Hormonal control of hyaluronic acid production in fibroblasts and its relation to nucleic acid and protein synthesis

Influence of laser irradiation on endogenous hyaluronan in rabbit iris and aqueous humor

PURPOSE

To monitor changes of endogenous hyaluronan in the iris tissue and aqueous humor after an isolated trauma to the iris by argon laser irradiation of the anterior surface of the iris.

METHODS

Iris and aqueous hyaluronan concentrations in rabbit were measured with a radiometric assay at different time points after laser irradiation.

RESULTS

Total hyaluronan content in iris tissue increased 3-fold to a peak concentration of 71-72 microg/g at 1 and 2 days after laser treatment. Aqueous hyaluronan increased to a maximum of about 1.6 microg/ml at 2 h and 12 h after laser irradiation of the iris.

CONCLUSIONS

The iris tissue responds with increased hyluronan synthesis to an isolated iris argon laser irradiation and it seems to be the most important source of aqueous hyaluronan.

Short-term impact of corticosteroids on hyaluronan and epithelial hyperplasia in the rabbit cornea after photorefractive keratectomy

PURPOSE

To investigate the impact of corticosteroids on subepithelial hyaluronan deposition and corneal epithelium thickness in the first 10 days after photorefractive keratectomy (PRK) and to analyze a possible contralateral effect of corticosteroids.

METHODS

Thirty-two New Zealand white rabbits were assigned into two groups and had a transepithelial 5.0-mm diameter, 8.00-diopter myopic PRK performed on one eye. The corticosteroid treatment group (16 animals) received 0.1 mL of methylprednisolone 4% subconjunctivally on the operation table, followed by 0.1% dexamethasone eye drops six times a day during the postoperative period. The sodium chloride (NaCl) treatment group received topical isotonic NaCl eye drops six times a day. In each treatment group, eight animals were killed after 3 and 9 days, respectively. The harvested specimens were stained for hyaluronan and the epithelial thickness was measured.

RESULTS

In contrast to the epithelial thickness, the subepithelial hyaluronan did not show a significant increase during the observation period. The corticosteroid treated group showed at both time-points significantly less subepithelial hyaluronan formation as well as a significantly thinner epithelium, when compared with the NaCl-treated group. At 9 days, the corticosteroid-treated group showed a mild epithelial hyperplasia in only one of eight eyes, whereas this was a common finding in the NaCl-treated group. We detected no hyaluronan deposits in any contralateral-untreated eye, and the epithelial thickness did not differ significantly between any of the four contralateral-untreated eye groups.

CONCLUSIONS

Corticosteroid medication during the first 10 days after operation reduces the amount of subepithelial hyaluronan production and inhibits the epithelial proliferation, and epithelial hyperplasia is prevented. Neither a contralateral hyaluronan deposition nor a contralateral corticosteroid effect could be detected.

Hyaluronan synthesis by mouse cumulus cells is regulated by interactions between follicle-stimulating hormone (or epidermal growth factor) and a soluble oocyte factor (or transforming growth factor beta1)

Expansion of the cumulus cell-oocyte complex (COC) in the preovulatory mammalian follicle requires a transient induction of hyaluronan (HA) synthesis by the cumulus cells. We studied the interactions of known factors that regulate this process by isolating compact COCs from mice and inducing their expansion in vitro. Maximum HA synthesis requires either follicle-stimulating hormone (FSH) or epidermal growth factor (EGF) in combination with either a soluble factor(s) produced by the oocyte or transforming growth factor beta1. FSH (or EGF) exerts its effects during the first 2 h of incubation, before HA synthesis actually begins. The oocyte factor(s) (or transforming growth factor beta1) exerts its effects from 2 h onwards and must be continuously present throughout the subsequent approximately 10 h to achieve a maximum level of HA synthesis. FSH stimulates intracellular cAMP synthesis, which correlates with net HA production up to approximately 14 fmol/COC at 5 ng/ml FSH; however, higher concentrations of FSH increase cAMP levels approximately 10-fold higher with no additional effect on HA synthesis. EGF at saturating concentrations for HA synthesis does not stimulate cAMP above basal levels. Tyrosine kinase inhibitors genistein and tyrphostin AG18 nearly abolish the HA synthesis response to EGF and inhibit the response to FSH by approximately 60%, suggesting that a tyrosine kinase activity is involved for both factors, whereas FSH also operates partially through another signaling pathway. Actinomycin D abolishes HA synthesis if added at the beginning of culture and reduces HA synthesis by approximately 50% if added between 6-12 h when HA synthesis is normally maximal. The results suggest that regulation of HA synthesis is primarily controlled at the transcriptional level.

