Crosstalk between metabolism and cell death in tumorigenesis.

It is generally recognized that tumor cells proliferate more rapidly than normal cells. Due to such an abnormally rapid proliferation rate, cancer cells constantly encounter the limits of insufficient oxygen and nutrient supplies. To satisfy their growth needs and resist adverse environmental events, tumor cells modify the metabolic pathways to produce both extra energies and substances required for rapid growth. Realizing the metabolic characters special for tumor cells will be helpful for eliminating them during therapy. Cell death is a hot topic of long-term study and targeting cell death is one of the most effective ways to repress tumor growth. Many studies have successfully demonstrated that metabolism is inextricably linked to cell death of cancer cells. Here we summarize the recently identified metabolic characters that specifically impact on different types of cell deaths and discuss their roles in tumorigenesis.

USP5 facilitates bladder cancer progression by stabilizing the c-Jun protein.

BACKGROUND: Bladder cancer is the second most common genitourinary malignancy worldwide. The death rate of bladder cancer has increased every year. However, the molecular mechanism of bladder cancer is not sufficiently studied. Deubiquitinating enzymes (DUBs) play an important role in carcinogenesis. Several studies have demonstrated that USP5 associated with malignancy and pathological progression in hepatocellular carcinoma, colorectal and non-small cell lung cancer. However, the role of USP5 in bladder cancer need to be explored. METHODS: The USP5 expression was analysed using the web server GEPIA. To explore USP5 function in bladder cancer, we constructed USP5-knockout cell lines in T24 cells. A FLAG-USP5 (WT USP5) plasmid and a plasmid FLAG-USP5 C335A (catalytic-inactive mutant) used to overexpress USP5 in EJ cells. CCK8, colony formation, transwell and scratch assays were used to assess cell viability, proliferation and migration. RNA sequencing (RNA-seq) and dual-luciferase reporter assays were performed to screen the pathway. Coimmunoprecipitation and immunofluorescence were used to explore the interaction between USP5 and c-Jun. Cycloheximide (CHX) chase assays were performed to establish the effect of USP5 on c-Jun stability. Xenograft mouse model was used to study the role of USP5 in bladder cancer. RESULTS: USP5 expression is increased in bladder cancer patients. Genetic ablation of USP5 markedly inhibited bladder cancer cell proliferation, viability, and migration both in vitro and in vivo. RNA-seq and luciferase pathway screening showed that USP5 activated JNK signalling, and we identified the interaction between USP5 and c-Jun. USP5 was found to activate c-Jun by inhibiting its ubiquitination. CONCLUSIONS: Our results show that high USP5 expression promotes bladder cancer progression by stabilizing c-Jun and that USP5 is a potential therapeutic target in bladder cancer.

Activation of CTNNB1 by deubiquitinase UCHL3-mediated stabilization facilitates bladder cancer progression.

BACKGROUND: The catenin beta 1 gene (CTNNB1) plays a crucial role in the malignant progression of various cancers. Recent studies have suggested that CTNNB1 hyperactivation is closely related to the occurrence and development of bladder cancer (BCa). As a member of the deubiquitinating enzyme (DUB) family, ubiquitin C-terminal hydrolase L3 (UCHL3) is abnormally expressed in various cancers. In this study, we discovered that UCHL3 is a novel oncogene in bladder cancer, suggesting it is a promising target against bladder cancer. METHODS: We utilized CRISPR‒Cas9 technology to construct cell lines with UCHL3 stably overexpressed or knocked out. The successful overexpression or knockout of UCHL3 was determined using Western blotting. Then, we performed CCK-8, colony formation, soft agar and Transwell migration assays to determine the impact of the UCHL3 gene on cell phenotype. RNA-seq was performed with UCHL3-depleted T24 cells (established via CRISPR-Cas9-mediated genomic editing). We analyzed differences in WNT pathway gene expression in wild-type and UCHL3-deficient T24 cell lines using a heatmap and by gene set enrichment analysis (GSEA). Then, we validated the effect of UCHL3 on the Wnt pathway using a dual fluorescence reporter. We then analyzed the underlying mechanisms involved using Western blots, co-IP, and immunofluorescence results. We also conducted nude mouse tumor formation experiments. Moreover, conditional UCHL3-knockout mice and bladder cancer model mice were established for research. RESULTS: We found that the overexpression of UCHL3 boosted bladder cancer cell proliferation, invasion and migration, while the depletion of UCHL3 in bladder cancer cells delayed tumor tumorigenesis in vitro and in vivo. UCHL3 was highly associated with the Wnt signaling pathway and triggered the activation of the Wnt signaling pathway, which showed that its functions depend on its deubiquitination activity. Notably, Uchl3-deficient mice were less susceptible to bladder tumorigenesis. Additionally, UCHL3 was highly expressed in bladder cancer cells and associated with indicators of advanced clinicopathology. CONCLUSION: In summary, we found that UCHL3 is amplified in bladder cancer and functions as a tumor promoter that enhances proliferation and migration of tumor cells in vitro and bladder tumorigenesis and progression in vivo. Furthermore, we revealed that UCHL3 stabilizes CTNNB1 expression, resulting in the activation of the oncogenic Wnt signaling pathway. Therefore, our findings strongly suggest that UCHL3 is a promising therapeutic target for bladder cancer.

