This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Lehoux, S. Articles by Tedgui, A. Search for Related Content PubMed PubMed Citation Articles by Lehoux, S. Articles by Tedgui, A. Related Collections Restenosis Mechanism of atherosclerosis/growth factors Pathophysiology Gene regulation Smooth muscle proliferation and differentiation

(Circulation Research. 2003;93:1139.)
© 2003 American Heart Association, Inc.

Editorials

All Strain, No Gain

Stretch Keeps Proliferation at Bay via the NF-B Response Gene iex-1

Stéphanie Lehoux, Alain Tedgui

From INSERM U541 and Institut Fédératif de Recherche–Paris-7, Hôpital Lariboisière, Paris, France.

Correspondence to Alain Tedgui, PhD, INSERM U541, 41, Bd de la Chapelle, 75010 Paris, France. E-mail tedgui{at}larib.inserm.fr

Key Words: mechanotransduction • stretch • smooth muscle cells • phenotype • nuclear factor-B

Blood vessels are continuously subjected to the action of mechanical forces in the form of shear stress and tensile stress. Shear stress results from the friction of blood against the vessel and is sensed by endothelial cells. Tensile stress is the product of blood pressure, and it is the major determinant of vessel stretch, to which a cyclic quality is added stemming from the pulsatile nature of blood pressure. Stretch affects both endothelial cells and smooth muscle cells (SMCs).

Alterations in the biomechanical environment induce transformations in the vessel wall to accommodate any new hemodynamic conditions and ultimately restore basal levels of tensile stress and shear stress.¹ In vivo studies and clinical observations suggest that decreased values of tensile and shear stress are correlated with enhanced cell proliferation and extracellular matrix production, whereas reestablishment of baseline tensile stress and shear stress levels is associated with restoration of vessel wall morphology and a return of SMCs to a differentiated state.² In fact, a distinct feature of vascular SMCs is their phenotypic plasticity: SMCs are capable of displaying broad changes in ultrastructure and marker protein expression depending on their environment.³ This feature is particularly evident in SMCs undergoing a transition from the contractile to the synthetic phenotype such as occurs during establishment of primary cell cultures or at the loci of vascular injury.³ For example, nonmuscle ß-actin mRNA and protein levels markedly increase in SMCs placed in culture.⁴ Interestingly, mechanical factors play a major role in determining SMC phenotype. Exposing cultured SMCs to cyclic stretch can restore the expression of high-molecular-weight caldesmon, as well as smooth muscle myosin heavy chains and myosin light chain kinase, all markers of differentiated SMCs.⁵ Similarly, experiments on isolated vessels demonstrate that a certain level of stretch is necessary and sufficient for the preservation of the SMC contractile state; high-molecular-weight caldesmon and filamin contents are maintained in SMCs of arterial segments maintained in organ culture for 3 days at physiological intraluminal pressure, whereas levels of these marker proteins decrease in relaxed vessels or vessels kept at low intraluminal pressure.⁶ Hence, loss of a threshold level of stretch is likely to contribute to SMC transformation in injured vessels and in cell culture.

Signaling pathways associated with mechanical forces, generally referred to as mechanotransduction, have been extensively studied in endothelial cells exposed to shear stress.⁷ Less is known regarding mechanotransduction in SMCs. Nevertheless, several reports indicate that stretch initiates complex signal transduction cascades leading to gene transcription and functional responses, via interaction of integrins with extracellular matrix proteins, or by stimulation of G protein receptors, tyrosine kinase receptors, or ion channels (reviewed in Reference ¹). Intracellular pathways reported to be activated by stretch in SMCs include the MAP (mitogen-activated protein) kinase cascades, which are relatively well characterized, and the NF-B (nuclear factor-B) pathway,^8,9 which has received much less attention thus far.

