Autophagy controls the kinetics and extent of mitochondrial apoptosis by regulating PUMA levels - PubMed (original) (raw)

Autophagy controls the kinetics and extent of mitochondrial apoptosis by regulating PUMA levels

Jacqueline Thorburn et al. Cell Rep. 2014.

Abstract

Macroautophagy is thought to protect against apoptosis; however, underlying mechanisms are poorly understood. We examined how autophagy affects canonical death receptor-induced mitochondrial outer membrane permeabilization (MOMP) and apoptosis. MOMP occurs at variable times in a population of cells, and this is delayed by autophagy. Additionally, autophagy leads to inefficient MOMP, after which some cells die through a slower process than typical apoptosis and, surprisingly, can recover and divide afterward. These effects are associated with p62/SQSTM1-dependent selective autophagy causing PUMA levels to be kept low through an indirect mechanism whereby autophagy affects constitutive levels of PUMA mRNA. PUMA depletion is sufficient to prevent the sensitization to apoptosis that occurs when autophagy is blocked. Autophagy can therefore control apoptosis via a key regulator that makes MOMP faster and more efficient, thus ensuring rapid completion of apoptosis. This identifies a molecular mechanism whereby cell-fate decisions can be determined by autophagy.

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Figures

Figure 1. Autophagy regulates the timing of MOMP and PUMA levels

(A) siRNA targeting ATG12 in HeLa cells inhibits trehalose-induced (Tre) autophagy shown by reduced accumulation of LC3-II after treatment with lysosomal protease inhibitors Pepstatin and E64D. (B) An example of complete and rapid MOMP visualized in HeLa cells stably expressing IMS-RP. Loss of punctate IMS-RP staining was used to determine when MOMP took place after TRAIL treatment. (C) Trehalose treatment delays MOMP after TRAIL treatment (25ng/ml) and this delay is blocked by autophagy inhibition using ATG12 siRNA. Cells were imaged individually after treatment with or without trehalose and siRNA as indicated. Bars indicate the mean time to MOMP, p values by Student’s t- test. (D) Western blot analysis of time course of TRAIL-treated HeLa cells −/+ Trehalose to analyze cleavage of initiator caspase-8 or BID. Autophagy induction by trehalose did not delay caspase-8 or Bid cleavage. (E) Autophagy inhibition by shRNA knockdown of ATG5, ATG7, ATG12, Vps34 or p62/SQSTM1 all cause an increase in PUMA levels. (F) PUMA knockdown is sufficient to delay MOMP in response to TRAIL treatment. Cells were infected with shRNA targeting PUMA in the presence or absence of trehalose as indicated and time to MOMP determined by imaging individual cells. PUMA knockdown is sufficient delay MOMP similarly to trehalose. Bars indicate mean time to MOMP, p values were calculated by t-test, ns indicates p>0.05. See also Supplementary Figures 1 and 2.

Figure 2. Autophagy is associated with slow cellular contraction after MOMP and cellular recovery

(A) Images from time-lapse series showing cellular contraction after MOMP displaying two distinct phenotypes. An example of rapid contraction and cellular breakup is shown in the upper series of images. An example of slower contraction after MOMP. (B) Quantification of the percent of cells displaying slow contraction after MOMP defined as cell area ≥60% of the original area 20 minutes after MOMP/incomplete MOMP. Cells were treated with trehalose in the presence or absence of ATG5 or ATG7 shRNAs as indicated. Trehalose increases the number of slow contracting cells and this is abolished by autophagy inhibition. (C) Continued monitoring of fast contracting cells after MOMP shows irreversible cell breakup and death from which they cannot recover. (D) Continued monitoring of slow contracting cells shows that they can recover normal morphology hours after MOMP.

Figure 3. PUMA depletion is sufficient to increase the likelihood of slow cellular contraction, cell recovery and can allow division after MOMP

(A) Quantitation of cell area 20 minutes after MOMP in cells treated with trehalose in the presence or absence of PUMA knockdown. The percentage of cells displaying slow contraction is increased by PUMA knockdown alone. (B) PUMA knockdown cells displaying slow contraction and recovery can go on to divide. Images from a time lapse experiment showing two cells that underwent MOMP followed by slow contraction, recovery of normal morphology and then cell division. See also Supplemental Movies 1 and 2.

Figure 4. Autophagy regulation of long term cell viability after TRAIL-induced apoptosis is explained by regulation of PUMA

(A) Quantitation of cell viability by continuous monitoring of mixed populations of control (Green) or shRNA knockdown (Red) cells. HeLa cells were marked with nuclear GFP or mCherry and cell viability after treatment with different doses of TRAIL was determined by continual monitoring using the IncuCyte system. shRNA knockdown of BID completely protects against TRAIL induced death. (B) shRNA knockdown of PUMA partially inhibits TRAIL induced apoptosis. (C) shRNA knockdown of ATG7 to inhibit autophagy potentiates TRAIL-induced apoptosis. (D) shRNA knockdown of Vps34 to inhibit autophagy potentiates TRAIL-induced apoptosis. (E) Cell viability quantified by the IncuCyte at different doses of TRAIL was determined in matched control, ATG7 or ATG7/PUMA knockdown cells. Autophagy inhibition by ATG7 knockdown increases TRAIL induced apoptosis and this is rescued by simultaneous knockdown of PUMA. At higher doses of TRAIL, the protective effect of autophagy inhibition is reduced. (F) Cell viability quantified by the IncuCyte in matched control, ATG12 or ATG12/PUMA knockdown cells. Autophagy inhibition by ATG12 knockdown increases TRAIL induced apoptosis that is rescued by simultaneous knockdown of PUMA. See also Supplemental Figures 3 and 4.

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Autophagy controls the kinetics and extent of mitochondrial apoptosis by regulating PUMA levels - PubMed (original) (raw)