Dirofilaria immitis: proteases produced by third- and fourth-stage larvae.

A model of cutaneous extracellular matrix was used to determine if live Dirofilaria immitis larvae secrete proteases which are active at physiological pH and capable of degrading macromolecules found in cutaneous tissue. After 72 hr, 100 third-stage larvae (L3) degraded 24% of the total matrix, while fourth-stage larvae (L4) degraded 10%. A sharp increase in the amount of matrix degraded by L3 corresponded with the onset of the molting process. L3 and L4 degraded comparable amounts of the glycoprotein and elastin components of the matrix, but molting L3 degraded nearly twice the amount of the collagen component (62% vs 35%). Characterization of proteases present in larval-soluble extracts and excretory-secretory products using synthetic substrates and protease inhibitors demonstrated cysteine-protease and metalloprotease activity. Cysteine protease activity was found in whole worm extracts of both L3 and L4. Metalloprotease was secreted at higher levels by molting L3, but was also secreted by L4. Partial separation of the metalloprotease by size-exclusion chromatography indicated that the molecular weight of the native enzyme was in the 49-54 kDa range. The cysteine protease activity was demonstrated in fractions corresponding to 34-39 kDa. The biological function of the D. immitis larval proteases remains to be conclusively determined; however, these data suggest that they are involved in degradation of components of cutaneous tissue and in the molting process.     

Adherence,proliferation and collagen turnover by human fibroblasts seeded into different types of collagen sponges

We describe an in vitro model that we have used to evaluate dermal substitutes and to obtain data on cell proliferation, the rate of degradation of the dermal equivalent, contractibility and de novo synthesis of collagen. We tested three classes of collagenous materials: (1) reconstituted non-crosslinked collagen, (2) reconstituted collagen that was chemically crosslinked with either glutaraldehyde, aluminium alginate or acetate, and (3) native collagen fibres, with or without other extracellular matrix molecules (elastin hydrolysate, hyaluronic acid or fibronectin). The non-crosslinked reconstituted collagen was degraded rapidly by human fibroblasts. Teh chemically crosslinked materials proved to be cytotoxic. Native collagen fibres were stable. In the absence of ascorbic acid, the addition of elastin hydrolysate to this type of matrix reduced the rate of collagen degradation. Both elastin hydrolysate and fibronectin partially prevented fibroblast-mediated contraction. Hyaluronic acid was only slightly effective in reducing the collagen degradation rate and more fibroblast-mediated contraction of the material was found than for the native collagen fibres with elastin hydrolysate and fibronectin. In the presence of ascorbate, collagen synthesis was enhanced in the native collagen matrix without additions and in the material containing elastin hydrolysate, but not in the material with hyaluronic acid. These results are indicative of the suitability of tissue substitutes for in vivo application.     

Action of the major protease from Entamoeba histolytica on proteins of the extracellular matrix

The action of the major protease from the parasitic protozoon Entamoeba histolytica, a cysteine protease of Mr 27,000-29,000, on some important proteins of the extracellular matrix has been studied. The isolated protease degraded the extracellular matrix proteins from human tissue collagen type IV and V as well as laminin and fibronectin with different velocities and specificities under native conditions. Whereas the degradation of fibronectin and laminin proceeded rapidly, yielding distinct fragment patterns, the breakdown of the collagen types happened more slowly and incompletely. The digestion of the denatured isolated alpha 2-chain of bovine collagen type I was very fast and unspecific requiring only 1/10 of the enzyme activities as compared with the other substrates mentioned above. Nearly 85% of the overall proteolytic activity of a soluble fraction of E. histolytica was strongly inhibited by antibodies against the purified histolytic protease as well as by cystatin from chicken egg white, a specific protein inhibitor of cysteine proteases. We conclude that the histolytic protease represents by far the highest portion of soluble proteolytic activity in E. histolytica which is sufficient to destroy the extracellular matrix of the host.     

