Evidence that tropoelastin is the primary precursor in elastin biosynthesis

Joel Rosenbloom and the revolution in biomedical research

This biography of Dr. Joel Rosenbloom is published on the occasion of the 50th anniversary of the journal. Dr. Rosenbloom presents the scientific milestones and achievements throughout his career emphasizing events that have spurred him to launch into a career in biomedical research and education. The biography spans several decades of the life and achievements of a distinguished physician scientist whose dedication to science demonstrates the development of new insights into a variety of connective tissues through technological advances and insightful approaches.

Elastin accumulation is regulated at the level of degradation by macrophage metalloelastase (MMP-12) during experimental liver fibrosis

Elastin has been linked to maturity of liver fibrosis. To date, the regulation of elastin secretion and its degradation in liver fibrosis has not been characterized. The aim of this work was to define elastin accumulation and the role of the paradigm elastase macrophage metalloelastase (MMP-12) in its turnover during fibrosis. Liver fibrosis was induced by either intraperitoneal injections of carbon tetrachloride (CCl(4) ) for up to 12 weeks (rat and mouse) or oral administration of thioacetamide (TAA) for 1 year (mouse). Elastin synthesis, deposition, and degradation were investigated by immunohistochemistry, quantitative polymerase chain reaction (qPCR), western blotting, and casein zymography. The regulation of MMP-12 elastin degradation was defined mechanistically using CD11b-DTR and MMP-12 knockout mice. In a CCl(4) model of fibrosis in rat, elastin deposition was significantly increased only in advanced fibrosis. Tropoelastin expression increased with duration of injury. MMP-12 protein levels were only modestly changed and in coimmunoprecipitation experiments MMP-12 was bound in greater quantities to its inhibitor TIMP-1 in advanced versus early fibrosis. Immunohistochemistry and macrophage depletion experiments indicated that macrophages were the sole source of MMP-12. Exposure of CCl(4) in MMP-12(-/-) mice led to a similar degree of overall fibrosis compared to wildtype (WT) but increased perisinusoidal elastin. Conversely, oral administration of TAA caused both higher elastin accumulation and higher fibrosis in MMP-12(-/-) mice compared with WT.

CONCLUSION

Elastin is regulated at the level of degradation during liver fibrosis. Macrophage-derived MMP-12 regulates elastin degradation even in progressive experimental liver fibrosis. These observations have important implications for the design of antifibrotic therapies.

Tropoelastin expression is up-regulated during activation of hepatic stellate cells and in the livers of CCl(4)-cirrhotic rats

BACKGROUND/AIMS

Activated hepatic stellate cells (HSC) are regarded as the principal cells synthesizing extracellular matrix components in fibrotic liver. Elastin content is increased in cirrhotic livers, but the cellular source is not known. Contribution of HSC to the production of elastin was investigated.

METHODS

Expression of elastin in CCl(4)-cirrhotic rat liver was studied by immunohistochemistry and in situ hybridization, liver myofibroblasts were identified in histological sections by alpha-smooth muscle actin (alpha-SMA) staining. LightCycler PCR and Northern blotting were used to detect tropoelastin mRNA in isolated HSC; tropoelastin protein was detected in the cells and in cell-conditioned medium by Western blotting.

RESULTS

HSC, isolated from normal rat liver, displayed increasing tropoelastin mRNA expression during transdifferentiation in culture. Expression of tropoelastin mRNA was accompanied by the production of tropoelastin protein in vitro. Increased levels of tropoelastin transcripts were found in the connective tissue septa of CCl(4)-cirrhotic rats and co-localized with alpha-SMA positive cells. Immunohistochemistry demonstrated elastin presence in the septa.

CONCLUSION

HSC express tropoelastin and its expression increases during transdifferentiation to myofibroblast-like cells.

Structure of the 3' region of the human elastin gene: great abundance of Alu repetitive sequences and few coding sequences

Two overlapping clones encompassing 8.5 kb of the human elastin gene were isolated from two genomic libraries constructed by partial digestion with either HaeIII/AluI or Sau3A and contained in lambda Charon 4A or EMBL3, respectively. The 6 kb of DNA comprising the most 3' portion of the gene were sequenced demonstrating an extremely low coding ratio since only three exons containing a total of 134 translated nucleotides were identified. Two exons totaling 78 bp of translated sequences which were previously found in the bovine gene were absent in the human gene. The 3' most exon encoded the unusual amino acid sequence, GGACLGKACGRKRK. The human gene was terminated by 1.2 kb of untranslated sequence which contained two polyadenylation attachment signals. The remainder of the 6 kb was composed of intervening sequences which were abundantly rich in Alu family repetitive sequences found in both orientations. This first report of the characterization of the human elastin gene suggests that significant variation in the gene may exist between species and raises the possibility of consequential polymorphism, mediated by recombination between Alu sequences, in the human population.

