Correlation of the biosynthesis of prostaglandin and cyclic AMP in monolayer cultures of rabbit articular chondrocytes

Investigation of the direct effects of salmon calcitonin on human osteoarthritic chondrocytes

BACKGROUND

Calcitonin has been demonstrated to have chondroprotective effects under pre-clinical settings. It is debated whether this effect is mediated through subchondral-bone, directly on cartilage or both in combination. We investigated possible direct effects of salmon calcitonin on proteoglycans and collagen-type-II synthesis in osteoarthritic (OA) cartilage.

METHODS

Human OA cartilage explants were cultured with salmon calcitonin [100 pM-100 nM]. Direct effects of calcitonin on articular cartilage were evaluated by 1) measurement of proteoglycan synthesis by incorporation of radioactive labeled 35SO4 [5 microCi] 2) quantification of collagen-type-II formation by pro-peptides of collagen type II (PIINP) ELISA, 3) QPCR expression of the calcitonin receptor in OA chondrocytes using four individual primer pairs, 4) activation of the cAMP signaling pathway by EIA and, 5) investigations of metabolic activity by AlamarBlue.

RESULTS

QPCR analysis and subsequent sequencing confirmed expression of the calcitonin receptor in human chondrocytes. All doses of salmon calcitonin significantly elevated cAMP levels (P < 0.01 and P < 0.001). Calcitonin significantly and concentration-dependently [100 pM-100 nM] induced proteoglycan synthesis measured by radioactive 35SO4 incorporation, with a 96% maximal induction at 10 nM (P < 0.001) corresponding to an 80% induction of 100 ng/ml IGF, (P < 0.05). In alignment with calcitonin treatments [100 pM-100 nM] resulted in 35% (P < 0.01) increased PIINP levels.

CONCLUSION

Calcitonin treatment increased proteoglycan and collagen synthesis in human OA cartilage. In addition to its well-established effect on subchondral bone, calcitonin may prove beneficial to the management of joint diseases through direct effects on chondrocytes.

Leukotriene A4 hydrolase and leukotriene C4 synthase activities in human chondrocytes: transcellular biosynthesis of Leukotrienes during granulocyte-chondrocyte interaction

OBJECTIVE

To investigate the cooperation of chondrocytes and polymorphonuclear cells (PMN) in the biosynthesis of leukotrienes (LT).

METHODS

PMN, resting and interleukin-1beta-stimulated cultured human chondrocytes, and mixtures of both cell types were incubated with A23187 and/or 14C-arachidonic acid (14C-AA). To explore the presence of LTC4 synthase and LTA4 hydrolase, the chondrocytes were incubated with authentic LTA4. Eicosanoids were analyzed using high performance liquid chromatography techniques.

RESULTS

Chondrocytes formed only prostaglandin E2 and minor amounts of 15-HETE and 11-HETE, the production of all of which was inhibited by 1 microM indomethacin. Incubation of PMN and chondrocytes produced more LTC4 from endogenous and exogenous AA, and more LTB4 from endogenous AA, than incubation of PMN alone, which was consistent with the presence of LTC4 synthase and LTA4 hydrolase activities in chondrocytes. Chondrocytes also slightly increased the level of PMN production of all 5-lipoxygenase (5-LO)-derived products from endogenous AA.

CONCLUSION

Human chondrocytes form eicosanoids from AA only by the cyclooxygenase pathway. Chondrocytes cooperate in the transcellular biosynthesis of LT since they possess LTA4 hydrolase and LTC4 synthase activities and increase metabolism by the 5-LO pathway in PMN.