Action of the Major Protease from Entamoeba histolytica on Proteins of the Extracellular Matrix

The action of the major protease from the parasitic protozoon Entamoeba histolytica , a cysteine protease of M, 27,000–29,000, on some important proteins of the extracellular matrix has been studied. The isolated protease degraded the extracellular matrix proteins from human tissue collagen type IV and V as well as laminin and fibronectin with different velocities and specificities under native conditions. Whereas the degradation of fibronectin and laminin proceeded rapidly, yielding distinct fragment patterns, the breakdown of the collagen types happened more slowly and incompletely. The digestion of the denatured isolated α2-chain of bovine collagen type I was very fast and unspecific requiring only 1/10 of the enzyme activities as compared with the other substrates mentioned above. Nearly 85% of the overall proteolytic activity of a soluble fraction of E. histolytica was strongly inhibited by antibodies against the purified histolytic protease as well as by cystatin from chicken egg white, a specific protein inhibitor of cysteine proteases. We conclude that the histolytic protease represents by far the highest portion of soluble proteolytic activity in E. histolytica which is sufficient to destroy the extracellular matrix of the host.     

Role of proteolysis and apoptosis in regression of pulmonary vascular remodeling

Remodeled pulmonary arteries return to normal structural conditions after the increase in pulmonary artery flow resistance is reversed. We studied whether proteolysis of extracellular matrix proteins and apoptosis occur during reversal of remodeling produced by chronic hypoxia in the rat. Main pulmonary arteries were removed at different times during a 10-day period of exposure to 10% O2 and 14 days after return to air. Content and rates of degradation of collagen and elastin as well as immunoreactive collagenase in tissue and isolated mast cells were measured. Immunoblots for collagenase and tissue inhibitor of metalloproteinases (TIMP) were performed. Apoptosis was assessed by cleavage of DNA and TUNEL assay. Excess collagen and elastin present at 10 days of hypoxia decreased to near normal levels after 3-5 days of air. Transient increases in collagenolytic and elastolytic enzyme activities accompanied the rapid decrease in matrix proteins. Mast cells containing collagenase accumulated in remodeled pulmonary arteries, and the active form of collagenase appeared at the time of peak proteolytic activity. TIMP increased during remodeling. Apoptosis was maximal 3 days after return to air. Our results suggest that activation of enzymes, which degrade matrix proteins, and apoptosis play a role in resolution of vascular remodeling.     

Liberation of desmosine and isodesmosine as amino acids from insoluble elastin by elastolytic proteases

The development of atherosclerotic lesions and abdominal aortic aneurysms involves degradation and loss of extracellular matrix components, such as collagen and elastin. Releases of the elastin cross-links desmosine (DES) and isodesmosine (IDE) may reflect elastin degradation in cardiovascular diseases. This study investigated the production of soluble elastin cross-linking structures by proteinases implicated in arterial diseases. Recombinant MMP-12 and neutrophil elastase liberated DES and IDE as amino acids from insoluble elastin. DES and IDE were also released from insoluble elastin exposed to monocyte/macrophage cell lines or human primary macrophages derived from peripheral blood monocytes. Elastin oxidized by reactive oxygen species (ROS) liberated more unconjugated DES and IDE than did non-oxidized elastin when incubated with MMP-12 or neutrophil elastase. These results support the exploration of free DES and IDE as biomarkers of elastin degradation.     

Echinostoma liei miracidia and Biomphalariaglabrata snails: Effect of egg age,habitat heterogeneity,water quality and volume on infectivity

Infectivity of Echinostoma liei miracidia to NIH albino Biomphalaria glabrata declines significantly from 62% with eggs incubated for 10–24 days to 3% for eggs incubated for 30–42 days. In mass exposures of 25 snails to 125 miracidia in 1 liter of water infectivity was high (54–66 %) and not affected by the presence of lettuce, plastic sheets, chalk, detritus or snail-conditioned water. In distilled water or snail-conditioned water the proportion of infected snails exposed singly to five miracidia per snail in 5 ml was not significantly different from the results of mass exposures of 25 snails in 1 liter to the same snail: miracidia ratio. Some evidence is presented suggesting that infected snails are less likely to suffer mortality than uninfected snails during the first 7–10 days post-exposure.The results suggest that Echinostoma liei miracidial searching efficiency is robust in volumes of at least 1 liter and in a heterogeneous habitat. These aspects enhance the competitive potential of echinostomes as possible biological control agents for Schistosoma mansoni.     

