Role of the cellular attachment domain of fibronectin in the phagocytosis of beads by human gingival fibroblasts in vitro

EGFR targeted thermosensitive liposomes: A novel multifunctional platform for simultaneous tumor targeted and stimulus responsive drug delivery

The epidermal growth factor receptor (EGFR) is a promising target for anti-cancer therapy. The aim of this study was to design thermosensitive liposomes (TSL), functionalized with anti-EGFR ligands for targeted delivery and localized triggered release of chemotherapy. For targeting, EGFR specific peptide (GE11) and Fab' fragments of cetuximab were used and the effect of ligand density on in vitro tumor targeting was investigated. Ligand conjugation did not significantly change the physicochemical characteristics of liposomes. Fab'-decorated TSL (Fab'-TSL) can specifically and more efficiently bind to the EGFR overexpressed cancer cells as compared to GE11 modified TSL. Calcein labeled Fab'-TSL showed adequate stability at 37°C in serum (<4% calcein released after 1h) and a temperature dependent release at above 40°C. FACS analysis and live cell imaging showed efficient and EGFR mediated cellular association as well as dramatic intracellular cargo release upon hyperthermia. Fab'-conjugation and hyperthermia induced enhanced tumor cell cytotoxicity of doxorubicin loaded TSL. The relative cytotoxicity of Fab'-TSL was also correlated to EGFR density on the tumor cells. These results suggest that Fab'-TSL showed great potential for combinational targeted and triggered release drug delivery.

Particle size influences fibronectin internalization and degradation by fibroblasts

The application of nanotechnology for drug targeting underlines the importance of controlling the kinetics and cellular sites of delivery for optimal therapeutic outcomes. Here we examined the effect of particle size on internalization and degradation of surface-bound fibronectin by fibroblasts using polystyrene nanoparticles (NPs; 51 nm) and microparticles (MPs; 1 μm). Fibronectin was strongly bound by NPs and MPs as assessed by immuno-dot blot analysis (5.1 ± 0.4 × 10(- 5)pg fibronectin per μm(2) of NP surface; 4.2 ± ± 0.3 × 10(-5)pg fibronectin per μm(2) of MP surface; p>0.2). We estimated that ~193 fibronectin molecules bound to a MP compared with 0.6 fibronectin molecules per NP, indicating that ~40% of nanoparticles were not bound by fibronectin. One hour after incubation, fibronectin-coated NPs and MPs were rapidly internalized by Rat-2 fibroblasts. MPs and NPs were engulfed partly by receptor-mediated endocytosis as indicated by decreased uptake when incubated at 4°C, or by depletion of ATP with sodium azide. Pulse-chase experiments showed minimal exocytosis of NPs and MPs. Internalization of NPs and MPs was inhibited by jasplakinolide, whereas internalization of MPs but not NPs was inhibited by latrunculin B and by integrin-blocking antibodies. Extraction of plasma membrane cholesterol with methyl β-cyclodextrin inhibited internalization of fibronectin-coated NPs but not MPs. Biotinylated fibronectin internalized by cells was extensively degraded on MPs but not NPs. Particle size affects actin and clathrin-dependent internalization mechanisms leading to fibronectin degradation on MPs but not NPs. Thus either prolonged, controlled release or an immediate delivery of drugs can be achieved by adjusting the particle size along with matrix proteins such as FN.

Dual targeted polymeric nanoparticles based on tumor endothelium and tumor cells for enhanced antitumor drug delivery

Some specific types of tumor cells and tumor endothelial cells represented CD13 proteins and act as receptors for Asn-Gly-Arg (NGR) motifs containing peptide. These CD13 receptors can be specifically recognized and bind through the specific sequence of cyclic NGR (cNGR) peptide and presented more affinity and specificity toward them. The cNGR peptide was conjugated to the poly(ethylene glycol) (PEG) terminal end in the poly(lactic-co-glycolic) acid PLGA-PEG block copolymer. Then, the ligand conjugated nanoparticles (cNGR-DNB-NPs) encapsulating docetaxel (DTX) were synthesized from preformed block copolymer by the emulsion/solvent evaporation method and characterized for different parameters. The various studies such as in vitro cytotoxicity, cell apoptosis, and cell cycle analysis presented the enhanced therapeutic potential of cNGR-DNB-NPs. The higher cellular uptake was also found in cNGR peptide anchored NPs into HUVEC and HT-1080 cells. However, free cNGR could inhibit receptor mediated intracellular uptake of NPs into both types of cells at 37 and 4 °C temperatures, revealing the involvement of receptor-mediated endocytosis. The in vivo biodistribution and antitumor efficacy studies indicated that targeted NPs have a higher therapeutic efficacy through targeting the tumor-specific site. Therefore, the study exhibited that cNGR-functionalized PEG-PLGA-NPs could be a promising approach for therapeutic applications to efficient antitumor drug delivery.

Interactions between the discoidin domain receptor 1 and β1 integrin regulate attachment to collagen

Collagen degradation by phagocytosis is essential for physiological collagen turnover and connective tissue homeostasis. The rate limiting step of phagocytosis is the binding of specific adhesion receptors, which include the integrins and discoidin domain receptors (DDR), to fibrillar collagen. While previous data suggest that these two receptors interact, the functional nature of these interactions is not defined. In mouse and human fibroblasts we examined the effects of DDR1 knockdown and over-expression on β1 integrin subunit function. DDR1 expression levels were positively associated with enhanced contraction of floating and attached collagen gels, increased collagen binding and increased collagen remodeling. In DDR1 over-expressing cells compared with control cells, there were increased numbers, area and length of focal adhesions immunostained for talin, paxillin, vinculin and activated β1 integrin. After treatment with the integrin-cleaving protease jararhagin, in comparison to controls, DDR1 over-expressing cells exhibited increased β1 integrin cleavage at the cell membrane, indicating that DDR1 over-expression affected the access and susceptibility of cell-surface β1 integrin to the protease. DDR1 over-expression was associated with increased glycosylation of the β1 integrin subunit, which when blocked by deoxymannojirimycin, reduced collagen binding. Collectively these data indicate that DDR1 regulates β1 integrin interactions with fibrillar collagen, which positively impacts the binding step of collagen phagocytosis and collagen remodeling.

Design of cyclic RKKH peptide-conjugated PEG liposomes targeting the integrin α₂β₁ receptor

Peptide conjugation to the surface of stealth liposomes has been studied for liposomal drug targeting to cells expressing specific receptors to provide a site-specific drug delivery. This study investigated the potential of peptide-conjugated liposomes for targeting cells expressing the human integrin α(2)β(1) receptor. A 12 amino acid head-to-tail cyclic peptide derived from the Jararhagin protein containing the Arg-Lys-Lys-His (RKKH)-specific binding site was conjugated to the distal ends of poly(ethylene glycol) (PEG) chains on PEGylated liposomes. Epithelial cells expressing the receptor showed increased cellular association and uptake of peptide-conjugated liposomes at 4 °C, compared to liposomes conjugated with a non-specific peptide. The interaction between cells and peptide-conjugated liposomes was significantly increased at 37 °C suggesting that a possible uptake mechanism might be energy-dependent endocytosis. In keratinocyte cell cultures, the ligand-conjugated liposomes loaded with the vitamin D(3) analogue calcipotriol induced transcription of the gene encoding the antimicrobial peptide cathelicidin, which is activated through the vitamin D(3) receptor upon binding of vitamin D(3) analogues. This suggests that the liposomes are internalized and that calcipotriol is delivered intracellularly and released in an active form. In conclusion, the 12 amino acid head-to-tail cyclic RKKH peptide seems promising for targeting of liposomes to the integrin α(2)β(1) receptor.