Effects of three avocado/soybean unsaponifiable mixtures on metalloproteinases, cytokines and prostaglandin E2 production by human articular chondrocytes

The in-vitro effects of avocado and soybean unsaponifiable residues on neutral metalloproteinase activity, cytokines and prostaglandin E2 (PGE2) production by human articular chondrocytes were investigated. Avocado and soybean unsaponifiable residues were mixed in three ratios: 1:2 (A1S2), 2:1 (A2S1) or 1:1 (A2S2). Freshly isolated human chondrocytes were cultured for 72 h in the absence or presence of interleukin-1beta, (IL-1beta) (17 ng/ml), with or without unsaponifiable residue mixtures at a concentration of 10 microg/ml. A/S unsaponifiable residues were also tested separately at concentrations of 3.3, 6.6 and 10 microg/ml. All A/S unsaponifiable mixtures reduced the spontaneous production of stromelysin, interleukin-6 (IL-6), interleukin-8 (IL-8) and prostaglandin E2 (PGE2) by chrondrocytes. At concentrations of 3.3 and 6.6 microg/ml, A/S residues, tested separately, were potent inhibitors of the production of IL-8 and PGE2. Nevertheless, only avocado residue inhibited IL-6 production at these concentrations. A/S unsaponifiable mixtures had a more pronounced inhibitory effect on cytokine production than avocado or soybean residues added alone. As anticipated, IL-1beta induced a marked release of collagenase, stromelysin, IL-6, IL-8 and PGE2. A/S unsaponifiable mixtures partially reversed the IL-1 effects on chrondrocytes. These findings suggest a potential role for A/S unsaponifiable extracts in mitigating the deleterious effects of IL-1beta: on cartilage.

Influence of interleukin-1 beta, tumour necrosis factor alpha and prostaglandin E2 on chondrogenesis and cartilage matrix breakdown in vitro

Inflammatory mediators such as the cytokines interleukin-1 (IL-1) or (TNF alpha), and prostaglandins [predominantly prostaglandin E2 (PGE2)] are generally considered to be involved in the breakdown of cartilage matrix in chondrodestructive diseases, especially rheumatoid arthritis and osteoarthritis. Their mode of action is not yet completely understood. Blastemal cells or differentiated chondroblasts/chondrocytes of limb buds from mouse embryos (day 12) in organoid cultures provide an efficient system to investigate the mechanism of action of these substances. Using recombinant human IL-1 beta, TNF alpha and PGE2 alone or together (in pairs) in this culture system, we found that none of these substances alone could affect chondrogenesis. TNF alpha, however, when combined with IL-1 beta, proved to be the more potent cytokine causing a transformation of embryonal chondrogenic cells into fibroblast-like cells and thus inhibiting the expression of the cartilage cell phenotype. This might be due to inhibition of both the morphogenetic and cytodifferentiation phases of chondrogenesis. The well-known synergistic interaction between both cytokines seems to be phase limited and may not occur in the postchondrogenesis phase. In addition, our results showed that TNF alpha alone or combined with PGE2 caused a marked breakdown of the cartilage matrix. These in vitro findings might be useful to elucidate the complexity of interactions between different cytokines and PGE2 involved in cartilage destruction processes in vivo.

Inhibition of the effects of rheumatoid synovial fluid cells on chondrogenesis and cartilage breakdown in vitro: possible therapeutical conclusions. A morphological--biochemical study

Short-term co-cultivation of blastemal cells from 12-day-old mouse limb buds and human rheumatoid synovial fluid cells in high density cultures (Trowell culture system) resulted, depending on when co-cultivation started, either in (1) an inhibition of chondrogenesis (co-cultivation right from the start) or in (2) an extensive breakdown of cartilaginous matrix (co-cultivation after formation of embryonic cartilage). These synovial effects were markedly impeded if Avarol (a dioxygenase inhibitor) was applied singly or in combination with PAI-2 (a u-PA-inhibitor). PAI-2 alone, however, had no effect on the synovial-induced inhibition of chondrogenesis, but produced a pronounced inhibitory effect on matrix breakdown. The effects of both inhibitors were studied electron microscopically and biochemically (determination of sulfated-glycosaminoglycans in the high density cultures by Alcian Blue binding assay). The results of this study are consistent with the presumption that rheumatoid synovial cells are capable of inhibiting chondrogenesis and enhancing the breakdown of the cartilaginous matrix. Amongst others, the possible mediators involved are prostaglandins and plasminogen activators. The response to the inhibitors Avarol and PAI-2 is compatible with their mode of action. The chondroprotective action of these substances may be useful in developing potential antirheumatic drugs.

