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

Emerging mechanisms of elastin transcriptional regulation

Elastin provides recoil to tissues that stretch such as the lung, blood vessels, and skin. It is deposited in a brief window starting in the prenatal period and extending to adolescence in vertebrates, and then slowly turns over. Elastin insufficiency is seen in conditions such as Williams-Beuren syndrome and elastin-related supravalvar aortic stenosis, which are associated with a range of vascular and connective tissue manifestations. Regulation of the elastin (ELN) gene occurs at multiple levels including promoter activation/inhibition, mRNA stability, interaction with microRNAs, and alternative splicing. However, these mechanisms are incompletely understood. Better understanding of the processes controlling ELN gene expression may improve medicine's ability to intervene in these rare conditions, as well as to replace age-associated losses by re-initiating elastin production. This review describes what is known about the ELN gene promoter structure, transcriptional regulation by cytokines and transcription factors, and posttranscriptional regulation via mRNA stability and micro-RNA and highlights new approaches that may influence regenerative medicine.

Morphometric analysis of thoracic aorta in Slc39a13/Zip13-KO mice

Ehlers-Danlos syndrome (EDS) is a collection of inheritable diseases involving the musculoskeletal, integumentary and visual systems. Spondylodysplastic EDS-ZIP13 (spEDS-ZIP13: OMIM 612350) was recently defined as a new form of EDS. Although vasculitis has been found in many spEDS-ZIP13 patients, vascular pathology has not been included as a pathognomonic lesion of this type of EDS. We investigate the morphometry of the thoracic aorta in wild-type and Zip13-knockout (Zip13-KO) mice. Our assessment found abnormalities in the number and morphology of elastic and cellular components in the aortic wall, especially the tunica media, of Zip13-KO mice, indicating aortic fragility. Accordingly, our major findings (vascular smooth muscle cells with small nuclei, small percentage of elastic membrane area per tunica media, many large elastic flaps) should be considered vulnerable characteristics indicating fragility of the aorta in patients with spEDS-ZIP13.

Differential and synergistic effects of mechanical stimulation and growth factor presentation on vascular wall function

We investigated the hypothesis that immobilizing TGF-β1 within fibrin hydrogels may act in synergy with cyclic mechanical stimulation to enhance the properties of vascular grafts. To this end, we engineered a fusion TGF-β1 protein that can covalently anchor to fibrin during polymerization upon the action of factor XIII. We also developed a 24-well based bioreactor in which vascular constructs can be mechanically stimulated by distending the silastic mandrel in the middle of each well. TGF-β1 was either conjugated to fibrin or supplied in the culture medium and the fibrin-based constructs were cultured statically for a week followed by cyclic distention for another week. The tissues were examined for myogenic differentiation, vascular reactivity, mechanical properties and ECM content. Our results showed that some aspects of vascular function were differentially affected by growth factor presentation vs. pulsatile force application, while others were synergistically enhanced by both. Overall, this two-prong biomimetic approach improved ECM secretion, vascular reactivity and mechanical properties of vascular constructs. These findings may be applied in other tissue engineering applications such as cartilage, tendon or cardiac regeneration where growth factors TGF-β1 and mechano-stimulation play critical roles.

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.

Elastin in large artery stiffness and hypertension

Large artery stiffness, as measured by pulse wave velocity, is correlated with high blood pressure and may be a causative factor in essential hypertension. The extracellular matrix components, specifically the mix of elastin and collagen in the vessel wall, determine the passive mechanical properties of the large arteries. Elastin is organized into elastic fibers in the wall during arterial development in a complex process that requires spatial and temporal coordination of numerous proteins. The elastic fibers last the lifetime of the organism but are subject to proteolytic degradation and chemical alterations that change their mechanical properties. This review discusses how alterations in the amount, assembly, organization, or chemical properties of the elastic fibers affect arterial stiffness and blood pressure. Strategies for encouraging or reversing alterations to the elastic fibers are addressed. Methods for determining the efficacy of these strategies, by measuring elastin amounts and arterial stiffness, are summarized. Therapies that have a direct effect on arterial stiffness through alterations to the elastic fibers in the wall may be an effective treatment for essential hypertension.

TGF-beta 1 is a potential regulator of vaginal tropoelastin production

INTRODUCTION AND HYPOTHESIS

Our aims were to correlate transforming growth factor (TGF)-β1 and elastin mRNA expression in the vagina of women and to measure the effects of TGF-β1 on vaginal smooth muscle cell (SMC) proliferation and tropoelastin production.

