Mechanisms of lipofuscinogenesis: Effect of the inhibition of lysosomal proteinases and lipases under varying concentrations of ambient oxygen in cultured rat neonatal myocardial cells

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Abstract
The objective of this study was to analyse the relative roles of oxidative stress and lysosomal lytic enzymes in lipofuscin formation in cultured neonatal rat cardiac myocytes. Myocytes were exposed to E-64 (an inhibitor of lysosomal cathepsins L. D and H), netilmicin (an inhibitor of lysosomal phospholipases A1 and C) and leupeptin (an inhibitor of cytosolic and lysosomal thiolproteinases) under varied conditions of oxidative stress (20% and 40% ambient oxygen) for up 14 days. Lipofuscin was quantified by microspectrofluorometry. The amount of lipofuscin accumulation was enhanced by the lytic enzyme inhibitors as well as by the increase in the ambient oxygen concentration. However, the effects of enzyme inhibitors was less obvious under 40% ambient oxygen than under 20% oxygen. Data are interpreted as suggesting that, under high levels of oxidative stress, intralysosomal peroxidative changes related to lipofuscin formation occur so rapidly that lytic activity assumes a minor role in lipofuscinogenesis whereas, under low oxidative stress, inhibition of lytic activity makes a greater contribution to lipofuscinogenesis by allowing a longer period of time for peroxidative changes. The results further substantiate the hypotheses that (a) lipofuscinogenesis is strongly related to oxidative stress, and (b) lipofuscin forms intralysosomally as a result of processes involving incomplete degradation of heterophagocytosed and or autophagocytosed material.