Regulation of the human elastin promoter in chick embryo cells. Tissue-specific effect of TGF-beta

Elastin, arterial mechanics, and cardiovascular disease

Large, elastic arteries are composed of cells and a specialized extracellular matrix that provides reversible elasticity and strength. Elastin is the matrix protein responsible for this reversible elasticity that reduces the workload on the heart and dampens pulsatile flow in distal arteries. Here, we summarize the elastin protein biochemistry, self-association behavior, cross-linking process, and multistep elastic fiber assembly that provide large arteries with their unique mechanical properties. We present measures of passive arterial mechanics that depend on elastic fiber amounts and integrity such as the Windkessel effect, structural and material stiffness, and energy storage. We discuss supravalvular aortic stenosis and autosomal dominant cutis laxa-1, which are genetic disorders caused by mutations in the elastin gene. We present mouse models of supravalvular aortic stenosis, autosomal dominant cutis laxa-1, and graded elastin amounts that have been invaluable for understanding the role of elastin in arterial mechanics and cardiovascular disease. We summarize acquired diseases associated with elastic fiber defects, including hypertension and arterial stiffness, diabetes, obesity, atherosclerosis, calcification, and aneurysms and dissections. We mention animal models that have helped delineate the role of elastic fiber defects in these acquired diseases. We briefly summarize challenges and recent advances in generating functional elastic fibers in tissue-engineered arteries. We conclude with suggestions for future research and opportunities for therapeutic intervention in genetic and acquired elastinopathies.

Nanoparticulate delivery of agents for induced elastogenesis in three-dimensional collagenous matrices

The degradation of elastic matrix in the infrarenal aortic wall is a critical parameter underlying the formation and progression of abdominal aortic aneurysms. It is mediated by the chronic overexpression of matrix metalloprotease (MMP)-2 and MMP-9, leading to a progressive loss of elasticity and weakening of the aortic wall. Delivery of therapeutic agents to inhibit MMPs, while concurrently coaxing cell-based regenerative repair of the elastic matrix represents a potential strategy for slowing or arresting abdominal aortic aneurysm growth. Previous studies have demonstrated elastogenic induction of healthy and aneurysmal aortic smooth muscle cells and inhibition of MMPs, following exogenous delivery of elastogenic factors such as transforming growth factor (TGF)-β1, as well as MMP-inhibitors such as doxycycline (DOX) in two-dimensional culture. Based on these findings, and others that demonstrated elastogenic benefits of nanoparticulate delivery of these agents in two-dimensional culture, poly(lactide-co-glycolide) nanoparticles were developed for localized, controlled and sustained delivery of DOX and TGF-β1 to human aortic smooth muscle cells within a three-dimensional gels of type I collagen, which closely simulate the arterial tissue microenvironment. DOX and TGF-β1 released from these nanoparticles influenced elastogenic outcomes positively within the collagen constructs over 21 days of culture, which were comparable to that induced by exogenous supplementation of DOX and TGF-β1 within the culture medium. However, this was accomplished at doses ~20-fold lower than the exogenous dosages of the agents, illustrating that their localized, controlled and sustained delivery from nanoparticles embedded within a three-dimensional scaffold is an efficient strategy for directed elastogenesis. Copyright © 2014 John Wiley & Sons, Ltd.

A cytokine axis regulates elastin formation and degradation

Underlying the dynamic regulation of tropoelastin expression and elastin formation in development and disease are transcriptional and post-transcriptional mechanisms that have been the focus of much research. Of particular importance is the cytokine-governed elastin regulatory axis in which the pro-elastogenic activities of transforming growth factor β-1 (TGFβ1) and insulin-like growth factor-I (IGF-I) are opposed by anti-elastogenic activities of basic fibroblast growth factor (bFGF/FGF-2), heparin-binding epidermal growth factor-like growth factor (HB-EGF), EGF, PDGF-BB, TGFα, tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β and noncanonical TGFβ1 signaling. A key mechanistic feature of the regulatory axis is that cytokines influence elastin formation through effects on the cell cycle involving control of cyclin-cyclin dependent kinase complexes and activation of the Ras/MEK/ERK signaling pathway. In this article we provide an overview of the major cytokines/growth factors that modulate elastogenesis and describe the underlying molecular mechanisms for their action on elastin production.