METHODS

Vaginal walls were sampled in women (n = 20). TGF-β1 and elastin mRNA expression was assessed by RT-PCR. SMC cultures were performed from vaginal wall biopsies. SMC were incubated with TGF-β1, and cell proliferation was assessed by MTT-assay. Tropoelastin production was measured by the Fastin Elastin Assay.

RESULTS

There was a significant positive correlation between TGF-β1 and elastin mRNA (r = 0.784, P < 0.01). SMC proliferation was significantly increased by 10 ng/mL TGF-β1 [relative cell number, mean ± SD, 198% ± 32% of control (P = 0.01)]. Tropoelastin production was significantly increased by TGF-β1 [mean ± SD, 645% ± 180% of control (P = 0.01)].

CONCLUSIONS

There is a positive correlation between TGF-β1 and elastin mRNA expression in the vaginal wall. In vitro, TGF-β1 increases vaginal tropoelastin production in vaginal SMC.

Contractile smooth muscle cells derived from hair-follicle stem cells

AIMS

We hypothesized that hair-follicle stem cells can differentiate toward smooth contractile muscle cells, providing an autologous cell source for cardiovascular tissue regeneration.

METHODS AND RESULTS

Smooth muscle progenitor cells (SMPCs) were obtained from ovine hair follicles using a tissue-specific promoter and fluorescence-activated cell sorting. Hair-follicle smooth muscle progenitor cells (HF-SMPCs) expressed several markers of vascular smooth muscle including alpha-actin, calponin, myosin heavy chain (MHC), caldesmon, smoothelin, and SM22. HF-SMPCs were highly proliferative and showed high clonogenic potential without any signs of chromosomal abnormalities as evidenced by karyotype analysis. HF-SMPCs compacted fibrin hydrogels to a similar extent as vascular smooth muscle cells from ovine umbilical veins (V-SMCs), indicating the development of the force-generating machinery. In addition, cylindrical tissue equivalents prepared with HF-SMPCs displayed significant contractility in response to vasoactive agonists including KCl and the thromboxane A2 mimetic U46619, suggesting that these cells had developed receptor and non-receptor-mediated pathways of contractility. Finally, transforming growth factor-beta1 promoted differentiation of HF-SMPCs toward a mature SMC phenotype as suggested by increased expression of MHC and enhanced matrix compaction.

CONCLUSION

Our results suggest that hair follicles may be an easily accessible, autologous, and rich source of functional SMPC for cardiovascular tissue engineering and regenerative medicine.

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.

Positive transcriptional regulatory element located within exon 1 of elastin gene

Elastin gene transcription is cell type specific and developmentally regulated, but the promoter often exhibits relatively weak activity in transient transfections of cells that express elastin at high levels. To search for positive-acting regulatory sequences, we isolated genomic clones spanning the mouse elastin gene and extensive 5'- and 3'-flanking regions. Restriction fragments of potential regulatory regions were ligated 5' or 3' relative to the active promoter to test for enhancer activity in transient transfections of fetal rat lung fibroblasts, which express elastin at high levels, and distal lung epithelial cells, which do not express detectable elastin. Fragments of intron 1 did not exhibit significant enhancer activity. Inclusion of the 84-bp exon 1 and adjacent 5'-untranslated region increased activity of the elastin promoter approximately sixfold compared with parental constructs. Transfections with constructs of varying promoter length showed that as little as 40 bp of the 5' end of exon 1 confers enhanced activity in elastin-expressing rat lung fibroblasts, but these constructs had variable activity in lung epithelial cell lines. This region, localized between the transcription start site and extending into exon 1, binds Sp1 in nuclear extracts from elastin-expressing cells. These studies indicate a role for the 5' end of the first exon of the elastin gene in regulating strong transcriptional activity in elastogenic cells.