OTUB1 facilitates bladder cancer progression by stabilizing ATF6 in response to endoplasmic reticulum stress.

The unfolded protein response (UPR) plays an important role in carcinogenesis, but the functional role and mechanism of UPR-associated bladder carcinogenesis remain to be characterized. Upon UPR activation, ATF6α is activated to upregulate the transcription of UPR target genes. Although the mechanism of ATF6 activation has been studied extensively, the negative regulation of ATF6 stabilization is not well understood. Here, we report that the deubiquitinase otubain 1 (OTUB1) facilitates bladder cancer progression by stabilizing ATF6 in response to endoplasmic reticulum stress. OTUB1 expression is raised in bladder cancer patients. Genetic ablation of OTUB1 markedly inhibited bladder cancer cell proliferation, viability, and migration both in vitro and in vivo. Mechanistically, luciferase pathway screening showed that ATF6 signaling was clearly activated compared with other pathways. OTUB1 was found to activate ATF6 signaling by inhibiting its ubiquitylation, thereby remodeling the stressed cells through transcriptional regulation. Our results show that high OTUB1 expression promotes bladder cancer progression by stabilizing ATF6 and that OTUB1 is a potential therapeutic target in bladder cancer.

Nemo-like kinase (NLK) negatively regulates NF-kappa B activity through disrupting the interaction of TAK1 with IKKß.

Stringent negative regulation of the transcription factor NF-κB is essential for maintaining cellular stress responses and homeostasis. However, the tight regulation mechanisms of IKKß are still not clear. Here, we reported that nemo-like kinase (NLK) is a suppressor of tumor necrosis factor (TNFα)-induced NF-κB signaling by inhibiting the phosphorylation of IKKß. Overexpression of NLK largely blocked TNFα-induced NF-κB activation, p65 nuclear localization and IκBα degradation; whereas genetic inactivation of NLK showed opposing results. Mechanistically, we identified that NLK interacted with IκB kinase (IKK)-associated complex, which in turn inhibited the assembly of the TAK1/IKKß and thereby, diminished the IκB kinase phosphorylation. Our results indicate that NLK functions as a pivotal negative regulator in TNFα-induced activation of NF-κB via disrupting the interaction of TAK1 with IKKß.

Carnosic acid, a novel food-source AT1R antagonist and its anti-hypertension mechanism.

Hypertension is the most prevalent non-communicable disease, affecting billions of people worldwide. Discovery and development of natural antihypertensive lead compounds or drugs are important to resolve the limitations of existing antihypertensive drug safety and resistance. This investigation verified that carnosic acid (CA), an important active ingredient of rosemary, an edible spice plant, indicates a significant anti-hypertensive activity in spontaneous hypertension rats by targeting AT1R. Moreover, our research indicated that CA shared a comparable antagonistic mechanism with established synthetic angiotensin II receptor blockers (ARBs), as it occupies the binding sites of Angiotensin II (AngII) at His6 and Pro7 within the AT1R's ligand-binding pocket. Notably, CA exerted better anti-hypertensive activity since it could not break the Asn1113.35-Asn2957.46 hydrogen bond to stabilize the AT1R inactive state. As the first potent AT1R antagonist identified in a natural food source, CA is poised to become a novel anti-hypertensive lead compound, distinguished by its unique skeleton structure different from conventional ARBs. This research lays a valuable theoretical groundwork for the future exploration of CA and rosemary extract in both fundamental studies and clinical applications.

CLK2 mediates IκBα-independent early termination of NF-κB activation by inducing cytoplasmic redistribution and degradation.

