Macromitophagy, neutral lipids synthesis, and peroxisomal fatty acid oxidation protect yeast from “liponecrosis”, a previously unknown form of programmed cell death

Abstract

We identified a form of cell death called "liponecrosis." It can be elicited by an exposure of the yeast Saccharomyces cerevisiae to exogenous palmitoleic acid (POA). Our data imply that liponecrosis is: (1) a programmed, regulated form of cell death rather than an accidental, unregulated cellular process and (2) an age-related form of cell death. Cells committed to liponecrotic death: (1) do not exhibit features characteristic of apoptotic cell death; (2) do not display plasma membrane rupture, a hallmark of programmed necrotic cell death; (3) akin to cells committed to necrotic cell death, exhibit an increased permeability of the plasma membrane for propidium iodide; (4) do not display excessive cytoplasmic vacuolization, a hallmark of autophagic cell death; (5) akin to cells committed to autophagic death, exhibit a non-selective en masse degradation of cellular organelles and require the cytosolic serine/threonine protein kinase Atg1p for executing the death program; and (6) display a hallmark feature that has not been reported for any of the currently known cell death modalities-namely, an excessive accumulation of lipid droplets where non-esterified fatty acids (including POA) are deposited in the form of neutral lipids. We therefore concluded that liponecrotic cell death subroutine differs from the currently known subroutines of programmed cell death. Our data suggest a hypothesis that liponecrosis is a cell death module dynamically integrated into a so-called programmed cell death network, which also includes the apoptotic, necrotic, and autophagic modules of programmed cell death. Based on our findings, we propose a mechanism underlying liponecrosis.

Mechanism of liponecrosis, a distinct mode of programmed cell death

“…1 Our analysis suggested the following hypothesis: (1) exogenous POA can be incorporated into phospholipids (PL) that accumulate within cellular membranes and into neutral lipids (NL) that amass in lipid droplets (LD); (2) an excessive accumulation of POA-containing PL within cellular membranes is a pro-death process which triggers liponecrosis by creating the extreme cellular stress; (3) POA incorporation into NL is a pro-survival process which prevents the buildup of POA-containing PL within cellular membranes by reducing the flow of POA into PL synthesis; and (4) mitophagy, an Atg32p-driven selective autophagic degradation of dysfunctional mitochondria, 5,6 is a pro-survival process which sustains a population of functional mitochondria needed for POA incorporation into NL deposited within LD. 1 As a first step toward verifying this hypothesis, we compared lipidomes of wildtype (WT) and mitophagy-deficient atg32D cells exposed to various concentrations of POA. We found that an exposure of WT cells to POA used at a low final concentration of 0.05 mM causes: (1) an increase in the cellular levels of triacylglycerols (TAG), a major class of NL that are synthesized in the endoplasmic reticulum (ER) and then deposited within LD; and (2) a decrease in the cellular levels of PL (Fig.…”

“…We named this previously unknown PCD subroutine "liponecrosis"; it can be instigated by a short-term exposure of the yeast Saccharomyces cerevisiae to exogenous palmitoleic acid (POA). 1 We demonstrated that the liponecrotic mode of cell death exhibits all 3 key features that have been recommended as mandatory for a cell death mode to be called "PCD". [2][3][4] In fact, we found that liponecrotic PCD: (1) is a genetically programmed, regulated cellular process which can be accelerated or decelerated by genetic manipulations impairing functionality of only certain proteins -in particular proteins enabling the maintenance of functional mitochondria, metabolism of certain lipid classes or autophagic degradation of some cellular constituents; (2) operates as a series of consecutive cellular events triggered by POA and following each other in a certain order; and (3) is likely to provide some benefits for the survival of a population of yeast cells because it represents an age-related mode of PCD -i.e., the degree to which it reduces cell viability following an exposure to POA appears to progress with the chronological age of a yeast cell.…”

“…1 Moreover, yeast cells committed to liponecrotic PCD display an excessive accumulation of lipid droplets where non-esterified fatty acids are deposited in the form of neutral lipids; this hallmark feature of liponecrotic PCD has not been reported for any of the presently known PCD modalities. 1 Collectively, these findings imply that liponecrosis in yeast is a previously unknown mode of PCD. In this study, we investigated a mechanism underlying liponecrotic PCD subroutine.…”

“…[2][3][4] In fact, we found that liponecrotic PCD: (1) is a genetically programmed, regulated cellular process which can be accelerated or decelerated by genetic manipulations impairing functionality of only certain proteins -in particular proteins enabling the maintenance of functional mitochondria, metabolism of certain lipid classes or autophagic degradation of some cellular constituents; (2) operates as a series of consecutive cellular events triggered by POA and following each other in a certain order; and (3) is likely to provide some benefits for the survival of a population of yeast cells because it represents an age-related mode of PCD -i.e., the degree to which it reduces cell viability following an exposure to POA appears to progress with the chronological age of a yeast cell. 1 Importantly, yeast cells committed to liponecrotic PCD do not exhibit a combination of morphological traits and biochemical features characteristic of cells committed to any of the presently known apoptotic, autophagic or necrotic subroutines of PCD. 1 Moreover, yeast cells committed to liponecrotic PCD display an excessive accumulation of lipid droplets where non-esterified fatty acids are deposited in the form of neutral lipids; this hallmark feature of liponecrotic PCD has not been reported for any of the presently known PCD modalities.…”

“…1 Importantly, yeast cells committed to liponecrotic PCD do not exhibit a combination of morphological traits and biochemical features characteristic of cells committed to any of the presently known apoptotic, autophagic or necrotic subroutines of PCD. 1 Moreover, yeast cells committed to liponecrotic PCD display an excessive accumulation of lipid droplets where non-esterified fatty acids are deposited in the form of neutral lipids; this hallmark feature of liponecrotic PCD has not been reported for any of the presently known PCD modalities. 1 Collectively, these findings imply that liponecrosis in yeast is a previously unknown mode of PCD.…”