Proteases and the gut barrier

Serine proteases, cysteine proteases, aspartic proteases and matrix metalloproteinases play an essential role in extracellular matrix remodeling and turnover through their proteolytic action on collagens, proteoglycans, fibronectin, elastin and laminin. Proteases can also act on chemokines, receptors and anti-microbial peptides, often potentiating their activity. The intestinal mucosa is the largest interface between the external environment and the tissues of the human body and is constantly exposed to proteolytic enzymes from many sources, including bacteria in the intestinal lumen, fibroblasts and immune cells in the lamina propria and enterocytes. Controlled proteolytic activity is crucial for the maintenance of gut immune homeostasis, for normal tissue turnover and for the integrity of the gut barrier. However, in intestinal immune-mediated disorders, pro-inflammatory cytokines induce the up-regulation of proteases, which become the end-stage effectors of mucosal damage by destroying the epithelium and basement membrane integrity and degrading the extracellular matrix of the lamina propria to produce ulcers. Protease-mediated barrier disruption in turn results in increased amounts of antigen crossing into the lamina propria, driving further immune responses and sustaining the inflammatory process.     

Extracellular matrix proteins: A positive feedback loop in lung fibrosis?

Lung fibrosis is characterized by excessive deposition of extracellular matrix. This not only affects tissue architecture and function, but it also influences fibroblast behavior and thus disease progression. Here we describe the expression of elastin, type V collagen and tenascin C during the development of bleomycin-induced lung fibrosis. We further report in vitro experiments clarifying both the effect of myofibroblast differentiation on this expression and the effect of extracellular elastin on myofibroblast differentiation.Lung fibrosis was induced in female C57Bl/6 mice by bleomycin instillation. Animals were sacrificed at zero to five weeks after fibrosis induction. Collagen synthesized during the week prior to sacrifice was labeled with deuterium. After sacrifice, lung tissue was collected for determination of new collagen formation, microarray analysis, and histology. Human lung fibroblasts were grown on tissue culture plastic or BioFlex culture plates coated with type I collagen or elastin, and stimulated to undergo myofibroblast differentiation by 0–10 ng/ml transforming growth factor (TGF)β₁. mRNA expression was analyzed by quantitative real-time PCR.New collagen formation during bleomycin-induced fibrosis was highly correlated to gene expression of elastin, type V collagen and tenascin C. At the protein level, elastin, type V collagen and tenascin C were highly expressed in fibrotic areas as seen in histological sections of the lung. Type V collagen and tenascin C were transiently increased. Human lung fibroblasts stimulated with TGFβ₁ strongly increased gene expression of elastin, type V collagen and tenascin C. The extracellular presence of elastin increased gene expression of the myofibroblastic markers α smooth muscle actin and type I collagen.The extracellular matrix composition changes dramatically during the development of lung fibrosis. The increased levels of elastin, type V collagen and tenascin C are probably the result of increased expression by fibroblastic cells; reversely, elastin influences myofibroblast differentiation. This suggests a reciprocal interaction between fibroblasts and the extracellular matrix composition that could enhance the development of lung fibrosis.     

The role of elastin in the mechanical properties of skin

The elastin fibers of rat skin samples were degraded by the use of a purified preparation of elastase to which soybean inhibitor was added, preventing the collagenolytic activity of the elastase on collagen. Control experiments ascertained degradation of elastin and no effect on collagen. The mechanical properties of the skin samples were studied before and after the enzymatic treatment and differences ascribed to the degraded elastin fibers. Elastin plays a role in the mechanical behaviour of rat skin at small stress values and small deformations. Especially, the elastin fibers are responsible for the recoiling mechanism after a stress or deformation has been applied.     

Elastin in a neonatal rat smooth muscle cell culture has greatly decreased susceptibility to proteolysis by human neutrophil elastase. An in vitro model of elastolytic injury

