Impaired lysosomal acidity maintenance in acid lipase-deficient cells leads to defective autophagy
Takahito Moriwaki, Seigo Terawaki, Takanobu Otomo
Abstract
The lysosome is an acid organelle that contains a variety of hydrolytic enzymes and plays a significant role in intracellular degradation to maintain cellular homeostasis. Genetic variants in lysosome-related genes can lead to severe congenital diseases, such as lysosomal storage diseases. In the present study, we investigated the impact of depleting lysosomal acid lipase A (LIPA), a lysosomal esterase that metabolizes esterified cholesterol or triglyceride, on lysosomal function. Under nutrient-rich conditions, LIPA gene knockout ( LIPA KO ) cells exhibited impaired autophagy, whereas, under starved conditions, they showed normal autophagy. The cause underlying the differential autophagic activity was increased sensitivity of LIPA KO cells to ammonia which was produced from L-glutamine in the medium. Further investigation revealed that ammonia did not affect upstream signals involved in autophagy induction, autophagosome-lysosome fusion, and hydrolytic enzyme activities in LIPA KO cells. On the other hand, LIPA KO cells showed defective lysosomal acidity upon ammonia loading. Microscopic analyses revealed that lysosomes of LIPA KO cells enlarged, whereas the amount of lysosomal proton pump V-ATPase did not proportionally increase. Since the enlargement of lysosomes in LIPA KO cells was not normalized under starved conditions, this is the primary change that occurred in the LIPA KO cells, and autophagy was affected by impaired lysosomal function under the specific conditions. These findings expand our comprehension of the pathogenesis of Wolman's disease, which is caused by a defect in the LIPA gene, and suggest that conditions, such as hyperlipidemia, may easily disrupt lysosomal functions.