Suppressed collagen gene expression and induction of alpha 2 beta 1 integrin-type collagen receptor in tumorigenic derivatives of human osteogenic sarcoma (HOS) cell line

Characterization of the roles of RHOC and RHOA GTPases in invasion, motility, and matrix adhesion in inflammatory and aggressive breast cancers

BACKGROUND

The 2 closely related small GTPases, RHOC and RHOA, are involved in mammary gland carcinogenesis; however, their specific roles in determining cancer cell adhesion and invasion have not been elucidated.

METHODS

RHOA and RHOC are highly homologous, thereby posing a major challenge to study their individual functions in cancer cells. By selectively knocking down these proteins, we have been able to alternatively inhibit RHOC and RHOA, while preserving expression of the other rho protein. Quantitative analyses of the growth patterns and invasion in the aggressive estrogen receptor negative cell lines MDA-231 and SUM149 were carried out on collagen I and Matrigel substrates.

RESULTS

RHOC, and not RHOA, modulates surface expression and colocalization of alpha2 and beta1 integrins in MDA-MB-231 on collagen I. Neither RHOC or RHOA affected integrin expression in the inflammatory breast cancer cell line SUM149, further highlighting the different regulation of adhesion and motility in inflammatory breast cancer.

CONCLUSIONS

This work shows that RHOC and RHOA play different roles in cell-matrix adhesion, motility, and invasion of MDA-MB-231 and reaffirms the crucial role of RHOC-GTPase in inflammatory breast cancer cell invasion.

Driven to death: Inhibition of farnesylation increases Ras activity and promotes growth arrest and cell death [corrected]

To improve cancer outcomes, investigators are turning increasingly to small molecule medicines that disrupt vital signaling cascades, inhibit malignant growth, or induce apoptosis. One vital signaling molecule is Ras, and a key step in Ras activation is membrane anchoring of Ras through prenylation, the C-terminal addition of a lipid anchor. Small molecule inhibitors of farnesyltransferase (FTI), the enzyme most often responsible for prenylating Ras, showed clinical promise, but development of FTIs such as tipifarnib has been stalled by uncertainty about their mechanism of action, because Ras seemed unimpeded in tipifarnib-treated samples. Interpretation was further complicated by the numerous proteins that may be farnesylated, as well as availability of an alternate prenylation pathway, geranylgeranylation. Our initial observations of varied response by cancer cell lines to tipifarnib led us to evaluate the role of FTI in Ras signal alteration using various tumor models. We describe our novel counterintuitive finding that endogenous Ras activity increases in cancer cell lines with low endogenous Ras activity when farnesyltransferase is inhibited by either tipifarnib or short hairpin RNA. In response to tipifarnib, variable growth arrest and/or cell death correlated with levels of activated extracellular signal–regulated kinase (ERK) and p38 mitogenactivated protein kinase (MAPK). Sensitivity to tipifarnib treatment was shown by growth inhibition and by an increase in subdiploid cell numbers; cells with such sensitivity had increased activation of ERK and p38 MAPK. Because Ras must be prenylated to be active, our findings suggest that geranylgeranylated N-Ras or K-Ras B interacts differently with downstream effector proteins in sensitive cancer cells responding to tipifarnib, switching the balance from cell proliferation to growth inhibition [corrected].

Rho proteins and cell-matrix interactions in cancer

Dynamic interactions and dissolution of cell-extracellular matrix contacts are required steps to support cell growth and survival during cancer cell metastasis. Malignant cells acquire the ability to remodel extracellular matrix (ECM) and to modulate the expression of ECM receptors. Integrins are cellular receptors for molecules in the extracellular matrix. Integrin signaling is known to regulate metastatic cancer phenotypes by interacting synergistically with several signaling pathways, including the growth factor receptor pathway, the Ras-MAP kinase (Ras-MAPK) pathway and the Rho-effector pathway. In this mini-review, we discuss the functions of the Rho proteins and their relationship with other signaling pathways in matrix remodeling and integrin signaling of highly motile and invasive cancer cells.