Summary A neonatal rat aorta smooth muscle cell culture system with a unique elastin-rich extracellular matrix was used as a model substrate for elastases. To study the susceptibility to solubilization of insoluble elastin, cultures were incubated in the presence of human neutrophil elastase (HNE) or porcine pancreatic elastase (PPE) and in the absence of serum for periods up to 45 min. Both the incubation media and cell layers were then assessed for elastin and collagen markers, total protein, and lactate dehydrogenase (LDH). Although HNE and PPE exhibited comparable activity against elastin purified from the cell layer, HNE exhibited a 6.7- to 25-fold reduction in its elastin solubilizing activity using intact cell layers as compared with the purified elastin, whereas PPE exhibited only a 1.5- to 2.5-fold reduction. This effect could not satisfactorily be explained as preferential inhibition of HNE activity in the culture system, because the amount of protein solubilized by HNE was 59% that of PPE. The mean elastin content of PPE-solubilized protein was 110% that of the elastin content of the corresponding cell layer; the value for HNE-solubilized protein was only 16%. Thus, the amount of elastin per microgram of solubilized protein for HNE was 15% that for PPE. Possible explanations for the greatly diminished elastolytic activity of HNE in the culture system include the preference of HNE for other substrates in the cell layer, the inability of HNE to penetrate sufficiently into the cell layer, and the presence of sulfated glycosaminoglycans in the vicinity of the elastin that act in an inhibitory fashion. Although there was extensive proteolytic damage to the extracellular matrix, LDH and DNA measurements indicated that little loss of cells or cell viability occurred. The observed differences in elastolytic activity of HNE and PPE in the culture system parallel the relative emphysema-inducing potency of the elastases in the hamster model of elastase-induced emphysema. Supported by National Heart, Lung and Blood Institute, Bethesda, MD, grants NIH-HL-25229, HL-19717, and HL-33522. Presented in part at the April 1985 meeting of the Federation of American Societies for Experimental Biology.     

The enzymatic digestion of elastin at acidic pH

The enzymatic degradation of insoluble elastin has been studied at several pH values using purified pepsin and cathepsin D, and neutrophil extracts. Pepsin degraded elastin throughout the pH range of 1.2-4.0 with the optimum pH below 2.0. Molecular sieve chromatography and gel electrophoresis indicated that a spectrum of molecular weight degradation products was produced. The degradation by pepsin was inhibited by sodium dodecyl sulfate (SDS), NaCl and pepstatin. Cathepsin D, which, like pepsin, degrades hemoglobin at acid pH and is inhibited by pepstatin, had no activity against insoluble elastin in the pH range of 3.2-7.2. Extracts of neutrophils degraded elastin above pH 4.0. The pH profile of elastin degradation by neutrophil extracts generally followed that of purified human leukocyte elastase. Our results suggest that during alimentation or pulmonary aspiration of gastric contents, extracellular elastin may be digested by gastric juice at acid pH. Inflammatory cells would not appear to be capable of contributing to such actions until local pH approaches neutrality. Cathepsin D, a major constituent of inflammatory cells, does not digest all types of connective tissue proteins.     

Expression of elastolytic cathepsins in human skin and their involvement in age-dependent elastin degradation

BackgroundSkin ageing is associated with structure-functional changes in the extracellular matrix, which is in part caused by proteolytic degradation. Since cysteine cathepsins are major matrix protein-degrading proteases, we investigated the age-dependent expression of elastolytic cathepsins K, S, and V in human skin, their in vitro impact on the integrity of the elastic fibre network, their cleavage specificities, and the release of bioactive peptides.MethodsCathepsin-mediated degradation of human skin elastin samples was assessed from young to very old human donors using immunohistochemical and biochemical assays, scanning electron microscopy, and mass spectrometry.ResultsElastin samples derived from patients between 10 and 86 years of age were analysed and showed an age-dependent deterioration of the fibre structure from a dense network of thinner fibrils into a beaded and porous mesh. Reduced levels of cathepsins K, S, and V were observed in aged skin with a predominant epidermal expression. Cathepsin V was the most potent elastase followed by cathepsin K and S. Biomechanical analysis of degraded elastin fibres corroborated the destructive activity of cathepsins. Mass spectrometric determination of the cleavage sites in elastin revealed that all three cathepsins predominantly cleaved in hydrophobic domains. The degradation of elastin was efficiently inhibited by an ectosteric inhibitor. Furthermore, the degradation of elastin fibres resulted in the release of bioactive peptides, which have previously been associated with various pathologies.ConclusionCathepsins are powerful elastin-degrading enzymes and capable of generating a multitude of elastokines. They may represent a viable target for intervention strategies to reduce skin ageing.     

The effect of ascorbic acid on the nature and production of collagen and elastin by rat smooth-muscle cells.

