Regulation of protein kinase C by transforming growth factor beta 1 in rat costochondral chondrocyte cultures

Lysophosphatidic acid signaling promotes proliferation, differentiation, and cell survival in rat growth plate chondrocytes

Growth plate cartilage is responsible for long bone growth in children and adolescents and is regulated by vitamin D metabolites in a cell zone-specific manner. Resting zone chondrocytes (RC cells) are regulated by 24,25-dihydroxyvitamin D3 via a phospholipase D-dependent pathway, suggesting downstream phospholipid metabolites are involved. In this study, we showed that 24R,25(OH)2D3 stimulates rat costochondral RC chondrocytes to release lysophosphatidic acid (LPA) and, therefore sought to determine the role of LPA signaling in these cells. RC cells expressed the G-protein coupled receptors LPA1-5 and peroxisome proliferator-activated receptor gamma (PPAR-gamma). LPA and the LPA1/3 selective agonist OMPT increased proliferation and two maturation markers, alkaline phosphatase activity and [35S]-sulfate incorporation. LPA and 24R,25(OH)2D3's effects were inhibited by the LPA1/3 selective antagonist VPC32183(S). Furthermore, apoptosis induced by either inorganic phosphate or chelerythrine was attenuated by LPA, based on DNA fragmentation, TUNEL staining, caspase-3 activity, and Bcl-2:Bax protein ratio. LPA prevented apoptotic signaling by decreasing the abundance, nuclear localization, and transcriptional activity of the tumor-suppressor p53. LPA treatment also regulated the expression of the p53-target genes Bcl-2 and Bax to enhance cell survival. Collectively, these data suggest that LPA promotes differentiation and survival in RC chondrocytes, demonstrating a novel physiological function of LPA-signaling.

1alpha,25(OH)2D3 is an autocrine regulator of extracellular matrix turnover and growth factor release via ERp60 activated matrix vesicle metalloproteinases

Growth plate chondrocytes produce proteoglycan-rich type II collagen extracellular matrix (ECM). During cell maturation and hypertrophy, ECM is reorganized via a process regulated by 1alpha,25(OH)(2)D(3) and involving matrix metalloproteinases (MMPs), including MMP-3 and MMP-2. 1alpha,25(OH)(2)D(3) regulates MMP incorporation into matrix vesicles (MVs), where they are stored until released. Like plasma membranes (PM), MVs contain the 1alpha,25(OH)(2)D(3)-binding protein ERp60, phospholipase A(2) (PLA(2)), and caveolin-1, but appear to lack nuclear Vitamin D receptors (VDRs). Chondrocytes produce 1alpha,25(OH)(2)D(3) (10(-8)M), which binds ERp60, activating PLA(2), and resulting lysophospholipids lead to MV membrane disorganization, releasing active MMPs. MV MMP-3 activates TGF-beta1 stored in the ECM as large latent TGF-beta1 complexes, consisting of latent TGF-beta1 binding protein, latency associated peptide, and latent TGF-beta1. Others have shown that MMP-2 specifically activates TGF-beta2. TGF-beta1 regulates 1alpha,25(OH)(2)D(3)-production, providing a mechanism for local control of growth factor activation. 1alpha,25(OH)(2)D(3) activates PKCalpha in the PM via ERp60-signaling through PLA(2), lysophospholipid production, and PLCbeta. It also regulates distribution of phospholipids and PKC isoforms between MVs and PMs, enriching the MVs in PKCzeta. Direct activation of MMP-3 in MVs requires ERp60. However, when MVs are treated with 1alpha,25(OH)(2)D(3), PKCzeta activity is decreased and PKCalpha is unaffected, suggesting a more complex feedback mechanism, potentially involving MV lipid signaling.

