Relationship of ricin and modeccin hepatotoxicity to accumulation rates by parenchymal and non-parenchymal liver cells

Abstract

The plant glycoproteins ricin, from castor beans (Ricinus communis), and modeccin, from the roots of the modecca flower (Adenia digitata), are both extremely toxic to eukaryote cells by a mechanism thought to be related to the irreversible inhibition of cellular protein synthesis (Olsnes & Pihl, 1976). The toxins are similar in that they comprise two polypeptide chains (A and B) and act by B-chain binding to cells followed by entry into the cytosol of the A chain, which inactivates the 60s ribosomal subunit thus arresting protein synthesis. However, in vim modeccin is a more potent toxin than ricin, especially to the liver where modeccin primarily produces hepatic damage to the parenchymal cells (Sperti et a[., 1979) and ricin causes lesions to the non-parenchymal sinusoidal cells which are more severe and produced earlier than in the parenchyma (Derenzini et al., 1976). Our earlier studies have shown that ricin is actively accumulated both in oivo and in vitro by hepatic nonparenchymal cells, which together with the extreme sensitivity to ricin of protein synthesis in these cells, probably explains the observed hepatotoxicity of this glycoprotein (Skilleter et al., 1981). Accordingly, the purpose of the present investigation was to compare the relative rates of modeccin uptake by parenchymal and non-parenchymal cells, both in vivo and in vitro, with those obtained from ricin to ascertain if this might explain differences in the liver toxicity of the two glycoproteins. Modeccin and ricin were purified as described previously (Nicolson et al., 1974; Gasperi-Campani et a / . , 1978) and then radiolabelled with NaltSI (Skilleter et al., 1981). Preliminary experiments established that in rats (Wistar, LAC P) 80-90% of an intravenous dose of either '251-labelled glycoprotein (1 pg/kg) was cleared from the blood within 10-30min, although r ich was more rapidly removed ( 1 5 min) than modeccin (1 5-20min). Little change in blood radioactive label content was observed over the next 1-2h. At 30min the amount of the toxins recovered in the liver corresponded to 25-30% and 1619% of the injected ricin and modeccin respectively. Following the intravenous administration of the more lethal dose of lOpg/kg the relative hepatocellular distribution of 251-labelled material was monitored at 30min and 2h in parenchymal and nonparenchymal cells isolated by a collagenase perfusion system described previously (Skilleter & Price, 1978). In the case of modeccin, at 30min the parenchymal and nonparenchymal cell contents were estimated at approximately 90 and 32Opg/1O6 cells respectively, compared with corresponding values for ricin of 150 and 92Opg/1O6 cells. Both sets of values decreased by 60% within 2 h. These results show that in vivo accumulation of ricin by non-parenchymal cells was 3 times that observed for modeccin, whereas in the same period uptake of ricin by parenchymal cells was only 70% greater than that of modeccin. Ricin and modeccin uptake by rat liver cells in vitro was examined in primary monolayer parenchymal and nonparenchymal cell cultures prepared according to the methods we have reported previously (Skilleter et al., 1981). Fig. 1 shows that although both toxins were taken up more readily by non-parenchymal cells the rates of modeccin uptake were comparable in both liver cell types, whereas the 35