Transforming growth factor beta 1 and hyaluronan oligomers synergistically enhance elastin matrix regeneration by vascular smooth muscle cells

Elastin is a vital structural and regulatory matrix protein that plays an important role in conferring elasticity to blood vessel wall. Previous tissue engineering approaches to regenerate elastin in situ or within tissue engineering constructs are curtailed by innate poor elastin synthesis potential by adult vascular smooth muscle cells (SMCs). Currently, we seek to develop cellular cues to enhance tropoelastin synthesis and improve elastin matrix yield, stability, and ultrastructure. Our earlier studies attest to the elastogenic utility of hyaluronan (HA)-based cellular cues, though their effects are fragment size dependent and dose dependent, with HA oligomers deemed most elastogenic. We presently show transforming growth factor beta 1 (TGF-beta1) and HA oligomers, when provided concurrently, to synergistically and dramatically improve elastin matrix regeneration by adult vascular SMCs. Together, these cues suppress SMC proliferation, enhance synthesis of tropoelastin (8-fold) and matrix elastin protein (5.5-fold), and also improve matrix elastin yield (45% of total elastin vs. 10% for nonadditive controls), possibly by more efficient recruitment of tropoelastin for crosslinking. The density of desmosine crosslinks within the elastin matrix was itself attenuated, although the cues together modestly increased production and activity of the elastin crosslinking enzyme, lysyl oxidase. TGF-beta1 and HA oligomers together induced much greater assembly of mature elastin fibers than they did separately, and did not induce matrix calcification. The present outcomes might be great utility to therapeutic regeneration of elastin matrix networks in situ within elastin-compromised vessels, and within tissue-engineered vascular graft replacements.

Regulation of elastin synthesis in pathological states

Elastin is rapidly deposited during late gestation in resilient tissues such as the arteries, lungs and skin owing to increased concentration of its mRNA. Pathological states can arise from congenital insufficiency or disorganization of elastin (cutis laxa). Other elastin deficiencies may be due to excess elastolysis or gene dosage effects. In the former, high turnover rates can be assessed by measurements of elastin degradation products in urine. Excess elastin accumulation by skin fibroblasts is characteristic of genetic diseases such as Buschke-Ollendorff syndrome, Hutchinson-Gilford progeria and keloid. Elastin expression is modulated by peptide growth factors, steroid hormones and phorbol esters, among which transforming growth factor beta (TGF-beta) is an especially potent up-regulator, acting largely through stabilization of mRNA. Recent evidence indicates cutis laxa fibroblasts that express little or no elastin have normal transcriptional activity but abnormal rates of elastin mRNA degradation. This defect is substantially reversed by TGF-beta through mRNA stabilization. Current studies explore the hypothesis that stability determinants lie within the 3' untranslated region of elastin mRNA. Post-transcriptional control of elastin expression appears to be a major regulatory mechanism.

LOXL as a target to increase the elastin content in adult skin: a dill extract induces the LOXL gene expression

