Isolation and sequencing of cDNAs for proteins with multiple domains of Gly-Xaa-Yaa repeats identify a distinct family of collagenous proteins

Angiogenesis inhibitor endostatin is a distinct component of elastic fibers in vessel walls

Theendothelial cell inhibitor endostatin (22 kDa) is part of the carboxyl-terminal globular domain of collagen XVIII and shows a widespread tissue distribution. Immunohistology of adult mouse tissues demonstrated a preferred localization in many vessel walls and some other basement membrane zones. A strong immunogold staining was observed across elastic fibers in the multiple elastic membranes of aorta and other large arteries. Staining was less strong along sparse elastic fibers of veins and almost none was observed in the walls of arterioles and capillaries. Strong evidence was also obtained for some intracellular and basement membrane associations. Immunogold double staining of elastic fibers showed a close colocalization of endostatin with fibulin-2, fibulin-1, and nidogen-2, but not with perlecan. Reasonable amounts of endostatin could be extracted from aorta and skin by EDTA, followed by detergents, with aorta being the richest source of the inhibitor identified so far. Solubilizations with collagenase and elastase were approximately fivefold less efficient. Immunoblots of aortic extracts detected major endostatin components of 22-25 kDa whereas skin extracts also contained some larger components. Solid-phase assays demonstrated distinct binding of recombinant mouse endostatin to the fibulins and nidogen-2, consistent with their tissue colocalization. Together, the data indicate several different ways for endostatin to be associated with the extracellular matrix, and its release may determine biological activation. This also defines a novel function for some elastic tissues.

Cytokine modulation of type XV collagen gene expression in human dermal fibroblast cultures

The expression of type XV collagen was studied in cultured human dermal fibroblasts exposed to transforming growth factor-beta (TGF-beta), tumor necrosis factor-alpha (TNF-alpha) or interleukin-1beta (IL-1beta), cytokines which have been shown previously to alter the expression of several extracellular matrix genes. TGF-beta enhanced the expression of the type XV collagen gene (COL15A1) in a time-dependent manner, up to 4.3-fold after 24 h of incubation, whereas TNF-alpha and IL-1beta reduced the mRNA steady-state levels by 32 and 80%, respectively. When TGF-beta and TNF-alpha were added together to the cultures, the stimulatory effect of TGF-beta on type XV collagen gene expression was abrogated, indicating antagonistic modulation by these 2 cytokines. These data suggest that the cytokines tested in this study may contribute to the regulation of type XV collagen synthesis in a variety of tissues which have recently been shown to express this particular collagen gene.

Inhibitory effect of full-length human endostatin on in vitro angiogenesis

Endostatin, a C-terminal product of collagen XVIII, is a very powerful angiogenesis inhibitor. In vivo experiments in mice indicate that endostatin dramatically reduces tumor mass without causing the onset of any resistance to the treatment. Recently, a 12-aa shorter human endostatin has been purified from plasma, but is ineffective in in vitro angiogenesis assays. Here we report that the full-length human recombinant endostatin has a potent inhibitory activity in in vitro angiogenesis assays. Two powerful angiogenic factors were used to stimulate endothelial cells: FGF-2 and VEGF-165. Endostatin prevented cell growth both in the basal condition and after stimulation with FGF-2 or VEGF-165. Migration of microvascular endothelial cells toward FGF-2 or VEGF-165 was impaired, both when cells were pretreated with the inhibitor and when endostatin was added together with the growth factors. Furthermore, experiments of inhibition of proliferation performed on nonmicroendothelial cells showed that endostatin was ineffective. This study indicates that human endostatin is a potent angiogenesis inhibitor and suggests its use in human anticancer therapy.