Activation of the NF-κB pathway is strictly regulated to prevent excessive inflammatory and immune responses. In a well-known negative feedback model, IκBα-dependent NF-κB termination is a delayed response pattern in the later stage of activation, and the mechanisms mediating the rapid termination of active NF-κB remain unclear. Here, we showed IκBα-independent rapid termination of nuclear NF-κB mediated by CLK2, which negatively regulated active NF-κB by phosphorylating the RelA/p65 subunit of NF-κB at Ser180 in the nucleus to limit its transcriptional activation through degradation and nuclear export. Depletion of CLK2 increased the production of inflammatory cytokines, reduced viral replication and increased the survival of the mice. Mechanistically, CLK2 phosphorylated RelA/p65 at Ser180 in the nucleus, leading to ubiquitin‒proteasome-mediated degradation and cytoplasmic redistribution. Importantly, a CLK2 inhibitor promoted cytokine production, reduced viral replication, and accelerated murine psoriasis. This study revealed an IκBα-independent mechanism of early-stage termination of NF-κB in which phosphorylated Ser180 RelA/p65 turned off posttranslational modifications associated with transcriptional activation, ultimately resulting in the degradation and nuclear export of RelA/p65 to inhibit excessive inflammatory activation. Our findings showed that the phosphorylation of RelA/p65 at Ser180 in the nucleus inhibits early-stage NF-κB activation, thereby mediating the negative regulation of NF-κB.

ALLN hinders HCT116 tumor growth through Bax-dependent apoptosis.

Continual high expression of cysteine proteases calpain I and II have been implicated in tumorigenicity; conversely, N-acetyl-leu-leunorleucinal (ALLN), which inhibits calpain I and II, should also influence tumor growth and carcinogenesis. To explore the role of ALLN against colon cancer and in promoting apoptosis, we used colon cancer HCT116 cell lines, p53 or Bax-deficient HCT116 cell lines. Cell viability and tumor growth decreased in a concentration-dependent manner when treated with 0-26µM ALLN. Treatment with ALLN induced apoptosis in HCT116 cell; however, flow cytometry showed that apoptosis significantly decreased in Bax-deficient HCT116 cell lines, but not in p53-deficient HCT116 cell lines. In addition, the ALLN-induced apoptosis response was through Bax translocation from cytosol to mitochondria. In this study we showed intraperitoneally injected ALLN to inhibit colon tumor formation in nude mice, and found ALLN to inhibit tumor growth in colon cancer cells, mainly through apoptosis that depends on translocation of Bax to a mitochondrial endogenous pathway; this implies a molecular mechanism for ALLN against human colon cancer. These results suggest that ALLN could become a novel agent for prevention of colon cancer.

Mitochondrial dynamics in neurological diseases: a narrative review.

Background and Objective: The mitochondrion is a crucial organelle for aerobic respiration and energy metabolism. It undergoes dynamic changes, including changes in its shape, function, and distribution through fission, fusion, and movement. Under normal conditions, mitochondrial dynamics are in homeostasis. However, once the balance is upset, the nervous system, which has high metabolic demands, will most likely be affected. Recent studies have shown that the imbalance of mitochondrial dynamics is involved in the occurrence and development of various neurological diseases. However, whether the regulation of mitochondrial dynamics can be used to treat neurological diseases is still unclear. We aimed to comprehensively analyze mitochondrial dynamics regulation and its potential role in the treatment of neurological diseases. Methods: A comprehensive literature review was carried out to understand the mechanisms and applications of mitochondrial dynamics in neurological diseases based on the literature available in PubMed, Web of Science, and Google Scholar. Key Content and Findings: This review discusses the molecular mechanisms related to mitochondrial dynamics and expounds upon the role of mitochondrial dynamics in the occurrence and development of neurodegenerative diseases, epilepsy, cerebrovascular disease, and brain tumors. Several clinically tested drugs with fewer side effects have been shown to improve the mitochondrial dynamics and nervous system function in neurological diseases. Conclusions: Disorders of mitochondrial dynamics can cause various neurological diseases. Elucidation of mechanisms and applications involved in mitochondrial dynamics will inform the development of new therapeutic targets and strategies for neurological diseases. Dynamin-related protein 1 (Drp1), as a highly relevant molecular for mitochondrial dynamics, might be a potential target for treating neurological diseases in the future.

USP14 promotes colorectal cancer progression by targeting JNK for stabilization.

