Is Lipofuscin Eliminated from Cells?

Katz et al. ¹ recently concluded that the age-related increase of lipofuscin within the retinal pigment epithelium (RPE) “results from an imbalance in the rates of lipofuscin formation and its disposal rather than from a complete absence of a disposal mechanism.” This leads the authors to discuss the possibility of reversing lipofuscin accumulation by enhancing the disposal processes. 

Katz et al. base their concept on the observation that lipofuscin-like bodies, formed within RPE cells after a single intravitreal injection of the protease inhibitor leupeptin, gradually disappear over time. These bodies, and those observed by Ivy et al. ^{2 3} (who developed the protease inhibition model of lipofuscin/ceroid formation), do resemble lipofuscin in ultrastructural and autofluorescence properties. However, there is no proof that these inclusions (representing large auto-/heterophagic vacuoles) arising from short-term leupeptin treatment and lipofuscin (age pigment) are identical. 

We recently have investigated the consequences of short- and long-term leupeptin treatment of neonatal rat cardiac myocytes and density-inhibited human fibroblasts. ^{4 5} A short-term (24 hour) administration of leupeptin does induce formation of electron-dense (osmiophilic) inclusions showing some yellowish autofluorescence when excited with blue light, which superficially resemble the authentic lipofuscin inclusions that form slowly under natural conditions. However, the vacuoles induced by short-term leupeptin treatment disappear relatively rapidly (within 4 days) from the cells when the drug is withdrawn, as also observed by Dr. Katz and colleagues. ¹ In contrast, when the leupeptin exposure is continued for 2 weeks, a portion of these leupeptin-induced vacuoles acquire new properties, including enhanced autofluorescence and, ultrastructurally, a predominance of crystalloid and myelin-like structures. Moreover, this material does not disappear when the cells are returned to standard culture conditions for another 2 weeks. 

Therefore, when auto-/heterophagocytosed material stays for a long time within lysosomes, it undergoes transformation into mature and nondegradable lipofuscin/ceroid. This may occur through the intermediacy of slow processes such as oxidative modification of macromolecules within lysosomes, eventually leading to lipofuscin/ceroid formation. Prolonged (but not short-term) inhibition of intralysosomal degradation by leupeptin appears to provide sufficient time for such modification. ^{4 6} This interpretation also is consistent with the fact that the formation of nondegradable material (authentic lipofuscin/ceroid) is much accelerated when the leupeptin-treatment is combined with oxidative stress. ⁵ Reactive oxygen species form continuously during normal oxygen metabolism and—despite the activity of cellular antioxidant systems—mild oxidative stress seems to be an inevitable side effect of aerobic life. However, under normal conditions, the formation of lipofuscin is relatively slow in postmitotic cells, since lysosomal degradation occurs much faster than that of leupeptin-treated ones. 

In a number of studies, lipofuscin/ceroid has been shown to be undegradable and not exocytosed. ^{4 5 7} Of course, one might dispute the validity of in vitro experiments or possibly unique characteristics of the lipofuscin-ceroid that accumulates in different cell types. Nevertheless, the catabolism and/or disposal of authentic pigment has not been demonstrated convincingly. The reduction of lipofuscin/ceroid formation by pharmacologically decreasing oxidative stress may represent a more promising approach to the problem.