Newly determined carboxy terminal sequences in tropoelastin: immunologic identification in insoluble elastin

The carboxy terminal sequence of sheep, bovine and human tropoelastin (GFPGGACLGKA/SCGRKRK) has been inferred in earlier studies from sequencing of cloned complementary and genomic DNA. However, this putative carboxy terminal sequence was not found previously in peptides recovered from tryptic digests of tropoelastin. In order to determine whether the amino acid sequence described above is found in insoluble elastin, antibodies were raised against the chemically synthesized peptides with the appropriate sequences and the antibodies were shown to react with peptides derived from human, bovine, porcine, dog and hamster insoluble elastins. These results strongly suggest that the sequence (GFPGGACLGKA/SCGRKRK) at the carboxy terminus of tropoelastin is found in the elastins of many species.

Analysis of the 3' region of the sheep elastin gene

The nucleotide sequences of a 1279-bp sheep elastin cDNA clone, pcSEL1 [Yoon et al. (1984) Biochem. Biophys. Res. Commun. 118, 261-269], and a 1230-bp sheep elastin genomic subclone, pSS1 [Davidson et al. (1984) Biochem. J. 220, 643-652], corresponding to a portion of the cDNA clone, were determined. These analyses permitted determination of the 100 amino acids at the carboxy terminus of sheep tropoelastin. A portion of this sequence showed strong homology to known sequences of pig tropoelastin, but most of the sequence had not been previously determined through protein sequencing. Novel aspects of the tropoelastin molecule which have been revealed by the present analyses are (i) the presence of an unusual sequence, KPPKP, which may contribute to crosslink formation; and (ii) the finding of cysteine within a sequence, CLGKSCGRKRK, at the putative carboxy terminus of tropoelastin. Because of the presence of these sequences, it is speculated that the carboxy-terminal region may be of importance in crosslinking tropoelastin molecules to themselves or to other matrix macromolecules. The nucleotide analyses revealed that sheep elastin mRNA contains a 974-bp untranslated sequence at the 3' end, which appears to be strongly conserved among species.

Structure of the 3' portion of the bovine elastin gene

A bovine genomic library constructed by partial Sau3A digestion and contained in lambda Charon 30 was screened by in situ hybridization with a 1.3-kilobase (kb) sheep elastin cDNA clone [Yoon, K., May, M., Goldstein, N., Indik, Z., Oliver, L., Boyd, C., & Rosenbloom, J. (1984) Biochem. Biophys. Res. Commun. 118, 261-269]. Three clones encompassing 10 kb of the bovine elastin gene were identified and characterized by restriction mapping and DNA sequencing of the 6.2 kb of the most 3' region of the gene. These analyses have permitted localization of eight exons in the 6.2 kb in which the translated exons vary in size from 27 to 69 base pairs, and there is an approximately 1-kb untranslated region at the 3' end. In addition to identification of sequences homologous to those found in porcine tropoelastin, the analyses defined a 58 amino acid sequence that forms the carboxy-terminal region of tropoelastin, and this sequence, which contains two cysteine residues, was previously not observed in the protein sequence data. The analyses also suggest that functionally distinct cross-link and hydrophobic domains of the protein are encoded in separate exons.

Secretion of elastin in the embryonic chick aorta as visualized by immunoelectron microscopy

Recently, significant advances have been made in characterizing the pathway of elastin biosynthesis from the biochemical point of view and a 70,000 dalton protein, designated tropoelastin, appears to be the primary translation product and soluble intermediate of the insoluble elastin. However, relatively little is known concerning the intracellular secretory pathway of tropoelastin. We previously developed an electron microscopic technique using elastin-specific antibody and ferritin-conjugated secondary antibody to identify intracellular elastin and to identify, provisionally, intracellular vesicles containing elastin ( Damiano et al., Conn. Tiss . Res. 8: 185-188, 1981). However, the method did not permit localization of elastin in other intracellular organelles. We now describe an improved post-embedding technique using the peroxidase-antiperoxidase method to detect the primary elastin antibody and have localized elastin in both the endothelial and medial cells of the embryonic chick aorta. Specific staining was visualized in the cisternae of the endoplasmic reticulum, in the Golgi apparatus, and in vesicles forming on the trans side of the Golgi. Some of these smaller vesicles appeared to fuse, forming larger vesicles which may have a storage function. Both types of vesicles were seen fusing with the cell plasma membrane, suggesting that elastin is secreted by an exocytotic process. These results suggest that tropoelastin follows the classical pathway for protein secretion.

