PLK1 (polo like kinase 1) inhibits MTOR complex 1 and promotes autophagy.

Mechanistic target of rapamycin complex 1 (MTORC1) and polo like kinase 1 (PLK1) are major drivers of cancer cell growth and proliferation, and inhibitors of both protein kinases are currently being investigated in clinical studies. To date, MTORC1's and PLK1's functions are mostly studied separately, and reports on their mutual crosstalk are scarce. Here, we identify PLK1 as a physical MTORC1 interactor in human cancer cells. PLK1 inhibition enhances MTORC1 activity under nutrient sufficiency and in starved cells, and PLK1 directly phosphorylates the MTORC1 component RPTOR/RAPTOR in vitro. PLK1 and MTORC1 reside together at lysosomes, the subcellular site where MTORC1 is active. Consistent with an inhibitory role of PLK1 toward MTORC1, PLK1 overexpression inhibits lysosomal association of the PLK1-MTORC1 complex, whereas PLK1 inhibition promotes lysosomal localization of MTOR. PLK1-MTORC1 binding is enhanced by amino acid starvation, a condition known to increase autophagy. MTORC1 inhibition is an important step in autophagy activation. Consistently, PLK1 inhibition mitigates autophagy in cancer cells both under nutrient starvation and sufficiency, and a role of PLK1 in autophagy is also observed in the invertebrate model organism Caenorhabditis elegans. In summary, PLK1 inhibits MTORC1 and thereby positively contributes to autophagy. Since autophagy is increasingly recognized to contribute to tumor cell survival and growth, we propose that cautious monitoring of MTORC1 and autophagy readouts in clinical trials with PLK1 inhibitors is needed to develop strategies for optimized (combinatorial) cancer therapies targeting MTORC1, PLK1, and autophagy.

Correction: Intestinal Autophagy Improves Healthspan and Longevity in C. elegans During Dietary Restriction.

[This corrects the article DOI: 10.1371/journal.pgen.1006135.].

Intestinal Autophagy Improves Healthspan and Longevity in C. elegans during Dietary Restriction.

Dietary restriction (DR) is a dietary regimen that extends lifespan in many organisms. One mechanism contributing to the conserved effect of DR on longevity is the cellular recycling process autophagy, which is induced in response to nutrient scarcity and increases sequestration of cytosolic material into double-membrane autophagosomes for degradation in the lysosome. Although autophagy plays a direct role in DR-mediated lifespan extension in the nematode Caenorhabditis elegans, the contribution of autophagy in individual tissues remains unclear. In this study, we show a critical role for autophagy in the intestine, a major metabolic tissue, to ensure lifespan extension of dietary-restricted eat-2 mutants. The intestine of eat-2 mutants has an enlarged lysosomal compartment and flux assays indicate increased turnover of autophagosomes, consistent with an induction of autophagy in this tissue. This increase in intestinal autophagy may underlie the improved intestinal integrity we observe in eat-2 mutants, since whole-body and intestinal-specific inhibition of autophagy in eat-2 mutants greatly impairs the intestinal barrier function. Interestingly, intestinal-specific inhibition of autophagy in eat-2 mutants leads to a decrease in motility with age, alluding to a potential cell non-autonomous role for autophagy in the intestine. Collectively, these results highlight important functions for autophagy in the intestine of dietary-restricted C. elegans.

C. elegans S6K Mutants Require a Creatine-Kinase-like Effector for Lifespan Extension.

Deficiency of S6 kinase (S6K) extends the lifespan of multiple species, but the underlying mechanisms are unclear. To discover potential effectors of S6K-mediated longevity, we performed a proteomics analysis of long-lived rsks-1/S6K C. elegans mutants compared to wild-type animals. We identified the arginine kinase ARGK-1 as the most significantly enriched protein in rsks-1/S6K mutants. ARGK-1 is an ortholog of mammalian creatine kinase, which maintains cellular ATP levels. We found that argk-1 is possibly a selective effector of rsks-1/S6K-mediated longevity and that overexpression of ARGK-1 extends C. elegans lifespan, in part by activating the energy sensor AAK-2/AMPK. argk-1 is also required for the reduced body size and increased stress resistance observed in rsks-1/S6K mutants. Finally, creatine kinase levels are increased in the brains of S6K1 knockout mice. Our study identifies ARGK-1 as a longevity effector in C. elegans with reduced RSKS-1/S6K levels.

Integrin-linked kinase modulates longevity and thermotolerance in C. elegans through neuronal control of HSF-1.