MAPK/JNK signaling is pivotal in carcinogenesis. However, ubiquitin-mediated homeostasis of JNK remains to be verified. Here, with results from RNA sequencing (RNA-seq) and luciferase reporter pathway identification, we show that USP14 orchestrates MAPK/JNK signaling and identify USP14 as a deubiquitinase that interacts and stabilizes JNK. USP14 is elevated in colorectal cancer patients and is positively associated with JNK protein and downstream gene expression. USP14 ablation reduces cancer cell proliferation in vitro and colorectal tumorigenesis in vivo by downregulating MAPK/JNK pathway activation. Moreover, USP14 expression is induced by TNF-α, forming a feedback loop with JNK and leading to tumor amplification. Our study suggests that elevated expression of USP14 promotes MAPK/JNK signaling by stabilizing JNK, which in turn augments colorectal carcinogenesis, indicating a potential therapeutic target for colorectal cancer patients with increased USP14 expression.

NLK is required for Ras/ERK/SRF/ELK signaling to tune skeletal muscle development by phosphorylating SRF and antagonizing the SRF/MKL pathway.

Serum response factor (SRF) regulates differentiation and proliferation by binding to RhoA-actin-activated MKL or Ras-MAPK-activated ELK transcriptional coactivators, but the molecular mechanisms responsible for SRF regulation remain unclear. Here, we show that Nemo-like kinase (NLK) is required for the promotion of SRF/ELK signaling in human and mouse cells. NLK was found to interact with and phosphorylate SRF at serine residues 101/103, which in turn enhanced the association between SRF and ELK. The enhanced affinity of SRF/ELK antagonized the SRF/MKL pathway and inhibited mouse myoblast differentiation in vitro. In a skeletal muscle-specific Nlk conditional knockout mouse model, forming muscle myofibers underwent hypertrophic growth, resulting in an increased muscle and body mass phenotype. We propose that both phosphorylation of SRF by NLK and phosphorylation of ELKs by MAPK are required for RAS/ELK signaling, confirming the importance of this ancient pathway and identifying an important role for NLK in modulating muscle development in vivo.

PTPN18 promotes colorectal cancer progression by regulating the c-MYC-CDK4 axis.

Protein tyrosine phosphatase non-receptor type 18 (PTPN18) is often highly expressed in colorectal cancer (CRC), but its role in this disease remains unclear. We demonstrated that PTPN18 overexpression promotes growth and tumorigenesis in CRC cells and that PTPN18 deficiency yields the opposite results in vitro. Moreover, a xenograft assay showed that PTPN18 deficiency significantly inhibited tumorigenesis in vivo. PTPN18 activated the MYC signaling pathway and enhanced CDK4 expression, which is tightly associated with the cell cycle and proliferation in cancer cells. Finally, we found that MYC interacted with PTPN18 and increased the protein level of MYC. In conclusion, our results suggest that PTPN18 promotes CRC development by stabilizing the MYC protein level, which in turn activates the MYC-CDK4 axis. Thus, PTPN18 could be a novel therapeutic target in the future.

UCHL3 promotes ovarian cancer progression by stabilizing TRAF2 to activate the NF-κB pathway.

The inflammatory response plays an important role in carcinogenesis. However, the functional role and mechanism of the UCHL3-associated inflammatory response in ovarian cancer remain to be characterized. Here, we report that increased expression of UCHL3 facilitates tumourigenesis by targeting TRAF2 protein, thereby enhancing the inflammatory response. The expression of UCHL3 is elevated in ovarian cancer patients and is associated with an unfavourable prognosis. Genetic ablation of UCHL3 was found to markedly block ovarian cancer cell proliferation, viability and migration both in vitro and in vivo. Mechanistically, luciferase pathway screening results show that NF-κB signalling is clearly activated compared with other pathways. UCHL3 was found to activate NF-κB signalling by deubiquitinating and stabilizing TRAF2, leading to tumourigenesis. Our results indicate that highly expressed UCHL3 enhances inflammation by stabilizing TRAF2, which in turn facilitates tumourigenesis in ovarian cancer, and that UCHL3 is a potential target for ovarian cancer patients with increased inflammation.

HDAC2 inhibits EMT-mediated cancer metastasis by downregulating the long noncoding RNA H19 in colorectal cancer.

