Inhibition of the membrane repair protein annexin-A2 prevents tumor invasion and metastasis

Cancer cells are exposed to major compressive and shearing forces during invasion and metastasis, leading to extensive plasma membrane damage. To survive this mechanical stress, they need to repair membrane injury efficiently. Targeting the membrane repair machinery is thus potentially a new way to prevent invasion and metastasis. We show here that annexin-A2 (ANXA2) is required for membrane repair in invasive breast and pancreatic cancer cells. Mechanistically, we show by fluorescence and electron microscopy that cells fail to reseal shear-stress damaged membrane when ANXA2 is silenced or the protein is inhibited with neutralizing antibody. Silencing of ANXA2 has no effect on proliferation in vitro, and may even accelerate migration in wound healing assays, but reduces tumor cell dissemination in both mice and zebrafish. We expect that inhibiting membrane repair will be particularly effective in aggressive, poor prognosis tumors because they rely on the membrane repair machinery to survive membrane damage during tumor invasion and metastasis. This could be achieved either with anti-ANXA2 antibodies, which have been shown to inhibit metastasis of breast and pancreatic cancer cells, or with small molecule drugs.

Regulation of prohepcidin processing and activity by the subtilisin-like proprotein convertases Furin, PC5, PACE4 and PC7

BACKGROUND AND AIMS

Hepcidin is an iron homoeostasis regulator peptide. Loss-of-function mutations cause juvenile haemochromatosis while its over-expression results in anaemia. However, the mechanism and function of preprohepcidin conversion to mature hepcidins (25, 22 and 20 amino acid C-terminal peptides) are not well known. After removal of the signal peptide, the first proteolytic cleavage occurs within the basic motif RRRRR(59)DT, suggesting the involvement of proprotein convertase (PC) family members in this process.

METHODS AND RESULTS

Using cell transfection experiments, the processing of preprohepcidin in the human hepatocyte line Huh-7 was found to be inhibited by the Furin inhibitors serpin alpha1-antitrypsin (alpha1-PDX) and prosegment preproFurin (ppFurin). Site-directed mutagenesis analysis confirmed the RRRRR(59)DT preprohepcidin cleavage site. In parallel, the lack of preprohepcidin processing found in the PC activity-deficient cell line LoVo was restored by the expression of Furin, paired basic amino acid cleaving enzyme 4 (PACE4), PC5 or PC7. This finding is consistent with the in vitro digestions of a synthetic peptide mimicking the cleavage site of preprohepcidin. In addition, during mouse embryonic development the major expression of hepcidin found in the liver coincided with that of Furin. While hepcidin induces the degradation of the iron transporter ferroportin, its RRRRR(59) to SSSSS(59) mutant is not active.

CONCLUSIONS

These results demonstrate the key role of the convertases Furin, PACE4, PC5 and/or PC7 in the generation and secretion of active hepcidin and suggest that the control of hepcidin processing as a potential therapeutic/diagnostic strategy in hepcidin-related disorders such as haemochromatosis, inflammatory diseases, anaemia and cancer.

Articular chondrocyte aging and endothelin-1

OBJECTIVE

To compare in cell culture endothelin-1 (ET-1) production, receptor density, and effect on macromolecular synthesis by articular chondrocytes (AC).

METHODS

AC were isolated from 1-month and 18-month old rats and cultured as monolayers. They were incubated with ET-1 without or with iNOS inhibitors, nitro-L-arginine methyl ester (L-NAME) or guanylate cyclase inhibitor, LY83583 and then [3H]thymidine, 35SO4 and [3H]proline incorporations were measured. The density and affinity for 125I-ET-1 of binding sites, and receptor isotypes were determined. The cells were also treated with interleukin-1beta (IL-1beta) or tumor necrosis factor-alpha (TNF-alpha), and then ET-1 productions measured. As well, the cells were challenged with NOC-5 (nitric oxide donor) or ET-1 and then ET-1 and NO respectively were measured.

