Growth Arrest In Response Research Articles

Ferric reduction by a CYBDOM protein counteracts increased iron availability in root meristems induced by phosphorus deficiency

To mobilize sparingly available phosphorus (P) in the rhizosphere, many plant species secrete malate to release P sorbed onto (hydr)oxides of aluminum and iron (Fe). In the presence of Fe, malate can provoke Fe over-accumulation in the root apoplast, triggering a series of events that inhibit root growth. Here, we identified HYPERSENSITIVE TO LOW P1 (HYP1), a CYBDOM protein constituted of a DOMON and a cytochrome b561 domain, as critical to maintain cell elongation and meristem integrity under low P. We demonstrate that HYP1 mediates ascorbate-dependent trans-plasma membrane electron transport and can reduce ferric and cupric substrates in Xenopus laevis oocytes and in planta. HYP1 expression is up-regulated in response to P deficiency in the proximal zone of the root apical meristem. Disruption of HYP1 leads to increased Fe and callose accumulation in the root meristem and causes significant transcriptional changes in roots. We further demonstrate that HYP1 activity overcomes malate-induced Fe accumulation, thereby preventing Fe-dependent root growth arrest in response to low P. Collectively, our results uncover an ascorbate-dependent metalloreductase that is critical to protect root meristems of P-deficient plants from increased Fe availability and provide insights into the physiological function of the yet poorly characterized but ubiquitous CYBDOM proteins.

The Arabidopsis katamari2 Mutant Exhibits a Hypersensitive Seedling Arrest Response at the Phase Transition from Heterotrophic to Autotrophic Growth.

Young seedlings use nutrients stored in the seeds to grow and acquire photosynthetic potential. This process, called seedling establishment, involves a developmental phase transition from heterotrophic to autotrophic growth. Some membrane-trafficking mutants of Arabidopsis (Arabidopsis thaliana), such as the katamari2 (kam2) mutant, exhibit growth arrest during seedling development, with a portion of individuals failing to develop true leaves on sucrose-free solid medium. However, the reason for this seedling arrest is unclear. In this study, we show that seedling arrest is a temporal growth arrest response that occurs not only in kam2 but also in wild-type (WT) Arabidopsis; however, the threshold for this response is lower in kam2 than in the WT. A subset of the arrested kam2 seedlings resumed growth after transfer to fresh sucrose-free medium. Growth arrest in kam2 on sucrose-free medium was restored by increasing the gel concentration of the medium or covering the surface of the medium with a perforated plastic sheet. WT Arabidopsis seedlings were also arrested when the gel concentration of sucrose-free medium was reduced. RNA sequencing revealed that transcriptomic changes associated with the rate of seedling establishment were observed as early as 4 d after sowing. Our results suggest that the growth arrest of both kam2 and WT seedlings is an adaptive stress response and is not simply caused by the lack of a carbon source in the medium. This study provides a new perspective on an environmental stress response under unfavorable conditions during the phase transition from heterotrophic to autotrophic growth in Arabidopsis.

Novel Compounds Derived from DFPM Induce Root Growth Arrest through the Specific VICTR Alleles of Arabidopsis Accessions.

The small compound [5-(3,4-dichlorophenyl) furan-2-yl]-piperidine-1-ylmethanethione (DFPM) inhibits ABA responses by activating effector-triggered immune signal transduction in Arabidopsis. In addition to the known function of DFPM as an antagonist of ABA signaling, DFPM causes accession-specific root growth arrest in Arabidopsis Columbia-0 via the TIR-NLR protein VICTR (VARIATION IN COMPOUND TRIGGERED ROOT growth response) in an EDS1/PAD4/RAR1/SGT1B-dependent manner. Although DFPM could control the specific steps of various cellular responses, the functional residues for the activity of DFPM or the existence of a stronger version of DFPM modification have not been characterized thoroughly. This study analyzed twenty-two DFPM derivatives during root growth arrest, inhibition of ABA signaling, and induction of biotic signal transduction to determine critical residues that confer the specific activity of DFPM. Furthermore, this study identified two more Arabidopsis accessions that generate significant root growth arrest in response to DFPM derivatives dependent on multiple amino acid polymorphisms in the coding region of VICTR. The isolation of novel compounds, such as DFPM-5, and specific amino acid polymorphisms critical for the compound-induced responses will help determine the detailed regulatory mechanism for how DFPM regulates abiotic and biotic stress signaling interactions.