Integrin-signaling complexes play important roles in cytoskeletal organization and cell adhesion in many species. Components of the integrin-signaling complex have been linked to aging in both Caenorhabditis elegans and Drosophila melanogaster, but the mechanism underlying this function is unknown. Here, we investigated the role of integrin-linked kinase (ILK), a key component of the integrin-signaling complex, in lifespan determination. We report that genetic reduction of ILK in both C. elegans and Drosophila increased resistance to heat stress, and led to lifespan extension in C. elegans without majorly affecting cytoskeletal integrity. In C. elegans, longevity and thermotolerance induced by ILK depletion was mediated by heat-shock factor-1 (HSF-1), a major transcriptional regulator of the heat-shock response (HSR). Reduction in ILK levels increased hsf-1 transcription and activation, and led to enhanced expression of a subset of genes with roles in the HSR. Moreover, induction of HSR-related genes, longevity and thermotolerance caused by ILK reduction required the thermosensory neurons AFD and interneurons AIY, which are known to play a critical role in the canonical HSR. Notably, ILK was expressed in neighboring neurons, but not in AFD or AIY, implying that ILK reduction initiates cell nonautonomous signaling through thermosensory neurons to elicit a noncanonical HSR. Our results thus identify HSF-1 as a novel effector of the organismal response to reduced ILK levels and show that ILK inhibition regulates HSF-1 in a cell nonautonomous fashion to enhance stress resistance and lifespan in C. elegans.

The TFEB orthologue HLH-30 regulates autophagy and modulates longevity in Caenorhabditis elegans.

Autophagy is a cellular recycling process that has an important anti-aging role, but the underlying molecular mechanism is not well understood. The mammalian transcription factor EB (TFEB) was recently shown to regulate multiple genes in the autophagy process. Here we show that the predicted TFEB orthologue HLH-30 regulates autophagy in Caenorhabditis elegans and, in addition, has a key role in lifespan determination. We demonstrate that hlh-30 is essential for the extended lifespan of Caenorhabditis elegans in six mechanistically distinct longevity models, and overexpression of HLH-30 extends lifespan. Nuclear localization of HLH-30 is increased in all six Caenorhabditis elegans models and, notably, nuclear TFEB levels are augmented in the livers of mice subjected to dietary restriction, a known longevity-extending regimen. Collectively, our results demonstrate a conserved role for HLH-30 and TFEB in autophagy, and possibly longevity, and identify HLH-30 as a uniquely important transcription factor for lifespan modulation in Caenorhabditis elegans.

Testing predictions of the oxidative stress hypothesis of aging using a novel invertebrate model of longevity: the giant clam (Tridacna derasa).

Bivalve species with exceptional longevity are newly introduced model systems in biogerontology to test evolutionarily conserved mechanisms of aging. Here, we tested predictions based on the oxidative stress hypothesis of aging using one of the tropical long-lived sessile giant clam species, the smooth giant clam (Tridacna derasa; predicted maximum life span: >100 years) and the short-lived Atlantic bay scallop (Argopecten irradians irradians; maximum life span: 2 years). The warm water-dwelling giant clams warrant attention because they challenge the commonly held view that the exceptional longevity of bivalves is a consequence of the cold water they reside in. No significant interspecific differences in production of H2O2 and O2- in the gills, heart, or adductor muscle were observed. Protein carbonyl content in gill and muscle tissues were similar in T derasa and A i irradians. In tissues of T derasa, neither basal antioxidant capacities nor superoxide dismutase and catalase activities were consistently greater than in A i irradians. We observed a positive association between longevity and resistance to mortality induced by exposure to tert-butyl hydroperoxide (TBHP). This finding is consistent with the prediction based on the oxidative stress hypothesis of aging. The findings that in tissues of T derasa, proteasome activities are significantly increased as compared with those in tissues of A i irradians warrant further studies to test the role of enhanced protein recycling activities in longevity of bivalves.

Autophagy induction extends lifespan and reduces lipid content in response to frataxin silencing in C. elegans.

Severe mitochondria deficiency leads to a number of devastating degenerative disorders, yet, mild mitochondrial dysfunction in different species, including the nematode Caenorhabditis elegans, can have pro-longevity effects. This apparent paradox indicates that cellular adaptation to partial mitochondrial stress can induce beneficial responses, but how this is achieved is largely unknown. Complete absence of frataxin, the mitochondrial protein defective in patients with Friedreich's ataxia, is lethal in C. elegans, while its partial deficiency extends animal lifespan in a p53 dependent manner. In this paper we provide further insight into frataxin control of C. elegans longevity by showing that a substantial reduction of frataxin protein expression is required to extend lifespan, affect sensory neurons functionality, remodel lipid metabolism and trigger autophagy. We find that Beclin and p53 genes are required to induce autophagy and concurrently reduce lipid storages and extend animal lifespan in response to frataxin suppression. Reciprocally, frataxin expression modulates autophagy in the absence of p53. Human Friedreich ataxia-derived lymphoblasts also display increased autophagy, indicating an evolutionarily conserved response to reduced frataxin expression. In sum, we demonstrate a causal connection between induction of autophagy and lifespan extension following reduced frataxin expression, thus providing the rationale for investigating autophagy in the pathogenesis and treatment of Friedreich's ataxia and possibly other human mitochondria-associated disorders.

Autophagy - An Emerging Anti-Aging Mechanism.