RESULTS

A concentration-dependent stimulation of DNA, PG, collagen and NO synthesis was obtained when cells were incubated with ET-1 for 24-h. Eighteen-month old chondrocytes incorporated per microg DNA more [3H]thymidine, 35SO4 and [3H]proline but less NO when challenged with ET-1 than the 1-month old cells. However, strong inhibition of this initial stimulation was seen after 48-h. L-NAME and LY83583 enhanced basal-, and ET-1-induced initial stimulation and completely suppressed late (at 48-h to 72-h) ET-1-induced inhibition, suggesting NO was responsible for this inhibitory effect. Eighteen-month old chondrocytes expressed per mug DNA more high affinity receptors of predominantly ET(A) subtype. They also produced more ET-1 but less NO under basal conditions and more ET-1 when challenged with IL-1beta and TNF-alpha. NOC-5 inhibited the production of ET-1.

CONCLUSIONS

Eighteen-month old chondrocytes produce more ET-1, possess more ET-1-specific receptors, and increase more DNA, PG and collagen synthesis when challenged during 24-h with ET-1. NO, which suppresses ET-1 production and the production of which is increased by ET-1, seems to account for the late ET-1-induced inhibition of macromolecular synthesis. The possible implication of ET-1 in aging as related to osteoarthritis is discussed.

Human synovium produces substances that inhibit DNA and stimulate proteoglycan and collagen synthesis by cultured human articular chondrocytes and synovial fibroblasts

The effects of synovial conditioned medium (SCM) on DNA, proteoglycan (PG), and protein-collagen synthesis and respective gene expressions, in human articular chondrocytes (AC) and DNA synthesis in synovial fibroblasts (SFb), were studied in monolayer culture. All SCM exhibited concentration-dependent inhibition of [3H]thymidine incorporation in both AC and SFb. In contrast, SCM from three OA patients stimulated [35S]SO4 and [3H]glycine incorporations and the expression (RT-PCR) of aggrecan- and type II collagen-specific mRNAs in AC. The production of agents that inhibit DNA synthesis was blocked by indomethacin and dexamethasone and stimulated by IL-1 beta and TNF-alpha. The inhibitory substances were not produced by heat-inactivated tissue nor cultured SFb or AC and were completely solubles in methanol. It is postulated that synovial tissue secretes lipids, most probably arachidonic acid metabolites. These may counteract growth of an inflammatory synovial pannus by inhibiting SFb proliferation and enhance repair of damaged tissues by stimulating the matrix synthesis.

Mechanism of inhibition of endothelin-1-stimulated proteoglycan and collagen synthesis in rat articular chondrocytes

The aim of this study was to determine the effects of endothelin-1 (ET-1) on proteoglycan (PG) and collagen synthesis by rat articular chondrocytes (RAC). PG and collagen synthesis was measured by [(35)S]-sulphate and [(3)H]-glycine incorporation, respectively into monolayers of confluent RAC exposed to ET-1 (10(-11) M-10(-7) M). ET-1 stimulated PG and collagen synthesis in these cells in a concentration-dependent manner during the first 24 h of incubation. Prolonged contact of the cells with ET-1 resulted in a gradual decrease, and finally, inhibition of ET-1 effects. This inhibition is mediated by nitric oxide (NO) released in response to ET-1 since: (1) nitric oxide synthase inhibitor, nitro-L-arginine-methyl ester (L-NAME), enhanced both basal and ET-1-induced [(35)S]-sulphate and [(3)H]-glycine incorporations; (2) sodium nitroprusside (SNP), which spontaneously releases NO, inhibited both basal and ET-1-induced incorporations, and was also able to suppress the effects of L-NAME; (3) NO levels in the culture media were also correlated with the inhibition of [(35)S]-sulphate and [(3)H]-glycine incorporation; and (4) SNP also inhibited aggrecan and collagen II transcriptions, probably via cGMP. This effect was mimicked with 8-bromo-cGMP. Interestingly, the LY83583, which blocks the NO-dependent production and release of cGMP, inhibited PG-collagen synthesis but had no effect on their mRNA expressions. Thus, normal levels of cGMP appeared to be necessary for PG-collagen synthesis, whereas decreased levels are detrimental. In conclusion, NO, produced by rat AC in response to ET-1, counteracts the stimulation and finally induces inhibition of PG-collagen synthesis by ET-1 in these cells but NO-induced cGMP is only partially responsible for this inhibition.

