Mechanisms through which lithocholic acid delays yeast chronological aging under caloric restriction conditions.

All presently known geroprotective chemical compounds of plant and microbial origin are caloric restriction mimetics because they can mimic the beneficial lifespan- and healthspan-extending effects of caloric restriction diets without the need to limit calorie supply. We have discovered a geroprotective chemical compound of mammalian origin, a bile acid called lithocholic acid, which can delay chronological aging of the budding yeast Saccharomyces cerevisiae under caloric restriction conditions. Here, we investigated mechanisms through which lithocholic acid can delay chronological aging of yeast limited in calorie supply. We provide evidence that lithocholic acid causes a stepwise development and maintenance of an aging-delaying cellular pattern throughout the entire chronological lifespan of yeast cultured under caloric restriction conditions. We show that lithocholic acid stimulates the aging-delaying cellular pattern and preserves such pattern because it specifically modulates the spatiotemporal dynamics of a complex cellular network. We demonstrate that this cellular network integrates certain pathways of lipid and carbohydrate metabolism, some intercompartmental communications, mitochondrial morphology and functionality, and liponecrotic and apoptotic modes of aging-associated cell death. Our findings indicate that lithocholic acid prolongs longevity of chronologically aging yeast because it decreases the risk of aging-associated cell death, thus increasing the chance of elderly cells to survive.

Caloric restriction delays yeast chronological aging by remodeling carbohydrate and lipid metabolism, altering peroxisomal and mitochondrial functionalities, and postponing the onsets of apoptotic and liponecrotic modes of regulated cell death.

A dietary regimen of caloric restriction delays aging in evolutionarily distant eukaryotes, including the budding yeast Saccharomyces cerevisiae. Here, we assessed how caloric restriction influences morphological, biochemical and cell biological properties of chronologically aging yeast advancing through different stages of the aging process. Our findings revealed that this low-calorie diet slows yeast chronological aging by mechanisms that coordinate the spatiotemporal dynamics of various cellular processes before entry into a non-proliferative state and after such entry. Caloric restriction causes a stepwise establishment of an aging-delaying cellular pattern by tuning a network that assimilates the following: 1) pathways of carbohydrate and lipid metabolism; 2) communications between the endoplasmic reticulum, lipid droplets, peroxisomes, mitochondria and the cytosol; and 3) a balance between the processes of mitochondrial fusion and fission. Through different phases of the aging process, the caloric restriction-dependent remodeling of this intricate network 1) postpones the age-related onsets of apoptotic and liponecrotic modes of regulated cell death; and 2) actively increases the chance of cell survival by supporting the maintenance of cellular proteostasis. Because caloric restriction decreases the risk of cell death and actively increases the chance of cell survival throughout chronological lifespan, this dietary intervention extends longevity of chronologically aging yeast.

Lithocholic bile acid accumulated in yeast mitochondria orchestrates a development of an anti-aging cellular pattern by causing age-related changes in cellular proteome.

We have previously revealed that exogenously added lithocholic bile acid (LCA) extends the chronological lifespan of the yeast Saccharomyces cerevisiae, accumulates in mitochondria and alters mitochondrial membrane lipidome. Here, we use quantitative mass spectrometry to show that LCA alters the age-related dynamics of changes in levels of many mitochondrial proteins, as well as numerous proteins in cellular locations outside of mitochondria. These proteins belong to 2 regulons, each modulated by a different mitochondrial dysfunction; we call them a partial mitochondrial dysfunction regulon and an oxidative stress regulon. We found that proteins constituting these regulons (1) can be divided into several "clusters", each of which denotes a distinct type of partial mitochondrial dysfunction that elicits a different signaling pathway mediated by a discrete set of transcription factors; (2) exhibit 3 different patterns of the age-related dynamics of changes in their cellular levels; and (3) are encoded by genes whose expression is regulated by the transcription factors Rtg1p/Rtg2p/Rtg3p, Sfp1p, Aft1p, Yap1p, Msn2p/Msn4p, Skn7p and Hog1p, each of which is essential for longevity extension by LCA. Our findings suggest that LCA-driven changes in mitochondrial lipidome alter mitochondrial proteome and functionality, thereby enabling mitochondria to operate as signaling organelles that orchestrate an establishment of an anti-aging transcriptional program for many longevity-defining nuclear genes. Based on these findings, we propose a model for how such LCA-driven changes early and late in life of chronologically aging yeast cause a stepwise development of an anti-aging cellular pattern and its maintenance throughout lifespan.

Mechanisms by which different functional states of mitochondria define yeast longevity.