Transforming growth factor-beta1 regulation of growth zone chondrocytes is mediated by multiple interacting pathways

Transforming growth factor beta 1 (TGF-beta1) affects growth plate chondrocytes through Smad-mediated mechanisms and has been shown to increase protein kinase C (PKC). This study determined if PKC mediates the physiological response of rat costochondral growth zone (GC) chondrocytes to TGF-beta1; if the physiological response occurs via type II or type III TGF-beta receptors, and, if so, which receptor mediates the increase in PKC; and the signal transduction pathways involved. Treatment of confluent GC cells with TGF-beta1 stimulated [(3)H]thymidine and [(35)S]sulfate incorporation as well as alkaline phosphatase (ALPase) and PKC specific activities. Inhibition of PKC with chelerythrine, staurosporine, or H-7 caused a dose-dependent decrease in these parameters, indicating that PKC signaling was involved. TGF-beta1-dependent PKC and the physiological response of GC cells to TGF-beta1 was reversed by anti-type II TGF-beta receptor antibody and soluble type II TGF-beta receptor, showing that TGF-beta1 mediates these effects through the type II receptor. The increase in [3H]thymidine incorporation and ALPase specific activity were also regulated by protein kinase A (PKA) signaling, since the effects of TGF-beta1 were partially blocked by the PKA inhibitor H-8. The mechanism of TGF-beta1 activation of PKC is through phospholipase A(2) (PLA(2)) and not through phospholipase C (PLC). Arachidonic acid increased PKC in control cultures and was additive with TGF-beta1. Prostanoids are required, as indomethacin blocked the effect of TGF-beta1, and Cox-1, but not Cox-2, is involved. TGF-beta1 stimulates prostaglandin E(2) (PGE(2)) production and exogenous PGE(2) stimulates PKC, but not as much as TGF-beta1, suggesting that PGE(2) is not sufficient for all of the prostaglandin effect. In contrast, TGF-beta1 was not regulated by diacylglycerol; neither dioctanoylglycerol (DOG) nor inhibition of diacylglycerol kinase with R59022 had an effect. G-proteins mediate TGF-beta1 signaling at different levels in the cascade. TGF-beta1-dependent increases in PGE(2) levels and PKC were augmented by the G protein activator GTP gamma S, whereas inhibition of G-protein activity via GDP beta S, pertussis toxin, or cholera toxin blocked stimulation of PKC by TGF-beta1, indicating that both G(i) and G(s) are involved. Inhibition of PKA with H-8 partially blocked TGF-beta1-dependent PKC, suggesting that PKA inhibition on the physiological response was via PKA regulation of PKC signaling. This indicates that multiple interacting signaling pathways are involved: TGF-beta1 stimulates PLA(2) and prostaglandin release via the action of Cox-1 on arachidonic acid. PGE(2) activates the EP2 receptor, leading to G-protein-dependent activation of PKA. PKA signaling results in increased PKC activity and PKC signaling regulates proliferation, differentiation, and matrix synthesis.

Parathyroid hormone and transforming growth factor-beta1 coregulate chondrocyte differentiation in vitro

Parathyroid hormone (1-34) (PTH(1-34) and transforming growth factor-beta1 (TGF-beta1) regulate chondrocyte proliferation, differentiation, and matrix synthesis. Both proteins mediate their effects in a dose- and time-dependent manner, and the effects are cell maturation specific. Moreover, similar signaling pathways are used, suggesting that there may be cross talk leading to coregulated cell response. To test this hypothesis, confluent cultures of rat costochondral resting zone and growth zone chondrocytes were treated with 0.22, 0.44, or 0.88 ng/mL of rhTGF-beta1 for 24 h, followed by treatment with 10(-11) to 10(-8) M PTH(1-34) for 10 min or 24 h. [3H]-Thymidine incorporation, specific activity of alkaline phosphatase (AP), and [35S]-sulfate incorporation were measured. PTH(1-34) had no effect on [3H]-thymidine incorporation by growth zone cells pretreated with 0.22 or 0.44 ng/mL of TGF-beta1, but in cultures treated with 0.88 ng/mL, PTH(1-34) caused a dose-dependent decrease that was maximal at the lowest concentration tested. By contrast, PTH(1-34) stimulated [3H]-thymidine incorporation by resting zone cells, and this effect was additive with the stimulation caused by 0.22 ng/mL of TGF-beta1. PTH(1-34) caused a synergistic increase in AP in growth zone cells treated with 0.44 or 0.88 ng/mL of TGF-beta1, but not in cells treated with 0.22 ng/mL of TGF-beta1. It had no effect on AP in resting zone cells pretreated with any concentration of TGF-beta1. PTH(1-34) increased [35S]-sulfate incorporation in growth zone and resting zone cell cultures treated with 0.22 ng/mL of TGF-beta1 to levels seen in cultures treated with 0.88 ng/mL of TGF-beta1 alone. These results support the hypothesis that PTH(1-34) and TGF-beta1 coregulate growth plate chondrocytes and that the effects are cell maturation dependent.