Cooperative regulation of the invasive and metastatic phenotypes by different domains of the type I insulin-like growth factor receptor beta subunit

The receptor for the type 1 insulin-like growth factor (IGF-I) regulates multiple cellular functions impacting on the metastatic phenotype of tumor cells, including cellular proliferation, anchorage-independent growth, survival, migration, synthesis of the 72-kDa type IV collagenase and invasion. We have used site-directed mutagenesis to generate domain-specific mutants of the receptor beta subunit to analyze the role of specific tyrosines in the regulation of the invasive/metastatic phenotype. Poorly invasive M-27 carcinoma cells expressing low receptor numbers were transfected with a plasmid vector expressing IGF-I receptor cDNA in which single or multiple tyrosine codons in the kinase domain, namely Tyr-1131, Tyr-1135, and Tyr-1136 or the C-terminal tyrosines 1250 and 1251 were substituted with phenylalanine. Changes in the invasive and metastatic properties were analyzed relative to M-27 cells expressing the wild type receptor. We found that cells expressing the Y1131F,Y1135F,Y1136F or Y1135F receptor mutants lost all IGF-IR-dependent functions and their phenotypes were indistinguishable from, or suppressed relative to, the parent line. The Y1250F,Y1251F substitution abolished anchorage-independent growth, cell spreading, and the anti-apoptotic effect of IGF-I whereas all other IGF-IR-dependent phenotypes were either unperturbed (i.e. mitogenicity) or only partially reduced (migration and invasion). The results identify three types of receptor-dependent functions in this model: those dependent only on an intact kinase domain (DNA synthesis), those dependent equally on kinase domain and Tyr-1250/1251 signaling (e.g. apoptosis, soft agar cloning) and those dependent on kinase domain and enhanced through Tyr-1250/1251 signaling (migration, invasion). They suggest that signals derived from both regions of the receptor cooperate to enhance tumor metastasis.

Granulocyte-macrophage colony stimulating factor activates proteoglycan, type II collagen, and cAMP production by rat articular chondrocytes through specific binding sites

OBJECTIVE

To evaluate the effects of granulocyte-macrophage colony stimulating factor (GM-CSF) on rat articular chondrocyte (AC) with respect to DNA synthesis, collagen type II and proteoglycan (PG) synthesis and expression, and cAMP production; to examine these cells for the presence of GM-CSF-specific binding sites; and to study their regulation by growth factors and cytokines.

METHODS

First passage monolayers of rat AC were incubated with various concentrations of recombinant human GM-CSF, and then [3H]-thymidine, [3H]-proline, and [35S]SO4 incorporation and cAMP production were measured. The density of GM-CSF-specific binding sites, the effects of growth factors and cytokines on receptor density, and the activation of certain post-receptor signaling pathways were also examined by labeling the cell monolayers with [125I]-GM-CSF.

RESULTS

GM-CSF (6-100 U/ml) inhibited (30%) [3H]-thymidine incorporation into DNA, and, in contrast, stimulated up to 3.6- and 2-fold [35S]SO4 and [3H]-proline incorporation into glycosaminoglycan side chains and collagen molecules, respectively. GM-CSF also increased aggrecan and type II collagen (Coll II) transcripts by 2- to 3-fold, respectively. These effects were associated with a concentration-dependent increase in cAMP production. A single class of high affinity (Kd = 98 pM; Bmax = 7.08 pM/microg DNA) binding sites of about 220 kDa were found. The [125I]-GM-CSF binding to the cells was slightly increased with phorbol 12-myristate 13-acetate (PMA), insulin-like growth factor-I, platelet derived growth factor, basic fibroblast growth factor, and tumor necrosis factor-alpha, and decreased with pertussis toxin, cholera toxin, and interleukin-1beta.