Collagen XVIII is a basement membrane heparan sulfate proteoglycan

The present study shows that collagen XVIII is, next to perlecan and agrin, the third basal lamina heparan sulfate proteoglycan (HSPG) and the first collagen/proteoglycan with heparan sulfate side chains. By using monoclonal antibodies to an unidentified HSPG in chick, 14 cDNA clones were isolated from a chick yolk sac library. All clones had a common nucleotide sequence that was homologous to the mRNA sequences of mouse and human collagen XVIII. The deduced amino acid sequence of the chick fragment shows an 83% overall homology with the human and mouse collagen XVIII. Similar to the human and mouse homologue, the chick collagen XVIII mRNA has a size of 4.5 kilobase pairs. In Western blots, collagen XVIII appeared as a smear with a molecular mass of 300 kDa. After treatment with heparitinase, the protein was reduced in molecular mass by 120 kDa to a protein core of 180 kDa. Collagen XVIII has typical features of a collagen, such as its existence, under non-denaturing conditions, as a non-covalently linked oligomer, and a sensitivity of the core protein to collagenase digestion. It also has characteristics of an HSPG, such as long heparitinase-sensitive carbohydrate chains and a highly negative net charge. Collagen XVIII is abundant in basal laminae of the retina, epidermis, pia, cardiac and striated muscle, kidney, blood vessels, and lung. In situ hybridization showed that the main expression of collagen XVIII HSPG in the chick embryo is in the kidney and the peripheral nervous system. As a substrate, collagen XVIII moderately promoted the adhesion of Schwann cells but had no such activity on peripheral nervous system neurons and axons.

Zinc-dependent dimers observed in crystals of human endostatin

The crystal structure of human endostatin reveals a zinc-binding site. Atomic absorption spectroscopy indicates that zinc is a constituent of both human and murine endostatin in solution. The human endostatin zinc site is formed by three histidines at the N terminus, residues 1, 3, and, 11, and an aspartic acid at residue 76. The N-terminal loop ordered around the zinc makes a dimeric contact in human endostatin crystals. The location of the zinc site at the amino terminus, immediately adjacent to the precursor cleavage site, suggests the possibility that the zinc may be involved in activation of the antiangiogenic activity following cleavage from the inactive collagen XVIII precursor or in the cleavage process itself.

Structure, function and tissue forms of the C-terminal globular domain of collagen XVIII containing the angiogenesis inhibitor endostatin

The C-terminal domain NC1 of mouse collagen XVIII (38 kDa) and the shorter mouse and human endostatins (22 kDa) were prepared in recombinant form from transfected mammalian cells. The NC1 domain aggregated non-covalently into a globular trimer which was partially cleaved by endogenous proteolysis into several monomers (25-32 kDa) related to endostatin. Endostatins were obtained in a highly soluble, monomeric form and showed a single N-terminal sequence which, together with other data, indicated a compact folding. Endostatins and NC1 showed a comparable binding activity for the microfibrillar fibulin-1 and fibulin-2, and for heparin. Domain NC1, however, was a distinctly stronger ligand than endostatin for sulfatides and the basement membrane proteins laminin-1 and perlecan. Immunological assays demonstrated endostatin epitopes on several tissue components (22-38 kDa) and in serum (120-300 ng/ml), the latter representing the smaller variants. The data indicated that the NC1 domain consists of an N-terminal association region (approximately 50 residues), a central protease-sensitive hinge region (approximately 70 residues) and a C-terminal stable endostatin domain (approximately 180 residues). They also demonstrated that proteolytic release of endostatin can occur through several pathways, which may lead to a switch from a matrix-associated to a more soluble endocrine form.

The short and long forms of type XVIII collagen show clear tissue specificities in their expression and location in basement membrane zones in humans

Two N-terminal ends of human type XVIII collagen chains have recently been identified. The two chains have different signal peptides and variant N-terminal noncollagenous NC1 domains of 493 (NC1-493) and 303 (NC1-303) amino acid residues, respectively, but share 301 residues of their NC1 domains as well as the collagenous and C-terminal noncollagenous portions of the molecule. Antibodies were produced against the NC1 region common to both human alpha1(XVIII) chain variants and against NC1 sequences specific to the long variant and were used in combination with in situ hybridization to localize this collagen in a number of human tissues. They were also used for Western blotting, which resulted in detection of overlapping high-molecular weight bands above the 200-kd standard in a kidney extract. Heparin lyase II and heparin lyase III digestions of kidney and placenta extracts indicated that at least in these tissues, type XVIII collagen contains heparin sulfate glycosaminoglycan side chains. Type XVIII collagen was found to be a ubiquitous basement membrane component, occurring prominently at vascular and epithelial basement membranes throughout the body. Comparison of the expression of the NC1-493 and NC1-303 variants revealed marked differences. The short variant was found in most conventional basement membranes, including blood vessels and the various epithelial structures, and around muscular structures. The long variant was expressed very strongly in liver, where it was virtually the only variant in the liver sinusoids, and it occurred only in minor amounts elsewhere. Thus, the 192 N-terminal residues specific to the long variant apparently confer some functional property needed above all in the liver sinusoids, but also at certain other locations.

