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(Hypertension. 2004;43:726.)
© 2004 American Heart Association, Inc.
Rapid Communication |
From the Department of Cardiovascular Physiology, University of Göttingen, Germany.
Correspondence to Dr Markus Hecker, Department of Cardiovascular Physiology, University of Göttingen, Humboldtallee 23, 37073 Göttingen, Germany. E-mail hecker{at}veg-physiol.med.uni-goettingen.de
| Abstract |
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Key Words: vascular smooth muscle cells gene expression remodeling
| Introduction |
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Although the aforementioned interrelation is well established in the clinic, little is known about SMC mechanotransduction, namely the activation of transcription factors that specifically link deformation of the cell to the increase in expression of mechanosensitive genes. This knowledge, however, could pave the way to developing drugs that specifically interfere with pressure-induced arterial remodeling. Therefore, we have analyzed the role of a putative mechanotransducer in rat aortic cultured SMC, the cytoskeletal protein zyxin.
| Methods |
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98% of cells revealing the typical pattern of
-SMC actin staining (according to immunofluorescence analysis) were used. They were cultured on BioFlex collagen I elastomers and exposed to cyclic stretch (15% elongation at 0.5 Hz) in a Flexercell FX-3000 strain unit as previously described.4 Alternatively, cells were treated with the cytokine combination interleukin-1ß (IL-1ß) (60 U/mL) plus tumor necrosis factor-
(TNF-
) (1000 U/mL) or exposed to osmotic stress (addition of 0.1 mL of 900 mmol/L sodium chloride per mL medium, resulting in a final osmolarity of 454 mosmol/L) for the times indicated.
Antisense Oligonucleotides
The antisense oligonucleotide (ODN) directed against zyxin had the sequence 5'-GGGGCCGCCATGGCC-3' (position -4 to 11 of the coding sequence; GenBank accession number NM_146002). As a control, a scrambled ODN (5'-CGCGCGGCA-CTCGGG-3') with no homology to zyxin was also designed. The terminal 4 bases of either ODN were linked by phosphorothioate esters for added stability. The ODN were added to the cultured SMCs at a concentration of 5 µmol/L for 48 hours without a transfection reagent before commencing of the experiments. This delivery method previously has been successfully used by us in rat aortic SMCs.5
Reverse-Transcription Polymerase Chain Reaction Analysis
Isolation of RNA and reverse-transcription polymerase chain reaction analysis of relative amounts of cDNA were performed as described previously.4 Primers used had annealing temperatures of 58°C and are listed in References 4 and 5 except those for cyclooxygenase-1 (COX-1), forward primer, 5'-TACTATCCATGCCAGAACCAGG-3'; reversed primer, 5'- TGGTAACTGCTTCTTCCCTTTG-3'; plasminogen activator inhibitor-1 (PAI-1), 5'-CCTCCAAAGACCGAAATGTG-3', 5'-GAATCTGGCTCTTTCCACCTC-3'; and tenascin-C, (5'-CAGTGCCATAGCAACAACAGC-3', 5'-TGTATTCCCAGACAC-TGTGCG-3').
Preparation of Nuclei and Western Blot Analysis
Proteins were prepared essentially as described previously,5 except that the nuclear and cytosolic fractions were subsequently precipitated with 3 volumes of ice-cold ethanol, solubilized with the appropriate volume of loading buffer containing 2% SDS, and subjected to SDS-PAGE.
Western blot analysis for zyxin and ß-actin as a loading control followed by densitometry was performed as described previously.5 For detection of zyxin, a primary rabbit polyclonal antibody (provided by Dr M. Beckerle, Department of Biology, University of Utah, Salt Lake City, Utah) or a mouse monoclonal antibody6 was used together with the corresponding secondary HRP-conjugated antibodies (Sigma, Deisenhofen, Germany) and the Super Signal Blaze chemiluminescence reagent (Pierce, St. Augustin, Germany).