Phagocytosis and remodeling of collagen matrices

The biodegradation of collagen and the deposition of new collagen-based extracellular matrices are of central importance in tissue remodeling and function. Similarly, for collagen-based biomaterials used in tissue engineering, the degradation of collagen scaffolds with accompanying cellular infiltration and generation of new extracellular matrix is critical for the integration of in vitro grown tissues in vivo. In earlier studies we observed significant impact of collagen structure on primary lung fibroblast behavior in vitro in terms of collagen uptake and matrix remodeling. Therefore, in the present work, the response of human fibroblasts (IMR-90) to the structural state of collagen was studied with respect to phagocytosis in the presence and absence of inhibitors. Protein content and transcript levels for collagen I (Col-1), matrix metalloproteinase 1 (MMP-1), matrix metalloproteinase 2 (MMP-2), tissue inhibitor of matrix metalloproteinase 1 (TIMP-1), tissue inhibitor of matrix metalloproteinase 2 (TIMP-2), and heat shock protein 70 (HSP-70) were characterized as a function of collagen matrix concentration, structure and cell culture time to assess effects on cellular collagen matrix remodeling processes. Phagocytosis of collagen was assessed quantitatively by the uptake of collagen-coated fluorescent beads incorporated into the collagen matrices. Significantly higher levels of collagen phagocytosis were observed for the cells grown on the denatured collagen versus native collagen matrices. Significant reduction in collagen phagocytosis was observed by blocking several phagocytosis pathways when the cells were grown on denatured collagen versus non-denatured collagen. Collagen phagocytosis inhibition effects were significantly greater for PDL57 IMR-90 cells versus PDL48 cells, reflecting a reduced number of collagen processing pathways available to the older cells. Transcript levels related to the deposition of new extracellular matrix proteins varied as a function of the structure of the collagen matrix presented to the cells. A four-fold increase in transcript level of Col-1 and a higher level of collagen matrix incorporation were observed for cells grown on denatured collagen versus cells grown on non-denatured collagen. The data suggest that biomaterial matrices incorporating denatured collagen may promote more active remodeling toward new extracellular matrices in comparison to cells grown on non-denatured collagen. A similar effect of cellular action toward denatured (wound-related) collagen in the remodeling of tissues in vivo may have significant impact on tissue regeneration as well as the progression of collagen-related diseases.

Methylglyoxal inhibits the binding step of collagen phagocytosis

Bacterial infection-induced fibrosis affects a wide variety of tissues, including the periodontium, but the mechanisms that dysregulate matrix turnover and mediate fibrosis are not defined. Since collagen turnover by phagocytosis is an important pathway for matrix remodeling, we studied the effect of the bacterial and eukaryotic cell metabolite, methylglyoxal (MGO), on the binding step of phagocytosis by periodontal fibroblasts. Type 1 collagen was treated with various concentrations of methylglyoxal, an important glucose metabolite that modifies Arg and Lys residues. The extent of MGO-induced modifications was authenticated by amino acid analysis, solubility, and cross-linking. Cells were incubated with fluorescent beads coated with collagen, and the percentage of phagocytic cells was estimated by flow cytometry. MGO inhibited collagen binding (20% of control for 10 mm MGO) in a time- and concentration-dependent manner. MGO-induced inhibition of binding was prevented by aminoguanidine, which blocks the formation of collagen cross-links. MGO reduced collagen binding strength and blocked intracellular calcium signaling. MGO modified the Arg residue in the critical alpha2beta1 integrin-binding recognition sequence of triple helical collagen peptides, whereas MGO-induced cross-linking of Lys residues played only a small role in binding inhibition. Thus, MGO modifications of Arg residues in collagen could be a key factor in the impaired degradation of collagen that promotes fibrosis in chronic infections, such as periodontitis.

In vitro phagocytosis of collagens by immortalised human retinal Müller cells

PURPOSE

This study is a first step to investigate phagocytosis of collagens by human retinal Müller cells, since Müller cells could be involved in remodelling of the vitreous and vitreoretinal interface in the human eye.

METHODS

Müller cells in culture were exposed to 2.0 microm fluorescent latex beads coated with BSA and human types I, II, and IV collagen and to non-coated beads for 2, 12, 24, and 48 h. To influence phagocytosis, cytochalasin B and anti-integrin subunits (alpha1, alpha2, and beta1) were added to the cells. Phagocytosis was evaluated by flow cytometry, transmission electron microscopy (TEM) and confocal microscopy.

RESULTS

Müller cells preferred to phagocytose beads coated with type II collagen compared with type IV collagen-, BSA- and non-coated beads. Phagocytosis of type I collagen-coated beads was intermediate. TEM and confocal microscopic evaluation confirmed phagocytosis of the beads. No significant differences were observed in phagocytosis of type II collagen-coated beads in the case of addition of cytochalasin B and anti-integrin subunits. Immunohistochemical analyses revealed that Müller cells were positive, under all tested circumstances, for vimentin and CRALBP. Less than 5% of the cells tested were GFAP positive.

CONCLUSIONS

Our observations demonstrate that human Müller cells in culture prefer to phagocytose type II collagen. In contrast, the phagocytosis of type IV collagen is comparable with the control coatings. We speculate that the relatively limited collagen phagocytosis by Müller cells supports a possible role for Müller cells in the slow process of vitreoretinal remodelling in adult human eyes.

Cellular unbinding forces of initial adhesion processes on nanopatterned surfaces probed with magnetic tweezers

To study the dependence of unbinding forces on the distance of molecularly defined and integrin specific c(-RGDfK-) ligand patches in initial cellular adhesion processes, we developed a magnetic tweezers setup for applying vertical forces of up to 200 pN to rat embryonic fibroblasts. The ligand patch distance is controlled with a hexagonally close packed pattern of biofunctionalized gold nanoparticles prepared by block-copolymer micelle nanolithography. Each gold nanoparticle potentially activates up to one alpha(v)beta(3)-integrin. The distances between the gold nanoparticles determine the separation of individual integrins and thus the assembly of integrin clusters. The results show an increase in cellular unbinding forces from approximately 6 to more than 200 pN for a decreasing ligand distance of 145 to 58 nm after 5 min of cell adhesion. Furthermore, we observe a strong dependence on adhesion time during the first 10 min of cell surface contact suggesting an active, cooperative cell response that is controlled by the spacing between individually activated integrins.

