Effects of prostaglandins on cyclic AMP levels in isolated cells from developing chick limbs

Effect of cocaine, ethanol or nicotine on ornithine decarboxylase activity in early chick embryo brain

Fetal drug exposure causes multiple deficits in the developing child. For both humans and animal models, the single most common drug-related problem is fetal growth suppression. This defect is associated with significant perinatal morbidity and mortality and may also be related to significant behavioral problems appearing later in life. Studies focussed on the molecular mechanism of fetal drug effects in placental models are complicated by multiple interactions of the drug with mother, placenta and fetus. Using early (76-168 h) chick embryos as a non-placental model, and three common drugs of abuse (nicotine, ethanol and cocaine) it was found that each drug suppressed the peak in fetal brain ornithine decarboxylase (ODC) activity that normally occurs at 120 h of development. For each drug, the decrease in ODC activity at 120 h was followed by a small but significant increase in ODC. Thus, although the drug-treated embryos were smaller in size, they appeared to be undergoing compensatory growth and, in fact, became equal in weight to the vehicle-treated animals, if allowed to hatch.

Effects of a putative prostaglandin E2 antagonist, AH6809, on chondrogenesis in serum-free cultures of chick limb mesenchyme

In the present study, we have examined the effects of a putative antagonist of prostaglandin E2 (PGE2), AH6809, on chondrogenesis in serum-free cultures of mesenchyme from distal tips of stage 25 chick limb buds in order to test the hypothesis that endogenous PGE2, through receptor-linked adenylate cyclase (AC), initiates differentiation of cartilage in limb mesenchyme. Daily addition of 10(-4) M concentrations of AH6809 produced marked inhibition of chondrogenesis over a 5-day period of cell culture as evaluated by Alcian green binding to cartilage matrix components. Inhibition of chondrogenesis by this compound was further shown to be reversible and treatment of cells with the antagonist limited to periods when chondrocytes had differentiated and were actively secreting cartilage-specific matrix components had little effect. Preincubation of control cells in 10(-4) M concentrations of AH6809 inhibited PGE2-induced activation of AC by greater than 80% without significant (P greater than .05) inhibition of basal activity by the antagonist. Responses to parathyroid hormone, which increased AC activity by 7-fold, and forskolin which increased AC activity by 23-fold in control cells, were also uninhibited by preincubation in AH6809. The results demonstrate that blockade of PGE2-AC linked receptors in prechondrogenic limb mesenchyme inhibits chondrogenesis supporting the hypothesis that endogenous PGE2 concentrations in undifferentiated limb mesenchyme play an initiating role in the differentiation of cartilage.

Development of PTH-responsive adenylate cyclase activity during chondrogenesis in cultured mesenchyme from chick limb buds

The present study investigated the development of parathyroid hormone (PTH)-responsive adenylate cyclase (AC) activity in chondrogenic cells differentiating from chick limb mesenchyme in culture. Mesenchyme from stage 25 chick embryos was removed from the distal tip (0.3 mm) of limb buds and cultured for a 6 day period in high density micromass cultures. Under these conditions, initial appearance of cartilage matrix and chondroblasts occurred on day 3 of culture and rapidly progressed over the next 3 days to produce, by day 6, a highly confluent and homogeneous layer of cartilage matrix and chondrocytes. Cells initially dissociated from limb mesenchyme on day 0 were essentially unresponsive to PTH, but development of AC-coupled, PTH receptors occurred rapidly during the initial 24 hours of culture. Based on data from dose-response experiments, prechondrogenic cells on day 1 of culture had synthesized their full complement of these receptors relative to fully differentiated chondrocytes in cultures at day 6. Inhibition of chondrocyte differentiation by retinoic acid did not significantly affect the initial development of AC-coupled, PTH receptors but it almost completely prevented synthesis of cartilage matrix. The results indicate that development of AC-coupled PTH receptors during chondrogenesis precedes, by at least 48 hours, overt differentiation of chondrocytes and the accumulation of cartilage-specific extracellular matrix and appears to represent one of the earliest reported events in chondrocyte differentiation. The lack of effect of retinoids on development of these receptors indicates that the inhibitory effects of retinoids on differentiating cartilage are at least somewhat specific for genes regulating synthesis of extracellular matrix molecules.

Effect of prostaglandin E2 on cyclic AMP levels in limb cells of mouse mutant brachypodism

Mouse embryo limb cells carrying either the brachypodism (bpH/bpH) mutation or its wild-type (+/+) allele were tested for their ability to accumulate cyclic AMP in response to prostaglandin E2 (PGE2) between Embryonic Days E12 and E14. Mutant cells exhibited a precocious increase in cyclic AMP. In the absence of PGE2 but in the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX), the brachypodism cells accumulated a significantly lower amount of cyclic AMP by Day E14. Limb cells carrying the bpH mutation may provide a useful experimental system to study the PGE2-cyclic AMP-cartilage differentiation interrelationship.