Fluctuations of intracellular iron modulate elastin production

Production of insoluble elastin, the major component of elastic fibers, can be modulated by numerous intrinsic and exogenous factors. Because patients with hemolytic disorders characterized with fluctuations in iron concentration demonstrate defective elastic fibers, we speculated that iron might also modulate elastogenesis. In the present report we demonstrate that treatment of cultured human skin fibroblasts with low concentration of iron 2-20 microm (ferric ammonium citrate) induced a significant increase in the synthesis of tropoelastin and deposition of insoluble elastin. Northern blot and real-time reverse transcription-PCR analysis revealed that treatment with 20 microm iron led to an increase of approximately 3-fold in elastin mRNA levels. Because treatment with an intracellular iron chelator, desferrioxamine, caused a significant decrease in elastin mRNA level and consequent inhibition of elastin deposition, we conclude that iron facilitates elastin gene expression. Our experimental evidence also demonstrates the existence of an opposite effect, in which higher, but not cytotoxic concentrations of iron (100-400 microm) induced the production of intracellular reactive oxygen species that coincided with a significant decrease in elastin message stability and the disappearance of iron-dependent stimulatory effect on elastogenesis. This stimulatory elastogenic effect was reversed, however, in cultures simultaneously treated with high iron concentration (200 microm) and the intracellular hydroxyl radical scavenger, dimethylthiourea. Thus, presented data, for the first time, demonstrate the existence of two opposite iron-dependent mechanisms that may affect the steady state of elastin message. We speculate that extreme fluctuations in intracellular iron levels result in impaired elastic fiber production as observed in hemolytic diseases.

Multi-species sequence comparison reveals dynamic evolution of the elastin gene that has involved purifying selection and lineage-specific insertions/deletions

Background

The elastin gene (ELN) is implicated as a factor in both supravalvular aortic stenosis (SVAS) and Williams Beuren Syndrome (WBS), two diseases involving pronounced complications in mental or physical development. Although the complete spectrum of functional roles of the processed gene product remains to be established, these roles are inferred to be analogous in human and mouse. This view is supported by genomic sequence comparison, in which there are no large-scale differences in the ~1.8 Mb sequence block encompassing the common region deleted in WBS, with the exception of an overall reversed physical orientation between human and mouse.

Results

Conserved synteny around ELN does not translate to a high level of conservation in the gene itself. In fact, ELN orthologs in mammals show more sequence divergence than expected for a gene with a critical role in development. The pattern of divergence is non-conventional due to an unusually high ratio of gaps to substitutions. Specifically, multi-sequence alignments of eight mammalian sequences reveal numerous non-aligning regions caused by species-specific insertions and deletions, in spite of the fact that the vast majority of aligning sites appear to be conserved and undergoing purifying selection.

Conclusions

The pattern of lineage-specific, in-frame insertions/deletions in the coding exons of ELN orthologous genes is unusual and has led to unique features of the gene in each lineage. These differences may indicate that the gene has a slightly different functional mechanism in mammalian lineages, or that the corresponding regions are functionally inert. Identified regions that undergo purifying selection reflect a functional importance associated with evolutionary pressure to retain those features.

Connection between elastin haploinsufficiency and increased cell proliferation in patients with supravalvular aortic stenosis and Williams-Beuren syndrome

To elucidate the pathomechanism leading to obstructive vascular disease in patients with elastin deficiency, we compared both elastogenesis and proliferation rate of cultured aortic smooth-muscle cells (SMCs) and skin fibroblasts from five healthy control subjects, four patients with isolated supravalvular aortic stenosis (SVAS), and five patients with Williams-Beuren syndrome (WBS). Mutations were determined in each patient with SVAS and in each patient with WBS. Three mutations found in patients with SVAS were shown to result in null alleles. RNA blot hybridization, immunostaining, and metabolic labeling experiments demonstrated that SVAS cells and WBS cells have reduced elastin mRNA levels and that they consequently deposit low amounts of insoluble elastin. Although SVAS cells laid down approximately 50% of the elastin made by normal cells, WBS cells deposited only 15% of the elastin made by normal cells. The observed difference in elastin-gene expression was not caused by a difference in the stability of elastin mRNA in SVAS cells compared with WBS cells, but it did indicate that gene-interaction effects may contribute to the complex phenotype observed in patients with WBS. Abnormally low levels of elastin deposition in SVAS cells and in WBS cells were found to coincide with an increase in proliferation rate, which could be reversed by addition of exogenous insoluble elastin. We conclude that insoluble elastin is an important regulator of cellular proliferation. Thus, the reduced net deposition of insoluble elastin in arterial walls of patients with either SVAS or WBS leads to the increased proliferation of arterial SMCs. This results in the formation of multilayer thickening of the tunica media of large arteries and, consequently, in the development of hyperplastic intimal lesions leading to segmental arterial occlusion.