Immunofluorescence Analysis
Cells were fixed with p-formaldehyde, and immunostaining with antibodies against vinculin (DPC, Bad Nauheim, Germany) and zyxin6 was performed by a standard protocol7 using Alexa Fluor 488-coupled anti-mouse antibodies (Molecular Probes via MoBiTec, Göttingen, Germany).
Statistical Analysis
Unless indicated otherwise, results are expressed as means±SEM of n observations with cells obtained from different animals. One-way analysis of variance followed by Dunnett post-hoc test was used to determine statistically significant differences between the means and/or the means and control, with P<0.05 considered significant (InStat v3.05; GraphPad Software, San Diego, Calif).
| Results |
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In contrast to stretching of the cells, and irrespective of the time of exposure (ranging from 15 minutes to 12 hours), neither osmotic stress (increase in osmolarity from 290 mosmol/L to 454 mosmol/L) nor simulation of a proinflammatory response (60 U/mL IL-1ß plus 1000 U/mL TNF-
) altered the intracellular localization of zyxin (Figure 2). Also, addition of leptomycin B (1 µmol/L),8 a specific inhibitor of the nuclear export of proteins harboring a nuclear export signal such as zyxin, did not result in an accumulation of zyxin in the nucleus of quiescent SMC during a 12- to 24-hour period (Figure 2), suggesting that, normally, the protein is effectively excluded from the nucleus in these cells.
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Zyxin and Mechanosensitive Gene Expression
To analyze whether the nuclear translocation of zyxin in SMCs exposed to cyclic stretch affects the expression of mechanosensitive genes, the antisense ODN approach was chosen. Apparently, the turnover of zyxin in quiescent rat aortic cultured SMCs is rather low, because the antisense ODN, but not the corresponding scrambled control ODN, was effective only (78±9% decrease in protein abundance, n=3, P<0.05) (Figure 3a) when the cells were stretched for 30 minutes followed by a 48-hour incubation period with the antisense ODN. This finding and the increase in zyxin mRNA on cyclic stretch (4.4±0.7-fold after 3 hours, n=5, P<0.05 versus static control) suggest that turnover of the protein in SMCs is greatly enhanced in response to mechanical deformation.
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As readout, 3 mechanosensitive gene products were selected whose expression in vascular SMCs is upregulated by cyclic stretch and thought to play a role in vascular remodeling, ie, the endothelin B receptor (ETB-R),1,4 the matrix protein tenascin-C,9,10 and plasminogen activator inhibitor-1 (PAI-1).9,11 Downregulation of zyxin protein abundance affected expression of these mechanosensitive gene products, albeit in a different manner (Figure 3b). Thus stretch-induced ETB-R mRNA expression was clearly suppressed, while that of tenascin-C was augmented. Stretch-induced expression of PAI-1 mRNA was insensitive to zyxin depletion. Moreover, stretch-induced expression of other gene products that are not considered to be truly mechanosensitive9,12 was also differentially affected by zyxin antisense ODN treatment (monocyte chemoattractant protein-1 [MCP-1], 110%±46% of scrODN control, n=9; COX-1, 41%±6% of scrODN control, n=9, P<0.01).
| Discussion |
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Mechanotransduction in Smooth Muscle Cells
Focal adhesions mediating cell-matrix interactions have been postulated to serve as a mechanosensor in vascular SMC both in vitro and in vivo.13 However, signaling through these structures appears to be rather complex, involving several protein kinases whose activation has also been implicated in the response of SMCs to other stimuli, such as proinflammatory cytokines or vasoactive peptides. In addition, transcription factors, such as activator protein-1 (AP-1) or CCAAT/enhancer binding protein ß that have been shown to be mechanosensitive,14 are also activated by such stimuli. The specific alterations in gene expression in vascular SMCs exposed to cyclic stretch, however, argue for the activation of a signaling molecule that, in addition to these common stress factors, acts as a specific mechanotransducer. This signaling molecule may, but not necessarily must, be associated with focal adhesions in vascular SMCs.
