Living Beyond Restriction: LBR promotes cellular immortalization by suppressing genomic instability and senescence.

Cellular immortalization is a complex process that requires multiple genetic alterations to overcome restricting barriers, including senescence. Not surprisingly, many of these alterations are associated with cancer; two tumor suppressor pathways, the cellular tumor antigen p53 and p16-Retinoblastoma (RB) pathways, are the best-characterized examples, but their mutations alone are known to be insufficient to drive full immortalization. En et al. identified a role for the lamin B receptor (LBR) in promoting cellular proliferation and immortalization in p53- and RB-deficient cells by maintaining their genome integrity and suppressing senescence. Thus, modulation of LBR could be exploited to treat cancer and potentially also to promote cell rejuvenation.

Metabolic remodeling in cancer and senescence and its therapeutic implications.

Cellular metabolism is a flexible and plastic network that often dictates physiological and pathological states of the cell, including differentiation, cancer, and aging. Recent advances in cancer metabolism represent a tremendous opportunity to treat cancer by targeting its altered metabolism. Interestingly, despite their stable growth arrest, senescent cells - a critical component of the aging process - undergo metabolic changes similar to cancer metabolism. A deeper understanding of the similarities and differences between these disparate pathological conditions will help identify which metabolic reprogramming is most relevant to the therapeutic liabilities of senescence. Here, we compare and contrast cancer and senescence metabolism and discuss how metabolic therapies can be established as a new modality of senotherapy for healthy aging.

A focused natural compound screen reveals senolytic and senostatic effects of Isatis tinctoria.

Natural products and their derivatives historically represent alternatives to conventional synthetic molecules for pharmacotherapy, ranging from cancer chemotherapeutics to cosmetic ingredients that exert anti-aging activities. Cellular senescence is considered a main driver of skin aging, yet natural products that target skin senescence in a specific manner are not thoroughly explored. Here, we performed a focused compound screen to identify natural products that exert anti-senescence effects. We found that Isatis tinctoria, woad extracts, displayed a senolytic effect on senescent human skin fibroblasts. Furthermore, treatment with woad extracts attenuated the expression of pro-inflammatory senescence-associated secretory phenotype (SASP), showing a senostatic activity. Intriguingly, woad extracts displayed only a marginal cytotoxic effect toward senescent human lung fibroblasts. Thus, our results reveal the potential activities of woad extracts for targeting skin senescence and suggest that woad extracts could be an attractive ingredient for cosmetics to prevent skin aging.

Targeting the stress support network regulated by autophagy and senescence for cancer treatment.

Autophagy and cellular senescence are two potent tumor suppressive mechanisms activated by various cellular stresses, including the expression of activated oncogenes. However, emerging evidence has also indicated their pro-tumorigenic activities, strengthening the case for the complexity of tumorigenesis. More specifically, tumorigenesis is a systemic process emanating from the combined accumulation of changes in the tumor support pathways, many of which cannot cause cancer on their own but might still provide excellent therapeutic targets for cancer treatment. In this review, we discuss the dual roles of autophagy and senescence during tumorigenesis, with a specific focus on the stress support networks in cancer cells modulated by these processes. A deeper understanding of such context-dependent roles may help to enhance the effectiveness of cancer therapies targeting autophagy and senescence, while limiting their potential side effects. This will steer and accelerate the pace of research and drug development for cancer treatment.

Sclerostin inhibits Wnt signaling through tandem interaction with two LRP6 ectodomains.

Low-density lipoprotein receptor-related protein 6 (LRP6) is a coreceptor of the ß-catenin-dependent Wnt signaling pathway. The LRP6 ectodomain binds Wnt proteins, as well as Wnt inhibitors such as sclerostin (SOST), which negatively regulates Wnt signaling in osteocytes. Although LRP6 ectodomain 1 (E1) is known to interact with SOST, several unresolved questions remain, such as the reason why SOST binds to LRP6 E1E2 with higher affinity than to the E1 domain alone. Here, we present the crystal structure of the LRP6 E1E2-SOST complex with two interaction sites in tandem. The unexpected additional binding site was identified between the C-terminus of SOST and the LRP6 E2 domain. This interaction was confirmed by in vitro binding and cell-based signaling assays. Its functional significance was further demonstrated in vivo using Xenopus laevis embryos. Our results provide insights into the inhibitory mechanism of SOST on Wnt signaling.

