Partial characterization of a collagen-binding, differentiation-related glycoprotein from skeletal myoblasts

Prolactin-induced changes in protein expression in human pancreatic islets

Ex vivo islet cell culture prior to transplantation appears as an attractive alternative for treatment of type 1 diabetes. Previous results from our laboratory have demonstrated beneficial effects of human prolactin (rhPRL) treatment on human islet primary cultures. In order to probe into the molecular events involved in the intracellular action of rhPRL in these cells, we set out to identify proteins with altered expression levels upon rhPRL cell treatment, using two-dimensional (2D) gel electrophoresis and mass spectrometry (MS). An average of 300 different protein spots were detected, 14 of which were modified upon rhPRL treatment (p<0.01), of which 12 were successfully identified using MS and grouped according to their biological functions. In conclusion, our study provides, for the first time, information about proteins that could be critically involved in PRL's action on human pancreatic islets, and facilitate identification of new and specific targets involved in islet cell function and proliferation.

Functional linkage between the endoplasmic reticulum protein Hsp47 and procollagen expression in human vascular smooth muscle cells

Hsp47 is a heat stress protein that interacts with procollagen in the lumen of the endoplasmic reticulum, which is vital for collagen elaboration and embryonic viability. The precise actions of Hsp47 remain unclear, however. To evaluate the effects of Hsp47 on collagen production we infected human vascular smooth muscle cells (SMCs) with a retrovirus containing Hsp47 cDNA. SMCs overexpressing Hsp47 secreted type I procollagen faster than SMCs transduced with empty vector, yielding a greater accumulation of pro alpha1(I) collagen in the extracellular milieu. Interestingly, the amount of intracellular pro alpha1(I) collagen was also increased. This was associated with an unexpected increase in the rate of pro alpha1(I) collagen chain synthesis and 2.5-fold increase in pro alpha1(I) collagen mRNA expression, without a change in fibronectin expression. This amplification of procollagen expression, synthesis, and secretion by Hsp47 imparted SMCs with an enhanced capacity to elaborate a fibrillar collagen network. The effects of Hsp47 were qualitatively distinct from, and independent of, those of ascorbate and the combination of both factors yielded an even more intricate fibril network. Given the in vitro impact of altered Hsp47 expression on procollagen production, we sought evidence for interindividual variability in Hsp47 expression and identified a common, single nucleotide polymorphism in the Hsp47 gene promoter among African Americans that significantly reduced promoter activity. Together, these findings indicate a novel means by which type I collagen production is regulated by the endoplasmic reticulum constituent, Hsp47, and suggest a potential basis for inherent differences in collagen production within the population.

Molecular and cell biology of porcine HSP47 during wound healing: complete cDNA sequence and regulation of gene expression

Heat shock protein (HSP) 47 is a major stress-inducible protein that is localized to the endoplasmic reticulum of avian and mammalian cells and is thought to act as a molecular chaperone specific for the processing of procollagen. However, limited information is available regarding the regulation of HSP47 during wound healing. Using a polymerase chain reaction strategy, screening of a cDNA library, and RACE-polymerase chain reaction approaches, the sequence of a full-length porcine HSP47 cDNA has been identified. The cDNA contained 2096 bp that encodes for an 18 amino acid signal peptide and a mature protein coding region consisting of 401 amino acid residues. It also included 108 bp of the 5' noncoding region and a 731-bp 3' noncoding region. The deduced amino acid is 83% identical to chicken, 87% identical to mouse, 88% identical to rat, and 91% identical to human HSP47. It also shares between 26% and 30% identity with different members of the serine protease inhibitor superfamily. The protein contains a RDEL endoplasmic reticulum retention signal, and two potential glycosylation sites. All of these features are characteristic of HSP47 in higher vertebrates. Heat shock treatment of porcine fibroblasts led to up-regulation of HSP47 at both the transcriptional and translational levels. HSP47 protein levels were also up-regulated during skin wound healing in a pig model. Moreover, a higher molecular weight complex at approximately 140 Kda containing HSP47 was detected at the stage of healing that was coincident with the maximal transcriptional expression of HSP47 during wound healing in this animal model. Further investigation of how HSP47 is regulated during normal and abnormal skin wound healing may lead to new therapeutic approaches to improve the healing process.

