Modulation of fibronectin gene expression in chondrocytes by viral transformation and substrate attachment

Identification of a CD36-related thrombospondin 1-binding domain in HIV-1 envelope glycoprotein gp120: relationship to HIV-1-specific inhibitory factors in human saliva

Human and non-human primate salivas retard the infectivity of HIV-1 in vitro and in vivo. Because thrombospondin 1 (TSP1), a high molecular weight trimeric glycoprotein, is concentrated in saliva and can inhibit the infectivity of diverse pathogens in vitro, we sought to determine the role of TSP1 in suppression of HIV infectivity. Sequence analysis revealed a TSP1 recognition motif, previously defined for the CD36 gene family of cell adhesion receptors, in conserved regions flanking the disulfide-linked cysteine residues of the V3 loop of HIV envelope glycoprotein gp120, important for HIV binding to its high affinity cellular receptor CD4. Using solid-phase in vitro binding assays, we demonstrate direct binding of radiolabeled TSP1 to immobilized recombinant gp120. Based on peptide blocking experiments, the TSP1-gp120 interaction involves CSVTCG sequences in the type 1 properdin-like repeats of TSP1, the known binding site for CD36. TSP1 and fusion proteins derived from CD36-related TSP1-binding domains were able to compete with radiolabeled soluble CD4 binding to immobilized gp120. In parallel, purified TSP1 inhibited HIV-1 infection of peripheral blood mononuclear cells and transformed T and promonocytic cell lines. Levels of TSP1 required for both viral aggregation and direct blockade of HIV-1 infection were physiologic, and affinity depletion of salivary TSP1 abrogated >70% of the inhibitory effect of whole saliva on HIV infectivity. Characterization of TSP1-gp120 binding specificity suggests a mechanism for direct blockade of HIV infectivity that might be exploited to retard HIV transmission that occurs via mucosal routes.

Involvement of alpha5beta1 integrin in matrix interactions and proliferation of chondrocytes

Integrins are cell surface receptors involved in cellular processes including adhesion, migration, and matrix assembly. In the present study, we analyzed the possible involvement of alpha 5 beta 1 integrin in the regulation of chondrocyte adhesion, spreading, and proliferation. We found that rabbit growth plate chondrocytes were able to attach to substrates coated with type I collagen, type II collagen, or fibronectin within 24 h of culture. During this time period, attachment to fibronectin appeared to be dependent on alpha 5 beta 1 integrin, whereas adhesion to collagens was not. By day 3 of culture, chondrocytes spread onto all the substrates tested. We found that regardless of the nature of the substrate, cell spreading was reversed by treatment with RGD peptide or antibodies against alpha 5 beta 1 or fibronectin, indicating that cell spreading involved alpha 5 beta 1 and fibronectin endogenously produced and deposited by the chondrocytes themselves. Colony formation by chondrocytes in soft agar was inhibited by treatment with RGD peptides or BIIG2, an antibody that interferes with alpha 5 beta 1 integrin-ligand interactions. Furthermore, DNA content was decreased by treatment with anti-fibronectin antibody in micromass culture of chondrocytes. Immunohistochemical analysis on tissue sections revealed that the alpha 5 subunit was particularly abundant in the proliferative and hypertrophic zones of growth plate. The results of the study indicate that alpha 5 beta 1 integrin plays multiple roles in chondrocyte behavior and function and appears to be involved in the regulation of both chondrocyte-matrix interactions and proliferation.

Fibronectin gene expression, synthesis and accumulation during in vitro differentiation of chicken osteoblasts

A well-defined chicken osteoblast culture system(18) has been used to examine fibronectin (FN) mRNA levels, synthesis, and accumulation during in vitro differentiation and matrix mineralization. Immunofluorescent staining of cells after 6 or 18 days in culture revealed that FN was initially associated with the cell surface and in partial coalignment with cytoskeletal elements while at the latter time most FN was associated with the extracellular matrix as a ubiquitous fibrillar network. Western blot analysis of total cell-associated proteins also detected FN at all culture times. However, when results were normalized to cellular DNA, FN levels increased until 12-16 and remained relatively constant thereafter. Similarly, FN synthesis as measured by [35S]-methionine labeling, and immunoprecipitation was greatest in early cultures (culture day 3) and then declined such that synthesis decreased 60% at day 18 and 94% after 24-31 days. FN mRNA levels as measured by Northern blot analysis were well correlated with FN synthesis. These results clearly show that FN is made by primary osteoblasts during their in vitro maturation. In contrast to other osteoblast markers such as alkaline phosphatase, osteocalcin, and osteopontin, whose expression increases as cells differentiate, FN accumulates in the matrix during periods of early cell growth and attachment and then remains proportional to cell number. Results with FN differ from those obtained with collagen which continues to accumulate in the extracellular matrix during osteoblast maturation. These results are consistent with FN being important for the initial attachment of early osteoblasts or osteoblast precursors to the pericellular matrix.