Collagen Phagocytosis by Fibroblasts Is Regulated by Decorin

Decorin is a small, leucine-rich proteoglycan that binds to collagen and regulates fibrillogenesis. We hypothesized that decorin binding to collagen inhibits phagocytosis of collagen fibrils. To determine the effects of decorin on collagen degradation, we analyzed phagocytosis of collagen and collagen/decorin-coated fluorescent beads by Rat-2 and gingival fibroblasts. Collagen beads bound to gingival cells by alpha2beta1 integrins. Binding and internalization of decorin/collagen-coated beads decreased dose-dependently with increasing decorin concentration (p < 0.001). Inhibition of binding was sustained over 5 h (p < 0.001) and was attributed to interactions between decorin and collagen and not to decorin-collagen receptor interactions. Both the non-glycosylated decorin core protein and the thermally denatured decorin significantly inhibited collagen bead binding (approximately 50 and 89%, respectively; p < 0.05). Mimetic peptides corresponding to leucine-rich repeats 1-3, encompassed by a collagen-binding approximately 11-kDa cyanogen bromide fragment of decorin and leucine-rich repeats 4 and 5, previously shown to bind to collagen, were tested for their ability to inhibit collagen bead binding. Although the synthetic peptide 3 alone exhibited saturable binding to collagen, neither peptides 3 nor 1 and 2 markedly inhibited phagocytosis. Leucine-rich repeat 3 bound to a triple helical peptide containing the alpha2 integrin-binding site of collagen. When collagen beads were co-incubated with peptides 3 and 4, inhibition of collagen phagocytosis (55%) was equivalent to intact native/recombinant core protein. Thus a novel collagen binding domain in decorin acts cooperatively with leucine-rich repeat 4 to mask the alpha2beta1 integrin-binding site on collagen, an important sequence for the phagocytosis of collagen fibrils.

3-D particle tracking in a two-photon microscope: application to the study of molecular dynamics in cells

We developed a method for tracking particles in three dimensions designed for a two-photon microscope, which holds great promise to study cellular processes because of low photodamage, efficient background rejection, and improved depth discrimination. During a standard cycle of the tracking routine (32 ms), the laser beam traces four circular orbits surrounding the particle in two z planes above and below the particle. The radius of the orbits is half of the x,y-width of the point spread function, and the distance between the z planes is the z-width of the point spread function. The z-position is adjusted by moving the objective with a piezoelectric-nanopositioner. The particle position is calculated on the fly from the intensity profile obtained during the cycle, and these coordinates are used to set the scanning center for the next cycle. Applying this method, we were able to follow the motion of 500-nm diameter fluorescent polystyrene microspheres moved by a nanometric stage in either steps of 20-100 nm or sine waves of 0.1-10 microm amplitude with 20 nm precision. We also measured the diffusion coefficient of fluorospheres in glycerol solutions and recovered the values expected according to the Stokes-Einstein relationship for viscosities higher than 3.7 cP. The feasibility of this method for live cell measurements is demonstrated studying the phagocytosis of protein-coated fluorospheres by fibroblasts.

Impact of collagen structure on matrix trafficking by human fibroblasts

Biodegradation of collagen biomaterial matrices and the deposition of new collagen extracellular matrix (ECM) are critical to the integration of in vitro bioengineered materials and tissues in vivo. In previous studies, we observed significant impact of collagen matrix structure on primary lung fibroblast behavior in vitro. In the present work, to begin to understand the mechanistic basis for our previous observation, the response of human fibroblasts (IMR-90) to the structural state of collagen matrices was studied with respect to cell proliferation, cell morphology, beta-galactosidase level, and transcript content for collagen (Col-1), matrix metalloproteinases (MMP-1, MMP-2), tissue inhibitors of matrix metalloproteinase (TIMP-1 and TIMP-2). Collagen digestion was assessed quantitatively by uptake of collagen-coated fluorescent beads incorporated in the preformed collagen matrix. Transcript levels related to the deposition of new ECM proteins varied as a function of the structure of the collagen matrix presented to the cells. Col-1 expression was 2-fold higher and expression for MMP-1, MMP-2, TIMP-1, and TIMP-2 increased for cells when grown on 156 microg/cm2 denatured collagen compared with cells grown on tissue culture (TC) plastic. On 156 microg/cm2 nondenatured (native) collagen, Col-1 expression was decreased by half and MMP-2 was increased by 2.5-fold compared with cells grown on TC plastic. On 78 microg/cm2 denatured collagen, Col-1 expression was 80% whereas the MMPs and TIMPs were increased by 1.25- to 2-fold compared with cells grown on TC plastic. On 78 microg/cm2 nondenatured collagen expression of all 5 transcripts was reduced 60-90% of the levels determined for the cells grown on TC plastic. Cell viability, based on cell morphology and beta-galactosidase activity, was improved on the denatured collagen. A higher level of collagen matrix incorporation was observed for cells grown on denatured collagen than on nondenatured collagen or TC plastic. These data suggest that tissue engineering matrices incorporating denatured collagen may promote more active remodeling toward new ECM in comparison to cells grown on nondenatured collagen or cells grown on TC plastic.

The major outer sheath protein of Treponema denticola inhibits the binding step of collagen phagocytosis in fibroblasts

Bacterial infections contribfute to the chronicity of connective tissue lesions in part by perturbing extracellular matrix remodelling processes. We examined a novel mechanism by which the major outer sheath protein (Msp) of the spirochaete Treponema denticola disrupts matrix remodelling mediated by intracellular digestion of collagen. The initial collagen-binding step of phagocytosis was examined in human gingival fibroblasts and Rat-2 fibroblasts. Cells were pretreated with Msp or vehicle, and binding of collagen-coated beads was measured by flow cytometry. Exposure to Msp induced a dose- and time-dependent decrease in cells that bound collagen beads; the inhibition of binding was reversed by absorption with anti-Msp antibodies. Msp-treated fibroblasts remained viable but underwent actin reorganization, including the assembly of a dense meshwork of subcortical actin filaments. Shear force assays showed that Msp abrogated collagen-binding interactions in the minimal affinity range required for stable adhesion. Fluorescence microscopy and immunoblotting showed equivalent amounts of beta1 integrin associated with collagen beads bound to Msp- and vehicle-treated cells. Photobleaching experiments found a similar percentage mobile fraction of beta1 integrins recovered in bleached areas of the plasma membrane. In contrast, Msp-induced inhibition of collagen binding was reversed by beta1 integrin affinity-activating antibodies and by latrunculin B, which prevented subcortical actin assembly. We conclude that native Msp of T. denticola inhibits the binding step of collagen phagocytosis in fibroblasts by inducing subcortical actin filament assembly and restricting affinity modulation of beta1 integrins. We suggest that, like Msp, bacterial toxins that target the cytoskeleton may also perturb the signalling networks required for cellular engagement of matrix ligands.