Transforming growth factor-beta1 regulation of resting zone chondrocytes is mediated by two separate but interacting pathways

Previous studies have shown that transforming growth factor-beta1 (TGF-beta1) stimulates protein kinase C (PKC) via a mechanism that is independent of phospholipase C or tyrosine kinase, but involves a pertussis toxin-sensitive G-protein. Maximal activation occurs at 12 h and requires new gene expression. To understand the signaling pathways involved, resting zone chondrocytes were incubated with TGF-beta1 and PKC activity was inhibited with chelerythrine, staurosporine or H-7. [(35)S]Sulfate incorporation was inhibited, indicating that PKC mediates the effects of TGF-beta1 on matrix production. However, there was little, if any, effect on TGF-beta1-dependent increases in [(3)H]thymidine incorporation, and TGF-beta1-stimulated alkaline phosphatase was unaffected, indicating that these responses to the growth factor are not regulated via PKC. TGF-beta1 caused a dose-dependent increase in prostaglandin E(2) (PGE(2)) production which was further increased by PKC inhibition. The increase was regulated by TGF-beta1-dependent effects on phospholipase A(2) (PLA(2)). Activation of PLA(2) inhibited TGF-beta1 effects on PKC, and inhibition of PLA(2) activated TGF-beta1-dependent PKC. Exogenous arachidonic acid also inhibited TGF-beta1-dependent increases in PKC. The effects of TGF-beta1 on PKC involve genomic mechanisms, but not regulation of existing membrane-associated enzyme, since no direct effect of the growth factor on plasma membrane or matrix vesicle PKC was observed. These results support the hypothesis that TGF-beta1 modulates its effects on matrix production through PKC, but its effects on alkaline phosphatase are mediated by production of PGE(2) and protein kinase A (PKA). Inhibition of PKA also decreases TGF-beta1-dependent proliferation. We have previously shown that PGE(2) stimulates alkaline phosphatase through its EP2 receptor, whereas EP1 signaling causes a decrease in PKC. Thus, there is cross-talk between the two pathways.

Growth plate chondrocytes store latent transforming growth factor (TGF)-beta 1 in their matrix through latent TGF-beta 1 binding protein-1

Osteoblasts produce a 100 kDa soluble form of latent transforming growth factor beta (TGF-beta) as well as a 290 kDa form containing latent TGF-beta binding protein-1 (LTBP1), which targets the latent complex to the matrix for storage. The nature of the soluble and stored forms of latent TGF-beta in chondrocytes, however, is not known. In the present study, resting zone and growth zone chondrocytes from rat costochondral cartilage were cultured to fourth passage and then examined for the presence of mRNA coding for LTBP1 protein. In addition, the matrix and media were examined for LTBP1 protein and latent TGF-beta. Northern blots, RT-PCR, and in situ hybridization showed that growth zone cells expressed higher levels of LTBP1 mRNA in vitro than resting zone cells. Immunohistochemical staining for LTBP1 revealed fine fibrillar structures around the cells and in the cell matrix. When the extracellular matrix of these cultures was digested with plasmin, LTBP1 was released, as determined by immunoprecipitation. Both active and latent TGF-beta1 were found in these digests by TGF-beta1 ELISA and Western blotting. Immunoprecipitation demonstrated that the cells also secrete LTBP1 which is not associated with latent TGF-beta, in addition to LTBP1 that is associated with the 100 kDa latent TGF-beta complex. These studies show for the first time that latent TGF-beta is present in the matrix of costochondral chondrocytes and that LTBP1 is responsible for storage of this complex in the matrix. The data suggest that chondrocytes are able to regulate both the temporal and spatial activation of latent TGF-beta, even at sites distant from the cell, in a relatively avascular environment.