1. The effects of various concentrations of ascorbic acid on the quality and quantity of the insoluble extracellular matrices produced by two strains of cultured rat smooth-muscle cells were studied. 2. Ascorbic acid was necessary for the appearance of insoluble collagen in the extracellular matrix. 3. Secretion of soluble collagen continued in the absence of ascorbic acid, but this soluble collagen was markedly underhydroxylated. 4. The amount of insoluble collagen present in the matrix was directly related to the ascorbic acid concentration. 5. The insoluble collagen that appeared in the matrix under conditions where ascorbic acid was limiting was no more than 7% underhydroxylated. 6. In contrast, the amount of insoluble elastin produced was inversely proportional to the ascorbic acid concentration. 7. The elastin produced in the absence of ascorbic acid had the expected amino acid composition, but hydroxyproline was absent. 8. The hydroxyproline content of elastin was also directly dependent on the ascorbic acid concentration. 9. Ascorbic acid had variable effects on the quantity of glycoprotein(s) present in the matrix. 10. The appearance of insoluble collagen in the extracellular matrices produced by cultured human fibroblasts and calf endothelial cells was also completely dependent on the presence of ascorbic acid.     

Comparison of collagen gels and mammary extracellular matrix as substrata for study of terminal differentiation in rabbit mammary epithelial cells

Mammary epithelial cells were prepared by collagenase digestion of tissue from mid-pregnant rabbits and cultured for up to 6 days on either collagen gels or an extracellular matrix prepared from the same tissue. The behaviour of the cells in serum-supplemented medium containing combinations of insulin, prolactin, hydrocortisone, estradiol and progesterone were monitored by measuring rates of casein synthesis, lactose synthesis, DNA synthesis and protein degradation. After 6 days, epithelial cells on floating collagen gels showed substantial increases in casein synthesis and DNA synthesis over freshly-prepared cells, following a decline during the first 3 days when the collagen gels are contracting. The optimum hormone combination for casein synthesis was insulin + prolactin + hydrocortisone, whereas for optimum DNA synthesis the additional presence of estradiol and progesterone was required. Cells on extracellular matrix showed increased rates of both casein synthesis and DNA synthesis by day 6 in the presence of insulin + prolactin + hydrocortisone, with additional estradiol + progesterone having an inhibitory effect. Whereas on day 2 rates of intracellular protein degradation were generally lower in cells on extracellular matrix, by day 6 rates of protein degradation were lowest in cells cultured on collagen gels with insulin + prolactin + hydrocortisone. In all cases, rates of lactose synthesis fell to low levels as the culture proceeded. Pulse-chase labelling of freshly-prepared cells with [32P]orthophosphate in medium containing serum and insulin + prolactin + hydrocortisone demonstrated that newly-synthesized casein was degraded during its passage through the epithelial cell. The influences of the collagen gels and extracellular matrix and of the hormone combinations on epithelial cell differentiation and secretory activity are discussed.     

Preeclampsia-associated decrease of potential collagenolytic and gelatinolytic activities in the wall of the umbilical cord vein.

Preeclampsia is the most common pathological syndrome associated with pregnancy. It is accompanied by remodelling of the extracellular matrix of the umbilical cord. A decrease of collagen content in the umbilical cord vein was described. This decrease may result from reduced collagen biosynthesis or enhanced collagen degradation. It was decided to evaluate whether or not this phenomenon is associated with alterations in the activities of collagenolytic, gelatinolytic and non-specific proteolytic enzymes that may be involved in collagen degradation, as well as the activity of prolidase which provides proline as a substrate for collagen biosynthesis. Studies were performed on the umbilical cord veins of newborns delivered by healthy mothers and those with preeclampsia. The control vein extract, activated with trypsin, degraded reconstituted collagen fibres (64.4+/-2.9 nmol Hyp x mg(-1) protein), whereas the preeclamptic material demonstrated only a trace activity. The venous wall extract contained a latent form of gelatinase that might have been activated by trypsin and 4-aminophenylmercuric acetate. A decrease in the gelatinolytic and proteolytic activities of preeclamptic vein extract at neutral pH was found. Prolidase activity was almost 3-fold lower in the preeclamptic extract (240.6+/-29.3 nmol Pro x min(-1) x mg(-1) protein) in comparison to the control (608.2+/-63.7 nmol Pro x min(-1) x mg(-1)protein). It was concluded that the umbilical cord vein contains a latent form of gelatinase A. The decrease in prolidase activity may reduce collagen biosynthesis, resulting in a decrease of this protein in the preeclamptic umbilical cord vein.     