Regulation of prostate cell collagen receptors by malignant transformation

Cell adhesion receptors, including the integrin-type collagen receptors (alpha1beta1, alpha2beta1, alpha10beta1 and alpha11beta1) participate in cancer progression and invasion. Quantitative RT-PCR indicated that all 4 receptors are abundantly expressed in sarcoma-derived cell lines, whereas most carcinoma-derived cells express alpha1beta1 and alpha2beta1 only. This was surprising because alpha11beta1 has been connected previously to the progression of lung adenocarcinomas. To test the hypothesis that alpha11 expression may not persist in cultured cancer cells we analyzed fresh tissue samples of 104 total prostatectomies, keeping in mind that prostate cancer cell lines showed negligible alpha11 mRNA levels. In prostate alpha2 expression was significantly lower in poorly differentiated carcinomas when compared to benign lesions (p = 0.0331). In immunohistochemistry the protein levels of alpha2 integrin decreased significantly (p = 0.0001) and the protein levels of alpha11 subunit increased significantly (p = 0.029) with the increasing grade of carcinoma. Thus alpha11beta1 may replace alpha2beta1 during tumor progression. Our observations support the idea that alpha11beta1 may be expressed in tumors but the corresponding cell lines may lose the expression of this integrin. Previous studies have shown that in cell culture androgen receptor (AR) controls alpha2beta1 expression. We measured AR mRNA levels and the number of AR positive nuclei in the prostate samples and the results showed a significant correlation between alpha2beta1 and AR. Androgen receptors may control the mechanisms regulating integrin expression in prostate.

The Fibril-associated Collagen IX Provides a Novel Mechanism for Cell Adhesion to Cartilaginous Matrix

Collagen IX is the prototype fibril-associated collagen with interruptions in triple helix. In human cartilage it covers collagen fibrils, but its putative cellular receptors have been unknown. The reverse transcription-PCR analysis of human fetal tissues suggested that based on their distribution all four collagen receptor integrins, namely alpha1beta1, alpha2beta1, alpha10beta1, and alpha11beta1, are possible receptors for collagen IX. Furthermore primary chondrocytes and chondrosarcoma cells express the four integrins simultaneously. Chondrosarcoma cells, as well as Chinese hamster ovary cells transfected to express alpha1beta1, alpha2beta1, or alpha10beta1 integrin as their only collagen receptor, showed fast attachment and spreading on human recombinant collagen IX indicating that it is an effective cell adhesion protein. To further study the recognition of collagen IX we produced recombinant alphaI domains in Escherichia coli. For each of the four alphaI domains, collagen IX was among the best collagenous ligands, making collagen IX exceptional compared with all other collagen subtypes tested so far. Rotary shadowing electron microscopy images of both alpha1I- and alpha2I-collagen IX complexes unveiled only one binding site located in the COL3 domain close to the kink between it and the COL2 domain. The recognition of collagen IX by alpha2I was considered to represent a novel mechanism for two reasons. First, collagen IX has no GFOGER motif, and the identified binding region lacks any similar sequences. Second, the alpha2I domain mutations D219R and H258V, which both decreased binding to collagen I and GFOGER, had very different effects on its binding to collagen IX. D219R had no effect, and H258V prevented type IX binding. Thus, our results indicate that collagen IX has unique cell adhesion properties when compared with other collagens, and it provides a novel mechanism for cell adhesion to cartilaginous matrix.

Binding of α2 monoclonal antibody to human cervical tumor cell (SiHa) surface α2β1 integrin modulates MMP-2 activity

OBJECTIVE(S)

The purpose was to study the interrelationship between cell surface integrin receptor (alpha2beta1) and matrixmetalloproteinases.

METHODS

Immunoprecipitation and cell adhesion assay were done to assay alpha2beta1 and alpha3beta1 on SiHa cell surface. Zymogram was developed to assay secreted MMP activity of cells grown in presence of alpha2 monoclonal antibody. Immunoblot was developed to assay expression of MMP-2, FAK, and p-FAK. Plasma membrane-dependent activation of MMP2 was performed by incubating pure MMP-2 with membrane-enriched fraction isolated from SiHa cells.

RESULTS

Immunoprecipitation and cell adhesion assay results confirmed the presence of alpha2beta1 receptor on SiHa cells. Zymographic analysis of serum-free media collected at different time points from SiHa cells grown on alpha2 monoclonal antibody-coated culture dishes showed the expression and activation of MMP-2 within 2-4 h, confirmed by immunoblot. Western blot of cells grown on alpha2-coated dishes for 30 min-4 h showed increased phosphorylation of FAK. Membrane-enriched fraction isolated from SiHa cells was found to specifically activate proMMP-2 to its activated forms within 30 min.

CONCLUSION(S)

The experimental findings strongly indicate that SiHa cell surface alpha2beta1 regulates MMP-2 expression. Increased phosphorylation of focal adhesion kinase (FAK) strongly indicates the possible role of FAK in signaling cascade. Incubation of SiHa cell membrane fraction with pure MMP-2 strongly confirms the cell membrane-dependent activation of proMMP2.

