Elastin secretion

Tropoelastin is the soluble precursor of elastin, secreted from cells as a 60-72 kDa protein in humans. The monomer has two major... 

After secretion from the cell, certain lysyl residues of tropoelastin are oxidatively deaminated to aldehydes by lysyl oxidase, the same enzyme involved in this process in collagen. However, the major cross-links formed in elastin are the desmosines, which result from the condensation of three of these lysine-derived aldehydes with an unmodified lysine to form a tetrafunctional cross-hnk unique to elastin. Once cross-linked in its mature, extracellular form, elastin is highly insoluble and extremely stable and has a very low turnover rate. Elastin exhibits a variety of random coil conformations that permit the protein to stretch and subsequently recoil during the performance of its physiologic functions. 

Our own skin is made up of specialized cells which become filled with microfibrils of keratin as they move outward and become the relatively dry nonliving external surface (Box 8-F). Internal epithelial cells secrete protein and carbohydrate materials that form a thin basement membrane around the exposed parts of the cells. The connective tissue that lies between organs and which also includes tendons, cartilage, and bone consists of a relatively small number of cells surrounded by a "matrix" consisting of the protein fibers collagen and elastin in a "ground substance" rich in proteoglycans (Chapter 4).616 618 in bone, the calcium phosphate is deposited within this matrix. 

In vivo elastin fiber formation requires the coordination of a number of important processes. These include the control of intracellular transcription and translation of tropoelastin, intracellular processing of the protein, secretion of the protein into the extracellular space, delivery of tropoelastin monomers to sites of elastogenesis, alignment of the monomers with previously accreted tropoelastin through associating microfibrillar proteins, and finally, the conversion to the insoluble elastin polymer through the crosslinking action of lysyl oxidase (Fig. 2). 

Elastogenesis occurs primarily during late fetal and early neonatal periods. Elastin is synthesized and secreted from several cell types including smooth muscle cells, fibroblasts, endothelial cells, chondroblasts, and mesothelial cells (Uitto et al, 1991) with tissue-specific induction of elastin expression during development (Swee et al, 1995). After elastin has been deposited, its synthesis ceases and very little turnover of elastin is seen during adult life, unless the elastic fibers are subject to injury. In this case,... 

Hinek, A., Keeley, F. W., and Callahan, J. (1995). Recycling of the 67-kDa elastin binding protein in arterial myocytes is imperative for secretion of tropoelastin. Exp. Cell Res. 220, 312-324. 

The MMPs are a family of zinc-dependent neutral endopep-tidases that share structural domains but differ in substrate specificity, cellular sources, and inductivity (Table I). All the MMPs are important for remodeling of the extra cellular matrix and share the following functional features (/) they degrade extracellular matrix components, including fibronectin, collagen, elastin, proteoglycans, and laminin, (//) they are secreted in a latent proform and require activation for proteolytic activity, (///) they contain zinc at their active site and need calcium for stability, (/V) they function at neutral pH, and (v) they are inhibited by specific tissue inhibitors of metalloproteinases (TIMPs). 

Elastin is a macromolecule synthesized as a 70,000 single peptide chain, termed tropoelastin and secreted into the extracellular matrix where it is rapidly crosslinked to form mature elastin. The carboxy-terminal end of elastin is highly conserved with the sequence Gly-Gly-Ala-Cys-Leu-Gly-Leu-Ala-Cys-Gly-Arg-Lys-Arg-Lys. The two Cys residues that form disulfide crosslinks are found in this region as well as a positively charged pocket of residues that is believed to be the site of interaction with microfibrillar protein residues. Hydrophobic alanine-rich sequences are known to form a helices in elastin these sequences are found near lysine residues that form crosslinks between two or more chains. Alanine residues not adjacent to lysine residues found near proline and other bulky hydrophobic amino acids inhibit a helix formation. Additional evidence exists for (3 structures and 3 turns within elastin thereby giving an overall model of the molecule that contains helical stiff segments connected by flexible segments. 