Autophagy is a cytoplasmic catabolic process that protects the cell against stressful conditions. Damaged cellular components are funneled by autophagy into the lysosomes, where they are degraded and can be re-used as alternative building blocks for protein synthesis and cellular repair. In contrast, aging is the gradual failure over time of cellular repair mechanisms that leads to the accumulation of molecular and cellular damage and loss of function. The cell's capacity for autophagic degradation also declines with age, and this in itself may contribute to the aging process. Studies in model organisms ranging from yeast to mice have shown that single-gene mutations can extend lifespan in an evolutionarily conserved fashion, and provide evidence that the aging process can be modulated. Interestingly, autophagy is induced in a seemingly beneficial manner by many of the same perturbations that extend lifespan, including mutations in key signaling pathways such as the insulin/IGF-1 and TOR pathways. Here, we review recent progress, primarily derived from genetic studies with model organisms, in understanding the role of autophagy in aging and age-related diseases.

Autophagy and lipid metabolism coordinately modulate life span in germline-less C. elegans.

BACKGROUND: The cellular recycling process of autophagy is emerging as a key player in several longevity pathways in Caenorhabditis elegans. Here, we identify a role for autophagy in long-lived animals lacking a germline and show that autophagy and lipid metabolism work interdependently to modulate aging in this longevity model. RESULTS: Germline removal extends life span in C. elegans via genes such as the lipase LIPL-4; however, less is known of the cellular basis for this life-span extension. Here, we show that germline loss induces autophagy gene expression via the forkhead box A (FOXA) transcription factor PHA-4 and that autophagy is required to extend longevity. We identify a novel link between autophagy and LIPL-4, because autophagy is required to maintain high lipase activity in germline-deficient animals. Reciprocally, lipl-4 is required for autophagy induction. Coordination between autophagy and lipolysis is further supported by the finding that inhibition of TOR (target of rapamycin), a major negative regulator of autophagy, induces lipl-4 expression, and TOR levels are reduced in germline-less animals. TOR may therefore function as a common upstream regulator of both autophagy and lipl-4 expression in germline-less animals. Importantly, we find that the link between autophagy and LIPL-4 is relevant to longevity, because autophagy is induced in animals overexpressing LIPL-4 and autophagy is required for their long life span, recapitulating observations in germline-less animals. CONCLUSIONS: Collectively, our data offer a novel mechanism by which autophagy and the lipase LIPL-4 interdependently modulate aging in germline-deficient C. elegans by maintaining lipid homeostasis to prolong life span.

Extreme longevity is associated with increased resistance to oxidative stress in Arctica islandica, the longest-living non-colonial animal.

We assess whether reactive oxygen species production and resistance to oxidative stress might be causally involved in the exceptional longevity exhibited by the ocean quahog Arctica islandica. We tested this hypothesis by comparing reactive oxygen species production, resistance to oxidative stress, antioxidant defenses, and protein damage elimination processes in long-lived A islandica with the shorter-lived hard clam, Mercenaria mercenaria. We compared baseline biochemical profiles, age-related changes, and responses to exposure to the oxidative stressor tert-butyl hydroperoxide (TBHP). Our data support the premise that extreme longevity in A islandica is associated with an attenuated cellular reactive oxygen species production. The observation of reduced protein carbonyl concentration in A islandica gill tissue compared with M mercenaria suggests that reduced reactive oxygen species production in long-living bivalves is associated with lower levels of accumulated macromolecular damage, suggesting cellular redox homeostasis may determine life span. Resistance to aging at the organismal level is often reflected in resistance to oxidative stressors at the cellular level. Following TBHP exposure, we observed not only an association between longevity and resistance to oxidative stress-induced mortality but also marked resistance to oxidative stress-induced cell death in the longer-living bivalves. Contrary to some expectations from the oxidative stress hypothesis, we observed that A islandica exhibited neither greater antioxidant capacities nor specific activities than in M mercenaria nor a more pronounced homeostatic antioxidant response following TBHP exposure. The study also failed to provide support for the exceptional longevity of A islandica being associated with enhanced protein recycling. Our findings demonstrate an association between longevity and resistance to oxidative stress-induced cell death in A islandica, consistent with the oxidative stress hypothesis of aging and provide justification for detailed evaluation of pathways involving repair of free radical-mediated macromolecular damage and regulation of apoptosis in the world's longest-living non-colonial animal.

Phosphorylation of ULK1 (hATG1) by AMP-activated protein kinase connects energy sensing to mitophagy.

Adenosine monophosphate-activated protein kinase (AMPK) is a conserved sensor of intracellular energy activated in response to low nutrient availability and environmental stress. In a screen for conserved substrates of AMPK, we identified ULK1 and ULK2, mammalian orthologs of the yeast protein kinase Atg1, which is required for autophagy. Genetic analysis of AMPK or ULK1 in mammalian liver and Caenorhabditis elegans revealed a requirement for these kinases in autophagy. In mammals, loss of AMPK or ULK1 resulted in aberrant accumulation of the autophagy adaptor p62 and defective mitophagy. Reconstitution of ULK1-deficient cells with a mutant ULK1 that cannot be phosphorylated by AMPK revealed that such phosphorylation is required for mitochondrial homeostasis and cell survival during starvation. These findings uncover a conserved biochemical mechanism coupling nutrient status with autophagy and cell survival.