Expression of an endogenous asialoglycoprotein receptor in a human intestinal epithelial cell line, Caco-2

We have previously shown that rat asialoglycoprotein receptor expressed in the intestine and liver differ in mRNA size, cell surface distribution, and ratio of compositional protein subunits. In this study, we examined a well characterized intestinal epithelial cell line, Caco-2, as a potential model for studying endogenous receptor in a polarized cell line. Both subunits H1 and H2 of human asialoglycoprotein receptor were detected in Caco-2 cells by Western blots using subunit-specific antisera raised against the hepatic receptor. Antigenic receptor level in fully differentiated Caco-2 cells was approx. 1 3 to 1 2 the level of hepatic HepG2 cells and H1 was the dominant subunit in both cell lines. The apparent size of H1 and H2 in Caco-2 cells was not the same as that in HepG2 cells, due to differences in N-linked glycosylation. Consistent with this finding, Northern blot analysis showed that receptor mRNA in the two cell types was of identical size. In pulse-chase experiments H1 was first detected as a 'high-mannose' precursor (40 kDa) in Caco-2 cells that was converted to mature H1 (43 kDa) with a half-life of approx. 60 min. Antigenic levels of H1 and H2 in undifferentiated Caco-2 cells were low, but increased rapidly during cell differentiation, reaching a peak level at 7 days after confluence. Immunocytochemical staining and domain-selective cell surface biotinylation assays showed that the ASGP-R was predominantly localized in the basolateral domain. The receptor in Caco-2 cells was capable of mediating specific uptake and degradation of [¹²⁵I]asialoorosomucoid. The ligand uptake capacity of the basolateral surface of was approx. 10-fold higher than the apical. These characteristics (H1 subunit and basolateral predominance) of the receptor in Caco-2 cells, resembles the hepatic receptor. We conclude that Caco-2 cells endogenously express an ectopic hepatic-type functional asialoglycoprotein receptor.