Corticosteroid Suppression of TraumaInduced Hyaluronan in Rabbit Cornea and Aqueous

BACKGROUND

Corneal and aqueous hyaluronan have recently been shown to react in response to several different types of trauma, including cataract surgery. In order to find ways to influence the reaction, we have evaluated the effect of topical dexamethasone (Isopto-Maxidex, Alcon Universal Ltd, Fort Worth, Tex) or indomethacin (Confortid, Dumex Ltd, Copenhagen, Denmark) on the postoperative hyaluronan concentration in rabbit cornea and aqueous after extracapsular lens extraction.

METHODS

The drugs were administered as topical eye drops three times daily. The hyaluronan concentration in rabbit cornea and aqueous after extracapsular lens extraction was measured with a radioligand assay.

RESULTS

Dexamethasone treatment (1 mg/mL) significantly suppressed the increase in corneal hyaluronan seen after extracapsular lens extraction, 2 (p < or = 0.0022) and 3 weeks (p < or = 0.0002) after surgery, while indomethacin did not induce any significant difference at 2 weeks. When the dexamethasone concentration was lowered to 0.1 and 0.2 mg/mL, there was still a significant decrease (p < or = 0.009) in hyaluronan concentration, but at lower concentrations of dexamethasone (0.01 and 0.02 mg/mL), no significant decrease was seen. The increase in aqueous hyaluronan concentration seen 2 days after surgery in untreated eyes was significantly lowered by both dexamethasone (p < or = 0.0076) and indomethacin (p < 0.036).

CONCLUSIONS

Dexamethasone lowers reactive corneal and aqueous hyaluronan concentration in vivo after extracapsular lens extraction.

Hyaluronic acid synthesis is absent in normal human endothelial cells irrespective of hyaluronic acid synthetase inhibitor activity, but is significantly high in transformed cells

The characteristics of glycosaminoglycan (GAG) synthesis in normal and transformed human endothelial cells were analyzed by the incorporation of [3H]glucosamine and by the activities of GAG synthetases. The GAG synthesized by normal endothelial cells consisted of mainly heparan sulfate (HS) and chondroitin sulfate/dermatan sulfate but little hyaluronic acid (HA) (less than 1%). The characteristics of GAG synthesis by normal cells reflected the synthetic enzyme activities for each individual GAG: the activity of HA synthetase was very low. In spite of this, the activity of HA synthetase inhibitor, induced in growth-retarded fibroblasts with low HA synthetase activity (Matuoka et al. (1987 J. Cell Biol., 104, 1105-1115), was very low in endothelial cells. In contrast to normal cells, transformed endothelial (ECV304) cells synthesized mainly HA (62% of total GAGs). These findings suggest that the regulatory system of GAG metabolism is cell type specific, and that transformation is accompanied by high levels of HA synthesis in endothelial cells.