Transforming growth factor-beta stabilizes elastin mRNA by a pathway requiring active Smads, protein kinase C-delta, and p38

Transforming growth factors (TGFs)-beta are multipotent in their biologic activity, regulating cell growth and differentiation as well as extracellular matrix deposition and degradation. Most of these activities involve modulation of gene transcription, but TGF-beta1 has been shown previously to substantially increase the expression of elastin by stabilization of tropoelastin mRNA through a signaling pathway that likely involves a phosphatidylcholine-specific phospholipase C, a protein kinase C, prenylated and acylated protein(s), and one or more tyrosine kinases. However, there is a 4- to 6-h lag period after the addition of TGF-beta1 before significant stimulation of elastin expression is observed and the question of whether the Smads are involved has not been addressed. In the present work, using cultured human fetal lung fibroblasts, we show through the use of specific inhibitors and transfection of a Smad 7 construct that in addition to de novo protein synthesis and active Smads, the extended activity of protein kinase C (PKC)-delta and the stress-activated protein kinase, p38, is required for TGF-beta1 to achieve elastin mRNA stabilization.

Umbilical vein blood flow in fetuses with normal and lean umbilical cord

OBJECTIVE

To evaluate whether umbilical vascular coiling is correlated with the umbilical vein blood flow profile and to investigate if this is different between fetuses with a lean and those with a normal umbilical cord.

METHODS

Consecutive women with a singleton gestation who delivered at term and who underwent an ultrasound examination within 24 h from delivery were studied. Umbilical cord and vessel areas were calculated. Umbilical vein blood flow parameters were obtained by digital color Doppler velocity profile integration. After delivery, the umbilical coiling index was calculated.

RESULTS

One hundred and sixteen women were studied. Twelve (10.3%) had a lean umbilical cord (area < 10th centile). A significant correlation was found between the umbilical coiling index and the umbilical vein blood flow (r = 0.67, P < 0.001). A significant difference between fetuses with and without a lean cord was found in terms of: umbilical coiling index (0.18 +/- 0.08 vs. 0.29 +/- 0.09, P < 0.005), cord area (87.6 +/- 5.1 mm2 vs. 200.6 +/- 34.6 mm2, P < 0.001), Wharton's jelly amount (25.7 +/- 10.3 mm2 vs. 122.1 +/- 33.4 mm2, P < 0.001), umbilical vein blood flow (93.7 +/- 17.8 ml/kg per min vs. 126.0 +/- 23.4 ml/kg per min, P < 0.001), and umbilical vein blood flow mean velocity (6.6 +/- 2.7 cm/s vs. 9.0 +/- 3.6 cm/s, P < 0.05). The proportion of fetuses with an umbilical vein blood flow < 80 ml/kg per min was higher when the cord was lean than when it was normal (25% vs. 1.9%, P < 0.01).

CONCLUSIONS

Lean umbilical cords differ from normal cords not only from a structural point of view but also in the umbilical vein blood flow characteristics. This could explain the increased incidence of intrapartum complications and fetal growth restriction among fetuses with a lean and/or hypocoiled cord.

Elastin gene expression is upregulated during pulmonary fibrosis

Elastin is a chief component of lung interstitium, and it is central to lung morphology and function. Efforts to understand the pathogenesis of pulmonary fibrosis have focused primarily upon collagen turnover in the lung; few studies have focused on elastin. In this study, we examined steady-state elastin mRNA levels after lung injury in the mouse induced by (1) butylated hydroxytoluene (BHT) which causes acute lung injury with recovery, (2) BHT + 70% O2 (fibrosis), or (3) 70% O2. Total lung elastin mRNA increased 70-80-fold on d10-14 after BHT/O2, but was unchanged after BHT or O2 alone. In situ hybridization studies localized elastin mRNA to cells in the muscularis of conducting airways and to scattered interstitial cells in fibrotic foci. Elastic fiber morphology was markedly distorted after BHT/O2. Thus, marked upregulation of elastin gene expression is correlated with the histopathology of fibrotic lung disease.

TGF-beta1 stimulation of fibronectin transcription in cultured human lung fibroblasts requires active geranylgeranyl transferase I, phosphatidylcholine-specific phospholipase C, protein kinase C-delta, and p38, but not erk1/erk2

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a variety of cell types, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. In the present work, we demonstrate that TGF-beta1 produces a fourfold increase in transcription of the fibronectin gene in cultured human fetal lung fibroblasts with only a small increase in mRNA stability resulting in a significant increase in fibronectin mRNA steady state level. A corresponding increase in production of fibronectin protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that geranylgeranylated, but not farnesylated or acylated protein(s), protein kinase C-delta, phosphatidylcholine-specific phospholipse C, tyrosine kinase activity, and stress-activated protein kinase p38 are required for this TGF-beta1 effect. Trimeric G proteins and mitogen-activated protein kinases erk1 and erk2 do not appear to be involved. While these results emphasize the complexities involved in the control of extracellular matrix synthesis by TGF-beta, they also identify reaction sites that may be amenable to pharmacologic modulation. Such modulation could be of great advantage in the treatment of a wide variety of undesirable fibrotic reactions.