Richard Lee’s group previously used DNA microarray technology to describe the transcriptional profile of mechanically induced genes in SMCs subjected to a uniform biaxial cyclic strain.^10,11 Cyclic stretch was found to stimulate the expression of a number of genes, including vascular endothelial growth factor (VEGF), cyclooxygenase-1, tenascin-C, and plasminogen activator inhibitor-1, but to negatively regulate others, such as matrix metalloproteinase-1 (MMP-1) and thrombomodulin.¹⁰ More recently, this group identified the immediate early response gene X-1 (iex-1) as being induced in cardiomyocytes during the early response to mechanical stimulation, through NF-B–dependent transcription.¹² In this issue of Circulation Research, Schulze et al¹³ report that (1) iex-1 is induced in human aortic SMCs exposed to cyclic stretch or to several growth factors; (2) stretch-induced iex-1 expression is regulated by NF-B; and (3) overexpression of iex-1 inhibits proliferation of SMCs in culture and modulates intimal hyperplasia induced by vascular injury in vivo. iex-1 gene expression in response to mechanical stimulation seems to be strictly controlled by NF-B, since overexpression of IB, the cytoplasmic inhibitor of NF-B activation, totally abolished the biomechanical induction of iex-1. However, in the absence of stretch, NF-B appears to be neither necessary nor sufficient for iex-1 gene expression in SMCs. PDGF, which does not activate NF-B,¹⁴ markedly enhanced iex-1 expression, whereas tumor necrosis factor- (TNF-), a potent NF-B activator, had no effect on iex-1 expression.

While iex-1 promotes apoptosis in HEK-293 and HeLa cells under serum deprivation in vitro,^15,16 it protects activated T cells from apoptosis during an immune response in vivo.¹⁷ This dichotomy also applies to the role of iex-1 in cell proliferation: iex-1 has been shown to inhibit cell proliferation in some cells but appears to accelerate cell cycle progression in others (reviewed in Wu¹⁸). In the study by Schulze et al,¹³ the antiproliferative function of iex-1 seemed to predominate, although it was clearly more significant in SMCs exposed to stretch than SMCs stimulated by growth factors. Intriguingly, in spite of the fact that endogenous iex-1 expression was itself induced, overexpression of iex-1 using adenoviral vectors was necessary to inhibit cyclic stretch-induced SMC proliferation, and even this strategy only marginally suppressed PDGF-induced mitogenesis. In the case of stretched cells, endogenous iex-1 may have effectively prevented proliferation at 24 hours, but no longer at 48 hours. Given that growth factor secretion and growth factor receptor activation are stimulated by cyclic stretch in SMCs, contributing to cell proliferation,^19,20 it is conceivable that the mitogenic pathways fueled by growth factors accumulating over 48 hours of cell culture eventually reach a threshold beyond which they overcome antiproliferative effects of endogenous iex-1. Dominant-negative strategies will be useful to test this hypothesis. In the case of balloon injury–induced vascular remodeling in vivo, iex-1 overexpression effectively abated intimal hyperplasia, but the role for NF-B–dependent iex-1 expression in this context is obscure because intimal hyperplasia can just as well be prevented using NF-B decoy strategies,²¹ which would presumably block iex-1 transcription. Again, specific iex-1 inhibition will be necessary to clarify its role in this context.

As stated by the investigators,¹³ the strain-dependent expression of iex-1 may represent a mechanically induced modulation of SMCs in culture leading toward a more physiological phenotype. Appropriately, iex-1 was found to be constitutively expressed in native, noninjured vessels, but was barely detectable in intimal hyperplasic lesions produced by balloon injury and endothelial denudation. Interestingly, in a recent study, we reported that NF-B inhibition induces apoptosis not only in vessels maintained in organ culture at elevated transmural pressure, but even in vessels cultured at physiological pressure.⁹ Taken together, these findings suggest a fundamental role for NF-B in the vessel, preventing cell proliferation through iex-1 transcription under physiological mechanical stimulation while maintaining SMC survival both in physiological conditions and in conditions of exaggerated vessel stretch, via antiapoptotic gene expression (Figure).