Senolytics and Senostatics: A Two-Pronged Approach to Target Cellular Senescence for Delaying Aging and Age-Related Diseases.

Aging is the most important single risk factor for many chronic diseases such as cancer, metabolic syndrome, and neurodegenerative disorders. Targeting aging itself might, therefore, be a better strategy than targeting each chronic disease individually for enhancing human health. Although much should be achieved for completely understanding the biological basis of aging, cellular senescence is now believed to mainly contribute to organismal aging via two independent, yet not mutually exclusive mechanisms: on the one hand, senescence of stem cells leads to exhaustion of stem cells and thus decreases tissue regeneration. On the other hand, senescent cells secrete many proinflammatory cytokines, chemokines, growth factors, and proteases, collectively termed as the senescence-associated secretory phenotype (SASP), which causes chronic inflammation and tissue dysfunction. Much effort has been recently made to therapeutically target detrimental effects of cellular senescence including selectively eliminating senescent cells (senolytics) and modulating a proinflammatory senescent secretome (senostatics). Here, we discuss current progress and limitations in understanding molecular mechanisms of senolytics and senostatics and therapeutic strategies for applying them. Furthermore, we propose how these novel interventions for aging treatment could be improved, based on lessons learned from cancer treatment.

Genetic interrogation of replicative senescence uncovers a dual role for USP28 in coordinating the p53 and GATA4 branches of the senescence program.

Senescence is a terminal differentiation program that halts the growth of damaged cells and must be circumvented for cancer to arise. Here we describe a panel of genetic screens to identify genes required for replicative senescence. We uncover a role in senescence for the potent tumor suppressor and ATM substrate USP28. USP28 controls activation of both the TP53 branch and the GATA4/NFkB branch that controls the senescence-associated secretory phenotype (SASP). These results suggest a role for ubiquitination in senescence and imply a common node downstream from ATM that links the TP53 and GATA4 branches of the senescence response.

Autophagy Is Pro-Senescence When Seen in Close-Up, but Anti-Senescence in Long-Shot.

When mammalian cells and animals face a variety of internal or external stresses, they need to make homeostatic changes so as to cope with various stresses. To this end, mammalian cells are equipped with two critical stress responses, autophagy and cellular senescence. Autophagy and cellular senescence share a number of stimuli including telomere shortening, DNA damage, oncogenic stress and oxidative stress, suggesting their intimate relationship. Autophagy is originally thought to suppress cellular senescence by removing damaged macromolecules or organelles, yet recent studies also indicated that autophagy promotes cellular senescence by facilitating the synthesis of senescence-associated secretory proteins. These seemingly opposite roles of autophagy may reflect a complex picture of autophagic regulation on cellular senescence, including different types of autophagy or a unique spatiotemporal activation of autophagy. Thus, a better understanding of autophagy process will lead us to not only elucidate the conundrum how autophagy plays dual roles in the regulation of cellular senescence but also helps the development of new therapeutic strategies for many human diseases associated with cellular senescence. We address the pro-senescence and anti-senescence roles of autophagy while focusing on the potential mechanistic aspects of this complex relationship between autophagy and cellular senescence.

A gain-of-function senescence bypass screen identifies the homeobox transcription factor DLX2 as a regulator of ATM-p53 signaling.

Senescence stimuli activate multiple tumor suppressor pathways to initiate cycle arrest and a differentiation program characteristic of senescent cells. We performed a two-stage, gain-of-function screen to select for the genes whose enhanced expression can bypass replicative senescence. We uncovered multiple genes known to be involved in p53 and Rb regulation and ATM regulation, two components of the CST (CTC1-STN1-TEN1) complex involved in preventing telomere erosion, and genes such as REST and FOXO4 that have been implicated in aging. Among the new genes now implicated in senescence, we identified DLX2, a homeobox transcription factor that has been shown to be required for tumor growth and metastasis and is associated with poor cancer prognosis. Growth analysis showed that DLX2 expression led to increased cellular replicative life span. Our data suggest that DLX2 expression reduces the protein components of the TTI1/TTI2/TEL2 complex, a key complex required for the proper folding and stabilization of ATM and other members of the PIKK (phosphatidylinositol 3-kinase-related kinase) family kinase, leading to reduced ATM-p53 signaling and senescence bypass. We also found that the overexpression of DLX2 exhibited a mutually exclusive relationship with p53 alterations in cancer patients. Our functional screen identified novel players that may promote tumorigenesis by regulating the ATM-p53 pathway and senescence.