CONCLUSION

These results suggest that GM-CSF may play a role in the regulation of chondrocyte metabolism as an anabolic agent and may stimulate cartilage healing under pathological conditions.

Inhibition of proprotein convertases is associated with loss of growth and tumorigenicity of HT-29 human colon carcinoma cells: importance of insulin-like growth factor-1 (IGF-1) receptor processing in IGF-1-mediated functions

Proprotein convertases (PCs) of the subtilisin/kexin family are responsible for the activation of prohormones, protrophic factors, and their receptors. We sought to determine whether loss of PC-mediated activities might affect the malignant phenotypes of cancer cells. Stable transfectants of alpha(1)-antitrypsin Portland (alpha(1)-PDX) cDNA, coding for a potent PC inhibitor, were analyzed in model HT-29 cells (HT-29/PDX) and in other cell lines. Expression of alpha(1)-PDX resulted in a proinsulin-like growth factor-1 receptor (pro-IGF-1R) processing blockade, hence inhibiting the ability of exogenous IGF-1 to induce tyrosine phosphorylation of its beta-subunit and insulin-related substrate-1. Coexpression of IGF-1R with four different PCs or the novel convertase SKI-1 in the furin-defective LoVo-C5 cells demonstrated that pro-IGF-1R ( approximately 200 kDa) cleavage into IGF-1R (beta-subunit, approximately 105 kDa) can be achieved by furin and PC5A, but not by PACE4, PC7, or SKI-1. Expression of alpha(1)-PDX resulted in reduction of DNA synthesis and in anchorage-independent growth. Following serum deprivation, the alpha(1)-PDX transfectants exhibited an enhanced apoptotic phenotype and were insensitive to IGF-1-mediated [(3)H]thymidine incorporation and protection against apoptosis. These cells showed reduced invasiveness that paralleled decreased mRNA levels of urokinase-type plasminogen activator and its receptor, tissue-type plasminogen activator, and plasminogen activator inhibitor-1. Comparative subcutaneous inoculation of cells in nude mice revealed that animals injected with HT-29/PDX cells exhibited delayed and lower incidence of tumor development as well as reduced tumor size. Immunohistochemical analysis of CD31 antigen expression, a marker of endothelial cells, revealed reduced HT-29/PDX tumor vascularization. These findings indicate that PCs actively contribute to the growth and malignant phenotypes of HT-29 tumors, suggesting that PC inhibition strategies may be a useful adduct to the arsenal of colorectal anticancer gene therapies.

Regulation of urokinase plasminogen activator/plasmin-mediated invasion of melanoma cells by the integrin vitronectin receptor alphaVbeta3

The integrin vitronectin receptor alphavbeta3 is a mediator of cellular migration and invasion and has been identified as a marker of progression in malignant melanoma. Using a human melanoma model, we have previously shown that this receptor was coordinately expressed with the receptor for the urokinase plasminogen activator (uPAR). In our present study, the link between these receptors was further investigated by assessing the effect of alphavbeta3 ligation on uPAR transcription and function. Using the reverse transcription-polymerase chain reaction, we found that receptor ligation by immobilized monoclonal antibodies (MAbs) induced a rapid increase (up to 4.5 fold) in uPAR mRNA levels, which was maximal 4 hr after cell attachment. An increase was also noted in plasminogen activator inhibitor type-1 (PAI-1) mRNA levels (2.7-fold), but none was noted in uPA levels. In addition, ligation of alphavbeta3 resulted in a significant increase in cell surface-associated plasmin levels, which coincided with a 2- to 3-fold increase in cell invasion as measured in the Matrigel invasion assay. This increase in invasion could in turn be abolished by antibodies directed to uPA and uPAR and by the plasmin inhibitors epsilon-aminocaproic acid and aprotinin. Furthermore, ligation of the integrin alphavbeta3 triggered a rapid increase of up to 12-fold in total cellular PKC activity, and this coincided with the redistribution of PKCbeta, but not PKCalpha, from the cytosol to the membrane. Treatment of the cells with the PKCbeta-specific inhibitor LY379196 blocked uPAR and PAI-1 mRNA induction and reduced the increase in cell invasion due to alphavbeta3 ligation, confirming the involvement of this isoform in the response. The results provide evidence that the vitronectin receptor can enhance invasion by regulating the uPAR/uPA/plasmin system of proteolysis and implicate PKCbeta as an intermediate in the activation pathway.