Mitochondrial functionality is vital to organismal physiology. A body of evidence supports the notion that an age-related progressive decline in mitochondrial function is a hallmark of cellular and organismal aging in evolutionarily distant eukaryotes. Studies of the baker's yeast Saccharomyces cerevisiae, a unicellular eukaryote, have led to discoveries of genes, signaling pathways and chemical compounds that modulate longevity-defining cellular processes in eukaryotic organisms across phyla. These studies have provided deep insights into mechanistic links that exist between different traits of mitochondrial functionality and cellular aging. The molecular mechanisms underlying the essential role of mitochondria as signaling organelles in yeast aging have begun to emerge. In this review, we discuss recent progress in understanding mechanisms by which different functional states of mitochondria define yeast longevity, outline the most important unanswered questions and suggest directions for future research.

Mechanism of liponecrosis, a distinct mode of programmed cell death.

An exposure of the yeast Saccharomyces cerevisiae to exogenous palmitoleic acid (POA) elicits "liponecrosis," a mode of programmed cell death (PCD) which differs from the currently known PCD subroutines. Here, we report the following mechanism for liponecrotic PCD. Exogenously added POA is incorporated into POA-containing phospholipids that then amass in the endoplasmic reticulum membrane, mitochondrial membranes and the plasma membrane. The buildup of the POA-containing phospholipids in the plasma membrane reduces the level of phosphatidylethanolamine in its extracellular leaflet, thereby increasing plasma membrane permeability for small molecules and committing yeast to liponecrotic PCD. The excessive accumulation of POA-containing phospholipids in mitochondrial membranes impairs mitochondrial functionality and causes the excessive production of reactive oxygen species in mitochondria. The resulting rise in cellular reactive oxygen species above a critical level contributes to the commitment of yeast to liponecrotic PCD by: (1) oxidatively damaging numerous cellular organelles, thereby triggering their massive macroautophagic degradation; and (2) oxidatively damaging various cellular proteins, thus impairing cellular proteostasis. Several cellular processes in yeast exposed to POA can protect cells from liponecrosis. They include: (1) POA oxidation in peroxisomes, which reduces the flow of POA into phospholipid synthesis pathways; (2) POA incorporation into neutral lipids, which prevents the excessive accumulation of POA-containing phospholipids in cellular membranes; (3) mitophagy, a selective macroautophagic degradation of dysfunctional mitochondria, which sustains a population of functional mitochondria needed for POA incorporation into neutral lipids; and (4) a degradation of damaged, dysfunctional and aggregated cytosolic proteins, which enables the maintenance of cellular proteostasis.

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

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.

Bile acids induce apoptosis selectively in androgen-dependent and -independent prostate cancer cells.

Prostate cancer is a prevalent age-related disease in North America, accounting for about 15% of all diagnosed cancers. We have previously identified lithocholic acid (LCA) as a potential chemotherapeutic compound that selectively kills neuroblastoma cells while sparing normal human neurons. Now, we report that LCA inhibits the proliferation of androgen-dependent (AD) LNCaP prostate cancer cells and that LCA is the most potent bile acid with respect to inducing apoptosis in LNCaP as well as androgen-independent (AI) PC-3 cells, without killing RWPE-1 immortalized normal prostate epithelial cells. In LNCaP and PC-3 cells, LCA triggered the extrinsic pathway of apoptosis and cell death induced by LCA was partially dependent on the activation of caspase-8 and -3. Moreover, LCA increased cleavage of Bid and Bax, down-regulation of Bcl-2, permeabilization of the mitochondrial outer membrane and activation of caspase-9. The cytotoxic actions of LCA occurred despite the inability of this bile acid to enter the prostate cancer cells with about 98% of the nominal test concentrations present in the extracellular culture medium. With our findings, we provide evidence to support a mechanism of action underlying the broad anticancer activity of LCA in various human tissues.

Mitochondrial membrane lipidome defines yeast longevity.

Our studies revealed that lithocholic acid (LCA), a bile acid, is a potent anti-aging natural compound that in yeast cultured under longevity-extending caloric restriction (CR) conditions acts in synergy with CR to enable a significant further increase in chronological lifespan. Here, we investigate a mechanism underlying this robust longevity-extending effect of LCA under CR. We found that exogenously added LCA enters yeast cells, is sorted to mitochondria, resides mainly in the inner mitochondrial membrane, and also associates with the outer mitochondrial membrane. LCA elicits an age-related remodeling of glycerophospholipid synthesis and movement within both mitochondrial membranes, thereby causing substantial changes in mitochondrial membrane lipidome and triggering major changes in mitochondrial size, number and morphology. In synergy, these changes in the membrane lipidome and morphology of mitochondria alter the age-related chronology of mitochondrial respiration, membrane potential, ATP synthesis and reactive oxygen species homeostasis. The LCA-driven alterations in the age-related dynamics of these vital mitochondrial processes extend yeast longevity. In sum, our findings suggest a mechanism underlying the ability of LCA to delay chronological aging in yeast by accumulating in both mitochondrial membranes and altering their glycerophospholipid compositions. We concluded that mitochondrial membrane lipidome plays an essential role in defining yeast longevity.