Non-natural CBP2 binding peptides and peptomers modulate carcinoma cell adhesion and invasion

A combinatorial approach that utilized a repertoire of bacteriophage-peptides has identified a number of non-natural CBP2 binding peptides. Moreover, co-localization of some of these peptides with CBP2 in a number of tumor cell lines demonstrated that the peptides were directed to an intracellular location spatially coincident with the normal distribution of CBP2 [Sauk et al., 2000]. From among these sequences WHYPWFQNWAMA and LDSRYSLQAAMY were the most effective CBP2 binding peptides and best fulfilled the combinatorial motif containing deep hydrophobic pockets. When the hydropathic profiles of collagen alpha1(IV) and alpha2 (IV) were compared with these dodecapeptides, the hydropathic profiles of WHYPWFQNWAMA and LDSRYSLQAAMY closely matched those of alpha1(IV) 414-452 and alpha1(IV)531-543. These peptides were shown to be functional peptidomimics and possessed the ability to alter cell adhesion and invasion of human squamous cell carcinoma cell lines. Peptomers were formed of these non-natural peptides to explore the role that a repetitive peptide may have on cell adhesion. The enhanced cell adhesion observed with the peptomers required both CBP2 antibodies and integrin antibodies for inhibition. The enhanced adhesion observed even in the face of combined antibody inhibition was consistent with such complexes possessing correspondingly slower dissociation rates. Thus, suggesting that peptomers may function in a like manner to multimeric peptide MHC complexes (tetramers) binding more than one cell receptor on a specific cell. These findings evoke both peptidomimics of native ligands and their peptomers as potential reagents by which to target tumor cells for chemotherapy, imaging, or retargeting viral vectors for gene therapy.

Heat shock genes and the heat shock response in zebrafish embryos

Heat shock genes exhibit complex patterns of spatial and temporal regulation during embryonic development in a wide range of organisms. Our laboratory has initiated an analysis of heat shock protein gene expression in the zebrafish, a model system that is now utilized extensively for the examination of early embryonic development of vertebrates. We have cloned members of the zebrafish hsp47, hsp70,\i and hsp90 gene families and shown them to be closely related to their counterparts in higher vertebrates. Whole mount in situ hybridization and Northern blot analyses have revealed that these genes are regulated in distinct spatial, temporal, and stress-specific manners. Furthermore, the tissue-specific expression patterns of the hsp47 and hsp90 alpha genes correlate closely with the expression of genes encoding known chaperone targets of Hsp47 and Hsp90 in other systems. The data raise a number of interesting questions regarding the function and regulation of these heat shock genes in zebrafish embryos during normal development and following exposure to environmental stress.

Cellular localization of gp46 at the human fetal-maternal interface

gp46 is a collagen-binding heat-shock glycoprotein with a possible role in the biosynthesis of collagen as well as in cell differentiation and fusion. In this study, the relative levels of gp46 protein and its mRNA transcript were examined, as well as the mRNA levels of collagen types I and IV in first trimester and term human placental tissues. Western blot analysis revealed substantially higher levels of gp46 in first trimester placentae than in term placentae. Similarly, elevated levels of type IV collagen transcript were detected in first trimester relative to term issues. Interestingly, the levels of gp46 and type I collagen mRNA remained unchanged. Immunohistochemical analysis of first trimester tissues demonstrated intense gp46 staining in mononucleated villous and extravillous cytotrophoblasts, decidual cells and in the villous connective tissue stroma. Syncytiotrophoblast in the same tissues also exhibited gp46 staining but at a reduced intensity. In chronic villi of term placentae, faint gp46 staining was only observed in the syncytiotrophoblast layer. However, as in the first trimester placentae, intense labelling was evident in the extravillous cytotrophoblasts and decidual cells of these tissues. These results suggest a developmental regulation of gp46 expression at the fetal-maternal interface during pregnancy and suggest a possible functional link between gp46 and collagen type IV. during gestation.

Structure of the gene encoding human colligin-2 (CBP2)

Colligins are collagen-binding proteins localized to the endoplasmic reticulum that belong to the superfamily of serine protease inhibitors and play a role in collagen biosynthesis. Previously, we cloned the human colligin-2 gene (CBP2) and mapped it to chromosome 11q13.15. To further characterize the CBP2 gene, we have determined its genomic structure and the 5'-flanking sequence. The CBP2 gene spanned approximately 11 kb of genomic DNA and consisted of five exons. The promoter sequence of the human gene showed significant homology to that of its murine counterpart, which contained several regulatory sequences including heat-shock and retinoic acid-responsive elements. These findings suggest colligin may function as a collagen-specific molecular chaperon and play a role in the process of retinoic acid-induced differentiation.