Articular chondrocytes from aging rats respond poorly to insulin-like growth factor-1: an altered signaling pathway

This study investigates the effect of insulin-like growth factor-1 (IGF-1) and phorbol 12-myrystate 13-acetate (PMA) on 3H-thymidine, 35SO(4) and 3H -glycine incorporations, adenosine 3':5'-cyclic monophosphate (cAMP) production and protein kinase C (PKC) activation in cultured rat articular chondrocyte monolayers (RACM) derived from animals of different ages. It was found that IGF-1 stimulates all these cellular functions in cultures derived from all age groups in a concentration dependent manner, although the cells from 14-month old animals responded poorly. IGF-1 also induces in cells from 1-month old rats an increase in the expression of mRNAs specific for aggrecan and type II collagen molecules as shown with RT-PCR. These effects are mediated via IGF-1 interaction with specific receptors because the monoclonal antibody against the receptor protein suppresses more than 60% of the ligand-induced DNA synthesis. PMA, a direct PKC activator, potentiated IGF-1-induced effects in all cells but much more strongly in cells from young than in cells from 14-month old animals. The age-related failure of RACM to respond adequately to IGF-1 was correlated with a decrease in IGF-1-induced cAMP production, and IGF-1-induced and PMA-induced PKC activations. These results show that IGF-1 regulates the synthesis of DNA, proteoglycans (PG) and collagen II at the level of transcription and suggest that the reduced response of cell monolayers derived from 14-month old rats to IGF-1 is probably due to a failure of old cells to adequately transduce IGF-1 receptor-generated downstream signaling.

Endothelin-1 in monolayer cultures of articular chondrocytes from young and old rats: regulation by growth factors and cytokines

The endothelin-1 (ET-1) concentrations were measured by RIA in the media of confluent monolayer cultures of rat articular chondrocyte (RAC) exposed to fetal calf serum (FCS) and several growth factors and cytokines. The cells were obtained from 1- and 18-month-old rats. First passage cells were starved in Dulbecco's modified Eagle's medium (DMEM) containing 0.2% FCS serum for 24 h and then incubated for 48 h in the same fresh medium with each of the following factors: fetal calf serum (FCS), transforming growth factor-beta (TGF-beta), platelet-derived growth factor (PDGF), epidermal growth factor (EGF), insulin-like growth factor-1 (IGF-1), interleukin-1 beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), lipopolysaccharide (LPS), and NO donor, sodium nitroprusside (SNP). The following was found: the cells from 18-month-old animals accumulated about twice as much ET-1 per microg DNA under basal (low serum) and stimulated conditions as cells from young rats. All, but PDGF and SNP produced concentration-dependent rise in ET-1 levels, the most effective being 10% FCS, IL-1beta, TNF-alpha, EGF, IGF-1 and LPS. TGF-beta caused the smallest stimulation and PDGF was ineffective or slightly inhibitory at high concentrations. SNP caused concentration-dependent decrease of ET-1 concentrations. ET-1-specific mRNA was identified by RT-PCR in cells incubated with the above factors and its concentration paralleled that of the peptide. This suggests that ET-1 found in the culture media of RAC stems, at least in part, from the synthesis. Increased immunoreactive peptide concentration and mRNA expression with the age of the donor rat and its regulation by several growth factors and cytokines suggest the involvement of ET-1 in chondrocytes' physiology and possibly pathology.