Zyxin, a Zinc-Finger Protein With Dual Function
Zyxin is predominantly located in focal adhesions and stress fibers. Besides the N-terminal domain mediating its association to focal adhesions,15 zyxin has a C-terminal LIM-domain comprising 3 zinc-finger motifs known to mediate proteinprotein and/or proteinDNA interactions.16 Originally, zyxin was characterized as a protein coordinating the organization of actin filaments at focal adhesions and cell-to-cell contacts in fibroblasts and epithelial cells. Thus, zyxin is likely to function as a structural protein in vascular SMCs, too.
The second function of zyxin, shown herein, is that of modulating stretch-induced gene expression in vascular SMCs. Thus, within minutes zyxin translocates to the nucleus of SMCs exposed to cyclic stretch and alters the expression of mechanosensitive genes. Osmotic stress or simulating a proinflammatory reaction, however, did not result in a detectable dissociation of zyxin from focal adhesions.
How its effect on mechanosensitive gene expression is brought about remains to be elucidated. The presence of the LIM-domains and the finding that a similar LIM-domain present in the zyxin homolog, lipoma-preferred protein, can directly induce gene expression in an artificial assay system17 suggest that zyxin may act as a transcription factor. Otherwise, zyxin may affect gene expression exclusively through proteinprotein interactions, as described for related zinc-finger proteins such as the GATA family of transcription factors.18
Zyxin-Induced Gene Expression in SMCs Exposed to Cyclic Stretch
A role for zyxin in mechanosensitive gene expression was evidenced by antisense ODN-based downregulation of zyxin protein abundance. Although only a small number of genes was analyzed whose expression had been shown to be mechanosensitive in vascular SMCs,4,9,12 the role of zyxin varied depending on the type of gene studied. Stretch-induced PAI-1 and MCP-1 expression appeared to be zyxin-insensitive, ETB-R and COX-1 expression were attenuated, and that of tenascin-C was unblocked.
The lack of effect of zyxin on PAI-1 and MCP-1 expression becomes intelligible when considering that regulation of these genes is normally mediated by common transcription factors, such as AP-1 or CCAAT/enhancer binding protein ß, which are mechanosensitive as well.14 Stretch-induced ETB-R expression, however, is only partially mediated by these transcription factors;14 therefore, it may rely on zyxin activation. SMC expression of the receptor along with stretch-induced synthesis of endothelin-1 by the endothelium results in SMC apoptosis,4 which together with the concomitant changes in matrix protein composition may form the basis for pressure-induced vascular remodeling.13
The role of COX-1 in this context is difficult to interpret.9 Stretch-induced COX-1 expression may counterbalance or further the remodeling process, depending on the type of prostanoid synthesized. In either case, regulation of the COX-1 gene, like that of the ETB-R, is positively controlled by zyxin, whereas the opposite seems true for the tenascin-C gene. Perhaps, the blockade of tenascin-C expression is a kinetic effect that wears off in conditions of enduring pressure overload. This would make sense insofar that premature upregulation of tenascin-C expression may antagonize SMC hypertrophy and/or hyperplasia.10
When combined, the aforementioned data indicate that zyxin not only contributes to organizing the actin cytoskeleton6,15 but also contributes to a second important feature of SMCs, ie, the changes in gene expression, hence, phenotype that occur as an adaptive response to enduring mechanical strain. The protein may thus serve to stabilize the vessel wall in conditions of a transient, eg, exercise-induced increase in blood pressure and during pressure overload, as in arterial hypertension.
Perspectives
The data presented herein are consistent with a role for zyxin in transducing mechanical stimuli from the cell membrane to the nucleus in vascular SMC. Future work must elucidate the mechanisms of stretch-induced zyxin activation and zyxin-mediated gene expression. This will lead to a better understanding of the principles of mechanotransduction in vascular cells and will also help to define a new therapeutic approach to limit the often-exaggerated pressure-induced remodeling of conduit and/or resistance-size arteries, such as in hypertension.
| Acknowledgments |
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Received September 24, 2003; first decision October 13, 2003; accepted January 14, 2004.
| References |
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