An open reading frame element mediates posttranscriptional regulation of tropoelastin and responsiveness to transforming growth factor beta1

Elastin, an extracellular component of arteries, lung, and skin, is produced during fetal and neonatal growth. We reported previously that the cessation of elastin production is controlled by a posttranscriptional mechanism. Although tropoelastin pre-mRNA is transcribed at the same rate in neonates and adults, marked instability of the fully processed transcript bars protein production in mature tissue. Using RNase protection, we identified a 10-nucleotide sequence in tropoelastin mRNA near the 5' end of the sequences coded by exon 30 that interacts specifically with a developmentally regulated cytosolic 50-kDa protein. Binding activity increased as tropoelastin expression dropped, being low in neonatal fibroblasts and high in adult cells, and treatment with transforming growth factor beta1 (TGF-beta1), which stimulates tropoelastin expression by stabilizing its mRNA, reduced mRNA-binding activity. No other region of tropoelastin mRNA interacted with cellular proteins, and no binding activity was detected in nuclear extracts. The ability of the exon-30 element to control mRNA decay and responsiveness to TGF-beta1 was assessed by three distinct functional assays: (i) insertion of exon 30 into a heterologous gene conferred increased reporter activity after exposure to TGF-beta1; (ii) addition of excess exon 30 RNA slowed tropoelastin mRNA decay in an in vitro polysome degradation assay; and (iii) a mutant tropoelastin cDNA lacking exon 30, compared to wild-type cDNA, produced a stable transcript whose levels were not affected by TGF-beta1. These findings demonstrate that posttranscriptional regulation of elastin production in mature tissue is conferred by a specific element within the open reading frame of tropoelastin mRNA.

Cutis laxa arising from frameshift mutations in exon 30 of the elastin gene (ELN)

Congenital cutis laxa, a rare syndrome with marked skin laxity and pulmonary and cardiovascular compromise, is due to defective elastic fiber formation. In several cases, skin fibroblast tropoelastin production is markedly reduced yet reversed in vitro by transforming growth factor-beta treatment. We previously showed that this reversal was due to elastin mRNA stabilization in one cell strain, and here this behavior was confirmed in skin fibroblasts from two generations of a second family. cDNA sequencing and heteroduplex analysis of elastin gene transcripts from three fibroblast strains in two kindreds now identify two frameshift mutations (2012DeltaG and 2039DeltaC) in elastin gene exon 30, thus leading to missense C termini. No other mutations were present in the ELN cDNA sequences of all three affected individuals. Transcripts from both alleles in each kindred were unstable and responsive to transforming growth factor-beta. Exons 22, 23, 26A, and 32 were always absent. Since exon 30 underwent alternative splicing in fibroblasts, we speculate that a differential splicing pattern could conceivably lead to phenotypic rescue. These two dominant-acting, apparently de novo mutations in the elastin gene appear to be responsible for qualitative and quantitative defects in elastin, resulting in the cutis laxa phenotype.

Signaling pathway by which TGF-beta1 increases expression of latent TGF-beta binding protein-2 at the transcriptional level

The cytokine transforming growth factor-beta has multiple effects on a wide variety of cell types. These effects include modulation of growth and regulation of gene transcription. In the present work, we demonstrate that TGF-beta1 increases transcription of the latent transforming growth factor-beta binding protein-2 ( LTBP-2) gene in cultured human fetal lung fibroblasts leading to a significant increase in LTBP-2 mRNA steady state level. The stability of LTBP-2 mRNA was not appreciably altered. A corresponding increase in production of LTBP-2 protein accompanied the increase in mRNA. Through the use of specific inhibitors, we demonstrate that a member of the Ras super family and a protein kinase C, probably of the atypical (non-diacylglycerol, non-Ca++ dependent) class are likely to be components in the signaling pathway. However, phospholipases, G proteins and extracellular-signal regulated kinases do not appear to be involved. These results combined with previous findings on elastin regulation by TGF-beta1 (Kucich et al. (1997). Am. J. Respir. Cell Mol. Biol., 17: 10-16) demonstrate that TGF-beta1 can coordinately increase the steady state levels of mRNAs encoding components of the elastic fiber, but through diverse mechanisms. In contrast to LTBP-2, increased elastin expression is achieved by message stabilization. Furthermore, the TGF-beta1 signaling pathways differ and while the pathway leading to increased LTBP-2 transcription shares components with those modulating transcription of other genes, it is unlikely to be precisely congruent with any other previously described one.