The DNA damage response induces inflammation and senescence by inhibiting autophagy of GATA4.

Cellular senescence is a terminal stress-activated program controlled by the p53 and p16(INK4a) tumor suppressor proteins. A striking feature of senescence is the senescence-associated secretory phenotype (SASP), a pro-inflammatory response linked to tumor promotion and aging. We have identified the transcription factor GATA4 as a senescence and SASP regulator. GATA4 is stabilized in cells undergoing senescence and is required for the SASP. Normally, GATA4 is degraded by p62-mediated selective autophagy, but this regulation is suppressed during senescence, thereby stabilizing GATA4. GATA4 in turn activates the transcription factor NF-κB to initiate the SASP and facilitate senescence. GATA4 activation depends on the DNA damage response regulators ATM and ATR, but not on p53 or p16(INK4a). GATA4 accumulates in multiple tissues, including the aging brain, and could contribute to aging and its associated inflammation.

Death-associated protein kinase (DAPK) and signal transduction: fine-tuning of autophagy in Caenorhabditis elegans homeostasis.

Autophagy is an evolutionarily conserved lysosomal pathway used to degrade and recycle long-lived proteins and cytoplasmic organelles. This homeostatic ability makes autophagy an important pro-survival mechanism in response to several stresses, such as nutrient starvation, hypoxia, damaged mitochondria, protein aggregation and pathogens. However, several recent studies have highlighted that autophagy also acts as a pro-death mechanism. What on the surface seem like conflicting roles of autophagy may be explained by the fact that the decision between pro-survival and pro-death is determined by the level of activation. A better understanding of autophagy signaling pathways will be helpful to elucidate how the level of autophagy is precisely regulated under different conditions and eventually how the final outcome is decided. In this review, we briefly discuss the pro-survival and pro-death roles of autophagy, and then discuss the mechanism by which autophagy is regulated, mainly focusing on death-associated protein kinase in the nematode Caenorhabditis elegans.

To be or not to be, the level of autophagy is the question: dual roles of autophagy in the survival response to starvation.

Autophagy is an evolutionally conserved lysosomal pathway used to degrade and turn over long-lived proteins and cytoplasmic organelles. Since autophagy was discovered, it has been thought to act as a pro-survival response to several stresses, especially starvation, at the cell and organism levels by providing recycled metabolic substrates to maintain energy homeostasis. However, several recent studies suggest that autophagy also plays a pro-death role through an autophagic cell death pathway mostly at the cellular level. The mechanism by which autophagy could perform these seemingly opposite roles as a pro-survival and a pro-death mechanism remained elusive until recently. Using C. elegans as a model system, we found that physiological levels of autophagy promote optimal survival of C. elegans during starvation, but either insufficient or excessive levels of autophagy render C. elegans starvation-hypersensitive. Furthermore, we found that muscarinic acetylcholine receptor signaling is important in modulating the level of autophagy during starvation, perhaps through DAP kinase and RGS-2. Our recent study provides in vivo evidence that levels of autophagy are critical in deciding its promotion of either survival or death: Physiological levels of autophagy are pro-survival, whereas insufficient or excessive levels of autophagy are pro-death.

Filamin is essential for shedding of the transmembrane serine protease, epithin.

Epithin is a type II transmembrane serine protease that exists in a soluble and membrane-bound form. Shedding is thought to be important in regulating its action, but little is known regarding the intracellular events that trigger such shedding. Here, we show that phorbol myristate acetate (PMA) causes the release of epithin. It also causes accumulation of the protein at the site of cell-cell contacts, and this accumulation is dependent on the formation of cortical actin. In addition, we have identified the actin-binding protein, filamin, as the linker between epithin and the actin cytoskeleton. The interaction of epithin and filamin was enhanced by PMA, and epithin was not released from filamin-deficient M2 cells. We also show that the release of epithin does not require its own activity and is blocked by a metalloprotease inhibitor, GM6001. These results show that filamin has an essential role in shedding by linking epithin to the as yet unidentified metalloprotease-shedding enzyme(s).