Suppression of mTORC1 activity in senescent Ras-transformed cells neither restores autophagy nor abrogates apoptotic death caused by inhibition of MEK/ERK kinases.

Elena Y Kochetkova,Galina I Blinova,Olga A Bystrova,Tatiana V Pospelova,Marina G Martynova,Valeriy A Pospelov

doi:10.18632/aging.101686

Abstract

Autophagy is conservative catabolic process that degrades organelles, in particular, mitochondria, and misfolded proteins within the lysosomes, thus maintaining cellular viability. Despite the close relationship between mitochondrial dysfunction and cellular senescence, it is unclear how mitochondria damage can induce autophagy in senescent cells. We show that MEK/ERK suppression induces mitochondria damage followed by apoptosis of senescent Ras-expressing cells. To understand the role of persistent mTORC1 signaling in breaking the cAMPK-induced autophagy caused by mitochondrial damage, we inhibited mTORС1 with low concentrations of pp242. mTORC1 suppression neither restores the AMPK-induced autophagy nor decreases the level of apoptosis upon MEK/ERK inhibition. We discovered the existence of an alternative autophagy-like way that partially increases the viability of senescent cells under suppressed mTORC1. The pp242-treated cells survive due to formation of the non-autophagous LC3-negative vacuoles, which contain the damaged mitochondria and lysosomes with the following excretion the content from the cell. MEK/ERK activity is required to implement this process in senescent cells. Senescent cells exhibit distinctive spatial distribution of organelles and proteins that provides uncoupling of final participants of autophagy. We show that this feature stops the process of cytoprotective autophagy in response to MEK/ERK suppression, thus allowing selective elimination of senescent Ras-expressing cells.

Highlights

Senescent cells are characterized by cell cycle arrest, an increase in cell size and mitochondrial mass together with mitochondrial dysfunction [1]
MTORC1 suppression does not increase viability of senescent cells treated by mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) inhibitor
We questioned whether mammalian Target of Rapamycin Complex 1 (mTORC1) suppression would rescue viability of senescent cells exposed to MEK/ERK inhibition

Summary

INTRODUCTION

Senescent cells are characterized by cell cycle arrest, an increase in cell size and mitochondrial mass together with mitochondrial dysfunction [1]. Lysosomes co-localize with mTORC1 in perinuclear region of senescent cells [22,23,24], mTORC1 is constitutively active, regardless of growth factors and nutrients supply [23]. It is unclear how mTORC1 remains constitutively active and what mechanisms provide “anchoring” the of lysosome-mTORC1 complex nearby nucleus. MTORC1 suppression by pp242 in senescent Ras–transformed cells results in mitochondria damage and an increase in lysosomal activity. Inhibition of mTORC1 in senescent cells is able to activate a rescue mechanism reminiscent of the process of accumulation of autophagic bodies in a special digestive vacuole during yeast starvation. Senescent cells require MEK/ERK activity to implement this process, as MEK1,2 inhibitor-treated cells fail to complete the rescue mechanism and die

RESULTS

DISCUSSION

MATERIALS AND METHODS