Treatment of resting zone chondrocytes with transforming growth factor-beta 1 induces differentiation into a phenotype characteristic of growth zone chondrocytes by downregulating responsiveness to 24,25-(OH)2D3 and upregulating responsiveness to 1,25-(OH)2D3

To determine if transforming growth factor-beta 1 (TGF-beta 1) can induce the differentiation of resting zone (RC) chondrocytes, confluent, fourth passage cultures of these cells were pretreated for 24, 36, 48, 72, and 120 h with TGF-beta 1. At the end of pretreatment, the media were replaced with new media containing 10(-10)-10(-8) mol/L 1,25-(OH)2D3 and the cells incubated for an additional 24 h. This second treatment was chosen because prior studies had shown that only the more mature growth zone (GC) chondrocytes respond to this vitamin D3 metabolite. The effect of TGF-beta pretreatment on cell maturation was assessed by measuring alkaline phosphatase (ALPase)-specific activity. In addition, changes in matrix protein synthesis were assessed by measuring collagen synthesis, as well as 35SO4 incorporation into proteoglycans. When RC cells were pretreated for 120 h with TGF-beta 1, treatment with 1,25-(OH)2D3 caused a dose-dependent increase in ALPase-specific activity and collagen synthesis, with no effect on proteoglycan production. RC cells pretreated with 1,25(OH)2D3 responded like RC cells that had not received any pretreatment. RC cells normally respond to 24,25-(OH)2D3; however, RC cultures pretreated for 120 h with TGF-beta 1 lost their responsiveness to 24,25-(OH)2D3. These results indicate that TGF-beta 1 directly regulates the maturation of RC chondrocytes into GC chondrocytes and support the hypothesis that this growth factor may play a significant role in regulating chondrocyte maturation during endochondral ossification.

17beta-estradiol regulation of protein kinase C activity in chondrocytes is sex-dependent and involves nongenomic mechanisms

17Beta-estradiol (E2) regulates growth plate chondrocyte differentiation in both a sex- and cell maturation-dependent manner, and the sex-specific effects of E2 appear to be mediated in part by membrane events. In this study, we examined whether E2 regulates protein kinase C (PKC) in a cell-maturation and sex-specific manner and whether E2 uses a nongenomic mechanism in regulating this enzyme. In addition, we determined if PKC mediates the E2-dependent stimulation of alkaline phosphatase activity seen in chondrocytes. Confluent, fourth passage resting zone (RC) and growth zone (GC) chondrocytes from male and female rat costochondral cartilage were treated with 10(-10) to 10(-7) M E2. E2 caused a dose-dependent increase in PKC in RC and GC cells from female rats. Peak stimulation was at 90 min. Increased PKC was evident by 3 min in both RC and GC and was still evident in RC cells at 720 min, but in GC cells activity returned to baseline by 270 min. Actinomycin D had no effect at 9, 90, 270, or 720 min, but there was a small decrease in E2-stimulated PKC in RC treated with cycloheximide at 90 and 270 min and in GC treated for 90 min. E2 increased cytosolic and membrane PKC at 9 min and by 90 min promoted translocation of PKC activity from the cytosol to the membranous compartment of female RC cells. Antibodies specific for the alpha, beta, delta, epsilon, and zeta isoforms of PKC revealed that PKCalpha in female GC and RC cells is activated by E2. There was a small, but statistically significant, increase in PKC in male RC cells in response to E2, but it was not dose-dependent, and no effect of E2 was noted in male GC cells. 17Alpha-estradiol, an inactive isomer of E2, did not affect PKC specific activity in RC or GC cells from either female or male rats. Chelerythrine, a specific inhibitor of PKC, inhibited E2-dependent alkaline phosphatase activity, indicating that E2 mediates its rapid effects on alkaline phosphatase via PKC.