BACKGROUND: Emerging evidence suggests that epithelial mesenchymal transition (EMT) and epigenetic mechanisms promote metastasis. Histone deacetylases (HDACs) and noncoding RNAs (ncRNAs) are important epigenetic regulators. Here, we elucidated a novel role of histone deacetylase 2 (HDAC2) in regulating EMT and CRC metastasis via ncRNA. METHODS: The expression of HDACs in CRC was analyzed using the public databases and matched primary and metastatic tissues, and CRC cells with different metastatic potentials (DLD1, HCT116, SW480 and SW620). Microarray analysis was used to identify differential genes in parental and HDAC2 knockout CRC cells. EMT and histone modifications were determined using western blot and immunofluorescence. Migration ability was assessed by transwell assay, and metastasis was assessed in vivo using a tail vain injection. Gene expression and regulation was assessed by RT-PCR, chromatin immunoprecipitation and reporter assays. Protein interaction was assessed by immunoprecipitation. Specific siRNAs targeting H19, SP1 and MMP14 were used to validate their role in HDAC2 loss induced EMT and metastasis. RESULTS: Reduced HDAC2 expression was associated with poor prognosis in CRC patients and found in CRC metastasis. HDAC2 deletion or knockdown induced EMT and metastasis by upregulating the long noncoding RNA H19 (LncRNA H19). HDAC2 inhibited LncRNA H19 expression by histone H3K27 deacetylation in its promoter via binding with SP1. LncRNA H19 functioned as a miR-22-3P sponge to increase the expression of MMP14. HDAC2 loss strongly promoted CRC lung metastasis, which was suppressed LncRNA H19 knockdown. CONCLUSION: Our study supports HDAC2 as a CRC metastasis suppressor through the inhibition of EMT and the expression of H19 and MMP14.

Cordycepin induces Bax­dependent apoptosis in colorectal cancer cells.

Cordycepin, or 3'­deoxyadenosine, is a derivative of the nucleoside adenosine. Initially extracted from the fungus Cordyceps militaris, cordycepin exhibits antitumor activity against certain cancer cell lines; however, the mechanism by which cordycepin counteracts colorectal cancer (CRC) remains poorly understood. The aim of the present study was to explore the underlying mechanisms of cordycepin against human CRC. To investigate the molecular mechanisms of cordycepin against colon cancer and in driving apoptosis, p53 and Bcl­2­like protein 4­null (Bax­/­) colon cancer HCT116 cell lines were used. Cell viability and growth were repressed in a dose­dependent manner in cells treated with cordycepin. Treatment with cordycepin resulted in increased apoptosis in HCT116 cells; however, flow cytometic analysis demonstrated that apoptosis was notably decreased in the Bax­/­ HCT116 cell lines, but not in the p53­/­ HCT116 cell lines. Furthermore, cordycepin exposure resulted in the translocation of Bax from the cytosol to the mitochondria and the subsequent release of cytochrome c from the mitochondria. Results from the present study demonstrated that cordycepin inhibited colon cancer cell growth in vitro and this appears to be through the endogenous Bax­dependent mitochondrial apoptosis pathway, which suggested a molecular mechanism for cordycepin against human colon cancer. These results indicated the possibility of cordycepin as a novel drug for the prevention of colon cancer.

DUB3 deubiquitinates and stabilizes NRF2 in chemotherapy resistance of colorectal cancer.

The transcription factor nuclear factor (erythroid-derived 2)-like 2 (NRF2) is one of the master regulators that control hundreds of genes containing antioxidant response elements (AREs). The NRF2-ARE pathway plays a complex role in colorectal cancer (CRC). NRF2 activity is known to be regulated by KEAP1-CUL3 E3 ligase-mediated ubiquitination, indicating the importance of deubiquitination regulation. However, the deubiquitinase (DUB) of NRF2 remains unknown. Here, by screening a DUB library, we identified DUB3 as a DUB that remarkably stabilized NRF2. Further experiments demonstrated that DUB3 promoted NRF2 stability and transcriptional activity by decreasing the K48-linked ubiquitination of NRF2. Coimmunoprecipitation studies revealed interactions between NRF2 and DUB3, as well as between KEAP1 and DUB3, indicating that NRF2, DUB3, and KEAP1 formed a large functional complex. Importantly, ectopic expression of DUB3 caused NRF2-dependent chemotherapy resistance in colon cancer cell lines. Thus, to the best of our knowledge, our findings are the first to identify DUB3 as a NRF2 DUB and may provide a new strategy against chemotherapy resistance in CRC and other NRF2-related diseases.

Cancer testis antigen 55 deficiency attenuates colitis-associated colorectal cancer by inhibiting NF-κB signaling.