Clinical correlations and prognosis based on hyaluronic acid serum levels in patients with progressive systemic sclerosis

The serum levels of hyaluronic acid (sHA) were measured using an affinoimmunoenzymatic assay in patients with distal (n = 16) and proximal (n = 15) progressive systemic sclerosis (PSS) and in 31 controls. The severity of PSS was evaluated using a standardized organ-involvement score. The mean sHA was significantly higher in the patients with PSS than in controls (mean +/- SD:80 +/- 43.4 micrograms/l vs. 42.3 +/- 19.1 micrograms/l, P less than 0.001). sHA was significantly higher in patients with proximal PSS than in patients with distal PSS (106.4 +/- 44.6 micrograms/l vs. 55.4 +/- 23.8 micrograms/l, P less than 0.001). A positive correlation was found between sHA and the disease score (r = 0.67, P less than 0.001). sHA was also correlated with lung diffusion capacity for carbon monoxide (r = 0.70, P less than 0.001), but only in the those patients who had abnormal lung function, and therefore presumably had lung PSS involvement. We suggest that sHA could be an indicator of the degree of systemic involvement in PSS. Its prognostic value and possible use in the follow up of patients with PSS remain to be clarified.

Sera of children with renal tumours contain low-molecular-mass hyaluronic acid

The molecular mass of hyaluronic acid (HA) rather than its serum concentration alone may be a hallmark of certain types of malignancy. A radiometric assay was used to measure HA levels in 35 children with renal tumours [33 Wilms' tumours and 2 bone metastasizing renal tumours of childhood (BMRTC)] and 20 normal siblings of children with cancer. The HA level in the sera of normal children was barely detectable and had a molecular mass of 1-5 x 10(5). In both Wilms' and BMRTC patients, very high levels of HA were found in preoperative serum samples; these fell dramatically following surgical excision of the tumours. A novel finding of our study was the presence of low-molecular-mass HA (similar to the angiogenic fragments of HA) in the sera of BMRTC patients. In contrast, high-molecular-mass HA (which is not angiogenic) was found in the sera of Wilms' patients (2 x 10(6) kDa). Following surgery in BMRTC patients, not only did serum HA levels fall to a value within normal ranges, but also the HA which remained was of high molecular mass.

A decrease in hyaluronic acid synthesis by aging human fibroblasts leading to heparan sulfate enrichment and growth reduction

Cultured normal human fibroblasts during in vitro aging exhibited increased proportions of heparan sulfate (HS; a glycosaminoglycan (GAG) species) in the cell-associated GAG pool, coincident with decreased cell growth activity. An analysis of GAG metabolism demonstrated that human fibroblasts during aging became relatively rich in HS due to an alteration in the profile of GAG synthesis. HS became relatively enriched and hyaluronic acid (HA) relatively depleted through a decrease in HA synthase activity. An experimental enrichment of human fibroblast cultures with exogenous HS brought about an arrest of the cells in the G0/G1 phase and a decrease in the rate of S phase entry, coincident with aged cell growth behaviour. These results suggest that the change in HA synthesis is responsible, at least to some extent, for the growth reduction during aging of normal human fibroblasts.

Polymorphonuclear leucocyte dysfunction during short term metabolic changes from normo- to hyperglycemia in type 1 (insulin dependent) diabetic patients

Polymorphonuclear leucocyte (PMN) ingestion of particles coated with lipopolysaccharide (LPS) from Escherichia coli was compared to other PMN functions in seven patients with insulin dependent diabetes mellitus (IDDM) during short-term controlled metabolic changes from normo- to hyperglycemia without ketoacidosis. Factors known to interfere with PMN functions were excluded. PMN ingestion of particles coated with both LPS and bovine serum albumin became reduced from normo- to hyperglycemia. PMN motility was impaired in IDDM, but did not seem to be affected by short-term changes in metabolic control. PMN metabolism did not change from normo-to hyperglycemia. Particle-uptake by diabetic PMN is impaired after short term hyperglycemia in the range normally occurring in diabetics in every-day life.