1,25(OH)2D3 regulates protein kinase C activity through two phospholipid-dependent pathways involving phospholipase A2 and phospholipase C in growth zone chondrocytes

We have previously shown that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a major role in growth zone chondrocyte (GC) differentiation and that this effect is mediated by protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 1,25(OH)2D3 to stimulate PKC activation. Confluent, fourth passage GC cells from costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of GC cultures with 1,25(OH)2D3 elicited a dose-dependent increase in both inositol-1,4,5-trisphosphate and diacylglycerol (DAG) production, suggesting a role for phospholipase C and potentially for phospholipase D. Addition of dioctanoylglycerol to plasma membranes isolated from GCs increased PKC activity. Neither pertussis toxin nor choleratoxin had an inhibitory effect on PKC activity in control or 1,25(OH)2D3-treated GCs, indicating that neither Gi nor Gs proteins were involved. Phospholipase A2 inhibitors, quinacrine, OEPC (selective for secretory phospholipase A2), and AACOCF3 (selective for cytosolic phospholipase A2), and the cyclooxygenase inhibitor indomethacin decreased PKC activity, while the phospholipase A2 activators melittin and mastoparan increased PKC activity in GC cultures. Arachidonic acid and prostaglandin E2, two downstream products of phospholipase A2 action, also increased PKC activity. These results indicate that 1,25(OH)2D3-dependent stimulation of PKC activity is regulated by two distinct phospholipase-dependent mechanisms: production of DAG, primarily via phospholipase C and production of arachidonic acid via phospholipase A2.

The synergistic effects of vitamin D metabolites and transforming growth factor-beta on costochondral chondrocytes are mediated by increases in protein kinase C activity involving two separate pathways

Transforming growth factor-beta (TGFbeta), as well as the vitamin D3 metabolites 1,25-dihydroxyvitamin D3 (1,25) and 24,25-dihydroxyvitamin D3 (24,25), regulate chondrocyte differentiation and maturation during endochondral bone formation. Both the growth factor and secosteroids also affect protein kinase C (PKC) activity, although each has its own unique time course of enzyme activation. Vitamin D3 metabolite effects are detected soon after addition to the media, whereas TGFbeta effects occur over a longer term. The present study examines the interrelation between the effects of 1,25, 24,25, and TGFbeta on chondrocyte differentiation, matrix production, and proliferation. We also examined whether the effect is hormone-specific and maturation-dependent and whether the effect of combining hormone and growth factor is mediated by PKC. This study used a chondrocyte culture model developed in our laboratory that allows comparison of chondrocytes at two stages of differentiation: the more mature growth zone (GC) cells and the less mature resting zone chondrocyte (RC) cells. Only the addition of 24,25 with TGFbeta showed synergistic effects on RC alkaline phosphatase-specific activity (ALPase). No similar effect was found when 24,25 plus TGFbeta was added to GC cells or when 1,25 plus TGFbeta were added to GC or RC cells. The addition of 1,25 plus TGFbeta and 24,25 plus TGFbeta to GC and RC cells, respectively, produced a synergistic increase in [35S]sulfate incorporation and had an additive effect on [3H]thymidine incorporation. To examine the signal transduction pathway involved in producing the synergistic effect of 24,25 and TGFbeta on RC cells, the level of PKC activity was examined. Addition of 24,25 and TGFbeta for 12 h produced a synergistic increase in PKC activity. Moreover, a similar effect was found when 24,25 was added for only the last 90 min of a 12-h incubation. However, a synergistic effect could not be found when 24,25 was added for the last 9 min or the first 90 min of incubation. To further understand how 24,25 and TGFbeta may mediate the observed synergistic increase in PKC activity, the pathways potentially leading to activation of PKC were examined. It was found that 24,25 affects PKC activity through production of diacylglycerol, not through activation of G protein, whereas TGFbeta only affected PKC activity through G protein. The results of the present study indicate that vitamin D metabolites and TGFbeta produced a synergistic effect that is maturation-dependent and hormone-specific. Moreover, the synergistic effect between 24,25 and TGFbeta was mediated by activation of PKC through two parallel pathways: 24,25 through diacylglycerol production and TGFbeta through G protein activation.

