ORGANIC ANION TRANSPORT IN HEPG2 CELLS - ABSENCE OF THE HIGH-AFFINITY, CHLORIDE-DEPENDENT TRANSPORTER

In previous studies, we identified a 55 kD organic anion-binding protein in liver cell sinusoidal plasma membrane subfractions. Other investigators identified another 55 kD bromosulfophthalein/bilirubin binding protein on the surface of rat hepatocytes and HepG2 cells and suggested that this protein served as a transporter for these ligands. In this study, transport of S-35-sulfobromophthalein by the human hepatoma cell line, HepG2, was quantified in the presence and absence of bovine serum albumin to further clarify the possible function of these plasma membrane binding proteins. In contrast to results in normal rat hepatocytes, virtually no uptake of S-35-sulfobromophthalein by HepG2 cells in the presence of bovine serum albumin was found. In the absence of albumin, HepG2 cells expressed temperature-dependent uptake of S-35-sulfobromophthalein. However, the high-affinity Cl--dependent sulfobromophthalein transport that characterizes normal rat hepatocytes was absent, as indicated by an approximately 95-fold lower affinity and 170-fold higher capacity of HepG2 cells for sulfobromophthalein compared with previous results with rat hepatocytes. These results suggest that 55 kD sulfobromophthalein/bilirubin-binding protein on the liver cell surface differs from organic anion-binding protein and is not responsible for sulfobromophthalein extraction in the presence of albumin, although it may play some role in lower affinity transport by cells. Immunoblot analysis and metabolic labeling of HepG2 cells demonstrated synthesis of organic anion-binding protein. However, light microscopic immunocytochemistry and immunoprecipitation of surface iodinated rat hepatocytes and HepG2 cells with antibody to a recombinant organic anion-binding protein fusion protein indicated absence of organic anion-binding protein on the surface of HepG2 cells. Because cell surface organic anion-binding protein and the mitochondrial F1-ATPase beta-subunit are immunologically highly cross-reactive, the presence of intracellular but not cell surface organic anion-binding protein immunoreactivity in HepG2 cells suggests that the mitochondrial and cell surface forms of this protein are independently regulated. These studies also indicate that demonstration of sulfobromophthalein transport alone cannot be used as evidence for the presence of the high-affinity transporter.