View larger version (22K): [in this window] [in a new window]   Physiological levels of intraluminal pressure induce vascular NF-B, protecting cells from apoptosis.9 It now appears that activation of NF-B by stretch also regulates SMC proliferation through iex-1 expression.13 iex-1 prevents growth factor–dependent degradation of p27Kip1 and Rb phosphorylation, undermining cell cycle progression. iex-1 can also attenuate NF-kB and PI3K/Akt pathway activation, leading to enhanced cell death.15 Hence, acting through different NF-B–dependent pathways, stretch contributes to SMC survival and helps maintain vascular cells in a quiescent, differentiated state.

Undoubtedly, further research is necessary to extend these initial observations and ascertain the functional roles of iex-1 in vascular biology. Loss-of-function experiments will be particularly useful to provide direct evidence for a role of iex-1 in SMC proliferation under physiological or pathological conditions.

Footnotes

See related article, pages 1210–1217

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.

References

1. Lehoux S, Tedgui A. Cellular mechanics and gene expression in blood vessels. J Biomech. 2003; 36: 631–643.[CrossRef][Medline] [Order article via Infotrieve]
2. Glagov S. Intimal hyperplasia, vascular modeling, and the restenosis problem. Circulation. 1994; 89: 2888–2891.[Free Full Text]
3. Berk BC. Vascular smooth muscle growth: autocrine growth mechanisms. Physiol Rev. 2001; 81: 999–1030.[Abstract/Free Full Text]
4. Kocher O, Gabbiani G. Expression of actin mRNAs in rat aortic smooth muscle cells during development, experimental intimal thickening, and culture. Differentiation. 1986; 32: 245–251.[CrossRef][Medline] [Order article via Infotrieve]
5. Birukov KG, Shirinsky VP, Stepanova OV, Tkachuk VA, Hahn AW, Resink TJ, Smirnov VN. Stretch affects phenotype and proliferation of vascular smooth muscle cells. Mol Cell Biochem. 1995; 144: 131–139.[CrossRef][Medline] [Order article via Infotrieve]
6. Birukov KG, Bardy N, Lehoux S, Merval R, Shirinsky VP, Tedgui A. Intraluminal pressure is essential for the maintenance of smooth muscle caldesmon and filamin content in aortic organ culture. Arterioscler Thromb Vasc Biol. 1998; 18: 922–927.[Abstract/Free Full Text]
7. Shyy JY, Chien S. Role of integrins in endothelial mechanosensing of shear stress. Circ Res. 2002; 91: 769–775.[Abstract/Free Full Text]
8. Hishikawa K, Oemar BS, Yang Z, Luscher TF. Pulsatile stretch stimulates superoxide production and activates nuclear factor-B in human coronary smooth muscle. Circ Res. 1997; 81: 797–803.[Abstract/Free Full Text]
9. Lemarie CA, Esposito B, Tedgui A, Lehoux S. Pressure-induced vascular activation of nuclear factor-B: role in cell survival. Circ Res. 2003; 93: 207–212.[Abstract/Free Full Text]
10. Feng Y, Yang JH, Huang H, Kennedy SP, Turi TG, Thompson JF, Libby P, Lee RT. Transcriptional profile of mechanically induced genes in human vascular smooth muscle cells. Circ Res. 1999; 85: 1118–1123.[Abstract/Free Full Text]
11. Lee RT. Functional genomics and cardiovascular drug discovery. Circulation. 2001; 104: 1441–1446.[Free Full Text]
12. De Keulenaer GW, Wang Y, Feng Y, Muangman S, Yamamoto K, Thompson JF, Turi TG, Landschutz K, Lee RT. Identification of IEX-1 as a biomechanically controlled nuclear factor-B target gene that inhibits cardiomyocyte hypertrophy. Circ Res. 2002; 90: 690–696.[Abstract/Free Full Text]