Hyaluronate synthetase inhibition by normal and transformed human fibroblasts during growth reduction

To establish the relation of glycosaminoglycan synthesis to cell proliferation, we investigated the synthesis of individual glycosaminoglycan species by intact cells and in a cell-free system, using normal and transformed human fibroblasts under differing culture conditions. Reducing serum concentration brought about a marked decline in the synthesis of hyaluronate (HA) as well as cell proliferation on both normal and transformed cells. Both HA synthesis and proliferation decreased with increasing cell densities markedly (in inverse proportion to cell density) in normal cells but gradually in transformed cells. This noticeable congruity of the changes in HA synthesis and proliferation indicates that the change in HA synthesis is related primarily to cell proliferation rather than to cell density or cellular transformation. Examination of HA synthesis in a cell-free system demonstrated that the activity of HA synthetase also fluctuated in conjunction with cell proliferation. Furthermore, growth-reduced cells (except crowded transformed cells) inhibited cell-free HA synthesis and this inhibition was induced coincidentally with a decrease in both HA synthetase activity and proliferation. These findings suggest that the change in HA synthesis is significant in the regulation of cell proliferation.

Inhibition of smooth muscle cell proliferation and endothelial permeability with flunarizine in vitro and in experimental atheromas

Repeated weak electrical stimulations of rabbit carotid artery walls with implanted electrodes cause intimal proliferations of smooth muscle cells (SMC) and lead to fibromuscular plaques beneath the anode. If the animals receive a cholesterol-enriched diet the plaques become typical atheromas. The endothelial lining is maintained. The procedure for the production of an atheroma with 11 +/- 4 layers of SMC lasts 4 weeks. Addition of the calcium antagonist Flunarizine to the food during the stimulation periods inhibits the growth of the plaque. The inhibition is dose-dependent. Whether the drug inhibits atherogenesis by direct action on SMC or via an effect on permeation of macromolecules through the endothelium has been studied by measurement of (1) peroxidase (MW 40,000 dalton) permeability through the stimulated endothelium of the artery and (2) the inhibitory effects of Flunarizine on cultures of arterial SMC. Endothelial permeability increases for some hours after stimulation mainly beneath the anode. Flunarizine inhibits the permeation of peroxidase through the endothelial lining for the most part by its action on intercellular transport. The drug also inhibits the growth of SMC in mass cultures and clone cultures. The inhibition of proliferation is not specific for SMC. Skin fibroblasts obtained from the same animals as the artery smooth muscle cells are also inhibited in mass cultures and clone cultures. From the results it can be concluded that Flunarizine exerts its inhibitory action not only by its effect on the permeation through the endothelial lining but by a combined action on permeability and proliferation of cells in the artery wall.

Change of hyaluronic acid synthesis during differentiation of myogenic cells and its relation to transformation of myoblasts by Rous sarcoma virus

Hyaluronic acid synthesis was examined in cultures of differentiating chick embryo muscle cells before, during and after fusion. Prior to fusion, hyaluronic acid was synthesized and secreted into the medium, but once fusion began this synthesis was reduced significantly. Synthesis then increased again after completion of fusion. Thus, production of hyaluronic acid was lowest at the time of or right before cell fusion. When myoblasts were transformed by Rous sarcoma virus (RSV), a higher amount of hyaluronic acid was synthesized, and cells were not able to fuse. The turnover rate of hyaluronic acid might be different between myotubes and RSV-transformed myoblasts. The addition of exogenous hyaluronic acid to myoblast cultures resulted in the partial inhibition of fusion. The effect was reversible because fusion took place after removal of the exogenous hyaluronic acid. These observations suggest that hyaluronic acid plays an important role in the differentiation of myogenic cells, and that elevated hyaluronic acid synthesis may partly be the reason for inhibition of myotube formation upon transformation by Rous sarcoma virus.