Decreased expression of alpha2 integrin in fibroblasts isolated from cyclosporin A- induced gingival overgrowth in rats

OBJECTIVES

Cyclosporin A (CsA), an immunosuppressive agent, induces fibrous gingival overgrowth through reduction of collagen phagocytosis by fibroblasts. Distinct receptors are involved in the binding of collagen to fibroblasts in collagen phagocytosis, and alpha2beta1 integrin serves as a specific receptor for type I collagen on fibroblasts. To elucidate the role of alpha2beta1 integrin in CsA-induced gingival overgrowth, we investigated collagen phagocytosis and alpha2beta1 integrin expression in rat gingival fibroblasts.

MATERIALS AND METHODS

Fibroblats were isolated from gingiva of rats fed a powdered diet containing or lacking CsA for 30 d. Flow cytometric analysis were performed to measure the collagen phagocytosis and the alpha2 integrin expression in fibroblasts. Furthermore, total RNAs were isolated from fibroblasts, and the reverse transcriptase-polymerase chain reaction was employed to investigate the mRNA levels of alpha2 integrin.

RESULTS

In vitro collagen phagocytosis assay revealed that CsA-treated and control fibroblasts contained a mean of 13.5% and 36.1% phagocytic cells, respectively. CsA-treated fibroblasts had 28% lower expression of alpha2 integrin than that of control. and mRNA expression of alpha2 integrin in CsA-treated fibroblasts was apparently lower than in the controls, but the mRNA expression of beta1 integrin was not affected.

CONCLUSION

These findings suggest that one etiological factor of gingival overgrowth may be inhibition of collagen phagocytosis by reducing alpha2 integrin expression in gingival fibroblasts.

Phagocytosis of titanium particles and necrosis in TNF-alpha-resistant mouse sarcoma L929 cells

In the oral cavity, titanium is an excellent biocompatible material. However, it is reported that high ratios of intracellular titanium particles can cause cell apoptosis or necrosis by as-yet unknown mechanisms. The purpose of this study was to investigate the response of tumor necrosis factor alpha (TNF-alpha)-resistant L929 fibroblasts to titanium particles. Cells were cultured in Eagle's medium supplemented with fetal bovine serum and L-glutamine. Titanium particle sizes were less than 9 micro. Cytotoxicity was assayed by a cell counting kit, trypan blue dye exclusion test and lactate dehydrogenase (LDH) leakage. The production of reactive oxygen species (ROS) was detected by a confocal laser scanning microscope (CLSM) using dichlorofluorescein diacetate as a fluorescent probe. Morphology was viewed by a CLSM and with an X-ray microanalyser (XMA). When titanium particles were added to cells, the viability decreased to around 50% at a particle concentration of 2.0%. The number of dead cells and LDH activity in the culture media increased significantly between 1 and 2 days. However, formation of active oxygen species did not occur, since no dichlorofluorescein fluorescence was observed. A scanning electron photomicrograph (SEM) revealed a large number of particles covering or adhering to cellular components in lysed cells compared with flattened control cells attached to the substrate. The XMA showed that the titanium accumulation was coincident with the deformed cell shape. The CLSM also confirmed that particles were within the cells. From these results it was concluded that titanium particles ingested in large quantities into the cell induced necrosis by a pathway other than by producing ROS.

Opsonization modulates Rac-1 activation during cell entry by Leishmania amazonensis

Lesions caused by Leishmania amazonensis normally heal, but relapses occur due to parasite persistence in host tissues. It has been proposed that infection of fibroblasts plays an important role in this process by providing the parasites with a safe haven in which to replicate. However, most previous studies have focused on the entry of Leishmania into macrophages, a process mediated by serum opsonins. To gain insight into a possible role of nonopsonic entry in the intracellular persistence of amastigotes, we examined the invasion of Chinese hamster ovary (CHO) cells. Amastigotes entered CHO cells by a cytochalasin D, genistein, wortmannin, and 2,3-butanedione monoxime-sensitive pathway and replicated within phagolysosomes. However, unlike most phagocytic processes described to date, amastigote internalization in CHO cells involved activation of the GTPases Rho and Cdc42 but not Rac-1. When uptake was mediated by fibronectin or when amastigotes were opsonized with immunoglobulin G and internalized by Fc receptor-expressing CHO cells, Rac-1 activation was restored and found to be required for parasite internalization. Given the essential role of Rac in assembly of the respiratory burst oxidase, invasion through this nonopsonic, Rac-1-independent pathway may play a central role in the intracellular survival of Leishmania in immune hosts.

Phagocytosis of fibronectin and collagens type I, III, and V by human gingival and periodontal ligament fibroblasts in vitro

BACKGROUND

Electron microscopic studies have suggested that the volume density of collagen-containing vacuoles in fibroblasts is higher in the periodontal ligament (PDL) than in the gingiva. Whether this difference reflects intrinsic differences in phagocytic capacity among the cells in these tissues is not known.

METHODS

PDL and gingival fibroblasts were isolated from subjects and cultured under identical conditions in the presence of fluorescent beads coated with collagen type I, III, or V or fibronectin. Control beads were coated with bovine serum albumin or an enamel matrix protein mixture that does not constitute part of the extracellular matrix of PDL and gingiva. After various time intervals (1 to 24 hours), the percentage of cells that had internalized beads was assessed by flow cytometry. Since alkaline phosphatase activity has been suggested to play a role in collagen phagocytosis, the activity of this enzyme was determined for all cell populations.

RESULTS

The results demonstrated the following order in the percentage of cells internalizing protein-coated beads: fibronectin > collagen type I > III > V. Internalization of collagen type I-coated beads exceeded that of beads coated with bovine serum albumin or enamel matrix proteins by 6 and 3 times, respectively. No differences were observed in collagen phagocytic activity between PDL and gingival fibroblasts, and no relationship could be demonstrated between collagen phagocytosis and alkaline phosphatase activity.

CONCLUSIONS

We conclude that differences in collagen phagocytosis between PDL and gingiva, as observed in vivo, are not likely to be explained in terms of intrinsic phagocytic capacities of these cells.

Cadherins mediate intercellular mechanical signaling in fibroblasts by activation of stretch-sensitive calcium-permeable channels

Cells in mechanically active environments form extensive, cadherin-mediated intercellular junctions that are important in tissue remodeling and differentiation. Currently, it is unknown whether adherens junctions in connective tissue fibroblasts transmit mechanical signals and coordinate multicellular adaptations to physical forces. We hypothesized that cadherins mediate intercellular mechanotransduction by activating calcium-permeable, stretch-sensitive channels. Human gingival fibroblasts in suspension were plated on established homotypic monolayer cultures. The cells formed intercellular adherens junctions. Controlled mechanical forces were applied to intercellular junctions by electromagnets acting on cells containing internalized magnetite beads. At early but not later stages of intercellular attachment, force application visibly displaced magnetite bead-loaded cells and induced robust Ca(2+) transients (65 +/- 9.4 nm above base line). Similar Ca(2+) transients were induced by force application to anti-N-cadherin antibody-coated magnetite beads. Ca(2+) responses depended on influx of extracellular Ca(2+) through mechanosensitive channels because both Ca(2+) chelation and gadolinium chloride abolished the response and MnCl(2) quenched fura-2 fluorescence after force application. Force application induced accumulation of microinjected rhodamine-actin at intercellular contacts; actin assembly was inhibited by buffering intracellular calcium fluxes. Our results indicate that mechanical forces applied to adherens junctions activate stretch-sensitive calcium-permeable channels and increase actin polymerization. We suggest that N-cadherins in fibroblasts are intercellular mechanotransducers.