Rapid induction of cytokine and E-selectin expression in the liver in response to metastatic tumor cells

The cytokine-inducible endothelial cell adhesion receptor E-selectin has been implicated in cancer metastasis. Previously, we reported that experimental liver metastasis of Lewis lung carcinoma subline H-59 cells could be abrogated in animals treated with an anti-E-selectin antibody. To gain further insight into the functional relevance of E-selectin expression to liver colonization, we investigated here the time course of cytokine and hepatic E-selectin expression after the intrasplenic/portal inoculation of H-59 cells by using a combination of reverse transcription-PCR, Northern blot analysis, immunohistochemistry, and in situ hybridization. In parallel, we analyzed cytokine induction in response to the injection of Lewis lung carcinoma subline M-27 and murine melanoma B16-F1 cells, which do not spontaneously metastasize to the liver. In livers derived from normal or saline-injected mice, only minimal basal levels of TNF-alpha and IL-1 mRNA were detectable by RT-PCR. Rapid cytokine mRNA induction was noted within 30-60 min of H-59 injection, reaching maximal levels at 4-6 h. This was followed by the appearance of E-selectin mRNA, which was detectable at 2 h after injection and reached maximal levels at 6-8 h, declining to basal levels by 24 h. In situ hybridization analysis and immunohistochemistry localized E-selectin mRNA and protein, respectively, to the sinusoidal endothelium. M-27 cells failed to induce cytokine or E-selectin expression, whereas B-16 cells elicited a delayed and more short-lived response. The results demonstrate that upon entry into the hepatic circulation, tumor cells can rapidly trigger a molecular cascade leading to the induction of E-selectin expression on the sinusoidal endothelium and suggest that E-selectin induction may contribute to the liver-colonizing potential of tumor cells.

Endothelin 1 receptors, signal transduction and effects on DNA and proteoglycan synthesis in rat articular chondrocytes

This study showed that endothelins (ETs) stimulate DNA and proteoglycan synthesis in monolayer culture of rat articular chondrocytes (AC) by interacting with specific cell surface receptors. The high affinity receptors bound [125I]ET-1 with a Kd of 0.54 nM and Bmax of 81.4 pM/microgram DNA (approximately 40 000 binding sites per cell) was demonstrated. [125I]ET-1 binding was completely inhibited by unlabelled ET-1 or ET-2, and by BQ123 (ETA receptor antagonist), whereas ET-3 and IRL1038 (ETB receptor antagonist) did so only weakly. SDS-PAGE of cell extracts containing [125I]ET-1 cross-linked to the receptors, followed by autoradiography of the gels revealed a single 50-kDa band. These findings indicate that most of the receptors are subtype ETA. Although mRNA transcripts specific for both ETA and ETB receptors were found by RT-PCR, the ETA mRNA was more abundant. ET-1 increased the production of cAMP, cGMP and prostaglandin E2 (PGE2) and protein kinase C (PKC) activity in a concentration- and time-dependent manner. ET-1, and to a lesser degree ET-2, stimulated DNA synthesis, whereas ET-3 was inactive. Stimulation of DNA synthesis by ET-1 was strongly inhibited in a concentration-dependent manner by BQ123 and, to a much lesser degree, by IRL1038, which is consistent with an ETA receptor. ET-1 also stimulated proteoglycan synthesis and increased the amount of mRNA specific for the aggrecan gene. These findings strongly suggest that ET-1 is involved in regulating chondrocyte proliferation and metabolism in health, and presumably in disease.

Basal and induced nitric oxide and cGMP productions are decreased in senescent cultured rat articular chondrocytes

The effects of interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-alpha), and lipopolysaccharide (LPS) on the productions of nitric oxide (NO) and cGMP by cultured articular chondrocyte (AC) monolayers from 1-, 8- and 18-month old male Wistar rats were studied. It was found that basal NO and cGMP productions decrease with the age of animals. The productions were more than 2-fold greater in cells from 1-month old rats then in cells from older animals. IL-1, TNF-alpha, and LPS stimulated all three types of cells to produce NO and cGMP in a time- and concentration-dependent manner. Although the cells from young animals produced more NO per microg DNA, the older counterparts were more sensitive to these agents since they produced more NO upon stimulation then the corresponding non-stimulated controls. At the concentration of 10(-3) M, the nitric oxide synthase (NOS) inhibitor, Ng-monomethyl-L-arginine (L-NMA), blocked, although incompletely, both the basal and stimulated NO and cGMP productions in cells from the 1 and 8-month old rats and only induced productions in 18-month old counterparts. These results show a decreased capacity of unstimulated- and stimulated-AC from old rats to produce NO and cGMP in culture, which may affect the ageing cells in some yet unknown way.