Supraphysiological Androgens Promote the Tumor Suppressive Activity of the Androgen Receptor through cMYC Repression and Recruitment of the DREAM Complex.

Determining the molecular pathways by which supraphysiological androgens promote growth arrest and treatment responses in prostate cancer provides opportunities for biomarker-selected clinical trials and the development of strategies to augment responses.

The BET inhibitor/degrader ARV-825 prolongs the growth arrest response to Fulvestrant + Palbociclib and suppresses proliferative recovery in ER-positive breast cancer.

Anti-estrogens or aromatase inhibitors in combination with cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors are the current standard of care for estrogen receptor-positive (ER+) Her-2 negative metastatic breast cancer. Although these combination therapies prolong progression-free survival compared to endocrine therapy alone, the growth-arrested state of residual tumor cells is clearly transient. Tumor cells that escape what might be considered a dormant or quiescent state and regain proliferative capacity often acquire resistance to further therapies. Our studies are based upon the observation that breast tumor cells arrested by Fulvestrant + Palbociclib enter into states of both autophagy and senescence from which a subpopulation ultimately escapes, potentially contributing to recurrent disease. Autophagy inhibition utilizing pharmacologic or genetic approaches only moderately enhanced the response to Fulvestrant+ Palbociclib in ER+ MCF-7 breast tumor cells, slightly delaying proliferative recovery. In contrast, the BET inhibitor/degrader, ARV-825, prolonged the growth arrested state in both p53 wild type MCF-7 cells and p53 mutant T-47D cells and significantly delayed proliferative recovery. In addition, ARV-825 added after the Fulvestrant + Palbociclib combination promoted apoptosis and demonstrated efficacy in resistant RB deficient cell lines. These studies indicate that administration of BET inhibitors/degraders, which are currently being investigated in multiple clinical trials, may potentially improve standard of care therapy in metastatic ER+ breast cancer patients and may further prolong progression-free survival.

Metabolic Reprogramming and Longevity in Quiescence.

Since Jacques Monod's foundational work in the 1940s, investigators studying bacterial physiology have largely (but not exclusively) focused on the exponential phase of bacterial cultures, which is characterized by rapid growth and high biosynthesis activity in the presence of excess nutrients. However, this is not the predominant state of bacterial life. In nature, most bacteria experience nutrient limitation most of the time. In fact, investigators even prior to Monod had identified other aspects of bacterial growth, including what is now known as the stationary phase, when nutrients become limiting. This review will discuss how bacteria transition to growth arrest in response to nutrient limitation through changes in transcription, translation, and metabolism. We will then examine how these changes facilitate survival during potentially extended periods of nutrient limitation, with particular attention to the metabolic strategies that underpin bacterial longevity in this state.

INTERMEDIUM-C mediates the shade-induced bud growth arrest in barley.

Tiller formation is a key agronomic determinant for grain yield in cereal crops. The modulation of this trait is controlled by transcriptional regulators and plant hormones, tightly regulated by external environmental conditions. While endogenous (genetic) and exogenous (environmental factors) triggers for tiller formation have mostly been investigated separately, it has remained elusive how they are integrated into the developmental program of this trait. The transcription factor gene INTERMEDIUM-C (INT-C), which is the barley ortholog of the maize domestication gene TEOSINTE BRANCHED1 (TB1), has a prominent role in regulating tiller bud outgrowth. Here we show that INT-C is expressed in tiller buds, required for bud growth arrest in response to shade. In contrast to wild-type plants, int-c mutant plants are impaired in their shade response and do not stop tiller production after shading. Gene expression levels of INT-C are up-regulated under light-limiting growth conditions, and down-regulated after decapitation. Transcriptome analysis of wild-type and int-c buds under control and shading conditions identified target genes of INT-C that belong to auxin and gibberellin biosynthesis and signaling pathways. Our study identifies INT-C as an integrator of the shade response into tiller formation, which is prerequisite for implementing shading responses in the breeding of cereal crops.