Patterns of integrin expression in a human mandibular explant model of osteoblast differentiation

Cell-matrix interaction is crucial in regulating osteoblast differentiation and function. These interactions are themselves regulated, at least in part, by integrins. Although there are some data from mammalian models, few studies have compared integrin expression at different stages of the osteoblast lineage. Here, primary human mandibular osteoblast cultures were grown in the presence of epidermal growth factor (EGF), giving a proliferative, less differentiated phenotype, or of vitamin D(3) and hydrocortisone (D+Hc), giving a more differentiated phenotype. These cultures were compared with those of cells prepared in the absence of EGF or D+Hc by fluorescence-activated cell sorter using a panel of monoclonal antibodies to specific integrin heterodimers. To provide in vivo correlation, the same panel of antibodies was used to stain fresh-frozen, undemineralised sections of human mandibular bone. Under baseline conditions the alpha(3), alpha(5), alpha(v), alpha(v)beta(3), beta(3) and beta(1) integrin subunits were expressed strongly by the cells, with low-level expression of the alpha(1), alpha(2) and alpha(4) subunits. In the presence of EGF there was increased alpha(2) expression. With D+Hc, alpha(3) and alpha(5) expression was elevated. Immunohistochemical analysis demonstrated alpha(2), alpha(3), alpha(5), alpha(v)beta(3), beta(1) and beta(3) subunits in cells of the osteoblast lineage; alpha(2) staining was restricted to cells close to the bone surface whilst alpha(v)beta(3) and beta(3) were most frequently localised in the osteocytes. The results provide evidence that cells at successive stages of the osteoblast lineage show different patterns of integrin expression. These integrins may be important in cell-matrix interactions leading to osteoblast differentiation.

Three-dimensional collagen regulates collagen gene expression by a mechanism that requires serine/threonine kinases and is independent of mechanical contraction

Integrin alpha1beta1, one of the cellular collagen receptors, can participate in the regulation of collagen accumulation by acting as a negative feedback regulator. The molecular mechanism behind this phenomenon has been unknown. We have plated cells inside three-dimensional collagen and analyzed a set of chemical inhibitors for various signal transduction pathways. Only two wide-spectrum serine/threonine kinase inhibitors, H-7 and iso-H-7 could prevent the down-regulation of alpha1(I) collagen mRNA levels in cells exposed to three-dimensional collagen. In monolayer iso-H-7 slightly down-regulated collagen gene expression, indicating that inside collagen it affected integrin signaling rather than having a direct stimulatory effect on collagen mRNA levels. The effect of iso-H-7 was not dependent on its ability to inhibit protein kinases A, C, or G. H-7 and iso-H-7 could also inhibit collagen gel contraction, but this mechanism was independent of collagen gene regulation. Three-dimensional collagen could also up-regulate the mRNA levels of several matrix metalloproteinases (MMPs) but H-7 and iso-H-7 had no effect on the regulation of MMP genes. Our data indicate that three-dimensional collagenous matrix regulates distinct cellular signaling pathways and that collagen gene regulation is independent of the other effects of the matrix.

Integrin expression by primary and immortalized human chondrocytes: evidence of a differential role for alpha1beta1 and alpha2beta1 integrins in mediating chondrocyte adhesion to types II and VI collagen

OBJECTIVE

Chondrocytes have been shown to express beta1-containing integrins both in vitro and in situ, but their role in regulating chondrocyte function is poorly understood. The objective of this study was to determine how the relative expression of different integrins may be modulated in relation to the differentiated state and proliferative capacity of the chondrocyte.

DESIGN

Integrin expression by four different cell lines of human chondrocytes immortalized with Simian virus 40 large T-antigen (SV40-TAg) was studied and compared to primary chondrocytes. Differences in alpha1 and alpha2 integrin subunit expression were utilized to further study the role of these integrins in mediating adhesion to types II and VI collagen.

RESULTS

The overall cell-surface levels of beta1-containing integrins were higher on all four immortalized cell lines which expressed over 10-fold higher levels of alpha2 and alpha3 integrin subunits compared to primary cells. However, primary cells expressed higher levels of the alpha1 integrin subunit which was not expressed by T/C28a4 cells and expressed at variable and lower levels in the other lines. Levels of the alpha3 integrin subunit were significantly greater on the highly proliferative juvenile costal chondrocyte lines (T/C-28a4, C-2812, and C-20a4) compared to primary articular chondrocytes and tsT/AC-62 cells which were derived from adult articular chondrocytes. Expression of alpha5 was similar among primary cells and cell lines except on C-20/A4 cells which had an average of over 4-fold higher levels. None of the primary or immortalized chondrocytes tested expressed significant levels of alpha4. Cell adhesion assays revealed that both alpha1beta1 and alpha2beta1 could serve as chondrocyte adhesion receptors for types II and VI collagen. In cell lines expressing both integrins, alpha1beta1 was the preferential receptor for type VI collagen while alpha2beta1 was the preferential receptor for type II collagen. Rather than inhibiting adhesion, incubation with the alpha3 blocking antibody P1B5 increased adhesion of C-28/12 cells to both fibronectin and type II collagen by 67% and 100% respectively.