The lysyl oxidases lysyl oxidase (LOX) and lysyl oxidase-like (LOXL) are responsible for elastin cross-linking. It was shown recently that LOXL is essential for the elastic fibres homeostasis and for their maintenance at adult age. We first determined whether or not elastin, LOX and LOXL are less expressed during adulthood. The LOX and LOXL mRNA level, quantified by real-time reverse transcriptase-polymerase chain reaction decreased in adult skin fibroblasts compared with fibroblasts from children. In contrast, the elastin mRNA level remains stable at all ages. The goal of this study was to induce elastogenesis at the adult age. Therefore, both enzymes, and in particular LOXL, of which expression is the most affected by age, could be targeted to induce elastogenesis in adult skin. We screened a library of about 1000 active ingredients to find activators capable to stimulate specifically the LOXL gene expression in adult dermal fibroblasts. The positive effect of selected active ingredients was confirmed on fibroblasts grown on monolayers and on dermal and skin equivalent cultures. One extract, obtained from dill (LYS'LASTINE V, Engelhard, Lyon, France), stimulates the LOXL gene expression in dermal equivalents (+64% increase in the LOXL mRNA level when compared with control). At the same time, the elastin detection is increased in dermal equivalents and under the dermal-epidermal junction of skin equivalents, without increase of the elastin mRNA. In conclusion, LOXL can be considered as a new target to reinduce elastogenesis. Its stimulation by a dill extract is correlated with increased elastin detection, suggesting an increase in elastogenesis efficiency.

Soluble transforming growth factor-beta type II receptor inhibits negative remodeling, fibroblast transdifferentiation, and intimal lesion formation but not endothelial growth

Using the rat balloon catheter denudation model, we examined the role of transforming growth factor-beta (TGF-beta) isoforms in vascular repair processes. By en face in situ hybridization, proliferating and quiescent smooth muscle cells in denuded vessels expressed high levels of mRNA for TGF-beta1, TGF-beta2, TGF-beta3, and lower levels of TGF-beta receptor II (TGF-betaRII) mRNA. Compared with normal endothelium, TGF-beta1 and TGF-beta2, as well as TGF-betaRII, mRNA were upregulated in endothelium at the wound edge. Injected recombinant soluble TGF-betaRII (TGF-betaR:Fc) localized preferentially to the adventitia and developing neointima in the injured carotid artery, causing a reduction in intimal lesion formation (up to 65%) and an increase in lumen area (up to 88%). The gain in lumen area was largely due to inhibition of negative remodeling, which coincided with reduced adventitial fibrosis and collagen deposition. Four days after injury, TGF-betaR:Fc treatment almost completely inhibited the induction of smooth muscle alpha-actin expression in adventitial cells. In the vessel wall, TGF-betaR:Fc caused a marked reduction in mRNA levels for collagens type I and III. TGF-betaR:Fc had no effect on endothelial proliferation as determined by reendothelialization of the denuded rat aorta. Together, these findings identify the TGF-beta isoforms as major factors mediating adventitial fibrosis and negative remodeling after vascular injury, a major cause of restenosis after angioplasty.

Expression, genomic structure and high resolution mapping to 19p13.2 of the human smooth muscle cell calponin gene

Smooth muscle cells (SMC) express a battery of cell-restricted differentiation genes, many of which are down-regulated during the course of vascular disease. Here, we present the mRNA expression, genomic structure and chromosomal mapping of the gene encoding human smooth muscle cell calponin (SMCC). Human SMCC transcripts are restricted to tissues and cells of SMC origin and, in the latter case, appear to be uniquely controlled in two distinct human SMC lines of uterine and aortic origin. Restriction mapping. Southern blot and PCR analysis of a 70-kb human bacterial artificial chromosome (BAC) revealed a genomic structure (seven exons spanning > 11 kb) very similar to that reported for the mouse SMCC gene. Using a variety of human-rodent somatic cell hybrid and radiation hybrid mapping panels, the human SMCC gene was mapped to a genomic interval of less than 1.32 Mb in 19p13.2. These results provide new information concerning the regulation of SMCC gene expression and demonstrate the utility of two human SMC lines for the further characterization of this gene's expression control. The identification of a BAC harboring the entire human SMCC locus represents an important reagent for future analysis of SMCC regulatory sequences. Finally, the localization of SMCC to a defined genomic interval will facilitate an analysis of its potential as a candidate gene for disease phenotypes mapping to 19p13.2.

Increase in elastin gene expression and protein synthesis in arterial smooth muscle cells derived from patients with Moyamoya disease

BACKGROUND AND PURPOSE

Moyamoya disease is a progressive cerebrovascular occlusive disease that is rare in all ages but frequently presents in children. The etiology of the disease is unknown. We examined elastin gene transcripts and elastin synthesis in cultured arterial smooth muscle cells (SMCs) derived from moyamoya patients and compared them with those in SMCs from age-matched control subjects.

