Summary The syndecans, a family of cell-surface heparan sulphate proteoglycans, have been proposed to mediate cellular interactions with extracellular effector molecules, such as growth factors and components of the extracellular matrix, during critical phases of development.Transcripts of all four syndecans are expressed at varying levels in the developing rat intestine and in a series of immature rat intestinal epithelial cell lines. In addition, we report the novel finding that, in the intestinal epithelial cell lines, expression of syndecan-1 transcript is up-regulated by transformation with activated H-ras. This is in contrast to other cell lines in which ras transformation is associated with a decrease in syndecan-1 levels. The observed increase in the syndecan-1 occurs as a result of increased transcription and can be correlated with the degree of transformation of the IEC-18 cells. Transformation is also associated with a decrease in apparent molecular weight and increased shedding of the proteoglycan into the culture medium. Increased shedding of syndecan-1 into the culture medium after transformation with H-ras may contribute to the disruption of proteoglycan interactions with the extracellular matrix, leading to alterations in cell adhesion and organization.Keywords: proteoglycan; syndecan; ras transformation; intestinal epithelial cell In the rat intestine, dramatic changes over the last few days of gestation result in the reorganization of a tubular structure into the mature crypt-villus structure in which the mesenchyme is lined by a simple columnar epithelium (Moog, 1979;Trier and Moxey, 1979;Madara et al, 1981). Underlying this morphogenesis and the maintenance of the adult crypt-villus structure are reciprocal interactions between the epithelium and mesenchyme (Kedinger et al, 1986) that involve recognition phenomena between cell-surface molecules and components of the pericellular environment. Membrane-bound proteoglycans contribute to the regulation of cell behaviour through interactions with extracellular matrix components or by acting as co-receptors for biologically active peptides (Klagsbrun and Baird, 1991;Rapraeger et al, 1991;Yayon et al, 1991;Aviezer et al, 1994).Two gene families encoding cell-surface heparan sulphate proteoglycans (HSPG) have been identified on the basis of sequence similarities between their core proteins: the glypicanrelated integral membrane proteoglycans (PGs) and syndecan-like integral membrane PGs (David, 1993). We have reported that OCI-5, a glypican-related HSPG (Filmus et al, 1995), is involved in intestinal development (Filmus et al, 1988). Although syndecans-1 and -4 have been detected in the adult intestine (Kim et al, 1994), the involvement of the syndecan family in intestinal development or differentiation is largely unknown. The role of syndecan-1 in epithelial-mesenchymal interactions during morphogenesis of a variety of tissues raises the possibility of a similar role in the developing intestine. The recent cloning of the four members of the syndecan...