Responsiveness of adenylate cyclase to PGE2 and forskolin in isolated cells from micromass cultures of chick limb mesenchyme during chondrogenesis

Exogenous PGE2 stimulation of adenylate cyclase (AC) in intact and enzymatically dissociated micromass cultures of mesenchymal cells derived from the distal tip of stage 25 chick limb buds was examined over a six day period of culture. Responsiveness to PGE2 was measured in both dissociated and intact cell layers in an effort to determine if an inhibitory interaction occurred between PGE2 receptors and the extracellular matrix synthesized by differentiating chondrocytes. PGE2 responsiveness was maximal in both dissociated and intact prechondrogenic mesenchyme after 24 hours in culture and declined significantly as chondrocyte differentiation occurred on days 3 and 6. Equivalent activation of AC activity by PGE2 at each time point examined was noted in both cell groups. In contrast to the decreased responsiveness of differentiating chondrocytes to PGE2, stimulation of AC by forskolin resulted in increased levels of activity in differentiating chondrocytes of both cell groups between days 3-6. The results of the present study demonstrate that the decline in PGE2 responsiveness of differentiating chondrocytes most likely involves specific changes in the PGE2 receptor complex and not in either the interaction of the receptor with extracellular matrix components or a reduction in the available pool of AC present.

Stimulation of limb cartilage differentiation by cyclic AMP is dependent on cell density

Cyclic AMP (cAMP) has been implicated in the regulation of limb cartilage differentiation. This study represents an attempt to clarify potential mechanisms by which cAMP might regulate chondrogenesis. We have found that the ability of cAMP to stimulate limb cartilage differentiation in vitro is dependent on cell density. Dibutyryl cAMP (dbcAMP) elicits a striking increase in the accumulation of Alcian blue, pH 1.0-positive cartilage matrix, and a corresponding three- to fourfold increase in the accumulation of 35S-labeled glycosaminoglycans (GAG) by limb mesenchymal cells cultured in low serum medium at densities greater than confluence (i.e. micromass cultures established with 1-2 x 10(5) cells in 10 microliters of medium). Moreover, dbcAMP causes a striking (two- to fourfold) increase in the steady-state cytoplasmic levels of mRNAs for cartilage-characteristic type II collagen and the core protein of cartilage-specific sulfated proteoglycan in these high density, supraconfluent cultures. In contrast, cAMP does not promote the chondrogenesis of limb mesenchymal cells cultured at subconfluent densities (i.e. cultures initiated with 2.5-5 x 10(4) cells in 10 microliters of medium). In these low density cultures, dbcAMP does not promote the formation of cartilage matrix, sulfated GAG accumulation or the accumulation of cartilage-specific mRNAs. These observations suggest that cAMP may exert its regulatory effect in part by facilitating cell-cell communication during the critical condensation phase of chondrogenesis.

Chondrogenesis in chick limb mesenchyme in vitro derived from distal limb bud tips: changes in cyclic AMP and in prostaglandin responsiveness

Chondrogenesis was monitored in micromass cultures of mesenchymal cells derived from the distal tip of stage-25 chick limb buds over a 6-day period. Alcian green staining and immunofluorescent localization of cartilage-specific proteoglycans revealed the appearance of cartilage matrix by day 3 of cell culture. By day 6, cultures contained a uniform and homogeneous population of fully differentiated chondrocytes throughout the cell layer, with only a narrow rim of nonchondrogenic cells around the extreme periphery of the culture. Synthesis of sulfated glycosaminoglycans also progressively increased between days 3 and 6, being 8-fold higher at day 6 than at day 1 of culture. Both adenylate cyclase (AC) activity and cAMP concentrations increased dramatically during the first 2 days of culture, reaching maximal levels by day 2, which remained elevated and stable throughout the remaining chondrogenic period (days 3-6). Responsiveness of both AC and cAMP concentrations of the cells to PGE2 was maximal by day 1 of culture and was increased over control cells by 12-fold and 8-fold respectively. Both responses, however, were dramatically reduced by day 3, at which time the initiation of cartilage formation was apparent. Responsiveness of cells during the prechondrogenic period to PGE2 was relatively specific in that no effects could be demonstrated with equivalent concentrations of PGF2 alpha or 6-keto-PGF1 alpha, although PGl2 did produce increases in cAMP concentrations of about 50% of those of PGE2. These results indicate that previously reported changes in the cAMP system in heterogeneous cell cultures derived from whole limb buds reflect changes occurring in the chondrogenic cell type and indicate further that peak responsiveness of the cAMP system of these cells to prostaglandins is restricted to prechondrogenic developmental periods.