Connective tissue activation. XXVII. The behavior of skin fibroblasts from patients with scleroderma

Four normal (NF) and 4 scleroderma skin fibroblast (SF) strains were compared with respect to 1) basal 14C-glucosamine and 35SO4-labeled glycosaminoglycan (GAG) synthesis, 2) responsiveness to autacoid mediators, and 3) performance following maximal stimulation. Under basal conditions, SF synthesized and secreted 2-3 times more radioactive hyaluronic acid than the NF (P less than 0.001); molecular volume by gel chromatography was similar and suggested a high molecular weight product. SF were essentially as responsive to normal lymphoid and platelet factors as were NF. No consistent qualitative or quantitative differences in sulfated GAG synthesis were noted between the 2 groups of cells. Incubation of NF and SF with a false "core protein" such as p-nitrophenyl-beta-D-xyloside suggested that synthesis of the core protein was rate limiting; SF and NF were equally facile in SO4-GAG chain synthesis in the presence of a beta-xyloside. SF appear to retain in vitro a partially activated state for many generations, at least with respect to hyaluronic acid synthesis.

Reduction of the hyaluronate concentration in rabbit aqueous humour by topical prednisolone

Rabbits were treated topically with prednisolone for 5 or 10 days and aqueous humour removed immediately after treatment as well as 10 and 20 days later. The hyaluronate concentration was significantly lower in the treated eyes compared to control eyes. It returned to the normal level after withdrawal of the drug. Repeated sampling of aqueous humour at intervals of 10 days did not influence the hyaluronate concentration in the control group.

Effect of low density lipoprotein on glycosaminoglycan secretion by cultured human smooth muscle cells and fibroblasts. Influence of serum concentration and cell proliferation rate

Glycosaminoglycan (GAG) secretion was studied in cultures of human fibroblasts and arterial smooth muscle cells. Supplementation of culture medium with whole human serum increased the secretion of GAG but this effect disappeared as cell density increased. Lipoprotein-free serum (LFS) supported cell growth but led to a decrease in GAG secretion and in cell cholesterol. Addition of human low density lipoprotein (LDL) to the medium containing 10% LFS produced increases in GAG secretion (200%) and cell cholesterol (300%) and a decrease (60%) in cell population. The effects of LDL were considerably smaller in medium containing 5% LFS; this was related to the lower rate of proliferation in this medium, since there was a close relationship between rate of proliferation and stimulation of GAG secretion by LDL independent of serum concentration. In addition, fetal smooth muscle cells showed a qualitatively different response to LDL in 5% LFS, with a biphasic dose-response of GAG secretion and cell number. It is concluded that: (1) whole human serum stimulates GAG secretion by sparse cell cultures, (2) LFS can support cell growth but not GAG secretion, (3) LDL stimulates GAG secretion but has a cytotoxic effect, (4) the degree of GAG stimulation by LDL is dependent on the proliferative state of cells, (5) at low serum concentrations fibroblasts and fetal smooth muscle cells show differences in response to LDL which are not evident at higher serum concentration.

Production of proteoglycans by human lung fibroblasts (IMR-90) maintained in a low concentration of serum

Maintenance of fibroblasts in 0.5% serum results in viable but non-proliferative cells that may be analogous to fibroblasts in vivo. The synthesis of proteoglycans by human embryo lung fibroblasts in Eagle's minimal essential medium with 0.5% newborn-bovine serum or with 10% serum has been compared. A similar amount of [35S]sulphate-labelled glycosaminoglycan per cell was secreted by fibroblasts in 10% or 0.5% serum. 35SO42-incorporation into sulphated glycosaminoglycans was enhanced in 0.5% serum when expressed per mg of cell protein, but [3H]glucosamine incorporation was decreased. The charge density of these glycosaminoglycans was not changed as determined by ion-exchange chromatography. It was concluded that decreased protein/ cell resulted in an apparent increase in 35S-labelled glycosaminoglycan synthesis/mg of cell protein, whereas decreased uptake of [3H]glucosamine resulted in a decrease in their glucosamine labelling. The proteoglycans secreted by fibroblasts in 0.5% serum were similar in glycosaminoglycan composition, chain length and buoyant density to the dermatan sulphate proteoglycan, which is the major secreted component of cells in 10% serum. Larger heparan sulphate and chondroitin sulphate proteoglycans, which comprise about 40% of the total secreted proteoglycans of cultures in 10% serum, were greatly diminished in the medium of cultures in 0.5% serum. The proteoglycan profile of medium from density-inhibited cultures in 10% serum resembles that of proliferating cultures, indicating that lack of proliferation was not responsible for the alteration. The dermatan sulphate proteoglycan, participating in extracellular matrix structure, may be the primary tissue product of lung fibroblasts in vivo.

