Vacuolin‐1 inhibits autophagy by activating rab5 (737.7)

Autophagy is a catabolic lysosomal degradation process essential for cellular homeostasis and cell survival. Dysfunctional autophagy has been associated with wide ranges of human diseases, e.g. cancer and neurodegenerative diseases. A large number of small chemicals that modulate autophagy have been widely used to dissect this process and some of them, e.g. chloroquine (CQ), might be ultimately applied to treat a variety of autophagy associated human diseases. Here we found that the small chemical vacuolin‐1 potently and reversibly inhibited the fusion between autophagosomes and lysosomes in mammalian cells, thereby inducing the accumulation of autophagosomes. Interestingly, vacuolin‐1 was less toxic but was at least 10 folds more potent in inhibiting autophagy compared to CQ. Vacuolin‐1 treatment also blocked the fusion between endosomes and lysosomes, resulting in a defect in general endosomal‐lysosomal degradation. Treatment of cells with vacuolin‐1 alkalinized lysosomal pH and decreased lysosomal Ca 2+ content, yet vacuolin‐1 had little effect on vacuolar ATPase activity. Instead, vacuolin‐1 treatment markedly activated Rab5 GTPase activity. Expression of a dominant negative mutant of Rab5A or Rab5A knockdown abolished vacuolin‐1 induced autophagosomal‐lysosomal fusion blockage, whereas expression of a constitutive active form of Rab5A inhibited the lysosomal‐autophagosomal fusion. These data indicate that vacuolin‐1 activates Rab5 to suppress autophagosomal‐lysosomal fusion. Vacuolin‐1 and its analogues present a novel class of drug that can potently and reversibly modulate autophagy. Grant Funding Source : This work was supported by Research Grant Council (RGC) grants (HKU 782709M, HKU 785911M, HKU 769912