Inhibition of tropoelastin secretion by incorporation of DL-3,4-dehydroproline

Ascorbate differentially regulates elastin and collagen biosynthesis in vascular smooth muscle cells and skin fibroblasts by pretranslational mechanisms

Ascorbate contributes to several metabolic processes including efficient hydroxylation of hydroxyproline in elastin, collagen, and proteins with collagenous domains, yet hydroxyproline in elastin has no known function. Prolyl hydroxylation is essential for efficient collagen production; in contrast, ascorbate has been shown to decrease elastin accumulation in vitro and to alter morphology of elastic tissues in vivo. Ascorbate doses that maximally stimulated collagen production (10-200 microM) antagonized elastin biosynthesis in vascular smooth muscle cells and skin fibroblasts, depending on a combination of dose and exposure time. Diminished elastin production paralleled reduced elastin mRNA levels, while collagen I and III mRNAs levels increased. We compared the stability of mRNAs for elastin and collagen I with a constitutive gene after ascorbate supplementation or withdrawal. Ascorbate decreased elastin mRNA stability, while collagen I mRNA was stabilized to a much greater extent. Ascorbate withdrawal decreased collagen I mRNA stability markedly (4.9-fold), while elastin mRNA became more stable. Transcription of elastin was reduced 72% by ascorbate exposure. Differential effects of ascorbic acid on collagen I and elastin mRNA abundance result from the combined, marked stabilization of collagen mRNA, the lesser stability of elastin mRNA, and the significant repression of elastin gene transcription.

Tropoelastin production and tropoelastin messenger RNA activity. Relationship to copper and elastin cross-linking in chick aorta

The elastin content of the chick thoracic aorta increases 2--3-fold during the first 3 weeks post-hatching. The deposition of elastin requires the covalent cross-linking of tropoelastin by means of lysine-derived cross-links. This process is sensitive to dietary copper intake, since copper serves as cofactor for lysyl oxidase, the enzyme that catalyses the oxidative deamination of the lysine residues involved in cross-link formation. Disruption of cross-linking alters tissue concentrations of both elastin and tropoelastin and results in a net decrease in aortic elastin content. Autoregulation of tropoelastin synthesis by changes in the pool sizes of elastin or tropoelastin has been suggested as a possible mechanism for the diminished aortic elastin content. Consequently, dietary copper deficiency was induced to study the effect of impaired elastin cross-link formation on tropoelastin synthesis. Elastin in aortae from copper-deficient chicks was only two-thirds to one-half the amount measured in copper-supplemented chicks, whereas copper-deficient concentrations of tropoelastin in aorta were at least 5-fold higher than normal. In spite of these changes, however, increased amounts of tropoelastin, copper deficiency and decreased amounts of elastin did not influence the amounts of functional elastin mRNA in aorta. Likewise, the production of tropoelastin in aorta explants was the same whether the explants were taken from copper-sufficient or -deficient birds. The lower accumulation of elastin in aorta from copper-deficient chicks appeared to be due to extracellular proteolysis, rather than to a decrease in the rate of synthesis. Electrophoresis of aorta extracts, followed by immunological detection of tropoelastin-derived products, indicated degradation products in aortae from copper-deficient birds. In extracts of aortae from copper-sufficient chicks, tropoelastin was not degraded and appeared to be incorporated into elastin without further proteolytic processing.

The secretion of tropoelastin by chick-embryo artery cells

Chymotryptic fingerprint analyses of tropoelastin a and tropoelastin b demonstrated a very close relationship between these two polypeptides synthesized in a cell-free system under the direction of chick-embryo polyribosomal mRNA. A similar study on tropoelastin polypeptides extracted in their hydroxylated and under-hydroxylated forms from artery cells incubated with [3H]valine in the absence and presence of alpha alpha'-bipyridine or 3,4-dehydroproline confirmed this close relationship and suggested that tropoelastins a and b are likely to be the products of a single gene. Pulse-chase experiments in which the synthesis and secretion of tropoelastin by artery cells were monitored demonstrated that, after a pulse with [3H]proline, the polypeptides rapidly appeared in the medium and the half-time of tropoelastin secretion was approx. 30 min. Further pulse-chase studies, in which [3H]tropoelastin contents of subcellular fractions were determined, showed that rough and smooth microsomal fractions contained maximal amounts of tropoelastin at different times. The quantity of tropoelastin in the smooth-microsomal fraction was always only a small proportion of that in the rough-microsomal fraction, suggesting rapid translocation of the polypeptides to the plasma membrane. Incubation of the cells with 0.1 mM-colchicine did not markedly alter the rate of secretion or the distribution of tropoelastin between the subcellular fractions, whereas when 1 microM-monensin was included in the incubations the polypeptides were retained in the rough microsomal fraction. The results are consistent with the proposal that tropoelastin may follow a pathway of secretion from rough endoplasmic reticulum to the plasma membrane via secretory vesicles.