EfgA is a conserved formaldehyde sensor that leads to bacterial growth arrest in response to elevated formaldehyde.

Normal cellular processes give rise to toxic metabolites that cells must mitigate. Formaldehyde is a universal stressor and potent metabolic toxin that is generated in organisms from bacteria to humans. Methylotrophic bacteria such as Methylorubrum extorquens face an acute challenge due to their production of formaldehyde as an obligate central intermediate of single-carbon metabolism. Mechanisms to sense and respond to formaldehyde were speculated to exist in methylotrophs for decades but had never been discovered. Here, we identify a member of the DUF336 domain family, named efgA for enhanced formaldehyde growth, that plays an important role in endogenous formaldehyde stress response in M. extorquens PA1 and is found almost exclusively in methylotrophic taxa. Our experimental analyses reveal that EfgA is a formaldehyde sensor that rapidly arrests growth in response to elevated levels of formaldehyde. Heterologous expression of EfgA in Escherichia coli increases formaldehyde resistance, indicating that its interaction partners are widespread and conserved. EfgA represents the first example of a formaldehyde stress response system that does not involve enzymatic detoxification. Thus, EfgA comprises a unique stress response mechanism in bacteria, whereby a single protein directly senses elevated levels of a toxic intracellular metabolite and safeguards cells from potential damage.

Cellular Senescence Triggers Altered Circadian Clocks With a Prolonged Period and Delayed Phases

Senescent cells, which show the permanent growth arrest in response to various forms of stress, accumulate in the body with the progression of age, and are associated with aging and age-associated diseases. Although the senescent cells are growth arrested, they still demonstrate high metabolic rate and altered gene expressions, indicating that senescent cells are still active. We recently showed that the circadian clock properties, namely phase and period of the cells, are altered with the establishment of replicative senescence. However, whether cellular senescence triggers the alteration of circadian clock properties in the cells is still unknown. In this study we show that the oxidative stress-induced premature senescence induces the alterations of the circadian clock, similar to the phenotypes of the replicative senescent cells. We found that the oxidative stress-induced premature senescent cells display the prolonged period and delayed phases. In addition, the magnitude of these changes intensified over time, indicating that cellular senescence changes the circadian clock properties. Our current results corroborate with our previous findings and further confirm that cellular senescence induces altered circadian clock properties, irrespective of the replicative senescence or the stress-induced premature senescence.

NMR-based metabonomic analysis of HUVEC cells during replicative senescence.

Cellular senescence is a physiological process reacting to stimuli, in which cells enter a state of irreversible growth arrest in response to adverse consequences associated with metabolic disorders. Molecular mechanisms underlying the progression of cellular senescence remain unclear. Here, we established a replicative senescence model of human umbilical vein endothelial cells (HUVEC) from passage 3 (P3) to 18 (P18), and performed biochemical characterizations and NMR-based metabolomic analyses. The cellular senescence degree advanced as the cells were sequentially passaged in vitro, and cellular metabolic profiles were gradually altered. Totally, 8, 16, 21 and 19 significant metabolites were primarily changed in the P6, P10, P14 and P18 cells compared with the P3 cells, respectively. These metabolites were mainly involved in 14 significantly altered metabolic pathways. Furthermore, we observed taurine retarded oxidative damage resulting from senescence. In the case of energy deficiency, HUVECs metabolized neutral amino acids to replenish energy, thus increased glutamine, aspartate and asparagine at the early stages of cellular senescence but decreased them at the later stages. Our results indicate that cellular replicative senescence is closely associated with promoted oxidative stress, impaired energy metabolism and blocked protein synthesis. This work may provide mechanistic understanding of the progression of cellular senescence.

