© 2001 American Heart Association, Inc.
Editorials |
Control of Vascular Cell Differentiation by Homeiobox Transcription Factors
From the Division of Surgical Oncology (D.H.G.), UMDNJ–Robert Wood Johnson Medical School, The Cancer Institute of New Jersey, New Brunswick, NJ, and Division of Cardiovascular Research (K.W.), Tufts University School of Medicine, St Elizabeth’s Medical Center, Boston, Mass.
Correspondence to Kenneth Walsh, PhD, Division of Cardiovascular Research, Tufts University School of Medicine, St Elizabeth’s Medical Center, 736 Cambridge St, Boston, MA 02135. E-mail kwalsh{at}opal.tufts.edu
Key Words: homeobox genes • vascular smooth muscle
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Introduction |
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 Top Introduction References |
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Pathological remodeling in the vasculature occurs during the development of restenosis after balloon angioplasty and atherosclerosis, where the migration of proliferating phenotypically modified vascular smooth muscle cells (VSMCs) from the media to the intima is an important contributor to the intimal thickening that narrows the vessel lumen.1 In the normal vessel wall, VSMCs are in the "contractile" state in that they are quiescent, do not migrate, and express smooth muscle–specific isoforms of contractile proteins.2 By contrast, in diseased vessels, VSMCs acquire a "synthetic" state, in which they are proliferative, migratory, and express lower levels of contractile proteins and higher levels of nonmuscle isoforms of myosin and actin. Synthetic VSMCs generally resemble their less-differentiated precursors in fetal blood vessels. Since 1992, investigators have sought to examine what roles homeobox genes may play in controlling the phenotypic modulation of VSMCs.3 First characterized in the study of Drosophila mutations that give rise to homeotic transformations,4 homeobox genes encode transcription factors with a common 60 amino acid DNA-binding motif that is referred to as the homeodomain.5 These homeodomain-containing transcription factors regulate proliferation, differentiation, and migration in multiple cell types and play an important role in organogenesis and pattern formation during embryogenesis.6 These features of homeobox proteins make them promising candidates as regulators of cellular differentiation involved in the final transcriptional control of the genes responsible for the phenotypic changes observed in VSMCs during development and in pathological states.7
In this issue of Circulation Research, Sekiguchi et al8 report observations suggesting that at least one homeobox gene, Hex (also called Prh), may play a previously unsuspected role in regulating the phenotype of VSMCs in addition to its previously postulated role in the differentiation of endothelial and hematopoietic cells.9 10 11 Hex is a proline-rich, divergent homeobox gene. It is expressed in a range of multipotent hematopoietic progenitor cells and cell lines9 and is an early marker of endothelial cell precursors that is transiently expressed in the endoderm, ventral foregut, and nascent blood islands of the visceral yolk sac and later in embryonic angioblasts and endocardium in the mouse embryo.10 The Xenopus laevis homologue of Hex, XHex, is expressed in vascular endothelial cells throughout the developing vascular network, with its expression commencing slightly after expression of the vascular endothelial growth factor receptor gene flk-1.11 Overexpression of XHex during embryogenesis produces an increase in endothelial cell number and leads to a disruption of vascular structures.11 These observations suggest a role for Hex in regulating the endothelial cell angiogenic phenotype and vascular patterning during embryogenesis. However, disruption of the Hex gene in mice does not produce detectable abnormalities in the cardiovascular system, although it is required for the development of endodermal tissues, including forebrain, liver, and thyroid.12
In their study, Sekiguchi et al8 demonstrate that Hex is more widely expressed in the vasculature than previously thought and report that it is expressed in VSMCs. Of more interest, Hex appears to be expressed mainly in cultured VSMCs in vitro and proliferating VSMCs in vivo. Indeed, its expression in native, uninjured aorta is essentially undetectable even by reverse transcriptase–polymerase chain reaction, but Hex protein is induced within 1 week after balloon injury. On the basis of the observation that the time course of Hex induction in arteries after balloon injury was similar to that of the nonmuscle-specific isoform of the myosin heavy chain, NMHC-B/SMemb (henceforth referred to as SMemb), a marker of VSMC dedifferentiation, they examined whether Hex could transactivate the SMemb promoter. Cotransfection experiments demonstrate that Hex can indeed transactivate the SMemb promoter and that the presence of the Hex homeodomain is required for this transactivation. Furthermore, they demonstrate that Hex-mediated activation of the SMemb promoter appears to occur through a protein kinase A–dependent mechanism involving a cAMP-response element.
The observation reported by Sekiguchi et
al8 that
Hex expression may be involved
in the dedifferentiation of VSMCs during proliferation and acute
vascular injury suggests that it may have similar functions to those of
another, more widely expressed homeobox gene,
HOXB7. Embryonic VSMCs, which
are closer to synthetic state, express
HOXB7 at a higher level than
observed in adult VSMCs.13
Furthermore, overexpression of
HOXB7 in C3H10T1/2 fibroblasts
results in increased proliferation and the induction of a VSMC-like
morphology, which is associated with the expression of early VSMC
markers calponin and SM22
, but not of the intermediate VSMC marker
smooth muscle myosin heavy
chain.14 The observation
that Hex expression is
upregulated in VSMCs in vivo after balloon injury, a stimulus that
causes them to revert to a more immature, synthetic phenotype, coupled
with its ability to activate the promoter of a gene associated with a
more immature ("synthetic") phenotype, suggests that
Hex, too, may have a role in
regulating the phenotype of VSMCs. However, it is unknown whether the
expression of Hex can activate
the endogenous SMemb gene in
VSMCs or act globally to promote the conversion of VSMCs to the
synthetic phenotype in a manner similar to
HOXB7.
Homeobox genes, such as
HOXB7 and possibly
Hex, are likely to be involved
in promoting dedifferentiation and proliferation of VSMCs. In contrast,
at least one other homeobox gene,
Gax, has been implicated in the
differentiation of VSMCs. In VSMCs,
Gax mRNA is rapidly
downregulated by mitogen stimulation in
vitro15 and by acute
vascular injury in vivo.16
Gax overexpression blocks VSMC
proliferation and results in G1 cell-cycle
arrest in vitro by activating the expression of the p21
cyclin-dependent kinase
inhibitor.17
Gax also controls the migration
of VSMCs toward chemotactic growth factors, an effect that can be
correlated with its ability to downregulate expression of integrins
Vß3 and
Vß5 both in vitro
and in vivo, which are induced in synthetic state
VSMCs.18 Thus, a model of
homeobox gene action in VSMCs can be postulated from these
observations, in which a subset of homeobox genes, such as
Hex and
HOXB7, promotes the synthetic
phenotype and another subset, containing
Gax, tends to promote a more
quiescent, contractile phenotype.
The present study by Sekiguchi et al8 identifies Hex, which, in addition to HOXB7 and Gax, is a potential homeobox gene regulator of VSMC phenotype. Furthermore, Sekiguchi et al demonstrate a potential downstream promoter target of Hex that is associated with a specific VSMC phenotype and a signaling pathway leading to the ability of Hex to activate that target. These findings are important, because little is known about the precise mechanisms by which homeodomain proteins regulate downstream target genes or how activation or repression of these target genes results in a change in cell phenotype or behavior. Given their importance in cell-cycle control, cell migration, and cell adhesion, it is reasonable to speculate that many more homeobox genes will be implicated in the regulation of VSMC differentiation and that these factors will have both complementary and synergistic roles during the remodeling that accompanies vascular disease and normal development.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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TopIntroductionReferences |
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3.
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7.
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8.
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