METHODS

We used six cell strains from moyamoya patients and four from controls. The expression of elastin protein was observed by Western blot analysis and metabolic labeling with 3H-valine. Elastin gene transcripts were identified by Northern blot analysis.

RESULTS

Elastin mRNA and protein levels were elevated in all SMCs from moyamoya patients compared with control SMCs. Although transforming growth factor-beta 1 (TGF-beta 1), a potent enhancer of the expression of elastin in arterial SMCs, upregulated elastin mRNA and protein levels in SMCs from both moyamoya patients and control subjects, the maximum levels of elastin synthesis and elastin gene transcripts in response to exogenous TGF-beta 1 were significantly greater in moyamoya SMCs than control SMCs. In addition, quiescent moyamoya SMCs secreted significantly more TGF-beta 1 into the culture medium than quiescent control SMCs (P < .01).

CONCLUSIONS

Our findings suggest that moyamoya disease may result, at least in part, from an abnormal regulation of extracellular matrix metabolism that leads to increased steady state levels of elastin mRNA and elastin accumulation in the intimal thickening and that increased elastin accumulation is a stable marker of SMCs from patients with moyamoya disease.

Exogenous and endogenous transforming growth factors-beta influence elastin gene expression in cultured lung fibroblasts

Elastin, an important structural protein of the extracellular matrix, confers elastic properties on the pulmonary alveolar interstitium. In the alveolar wall, elastin is primarily produced postnatally by fibroblasts. The mechanisms that regulate lung fibroblast (LF) elastin gene expression have not been completely defined, although both transcriptional and posttranscriptional mechanisms appear to be involved. Transforming growth factors-beta (TGF-beta s) have been shown to increase elastin production by cultured neonatal rat LF. Analyses of elastin gene transcription and mRNA stability indicate that exogenous TGF-beta 1 increases the half-life of tropoelastin mRNA by 1.5-fold and does not alter elastin gene transcription. Interference with the functions of endogenous TGF-beta 1 in cultured LF, through the addition of neutralizing antibodies or antisense oligodeoxynucleotides, decreases tropoelastin and tropoelastin mRNA production by these cells. The content of total (latent plus active) TGF-beta s was approximately 4.5-fold greater in lungs obtained from rats on postnatal day 8 than in lungs obtained from adults. These findings indicate that endogenous TGF-beta s, in cultured LF, regulate elastin gene expression, most likely by a posttranscriptional mechanism. Since others have shown that elastin mRNA appears to have a longer half-life in neonatal than in adult rat lungs, we hypothesize that the higher content of TGF-beta s could contribute to the greater elastin mRNA stability in neonatal lungs.

Elastin in lung development

Elastin is a critical component of the lung interstitium, providing the property of recoil to the vascular, conducting airway, and terminal airspace compartments of the lung. Elastic fibers, consisting of soluble tropoelastin monomers cross-linked on a preexisting scaffold of microfibrils, are produced primarily during late fetal and neonatal stages of development. The factors and molecular mechanisms regulating the cell type-specific and tightly temporally regulated expression of tropoelastin are currently under investigation. The onset and inductive phase of tropoelastin expression are characterized by increased transcription of the tropoelastin gene. Glucocorticoids accelerate this induction in fetal rats during the canalicular stage of lung development. Many additional factors regulate tropoelastin expression in cultured lung fibroblasts and vascular smooth muscle cells, but the in vivo roles of such mediators are still under investigation. Cell-cell interactions may also promote elastogenesis during lung development, as localization of tropoelastin mRNA in pseudo-glandular and canalicular lungs demonstrates a close spatial relationship between epithelium and adjacent elastogenic mesenchyme. Elastin metabolism is altered in several experimental models of bronchopulmonary dysplasia, characterized by abnormal lung morphological development, suggesting that normal elastin production and deposition is necessary for proper development of alveoli. Studies employing reverse genetics may prove useful in further defining the role of elastin in lung development.