Characterization of a sheep elastin cDNA clone containing translated sequences

mRNA, isolated from the ligamentum nuchae of fetal sheep by guanidine HCl extraction and oligo(dT) cellulose chromatography, was used to synthesize blunt-ended cDNA molecules by the successive application of AMV reverse transcriptase, DNA polymerase and S1 nuclease. The cDNA was centrifuged on a 15-30% sucrose gradient and molecules greater than 700 bp were tailed with dCTP and cloned into the PstI site of pBR322 which had been tailed with dGTP. Ampicillin-sensitive and tetracycline-resistant colonies were screened by in situ hybridization with elastin-enriched mRNA that had been terminally labeled with 32p. Recombinant plasmids prepared from strongly hybridizing colonies were characterized by restriction mapping and the plasmid with the largest insert (1300 bp) thought to contain elastin sequences was characterized in more detail. The nick-translated cDNA hybridized to a single 3.5 kb mRNA species upon blot hybridization, a size identical to that previously identified for chick elastin mRNA (Burnett et al. (1982) J. Biol. Chem. 259, 1569-1572). Nucleotide sequencing of the 5' end of the cDNA demonstrated a sequence which was extremely GC rich and which corresponded to an amino acid sequence partially homologous to that previously identified in porcine tropoelastin (Foster et al. (1973) J. Biol. Chem. 248, 2876-2879). This is the first report of the identification of a plasmid containing sequences complementary to a translated region of elastin mRNA.

Elastin in diseases

Previous morphologic observations have suggested abnormalities in the elastic fibers in a number of both inherited and acquired diseases. Recent progress made in understanding of the normal biology of elastin has allowed us to examine these diseases by biochemical means. In this review we are discussing the current status of the research on the elastin diseases with particular emphasis on clinical conditions affecting skin, as for example, cutis laxa, pseudoxanthoma elasticum, and the Buschke-Ollendorff syndrome. In addition, we present new data which appears to be the first demonstration of an elastin abnormality in the Marfan syndrome.

Control of elastin synthesis

mRNA was isolated from the thoracic aortas of 16-day chick embryos and used to synthesize blunt-ended heteroduplex molecules consisting of one strand of mRNA and one of cDNA using AMV reverse transcriptase and S1 nuclease. The duplexes were tailed with dCTP and hybridized to the plasmid pBR322 which had been restricted with Pst I and tailed with dGTP. Recombinant plasmids were used to transform E. coli C600 and colonies containing elastin cDNA were selected by in situ hybridization with 32P labeled elastin mRNA and by hybrid selected translation using the nuclease-treated reticulocyte lysate system. mRNA recovered from hybridization to DNA of one clone, pWB1, markedly stimulated incorporation of [3H]valine into a protein which was immunoprecipitable with elastin-specific antibody and had a molecular weight of 72,000, characteristic of tropoelastin. The 230 bp insert of pWB1 was sequenced by the technique of Maxam and Gilbert and found to be derived from a nontranslated region of the 3' end of the mRNA. Nick-translated pWB1 was used to identify and to estimate the relative amounts of elastin mRNA in the developing chick embryo aorta by blot hybridization. A single mRNA species of 3.5 kb hybridized to the pWB1 probe and this species increased greatly in amount between day 7 and day 14. This increase was paralleled by an increase in translatable elastin mRNA and by the rate of elastin synthesis of aortas from various age embryos incubated in vivo. The injection of 150 microgram of hydrocortisone 21-phosphate into 8-day eggs produced a significant increase in both the relative rate of tropoelastin synthesized by the isolated aortas and the relative amount of elastin mRNA. These results suggest that the observed changes in elastin synthesis during development and after hydrocortisone administration are governed by the elastin mRNA content of the aortas.

Identification of the primary translation product of elastic mRNA

Poly (A) RNA was prepared from matrix free cells derived from the major thoracic arteries of 17-day old chick embryos. This was translated in a messenger-dependent reticulocyte cell-free system in the presence of [35S]-methionine, [3H]-proline or [3H]-valine. The translation products were analyzed by SDS polyacrylamide gel electrophoresis and fluorography and elastin-related products were immunoprecipitated by the addition of anti-elastin sheep antiserum in the presence of proteinase inhibitors and the precipitate analyzed by the same technique. The elastin-related products were found to consist of a doublet of two closely related polypeptides (mol. wts. 70,000 and 73,000) comigrating with a tropoelastin standard. Unhydroxylated pro alpha 1 and pro alpha 2 collagen polypeptides were also found, but no elastin related product corresponding to proelastin. It is concluded that the primary translation product of elastin mRNA consists of two polypeptides between 70,000 and 73,000 mol wt. which are closely related in size and immunoreactivity to tropoelastin.

Biosynthesis of tropoelastin by elastic cartilage

Elastic ear cartilage and aortae from nine-day-old hamsters were incubated in Krebs-Ringer medium containing ascorbate, beta-aminopropionitrile and either [14C]proline or [3H]valine. The [14C]proline-labeled proteins were separated by chromatography on Agarose A-5 m in sodium dodecyl sulfate and their [14C]-hydroxyproline content measured. A fraction having an approximate molecular weight of 70,000 measured by polyacrylamide gel electrophoresis had a [14C]hydroxyproline content of 9.1% in the cartilage and 11.8% in the aorta. This fraction was also relatively heavily labeled with [3H]valine. The 70,000 dalton, [3H]valine-labeled protein of both the aorta and ear cartilage was precipitated by elastin-specific antibody prepared against hamster insoluble aorta elastin, but no higher molecular weight protein was immunoprecipitated. Based on these results we identify the 70,000 dalton protein from the elastic cartilage as tropoelastin.