Complete exon-intron organization of the human gene for the alpha1 chain of type XV collagen (COL15A1) and comparison with the homologous COL18A1 gene

The human gene for the alpha1 chain of type XV collagen (COL15A1) is about 145 kilobases in size and contains 42 exons. The promoter is characterized by the lack of a TATAA motif and the presence of several Sp1 binding sites, some of which appeared to be functional in transfected HeLa cells. Comparison with Col18a1, which encodes the alpha1(XVIII) collagen chain homologous with alpha1(XV), indicates marked structural homology spread throughout the two genes. The mouse Col18a1 contains one exon more than COL15A1, due to the fact that COL15A1 lacks sequences corresponding to exon 3 of Col18a1, which encodes a cysteine-rich sequence motif. Twenty-five of the exons of the two genes are almost identical in size, six of them contain conserved split codons, and the locations of the respective exon-intron junctions are identical or almost identical in the two genes. The homologous exons include the closely adjacent first pair of exons and the exons encoding a thrombospondin-1 homology found in the N-terminal noncollagenous domain 1, which are followed by the most variable part of the two genes, covering the C-terminal half of their noncollagenous domain 1 and the beginning of the collagenous portion, after which most of the exons are homologous. The lengths of the introns are not similar in these genes, with two exceptions, namely the first intron, which is very short, less than 100 base pairs, and the second intron, which is very large, about 50 kilobases, in both genes. It can be concluded that COL15A1 and Col18a1 are derived from a common ancestor.

Collagen XVIII is localized in sinusoids and basement membrane zones and expressed by hepatocytes and activated stellate cells in fibrotic human liver

Type XVIII collagen is a recently discovered nonfibrillar collagen associated with basement membranes in mice and expressed at high levels in human liver. We studied the origin, distribution, and RNA levels of type XVIII collagen in normal and fibrotic human livers by in situ hybridization, immunohistochemistry, and Northern and dot blots and compared procollagen alpha1(XVIII) RNA levels with those of procollagen alpha1(IV) and laminin gamma1, the two major components of liver basement membranes. In normal liver, type XVIII collagen was heavily deposited in perisinusoidal spaces and basement membrane zones. The major source of type XVIII collagen was hepatocytes and, to a lesser extent, endothelial, biliary epithelial, and vascular smooth muscle cells and peripheral nerves. In cirrhosis, type XVIII collagen formed a thick deposit along capillarized sinusoids. Grain counts after in situ hybridization showed myofibroblasts to increase their expression 13-fold in active and twofold in quiescent fibrosis, whereas hepatocytes increased their expression only twofold in both active and quiescent fibrosis. Activated stellate cells in vitro expressed type XVIII collagen at high levels. These data indicate that type XVIII collagen is a component of the perisinusoidal space and is associated with basement membrane remodeling. Hepatocytes and activated stellate cells are important sources of type XVIII collagen in normal and fibrotic liver respectively, which suggests tissue-specific regulation of its expression.

Crystal structure of the angiogenesis inhibitor endostatin at 1.5 A resolution

A number of extracellular proteins contain cryptic inhibitors of angiogenesis. Endostatin is a 20 kDa C-terminal proteolytic fragment of collagen XVIII that potently inhibits endothelial cell proliferation and angiogenesis. Therapy of experimental cancer with endostatin leads to tumour dormancy and does not induce resistance. We have expressed recombinant mouse endostatin and determined its crystal structure at 1.5 A resolution. The structure reveals a compact fold distantly related to the C-type lectin carbohydrate recognition domain and the hyaluronan-binding Link module. The high affinity of endostatin for heparin is explained by the presence of an extensive basic patch formed by 11 arginine residues. Endostatin may inhibit angiogenesis by binding to the heparan sulphate proteoglycans involved in growth factor signalling.