Human Skin Keratinocytes on Sustained TGF-β Stimulation Reveal Partial EMT Features and Weaken Growth Arrest Responses.

Defects in wound closure can be related to the failure of keratinocytes to re-epithelize. Potential mechanisms driving this impairment comprise unbalanced cytokine signaling, including Transforming Growth Factor-β (TFG-β). Although the etiologies of chronic wound development are known, the relevant molecular events are poorly understood. This lack of insight is a consequence of ethical issues, which limit the available evidence to humans. In this work, we have used an in vitro model validated for the study of epidermal physiology and function, the HaCaT cells to provide a description of the impact of sustained exposure to TGF-β. Long term TGF-β1 treatment led to evident changes, HaCaT cells became spindle-shaped and increased in size. This phenotype change involved conformational re-arrangements for actin filaments and E-Cadherin cell-adhesion structures. Surprisingly, the signs of consolidated epithelial-to-mesenchymal transition were absent. At the molecular level, modified gene expression and altered protein contents were found. Non-canonical TGF-β pathway elements did not show relevant changes. However, R-Smads experienced alterations best characterized by decreased Smad3 levels. Functionally, HaCaT cells exposed to TGF-β1 for long periods showed cell-cycle arrest. Yet, the strength of this restraint weakens the longer the treatment, as revealed when challenged by pro-mitogenic factors. The proposed setting might offer a useful framework for future research on the mechanisms driving wound chronification.

SENESCENCE-ASSOCIATED SECRETOME COMPONENTS AS BIOMARKERS FOR AGING

Cellular senescence is a state of stable growth arrest in response to stress, which is a fundamental process of biological aging. They secrete products, the Senescence-Associated Secretory Phenotype (SASP), which consists of inflammatory cytokines, chemokines, growth factors and matrix remodeling proteins. Senescent cells accumulate with advancing age and partial elimination of senescent cells can reverse age-related dysfunction and increase mean lifespan in mice. However, it is not clear whether components of the SASP can be measured in human plasma and serve as aging biomarkers. Here we generated a candidate panel of senescence markers based on a multiplexed bead-based assay of proteins secreted by senescent preadipocytes, endothelial cells, fibroblasts, myoblasts, preadipocytes, and epithelial cells compared to non-senescent controls. The SASP undoubtedly varies by cell type; however, we observed that multiple components of the SASP are conserved. We then assessed circulating SASP components in human plasma samples from Mayo Clinic Biobank participants (n=280, 20 male and 20 female per decade, age 20-90) using the same method. We confirmed that components of the SASP can be quantified in human plasma with the multiplexed bead-based assay and observed several SASP components robustly increase with chronological age in humans. Our study illustrates that senescence-associated secretome components are detectable in human plasma and could potentially serve as biomarkers of systemic aging and senescent cell burden.

Senescent Kidney Cells in Hypertensive Patients Release Urinary Extracellular Vesicles.

BackgroundHypertension may be associated with renal cellular injury. Cells in distress release extracellular vesicles (EVs), and their numbers in urine may reflect renal injury. Cellular senescence, an irreversible growth arrest in response to a noxious milieu, is characterized by release of proinflammatory cytokines. We hypothesized that EVs released by senescent nephron cells can be identified in urine of patients with hypertension.Methods and ResultsWe recruited patients with essential hypertension (EH) or renovascular hypertension and healthy volunteers (n=14 each). Renal oxygenation was assessed using magnetic resonance imaging and blood samples collected from both renal veins for cytokine‐level measurements. EVs isolated from urine samples were characterized by imaging flow cytometry based on specific markers, including p16 (senescence marker), calyxin (podocytes), urate transporter 1 (proximal tubules), uromodulin (ascending limb of Henle's loop), and prominin‐2 (distal tubules). Overall percentage of urinary p16+ EVs was elevated in EH and renovascular hypertension patients compared with healthy volunteers and correlated inversely with renal function and directly with renal vein cytokine levels. Urinary levels of p16+/urate transporter 1+ were elevated in all hypertensive subjects compared with healthy volunteers, whereas p16+/prominin‐2+ levels were elevated only in EH versus healthy volunteers and p16+/uromodulin+ in renovascular hypertension versus EH.ConclusionsLevels of p16+ EVs are elevated in urine of hypertensive patients and may reflect increased proximal tubular cellular senescence. In EH, EVs originate also from distal tubules and in renovascular hypertension from Henle's loop. Hence, urinary EVs levels may be useful to identify intrarenal sites of cellular senescence.