Tissue origin and extracellular matrix control neutral proteinase activity in human fibroblast three-dimensional cultures

Remodeling of the extracellular matrix by fibroblasts is an important step in the process of wound healing and tissue repair. We compared the behavior of fibroblasts from two different tissues, dermis and gingiva, in three-dimensional lattices made of two different extracellular matrix macromolecules, collagen and fibrin. Cells were grown in monolayer cultures from normal skin or gingiva and seeded in three-dimensional lattices made of either collagen or fibrin. Photonic and scanning electron microscopy did not reveal any morphological differences between the two types of fibroblasts in both sets of lattices. Both types of fibroblasts retracted collagen lattices similarly and caused only a slight degradation of the collagen substratum. By contrast, when seeded in fibrin lattices, gingival fibroblasts completely digested their substratum in less than 8 days, whereas only a slight fibrin degradation was observed with dermal fibroblasts. The ability of gingival but not dermal fibroblasts to express high levels of tissue plasminogen activators (tPA) when cultured in fibrin lattices was assessed on an immunological basis. Also, deprivation of plasminogen-contaminating fibrinogen preparations or use of tPA inhibitors markedly inhibited both fibrinolysis and retraction rates of fibrin lattices by gingival fibroblasts. Casein-zymography confirmed the intense proteolytic activity induced by fibrin in gingival fibroblasts. It was inhibited by aprotinin and phenyl methylsulfonyl fluoride (PMSF), two non-specific inhibitors of serine proteinases, and by η-amino-caproic acid (ηACA), an inhibitor of plasminogen activators. Monolayer cultures exhibited only trace amounts of caseinolytic activity. Our results demonstrate that the expression of proteinases by fibroblasts is dependent not only on their tissue origin but also on the surrounding extracellular matrix. The intense fibrinolytic activity of gingival fibroblasts in fibrin lattices may explain partially the high rate of healing clinically observed in gingiva. © 1996 Wiley-Liss, Inc.     

Responses of vascular smooth muscle cell to extracellular matrix degradation.

Vessels remodel to compensate for increases in blood flow/pressure. The chronic exposure of blood vessels to increased flow and circulatory redox-homocysteine may injure vascular endothelium and disrupt elastic laminae. In order to understand the role of extracellular matrix (ECM) degradation in vascular structure and function, we isolated human vascular smooth muscle cells (VSMC) from normal and injured coronary arteries. The apparently normal vessels were isolated from explanted human hearts. The vessels were injured by inserting a blade into the lumen of the vessel, which damages the inner elastic laminae in the vessel wall and polarizes the VSMC by producing a pseudopodial phenotypic shift in VSMC. This shift is characteristic of migratory, invasive, and contractile nature of VSMC. We measured extracellular matrix metalloproteinases (MMPs), tissue plasminogen activator (tPA), tissue inhibitor of metalloproteinase (TIMP), and collagen I expression in VSMC by specific substrate zymography and Northern blot analyses. The injured and elastin peptide, val-gly-val-ala-pro-gly, treated VSMC synthesized active MMPs and reduced expression of TIMP. The level of tPA and collagen type I was induced in the injured, invasive VSMC and in the val-gly-val-ala-pro-gly treated cells. To demonstrate the angiogenic role of elastin peptide to VSMC we performed in vitro organ culture with rings from normal coronary artery. After 3 days in culture the vascular rings in the collagen gel containing elastin peptide elaborated MMP activity and sprouted and grew. The results suggest that val-gly-val-ala-pro-gly peptide generated at the site of proteolysis during vascular injury may have angiogenic activity.     

Alteration of the extracellular matrix of smooth muscle cells by ascorbate treatment

The protein composition in the extracellular matrix of cultured neonatal rat aortic smooth muscle cells has been monitored over time in culture. The influence of ascorbate on insoluble elastin and collagen has been described. In the absence of ascorbate, the cells accumulate an insoluble elastin component which can account for as much as 50% of the total protein in the extracellular matrix. In the presence of ascorbate, the amount of insoluble collagen increases, while the insoluble elastin content is significantly less. When ascorbate conditions are varied at different times during the culture, the extracellular matrices are altered with respect to collagen and elastin ratios. The decrease in elastin accumulation in the presence of ascorbate may be explained by an overhydroxylation of tropoelastin. Approximately 1/3 of the prolyl residues in the soluble elastin fractions isolated from cultures grown in the presence of ascorbate are hydroxylated. Since the insoluble elastin accumulated in these cultures contain the unique lysine-derived cross-links in amounts comparable to aortic tissue, this culture system proves ideal for studying the influence of extracellular matrix elastin on cell growth and metabolism.