IRON LOADING OF ISOLATED RAT HEPATOCYTES INHIBITS ASIALOGLYCOPROTEIN RECEPTOR DYNAMICS AND INDUCES FORMATION OF RAT HEPATIC LEPTIN-1 (RHL-1) OLIGOMERS

The major subunit [rat hepatic lectin-l (RHL-1)] of the asialoglycopro tein (ASGP) receptor mediates endocytosis of the iron-binding protein lactoferrin (Lf) by isolated rat hepatocytes, yet iron loading of cult ured adult rat hepatocytes increases the binding and endocytosis of Lf while greatly inhibiting the uptake of desialylated ligand. In the pr esent study, we determined whether the iron-induced Lf-binding site is RHL-1 and examined the nature of the iron-induced block in ASGP recep tor endocytic function. Isolated rat hepatocytes increased their non-h aem iron content from 70 to 470 p.p.b. following incubation with ferri c ammonium citrate (less than or equal to 100 mu g/ml). These conditio ns blocked internalization of I-125-asialo-orosomucoid (ASOR) by appro ximate to 90 % but increased I-125-Lf endocytosis by 40 %. ASOR and an ti-RHL-1 sera blocked the binding and endocytosis of I-125-Lf On contr ol cells but not on iron-loaded cells, indicating that the iron-induce d Lf-binding site on hepatocytes is not RHL-1. Iron-loading of hepatoc ytes in the presence or absence of excess ASOR did not significantly a lter the number of active ASGP receptors on the cell surface. In contr ast, iron-loading decreased the number of active intracellular recepto rs by 40 % and blocked the uptake of I-125-ASOR prebound to the cells by approximate to 80 %. Under these conditions, we found an iron-depen dent evolution of 88 and 140 kDa RHL-1-containing, beta-mercaptoethano l-sensitive multimers that constituted up to 34 and 23 %, respectively , of total immunodetectable RHL-1. We propose that iron-induced format ion of cystinyl-linked RHL-1-containing multimers inhibits ASGP recept or movement between cell surface and interior and disrupts acylation o f intracellular receptors.