Abscisic acid-dependent histone demethylation during postgermination growth arrest in Arabidopsis.

After germination, seedlings undergo growth arrest in response to unfavourable conditions, a critical adaptation enabling plants to survive harsh environments. The plant hormone abscisic acid (ABA) plays a key role in this arrest. To arrest growth, ABA-dependent transcription factors change gene expression patterns in a flexible and reversible manner. Although the control of gene expression has important roles in growth arrest, the epigenetic mechanisms in the response to ABA are not fully understood. Here, we show that the histone demethylases JUMONJI-C domain-containing protein 30 (JMJ30) and JMJ32 control ABA-mediated growth arrest in Arabidopsis thaliana. During the postgermination stage (2-3days after germination), the ABA-dependent transcription factor ABA-insensitive3 (ABI3) activates the expression of JMJ30 in response to ABA. JMJ30 then removes a repressive histone mark, H3 lysine 27 trimethylation (H3K27me3), from the SNF1-related protein kinase 2.8 (SnRK2.8) promoter, and hence activates SnRK2.8 expression. SnRK2.8 encodes a kinase that activates ABI3 and is responsible for JMJ30- and JMJ32-mediated growth arrest. A feed-forward loop involving the ABI3 transcription factor, JMJ histone demethylases, and the SnRK2.8 kinase fine-tunes ABA-dependent growth arrest in the postgermination phase. Our findings highlight the importance of the histone demethylases in mediating adaptation of plants to the environment.

A cellular threshold for active ERK1/2 levels determines Raf/MEK/ERK-mediated growth arrest versus death responses

In addition to its conventional role for cell proliferation and survival, the Raf/MEK/Extracellular signal-regulated kinase (ERK) pathway can also induce growth arrest and death responses, if aberrantly activated. Here, we determined a molecular basis of ERK1/2 signaling that underlies these growth inhibitory physiological outputs. We found that overexpression of ERK1 or ERK2 switches ΔRaf-1:ER-induced growth arrest responses to caspase-dependent apoptotic death responses in different cell types. These death responses, however, were reverted to growth arrest responses upon titration of cellular phospho-ERK1/2 levels by the MEK1/2 inhibitor AZD6244. These data suggest that a cellular threshold for active ERK1/2 levels exists and affects the cell fate between death and growth arrest. We also found that death-mediating ability of ERK2 is abolished by the catalytic site-disabling Lys52Arg replacement or significantly attenuated by the F-site recruitment site-disabling Tyr261Asn replacement, although unaffected by the mutations that disable the common docking groove or the dimerization interface. Therefore, ERK1/2 mediates death signaling dependently of kinase activity and specific physical interactions. Intriguingly, Tyr261Asn-replaced ERK2 could still mediate growth arrest signaling, further contrasting the molecular basis of ERK1/2-mediated growth arrest and death signaling. These data reveal a mechanism underlying the role of ERK1/2 as a focal point of Raf/MEK/ERK-mediated growth arrest and death signaling.

Persistence of senescent prostate cancer cells following prolonged neoadjuvant androgen deprivation therapy.