Isolation and characterization of the circulating form of human endostatin

Recently, fragments of extracellular proteins, including endostatin, were defined as a novel group of angiogenesis inhibitors. In this study, human plasma equivalent hemofiltrate was used as a source for the purification of high molecular weight peptides (10-20 kDa), and the isolation and identification of circulating human endostatin are described. The purification of this C-terminal fragment of collagen alpha1(XVIII) was guided by MALDI-MS and the exact molecular mass determined by ESI-MS was found to be 18 494 Da. N-terminal sequencing revealed the identity of this putative angiogenesis inhibitor and its close relation to mouse endostatin. The cysteine residues 1-3 and 2-4 in the molecule are linked by disulfide bridges. In vitro biological characterization of the native protein demonstrated no anti-proliferative activity on different endothelial cell types. These data indicate that human endostatin, which is a putative angiogenesis inhibitor, is present in the circulation.

Cloning of mouse type XV collagen sequences and mapping of the corresponding gene to 4B1-3. Comparison of mouse and human alpha 1 (XV) collagen sequences indicates divergence in the number of small collagenous domains

We report on full-length mouse type XV collagen cDNAs that encode a 1367-residue alpha 1(XV) chain. The amino acid sequences of the mouse and previously characterized human alpha 1(XV) chains exhibit an overall identity of 72%. The highest homology between these chains and to the structurally related type XVIII collagen is observed in their C-terminal noncollagenous domains. Although the mouse and human alpha 1(XV) chains are highly homologous and similar in their overall domain structure, the mouse chain contains only seven collagenous domains, whereas the human chain contains nine. Northern analysis of several mouse tissues indicated strong hybridization in the case of heart and skeletal muscle RNAs and moderate signals with kidney, lung, and testis RNAs. Analysis of type XV collagen mRNA levels at different stages of mouse embryonic development indicated a marked increase in the level between 11 and 15 days of development, which coincides with pronounced development of the muscles, heart, and vascular system in the mouse embryo. The mouse gene for type XV collagen was mapped by fluorescence in situ hybridization to chromosome 4, band B1-3. This result indicates that the mouse type XV collagen gene and its human counterpart are located in the chromosomal segments with conserved syntenies.

Location of type XV collagen in human tissues and its accumulation in the interstitial matrix of the fibrotic kidney

An antipeptide antibody was produced against the carboxyl-terminal noncollagenous domain of human type XV collagen and used to localize this recently described collagen in a number of human tissues. The most conspicuous findings were powerful staining of most of the capillaries and staining of the basement membrane (BM) zones of muscle cells. Not all of the BM zones were positive, however, as shown by the lack of staining in the developing fetal alveoli and some of the tubules in developing kidney. Nor was type XV collagen staining restricted to the BM zones, as some could be observed in the fibrillar collagen matrix of the papillary dermis and placental villi, for example. Interestingly, differences in the expression of type XV collagen could be observed during kidney development, and staining of fetal lung tissue suggested that changes in its expression may also occur during the formation of vascular structures. Another intriguing finding was pronounced renal interstitial type XV collagen staining in patients with kidney fibrosis due to different pathological processes. This suggests that the accumulation of type XV collagen may accompany fibrotic processes. Full-length human type XV collagen chains with an apparent molecular mass of approximately 200 kd were produced in insect cells using a baculovirus expression system. The fact that these had a markedly higher molecular mass than the 100- to 110-kd type XV collagen chains found in homogenates of heart and kidney tissue suggests either proteolytic processing during the synthesis of type XV collagen or an inability to solubilize complete molecules from tissues.

Endostatin: an endogenous inhibitor of angiogenesis and tumor growth

We previously identified the angiogenesis inhibitor angiostatin. Using a similar strategy, we have identified endostatin, an angiogenesis inhibitor produced by hemangioendothelioma. Endostatin is a 20 kDa C-terminal fragment of collagen XVIII. Endostatin specifically inhibits endothelial proliferation and potently inhibits angiogenesis and tumor growth. By a novel method of sustained release, E. coli-derived endostatin was administered as a nonrefolded suspension. Primary tumors were regressed to dormant microscopic lesions. Immunohistochemistry revealed blocked angiogenesis accompanied by high proliferation balanced by apoptosis in tumor cells. There was no toxicity. Together with angiostatin data, these findings validate a strategy for identifying endogenous angiogenesis inhibitors, suggest a theme of fragments of proteins as angiogenesis inhibitors, and demonstrate dormancy therapy.