Colitis-associated cancer (CAC), a prototype of inflammation-associated cancer, is one of the most common gastrointestinal tumors. As a potential cancer testis antigen (CT antigen), cancer testis antigen 55 (CT55) is expressed in different tumors and normal testes. However, its role in CAC remains unknown. Here, we identified CT55 as a new potent promoter of CAC. We discovered that Ct55 deficiency alleviated inflammatory responses, decreased cell proliferation and colitis-associated tumorigenesis in an azoxymethane/dextran sulfate sodium (AOM/DSS) mouse model. Mechanistically, CT55 acts as an accelerator of tumor necrosis factor (TNF)-α-induced nuclear factor-κB (NF-κB) signaling. Upon stimulation with TNF-α, CT55 interacts with the IκB kinase (IKK) complex, which increases the phosphorylation of IKKα/ß and activates IKK-p65 signaling, while knockout of CT55 blocks IKK-p65 signaling. Notably, inhibition of IKK abolished the positive effect of CT55 on NF-κB activation. Collectively, our findings strongly indicate that CT55 deficiency suppresses the development of CAC and that the CT55-TNF-α-induced NF-κB axis may represent a promising target for CAC therapy.

Phosphorylation of MAVS/VISA by Nemo-like kinase (NLK) for degradation regulates the antiviral innate immune response.

MAVS is essential for antiviral immunity, but the molecular mechanisms responsible for its tight regulation remain poorly understood. Here, we show that NLK inhibits the antiviral immune response during viral infection by targeting MAVS for degradation. NLK depletion promotes virus-induced antiviral cytokine production and decreases viral replication, which is potently rescued by the reintroduction of NLK. Moreover, the depletion of NLK promotes antiviral effects and increases the survival times of mice after infection with VSV. NLK interacts with and phosphorylates MAVS at multiple sites on mitochondria or peroxisomes, thereby inducing the degradation of MAVS and subsequent inactivation of IRF3. Most importantly, a peptide derived from MAVS promotes viral-induced IFN-ß production and antagonizes viral replication in vitro and in vivo. These findings provide direct insights into the molecular mechanisms by which phosphorylation of MAVS regulates its degradation and influences its activation and identify an important peptide target for propagating antiviral responses.

Nemo-like kinase (NLK) primes colorectal cancer progression by releasing the E2F1 complex from HDAC1.

Control of E2F1 activity is restricted via its interactions with RB1 and HDAC1. However, the detailed regulatory mechanisms underlying the E2F1/HDAC1 complex remain elusive. Here, we report that Nemo-like kinase (NLK) boosts cell cycle progression, which facilitates tumor development by releasing the E2F1 protein from HDAC1. Deletion of NLK largely blocks colorectal tumor proliferation and development. Moreover, RNA-seq shows that cell cycle is arrested at the G1/S phase in NLK-deficient cells and that the expression of E2F complex-targeted genes are affected, whereas overexpression of NLK but not an NLK mutant restores the wild-type phenotype. Mechanistically, we show that NLK interacts with the E2F1 complex, leading to disassembly of the E2F1/HDAC1 complex and thus diminishing the ability of E2F1 to bind to target gene promoters. Our results indicate that NLK boosts cell proliferation and E2F1 activity and controls the cell cycle switch by releasing HDAC1 from the E2F1 complex.

Stabilization of ATF5 by TAK1-Nemo-like kinase critically regulates the interleukin-1ß-stimulated C/EBP signaling pathway.

Interleukin-1ß (IL-1ß) is a key proinflammatory cytokine that initiates several signaling cascades, including those involving CCAAT/enhancer binding proteins (C/EBPs). The mechanism by which IL-1ß propagates a signal that activates C/EBP has remained elusive. Nemo-like kinase (NLK) is a mitogen-activated protein kinase (MAPK)-like kinase associated with many pathways and phenotypes that are not yet well understood. Using a luciferase reporter screen, we found that IL-1ß-induced C/EBP activation was positively regulated by NLK. Overexpression of NLK activated C/EBP and potentiated IL-1ß-triggered C/EBP activation, whereas knockdown or knockout of NLK had the opposite effect. NLK interacted with activating transcription factor 5 (ATF5) and inhibited the proteasome-dependent degradation of ATF5 in a kinase-independent manner. Consistently, NLK deficiency resulted in decreased levels of ATF5. NLK cooperated with ATF5 to activate C/EBP, whereas NLK could not activate C/EBP upon knockdown of ATF5. Moreover, TAK1, a downstream effector of IL-1ß that acts upstream of NLK, mimicked the ability of NLK to stabilize ATF5 and activate C/EBP. Thus, our findings reveal the TAK1-NLK pathway as a novel regulator of basal or IL-1ß-triggered C/EBP activation though stabilization of ATF5.