Differential effects of interleukin-1 alpha and beta on the arachidonic acid cascade in human synovial cells and chondrocytes in culture

The effects of interleukin-1 alpha and beta were tested on the [3H]-arachidonic acid release and the prostaglandin synthesis by human cultured synovial cells and chondrocytes. Both forms of interleukin-1 stimulated the arachidonic acid release but interleukin-1 beta was more potent than IL-1 alpha. Human synovial cells and chondrocytes synthesized three types of prostaglandins upon stimulation with interleukin-1 alpha or beta: prostaglandin E2, F2 alpha and 6-keto-prostaglandin F1 alpha. Regarding the synthesis of these prostaglandins, IL-1 beta was again more potent than IL-1 alpha. A comparison between interleukin-1-stimulated synovial cells and chondrocytes revealed neither significant quantitative nor qualitative differences in both the arachidonic acid release and the prostaglandin synthesis.

Characterisation of chondrocyte activation in response to cytokines synthesised by a synovial cell line

The lapine, synovial cell line, HIG-82, secretes 'chondrocyte activating factors' (CAF) which induce the synthesis of collagenase (EC 3.4.24.7), gelatinase, caseinase and prostaglandin E2 (PGE2) by confluent, monolayer cultures of lapine, articular chondrocytes. Partially purified CAF increased the production of PGE2 by chondrocytes within 3 h; in certain cultures this occurred in as little as 1 h. Increased levels of the three neutral metalloproteinases, in contrast, were only measurable in the conditioned medium after a delay of 9-18 h. After removal of the CAF, the synthesis of PGE2 reverted to basal levels within 1-4 h, but synthesis of the three proteinases remained high for an additional 4 days. Indomethacin, at concentrations which completely inhibited PGE2 synthesis, had no effect upon the coordinate induction of collagenase, gelatinase and caseinase. However, cycloheximide, alpha-amanitin and 5,6-dichlororibosylbenzimidazole (DRB) suppressed induction of these proteinases suggesting that CAF derepressed the genes coding for these enzymes. Once the chondrocytes had been activated by CAF, the inhibitors of transcription had a much weaker effect on the production of the neutral proteinases, indicating that their mRNAs may be relatively stable. In the presence of CAF, inhibition under these conditions was weaker still, possibly due to stabilisation of these mRNA molecules. Experiments with a number of compounds which modulate cellular Ca2+, cAMP or cGMP failed to support a straightforward role for these mediators in the induction of neutral metalloproteinases in chondrocytes. High concentrations of phorbol myristate acetate (PMA) provoked only a slight synthesis of these enzymes.

Calcium ionophore and phorbol myristate acetate synergistically inhibited proteoglycan biosynthesis in articular chondrocytes by prostaglandin independent mechanism

In rabbit articular chondrocytes, phorbol myristate acetate (PMA), 1,2-dioctanoyl-sn-glycerol (DG) and calcium ionophore (A23187), reduced the proteoglycan synthesis, in a dose-dependent manner. The combined treatment by PMA and A23187 resulted in an enhanced inhibition of proteoglycan production, indicating a synergistic effect. In presence of PMA or A23187, the release of prostaglandin E2 (PGE2) was dramatically increased. The addition of indomethacin and BW755c to chondrocytes stimulated by PMA or A23187, suppressed the liberation of PGE2, but did not stop the decrease of proteoglycan synthesis.

Evidence for both histamine H1 and H2 receptors on human articular chondrocytes

Using specific histamine H1 and H2 receptor antagonists, evidence is presented for the existence of both H1 and H2 receptors on human articular chondrocytes in vitro. Stimulation of the H1 receptor by histamine (range 0.18 to 17.8 mumol/l) significantly increased prostaglandin E (PGE) production, while activation of the histamine H2 receptor increased intracellular cyclic adenosine-5'-monophosphate (AMP). The histamine H1 antagonists mepyramine and tripelennamine blocked the histamine induced increase in PGE production, and the H2 antagonists cimetidine and ranitidine prevented the increase in intracellular cyclic AMP. These observations suggest that mast cell-chondrocyte interactions mediated via histamine may contribute to some of the pathophysiological changes observed in joint disease.

