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A new study identifies the extracellular matrix (ECM) component, SPARC (secreted protein acidic, rich in cysteine), as a critical determinant of tumour burden in the APCmin/+ model of intestinal cancer.
The genetic lesions that initiate colorectal cancer are relatively well understood. In the overwhelming majority of cases, colorectal cancer cells carry mutations in components of the Wnt signaling pathway (most often in the adenomatous polyposis coli (APC) gene), which confers upon them a marked growth advantage over surrounding normal epithelial cells.1,2 Although this initial burst in proliferative potential represents an important event in the progression towards adenomas and later carcinomas, clearly other contributing factors must also play their part. Over the years, the classical APCmin/+ model of intestinal tumorigenesis has provided researchers with an excellent tool to test genetically whether candidate genes can enhance or repress adenoma formation in vivo.3,4 Phenotypically, APCmin mice resemble familial adenomatous polyposis patients, in that they carry inactivating mutations in the APC gene and spontaneously develop numerous polyps along their intestinal tract.5
The number of APCmin tumours is strongly influenced by the genetic background. On the basis of this observation, linkage studies have identified loci that modify tumour load in APCmin/+ mice, most prominent of which is the secretory phospholipase A2 (Pla2g2a).6,7,8,9 A more effective strategy for identifying modifiers has, however, come through candidate approaches. For a start, several laboratories have looked at the role of genetic surveillance factors such as Mbd4,10 Myh,11 Fen112 and Msh2.13 These proteins help repair DNA errors, and by doing so, reduce the risk of potentially tumorigenic mutations from occurring. Accordingly, the inactivation of such genes results in accelerated polyp formation in APCmin/+ mice. Another way to augment tumorigenicity is through aneuploidy, a consequence of missegregated chromosomes during cell division. One study has indeed shown that loss of a single allele of the spindle checkpoint component, BubR1, causes greater numbers of colonic adenomas with enhanced genomic instability,14 but paradoxically BubR1 deficiency also led to lower polyp counts in the small intestine. Similarly, the decreased expression of the transcription factor Cdx2 in APC‐deficient mice is also associated with higher rates of chromosomal aberrations and increased colonic polyp numbers, whereas in the small intestine adenoma formation was reduced.15
Another important class of modifiers of the APCmin phenotype constitutes genes that are transcriptionally regulated by the Wnt signaling pathway (see table 11).). Inactivating the APC gene leads to a robust increase in β‐catenin/Tcf transcriptional activity.16 In recent years hundreds of genes regulated by β‐catenin/Tcf have been reported, some of which can modulate adenoma formation (a partial list includes cMyc17,18 EphB2/B3,19 Tcf1,20 Tiam1,21 and Cox222). For an updated list of Wnt target genes see http://www.stanford.edu/˜rnusse/wntwindow.html. Interestingly, certain Wnt target genes (i.e. EphB receptors and Tiam1) stimulate the invasiveness of adenomas upon inactivation.21,23 It is worth noting here that the progression of APCmin adenomas to adenocarcinomas is also associated with parallel signaling cascades such as those regulated by Ras and Tgfβ.24,25
The regulation of gene expression on a more general level also plays a role in intestinal neoplasia. For example, deficiencies in global regulators of transcription such as the histone deacetylase, HDAC‐2;26 the DNA methyltransferase, Dnmt1;27 and the methyl binding domain protein, Mbd2,28 all result in impaired tumorigenesis in the APCmin background. A similar reduction in APCmin tumours has been noted in mice lacking the AP‐1 component, c‐Jun,29 and the partner of p120‐catenin, Kaiso.30 In the case of c‐Jun, the suppression of adenoma formation may be caused by the ability of c‐Jun to promote β‐catenin/Tcf activity. Finally, a more specialized transcription factor such as FoxL1 has the opposite effect.31 The deletion of FoxL1 promotes tumour formation in the intestine and also initiates gastric tumours. The fact that FoxL1 is expressed in stromal cells suggests that it might regulate the expression of secreted factors that favour tumour development in a paracrine fashion.
The interface between tumour cells and the extracellular matrix (ECM) has long been appreciated as an important determinant of tumour progression.35 Note that the ECM in this context simply refers to all material constituents surrounding any given cell. The first ECM component linked to tumorigenesis in the APCmin model was the matrix metalloproteinase and Wnt target gene, MMP7/Matrilysin.33 In this issue of Gut, Owen Sansom and colleagues identified SPARC (secreted protein acidic, rich in cysteine), as a novel ECM factor involved in adenoma formation. Their results show that SPARC protein levels are upregulated in polyps arising in APCmin/+ mice and in human colorectal adenocarcinomas. The significance of these findings was confirmed by demonstrating that the removal of SPARC in APCmin/+ mice suppressed the multiplicity of adenomas, while not affecting the size of tumours. Finally, the authors also measured migration rates of epithelial cells and found that SPARC‐deficient cells transited more rapidly up the crypt–villus axis, a result implying higher turnover rates of the intestinal epithelium.
SPARC is a so‐called matricellular protein and is part of a family of structurally unrelated extracellular modulators of cellular function.36 Members include thrombospondin 1 and 2, osteopontin, cyr61, CTGF, CCN1 and the tenascins. The distinguishing feature of this diverse group is that they are all secreted macromolecules, which appear not to serve any structural roles. Instead, matricellular proteins regulate cell–matrix interactions, bind numerous constituents of the ECM and are generally involved in modulating adhesion.
The biological function of SPARC has been studied in cultured cells as well as in knockout mice. In vitro SPARC promotes de‐adhesion, a phenomenon characterized by the transition between a state of strong to intermediate adhesion, accompanied by a restructuring of focal adhesions and stress fibres.37SPARC null mice display altered responses to cutaneous wounds as well as varied developmental defects affecting the formation of the lens capsule, dermis, adipocytes, and bone.38SPARC mutant mice have also been useful in establishing a role for SPARC in neoplasia. In brief, the loss of function of SPARC seems to have disparate effects depending on the cellular context. For example, the subcutaneous injection of tumour cells in SPARC‐deficient mice resulted in enhanced tumour growth, with evidence of reduced collagen deposition and macrophage infiltration.39 Whereas in a mammary carcinoma model, tumour size, as well as collagen deposition and vascularization were reduced in SPARC−/− mice.40
What do all these studies tell us about the function of SPARC in intestinal neoplasia? In a system with a high turnover rate, such as the intestinal epithelium, tumour cells must find a way to embed or seed themselves into the mucosa in order to avoid the general flux that pushes enterocytes out into the lumen. It has been argued that SPARC and probably other ECM components might be essential in this ‘seeding' effect.36 In other words, through SPARC's ability to regulate processes such as the deposition of ECM components and vascular growth and remodeling, SPARC might be essential in preparing the stromal compartment in the intestine for efficient tumour growth and invasion. An alternative explanation may depend on the recognized role of SPARC in regulating adhesion. Indeed, Sansom and colleagues propose that the higher migration rates of SPARC−/− enterocytes may contribute to eliminating deleterious cells and thus suppressing adenoma formation. Finding out what precisely SPARC does during intestinal tumorigenesis will be the next challenge.
Conflict of interest: None declared.