Alteration of the extracellular matrix of smooth muscle cells by ascorbate treatment

The protein composition in the extracellular matrix of cultured neonatal rat aortic smooth muscle cells has been monitored over time in culture. The influence of ascorbate on insoluble elastin and collagen has been described. In the absence of ascorbate, the cells accumulate an insoluble elastin component which can account for as much as 50% of the total protein in the extracellular matrix. In the presence of ascorbate, the amount of insoluble collagen increases, while the insoluble elastin content is significantly less. When ascorbate conditions are varied at different times during the culture, the extracellular matrices are altered with respect to collagen and elastin ratios. The decrease in elastin accumulation in the presence of ascorbate may be explained by an overhydroxylation of tropoelastin. Approximately 1/3 of the prolyl residues in the soluble elastin fractions isolated from cultures grown in the presence of ascorbate are hydroxylated. Since the insoluble elastin accumulated in these cultures contain the unique lysine-derived cross-links in amounts comparable to aortic tissue, this culture system proves ideal for studying the influence of extracellular matrix elastin on cell growth and metabolism.

Effects of ascorbate on insoluble elastin accumulation and cross-link formation in rabbit pulmonary artery smooth muscle cultures

Cultured pulmonary artery smooth muscle cells derived from the medial vessel layer of weanling rabbits were grown in the presence or absence of sodium ascorbate. The connective tissue elements insoluble elastin and collagen were identified and quantified. Formation and accumulation of alpha-aminoadipic acid gamma-semialdehyde (allysine) and the intermolecular cross-links desmosine (Des), isodesmosine (Ides), and aldol condensation product (Aldol) were evaluated from [14C]lysine pulse-chase experiments. [14C]Des, [14C]Ides, peptide-bound [14C]lysine, [14C]allysine, and [14C]Aldol were determined from amino acid analysis. The latter two components were determined after reduction with NaBH4. [14C]Proline conversion to hydroxy[14C]proline and collagenase susceptibility were used to identify and quantify collagen synthesis. Ascorbate dramatically affects insoluble elastin synthesis, accumulation, and cross-link formation. Cells grown in the presence of ascorbate synthesize and accumulate significantly less insoluble elastin than non-ascorbate cultures. Those elastin molecules which do become incorporated into the extracellular matrix in the presence of ascorbate contain a slightly elevated content of hydroxyproline and lysine and, most importantly, are turned over more rapidly.

Immuno electron microscopic studies on cells synthesizing elastin

Embryonic chick and hamster aortas were examined in the electron microscope using ferritin-conjugated, elastin-specific antibodies after etching of the plastic, thin sections with a dilute solution of benzene, methanol and ethanol. Specific staining of intracellular vesicles was observed in both smooth muscle and endothelial cells in addition to extracellular elastin fibers. In the chick cells, these ferritin-stained vesicles had a lipid-laden appearance. In both species the vesicles appeared to fuse with the plasma membrane and discharge their contents into the extracellular space suggesting elastin is secreted via vesicular structures.

In vitro studies of elastin metabolism

Neonatal pig and rat aortas were studied for their ability to synthesize elastin in an in vitro situation. Smooth muscle cells from the rat aorta produced excellent multilayered cultures and produced soluble elastin (tropoelastin), insoluble elastin, and small amounts of collagen. BAPN proved to be toxic to these cells, adversely affecting the level of extracellular protein production. Tissue minces from pig aorta continued to synthesize elastin for two hours after removal. However, a 24 hour study indicated that elastin synthesis had almost completely shut down and that collagen synthesis continued in an apparently normal fashion. It is concluded that in vitro elastin synthesis is an extremely sensitive process easily altered by culture conditions and the addition of extraneous substances such as BAPN, and also highly influenced by the past history of the smooth muscle cells involved.