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.

Changes in cell-surface glycosaminoglycans in human diploid fibroblasts during in vitro aging

Changes in glycosaminoglycans during in vitro aging were investigated in human diploid fibroblasts. The cells were found to produce predominantly hyaluronate and smaller amounts of chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate and heparan sulfate. Accumulation of heparan sulfate on the cell surface was notable during aging. Total glycosaminoglycan production in preconfluent culture did not change with population doubling level (PDL), while in confluent culture a decline in glycosaminoglycan production was observed. In contrast with this, heparan sulfate on the cell surface increased as a function of PDL in both confluent and preconfluent cultures. The distribution pattern of heparan sulfate in medium and cell surface indicated that the increase in heparan sulfate on the cell surface could be attributed to an increased accumulation on the cell surface, but not to an elevated production. Thus, we conclude that the increased accumulation of heparan sulfate on the cell surface might be involved in an age-related alteration in the cell membrane.

Effects of depletion of K+, Na+, or Ca2+ on DNA synthesis and cell cation content in chick embryo fibroblasts

Decreasing the K+ concentration of the medium from 5 mM to 0.59 mM decreased the K+ content of chick embryo fibroblasts to 22% of control values and increased the Na+ content to 820% of control values. The alteration of monovalent cation content occurred within two hours but had no effect on the rate of DNA synthesis, as measured by 3H-thymidine incorporation, for at least 16 hours. By decreasing the Na+ concentration in the medium, a 50% reduction in cellular Na+ could be obtained with no effect on thymidine incorporation. Since these changes in cellular Na+ or K+ are much larger than any known to occur under physiological conditions but have no effect on thymidine incorporation, we conclude that Na+ and K+ do not play a critical role in determining multiplication rate. Addition of 1.8 mEGTA to cells in media containing 1.7 mM Ca2+ and 0.8 mM Mg2+ inhibited thymidine incorporation and sharply decreased cellular K+ and increased cellular Na+ content. However, there was no reduction in total cellular Ca2+ levels. Likewise, decreasing the Ca2+ concentration of the medium below 0.01 mM inhibited thymidine incorporation, decreased cellular K+ and Mg2+, and increased cellular Na+ but did not affect total cellular Ca2+ levels. Inhibition of DNA synthesis, therefore, could not be correlated with changes in cellular Ca2+ levels.

The antinflammatory effects of glucocorticosteroids. A brief review of the literature

The effects of glucocorticosteroids on immune and inflammatory responses are reviewed. The steroids seem to change the number or function of cell receptors for regulating agents, so that in areas of inflammation: (a) blood vessels dilate less, (b) lymphocytes proliferate less, (c) all leukocytes infiltrate less, (d) macrophages become less activated (digesting and secreting less), and (e) fibroblasts produce less collagen and ground substance. In addition, the corticosteroids seem to alter the response of cells to various signals from their receptors by affecting the prostaglandin system, cyclic nucleotides, and perhaps other internal mediators.

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.

Coordinate control of Balb/c3T3 cell survival and multiplication by serum or calcium pyrophosphate complexes