Early development of intimal thickening in superficial temporal arteries in patients with moyamoya disease

BACKGROUND AND PURPOSE

Moyamoya disease is a progressive cerebrovascular occlusive disease that occurs in children. The etiology is unknown. We examined the superficial temporal arteries from patients with moyamoya disease, particularly children, to determine whether the extracranial arteries as well as the intracranial arteries are involved in this disease.

METHODS

Small branches of the superficial temporal arteries were obtained from 22 patients with moyamoya disease during indirect arterial bypass surgery. Histological examinations were performed, and the findings were compared with those of arteries from 12 control patients.

RESULTS

Intimal thickening was observed in 9 of 17 patients with moyamoya disease younger than 20 years but in none of 7 control patients under the age of 20 years (P < .02, Fisher's exact test). Intimal thickening appeared from age 20 years in control patients. The arteries of moyamoya patients showed fibrocellular intimal thickening with a paucity of lipid. The arteries from moyamoya patients contained strongly stained multilayered elastic fibers in the thickened intima, while those from control patients showed only weakly stained elastic fibers in the intima.

CONCLUSIONS

Our findings suggest that moyamoya disease is a systemic vascular disease. The results indicate systemic etiologic factors that may promote the early development of intimal thickening in moyamoya disease.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Decreased elastin synthesis in normal development and in long-term aortic organ and cell cultures is related to rapid and selective destabilization of mRNA for elastin

We have previously shown that aortic organ cultures from 1- to 3-day-old chickens initially mimic the high levels of elastin production seen in vivo. However, more prolonged incubation of these tissues results in decreased synthesis of elastin. In the present study, we demonstrate that decreased production of elastin in these aortic organ cultures is selective for elastin compared with collagen and is correlated with decreased steady state levels of mRNA for elastin. These decreases in steady state levels of elastin mRNA are due at least in part to a rapid and selective destabilization of mRNA for elastin, the half-life of which falls from approximately 25 hours in fresh aortic tissues to approximately 15 hours after incubation for only 8 hours. Destabilization of elastin mRNA can be prevented by incubation in the presence of blockers of DNA transcription (5,6-dichlorobenzimidazole riboside and actinomycin D) and mRNA translation (cycloheximide). Furthermore, the half-life of aortic elastin mRNA decreases from approximately 25 hours in the 1-day-old chicken to approximately 7 hours in the 8-week-old chicken, demonstrating that destabilization of mRNA is an important contributing factor in the decline in production of aortic elastin taking place during normal postnatal growth.

Identification of a recognition element for CAAT-enhancer binding proteins (C/EBPs) in the elastin promoter

DNase I footprinting experiments with a DNA fragment of the human elastin promoter have revealed a protected segment comprised between -156 and -172 nucleotides from the translation start site. Various types of gel retardation experiments indicate that the protected element binds different members of the C/EBP family of transcription factors. CAT (chloramphenicol acetyltransferase) fusion constructs carrying the wild type or a mutated promoter sequence were transfected into NIH3T3 and chick embryo aorta cells. The mutation significantly lowered CAT expression in NIH3T3 cells, but was ineffective in aorta cells. Cotransfection of the CAT promoter constructs with eucaryotic vectors expressing C/EBPs, did not affect the production of the reporter gene in NIH3T3 cells; on the contrary a several-fold increase of CAT activity was observed in aortic cells. This increase, however, was identical for the wild type and the mutated constructs. Taken together the data indicate that the elastin promoter contains a recognition site for proteins of the C/EBP family and that the function of this cis-acting element on basal elastin transcription varies with the cell type.