CONCLUSIONS

Immortalization with SV40-TAg results in altered integrin expression by chondrocytes. Changes in the relative levels of alpha1, alpha2, and alpha3 subunits may significantly alter the manner in which chondrocytes interact with types II and VI collagen in the extracellular matrix.

Effect of gel re-organization and tensional forces on alpha2beta1 integrin levels in dermal fibroblasts

Mechanical forces are known to play an important role in regulating cell function in a wide range of biological systems. This is of particular relevance to dermal fibroblast function, given that the skin is known to be held under an intrinsic natural tension. To understand more about the generation of force by dermal fibroblasts and their ability to respond to changes in it, we have studied the role of the beta1 integrin receptors expressed by dermal fibroblasts in their ability to generate tensional forces within a collagen type I matrix and the effect of altered tensional force on integrin expression by dermal fibroblasts. Using a purpose-built culture force monitor, function-blocking antibodies directed towards the beta1 receptors dramatically reduced the tensional forces generated by dermal fibroblasts in a 3D collagen I matrix. However, the specific involvement of alpha1 or alpha2 subunits could not be demonstrated. Analysis of cellular response demonstrated that cells isolated from contracting collagen gels expressed fourfold higher levels of alpha2 mRNA than cells isolated from fully restrained gels. The levels of beta1 messenger RNA were relatively unaffected by reductions in force. Cells exposed to single reductions in force, however, did not exhibit alterations in either alpha1 or beta1 mRNA levels. We propose, therefore that alpha2beta1 integrin receptor levels in dermal fibroblasts are not altered in response to single reductions of gel tension, but do change following a continual change in force and associated matrix re-organization

Positive and negative regulations of the renal 25-hydroxyvitamin D3 1alpha-hydroxylase gene by parathyroid hormone, calcitonin, and 1alpha,25(OH)2D3 in intact animals

Reflecting the prime role of 1alpha,25(OH)2D3 in calcium homeostasis, the activity of 25-hydroxyvitamin D3 1alpha-hydroxylase, a key enzyme for 1alpha,25(OH)2D3 biosynthesis, is tightly regulated by 1alpha,25(OH)2D3, PTH and calcitonin. Its significant activity is found in kidney, though the enzymatic activity is also reported in extra-renal tissues. In the present study, we found that the 1alpha-hydroxylase gene abundantly expresses in kidney, and at low levels in other tissues and in some cell lines. Positive and negative regulations of 1alpha-hydroxylase gene by PTH, calcitonin, or 1alpha,25(OH)2D3 were observed at transcriptional levels in kidneys of animals and in a mouse proximal tubule cell line. Moreover, the protein kinase A inhibitor abrogated the PTH-mediated positive regulation. In mice lacking the vitamin D receptor, the 1alpha-hydroxylase gene expression was overinduced, and the inducible effect of either PTH or calcitonin, but not the repression by 1alpha,25(OH)2D3, was evident. Thus, vitamin D receptor is essential for the negative regulation by 1alpha,25(OH)2D3. Moreover, we demonstrate that renal 1alpha-hydroxylase gene expression in chronic renal failure model rats was decreased and the positive effect by PTH and calcitonin was diminished. The present study demonstrates that PTH and calcitonin positively regulate renal 1alpha-hydroxylase gene expression via PKA-dependent and independent pathway, respectively, and that 1alpha,25(OH)2D3 negatively regulates it mediated by vitamin D receptor. Furthermore, in a moderate state of chronic renal failure, renal cells expressing the 1alpha-hydroxylase gene appear to have diminished potential in response to PTH and calcitonin.

Transcription of alpha2 integrin gene in osteosarcoma cells is enhanced by tumor promoters

Integrin alpha2beta1 is a heterodimeric transmembrane receptor for collagens. In osteogenic cells the expression of alpha2beta1 integrin is induced by both Kirsten sarcoma virus and chemical transformation. The association of alpha2 integrin with transformed cell phenotype was studied further by testing the effects of two tumor promoters, 12-O-tetradecanoylphorbol 13-acetate (TPA) and okadaic acid (OA), on human MG-63 osteosarcoma cells. TPA, an activator of protein kinase C, increased the cell surface expression of alpha2 integrin and the corresponding mRNA levels. Nuclear run-on assays indicated that TPA activated the transcription of alpha2 integrin gene. TPA also slightly increased the expression of alpha3 integrin but had no effect on the transcription of alpha5, alphav, or beta1 integrin subunits. OA, an inhibitor of serine/threonine phosphatases, increased alpha2 integrin gene transcription and mRNA levels, but in contrast to TPA, OA decreased alpha3 integrin expression. The increased expression of alpha2 integrin on TPA-treated MG-63 cells led to faster cell spreading on type I collagen. Our results link the enhanced transcription of alpha2 integrin gene to tumor progression and show the independent regulation of alpha2 integrin compared to other integrin genes.