Cloning and characterization of a cDNA encoding the collagen-binding stress protein hsp47 in zebrafish

Hsp47 is a major stress-inducible protein that is localized to the endoplasmic reticulum of avian and mammalian cells and is thought to act as a molecular chaperone specific for the processing of procollagen. Although hsp47 is coordinately expressed together with several collagen types, and vertebrate embryos are known to express collagen genes in complex spatial and temporal patterns, limited information is available regarding the function or regulation of hsp47 during early embryonic development. We have initiated an examination of hsp47 in the zebrafish, Danio rerio, which offers a number of features that make it attractive as a model developmental system with which to examine the expression and function of hsp47. A polymerase chain reaction (PCR)-based cloning strategy was used to isolate a hsp47 cDNA from an embryonic zebrafish cDNA library. The deduced translation product of the cDNA is a 404-amino-acid polypeptide that is 72% identical to chicken, 64% identical to mouse and rat, and 69% identical to human hsp47. The protein contains a typical hydrophobic signal sequence, an RDEL endoplasmic reticulum retention signal, and a serine protease inhibitor signature sequence, all of which are characteristic of hsp47 in higher vertebrates. Thus, it is likely that hsp47 in zebrafish is also localized to the endoplasmic reticulum and may play a similar role to its counterpart in higher vertebrates. Northern blot analysis revealed that the hsp47 gene is expressed at relatively low levels in embryos during normal development but is strongly induced following exposure to heat shock at the gastrula, midsomitogenesis, 2-day, and 3-day larval stages. The level of induction was much higher than has previously been reported in chicken and mouse cells.

Developmental expression of the collagen-binding heat-shock protein GP46 and collagen types I and IV in rat tissues

The temporal expression of protein and mRNA encoding the collagen-binding heat-shock glycoprotein, gp46, were determined in the heart, kidney, and lung during early rat postnatal development. The steady-state levels of collagen types I and IV mRNA expression were also examined to determine if gp46 and these collagen types are co-regulated during ontogenesis. Western blot analysis using a monoclonal antibody to gp46 revealed that gp46 levels are developmentally regulated. In heart and kidney, gp46 levels were high on days 3 and 8, reduced significantly on day 25, and low to undetectable on day 69. Protein levels of gp46 in the lung exhibited a similar temporal pattern except on day 3, when very low levels of gp46 were detected. mRNA expression of gp46 during early postnatal development did not correlate with gp46 protein accumulation in these tissues, suggesting a complex pre- and post-translational regulatory scheme. In the heart, protein levels of gp46 correlated well with collagen type I alpha 1(I) mRNA expression but not with collagen type IV alpha 1(IV). In contrast, gp46 protein levels closely paralleled alpha 1(IV) expression in the kidney. Gp46 levels exhibited no apparent correlation with either alpha 1(I) or alpha 1(IV) levels in the lung. These results show that gp46 is developmentally regulated at both the protein and mRNA levels in a tissue specific manner. The relationship between gp46 and collagen alpha 1(I) and alpha 1(IV) chain mRNA expression also has been shown to be tissue specific.

Interaction of procollagen I and other collagens with colligin

Colligin is a collagen-binding glycoprotein of molecular mass 46000 Da localized to the endoplasmic reticulum (ER) of diverse kinds of cells that produce collagen I. In order to help define its role in collagen biosynthesis and to study the interaction of colligin with procollagen I in detail, the binding characteristics of colligin purified from L6 myoblasts have been studied. A total of 3 mol were found to bind/mol of procollagen I, with a Kd of about 25 nM. Both pure and separated pro alpha 1(I) and procollagen alpha 2 (I) chains were able to compete with procollagen I for binding to colligin. However, colligin binds to pro alpha 2 (I) with higher affinity than to pro alpha 1 (I). To find if the binding activity of colligin was altered during purification, an assay to measure colligin binding to procollagen in crude myoblast cell extracts was developed. This procedure gave the same binding parameters as did the highly purified colligin. Among different collagen types, colligin was found to bind to collagen I and collagen IV, but not to collagen III. In order to examine whether glycosylation or phosphorylation of colligin were required for the binding of colligin to procollagen I and to obtain enough colligin for further studies, recombinant protein was produced in Escherichia coli. An immunoaffinity purification scheme was used to get virtually pure protein in milligram yields. Comparison of the recombinant colligin with that isolated from L6 myoblasts showed that both types existed in solution as monomers and dimers. In addition, both types of colligins showed identical properties with regard to their binding to procollagen I and the isolated pro alpha 1(I) and pro alpha 2(I) chains. Post-translational modifications of colligin were thus not essential for binding to procollagen I.