Stimulation of sulfated-proteoglycan synthesis by forskolin in monolayer cultures of rabbit articular chondrocytes

Forskolin, a plant cardiotonic diterpene, stimulated proteoglycan biosynthesis by chondrocytes in monolayer culture. The quantitative increase in proteoglycans was dependent on the concentration of forskolin, but was relatively independent of the presence of serum. At forskolin concentrations that stimulated proteoglycan synthesis, a significant stimulation of adenylate cyclase and cAMP was also measured. The quantitative increase in proteoglycans was characterized, qualitatively, by an increased deposition of newly synthesized proteoglycan in the cell-associated fraction. An analysis of the most dense proteoglycans (fraction dA1) in the cell-associated fraction showed that more of the proteoglycans eluted in the void volume of a Sepharose CL-2B column, indicating that an increased amount of proteoglycan aggregate was synthesized in forskolin-treated cultures. The proteoglycan monomer dA1D1 secreted into the culture medium of forskolin-stimulated cultures overlapped in hydrodynamic size with that of control cultures, although cultures stimulated with forskolin and phosphodiesterase inhibitors produced even larger proteoglycans. The hydrodynamic size of 35SO4 and 3H-glucosamine-labelled glycosaminoglycans isolated from the dA1D1 fraction of the culture medium was greater in forskolin-treated chondrocytes, especially from those in which phosphodiesterase inhibitors had been added. These results indicated that forskolin, a direct activator of chondrocyte adenylate cyclase mimicked the effects of cAMP analogues on chondrocyte proteoglycan synthesis previously reported. These results implicate activation of adenylate cyclase as a regulatory event in the biosynthesis of cartilage proteoglycans, and more specifically in the production of hydrodynamically larger glycosaminoglycans.

Articular cartilage response to arthroscopic surgery: a review of current knowledge

This review is intended to point out the many variables affecting articular cartilage around the time of arthroscopic surgery. Despite the rapid clinical recovery of arthroscopic surgery patients, there may be considerable articular cartilage inhibition related to preoperative, intraoperative, and postoperative factors. Risks to the patient and his joint can be minimized by careful management of these known variables.

Effect of a interleukin-1 like factor (mononuclear cell factor) on proteoglycan synthesis in cultured human articular chondrocytes

A partially purified monocyte factor, with Interleukin-1 properties (MCF/IL-1), enhances the proteoglycan synthesis of human neonatal articular chondrocytes in culture, and changes the repartition of these macromolecules between medium and cell layer. The size of the proteoglycan monomers, the length of the glycosaminoglycan chains and the respective levels of chondroitin-6-sulfate, chondroitin-4-sulfate and non sulfated chondroitin remain unchanged under MCF/IL-1 exposures. The addition of indomethacin reduces the stimulation effect by 60-70% only, suggesting that the MCF/IL-1 action is partially dependent on prostaglandins but seems also related to mechanisms distinct from the cyclooxygenase pathway.

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)

Histamine H2 receptors on chondrocytes derived from human, canine and bovine articular cartilage

Histamine (1-100 microM) induced a concentration-dependent increase in intracellular cyclic AMP in monolayer cultures of human, canine and foetal-bovine articular chondrocytes. The dose-response curve for histamine in each culture was progressively displaced to the right with increasing concentrations of cimetidine, an H2-receptor antagonist. The histamine-induced cyclic AMP elevation in human articular chondrocytes was also significantly decreased by ranitidine, another H2 antagonist, but not by the H1 antagonists mepyramine and chlorpheniramine. These findings indicate that histamine activates chondrocyte adenylate cyclase through an H2 receptor. The cyclic AMP response of human chondrocytes to histamine was many times greater than that measured for synovial fibroblasts under similar conditions. Such findings suggest that mast-cell-chondrocyte interactions in vivo may contribute to changed chondrocyte metabolism in joint disease.