Modulation of in vivo migratory function of alpha 2 beta 1 integrin in mouse liver

We report herein that expression of alpha 2 beta 1 integrin increased human erythroleukemia K562 transfectant (KX2C2) cell movement after extravasation into liver parenchyma. In contrast, a previous study demonstrated that alpha 2 beta 1 expression conferred a stationary phenotype to human rhabdomyosarcoma RD transfectant (RDX2C2) cells after extravasation into the liver. We therefore assessed the adhesive and migratory function of alpha 2 beta 1 on KX2C2 and RDX2C2 cells using a alpha 2 beta 1-specific stimulatory monoclonal antibody (mAb), JBS2, and a blocking mAb, BHA2.1. In comparison with RDX2C2 cells, KX2C2 were only weakly adherent to collagen and laminin. JBS2 stimulated alpha 2 beta 1-mediated interaction of KX2C2 cells with both collagen and laminin resulting in increases in cell movement on both matrix proteins. In the presence of Mn2+, JBS2-stimulated adhesion on collagen beyond an optimal level for cell movement. In comparison, an increase in RDX2C2 cell movement on collagen required a reduction in its adhesive strength provided by the blocking mAb BHA2.1. Consistent with these in vitro findings, in vivo videomicroscopy revealed that alpha 2 beta 1-mediated postextravasation cell movement of KX2C2 cells in the liver tissue could also be stimulated by JBS2. Thus, results demonstrate that alpha 2 beta 1 expression can modulate postextravasation cell movement by conferring either a stationary or motile phenotype to different cell types. These findings may be related to the differing metastatic activities of different tumor cell types.

Inhibition of melanoma cell binding to type IV collagen by analogs of cell adhesion regulator

Integrin-mediated tumor cell adhesion to type IV collagen is believed to play a role in the invasion of basement membrane proteins and the subsequent metastatic process. The cellular protein CAR (cell adhesion regulator) has been proposed to influence integrin-mediated binding to extracellular matrix proteins, including basement membrane (type IV) collagen. Three analogs of the CAR138-142 have been tested for activity. The first contains the 138-142 sequence (CAR138-142, Val-Glu-Ile-Leu-Tyr-NH2), the second contains the 138-142 sequence with a phosphorylated Tyr [pCAR138-142, Val-Glu-Ile-Leu-Tyr(PO3H2)-NH2], and the third contains the reversed 138-142 sequence (rCAR138-142, Tyr-Leu-Ile-Glu-Val-NH2). When added extracellularly, none of the analogs had a significant affect on cell adhesion to type IV collagen. Using a novel reversible cell permeabilization method, we found that intracellular incorporation of both CAR138-142 and pCAR138-142 resulted in inhibition of cell adhesion in a dose-dependent fashion. The IC50 values were approximately 90 and approximately 10 microM for CAR138-142 and pCAR138-142, respectively. Intracellular incorporation of the rCAR138-142 peptide had no affect on cell adhesion. Fluorescence microscopy of a fluorescein-labeled CAR138-142 peptide revealed that the reversible permeabilization procedure resulted in the peptides crossing the cell membrane. Affinity chromatography of melanoma cell lysates with pCAR138-142 or rCAR138-142 attached to a solid support of magnetic beads suggested that one protein was bound uniquely by pCAR138-142. Immunoprecipitation analysis identified vinculin, a protein associated with the actin cytoskeleton, as the protein specifically bound by pCAR138-142. Immunoprecipitation with pp125FAK- or beta 1-integrin-derived mAbs gave negative results. Our study suggests that a possible therapeutic approach for inhibition of melanoma cell adhesion adhesion to extracellular matrix proteins is the use of CAR peptide analogs intracellularly.