Essential roles of peroxisomally produced and metabolized biomolecules in regulating yeast longevity.

The essential role of the peroxisome in oxidizing fatty acids, maintaining reactive oxygen species homeostasis and replenishing tricarboxylic acid cycle intermediates is well known. Recent findings have broadened a spectrum of biomolecules that are synthesized and metabolized in peroxisomes. Emergent evidence supports the view that, by releasing various biomolecules known to modulate essential cellular processes, the peroxisome not only operates as an organizing platform for several developmental and differentiation programs but is also actively involved in defining the replicative and chronological age of a eukaryotic cell. The scope of this chapter is to summarize the evidence that the peroxisome defines yeast longevity by operating as a system controller that: (1) modulates levels of non-esterified fatty acids and diacylglycerol; (2) replenishes tricarboxylic acid cycle intermediates destined for mitochondria; and (3) contributes to the synthesis of polyamines. We critically evaluate molecular mechanisms underlying the essential role of peroxisomally produced and metabolized biomolecules in governing cellular aging in yeast.

Macromitophagy is a longevity assurance process that in chronologically aging yeast limited in calorie supply sustains functional mitochondria and maintains cellular lipid homeostasis.

Macromitophagy controls mitochondrial quality and quantity. It involves the sequestration of dysfunctional or excessive mitochondria within double-membrane autophagosomes, which then fuse with the vacuole/lysosome to deliver these mitochondria for degradation. To investigate a physiological role of macromitophagy in yeast, we examined how theatg32Δ-dependent mutational block of this process influences the chronological lifespan of cells grown in a nutrient-rich medium containing low (0.2%) concentration of glucose. Under these longevity-extending conditions of caloric restriction (CR) yeast cells are not starving. We also assessed a role of macromitophagy in lifespan extension by lithocholic acid (LCA), a bile acid that prolongs yeast longevity under CR conditions. Our findings imply that macromitophagy is a longevity assurance process underlying the synergistic beneficial effects of CR and LCA on yeast lifespan. Our analysis of how the atg32Δ mutation influences mitochondrial morphology, composition and function revealed that macromitophagy is required to maintain a network of healthy mitochondria. Our comparative analysis of the membrane lipidomes of organelles purified from wild-type and atg32Δ cells revealed that macromitophagy is required for maintaining cellular lipid homeostasis. We concluded that macromitophagy defines yeast longevity by modulating vital cellular processes inside and outside of mitochondria.

Integration of peroxisomes into an endomembrane system that governs cellular aging.

The peroxisome is an organelle that has long been known for its essential roles in oxidation of fatty acids, maintenance of reactive oxygen species (ROS) homeostasis and anaplerotic replenishment of tricarboxylic acid (TCA) cycle intermediates destined for mitochondria. Growing evidence supports the view that these peroxisome-confined metabolic processes play an essential role in defining the replicative and chronological age of a eukaryotic cell. Much progress has recently been made in defining molecular mechanisms that link cellular aging to fatty acid oxidation, ROS turnover, and anaplerotic metabolism in peroxisomes. Emergent studies have revealed that these organelles not only house longevity-defining metabolic reactions but can also regulate cellular aging via their dynamic communication with other cellular compartments. Peroxisomes communicate with other organelles by establishing extensive physical contact with lipid bodies, maintaining an endoplasmic reticulum (ER) to peroxisome connectivity system, exchanging certain metabolites, and being involved in the bidirectional flow of some of their protein and lipid constituents. The scope of this review is to summarize the evidence that peroxisomes are dynamically integrated into an endomembrane system that governs cellular aging. We discuss recent progress in understanding how communications between peroxisomes and other cellular compartments within this system influence the development of a pro- or anti-aging cellular pattern. We also propose a model for the integration of peroxisomes into the endomembrane system governing cellular aging and critically evaluate several molecular mechanisms underlying such integration.

Lithocholic acid extends longevity of chronologically aging yeast only if added at certain critical periods of their lifespan.

