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Control of Vascular Cell Differentiation by Homeobox
Transcription Factors
David H. Gorski, Kenneth Walsh
P
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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
athological 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 DNAbinding 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 –11 Hex is a proline-rich,
divergent homeobox gene. It is expressed in a range of
multipotent hematopoietic progenitor cells and cell lines9 and
The opinions expressed in this editorial are not necessarily those of the
editors or of the American Heart Association.
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@opal.
tufts.edu
(Circ Res. 2001;88:7-8.)
© 2001 American Heart Association, Inc.
Circulation Research is available at http://www.circresaha.org
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Circulation Research
January 5/19, 2001
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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 ␣V␤3
and ␣V␤5 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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KEY WORDS: homeobox genes
䡲
vascular smooth muscle
Control of Vascular Cell Differentiation by Homeiobox Transcription Factors
David H. Gorski and Kenneth Walsh
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Circ Res. 2001;88:7-8
doi: 10.1161/01.RES.88.1.7
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