Requirement for geranylgeranyl transferase I and acyl transferase in the TGF-beta-stimulated pathway leading to elastin mRNA stabilization

The TGF-betas are multipotent in their biological activity, modulating cell growth and differentiation as well as extracellular matrix deposition and degradation. Most of these activities involve modulation of gene transcription. However, TGF-beta1 has been shown previously to substantially increase the expression of elastin by stabilization of tropoelastin mRNA through a signaling pathway which involves a phosphatidylcholine-specific phospholipase and a protein kinase C. The present results, through the use of specific inhibitors of geranylgeranyl transferase I, farnesyl transferase, and acyl transferase, demonstrate that geranylgeranylated and acylated, but not farnesyslated protein(s) is required for this TGF-beta1 effect. In addition, the general tyrosine kinase inhibitor genistein completely blocked this TGF-beta1 effect. The results suggest that the TGF-beta1 signaling pathway requires not only receptor ser/thr kinase activity, but also tyrosine kinase and small GTPase activities.

Stabilization of elastin mRNA by TGF-beta: initial characterization of signaling pathway

The cytokine transforming growth factor-beta (TGF-beta) has multiple effects on a wide variety of cell types. These effects include modulation of growth and regulation of gene transcription. In a few instances, TGF-beta has also been shown to regulate gene expression posttranscriptionally by altering message stability, but the pathway by which this activity is executed remains largely unknown. In the present work, we demonstrate that TGF-beta 1 has no effect on transcription of the elastin gene in cultured human fetal lung fibroblasts, but does stabilize elastin messenger RNA (mRNA), leading to a dramatic increase in the steady-state level of elastin mRNA. A corresponding increase in production of tropoelastin accompanies the increase in elastin mRNA. Through the use of specific inhibitors, we demonstrate that phosphatidylcholine (PC)-specific phospholipase C (PLC) and protein kinase C (PKC) are involved in mediating the elastin message stabilization. In contrast, G proteins and extracellularly regulated kinases do not appear to be involved. These results suggest that although the TGF-beta signaling pathway leading to message stabilization shares components with that modulating transcription, the message-stabilization pathway also contains diverse other elements.

Transcriptional and post-transcriptional control of lysyl oxidase expression in vascular smooth muscle cells: effects of TGF-beta 1 and serum deprivation

Transforming growth factor-beta 1 (TGF-beta 1) markedly reduced cell proliferation and elevated steady state lysyl oxidase (LO) mRNA 3-fold in neonatal rat aorta smooth muscle cells cultured in medium containing 10% fetal bovine serum. The increase in LO mRNA was prevented by the presence of cycloheximide, indicative of controlling events at the level of protein synthesis. The basal level of mRNA in cells proliferating in 10% fetal bovine serum in the absence of TGF-beta 1 was enhanced 7-fold upon decreasing growth by shifting to medium containing 0.5% serum. Changes in LO activity paralleled those in LO mRNA. Nuclear run-on assays revealed that the stimulation of expression in 0.5% serum involved increased gene transcription whereas that caused by TGF-beta 1 was mostly post-transcriptional in origin. LO mRNA was quite labile (t1/2 approximately 3 h) in 10% serum but was markedly stabilized (t1/2 > 12 h) by the presence of TGF-beta 1 in the 10% serum medium. LO mRNA was also considerably more stable under retarded growth conditions (0.5% serum) in the absence of TGF-beta 1. LO promoter activity in luciferase reporter constructs transfected into these cells was low and not significantly affected by the addition of TGF-beta 1 to the 10% serum medium but was markedly elevated by shifting from 10 to 0.5% serum in the absence of TGF-beta 1. Thus, LO expression is inversely correlated with cell proliferation, and is subject to control at transcriptional and post-transcriptional levels. TGF-beta 1 enhances LO expression in these cells by dramatically stabilizing LO mRNA.

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.