Parallel regulation of procollagen I and colligin, a collagen-binding protein and a member of the serine protease inhibitor family

A potential regulatory linkage between the biosynthesis of colligin, a collagen-binding protein of the ER, and procollagen I was examined under a variety of experimental conditions. Cell lines which did not produce a significant amount of procollagen I mRNA also lacked the capacity to produce colligin mRNA. Anchorage-dependent cell lines like L6 myoblasts and normal rat kidney fibroblasts produced both colligin and procollagen I mRNA, but the level of both was concurrently reduced considerably in their ras-transformed counterparts. Similarly, during the differentiation of L6 myoblasts, levels of both colligin and procollagen declined together. Treatment of myoblasts by dexamethasone or EGF led to a decrease in the steady-state levels of procollagen I mRNA, and this was, again, accompanied by a decrease in colligin mRNA synthesis. On the other hand, when the rate of procollagen I synthesis was stimulated by treatment of myoblasts with TGF beta, it led to the concurrent augmentation of both the mRNA and protein levels of colligin. A linkage between the regulation of synthesis of procollagen I and colligin thus seems to exist. The only exception to this generalization is provided by the heat induction behavior of the two proteins. Treatment of myoblasts for a very short period leads to an increase in the synthesis of both the mRNA and protein levels of colligin. This, however, is not accompanied by a change in the mRNA levels of procollagen I. These studies establish that colligin and procollagen are generally tightly co-regulated except after heat shock, suggesting an important functional linkage.

Cloning of a human collagen-binding protein, and its homology with rat gp46, chick hsp47 and mouse J6 proteins

Several cDNA clones encoding a collagen-binding protein were isolated from human fibroblasts. The cDNA encoded a 417 amino acid protein, containing two potential N-linked oligosaccharide binding sites and a C-terminal RDEL sequence, which has been shown to act as an endoplasmic retention signal in other systems. The derived amino acid sequence of the protein shows close homology with gp46 from rat skeletal myoblasts, J6 protein from mouse F9 embryonal carcinoma cells and hsp47 from chick embryo fibroblasts. It also shows sequence similarity with members of the serpin family.

Analysis of the phosphorylation state of a collagen-binding heat-shock glycoprotein from L6 myoblasts by isoelectric focusing

A major collagen-binding glycoprotein from rat L6 skeletal myoblasts, designated gp46, is phosphorylated in vivo. In this report the relative phosphorylation state of gp46 was examined using isoelectric focusing to identify the phosphorylated and unphosphorylated forms of gp46. Two major and one minor isoform of gp46 were identified that could be related to the phosphorylation state of gp46. The relative percentage of unphosphorylated to phosphorylated gp46 increased 10% in myoblasts heat-shocked at 42 degrees C for 24 h. Treatment of myoblasts with phorbol ester or dibutyryl-cAMP had no effect on the phosphorylation ratio of gp46. Transformation of L6 myoblasts with Rous sarcoma virus, likewise, had no effect on the phosphorylation ratio. However, ras-transformed L6 myoblasts showed a 12% increase in phosphorylation of gp46. These results indicate that gp46 does not undergo large changes in phosphorylation status. Pulse-chase labelling showed that the phosphorylation of gp46 occurred either co-translationally or soon after translation, suggesting that gp46 was phosphorylated by a constitutively active protein kinase.

Purification and reconstitution of a collagen-binding heat-shock glycoprotein from L6 myoblasts

A major collagen-binding heat-shock glycoprotein from L6 myoblasts, designated gp46, was purified by gelatin-agarose chromatography and ion-exchange chromatography. Purified gp46 was functionally active, as shown by its ability to rebind to gelatin-agarose, and was homogeneous as determined by SDS/polyacrylamide-gel electrophoresis. This is the first reported purification of myoblast gp46 in an active state. Triton X-100-soluble gp46 was found to bind preferentially to immobilized pepsin-treated type IV collagen compared with native type I collagen. gp46, reconstituted into phospholipid vesicles, retained collagen-binding activity. This activity could be destroyed by chemical modification with chloramine-T, but was decreased by only 20-30% following treatment with iodoacetamide or N-ethylmaleimide. Since gp46 is a heat-shock protein, we examined the possibility that it may confer protection on type I collagen from thermal denaturation at temperatures above its normal melting temperature of 41 degrees C. In the presence of gp46 liposomes the apparent melting temperature of type I collagen was marginally increased to 42 degrees C. This change was considered to be too small to be of physiological significance. We have therefore concluded that the role of gp46 in collagen metabolism is unlikely to be related to any thermal-stabilizing function.