PurposeAndrogen deprivation therapy (ADT) commonly leads to incomplete cell death and the fate of persistent cells involves, in part, a senescent phenotype. Senescence is terminal growth arrest in response to cell stress that is characterized by increased lysosomal-β-galactosidase (GLB1) the origin of senescence associated-β-gal activity (SA-β-gal). In the current study senescence is examined in vivo after ADT use in a neoadjuvant trial.Methods and materialsTissue microarrays were generated from prostate cancer specimens (n = 126) from a multicenter neoadjuvant ADT trial. Arrays were subjected to multiplexed immunofluorescent staining for GLB1, Ki67, cleaved caspase 3 (CC3) and E-cadherin. Automated quantitative imaging was performed using Vectra™ and expression correlated with clinicopathologic features.ResultsTissue was analyzed from 59 patients treated with neoadjuvant ADT and 67 receiving no therapy preoperatively. Median follow-up was 85.3 mo and median ADT treatment was 5 mo. In PC treated with neoadjuvant ADT, GLB1 expression increased in intermediate Gleason score (GS 6–7; p = 0.001), but not high grade (GS 8–10) cancer. Significantly higher levels of GLB1 were seen in tissues undergoing neoadjuvant ADT longer than 5 months compared to untreated tissues (p = 0.002). In contrast, apoptosis significantly increased earlier (1–4 mo) after ADT treatment (p<0.5).ConclusionsIncreased GLB1 after neoadjuvant ADT occurs primarily among more clinically favorable intermediate grade cancers and enrichment of the phenotype occurs in a temporally prolonged fashion. Senescence may explain the persistence of PCa cells after ADT. Given concerns for the detrimental longer term presence of senescent cells, targeting these cells for removal may improve outcomes.

MicroRNA Regulation of Oxidative Stress‐Induced Cellular Senescence

Aging is a time-related process of functional deterioration at cellular, tissue, organelle, and organismal level that ultimately brings life to end. Cellular senescence, a state of permanent cell growth arrest in response to cellular stress, is believed to be the driver of the aging process and age-related disorders. The free radical theory of aging, referred to as oxidative stress (OS) theory below, is one of the most studied aging promoting mechanisms. In addition, genetics and epigenetics also play large roles in accelerating and/or delaying the onset of aging and aging-related diseases. Among various epigenetic events, microRNAs (miRNAs) turned out to be important players in controlling OS, aging, and cellular senescence. miRNAs can generate rapid and reversible responses and, therefore, are ideal players for mediating an adaptive response against stress through their capacity to fine-tune gene expression. However, the importance of miRNAs in regulating OS in the context of aging and cellular senescence is largely unknown. The purpose of our article is to highlight recent advancements in the regulatory role of miRNAs in OS-induced cellular senescence.

An E3Ubiquitin Ligase-BAG Protein Module Controls Plant Innate Immunity and Broad-Spectrum Disease Resistance.

Programmed cell death (PCD) and immunity in plants are tightly controlled to promote antimicrobial defense while preventing autoimmunity. However, the mechanisms contributing to this immune homeostasis are poorly understood. Here, we isolated a rice mutant ebr1 (enhanced blight and blast resistance 1) that shows enhanced broad-spectrum bacterial and fungal disease resistance, but displays spontaneous PCD, autoimmunity, and stunted growth. EBR1 encodes an E3 ubiquitin ligase that interacts with OsBAG4, which belongs to the BAG (Bcl-2-associated athanogene) family that functions in cell death, growth arrest, and immune responses in mammals. EBR1 directly targets OsBAG4 for ubiquitination-mediated degradation. Elevated levels of OsBAG4 in rice are necessary and sufficient to trigger PCD and enhanced disease resistance to pathogenic infection, most likely by activating pathogen-associated molecular patterns-triggered immunity (PTI). Together, our study suggests that an E3-BAG module orchestrates innate immune homeostasis and coordinates the trade-off between defense and growth in plants.

53BP1 and USP28 mediate p53 activation and G1 arrest after centrosome loss or extended mitotic duration.