The mechanism of inhibition of endothelin-1-induced stimulation of DNA synthesis in rat articular chondrocytes

Endothelin-1 (ET-1) is a potent mitogen for rat articular chondrocytes (AC) in short term culture (24 h). Prolonged incubation (72 h) of AC with ET-1 resulted in inhibition of [3H]thymidine incorporation. This inhibition seemed to be mediated by prostaglandins (PGs) released in response to ET-1, since indomethacin (INDO) enhanced ET-1-induced [3H]thymidine incorporation. In agreement with this hypothesis, exogenous prostaglandins (PGE2, PGF2alpha and TxB2) blocked all basal, ET-1-induced and ET-1 induced-INDO-enhanced [3H]thymidine incorporation and ET-1 stimulated PGE2 release in a time and concentration-dependent manner. INDO also blocked cGMP production and 6-anilino-5,8-quinolinedione, a relatively specific inhibitor of cGMP formation, enhanced the stimulation and suppressed the inhibition of ET-1-induced DNA synthesis. In addition, 8-bromo-cGMP, an analogue of cGMP, blocked at all time periods studied, both basal and ET-1-induced incorporations of [3H]thymidine. Thus, PGs produced in response to ET-1 counteract the ET-1-induced stimulation of [3H]thymidine incorporation into rat AC by increasing cGMP production.

Constitutive and inducible expression of endothelin-1 in primary rat articular chondrocyte culture

Endothelin-1 (ET-1) mRNA expression and protein production were examined in primary rat articular chondrocyte (AC) cultures by RT-PCR and radioimmunoassay, respectively. We found that serum-starved rat AC express ET-1 mRNA and produce the peptide constitutively. Treatment of cells with 10% FCS resulted in a marked increase in ET-1 levels with a peak at 48 h (5.6-fold). A similar concentration-dependent effect was also obtained in the presence of interleukin 1beta (3.1-fold), tumour necrosis factor alpha (3. 5-fold), lipopolysaccharide (2.7-fold), transforming growth factor beta1 (3.5-fold), epidermal growth factor (5.0-fold) and insulin-like growth factor-I (4.4-fold). In addition, ET-1 was found to induce, over a period of 24 h, a potent concentration-dependent stimulation of DNA synthesis in rat AC. These findings demonstrate for the first time the constitutive expression and production of ET-1 by rat AC which could be modulated by several cytokines and growth factors, suggesting a possible role for ET-1 in autocrine regulation of chondrocyte function.

The mechanism of inhibition of DNA synthesis in articular chondrocytes from young and old rats by nitric oxide

The mechanism by which nitric oxide inhibits the incorporation of [3H]thymidine into rat articular chondrocytes (AC) in culture was studied. First-passage articular chondrocytes, isolated by collagenase digestion of cartilage fragments from humeral and femoral heads of 1- and 18-month-old rats, were used in all experiments. NO-generating compounds, isosorbide dinitrate or sodium nitoprusside, inhibited the incorporation of [3H]thymidine and the release of prostaglandin E2 (PGE2) and stimulated cyclic guanosine monophosphate (cGMP) production by rat AC monolayers in a concentration-dependent manner. The cells from old rats were much less sensitive to NO donors and also produced less PGE2 and cGMP. Blocking the production of endogenous NO with NG-monomethyl-L-arginine (L-NMA), an inhibitor of NO synthase, stimulated DNA synthesis. cGMP was found to be a key mediator of the inhibition of DNA synthesis by NO donors in rat AC. 6-Anilino-5,8-quinolinedione (LY83583), an inhibitor of NO-dependent cGMP release, stimulated [3H]-thymidine incorporation, whereas the cGMP analog, 8- bromo-cGMP, inhibited L-NMA-induced or LY83583-induced stimulation of [3H]thymidine incorporation. NO donors blocked the stimulation of DNA synthesis induced by L-NMA and only marginally blocked that of LY83583. Indomethacin had no effect on the inhibition of DNA synthesis by NO or 8-bromo-cGMP. These results show that NO donors induce inhibition of DNA synthesis probably by elevating cGMP. The relative insensitivity of senescent cells to NO donors may be due, at least in part, to their decreased capacity to produce cGMP.