Extracellular ATP modulates [Ca2+]i in retinoic acid-treated embryonic chondrocytes

When treated with low doses of retinoic acid (RA), cephalic chondrocytes of the chick embryonic sternum mature and express phenotypic characteristics of postmitotic hypertrophic cells. In concert with these maturation-dependent changes, cells release adenine nucleotides into the culture medium. To ascertain if these compounds modulate chondrocyte function, we challenged chondrocytes with nucleotides and measured one determinant of the signal transduction pathway, intracellular Ca2+ concentration ([Ca2+]i). In the presence of micromolar concentrations of ATP, there was a dose-dependent elevation in chondrocyte [Ca2+]i; ADP caused a small but significant rise in the peak [Ca2+]i response. We found that the change in the [Ca2+]i response is linked to retinoid-dependent maturation of chondrocytes. Thus the [Ca2+]i rise was dependent on the RA concentration and treatment time. Immature caudal chondrocytes, cells that were not affected by RA, were used as control cells for this study. When treated with ATP, these cells did not exhibit a [Ca2+]i response. Although the purinergic subtype receptor was not characterized, the observation that cells responded to ATP and ADP but were refractory to AMP and adenosine suggested that P2 purinoceptors were expressed by chondrocytes. Because, during the same culture period, chondrocytes exhibited many of the unique characteristics of the terminally differentiated cell, the acquisition of purinergic receptors represents a new feature associated with expression of the mature phenotype. Finally, to ascertain if the ATP-dependent response was due to release of Ca2+ from intracellular stores, cells were treated with thapsigargin. Since this compound significantly reduced the [Ca2+]i signal, we concluded that the ATP response is mediated by release of cation, from the endoplasmic reticulum.

Consequences of vitamin D receptor regulation for the 1,25-dihydroxyvitamin D3-induced 24-hydroxylase activity in osteoblast-like cells: initiation of the C24-oxidation pathway

A direct relationship between vitamin D receptor (VDR) level and target cell responsiveness to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) has been shown in osteoblast-like cell lines. However, we previously found an inverse relationship between the TGF beta-induced VDR up-regulation and subsequent 1,25-(OH)2D3-induced biological responses. A clear inhibition of the 1,25-(OH)2D3-induced stimulation of osteocalcin and osteopontin expression was observed. A biological response that has formerly been shown to be coupled to VDR level is 24-hydroxylase activity. This enzyme initiates the C24 oxidation of the side-chain, followed by cleavage and ultimate metabolic clearance of both 25-(OH)D3 and its metabolite 1,25-(OH)2D3. With UMR 106 (rat) and MG 63 (human) osteoblast-like cells, we show that after preincubation with TGF beta, which causes an increase in VDR level, 1,25-(OH)2D3 induction of 24-hydroxylase activity is also stimulated. In addition, we provide evidence that variations in VDR level induced by other means (PTH, EGF, medium change) are also closely associated with 1,25-(OH)2D3-induced 24-hydroxylase activity. Furthermore, we show that in MG 63 cells, but not in UMR 106 cells, TGF beta itself was able to increase the activity of the enzyme 24-hydroxylase. As 24-hydroxylation is the initial step in the further C24 oxidation of 1,25-(OH)2D3, our results indicate a close coupling of VDR level and the degradation of its ligand, 1,25-(OH)2D3. This mechanism may provide an important regulatory feedback in the action of 1,25-(OH)2D3 at target tissue/cell level.