Transplantation of labeled periodontal ligament cells promotes regeneration of alveolar bone

Regeneration of damaged periodontal tissues is mediated by periodontal cells, but a major sub-population comprises highly differentiated cells that do not renew. To overcome the loss of specialized cell types caused by disease, various therapeutic approaches including cell transplants have been developed to promote cell re-population in periodontal tissues. As previous transplantation studies used unlabeled cells, that are indistinguishable from host cells, it has been difficult to assess the contributions of transplanted cells to the healing processes. To track the fate and differentiation of rat periodontal cells transplanted into periodontal wounds, we used collagen-coated fluorescent beads as a permanent endocytosed marker, or cells constitutively expressing beta-galactosidase. We assessed osteogenic cell differentiation with immunohistochemical staining for osteopontin and bone sialoprotein. Cells were transplanted into periodontal wounds created in Sprague--Dawley male rats that are null for beta-galactosidase. Defects were allowed to heal spontaneously (controls), or were closed with collagen implants mixed with beta-galactosidase-positive (Lac-Z) periodontal cells, or closed with collagen implants mixed with periodontal cells loaded with fluorescent beads. Animals were killed at 1 and 2 weeks after surgery and tissues were prepared for morphometric assessment and immunostaining for osteopontin (OPN) and bone sialoprotein (BSP). Transplanted cells were easily distinguished by fluorescent beads or by beta-galactosidase-positive expression and were distributed throughout the regenerating periodontal ligament (PL) and alveolar bone. At 1 week after wounding, animals treated with beta-galactosidase-positive cells exhibited a slightly higher percentage of labeled cells in the PL compared with the fluorescent bead-labeled cell implant group (2% vs. 1% respectively; P > 0.2). At Week 2 percentages of labeled cells were slightly increased in the regenerating PL (approximately 3% for both groups, P > 0.2). In regenerating alveolar bone at 1 week, animals that were treated with beta-galactosidase-positive cells and fluorescent bead-loaded cells exhibited approximately 30% and 25% of labeled cells respectively. At 2 weeks after wounding there was an increase in the percentage of transplanted beta-galactosidase-positive cells (approximately 39% at week 2; P < 0.05), but not of transplanted cells with fluorescent beads (approximately 25% at week 2). In sites with transplanted cells there were higher percentages of OPN positive and BSP positive cells in nascent bone and more newly formed bone than in controls (>40%; P < 0.05). Transplantation of beta-galactosidase-positive cells or cells loaded with fluorescent beads is a useful method for assessing the fate and differentiation of periodontal cells in vivo. Fluorescent beads, however, are diluted at mitosis and this method underestimates the percentage of transplanted cells. As transplanted periodontal cells in both groups promoted regeneration of alveolar bone, cell transplantation could improve the restoration of periodontium destroyed by periodontitis.

Involvement of actin filaments and integrins in the binding step in collagen phagocytosis by human fibroblasts

In physiological conditions, collagen degradation by fibroblasts occurs primarily via phagocytosis, an intracellular pathway that is thought to require collagen receptors and actin assembly for fibril internalization and degradation. Currently it is unclear which specific steps of collagen phagocytosis in fibroblasts involve actin filament assembly. As studies of phagocytosis in fibroblasts are complicated by the relatively slow rate of particle internalization compared to professional phagocytes, we have examined the role of collagen receptors and actin only in the initial collagen binding step. Prior to the binding of collagen-coated fluorescent beads by human gingival fibroblasts, a cell type that is avidly phagocytic in vitro, cells were treated with cytochalasin D (actin filament barbed-end capping) or swinholide A (actin dimer sequestering and severing) or latrunculin B (actin monomer sequestering). Bead binding and immunostaining of (alpha)(2)(beta)(1) and (alpha)(3)(beta)(1) integrin collagen receptors were measured by flow cytometry. After 1–3 hours of coincubation with beads, cytochalasin D or swinholide A eliminated actin filaments stained by rhodamine-phalloidin and inhibited collagen bead binding (reductions of 25% and 50%, respectively), possibly because of cell rounding and restricted interactions with beads. In contrast, latrunculin enhanced binding dose-dependently over controls (twofold at 1 microM) and induced the formation of brightly staining aggregates of actin and the retention of long cytoplasmic extensions. Latrunculin also reduced surface (beta)(1), (alpha)(2) and (alpha)(3) integrin staining up to 40% in bead-free and bead-loaded cells, indicating that latrunculin enhanced collagen receptor internalization. As determined by fluorescence recovery after photobleaching, latrunculin increased the mobility of surface-bound (beta)(1) integrin. The stimulatory effect of latrunculin on collagen bead binding was reduced to control levels by treatment with a (beta)(1) integrin inactivating antibody while a (beta)(1) integrin blocking antibody abrogated both bead binding and the latrunculin-induced stimulation. Immunoblotting of bead-associated proteins showed that latrunculin completely eliminated binding of (beta)-actin to collagen beads but did not affect (beta)(1) integrin binding. These data indicate that latrunculin-induced sequestration of actin monomers facilitates the disengagement of actin from (beta)(1) integrin receptors, increases collagen bead binding and enhances collagen receptor mobility. We suggest that these alterations increase the probability of adhesive bead-to-cell interactions.

Decreased capacity of asthmatic bronchial fibroblasts to degrade collagen

The mechanisms of fibrillar collagen accumulation in asthmatic bronchi remain unclear, an imbalance between synthesis and degradation of collagen may be implicated in this process. The aim of this study was to compare the capacities of normal (BNF) and asthmatic (BAF) bronchial fibroblasts to degrade collagen. Metalloproteinases and their inhibitors were measured by ELISA, types I and III procollagen synthesis was determined by liquid RIA and, finally, zymography was used to assess the presence of active and latent forms of MMPs. The capacity of fibroblasts to degrade collagen coated onto latex beads was evaluated by flow cytometry. Our results showed that MMP-2 secretion was significantly higher in BNF when compared to BAF and this was confirmed by gelatin zymography. In BNF culture, TIMP-1 and MMP-1 secretions positively correlated with types I and III procollagen synthesis. However, in BAF, this correlation was negative. This suggests that a balance exists between collagen synthesis and degradation in BNF and that this balance is compromised in BAF. On the other hand, BAF did show significantly reduced capacity to degrade collagen when compared to that of BNF. This reduced phagocytic activity was not associated with a decrease in collagen receptor expression. This study establishes for the first time that a relationship exists between metalloproteinases enzyme dysregulation and the reduced capacity of asthmatic bronchial fibroblast to degrade collagen. These events may shed light on why accumulation of collagen can be observed in asthmatic airways.