Rapid induction of type X collagen gene expression in cultured chick vertebral chondrocytes

During endochondral ossification, small rapidly proliferating chondrocytes mature into flattened disc-shaped cells and then into large round hypertrophic cells. These morphological changes are accompanied by a decrease in the rate of cell proliferation. Type X collagen synthesis is initiated during chondrocyte maturation and reaches very high levels in the hypertrophic cells. We have analyzed type X collagen gene expression in chick embryo vertebral chondrocytes that were allowed to mature in monolayer culture and were then switched to suspension culture. The resuspended chondrocytes changed in shape from flat to round and decreased the proliferation rate as they do in vivo. These events were accompanied by a rapid, dramatic increase in type X collagen gene expression at the levels of transcription, steady-state mRNA and protein synthesis, as well as an increase in the number of cells producing type X collagen. The amount of type X collagen gene expression in resuspended chondrocytes was comparable to that in mineralizing cartilage in vivo. These results indicate that events accompanying the switch from monolayer to suspension culture (for example, the change from a flat to a round shape and/or the decrease in proliferation rate) may play a role in the induction of type X collagen gene expression during chondrocyte maturation. Thus we have developed an in vitro system that appears to mimic the events occurring during in vivo chondrocyte maturation. This in vitro model may provide an ideal system for further examination of the parameters regulating chondrocyte maturation and type X collagen gene expression.

Type X collagen gene expression is transiently up-regulated by retinoic acid treatment in chick chondrocyte cultures

During endochondral ossification, resting and proliferating chondrocytes mature into hypertrophic chondrocytes that initiate synthesis of type X collagen. The mechanisms regulating the differential expression of type X collagen gene were examined in confluent Day 12 secondary cultures of chick vertebral chondrocytes in monolayer treated with the vitamin A analog retinoic acid (RA). Preliminary results showed that major effects of RA on chondrocyte gene expression occurred between 24 and 48 h of treatment. Thus in subsequent experiments cultures were treated for 24, 30, 36, 42, 48, 72, 96, and 120 h. Total RNAs were isolated and analyzed by hybridization with 32P-labeled plasmid probes coding for five matrix macromolecules including type X collagen. We found that the steady-state levels of mRNAs for the large keratan sulfate/chondroitin sulfate proteoglycan (KS:CS-PG) core protein and type II collagen decreased several fold between 24 and 48 h of treatment compared to untreated cells, and remained low with further treatment. In sharp contrast, the level of type X collagen mRNA increased threefold by 42 h of treatment; thereafter it began to decrease and reached minimal levels by 72-120 h of treatment. The changes in steady-state mRNA levels during RA regimen paralleled similar changes in relative rates of protein synthesis. The transient up-regulation of type X collagen gene expression at 42 h of treatment was preceded by a five-fold increase in fibronectin gene expression, was followed by a several fold increase in type I collagen gene expression, and was accompanied by cell flattening and loss of the pericellular proteoglycan matrix. Thus, RA treatment leads to a unique biphasic modulation of type X collagen gene expression in maturing chondrocyte cultures. The underlying, RA-sensitive mechanisms effecting this modulation may reflect those normally regulating the differential expression of this collagen gene during endochondral ossification.