Transcriptional activation of the alpha 1(VI) collagen gene during myoblast differentiation is mediated by multiple GA boxes

During differentiation of ClC12 myoblasts in vitro, expression of alpha 1(VI) collagen mRNA was transiently stimulated severalfold. Promoter assays on cells transfected with chloramphenicol acetyltransferase (CAT) chimeric constructs have identified a region of the alpha 1(VI) a collagen promoter that increases CAT activity about 8-fold during differentiation. The region, which overlaps with transcription initiation sites, was shown to contain three protected segments (A, B, and C) in DNase I footprinting assays. The contact points between nuclear factors and the protected segments were determined by methylation interference assay and included the sequence GGGAGGG (GA box) in all segments. Experiments in which CAT constructs were cotransfected with double-stranded oligonucleotides containing the GA box suggested that this motif was necessary for induction. Transfections with deletion constructs of the natural promoter and with minipromoters made of three copies of A, B, or C showed that the elements have inducing activity and that elements C and, to a lower extent, B are stimulatory for basal transcription, whereas the contribution of A in this process is limited. Electrophoretic mobility shift assays with nuclear extracts from C2C12 cells indicated that the three GA box-containing elements bound several transcription factors, including Sp1. Comparison of the properties of the bands shifted under different experimental conditions (presence of 10 mM EDTA, heating of the nuclear extracts, addition of different concentrations of competitor oligonucleotides) established that A, B, and C probes form nine, eight and five main retarded complexes, respectively, and indicated that nuclear factors binding to C and B are subsets of proteins binding to A. UV cross-linking assays identified several peptides (seven with probe A, six with B, And five with C) in the range of 150-32 kDa. Comparison of the gel retardation pattern obtained with nuclear extracts from proliferating and differentiating cells revealed a particular increased intensity of two retarded bands. The data establish that multiple GA boxes mediate induction of the alpha 1(VI) collagen promoter during myoblast differentiation and suggest the attractive hypothesis that the effect may be related to variations of expression of transcription factors binding to these motifs.

Transforming growth factor-beta reverses a posttranscriptional defect in elastin synthesis in a cutis laxa skin fibroblast strain

Skin fibroblasts from two cases of autosomal recessive cutis laxa (CL), having insignificant elastin production and mRNA levels, were challenged with transforming growth factor beta-1 (TGF-beta 1). Elastin production was brought from undetectable values to amounts typical of normal human skin fibroblasts in a dose-dependent fashion. Basic fibroblast growth factor (100 ng/ml) alone or in combination with TGF-beta 1 reduced elastin production and mRNA expression in CL skin fibroblasts more extensively than in normal cells. In situ hybridization showed that these effects were at the transcript level. One of the CL strains was examined in detail. Transcription rates for elastin were similar in normal and CL and unchanged by TGF-beta 1 or TGF-beta 2 (10 ng/ml), while in CL elastin mRNA half-life was increased > 10-fold by TGF-beta 2 and reduced 6-fold after TGF-beta 2 withdrawal, as compared with a control strain. Cycloheximide partially reversed elastin mRNA instability. These data are consistent with a defect in elastin mRNA stability that requires synthesis of labile factors or intact translational machinery, resulting in an extremely low steady state level of mRNA present in this strain of CL. Furthermore, TGF-beta can relieve elastin mRNA instability in at least one CL strain and elastin production defects in both CL strains.

Evidence for a cell-specific negative regulatory element in the first intron of the gene for bovine elastin

A cell-specific negative regulatory element has been identified in the first intron of the gene for elastin in a region between 442 and 464 bp from the translational start site. This regulatory element functions both when it is located 5' of the promoter and 3' of the chloramphenicol acetyltransferase (CAT) gene. The inhibition is observed both with the homologous elastin promoter and the heterologous SV1 promoter in transient expression experiments using rat aortic smooth-muscle cells. No inhibition was observed with NIH 3T3, Hep G2 and little, if any, with HeLa cells. Cell specificity was further confirmed by DNA mobility shift assays and the position of the negative regulatory element was localized with the use of synthetic duplex oligomers. It is proposed that this negative element plays a significant role in the modulation of the expression of the gene for elastin in the smooth-muscle cells of the aorta during development.