Biosynthesis and processing of type XVI collagen in human fibroblasts and smooth muscle cells

The alpha 1(XVI) collagen chain, recently identified by cDNA cloning, exhibits structural similarity to a subgroup of collagens that associate with collagen fibrils. Recombinant alpha 1(XVI) collagen chains produced in embryonic kidney cells are able to form stable homotrimers, which are rapidly converted into smaller polypeptides after secretion into the culture medium. In this study, we investigated the biosynthesis of native type XVI collagen by immunoprecipitation of metabolically labeled human cells. Dermal fibroblasts and arterial smooth muscle cells were precipitated with three antibodies raised against distinct regions in the N- and C-terminal part of the human alpha 1(XVI) collagen chain. A disulfide-bonded polypeptide of 220 kDa was obtained from the culture medium, cells and extracellular matrix with all three antibodies. This polypeptide is sensitive to bacterial collagenase digestion and partially resistant to pepsin digestion, suggesting that it is the endogenous alpha 1(XVI) collagen chain. Pulse/chase experiments showed that the newly synthesized alpha 1(XVI) chains are secreted into the medium and deposited in the extracellular matrix in a time-dependent manner. Unlike the recombinant chain, the native type XVI collagen does not undergo extensive proteolytic processing upon secretion. Both cell types deposit a substantial amount of the newly synthesized alpha 1(XVI) chain into the extracellular matrix, in which the 220-kDa polypeptide is the only product immunoprecipitated. There is little evidence for the presence of another constituent chain. The data are consistent with a nomotrimeric chain composition for type XVI collagen. No apparent difference exists in the rate of synthesis and secretion between fibroblasts and smooth muscle cells. Indirect immunofluorescence microscopy showed an extracellular distribution of type XVI collagen, which is located close to cells but not associated with fibrillar structures.

Distribution of type XV collagen transcripts in human tissue and their production by muscle cells and fibroblasts

Type XV collagen is a recently identified member of the diverse family of collagens, its structure being characterized by extensive interruptions in the collagenous sequences. A combination of Northern blot hybridization of fetal and adult human tissues and in situ hybridization analyses of a fetus with Down's syndrome, several placentas, and adult skin were used to localize expression of its mRNAs. Northern blot analysis revealed marked expression in heart, skeletal muscle, and placenta tissues and moderate levels in the kidney and pancreas. Clear in situ hybridization signals were detected in fibroblasts and endothelial cells in all tissues studied. Examination of fetal heart, skeletal muscle, and smooth muscle tissues showed that the high type XV collagen mRNA level in the muscle RNA was localized not only to fibroblasts residing in the endomysium but also to myoblasts. Interestingly, type XV collagen mRNAs were also synthesized by certain epithelial cells in kidney, lung, pancreas, and placenta. It was the morphologically immature glomeruli in the kidney and the lower parts of the nephron, especially the collecting ducts, that contained these mRNAs but not the mature glomeruli or proximal tubules, suggesting differences in expression during development. These findings indicate a wide distribution of type XV collagen transcripts, the main producers being mesenchymally derived cells, particularly muscle cells and fibroblasts.

Mouse Col18a1 is expressed in a tissue-specific manner as three alternative variants and is localized in basement membrane zones

We have isolated overlapping cDNAs encoding the N-terminal non-triple-helical region of mouse alpha 1(XVIII) collagen and shown that three different variants of alpha 1(XVIII) collagen exist. Each of the three variants shows characteristic tissue-specific expression patterns. Immunohistochemical studies show positive staining for alpha 1(XVIII) collagen along the basement membrane zones of vessels in the intestinal villi, the choroid plexus, skin, liver, and kidney. Thus, we conclude that alpha 1(XVIII) collagen may interact (directly or indirectly) with components in basement membrane zones or on the basal surface of endothelial/epithelial cells.