Cyclosporin A decreases the degradation of type I collagen in rat gingival overgrowth

Cyclosporin A (CsA) is used as an immunosuppressive agent and its prominent side effect is the induction of fibrous gingival overgrowth. The purpose of this study was to investigate the effect of CsA on the type I collagen metabolism in the gingiva of rats fed a powdered diet either containing or lacking CsA. Immunohistochemical analysis revealed that type I collagen was more prevalent in the connective tissue of CsA-treated gingiva than in those of control rats on days 15, 30, and 55 after the start of feeding. Total RNAs were isolated from mandibular molar gingiva on days 0, 3, 8, 15, 30, and 55. Quantitative analysis of mRNA by reverse transcriptase-polymerase chain reaction revealed that the CsA-treated groups showed a gradual decrease in expression of type I collagen and collagenase mRNAs, 0.4% and 18.0% on day 55 compared with those on day 0, respectively. In the control groups, type I collagen and collagenase mRNAs also decreased to 19.7% and 63.0%, respectively, however, both mRNA expressions were significantly lower in the CsA-treated group than in the controls. An electron microscopic analysis of fibroblasts was performed to count the number of cells with collagen fibrils in the cytoplasm, a marker of phagocytosis of collagen by fibroblasts. The collagen fibrils were detected in 4.7% +/- 2.7% and 24.3% +/- 13.7% of fibroblasts in the overgrown gingiva treated with CsA rat for 8 days and 30 days, but in 57.0% +/- 5.3% and 81.3% +/- 9.2% of fibroblasts in the each control group gingiva, respectively. Furthermore, in vitro analysis was performed to measure the phagocytosis of cultured fibroblasts by flow cytometry using collagen-coated latex beads. Fibroblasts isolated from CsA-treated gingiva on day 8 and day 30 contained 5.7% +/- 0.6% and 9.9% +/- 1.5% phagocytic cells, whereas control fibroblasts contained 50.3% +/- 5.5% and 33.3% +/- 4.9% phagocytic cells, respectively. The inhibition rate of phagocytic activity was similar between in vivo and in vitro assays. These findings suggest that the decrease of the collagen degradation due to the lower phagocytosis and the lower collagenase mRNA expression are closely associated with the increase of type I collagen accumulation in CsA-treated rat gingiva.

Treponema denticola outer membrane enhances the phagocytosis of collagen-coated beads by gingival fibroblasts

Human gingival fibroblasts (HGFs) degrade collagen fibrils in physiological processes by phagocytosis. Since Treponema denticola outer membrane (OM) extract perturbs actin filaments, important structures in phagocytosis, we determined whether the OM affects collagen phagocytosis in vitro by HGFs. Phagocytosis was measured by flow cytometric assessment of internalized collagen-coated fluorescent latex beads. Confluent HGFs pretreated with T. denticola ATCC 35405 OM exhibited an increase in the percentage of collagen phagocytic cells (phagocytosis index [PI]) and in the number of beads per phagocytosing cell (phagocytic capacity [PC]) compared with untreated controls. The enhancement was swift (within 15 min) and was still evident after 1 day. PI and PC of HGFs for bovine serum albumin (BSA)-coated beads were also increased, indicating a global increase in phagocytic processes. These results contrasted those for control OM from Veillonella atypica ATCC 17744, which decreased phagocytosis. The T. denticola OM-induced increase in bead uptake was eliminated by heating the OM and by depolymerization of actin filaments by cytochalasin D treatment of HGFs. Fluid-phase accumulation of lucifer yellow was enhanced in a saturable, concentration-dependent, transient manner by the T. denticola OM. Our findings were not due to HGF detachment or cytotoxicity in response to the T. denticola OM treatment since the HGFs exhibited minimal detachment from the substratum; they did not take up propidium iodide; and there was no change in their size, granularity, or content of sub-G1 DNA. We conclude that a heat-sensitive component(s) in T. denticola OM extract stimulates collagen phagocytosis and other endocytic processes such as nonspecific phagocytosis and pinocytosis by HGFs.

Deficiency in collagen and fibronectin phagocytosis by human buccal mucosa fibroblasts in vitro as a possible mechanism for oral submucous fibrosis

Oral submucous fibrosis (OSF), a chronic oral mucosal condition commonly found in south Asians, is a disorder characterized by a quantitative as well as a qualitative alteration of collagen deposition within the subepithelial layer of the oral mucosa. Since degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues, we have investigated phagocytosis of collagen- and fibronectin-coated latex beads by fibroblast cultures with an in vitro model system. Coated fluorescent latex beads were incubated with human oral mucosa fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 16 h contained a mean of 75% collagen phagocytic cells and 70% fibronectin phagocytic cells; however, about 15% and 10% of phagocytic cells individually contained more than twice the mean number of beads per cell. In contrast, cells from OSF tissues exhibited a 40% reduction of the proportions of collagen phagocytic cells (mean=35%) and a 48% decrease of the proportions of fibronectin phagocytic cells (mean=22%), none of the cells having a high number of beads as compared to normal fibroblasts. OSF lesions appear to contain fibroblasts with marked deficiencies in collagen and fibronectin phagocytosis. To investigate if inhibition of phagocytosis could be demonstrated in vitro, normal fibroblast cultures were incubated with areca nut alkaloids (arecoline, arecaidine). The cultures had a dose-dependent reduction in the proportions of phagocytic cells. On the other hand, corticosteroid used in the treatment of OSF exhibited a dose-dependent enhancement in the proportion of phagocytic cells. Therefore, our hypothesis for OSF, although oversimplified, is that betel nut alkaloids (arecoline, arecaidine) inhibit fibroblast phagocytosis and this provides a mechanism for the development of OSF. The benefit of a local intralesional injection of corticosteroid is also possibly, at least in part, through an enhancement of fibroblast collagen phagocytosis.

Continuity between wound macrophage and fibroblast phenotype: analysis of wound fibroblast phagocytosis

Analysis of phagocytic activity in wound fibroblasts was chosen as a means to assess the possible continuity between macrophage and fibroblast phenotypes. Fibroblast phagocytosis of uncoated, IgG-coated, or collagen-coated fluorescent beads was analyzed by flow cytometry in vivo and in vitro. Phagocytosis of fluorescent beads by procollagen I-positive cells (fibroblasts) was evaluated in vivo by injecting beads into subcutaneously implanted sponge wounds in anesthetized Fisher rats. Phagocytic activity of a purified population of wound fibroblasts was measured in vitro and correlated with oxidation state using hydroethidium. In the wound environment, 50-60% of the cells that engulfed uncoated, IgG-coated, or collagen-coated beads were procollagen I-positive cells (i.e., fibroblasts). Procollagen I-positive cells engulfed uncoated and IgG-coated beads in preference to collagen-coated beads in vivo. Cultured wound fibroblasts engulfed uncoated, IgG-coated, and collagen-coated particles. The majority of fibroblasts that engulfed beads were in an elevated oxidation state. We conclude that substantial fibroblast phagocytosis occurs in the wound, but scavenger receptor-mediated fibroblast phagocytosis is different from that of macrophages. Additional markers will be helpful in defining the macrophage fibroblast continuum.