The effect of heparin on fibronectin and thrombospondin synthesis and mRNA levels in cultured human endothelial cells

Studies to eludicate the effect of heparin on the synthesis of extracellular matrix components by cultured human umbilical vein endothelial cells (EC) were conducted. Using pulse-labeling and ELISA techniques, we found that EC grown in the presence of heparin (90 micrograms/ml) and endothelial cell growth factor (ECGF) synthesized 50% less fibronectin (FN) than did ECGF-treated control cultures. No change in the synthesis of thrombospondin (TSP) was induced by heparin. The effect of heparin on EC FN synthesis was independent of whether the cells were cultivated on plastic or gelatin substrates. However, ECGF modulates the effect of heparin on EC synthesis of FN. RNA slot-blot analysis demonstrated that heparin treatment specifically decreased the steady-state mRNA levels for both FN and TSP in the cells. Steady-state levels of mRNA for two intracellular proteins, actin and tubulin, were unchanged. These data suggest that heparin decreases EC expression of FN at least in part by decreasing the amount of FN mRNA available for translation. The failure of heparin to inhibit TSP expression, although it reduces TSP mRNA levels, points to the possibility that the rate of EC synthesis of TSP is translationally or post-translationally regulated.

Comparative morphological and biochemical analysis of hypertrophic, non-hypertrophic and 1,25(OH)2D3 treated non-hypertrophic chondrocytes

A comparative study of Type X collagen expression, chondrocyte morphology, and the expression of two genes controlling chondrocyte morphology (beta-actin and fibronectin) was carried out on chondrocytes derived from a tissue that remains as permanent hyaline cartilage in vivo (embryonic chicken caudal sterna), from a tissue that undergoes endochondral replacement (embryonic chicken ventral vertebrae) and caudal sternal chondrocytes treated with 1,25(OH)2D3. Under identical in vitro growth conditions and times, sternal chondrocytes grew as rounded non-adherent cells, and vertebral chondrocytes grew as adherent polygonal cells. Upon treatment with 10(-8) M 1,25(OH)2D3 over a twelve day period the sternal chondrocytes showed complete adherence and took on an identical appearance as the vertebral chondrocytes. Cellular adherence of both vertebral and 1,25(OH)2D3 treated sternal chondrocytes was associated with 10 X increased beta-actin, fibronectin and their corresponding mRNA's. Changes in connective tissue expression were observed with altered cellular morphology. Total collagen synthesis was 35-50% lower in both hormone treated and vertebral chondrocytes. Type II collagen was the major collagen type produced by all chondrocyte cultures; however, in both vertebral and 1,25(OH)2D3 treated sternal chondrocytes, a 60 kD collagenous protein was identified. This short chain collagen was determined to be Type X collagen based on its molecular weight and its CNBr peptide maps. Analysis of Type X mRNA levels using a 33 base pair anti sense oligonucleotide sequence to Type X, demonstrated that vertebral cells showed six to seven times more mRNA than sternal chondrocytes. However, the low mRNA levels of type X mRNA in sternal chondrocytes were increased two to three times by 1,25(OH)2D3 treatment. These studies demonstrate that the steroid hormone 1,25(OH)2 vitamin D3 induced morphological, biochemical and molecular changes indicative of chondrocyte maturation from a hyaline to a more hypertrophic phenotype.

Heparin increases mRNA levels of thrombospondin but not fibronectin in human vascular smooth muscle cells

The effects of heparin (180 micrograms/ml) on steady state mRNA levels for fibronectin, thrombospondin, actin and collagen types I and III were investigated in human umbilical artery smooth muscle cells. Heparin caused a 120% increase in thrombospondin mRNA levels and a 60% and 180% increase in the mRNA levels of procollagen chains alpha 2(I) and alpha 1(III), respectively. No change in fibronectin or actin mRNA levels resulted from heparin treatment. We reported earlier (Biochem. Biophys. Res. Comm. 148:1264, 1987) that heparin increases smooth muscle cell synthesis of both fibronectin and thrombospondin. These data show that heparin coordinately regulates thrombospondin mRNA and protein levels. The heparin induced increase in fibronectin biosynthesis apparently reflects control at the translational or post-translational level.

Effects of cell shape on type X collagen gene expression in hypertrophic chondrocytes

During endochondral ossification, small flat resting and proliferating chondrocytes mature into large round hypertrophic chondrocytes that synthesize a unique collagen, type X. We have asked whether this change in cell shape during chondrocyte maturation regulates type X collagen gene expression, using immature chick vertebral chondrocytes grown in monolayer or in suspension. The freshly isolated chondrocytes contained no type X collagen RNA, but after 30 days of culture, both attached and suspended cells contained a similar large amount. However, in cells that were grown in monolayer and then resuspended three days before harvest, type X collagen gene expression increased a further 6 fold. These results suggest that the change from a flat to a round shape that occurs during chondrocyte maturation in vivo may be important for maximal expression of the type X collagen gene.