© 2001 American Heart Association, Inc.
Editorial |
Regulation of Vascular Cell Behavior by Collagen
Form Is Function
From the John P. Robarts Research Institute (Vascular Biology Group), London Health Science Centre and Departments of Medicine (Cardiology), Biochemistry, and Medical Biophysics, University of Western Ontario, London, Ontario, Canada.
Correspondence to J. Geoffrey Pickering, MD, PhD, FRCP(C), London Health Science Centre, 339 Windermere Rd, London, Ontario N6A 5A5, Canada. E-mail gpickering{at}rri.on.ca
Key Words: collagen • smooth muscle cell • gene expression • migration
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Introduction |
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 Top Introduction References |
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The vascular extracellular matrix (ECM) is a complex mixture of collagens, elastin, glycoproteins, and proteoglycans. These constituents not only provide mechanical integrity to the vessel wall but comprise a repertoire of insoluble ligands that can signal the cell to control proliferation, migration, differentiation, and survival. In the normal adult artery wall, the basement membrane is the primary ECM compartment that interacts with the vascular smooth muscle cell (SMC), and its components are believed to be important in maintaining a stable and well-differentiated SMC.1 However, during conditions of arterial restructuring, the ECM can be quickly remodeled through a combination of synthesis of new ECM molecules, regulated assembly of these molecules, and proteolytic degradation and editing of existing structures. Together, these actions provide a new and dynamic set of ECM stimuli that can have a profound effect on SMC behavior.
One of the most abundant ECM molecules of both healthy and
diseased arteries is type I collagen, a fibril-forming, heterotrimeric
molecule comprised of two 
1(I) collagen chains and one 2(I)
collagen chain. These chains are synthesized as soluble propeptides
that wind around each other in the endoplasmic reticulum to form type I
procollagen, with its long triple helical domain. As type I procollagen
molecules are transported to the cell surface, they associate with each
other.2 These aggregates
nonetheless remain in solution, by virtue of the nonhelical domains of
procollagen at the amino and carboxy termini. Outside the cell, the
propeptide termini are proteolytically removed, on which collagen
molecules fall out of solution and polymerize into a highly ordered
collagen fibril.3 The size of
the fibrils and their organization within the ECM will vary depending
on mechanical influences, the stage of tissue development, and
interactions with other ECM molecules, such as decorin, type III
collagen, and
lumican.4 5 6
Nevertheless, as discussed below, the fibrils are a key determinant of
SMC function.
The interplay between vascular cells and type I collagen is
often examined using tail or skin collagen that has been solubilized in
acid and then coated onto the culture dish as a planar surface of
monomers or oligomers. This is a convenient approach to eliciting
cell-collagen interactions and has yielded important insights. For
example, it has been determined that vascular SMCs use
2ß1 integrin to
crawl on type I collagen7 and
that this motility can be enhanced through localized proteolysis of the
collagen substrate.8 However,
the planar substrate does not provide a close representation of
the collagen fibril network that assembles in vivo. Moreover, there is
considerable evidence that polymerized collagen fibrils can influence
cell behavior differently than a monolayer of collagen. Fibroblasts in
a collagen fibril gel, in contrast to those on planar collagen, were
found to stop replicating, reduce their expression of type I and III
collagens, and induce the expression of collagenase-1
(matrix metalloproteinase-1
[MMP-1]).9 10 It
has also been shown in various cell types that polymerized collagen
fibrils can induce the expression of collagenase-3
(MMP-13)11 and the
2-integrin
subunit,12 suppress the
expression of heat shock protein (Hsp)
47,13 activate
mitogen-activated protein kinases and protein kinase
C-
,11 12 and
attenuate responses to growth
factors.14 Furthermore, type
I collagen in its polymerized, but not monomeric, form is a potent
inhibitor of SMC proliferation, a process mediated by
upregulation of cyclin-dependent kinase-2 (cdk-2)
inhibitors.15
Thus, in regulating SMC behavior, collagen form dictates collagen
function.
In this issue of Circulation Research, Ichii et al16 broaden our understanding of the interactions between polymerized collagen fibrils and vascular SMCs. These investigators cultured human SMCs for 24 hours on bovine skin collagen in either a monomeric or polymerized form and assessed the differential consequences for gene expression using suppressive subtraction hybridization. Twenty-two differentially expressed transcripts were found, and the authors provide good confirmation of the differences by Northern blot analyses. They also showed that 9 of the transcripts that were downregulated by fibrillar collagen, relative to monomeric collagen, were upregulated in the rat carotid artery after balloon injury. This does not prove that the shifts in gene expression in the artery wall were attributable to altered cellular interaction with fibrillar collagen. It does, however, provide a meaningful in vivo context for the specific genes identified by the in vitro screen.
As with any expression screen, definitive statements regarding the function, or even importance, of the individual genes identified cannot be made. However, the patterns of gene expression themselves can be informative and hypothesis-generating, and the findings of Ichii et al16 illustrate this. For example, whereas polymerized collagen suppressed the expression of 18 genes relative to monomeric collagen, it stimulated the expression of only 4. This net suppression may reflect the more physiological environment afforded by the collagen gel and implicates collagen fibrils as structures that can terminate some of the cellular processes that were activated during vascular remodeling. At the same time, it may be particularly informative to pursue the significance of the minority, upregulated genes, including tissue-type plasminogen activator, angiopoietin-related protein-2, and microfibril-associated glycoprotein 4.
It is also noteworthy that more than half of the differentially expressed transcripts could be categorized as encoding either ECM proteins or cytoskeleton-associated proteins. The authors established that fibronectin, thrombospondin-1, and tenascin-C were downregulated by polymerized collagen, highlighting a phenomenon whereby the ECM environment can regulate its own constituents. Furthermore, the associated change in cytoskeletal gene expression is consistent with a linkage between the ECM, SMC shape, and gene expression. Indeed, the differences in gene expression between SMCs on monomeric or polymerized collagen may be due to differences in cell geometry and in the different mechanical forces exerted on the cell surface by the 2 substrates.
Ichii et al16
also show that SMCs that have been cultured on fibrillar collagen
subsequently manifest a repressed ability to directionally migrate to
thrombospondin-1 on a substrate of vitronectin or
osteopontin. The authors suggest that because SMC migration on
vitronectin or osteopontin is mediated, at least in part,
by vß3 integrin and
because polymerized collagen had no effect on
vß3 integrin
expression, the repressed motility may be due to inhibition of
vß3 integrin
function. The authors observed that SMCs on polymerized collagen had
smaller focal adhesions with less -actinin, and they postulate that
this may repress vß3
integrin–mediated migration. This remains speculative, because,
depending on the initial force with which the cell is attached to the
substrate, a reduction in focal adhesion assembly can actually favor
motility over stationary
adhesion.17 Nevertheless,
the findings are consistent with an interesting paradigm
whereby one ECM constituent suppresses the cellular responses to
another. This filtering-out of specific signals may be an important
mode of cell regulation in vascular disease. Mechanistically, a form of
crosstalk between integrins or other ECM-initiated signaling components
seems possible and warrants additional investigation.
Clearly, there is a wealth of information in the insoluble ECM of the artery wall. However, as highlighted by the data of Ichii et al,16 the mere presence of a given ECM molecule at the surface of the cell may not define the consequences for that cell. Higher levels of control must be considered, including the extent of ECM assembly and a dynamic reciprocity between the cell and the substrate that regulates both the constituents of the ECM and the responsiveness of the cell to those constituents. Elucidating the complex networks that underlie this control remains an exciting challenge.
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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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