Expression of small extracellular chondroitin/dermatan sulfate proteoglycans is differentially regulated in human endothelial cells

We have examined the expression of the small extracellular chondroitin/dermatan sulfate proteoglycans (CS/DS PGs), biglycan, decorin, and PG-100, which is the proteoglycan form of colony stimulating factor-1, in the human endothelial cell line EA.hy 926. We have also examined whether modulation of the phenotype of EA.hy 926 cells by tumor necrosis factor-alpha (TNF-alpha) is associated with specific changes in the synthesis of these PGs. We demonstrate that EA.hy 926 cells, when they form monolayer cultures typical of macrovascular endothelial cells, express and synthesize detectable amounts of biglycan and PG-100, but not decorin. On SDS-polyacrylamide gel electrophoresis both PGs behave like proteins of the relative molecular weight of approximately 250,000. TNF-alpha that changed the morphology of the cells from a polygonal shape into a spindle shape and that also stimulated the detachment of the cells from culture dish, markedly decreased the net synthesis of biglycan, whereas the net synthesis of PG-100 was increased. These changes were parallel with those observed at the mRNA level of the corresponding PGs. The proportions of the different sulfated CS/DS disaccharide units of PGs were not affected by TNF-alpha. Several other growth factors/cytokines, such as interferon-gamma, fibroblast growth factors-2 (FGF-2) and -7 (FGF-7), interleukin-1beta, and transforming growth factor-beta, unlike TNF-alpha, modulated neither the morphology nor the biglycan expression of EA.hy 926 cells under the conditions used in the experiments. However, PG-100 expression was increased also in response to FGF-2 and -7 and transforming growth factor-beta. None of the above cytokines, including TNF-alpha, was able to induce decorin expression in the cells. Our results indicate that the regulatory elements controlling the expression of the small extracellular CS/DS PGs in human endothelial cells are different.

Bone morphogenetic protein-2 is a regulator of cell adhesion

Bone morphogenetic proteins (BMPs) are a group of peptide growth factors closely related to transforming growth factors-beta. The BMPs are suggested to play an essential role in bone development and they are strong candidate molecules to be used clinically to improve fracture healing. BMPs are also involved in the differentiation of several other tissues during embryogenesis. Here, we show that human recombinant BMP-2 regulates cell-matrix interactions by modifying the expression of integrin-type receptors. The synthesis of alpha3 integrin was down-regulated by BMP-2 in two osteogenic sarcoma-derived cell lines, Saos-2 and HOS, and also in human fetal chondrocytes. BMP-2 had no effect on the expression of alpha1, alpha2, alpha5, alpha6, and alphaV integrins. BMP-2 reduced the expression of alpha3 integrin subunit at mRNA level. Laminin-5 was shown to be the ligand for alpha3beta1 integrin on Saos cells and BMP-2 decreased the ability of Saos cells to attach to it. These results suggest that BMP-2 has a specific effect on the alpha3beta1 integrin-mediated cell adhesion to laminin-5. Given the fact that BMP-2 is expressed in osteosarcomas, in addition to in bone, this mechanism is putatively important especially in bone development and tumors. We also studied the effect of BMP-2 on a human keratinocyte cell line, HaCaT. In HaCaT cells, the expression of alpha2 integrin was strongly down-regulated by BMP-2, whereas its effect on the expression of alpha3 integrin was smaller. We suggest that the effects of BMP-2 may be partially mediated by specifically altered cell adhesion.

Okadaic acid-induced transcriptional downregulation of type I collagen gene expression is mediated by protein phosphatase 2A

Expression of type I collagen genes is highly regulated and becomes abnormal in various pathological conditions, from excessive collagen production in fibrotic diseases to their downregulation in transformed cells. Some inflammatory cytokines and other ligands, capable of eliciting intracellular phosphorylation, can profoundly alter collagen gene expression. We investigated the role of serine/threonine protein phosphatases (PP) in the regulation of collagen gene expression. Biosynthesis of the endogenous type I procollagen, and expression of Pro alpha 1(I) promoter-luciferase (Luc) constructs transfected in NIH3T3 fibroblasts, were evaluated in response to PP2A and PP1 inhibitor okadaic acid (OA) and exogenously expressed PP catalytic subunits. OA suppressed type I collagen gene expression as judged by reduced rates of protein synthesis, steady state levels of Pro alpha 1(I) collagen mRNA and expression of Luc driven by Pro alpha 1 (I) collagen promoter in OA-treated cells. Co-transfection of Pro alpha 1(I)-Luc with expression vectors containing PP2A, but not PP1, stimulated collagen promoter activity. These results strongly suggest that OA acts via PP2A-mediated dephosphorylation of an unidentified transcription factor(s) or cofactor(s) needed to activate Pro alpha 1(I) collagen promoter.

Biology of tumor cell invasion: interplay of cell adhesion and matrix degradation

Invasion of malignant cells requires altered cellular interaction with extracellular matrix. Integrin-type cell adhesion receptors play an important role in this process. Integrin-related cell biological phenomena explain cancer cell migration, and recent developments in the field have made it possible to propose that integrins are also involved in the penetration through basement membranes and other molecular barriers. Finally, malignant melanoma has been used here as an example to speculate on the function of each integrin in light of information from different experimental models.

