Abstract
Tight junctions have recently emerged as essential signaling regulators of proliferation and differentiation in epithelial tissues. Here, we aimed to identify the factors regulating claudin-7 expression in the colon, and analyzed the consequences of claudin-7 overexpression in colorectal carcinoma (CRC). In healthy human colonic crypts, claudin-7 expression was found to be low in the stem/progenitor cell compartment, where Tcf-4 activity is high, but strong in differentiated and postmitotic cells, where Tcf-4 is inactive. In contrast, claudin-7 was overexpressed in areas with high Tcf-4 target gene levels in CRC samples. In vitro, Tcf-4 was able to repress claudin-7 expression, and the high mobility group-box transcription factor Sox-9 was identified as an essential mediator of this effect. Claudin-7 was strongly expressed in the intestine of Sox-9-deficient mice and in CRC cells with low Sox transcriptional activity. Sox-9 overexpression in these cells reinstated claudin-7 repression, and residual claudin-7 was no longer localized along the basolateral membrane, but was instead restricted to tight junctions. Using HT-29Cl.16E CRC cell spheroids, we found that Sox-9-induced polarization was completely reversed after virus-mediated claudin-7 overexpression. Claudin-7 overexpression in this context increased Tcf-4 target gene expression, proliferation, and tumorigenicity after injection in nude mice. Our results indicate that Tcf-4 maintains low levels of claudin-7 at the bottom of colonic crypts, acting via Sox-9. This negative regulation seems to be defective in CRC, possibly due to decreased Sox-9 activity, and the resulting claudin-7 overexpression promotes a loss of tumor cell polarization and contributes to tumorigenesis.
Highlights
IntroductionThis is notably the case for tight junction (TJ) proteins, which have been identified as regulators of cell proliferation in kidney and intestinal cells [3, 4]
In recent years, several groups of proteins involved in cell/cell adhesion complexes were shown to play an important role inNote: Supplementary data for this article are available at Cancer Research Online.Current address for M
The first important finding of the present work is that the Wnt/ Tcf-4 pathway negatively regulates the expression of claudin-7, resulting in the presence of an increasing gradient of expression for this protein from the bottom of human colonic crypts towards the differentiated surface epithelium
Summary
This is notably the case for tight junction (TJ) proteins, which have been identified as regulators of cell proliferation in kidney and intestinal cells [3, 4] Among these proteins is the large family of claudin isoforms, which have been described as essential modulators of paracellular permeability, and whose role on intracellular signaling [5, 6] and on the expression of the cancer cell phenotype [7] is slowly emerging. Both overexpression and down-regulation of selective claudin isoforms have been identified in various types of cancers, resulting in a disruption of TJ strands and an increase in epithelial permeability [8, 9]. Several processes have been suggested to play a role in the promotion of tumor development upon claudin/TJ disruption, including a facilitation of growth factor leakage from the lumen [8], a promotion of proliferation [2, 7], a role in the loss of polarity [9] and the appearance of invasive behavior [10] in epithelial cells
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