The other major protein in the extracellular matrix is elastin, which is the main component of elastic fibers found in ligaments, large arteries, and lungs. After synthesis and partial hydroxylation of proline residues, a 72 kDa molecule of tropoelastin is secreted into the matrix. This protein is rich in nonpolar amino acids and contains repeating sequences, such as (Val-Pro-Gly-Val-Gly). These sections form an amorphous, random-coiled structure with frequent reverse turns. Other recurrent sequences are rich in alanine with paired lysine residues, e.g., -Ala-Ala-Ala-Ala-Lys-Ala-Ala-Lys-... 

Elastic fibers are usually found in tissues rich in smooth muscle or tissues containing fibroblasts possessing some of characteristics of smooth muscle cells (4). There is a recent report, however, that suggests elastin-like proteins may be secreted from chondrocytes (25). When elastin is secreted, it is accompanied by other proteins that appear to be important to its alignment into fibrils. One of these proteins is referred to as microfibrillar protein (cf. Table I, ref. 2). When elastin is secreted, it combines with the microfibrillar protein to form a complex which is initially rich in the microfibrillar protein. 

The ratio of microfibrillar protein to elastin, however, appears to decrease upon maturation (2). Other proteins are also secreted with microfibrillar protein and elastin. It is now clear that bound to elastin in its non-crosslink form(s) is a trypsin-like neutral proteinase (26). This proteinase effects... 

The exact form in which non-crosslinked elastin is secreted from smooth muscle cells is yet to be clearly defined. Foster et al. (36) have suggested that a non-cross linked elastin (pro-elastin) is secreted from smooth muscle cells in a form that is approximately 120,000 to 140,000 daltons. They have suggested that proelastin is cleaved to smaller molecular weight forms of non-crosslinked elastin. It should be noted, however, that this view is not entirely supported by data from other laboratories. There are two reports on the use of isolated mRNA from chick aorta suggesting only a 70,000 dalton non-cross linked elastin is the major product of translation (37,38). There is also a recent report suggesting that aortic mRMA translates a 200,000 dalton putative elastin product (39). We have recently isolated a non-crosslinked elastin from the aortas of copper deficient chicks that appears to be 100,000 daltons (27). Its amino acid composition is similar to that for tropoelastin (Table III). A major problem in resolving these points is that the trypsin-like proteinase associated with elastin is not easily denatured or separated from the non-crosslinked forms of elastin. The proteinase is also not readily inhibited by commonly used inhibitors for trypsin-like proteinases (26). 

HLE demonstrates a vast array of biologic activities besides the degradation of elastin. A partial, and by no means inclusive, list of some of these activities includes degradation of fibronectin [11], laminin [12], collagen and proteoglycans [13], and immunoglobulins (IgM, IgA and secretory IgA) [14] the activation and hydrolysis of complement components and neutrophil complement receptors [15] stimulation of mucus secretion and induction of mucous gland hypertrophy [16] and decreases in ciliary beat frequency and mucociliary clearance [17]. 

In particular, excessive proteolysis of elastin by HLE has been implicated in pulmonary emphysema [19]. In this case, the imbalance appears to result from reduced levels of active extracellular alpha,-proteinase inhibitor (a,-PI), the primary plasma inhibitor of HLE. This decrease is caused either by a genetic disorder (PiZZ phenotype individuals) or by reduction in the elastase inhibitory capacity (EIC) of ai-PI due to its oxidative inactivation by tobacco smoke [20]. The detailed evidence supporting the potential role of elastase in the development of emphysema has been extensively reviewed [21] and will not be repeated here. The fact that HLE is also a potent secretagogue [22] may play a role in several disease states, including cystic fibrosis [23], chronic bronchitis [24], and acute respiratory distress syndrome (ARDS) [25]. The mechanism of the secretagogue activity is not known, but, since the HLE-induced secretion can be blocked by specific HLE inhibitors, it appears to require catalytic activity by the enzyme [26]. 

Hlastin is a hydroxyproline containing protein of connective tissue. Unlike collagen, elastin does not form a triple helix. With a vitamin deficiency elaslin continues to be produced and secreted from the cell, but in an underhydroxylated state. The function of the hydroxyl group in eJastin is not dear. 

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