EXPRESSION OF AN ENDOGENOUS ASIALOGLYCOPROTEIN RECEPTOR IN A HUMAN INTESTINAL EPITHELIAL-CELL LINE, CACO-2

We have previously shown that rat asialoglycoprotein receptor expresse d in the intestine and liver differ in mRNA size, cell surface distrib ution, 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 recepto r were detected in Caco-2 cells by Western blots using subunit-specifi c antisera raised against the hepatic receptor. Antigenic receptor lev el in fully differentiated Caco-2 cells was approx. 1/3 to 1/2 the lev el 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 sam e as that in HepG2 cells, due to differences in N-linked glycosylation . Consistent with this finding, Northern blot analysis showed that rec eptor mRNA in the two cell types was of identical size. In pulse-chase experiments H1 was first detected as a 'high-mannose' precursor (40 k Da) in Caco-2 cells that was converted to mature H1 (43 kDa) with a ha lf-life of approx. 60 min. Antigenic levels of H1 and H2 in undifferen tiated Caco-2, cells were low, but increased rapidly during cell diffe rentiation, reaching a peak level at 7 days after confluence. Immunocy tochemical staining and domain-selective cell surface biotinylation as says showed that the ASGP-R was predominantly localized in the basolat eral domain. The receptor in Caco-2 cells was capable of mediating spe cific uptake and degradation of [I-125]asialoorosomucoid. The ligand u ptake capacity of the basolateral surface of was approx. 10-fold highe r than the apical. These characteristics (Hi subunit and basolateral p redominance) of the receptor in Caco-2 cells, resembles the hepatic re ceptor. We conclude that Caco-2 cells endogenously express an ectopic hepatic-type functional asialoglycoprotein receptor.