Deregulation of collagen phagocytosis in aging human fibroblasts: effects of integrin expression and cell cycle

Intracellular degradation of collagen by phagocytosis in fibroblasts is essential for physiological remodeling of the extracellular matrix in a wide variety of connective tissues but imbalances between degradation and synthesis can lead to loss of tissue collagen. As aging is associated with loss of dermal and periodontal collagen and with increased lysomomal enzyme content in fibroblasts, we examined the regulation of collagen phagocytosis by integrin expression and the cell cycle in an in vitro fibroblast aging model. Two different fibroblast lines (CL1; CL2) at the fourth subculture were passaged up to replicative senescence to model aging processes in vitro. Cells were incubated with collagen-coated or BSA-coated green fluorescent beads for 3 h to assess alpha 2 beta 1-integrin-mediated or nonspecific phagocytosis, respectively. Single-cell suspensions were stained with DAPI and sulforhodamine 101 to separate cycling G1 and noncycling G0 cells. Staining for alpha 2-integrin, bead internalization, and bivariate analyses of DNA/protein content were measured by three-color flow cytometry. Serum deprivation was used to induce increases in the proportion of G0 cells. For G1 cells, the proportion of collagen phagocytic cells was > 50% for all passages and collagen beads were internalized > 5-fold more frequently than BSA beads. In contrast, G0 cells with diploid DNA content but low protein content exhibited greatly reduced phagocytic capacity (< 10% of cells internalized collagen or BSA beads), the number of beads per cells was 4-fold less, and alpha 2 integrin expression was very low compared to G1 cells. The proportion of collagen phagocytic cells and the proportion of alpha 2-integrin-positive cells increased with transit through the cell cycle. At higher passage numbers mean cell volume and cytoplasmic granularity were reduced approximately 30% but at replicative senescence cells with large surface area and subdiploid DNA predominated. The proportion of collagen and BSA phagocytic G1 cells increased 1.5- and 5-fold, respectively, and the number of beads per cell increased < 3-fold. However, surface alpha 2-integrin staining remained unchanged. These data indicate that the collagen and nonspecific internalization pathways were greatly unregulated, independent of cell cycle phase, and that cellular aging in vitro strongly influences the specificity and rate of phagocytic processes in fibroblasts. We suggest that age-related loss of collagen in connective tissues undergoing turnover may be a manifestation of a deregulated increase of collagen phagocytosis in which the net loss of degraded collagen exceeds new synthesis.

Collagen phagocytosis and apoptosis are induced by high level alkaline phosphatase expression in rat fibroblasts

Study of fibroblast origins and lineages is complicated by the lack of unambiguous markers that could be used to identify discrete subpopulations on the basis of functional attributes. We have studied the role of the membrane-anchored hydrolytic enzyme tissue-nonspecific alkaline phosphatase (TN-AP) and the placental alkaline phosphatase (PL-AP) in collagen phagocytosis and in the deletion of cells by apoptosis. Rat-2 cells, which do not constitutively express AP, were transfected with full-length rat TN-AP or PL-AP cDNAs to determine the impact of the TN-AP collagen-binding domain on cell function. Various levels of expression were driven by early (strong) or late (weak) SV40 promoters in the plasmid construct. Controls were transfected with plasmids that did not contain AP cDNA. AP expression in transfected cells was confirmed by Northern blotting, histochemical analysis, and SDS-PAGE analysis of membrane-anchored enzyme released by phosphatidyl inositol phospholipase C. Low levels of TN-AP expression increased cell spreading slightly, nearly doubled the percentage of collagen phagocytic cells (up to 80%), and increased the number of internalized collagen-coated fluorescence beads per cell. In cells transfected with PL-AP (i.e., no collagen-binding domain), collagen phagocytosis was not affected. Internalization of BSA beads was also not affected by either AP isozyme, indicating that AP was selective for integrin-mediated phagocytosis. In single cells, histochemically demonstrable TN-AP activity on cell membranes was colocalized with the binding of collagen beads, but this colocalization was not detected in cells transfected with PL-AP. Phagocytosis was inhibited by antibodies to the alpha 2 integrin and to AP but not by levamisole, an inhibitor of AP phosphohydrolytic activity. High-level TN-AP expression caused a fivefold reduction of cell proliferation and was associated with the development of cells with sub-G1 DNA content, nuclear condensation, and nuclear budding. In AP-positive cultures, there was a greatly increased number of floating cells; nick-labeling of DNA by terminal transferase and biotinylated dUTP showed a 15-fold increase of stained cells. These data indicate that low-level TN-AP expression enhances collagen phagocytosis, presumably through the TN-AP collagen-binding domain. High-level AP expression promotes cell deletion by apoptosis. We suggest that the expression of AP by fibroblasts indicates a novel role for this enzyme in collagen degradation by phagocytosis.

Role of integrins in regulation of collagen phagocytosis by human fibroblasts

Phagocytosis of collagen fibrils by fibroblasts is an important pathway for degradation of extracellular matrix in mature connective tissues. To study regulatory mechanisms in phagocytosis, 2-microns fluorescent beads coated with either collagen (COL) or bovine serum albumin (BSA) were incubated with human gingival fibroblasts in vitro. For these studies single cell suspensions were prepared by trypsinization, and bead internalization and collagen receptor expression were assessed by flow cytometry. After 3-h incubations, up to 8-fold more cells internalized COL beads than BSA-coated beads. Increased collagen coating concentration was associated with elevated proportions of cells that internalized COL beads, and was observed also in the presence of competing fibronectin-coated beads. The number of beads per cell and the percent of phagocytic cells increased proportionally with higher bead loadings. At > 4 beads per cell a maximum of approximately 80% of cells were phagocytic. Cells reacted with mAbs against the alpha 1, alpha 2, and alpha 3 integrin subunits were, respectively, 5%, 98% and 93% positively stained above background controls. All cells that internalized COL beads exhibited alpha 2 staining but there were large proportions of phagocytic cells that were not stained for alpha 1. In unfixed cells, bead internalization caused an immediate reduction of surface staining of membrane-bound alpha 2 by approximately 55% which returned to control levels within 3 h, indicating that cell-surface alpha 2 was internalized by phagocytosis. Preincubation of cells with up to 8 COL beads per cell reduced the proportion of phagocytic cells and the number of internalized beads after a second COL bead incubation 4 h later. To assess the relationship between the percent of phagocytic cells and alpha 2 integrin levels, serum starvation and cycloheximide experiments were conducted. Compared to controls, serum starvation for 24 h induced a 3.2-fold increase of cells internalizing COL beads but did not alter alpha 2 staining levels. In contrast, 3 h cycloheximide treatment reduced alpha 2 staining to 60% of control levels and this treatment also inhibited COL bead internalization. GRGDTP peptide as well as mAbs against the alpha 1 and alpha 2 subunits significantly reduced internalization of COL beads by 1.8 to 2.6-fold, whereas GRGESP peptide and alpha 3 mAb exerted no effect. Internalization of BSA beads was not affected by any of these treatments. Collectively, these data indicate that the alpha 2 integrin, along with other, as yet unidentified components, is likely involved in COL bead internalization. The alpha 2 integrin subunit is rapidly recycled or synthesized following a phagocytic load. In contrast, the alpha 1 integrin is not directly required for phagocytosis but may regulate the internalization step.