Nonsteroidal anti-inflammatory drugs and modulation of cartilaginous changes in osteoarthritis and rheumatoid arthritis. Clinical implications

Nonsteroidal anti-inflammatory drugs have a potential for modifying the complex pathophysiologic events leading to cartilage destruction in various forms of arthritis. Following an evaluation of basic mechanisms in the pathogenesis of cartilaginous destructive lesions, the effects of nonsteroidal anti-inflammatory drugs on normal chondrocyte metabolism are discussed. Their capacity to modulate cartilage and bone lesions in experimental forms of arthritis is addressed, as is the manner in which they may modify the pathophysiology of cartilage destruction in human forms of arthritis. Different classes of nonsteroidal anti-inflammatory drugs produce different effects in certain in vivo or in vitro settings.

Effect of a monocyte cell factor (MCF) on collagen production in cultured articular chondrocytes: role of prostaglandin E2

A monocyte cell factor (MCF) inhibited the incorporation of (3H)proline into collagen of rabbit articular chondrocytes in culture, without significant effect on non-collagen protein. In addition, MCF produced a new compartmental repartition of collagen between cell layer and medium. No MCF-induced shift was observed in the relative proportion of collagens synthesized, type II remaining the major collagenous product. The inhibitory effect of MCF was not completely suppressed when prostaglandin synthesis was blocked by indomethacin. Addition of PGE2 at 12.5-25 micrograms/ml to the cultures resulted in a decrease of total collagen. Lower concentrations (0.42-0.85 microgram/ml) did not affect the total synthesis of collagen but changed its distribution between cells and medium in the same way as MCF. These results suggest that the MCF-stimulated release of PGE2 may be partially involved in the inhibitory effect observed on collagen synthesis.

Stimulation of cyclic AMP in chondrocyte cultures: effects on sulfated-proteoglycan synthesis

The effects of N6,O2'-dibutyryladenosine 3':5'-cyclic monophosphate (DBcAMP), 8-bromoadenosine 3':5'-cyclic monophosphate (8Br-cAMP), 3':5'-cyclic monophosphate (cAMP), L-isoproterenol and L-epinephrine on sulfated-proteoglycan synthesis by rabbit articular chondrocytes were compared. DBcAMP and 8Br-cAMP in the presence or absence of 3-isobutyl-1-methylxanthine (IBMX) stimulated sulfated-proteoglycan biosynthesis after 20 h of incubation. cAMP had no significant effect. Both DBcAMP and 8Br-cAMP increased the hydrodynamic size of the newly synthesized proteoglycan monomer (A1D1) relative to control cultures. By contrast, although isoproterenol and epinephrine stimulated total cAMP synthesis, neither stimulated sulfated-proteoglycan synthesis. Whereas intracellular cAMP accumulated after incubation with DBcAMP and 8Br-cAMP, this was not the case with isoproterenol whether IBMX was present or not. Thus, stimulation of sulfated-proteoglycan synthesis by cAMP analogues in chondrocyte cultures appears to be dependent on increased intracellular cAMP accumulation rather than total cAMP biosynthesis.

In vitro response of chondrocytes to mechanical loading. The effect of short term mechanical tension

Chick epiphyseal chondrocytes were grown in high density cultures for 14 days, after which the cell layers were placed in a cyclical stretching apparatus and subjected to a strain of 5.5% at a frequency of 0.2 Hz. There was a 1.4- and 1.7-fold increase in the incorporation of 35SO4 and 14C-glucosamine, respectively, into glycosaminoglycans in cultures subjected to mechanical loading for 24 h. No significant change was observed in the hydrodynamic size of the proteoglycans synthesized by chondrocytes subjected to mechanical loading. In this time period there was no increase in 3H-glycine incorporation into acid-insoluble protein, but there was a 2.4-fold increase of 3H-thymidine into DNA in cultures subjected to tensional strain. Concomitant with these changes, the cellular levels of cyclic AMP increased 2.2 times in the mechanically loaded cultures. This is discussed as a possible mechanism whereby chondrocytes respond to mechanical stimuli.