Identification of three N-terminal ends of type XVIII collagen chains and tissue-specific differences in the expression of the corresponding transcripts. The longest form contains a novel motif homologous to rat and Drosophila frizzled proteins

Transcripts for the alpha 1 chain of mouse type XVIII collagen were found to be heterogeneous at their 5'-ends and to encode three variant N-terminal sequences of the ensuing 1315-, 1527-, or 1774-residue collagen chains. The variant mRNAs appeared to originate from the use of two alternate promoters of the alpha 1(XVIII) chain gene, resulting in the synthesis of either short or long N-terminal non-collagenous NC1 domains, the latter being further subject to modification due to alternative splicing of the transcripts. As a result, the 1527- and 1774-residue polypeptides share the same signal peptide, and the lengths of their NC1 domains are 517 or 764 amino acid residues, respectively, while the 1315-residue polypeptide has a different signal peptide and a 301-residue NC1 domain. The longest NC1 domain was strikingly characterized by a 110-residue sequence with 10 cysteines, which was found to be homologous with the previously identified frizzled proteins belonging to the family of G-protein-coupled membrane receptors. Thus, it is proposed that the cysteine-rich motif, termed fz, represents a new sequence motif that can be found in otherwise unrelated proteins. Tissues containing mainly one or two NC1 domain mRNA variants or all three NC1 domains were identified, indicating that there is tissue-specific utilization of two alternate promoters and alternative splicing of alpha 1(XVIII) transcripts.

Recombinant analysis of human alpha 1 (XVI) collagen. Evidence for processing of the N-terminal globular domain

The N-terminal non-collagenous domain NC11 of the human collagen alpha 1 (XVI) chain was obtained as a recombinant 35-kDa protein from stably transfected kidney cell clones. This form had undergone proteolytic trimming at a basic cleavage motif indicating a similar release in vivo. Domain NC11 showed a globular shape after rotary shadowing and was resistant to neutral proteases. Specific antibodies could be raised against recombinant NC11 and were used for the analysis of other cell clones transfected with the full-length alpha 1 (XVI) chain. Immunoprecipitation of detergent extracts of metabolically labelled cells demonstrated the presence of disulfide-bonded 200-kDa polypeptides possessing NC11 epitopes. This material was partially resistant to pepsin, indicating the formation of alpha 1 (XVI) chain homotrimers with a triple-helical conformation. Yet a substantial proportion of these homotrimers was degraded to fragments of variable size (35-150 kDa) when secreted into the culture medium. Several of these fragments could be obtained on a semi-preparative scale from cells grown in hollow fiber cassettes and showed substantial hydroxylation of proline, consistent with triple-helix formation. Edman degradation demonstrated the origin of some from the N-terminal and of one from a more C-terminal position of collagen XVI. This extensive degradation may be explained by the release of NC11 and by further cleavages within some of the nine interruptions of the triple-helical domain of the alpha 1(XVI) chain. Whether this process also occurs in situ remains to be shown.

Alpha 1(XVIII), a collagen chain with frequent interruptions in the collagenous sequence, a distinct tissue distribution, and homology with type XV collagen

We report on the isolation of mouse cDNA clones which encode a collagenous sequence designated here as the alpha 1 chain of type XVIII collagen. The overlapping clones cover 2.8 kilobases and encode an open reading frame of 928 amino acid residues comprising a putative signal peptide of 25 residues, an amino-terminal noncollagenous domain of 301 residues, and a primarily collagenous stretch of 602 residues. The clones do not cover the carboxyl-terminal end of the polypeptide, since the translation stop codon is absent. Characteristic of the deduced polypeptide is the possession of eight noncollagenous interruptions varying in length from 10 to 24 residues in the collagenous amino acid sequence. Other features include the presence of several putative sites for both N-linked glycosylation and O-linked glycosaminoglycan attachment and homology of the amino-terminal noncollagenous domain with thrombospondin. It is of particular interest that five of the eight collagenous sequences of type XVIII show homology to the previously reported type XV collagen, suggesting that the two form a distinct subgroup among the diverse family of collagens. Northern blot hybridization analysis revealed a striking tissue distribution for type XVIII collagen mRNAs, as the clones hybridized strongly with mRNAs of 4.3 and 5.3 kilobases that were present only in lung and liver of the eight mouse tissues studied.