The effect of prostaglandins on the cyclic AMP content of limb mesenchymal cells

We have been investigating the hypothesis that prostaglandins including prostaglandin E2 (PGE2) produced during the critical condensation phase of limb chondrogenesis are involved in the regulation of cartilage differentiation by acting as local modulators of cyclic AMP (cAMP) accumulation. The purpose of the present study was to determine directly whether PGE2 and other prostanoids which had previously been shown to stimulate in vitro chondrogenic differentiation do indeed elevate the cAMP content of limb mesenchymal cells, and to determine whether the ability of various prostanoids to increase cAMP production by these cells directly reflects the potencies of these same molecules in stimulating chondrogenesis. We have found that PGE2 does indeed elicit a striking elevation in the cAMP content of subridge mesenchymal cells, indicating that the cells possess adenylate cyclase-coupled receptors for this molecule. The effect of PGE2 on cAMP accumulation is potentiated by a phosphodiesterase inhibitor, thus paralleling the potentiating effect phosphodiesterase inhibitors have on PGE2-stimulated in vitro chondrogenesis. The effect of PGE2 on cAMP content is dose-dependent with a 3-fold increase seen at 10(-8)M, which is the lowest concentration at which PGE2 effectively stimulates chondrogenesis. PGE1, which is just as effective as PGE2 in stimulating chondrogenesis, is just as effective as PGE2 in stimulating cAMP accumulation. PGA1, which is a much less effective stimulator of chondrogenesis than PGE2 or PGE1, is less than half as potent as these molecules in elevating cAMP levels. PGF1 alpha, 6-keto PGF1 alpha, and thromboxane B2, which have little or no effect on chondrogenesis, have little or no effect on cAMP content.(ABSTRACT TRUNCATED AT 250 WORDS)

Limb development in chick embryos: cyclic AMP-dependent protein kinase activity, cyclic AMP, and prostaglandin concentrations during cytodifferentiation and morphogenesis

The effects of prostaglandin E2 (PGE2) on cyclic AMP (cAMP) concentrations of chick limb bud cells obtained from limbs at various stages of development were investigated. In addition, endogenous concentrations of PGE2 were examined in whole limbs from comparable stages. Prior to either chondrogenesis or myogenesis (stages 20-23), cells were more responsive to PGE2, in terms of cAMP levels, than those of differentiated phenotypes, obtained at stages 25-28. This greater responsiveness to PGE2 of undifferentiated cells was correlated with endogenous concentrations of PGE2 which were significantly higher in undifferentiated limbs than in limbs containing differentiated cartilage and muscle. Cyclic AMP-dependent protein kinase (PKA) activity was detectable in cell homogenates at each stage examined and did not appear to change in cAMP dependency at any stage. The majority (80-85%) of total enzyme activity was localized in soluble fractions of cell homogenates while the residual activity was localized to membrane-enriched, particulate fractions. The results demonstrate that both responsiveness of limb mesenchyme to PGE2 and endogenous concentrations of PGE2 are maximal prior to cytodifferentiation of limb tissues. The presence of cAMP-dependent protein kinase in these undifferentiated cells supports a regulatory role for both PGE2 and a cAMP-protein phosphorylation system in the differentiation of limb tissues.

Effects of compressive forces on proliferation and matrix synthesis in mandibular condylar cartilage of the rat in vitro

The cartilage from 4-day-old rats were exposed to continuous and intermittent compressive forces. Proliferative activity and matrix synthesis in vitro were determined by measuring the incorporation of [3H]-thymidine, [35S]-sulphate and [3H]-proline by autoradiography and liquid-scintillation counting. The findings suggest that a continuously-applied force of approx. 0.5 g stimulated proliferation in the condylar cartilage but reduced the synthesis of the sulphated glycosaminoglycans and collagen. A stimulatory influence on the synthesis of these matrix components, on the other hand, was observed when the condylar cartilage was exposed to an intermittent force (0.7 Hz) of approx. 0.5-1.0 g; this intermittent force, however, reduced the proliferative activity. It appears that, at least in vitro, the basic growth processes in the condylar cartilage can be regulated by compressive forces.

Chondrogenesis of chick limb mesenchyme in vitro. Effects of prostaglandins on cyclic AMP

The effects of prostaglandin E2 (PGE2) on cyclic AMP (cAMP) levels of chick limb bud cells during various stages of chondrogenesis were studied utilizing high density, micro-mass, cell culture. Concentrations of PGE2 in cell cultures at these same stages were measured by radio-immunoassay. Both basal levels of cAMP, as well as PGE2-stimulated changes in cAMP, increased maximally during the first 3 days of culture; this increase was associated with the formation of cell aggregates. Concentrations of PGE2 were also highest during this period. By 6 days of culture, nodules, containing cartilage matrix components, predominated. Both basal levels of cAMP and PGE2-stimulated cAMP levels were significantly decreased at this stage, relative to cultures at day 3. Concentrations of PGE2 fell dramatically in the 6-day cultures containing differentiated cartilage. These results support a regulatory role for both PGE2 and cAMP in the early events associated with chondrogenesis.