Our studies revealed that LCA (lithocholic bile acid) extends yeast chronological lifespan if added to growth medium at the time of cell inoculation. We also demonstrated that longevity in chronologically aging yeast is programmed by the level of metabolic capacity and organelle organization that they developed before entering a quiescent state and, thus, that chronological aging in yeast is likely to be the final step of a developmental program progressing through at least one checkpoint prior to entry into quiescence. Here, we investigate how LCA influences longevity and several longevity-defining cellular processes in chronologically aging yeast if added to growth medium at different periods of the lifespan. We found that LCA can extend longevity of yeast under CR (caloric restriction) conditions only if added at either of two lifespan periods. One of them includes logarithmic and diauxic growth phases, whereas the other period exists in early stationary phase. Our findings suggest a mechanism linking the ability of LCA to increase the lifespan of CR yeast only if added at either of the two periods to its differential effects on various longevity-defining processes. In this mechanism, LCA controls these processes at three checkpoints that exist in logarithmic/diauxic, post-diauxic and early stationary phases. We therefore hypothesize that a biomolecular longevity network progresses through a series of checkpoints, at each of which (1) genetic, dietary and pharmacological anti-aging interventions modulate a distinct set of longevity-defining processes comprising the network; and (2) checkpoint-specific master regulators monitor and govern the functional states of these processes.

Caloric restriction extends yeast chronological lifespan by altering a pattern of age-related changes in trehalose concentration.

The non-reducing disaccharide trehalose has been long considered only as a reserve carbohydrate. However, recent studies in yeast suggested that this osmolyte can protect cells and cellular proteins from oxidative damage elicited by exogenously added reactive oxygen species (ROS). Trehalose has been also shown to affect stability, folding, and aggregation of bacterial and firefly proteins heterologously expressed in heat-shocked yeast cells. Our recent investigation of how a lifespan-extending caloric restriction (CR) diet alters the metabolic history of chronologically aging yeast suggested that their longevity is programmed by the level of metabolic capacity - including trehalose biosynthesis and degradation - that yeast cells developed prior to entry into quiescence. To investigate whether trehalose homeostasis in chronologically aging yeast may play a role in longevity extension by CR, in this study we examined how single-gene-deletion mutations affecting trehalose biosynthesis and degradation impact (1) the age-related dynamics of changes in trehalose concentration; (2) yeast chronological lifespan under CR conditions; (3) the chronology of oxidative protein damage, intracellular ROS level and protein aggregation; and (4) the timeline of thermal inactivation of a protein in heat-shocked yeast cells and its subsequent reactivation in yeast returned to low temperature. Our data imply that CR extends yeast chronological lifespan in part by altering a pattern of age-related changes in trehalose concentration. We outline a model for molecular mechanisms underlying the essential role of trehalose in defining yeast longevity by modulating protein folding, misfolding, unfolding, refolding, oxidative damage, solubility, and aggregation throughout lifespan.

Interspecies Chemical Signals Released into the Environment May Create Xenohormetic, Hormetic and Cytostatic Selective Forces that Drive the Ecosystemic Evolution of Longevity Regulation Mechanisms.

Various organisms (i.e., bacteria, fungi, plants and animals) within an ecosystem can synthesize and release into the environment certain longevity-extending small molecules. Here we hypothesize that these interspecies chemical signals can create xenohormetic, hormetic and cytostatic selective forces driving the ecosystemic evolution of longevity regulation mechanisms. In our hypothesis, following their release into the environment by one species of the organisms composing an ecosystem, such small molecules can activate anti-aging processes and/or inhibit pro-aging processes in other species within the ecosystem. The organisms that possess the most effective (as compared to their counterparts of the same species) mechanisms for sensing the chemical signals produced and released by other species and for responding to such signals by undergoing certain hormetic and/or cytostatic life-extending changes to their metabolism and physiology are expected to live longer then their counterparts within the ecosystem. Thus, the ability of a species of the organisms composing an ecosystem to undergo life-extending metabolic or physiological changes in response to hormetic or cytostatic chemical compounds released to the ecosystem by other species: 1) increases its chances of survival; 2) creates selective forces aimed at maintaining such ability; and 3) enables the evolution of longevity regulation mechanisms.

Lithocholic bile acid selectively kills neuroblastoma cells, while sparing normal neuronal cells.

