KEY ROLE OF THE CDX2 HOMEOBOX GENE IN EXTRACELLULAR MATRIX-MEDIATED INTESTINAL-CELL DIFFERENTIATION

To explore the role of homeobox genes in the intestine, the human colo n adenocarcinoma cell line Caco2-TC7 has been stably transfected with plasmids synthesizing Cdx1 and Cdx2 sense and antisense RNAs. Cdx1 ove rexpression or inhibition by antisense RNA does not markedly modify th e cell differentiation markers analyzed in this study. In contrast, Cd x2 overexpression stimulates two typical markers of enterocytic differ entiation: sucrase-isomaltase and lactase. Cells in which the endogeno us expression of Cdx2 is reduced by antisense RNA attach poorly to the substratum. Conversely, Cdx-2 overexpression modifies the expression of molecules involved in cell-cell and cell-substratum interactions an d in transduction process: indeed, E-cadherin, integrin-beta 4 subunit , laminin-gamma 2 chain, hemidesmosomal protein, APC, and alpha-actini n are upregulated. Interestingly, most of these molecules are preferen tially expressed in vivo in the differentiated villi enterocytes rathe r than in crypt cells. Cdx2 overexpression also results in the stimula tion of HoxA-9 mRNA expression, an homeobox gene selectively expressed in the colon, In contrast, Cdx2-overexpressing cells display a declin e of Cdx1 mRNA, which is mostly found in vivo in crypt cells. When imp lanted in nude mice, Cdx2-overexpressing cells produce larger tumors t han control cells, and form glandular and villus-like structures. Lami nin-l is known to stimulate intestinal cell differentiation in vitro. In the present study, we demonstrate that the differentiating effect o f laminin-l coatings on Caco2-TC7 cells is accompanied by an upregulat ion of Cdx2. To further document this observation, we analyzed a serie s of Caco2 clones in which the production of laminin-alpha 1 chain is differentially inhibited by antisense RNA, We found a positive correla tion between the level of Cdx2 expression, that of endogenous laminin- alpha 1 chain mRNA and that of sucrase-isomaltase expression in these cell lines. Taken together, these results suggest (a) that Cdx1 and Cd x2 homeobox genes play distinct roles in the intestinal epithelium, (b ) that Cdx2 provokes pleiotropic effects triggering cells towards the phenotype of differentiated villus enterocytes, and (c) that Cdx2 expr ession is modulated by basement membrane components. Hence, we conclud e that Cdx2 plays a key role in the extracellular matrix-mediated inte stinal cell differentiation.