In normal human cells, centrosome loss induced by centrinone-a specific centrosome duplication inhibitor-leads to irreversible, p53-dependent G1 arrest by an unknown mechanism. A genome-wide CRISPR/Cas9 screen for centrinone resistance identified genes encoding the p53-binding protein 53BP1, the deubiquitinase USP28, and the ubiquitin ligase TRIM37. Deletion of TP53BP1, USP28, or TRIM37 prevented p53 elevation in response to centrosome loss but did not affect cytokinesis failure-induced arrest or p53 elevation after doxorubicin-induced DNA damage. Deletion of TP53BP1 and USP28, but not TRIM37, prevented growth arrest in response to prolonged mitotic duration. TRIM37 knockout cells formed ectopic centrosomal-component foci that suppressed mitotic defects associated with centrosome loss. TP53BP1 and USP28 knockouts exhibited compromised proliferation after centrosome removal, suggesting that centrosome-independent proliferation is not conferred solely by the inability to sense centrosome loss. Thus, analysis of centrinone resistance identified a 53BP1-USP28 module as critical for communicating mitotic challenges to the p53 circuit and TRIM37 as an enforcer of the singularity of centrosome assembly.

Sphingosine Kinase‐2/Sphingosine 1‐Phosphate Signaling Regulates Telomere Dysfunction through GADD45 and TCF21 Mediated Tumor Suppressor Mechanisms in Lung Cancer

Sphingosine 1‐phosphate (S1P), a pro‐proliferative sphingolipid generated by sphingosine kinases (SK1 and SK2) has been shown to be up regulated in many cancers. Telomerase, a ribonucleoprotein extends the ends of chromosomes (telomeres), which is involved in cancer pathogenesis and hTERT, the catalytic subunit of telomerase localizes to nucleus to promote lung cancer growth/metastasis. Previously, we have demonstrated that SK2 generated S1Pbinds and stabilizes telomerase in the nuclear periphery by C′3‐OH of S1P and D684 residue of hTERT. Mechanistically, nuclear S1P binding prevented E3ubiquitin ligase MKRN1‐hTERT association, and protected hTERT from degradation, preventing telomere damage, and delaying senescence in non‐cancerous lung fibroblasts (Panneer Selvam et al, Sci Signal, 2015). The objective of the following study is to understand the molecular mechanisms of lung tumor suppression mediated by SK2‐S1P pathway. First, we identified the protein levels of SK2 and hTERT to be upregulated in both non‐small cell (N=48) and small cell lung cancer (N=40) tumor tissues. Mechanistically, pharmacological inhibition of SK2 by ABC294640 drug (in Phase I clinical trials) leads to telomere dysfunction induced foci (TIF) formation in A549 cells as observed by colocalization of γ‐H2AX and TRF‐2 and interestingly wild type hTERT overexpression prevented TIF formation whereas hTERTD684A expression (lacks S1P binding) did not. In vivo, shRNA mediated SK2 knockdown tumors displayed reduced tumor volume, decreased hTERT protein levels and increased TUNEL positive cells. Moreover, pharmacological inhibition of SK2 by ABC294640 (100mg/kg/day body weight for 16days) in an orthotopic mice model for lung cancer showed decreased cancer proliferation as measured by bioluminescence. Interestingly, qPCR based gene array studies revealed ~5 fold increase in the expression levels of epigenetically silenced tumor suppressor TCF21 in stable shSK2 tumors compared to scrambled control tumors. Moreover, we observed increased protein expression of GADD45 (growth arrest and DNA damage response protein), an upstream regulator of TCF21 with concomitant increase in the mRNA levels of TCF21 following SK2 inhibition. Interestingly, Stable shRNA mediated knockdown of TCF21 in A549cells following xenograft studies in SCID mice showed significant protection against ABC294640 mediated tumor suppression. Interestingly, in in vitro experiments we found caspase‐3activation following treatment with ABC294640 and stable shRNA mediated knockdown of GADD45 and TCF21 prevented caspase‐3 activation suggesting caspase‐3activation downstream of GADD45 and TCF21. Also, ABC294640 treated cells showed decreased cell viability and shGADD45 and shTCF21 cells showed significant protection against cell death. Overall, our data suggest that SK2 generated S1Pregulates telomere dysfunction in cancer cells through hTERT thereby leading to GADD45 and TCF21 mediated tumor suppression in lung cancer.Support or Funding InformationNCI 88932‐06