Phagocytosis and intracellular digestion of collagen, its role in turnover and remodelling

Collagens of most connective tissues are subject to continuous remodelling and turnover, a phenomenon which occurs under both physiological and pathological conditions. Degradation of these proteins involves participation of a variety of proteolytic enzymes including members of the following proteinase classes: matrix metalloproteinases (e.g. collagenase, gelatinase and stromelysin), cysteine proteinases (e.g. cathepsin B and L) and serine proteinases (e.g. plasmin and plasminogen activator). Convincing evidence is available indicating a pivotal role for matrix metalloproteinases, in particular collagenase, in the degradation of collagen under conditions of rapid remodelling, e.g. inflammation and involution of the uterus. Under steady state conditions, such as during turnover of soft connective tissues, involvement of collagenase has yet to be demonstrated. Under these circumstances collagen degradation is likely to take place particularly within the lysosomal apparatus after phagocytosis of the fibrils. We propose that this process involves the following steps: (i) recognition of the fibril by membrane-bound receptors (integrins?), (ii) segregation of the fibril, (iii) partial digestion of the fibril and/or its surrounding non-collagenous proteins by matrix metalloproteinases (possibly gelatinase), and finally (iv) lysosomal digestion by cysteine proteinases, such as cathepsin B and/or L. Modulation of this pathway is carried out under the influence of growth factors and cytokines, including transforming growth factor beta and interleukin 1 alpha.

Deficiencies in collagen phagocytosis by human fibroblasts in vitro: a mechanism for fibrosis?

Degradation of collagen by fibroblast phagocytosis is an important pathway for physiological remodelling of soft connective tissues. Perturbations of this pathway may provide a mechanism for the development of fibrotic lesions. As collagen phagocytosis may be regulated by either a change of the proportions or the activity of phagocytic cells, we quantified phagocytosis with an in vitro model system. Collagen-coated fluorescent latex beads were incubated with human gingival fibroblasts and the fluorescence associated with internalized beads was measured by flow cytometry. Cells from normal tissues that had been incubated with beads for 3 hours contained a mean of 64% phagocytic cells; however, a small subpopulation (10% of phagocytic cells) contained more than threefold higher numbers of beads per cell than the mean. In contrast, cells from fibrotic lesions exhibited a large reduction of the proportions of phagocytic cells (mean = 13.8%) and there were no cells with high numbers of beads. On the basis of 3H-Tdr labeling, cells from fibrotic lesions that had internalized beads failed to proliferate, in contrast to phagocytic cells from normal tissues, which underwent repeated cell divisions. This result was not due to variations of cell cycle phase as there was no preferential internalization of beads during different phases of the cell cycle. The low phagocytic rate of cells from fibrotic lesions was also not due to asymmetric partitioning of phagosomes at mitosis as videocinemicrography of bead-labeled phagosomes in single, pre-mitotic cells demonstrated that > 90% of phagocytic cells equally partitioned beads to daughter cells. To investigate if inhibition of phagocytosis could be replicated in vitro, cells were incubated with the fibrosis-inducing drugs nifedipine or dilantin. These cultures exhibited marked (15-75%), dose-dependent reductions in the proportions of phagocytic cells, but there was no reduction in bead number per cell. Fibrotic lesions appear to contain fibroblasts with marked deficiencies in phagocytosis and the reduced phagocytic activity of these cells may contribute to unbalanced degradation and fibrosis.

Uptake of toxic silica particles by isolated rat liver macrophages (Kupffer cells) is receptor mediated and can be blocked by competition

Silica particles (quartz dust) are toxic to macrophages after their uptake into these cells. These experiments describe the opsonization mechanism(s) and macrophage receptor(s) involved in silica uptake. Freshly isolated rat liver macrophages (Kupffer cells) were incubated at 37 degrees C with silica particles in the presence or absence of autologous or heterologous plasma or purified plasma fibronectin and cell viability was assessed at various times. Within 60 min of coincubation, > 80% of macrophages were lysed in the presence of plasma or purified fibronectin but not in their absence (viability > 90%). Lysis was slower with defibronectinized plasma (28% in 60 min). Macrophages could be protected from lysis by addition of the monosaccharide N-acetyl-D-galactosamine but not by N-acetyl-D-glucosamine. Galactosylated serum albumin but not mannosylated albumin or native albumin exerted full protection from lysis. The pentapeptide GRGDS also prevented macrophage lysis in synergy with N-acetyl-galactosamine. Enzymatic deglycosylation of fibronectin reduced lysis significantly. These findings indicate an important opsonizing activity for fibronectin and dual recognition via the lectin-like galactose-specific binding activity of membrane-associated C-reactive protein and by integrin receptor(s). Binding experiments (at 4 degrees C) revealed initial binding as primarily galactose-inhibitable, suggesting integrin-mediated binding as a later event necessary for effective uptake.

Interaction of fibronectin and fibronectin binding protein (FnBP) of Staphylococcus aureus with murine phagocytes and lymphocytes

In the present study we examined the in vitro and in vivo interactions of a cloned staphylococcal fibronectin binding protein (FnBP) and plasma fibronectin (Fn) with polymorphonuclear cells (PMNs) and macrophages, and how antibodies against FnBP affect the phagocytosis process in vitro. Moreover, the interaction of FnBP and Fn coupled on latex beads as 'artificial bacteria' and 'artificially opsonized bacteria' with murine spleen cells and peritoneal macrophages was tested. The major finding of the present study is that antibodies against the FnBP of Staphylococcus aureus (S. aureus) and low concentration of antibodies recognized two IgG-binding domains of protein A (SpA) are effective in the promotion of phagocytosis in vitro. It was also observed that FnBP has a chemoattractant activity and causes accumulation of PMNs and macrophages in the mouse peritoneal cavity when injected 24 h before irritation of peritoneal exudate. It seems likely that this activity is connected with binding to Fn molecules since formalin inactivation of FnBP (60%) abolished it. In the in vitro phagocytosis assay in the presence of FnBP (in a medium supplemented with serum depleted of Fn), ingestion of bacteria by phagocytes was identical to assay carried out in the presence of BSA. However, addition of plasma fibronectin caused an increased uptake of bacteria by macrophages and to a lesser degree by PMNs. We observed that in a population of normal splenocytes, those cells that effectively bound FnBP- and Fn-coated latex beads were mostly those cells exhibiting macrophage and dendritic morphology. In populations of spleen cells of animal infected with S. aureus, T lymphocytes were also found to bind FnBP- and Fn-coated latex beads. These data suggest that FnBP may have the ability to promote aggregation of immune cells, either directly or by interaction with plasma Fn, which can be helpful in certain cell-cell interactions taking place at the initial stages of specific immune response.