24,25-(OH)2D3 regulates protein kinase C through two distinct phospholipid-dependent mechanisms

We have previously shown that 24,25-(OH)2D3 plays a major role in resting zone (RC) chondrocyte differentiation and that this vitamin D metabolite regulates protein kinase C (PKC). The aim of the present study was to identify the signal transduction pathway used by 24,25-(OH)2D3 to stimulate PKC activation. Confluent, fourth passage RC cells from rat costochondral cartilage were used to evaluate the mechanism of PKC activation. Treatment of RC cultures with 24,25-(OH)2D3 for 90 min produced a dose-dependent increase in diacylglycerol (DAG). Addition of R59022, a diacylglycerol kinase inhibitor, significantly increased PKC activity in cultures treated with 24,25-(OH)2D3. Addition of dioctanoylglycerol (DOG) to plasma membranes isolated from RC increased PKC activity 447-fold. Addition of pertussis toxin or cholera toxin to control cultures elevated basal PKC activity. When added together with 10(-9) M 24,25-(OH)2D3, there was an additive effect on PKC activity but in cultures treated with 10(-8) M 24,25-(OH)2D3, only the hormone-dependent stimulation of PKC was observed. The phospholipase C inhibitor, U73-122, had no effect on PKC activity, indicating that the DAG produced in response to 24,25-(OH)2D3 is not derived from phosphatidylinositol. Addition of the tyrosine kinase inhibitor, genistein, also had no effect on 24,25-(OH)2D3-stimulated PKC, further supporting the hypothesis that phospholipase C is not involved in the mechanism and that phospholipase D is responsible for the increase in DAG production. Phospholipase A2 inhibitors, quinacrine and AACOCF3, and the cyclooxygenase inhibitor indomethacin increased PKC activity in the RC cultures. Exogenous PGE2, one of the downstream products of phospholipase A2 action, inhibited PKC activity. These results suggest that 24,25-(OH)2D3 regulates PKC activity by two distinct phospholipid-dependent mechanisms: production of DAG via phospholipase D and inhibition of the production of PGE2 via inhibition of phospholipase A2 and cyclooxygenase.

The synergistic effect of TGF beta and 24,25-(OH)2D3 on resting zone chondrocytes is metabolite-specific and mediated by PKC

Transforming growth factor-beta (TGF beta) and 24,25-(OH)2D3 play a major role in chondrocyte maturation during endochondral bone formation. TGF beta and vitamin D metabolites when added separately to resting zone (RC) or growth zone (GC) chondrocyte cultures, activate protein kinase C (PKC). The present study determined whether there is an additive or synergistic effect of vitamin D3 metabolites and TGF beta on alkaline phosphatase and PKC specific activities and whether this effect is cell maturation-dependent. GC and RC chondrocytes were isolated from rat costochondral cartilage and cultured to fourth passage. The cells were incubated with vitamin D3 metabolites and TGF beta alone or in combination, and the specific activity of alkaline phosphatase, as well as the specific activity of PKC, were measured. The addition of 24,25-(OH)2D3 with TGF beta to RC cells caused a synergistic effect on alkaline phosphatase activity; this result was not found if the vitamin D3 metabolite was replaced by 1,25-(OH)2D3. The addition of 1,25-(OH)2D3 or 24,25-(OH)2D3 with TGF beta on GC cells had no synergistic or additive effect. The addition of 24,25-(OH)2D3 and TGF beta for 12 hours caused a synergistic effect on PKC activity; this effect was also observed if TGF beta was added first for 12 hours and 24,25-(OH)2D3 for the last 90 min. However, the addition of 24,25-(OH)2D3 for 90 min followed by the addition of TGF beta for an additional 10.5 hours had no synergistic effect. This study indicates that TGF beta and 24,25-(OH)2D3 have a synergistic effect on chondrocyte differentiation as well as on PKC activity, suggesting that the synergistic effect of 24,25-(OH)2D3 and TGF beta on chondrocyte differentiation may be mediated through activation of PKC. The synergistic effect of 24,25-(OH)2D3 and TGF beta was cell maturation dependent.