Mitogenic and metabolic actions of epidermal growth factor on rat articular chondrocytes: modulation by fetal calf serum, transforming growth factor-beta, and tyrphostin

The effects of human recombinant epidermal growth factor (EGF) on rat articular chondrocytes from humeral and femoral head cartilage of 21-day-old Wistar rats were analyzed. The cells were cultured under standard conditions as monolayers. Cell proliferation was studied by [3H]thymidine incorporation and determination of DNA content, proteoglycan synthesis by [35S]sulfate incorporation, and collagen synthesis by [3H]proline incorporation. The presence of specific receptors was confirmed by [125I]-EGF binding and that of EGF and EGF-receptor (EGF-R) mRNA by reverse transcription and the polymerase chain reaction. EGF (0.5-2.5 ng/ml) stimulated [3H]thymidine incorporation and increased DNA content of cultures. The effect was strongest when serum concentration was low (< or =1%) and was lost at high (> or =7.5%) serum concentrations. The EGF-induced effect on deoxynucleic acid synthesis was inhibited by transforming growth factor-beta and tyrphostin, a tyrosine kinase inhibitor that blocks the phosphorylation of tyrosine residues on EGF-R. Cultured rat articular chondrocytes possess a single class of high-affinity binding sites (Kd 0.18 nM). There were about 4.5 x 10(9) binding sites per microgram of DNA or about 37,800 binding sites per cell with 8.3 pg DNA per cell. Cultured cells contained EGF mRNA and EGF-R mRNA. Incubation of cells with EGF for 24 h decreased the EGF mRNA transcripts and increased the EGF-R mRNA levels. These findings suggest that EGF probably takes part in the regulation of chondrocyte activity under normal and presumably pathological conditions.

Degradation of hyaluronic acid by photosensitized riboflavin in vitro. Modulation of the effect by transition metals, radical quenchers, and metal chelators

The effect of photoexcited riboflavin (RF) on the viscosity of hyaluronic acid (HA) solutions has been investigated. UV irradiation of RF causes under aerobic conditions fragmentation of HA and a decrease in the viscosity of its solutions. A decrease of HA viscosity occurs in PO(4)-buffered solutions and is accelerated by high pH, Fe2+ (but much less so by Fe3+), certain metal chelators, and horseradish peroxidase (HRP); it is partially inhibited by catalase and less so by superoxide dismutase (SOD). The reactivity of the system was completely blocked by Tris, ethanol, aspirin, d-manitol, dimethylthiourea (DMTU), dimethylsulfoxide (DMSO), and sodium azide. These results indicate that the most likely chemical species involved in the reaction is the hydroxyl radical. Singlet oxygen ((1)O(2)) generation is suggested by the ability of NaN3 and DMSO to completely inhibit the reactivity of the system. These two agents, however, may also interact with OH. radical, as well and suppress the reactivity of the system. H(2)O(2) and O(2).- seem also to be produced in significant amounts, because catalase and SOD partially block the reactivity of the system. The effect of HRP may be due to hydrogen subtraction from HA and H(2)O(2) reduction to water. Photoexcitation of RF may potentially occur in vitro and in vivo in the organs and tissues that are permeable to light, such as the eye or skin, and damage HA and other cell-matrix components causing inflammation and accelerating aging.