Lysosome as a Central Hub for Rewiring PH Homeostasis in Tumors

Abstract

Cancer cells generate large quantities of cytoplasmic protons as byproducts of aberrantly activated aerobic glycolysis and lactate fermentation. To avoid potentially detrimental acidification of the intracellular milieu, cancer cells activate multiple acid-removal pathways that promote cytosolic alkalization and extracellular acidification. Accumulating evidence suggests that in addition to the well-characterized ion pumps and exchangers in the plasma membrane, cancer cell lysosomes are also reprogrammed for this purpose. On the one hand, the increased expression and activity of the vacuolar-type H⁺-ATPase (V-ATPase) on the lysosomal limiting membrane combined with the larger volume of the lysosomal compartment increases the lysosomal proton storage capacity substantially. On the other hand, enhanced lysosome exocytosis enables the efficient release of lysosomal protons to the extracellular space. Together, these two steps dynamically drive proton flow from the cytosol to extracellular space. In this perspective, we provide mechanistic insight into how lysosomes contribute to the rewiring of pH homeostasis in cancer cells.

Keywords: V-ATPase; lysosomal exocytosis; lysosome; pH regulation.

Figure 1

Structure of V-ATPase complex on lysosomal membranes and V-ATPase assembly machinery. Subunits A to H form V1 domain which hydrolyzes ATP. Subunits a to f form V0 domain. Protons are transported by V-ATPase against the electrochemical gradient to maintain proton equilibrium across lysosomal membranes. Two glycolytic enzymes (phosphofructokinase & aldolase) and RAVE are required for V-ATPase reassembly upon glucose readdition in yeast.

Figure 2

Lysosomes contribute to the maintenance of pH gradient reversal in three means: (1) increased proton storage capacity; (2) overactivated exocytosis; (3) plasma membrane-localized V-ATPase.