13. Schulze PC, de Keulenaer GW, Kassik KA, Takahashi T, Chen Z, Simon DI, Lee RT. Biomechanically induced gene iex-1 inhibits vascular smooth muscle cell proliferation and neointima formation. Circ Res. 2003; 93: 1210–1217.[Abstract/Free Full Text]
14. Jiang B, Xu S, Brecher P, Cohen RA. Growth factors enhance interleukin-1ß–induced persistent activation of nuclear factor-B in rat vascular smooth muscle cells. Arterioscler Thromb Vasc Biol. 2002; 22: 1811–1816.[Abstract/Free Full Text]
15. Arlt A, Kruse ML, Breitenbroich M, Gehrz A, Koc B, Minkenberg J, Folsch UR, Schafer H. The early response gene IEX-1 attenuates NF-B activation in 293 cells, a possible counter-regulatory process leading to enhanced cell death. Oncogene. 2003; 22: 3343–3351.[CrossRef][Medline] [Order article via Infotrieve]
16. Arlt A, Grobe O, Sieke A, Kruse ML, Folsch UR, Schmidt WE, Schafer H. Expression of the NF-B target gene IEX-1 (p22/PRG1) does not prevent cell death but instead triggers apoptosis in Hela cells. Oncogene. 2001; 20: 69–76.[CrossRef][Medline] [Order article via Infotrieve]
17. Zhang Y, Schlossman SF, Edwards RA, Ou CN, Gu J, Wu MX. Impaired apoptosis, extended duration of immune responses, and a lupus-like autoimmune disease in IEX-1-transgenic mice. Proc Natl Acad Sci U S A. 2002; 99: 878–883.[Abstract/Free Full Text]
18. Wu MX. Roles of the stress-induced gene IEX-1 in regulation of cell death and oncogenesis. Apoptosis. 2003; 8: 11–18.[CrossRef][Medline] [Order article via Infotrieve]
19. Standley PR, Obards TJ, Martina CL. Cyclic stretch regulates autocrine IGF-I in vascular smooth muscle cells: implications in vascular hyperplasia. Am J Physiol. 1999; 276: E697–E705.[Medline] [Order article via Infotrieve]
20. Hu Y, Bock G, Wick G, Xu Q. Activation of PDGF receptor in vascular smooth muscle cells by mechanical stress. FASEB J. 1998; 12: 1135–1142.[Abstract/Free Full Text]
21. Yamasaki K, Asai T, Shimizu M, Aoki M, Hashiya N, Sakonjo H, Makino H, Kaneda Y, Ogihara T, Morishita R. Inhibition of NFB activation using cis-element ‘decoy’ of NFB binding site reduces neointimal formation in porcine balloon-injured coronary artery model. Gene Ther. 2003; 10: 356–364.[CrossRef][Medline] [Order article via Infotrieve]

This article has been cited by other articles:

				S. L. Sommer, T. J. Berndt, E. Frank, J. B. Patel, M. M. Redfield, X. Dong, M. D. Griffin, J. P. Grande, J. M. A. van Deursen, G. C. Sieck, et al. Elevated blood pressure and cardiac hypertrophy after ablation of the gly96/IEX-1 gene J Appl Physiol, February 1, 2006; 100(2): 707 - 716. [Abstract] [Full Text] [PDF]

				C. J. Thompson, S. M. Ross, J. Hensley, K. Liu, S. C. Heinze, S. S. Young, and K. W. Gaido Differential Steroidogenic Gene Expression in the Fetal Adrenal Gland Versus the Testis and Rapid and Dynamic Response of the Fetal Testis to Di(n-butyl) Phthalate Biol Reprod, November 1, 2005; 73(5): 908 - 917. [Abstract] [Full Text] [PDF]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Lehoux, S. Articles by Tedgui, A. Search for Related Content PubMed PubMed Citation Articles by Lehoux, S. Articles by Tedgui, A. Related Collections Restenosis Mechanism of atherosclerosis/growth factors Pathophysiology Gene regulation Smooth muscle proliferation and differentiation