ON THE DEGRADABILITY AND EXOCYTOSIS OF CEROID LIPOFUSCIN IN CULTURED RAT CARDIAC MYOCYTES/

The accumulation of lipofuscin (LF)-a polymeric, electron-dense, autof luorescent substance-within postmitotic cells is a characteristic mani festation of aging. II is generally believed that LF is undegradable a nd formed due to peroxidative alterations of various macromolecules un der intralysosomal autophagic degradation. We report here that a short -term exposure of cultured neonatal rat cardiac myocytes to the thiol protease-inhibitor leupeptin, causes an accumulation of numerous elect ron-dense autophagic lysosomes within the cells. Although very similar to LF by ultrastructure, these inclusions do not display LF-specific, yellow-orange autofluorescence when excited with blue light. Moreover , they rapidly disappear from the cells upon re-establishment of norma l culture conditions. In contrast, prolonged leupeptin treatment resul ts in an accumulation of dense lysosomes that also show LF-typical aut ofluorescence. This autofluorescent material remains in the cells afte r the end of leupeptin action. The results suggest that: (i) a certain amount of time is needed for autophagocytosed material to become pero xidized, autofluorescent and undegradable, i.e. to acquire properties typical of LF; (ii) protease-inhibition by itself does not lead to LF- formation but rather allows the prolonged time needed for oxidative mo dification of autophagocytosed material; (iii) mature LF is probably n ot subjected to either degradation or exocytosis. (C) 1998 Elsevier Sc ience Ireland Ltd.