Functional analysis of the promoter and first intron of the human lysyl oxidase gene

Alterations in the synthesis and activity of lysyl oxidase occur concomitant with developmental changes in collagen and elastin deposition and with the pathogenesis of several acquired and heritable connective tissue disorders. To begin to unravel the mechanisms that control lysyloxidase gene expression, we have previously reported the complete exon-intron structure of the human lysyl oxidase gene. We have now sequenced this entire gene, including all six introns and 4 kb of DNA 5' of exon 1. Analysis of over 13 kb of intervening sequence and 5' flanking sequence revealed a concentration of conserved consensus sequence elements within the first intron and 1 kb immediately 5' of exon 1. Analysis of intron 1 and the 5' flanking domain, using recombinant plasmids containing the chloramphenicol acetyl transferase (CAT) reporter gene, identified functional DNA sequence elements within these non-coding domains responsible for inhibition and up-regulation of CAT activity in primary cultures of human skin fibroblasts, in smooth muscle cells, revertant cells derived from an osteosarcoma cell line and malignant c-Ha-ras-transformed osteosarcoma cells. DNA sequence elements within intron 1, in particular, resulted in a marked increase in CAT reporter activity in cultured fibroblasts, smooth muscle cells and osteosarcoma cells. In c-Ha-ras-transformed osteosarcoma cells, however, no such enhancer activity of intron 1 sequence was observed. Ras-transformed osteosarcoma cells exhibited reduced steady-state levels of lysyl oxidase mRNA that was primarily controlled through reduced transcription of the lysyl oxidase gene. The lack of any up-regulation of CAT activity in these ras-transformed cells by sequence elements within intron 1 suggests a complex interaction between cis-acting domains and trans-acting transcriptional factors in the 5' promoter domain and the first intron of the lysyl oxidase gene.

A peptide model of basement membrane collagen alpha 1 (IV) 531-543 binds the alpha 3 beta 1 integrin

Tumor cell adhesion to the triple-helical domain of basement membrane (type IV) collagen occurs at several different regions. Cellular recognition of the sequence spanning alpha 1(IV)531-543 has been proposed to be independent of triple-helical conformation (Miles, A. J., Skubitz, A. P. N., Furcht, L. T., and Fields, G. B. (1994) J. Biol. Chem. 269, 30939-30945). In the present study, integrin interactions with a peptide analog of the alpha 1(IV)-531-543 sequence have been analyzed. Tumor cell adhesion (melanoma, ovarian carcinoma) to the alpha 1(IV)531-543 chemically synthesized peptide was inhibited by a monoclonal antibody against the alpha 3 integrin subunit, and to a lesser extent by monoclonal antibodies against the beta 1 and alpha 2 integrin subunits. An anti-alpha 5 monoclonal antibody and normal mouse IgG were ineffective as inhibitors of tumor cell adhesion to the peptide. Two cell surface proteins of 120 and 150 kDa bound to an alpha 1(IV)531-543 peptide affinity column and were eluted with 20 mM EDTA. When the eluted proteins were incubated with monoclonal antibodies against either the alpha 3 or beta 1 integrin subunit, proteins corresponding in molecular weight to alpha 3 and beta 1 integrin subunits were precipitated. No proteins were immunoprecipated with monoclonal antibodies against the alpha 2 or alpha 5 integrin subunits. Thus, the alpha 3 beta 1 integrin from two tumor cell types has been shown to bind directly to the alpha 1 (IV)531-543 peptide. The alpha 1(IV)531-543 peptide is the first collagen-like sequence that has been shown to bind the alpha 3 beta 1 integrin.

Integrin alpha 2 beta 1 is a positive regulator of collagenase (MMP-1) and collagen alpha 1(I) gene expression

A classical model for studying the effects of extracellular matrix is to culture cells inside a three-dimensional collagen gel. When surrounded by fibrillar collagen, many cell types decrease the production of type I collagen, and the expression of interstitial collagenase (matrix metalloproteinase-1; MMP-1) is simultaneously induced. To study the role of the collagen-binding integrins alpha 1 beta 1 and alpha 2 beta 1 in this process, we used three different osteogenic cell lines with distinct patterns of putative collagen receptors: HOS cells, which express only alpha 1 beta 1 integrin, MG-63 cells, which express only alpha 2 beta 1 integrin, and KHOS-240 cells, which express both. Inside collagen gels, alpha 1 (I) collagen mRNA levels were decreased in HOS and KHOS-240 cells but not in MG-63 cells. In contrast, MMP-1 expression was induced in KHOS-240 and MG-63 cells but not in HOS cells. Transfection of MG-63 cells with alpha 2 integrin cDNA in an antisense orientation reduced the expression level of alpha 2 integrin. These cell clones showed induction and reduction of mRNA levels for MMP-1, respectively. HOS cells normally lacking alpha 2 beta 1 integrin were forced to express it, and this prevented the down-regulation in the levels of alpha 1 (I) collagen mRNA when cells were grown inside collagen gels. The data indicate that the level of MMP-1 expression is regulated by the collagen receptor alpha 2 beta 1 integrin. The down-regulation of collagen alpha 1 (I) is mediated by another receptor. Integrin alpha 2 beta 1 may compete with it and thus be a positive regulator of collagen synthesis.