Balb/c3T3 cells in crowded cultures detach from the dish when deprived of serum, and the survivors incorporate 3H-thymidine at a reduced rate. The detachment becomes pronounced two hours after removal of serum, and reaches its maximum rate between two and four hours. Cells in sparse culture are not detached by serum removal, and their rate of 3H-thymidine incorporation is only slightly reduced. As the sparse cultures grow into more crowded cultures, and the serum is depleted, increasing numbers detach. The detached cells are incapable of reattaching when placed in a new dish with ample fresh serum. The cells are leaky to cellular constituents and appear to be dead. Detachment is a consequence rather than the cause of cell death, and can be produced by agents which inhibit cellular energy metabolism. The cells on the dish which survive serum deprivation are fully viable and grow rapidly when serum is added. When they become crowded they are as sensitive to serum deprivation as was the original population. They are therefore not selected for a low serum requirement but apparently survive because they spread into the space vacated by the detaching cells and then behave as sparse cultures in response to serum variations. Insoluble complexes of Ca2+ and pyrophosphate (Ca2+-PPi) show the same concentration dependence in promoting cell survival as in stimulating 3H-thymidine incorporation, showing that a single substance can be responsible for both activities. It is concluded that survival and growth are part of the coordinate response of 3T3 cells to single external effectors. The results are discussed in terms of a simple model in which the coordinate response is regulated by the availability of Mg2+ for transphosphorylation reactions within the cell, and the availability depends on the binding affinity of cellular membranes for Mg2+. The difference between survival and multiplication is postulated to be in the intensity and duration rather than the kind of stimulus.

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.

Stimulatory effect of prostaglandins on the production of hexosamine-containing substances by cultured fibroblasts (4) adenosine 3':5'-cyclic monophosphate independent stimulation by prostaglandin F2alpha

The mechanism of the stimulatory effect of prostaglandin PG) F2alpha on the production of hexosamine-containing substances by cultured fibroblasts was studied with special reference to adenosine 3':5'-cyclic monophosphate (cAMP). At the stationary phase, the cells were exposed for 6 hrs to PGF2alpha, E1, cAMP or dibutyryl-cAMP in a wide range of concentrations. cAMP itself showed a slight stimulation on the production of hexosamine-containing substances, and the effect was enhanced by using the dibutyryl derivative. PGF2alpha had much a greater capacity than either the exogeneous cAMP or the dibutyryl-cAMP for enhancing the production of hexosamine-containing substances. To know whether cAMP is involved in the stimulatory effect of PGF2alpha, intracellular cAMP level was concomitantly measured in both PGF2alpha and PGE1 treated cultures. Although the cellular cAMP level in PGE1 treated cultures was much higher than that in the PGF2alpha treated cultures, the stimulatory effect on the production of hexosamine-containing substances in PGE1 treated cultures was always much smaller than that in the PGF2alpha treated cultures. Moreover, PGF2alpha had a significant stimulatory effect on the production of hexosamine-containing substances even at a low concentration as 100 pg/ml, which is small enough not to increase any cellular cAMP level. From these results, it was concluded that the stimulatory effect of PGF2alpha on the production of hexosamine-containing substances by cultured fibroblasts is not mediated by cAMP and is caused by a mechanism different from that caused by cAMP.

Reversible regulation by magnesium of chick embryo fibroblast proliferation

The rate of 3H-thymidine incorporation and of cell proliferation in chick embryo fibroblast cultures are reduced coordinately when the [Mg2+] of the external medium is reduced below the physiological concentration of about 0.8 mM. These effects of moderately reduced [Mg2+] and the accompanying change in appearance of the cells, resemble the effects produced by lowering the [serum] of the medium. Cells subjected to severe Mg2+ deprivation, especially at low [Ca2+], die and detach from the culture dish. Cells kept at a reduced rate of proliferation for three days by moderate Mg2+ deprivation are quickly restored to rapid proliferation upon restoration of the normal [Mg2+] of the medium. The rate of proliferation of the chick embryo cells is reduced markedly by lowering [Ca2+] about 100-fold, but unlike the case of Mg2+-deprivation this can occur without significant effect on the rate of 3H-thymidine incorporation. More severe Ca2+ deprivation, which does lower the rate of 3H-thymidine incorporation, produces retraction of cells from one another and from the dish, and results in a distinctly abnormal, rounded appearance. The results lend weight to the thesis that free [Mg2+] plays a central role within the cell in the coordinate control of metabolism and growth. They also suggest that the effects produced by varying [Ca2+] in the medium are caused by changes at the external surface of the cell.