We have taken advantage of the presence of hepatic receptors for galactose-terminal (asialo-)glycoproteins to achieve targeted rescue of differentiated hepatocytes from acetaminophen-induced toxicity in uitro. To accomplish this, a conjugate was formed by covalent coupling of N-acetylcysteine (an acetaminophen antagonist) to galactose-terminal (asialo-)fetuin. We used two human hepatocyte-derived cell lines to test our targeted-rescue method: Hep G2 cells are capable of receptor-mediated endocytosis of galactose-terminal glycoproteins and PLC/PRF/5 cells are not. In the presence of acetaminophen alone, both cell lines demonstrated a similar concentration-dependent sensitivity. Growth rates of both cell lines became normal when N-acetylcysteine was administered in equimolar quantities with acetaminophen indicating that both cell lines had the potential of responding to the antagonist. When asialofetuin-N-acetylcysteine conjugate was given to both cell lines in the presence of acetaminophen, PLC/PRF/S, receptor (-) cells failed to respond. However, Hep G2, receptor (+) cells treated with asialofetuin-N-acetylcysteine conjugate under identical conditions, increased their populations and eventually reached confluence. Control conjugate fetuin-N-acetylcysteine as well as asialofetuin alone had no effect on either cell line.
Current chemotherapy for hepatocellular carcinoma has not enjoyed much clinical success (1). This has been due, in part, to a lack of specificity of these agents resulting in damage to normal as well as malignant cells. Although in some cases side effects can be reduced by administration of antagonists (21, the beneficial effects are often also reduced by this maneuver.
Therefore, in order to be effective, treatment of hepatocellular carcinoma, and of cancer in general, requires that advantage be taken of differences between normal and malignant cells. One such difference is the ability of normal, differentiated cells, and inability of most transformed cells, to recognize and internalize certain glycoproteins. Such glycoproteins are bound by specific sites,