Transforming growth factor-beta regulates collagen gel contraction by increasing alpha 2 beta 1 integrin expression in osteogenic cells

The contraction of floating collagen gels is suggested to mimic the reorganization of collagenous matrix during development and tissue healing. Here, we have studied two osteogenic cell lines, namely MG-63 and HOS, and a chemically transformed subclone of HOS cells, HOS-MNNG. Transforming growth factor-beta (TGF-beta), a putative regulator of bone fracture healing, increased collagen gel contraction by MG-63 and HOS-MNNG, but not by HOS cells. Our data show that TGF-beta-induced fibronectin synthesis is not sufficient for the process. Instead, anti-beta 1 integrin antibodies could prevent the contraction. There are three different integrin heterodimers that are known to mediate the cell-collagen interaction, namely alpha 1 beta 1, alpha 2 beta 1, and alpha 3 beta 1. In MG-63 cells TGF-beta increased the expression of alpha 2 beta 1 integrin and decreased the expression of alpha 3 beta 1 integrin, whereas alpha 1 beta 1 integrin is not expressed. HOS cells had no alpha 2 beta 1 integrin, neither did TGF-beta induce its expression. However, HOS-MNNG cells expressed more alpha 2 beta 1 integrin when treated with TGF-beta. Thus, we suggest that the mechanism of the enhanced collagen gel contraction by TGF-beta is the increased expression of alpha 2 beta 1 integrin heterodimer. To further test this hypothesis, we expressed a full-length alpha 2 integrin cDNA in HOS cells and in MG-63 cells. We obtained HOS cell clones that expressed alpha 2 beta 1 heterodimer, and the ability of these cells to contract collagen gels was greatly enhanced. Furthermore, the contraction by MG-63 cells transfected with alpha 2 integrin cDNA was enhanced, and the contraction by cells transfected with antisense oriented alpha 2 integrin cDNA was decreased. Thus, both in MG-63 and HOS cells the increased alpha 2 integrin expression alone was sufficient for the enhanced contraction of collagen gels. Furthermore, the amount of alpha 2 integrin is critical for the process, and its decrease leads to diminished ability to contract gels.

Altered interaction of human granulation-tissue fibroblasts with fibronectin is regulated by alpha 5 beta 1 integrin

Granulation-tissue fibroblasts express an unique phenotype distinct from normal fibroblasts. Due to the importance of the cell-matrix interactions in the regulation of cell morphology and behavior, we have compared the cell adhesion apparatus, especially integrin-type receptors, in fibroblasts cultured from healthy human periodontal connective tissues and from chronic and wound granulation tissues. The spreading of granulation-tissue cells on fibronectin, but not on type I collagen or laminin, was slower when compared with the normal fibroblasts. Cell spreading on fibronectin could be inhibited by RGD-containing peptide, suggesting integrin-mediated interaction. Both cell types expressed beta 1 integrin subunit, which associated with several integrin alpha subunits, namely alpha 1, alpha 2, alpha 3, alpha 5 and alpha v. In addition to beta 1 subunit, alpha v chain formed heterodimers with beta 3 and beta 5 subunits. Thus, these cells have multiple putative fibronectin, laminin, collagen, and vitronectin receptors. Cell spreading of both cell types on fibronectin was inhibited with anti-beta 1 and anti-alpha 5 antibodies, but antibodies against other putative FN-binding integrins (alpha 3, alpha v, and alpha v beta 3) had no effects. Furthermore, granulation-tissue fibroblasts showed delayed spreading on substrates coated with anti-beta 1 or anti-alpha 5 integrin antibodies. On substrates coated with anti-alpha 3 antibody, both cell types spread equally well. By FACS analysis, the amount of beta 1 and alpha 5 integrin subunits expressed on the cell surfaces was slightly elevated in GTFs compared with HGFs. Thus, the findings in this study indicate that the weakened interaction of granulation-tissue fibroblasts with fibronectin is regulated by altered function of alpha 5 beta 1 integrin.