Aging is one of the major risk factors of cancer. The onset of cancer can be postponed by pharmacological and dietary anti-aging interventions. We recently found in yeast cellular models of aging that lithocholic acid (LCA) extends longevity. Here we show that, at concentrations that are not cytotoxic to primary cultures of human neurons, LCA kills the neuroblastoma (NB) cell lines BE(2)-m17, SK-n-SH, SK-n-MCIXC and Lan-1. In BE(2)-m17, SK-n-SH and SK-n-MCIXC cells, the LCA anti-tumor effect is due to apoptotic cell death. In contrast, the LCA-triggered death of Lan-1 cells is not caused by apoptosis. While low concentrations of LCA sensitize BE(2)-m17 and SK-n-MCIXC cells to hydrogen peroxide-induced apoptotic cell death controlled by mitochondria, these LCA concentrations make primary cultures of human neurons resistant to such a form of cell death. LCA kills BE(2)-m17 and SK-n-MCIXC cell lines by triggering not only the intrinsic (mitochondrial) apoptotic cell death pathway driven by mitochondrial outer membrane permeabilization and initiator caspase-9 activation, but also the extrinsic (death receptor) pathway of apoptosis involving activation of the initiator caspase-8. Based on these data, we propose a mechanism underlying a potent and selective anti-tumor effect of LCA in cultured human NB cells. Moreover, our finding that LCA kills cultured human breast cancer and rat glioma cells implies that it has a broad anti-tumor effect on cancer cells derived from different tissues and organisms.

Chemical genetic screen identifies lithocholic acid as an anti-aging compound that extends yeast chronological life span in a TOR-independent manner, by modulating housekeeping longevity assurance processes.

In chronologically aging yeast, longevity can be extended by administering a caloric restriction (CR) diet or some small molecules. These life-extending interventions target the adaptable target of rapamycin (TOR) and cAMP/protein kinase A (cAMP/PKA) signaling pathways that are under the stringent control of calorie availability. We designed a chemical genetic screen for small molecules that increase the chronological life span of yeast under CR by targeting lipid metabolism and modulating housekeeping longevity pathways that regulate longevity irrespective of the number of available calories. Our screen identifies lithocholic acid (LCA) as one of such molecules. We reveal two mechanisms underlying the life-extending effect of LCA in chronologically aging yeast. One mechanism operates in a calorie availability-independent fashion and involves the LCA-governed modulation of housekeeping longevity assurance pathways that do not overlap with the adaptable TOR and cAMP/PKA pathways. The other mechanism extends yeast longevity under non-CR conditions and consists in LCA-driven unmasking of the previously unknown anti-aging potential of PKA. We provide evidence that LCA modulates housekeeping longevity assurance pathways by suppressing lipid-induced necrosis, attenuating mitochondrial fragmentation, altering oxidation-reduction processes in mitochondria, enhancing resistance to oxidative and thermal stresses, suppressing mitochondria-controlled apoptosis, and enhancing stability of nuclear and mitochondrial DNA.

A novel function of lipid droplets in regulating longevity.

Growing evidence supports the view that LDs (lipid droplets) are dynamic organelles that can serve both as an intracellular signalling compartment and as an organizing platform orchestrating many vital processes in eukaryotic cells. It has become clear that the LDs-confined deposition and lipolytic degradation of neutral lipids define longevity in multicellular eukaryotic organisms and yeast. We summarize the evidence in support of the essential role that LDs play in longevity regulation and propose several molecular mechanisms by which these dynamic organellar compartments control the aging process in multicellular eukaryotes and yeast.

Effect of calorie restriction on the metabolic history of chronologically aging yeast.

Aging is a highly complex, multifactorial process. We use the yeast Saccharomyces cerevisiae as a model to study the mechanisms of cellular aging in multicellular eukaryotes. To address the inherent complexity of aging from a systems perspective and to build an integrative model of aging process, we investigated the effect of calorie restriction (CR), a low-calorie dietary regimen, on the metabolic history of chronologically aging yeast. We examined how CR influences the age-related dynamics of changes in the intracellular levels of numerous proteins and metabolites, carbohydrate and lipid metabolism, interorganellar metabolic flow, concentration of reactive oxygen species, mitochondrial morphology, essential oxidation-reduction processes in mitochondria, mitochondrial proteome, cardiolipin in the inner mitochondrial membrane, frequency of mitochondrial DNA mutations, dynamics of mitochondrial nucleoid, susceptibility to mitochondria-controlled apoptosis, and stress resistance. Based on the comparison of the metabolic histories of long-lived CR yeast and short-lived non-CR yeast, we propose that yeast define their long-term viability by designing a diet-specific pattern of metabolism and organelle dynamics prior to reproductive maturation. Thus, our data suggest that longevity in chronologically aging yeast is programmed by the level of metabolic capacity and organelle organization they developed, in a diet-specific fashion, prior to entry into a non-proliferative state.