nor DPH inhibits the release of [3H]arachidonic acid prelabeled in these cells, as they do in thymocytes which have the common receptor. Thus, it appears that glucocorticoid receptor IB binds DEX and DPH as glucocorticoid agonists mediating the anti-inflammatory and teratogenic action of these drugs, while receptor II apparently is responsible for the induction of tyrosine aminotransferase by DEX.