Arachidonic acid metabolism by rabbit synovial cells in culture. Studies of non-cyclooxygenase pathways

We have investigated arachidonic acid (20:4) metabolism by rabbit synovial cells in culture. The lipoxygenase products 5-HETE, 12-HETE and 15-HETE were not detected, despite the presence of a cyclooxygenase inhibitor sodium meclofenamate (20 microM), nor after incubation with ionophore A23187 (1 microM), 20:4 (10 microM), prostaglandin E2, (1 microM), N-formylmethionylleucylphenylalanine (0.01 microM), or murine spleen cell-conditioned medium. [3H]20:4 (10 microM) was incorporated into phospholipids, triacylglycerols and diacylglycerols. A majority of the 3H content of phosphatidylinositol/phosphatidylserine and of diacylglycerols was already present at 1 min, in contrast to the slower accumulation of 3H in triacylglycerols, phosphatidylcholine and phosphatidylethanolamine. The diacylglycerol fraction contained sn-glycerol-1-acyl-2-20:4. These observations are consistent with phospholipase C activity in synovial cells under those culture conditions. The products generated by these enzymes may play important roles in the physiological processes of synovium.

Estradiol and tamoxifen stimulation of lapine articular chondrocyte prostaglandin synthesis

The effect of estradiol and tamoxifen on prostaglandin (PG) synthesis by rabbit articular chondrocytes in secondary monolayer cultures was investigated. Radioimmunoassay for PGE2, PGF2 alpha, 6-oxo-PGF1 alpha and thromboxane B2 was performed on media from cultures containing estradiol and tamoxifen (10-12M-10-7M). Radiometric thin-layer chromatography was also carried out. The time course of estradiol/tamoxifen effect on chondrocyte PG synthesis was evaluated and its relationship to cell density in culture examined. Estradiol stimulated the synthesis of PGs by chondrocytes. Stimulation was noted at picomolar concentrations of estradiol without further stimulation at markedly higher concentrations. In time studies, after a lag, the effect of estradiol was present fully by 5 hrs, remained steady for 24 hrs and then declined by 48 hrs. Estradiol stimulation of PG synthesis was dependent upon chondrocyte culture plating density. Tamoxifen stimulated chondrocyte PG synthesis to relatively lower levels than estradiol. The characteristics of estradiol/tamoxifen stimulation of chondrocyte PG synthesis suggest a mechanism involving estradiol cytoplasmic receptors.

Histamine H2 receptors on foetal-bovine articular chondrocytes

The dose-response curve of histamine-induced cyclic AMP elevation in monolayer cultures of primary foetal-bovine articular chondrocytes was displaced to the right by cimetidine. In addition, H2 but not H1 antagonists prevented the histamine-induced cyclic AMP elevation, suggesting histamine activates chondrocyte adenylate cyclase through an H2 receptor.

The effect of prostaglandins on in vitro limb cartilage differentiation

A variety of studies indicate that a key event in limb chondrogenic differentiation is a cellular condensation process during which an intimate cell-cell interaction occurs that triggers cartilage differentiation by elevating cAMP levels. It has recently been demonstrated that when limb mesenchymal cells are subjected to high density monolayer culture under conditions conducive to chondrogenesis, they synthesize several prostaglandins, including PGE2 and prostacyclin, which are important local modulators of cAMP formation in a number of cells and tissues. In the present study, we demonstrate that exogenous PGE2 stimulates the in vitro chondrogenic differentiation of the subridge mesoderm of the embryonic chick limb bud. The stimulatory effect of PGE2 is greatly potentiated by the phosphodiesterase inhibitor, theophylline, suggesting its influence on chondrogenesis is mediated by its ability to increase cAMP levels. The stimulatory effect of PGE2 is dose-dependent and can be detected at a concentration as low as 10(-8)M. PGE1 is just as effective as PGE2 in stimulating in vitro chondrogenesis, whereas PGA1 and PGF1 alpha are less than half as effective. Thromboxane B2 has no effect on chondrogenesis. On the basis of our results, the possibility that endogenous prostaglandins might regulate limb cartilage differentiation by acting as local regulators of cAMP content is discussed.