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Oblongifolin C suppresses lysosomal function independently of TFEB nuclear translocation

Authors: Man Wu1,2, Yuan-zhi Lao1,2, Hong-sheng Tan1,2, Guang Lu3, Yi Ren3, Zhao-qing Zheng1,2, Juan Yi3, Wen-wei Fu1,2, Han-ming Shen3, Hong-xi Xu1,2
1 School of Pharmacy, Shanghai University of Traditional Chinese Medicine, Shanghai 201203, China
2 Engineering Research Center of Shanghai Colleges for TCM New Drug Discovery, Shanghai 201203, China
3 Department of Physiology, Yong Loo Lin School of Medicine, National University of Singapore, Singapore, Singapore
Correspondence to: Han-ming Shen: phsshm@nus.edu.sg, Hong-xi Xu: xuhongxi88@gmail.com,
DOI: 10.1038/s41401-018-0167-7
Received: 25 April 2018
Accepted: 29 August 2018
Advance online: 17 October 2018

Abstract

Lysosomes are the terminal organelles of the autophagic-endocytic pathway and play a key role in the degradation of autophagic contents. We previously reported that a natural compound oblongifolin C (OC) increased the number of autophagosomes and impaired the degradation of P62, most likely via suppression of lysosomal function and blockage of autophagosome-lysosome fusion. However, the precise mechanisms of how OC inhibits the lysosome-autophagy pathway remain unclear. In the present study, we investigated the effect of OC on transcription factor EB (TFEB), a master regulator of lysosomal biogenesis, lysosomal function and autophagy. We showed that treatment with OC (15 μM) markedly enhanced the nuclear translocation of TFEB in HeLa cells, concomitantly reduced the interaction of TFEB with 14-3-3 proteins. We further demonstrated that OC caused significant inhibition of mTORC1 along with TFEB nuclear translocation, and OC-mediated TFEB nuclear translocation was dependent on mTORC1 suppression. Intriguingly, this increased nuclear TFEB was accompanied by reduced TFEB luciferase activity, increased lysosomal pH and impaired cathepsin enzyme activities. In HeLa cells, treatment with OC (7.5 μM) resulted in about 30% of cell death, whereas treatment with hydroxycitrate, a caloric restriction mimetic (20 μM) did not affect the cell viability. However, cotreatment with OC and hydroxycitrate caused significantly great cytotoxicity (>50%). Taken together, these results demonstrate that inhibition of lysosome function is mediated by OC, despite evident TFEB nuclear translocation.
Keywords: oblongifolin C; autophagy; TFEB; mTORC1; lysosome

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