Metabolism of [3H]leupeptin by rat liver

Leupeptin causes an accumulation of lipofuscin-like substances in liver cells of young rats

Leupeptin, a thiol protease inhibitor, has previously been shown to cause a dense accumulation of substances resembling age pigment and called ceroid-lipofuscin, in brain cells of young rats. Thus far, however, attempts to produce age pigments in hepatocytes of normal young rats with protease inhibitor(s) have not been successful. The present study provides the first demonstration that leupeptin induces lipofuscin-like substances in normal young rat hepatocytes. Male Fischer-344 rats (age 4-6 weeks) were continuously infused with leupeptin or saline i.p. for 2 weeks by an osmotic minipump (dosage, 1-50 mg/100 g per day). Liver tissues were then examined by light, fluorescence and electron microscopy. Both hepatocytes and non-parenchymal cells of livers treated with leupeptin, but not saline, showed a dense accumulation of pigments which stained deeply with toluidine blue, were PAS-positive and were brightly autofluorescent. After UV excitation the pigments had an emission spectrum with a broad peak at 480-540 nm extending to 650 nm resembling the spectrum of age pigment from livers of normal aged rats. Electron microscopic examination revealed numerous lipofuscin-like deposits with heterogeneous morphology in the cytoplasm of both hepatocytes and non-parenchymal cells; lipid and myelin-like bodies were also present in hepatocytes. The results indicate that the perturbation of proteolytic activity in liver by leupeptin causes an accumulation of substances which by several criteria resemble lipofuscin. These results thus provide further support for the 'Protease Inhibitor Model of Lipofuscin Formation' as well as a potential experimental model for studying hepatocellular aging processes.

Morphological, physiological and biochemical alterations in livers of rodents induced by protease inhibitors: a comparison with old livers

A "Protease inhibitor model of aging" has been proposed primarily based on observations on brain tissues exposed to a thiol protease inhibitor, leupeptin (Ivy et al., 1984a). In order to validate this model in terms of a mechanism of cellular aging, as well as of lipofuscin formation in particular, attempts have been made to induce lipofuscin in hepatocytes in young rodent (rat and mouse) livers by continuous i.p. infusion of two different thiol protease inhibitors, leupeptin and E-64C. With doses of leupeptin higher than 1.0 mg/100g/day for 2 wks, a fine granular lipofuscin-like deposition with distinct yellowish-green fluorescence was induced in young rat hepatocytes. The deposition became greater in degree with increasing leupeptin doses. In Kupffer cells and other endothelial cells, fluorescent granules were also induced. In contrast to rat livers, lipofuscin-like pigments induced in hepatocytes in mice were much less, even with a higher dose (20 mg/100 g/day). E-64C also induced the accumulation of lipofuscin-like pigments at a dose of 5 mg/100 g/day, their characteristics being very similar to those induced by leupeptin, but the accumulation being smaller in degree. The fluorescence of leupeptin induced lipopigments was yellowish-green having a peak around 520 nm in emission profile, closely resembling that observed in old rat livers. The hepatobiliary transport functions such as biliary transport maximum (Tm) for sulfobromophthalain and the biliary recovery of iv injected ouabain which are known to decline with age tended to decline in young (6-wk-old) rats administered with leupeptin at a dose of 5 mg/100 g/day for 2 wks. On the other hand, dolichol concentration in leupeptin treated livers was not increased in comparison to control livers, whereas in old rat livers, the dolichol concentration was more than 2 times greater than in young livers. A clear-dose-dependent deposition of ceroid-lipofuscin induced in young rodent livers by protease inhibitors strongly suggests that the "Protease inhibitor model" is generally valid not only for the brain but for other tissues such as the liver, and for two different thiol protease inhibitors.