On the degradability and exocytosis of ceroid/lipofuscin in cultured rat cardiac myocytes.
The accumulation of lipofuscin (LF)--a polymeric, electron-dense, autofluorescent substance--within postmitotic cells is a characteristic manifestation of aging. It is generally believed that LF is undegradable and formed due to peroxidative alterations of various macromolecules under 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 electron-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 normal culture conditions. In contrast, prolonged leupeptin treatment results in an accumulation of dense lysosomes that also show LF-typical autofluorescence. This autofluorescent material remains in the cells after the end of leupeptin action. The results suggest that: (i) a certain amount of time is needed for autophagocytosed material to become peroxidized, 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 modification of autophagocytosed material; (iii) mature LF is probably not subjected to either degradation or exocytosis.
Terman
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A
Department of Pathology II, Faculty of Health Sciences, Linköping University, Sweden.
Brunk
U T
UT
D000818:Animals; D002478:Cells, Cultured; D002566:Ceroid / Q000378:metabolism; D005089:Exocytosis; D007976:Leupeptins / Q000494:pharmacology; D008062:Lipofuscin / Q000378:metabolism*; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013997:Time Factors
D002566:Ceroid; D007976:Leupeptins; D008062:Lipofuscin; C032854:leupeptin