LKB1--a master tumour suppressor of the small intestine and beyond

TBC1D23 mediates Golgi-specific LKB1 signaling

Liver kinase B1 (LKB1), an evolutionarily conserved serine/threonine kinase, is a master regulator of the AMPK subfamily and controls cellular events such as polarity, proliferation, and energy homeostasis. Functions and mechanisms of the LKB1-AMPK axis at specific subcellular compartments, such as lysosome and mitochondria, have been established. AMPK is known to be activated at the Golgi; however, functions and regulatory mechanisms of the LKB1-AMPK axis at the Golgi apparatus remain elusive. Here, we show that TBC1D23, a Golgi-localized protein that is frequently mutated in the neurodevelopment disorder pontocerebellar hypoplasia (PCH), is specifically required for the LKB1 signaling at the Golgi. TBC1D23 directly interacts with LKB1 and recruits LKB1 to Golgi, promoting Golgi-specific activation of AMPK upon energy stress. Notably, Golgi-targeted expression of LKB1 rescues TBC1D23 deficiency in zebrafish models. Furthermore, the loss of LKB1 causes neurodevelopmental abnormalities in zebrafish, which partially recapitulates defects in TBC1D23-deficient zebrafish, and LKB1 sustains normal neuronal development via TBC1D23 interaction. Our study uncovers a regulatory mechanism of the LKB1 signaling, and reveals that a disrupted Golgi-LKB1 signaling underlies the pathogenesis of PCH.

Death-associated protein 3 in cancer-discrepant roles of DAP3 in tumours and molecular mechanisms

Cancer, ranks as the secondary cause of death, is a group of diseases that are characterized by uncontrolled tumor growth and distant metastasis, leading to increased mortality year-on-year. To date, targeted therapy to intercept the aberrant proliferation and invasion is crucial for clinical anticancer treatment, however, mutant expression of target genes often leads to drug resistance. Therefore, it is essential to identify more molecules that can be targeted to facilitate combined therapy. Previous studies showed that death associated protein 3 (DAP3) exerts a pivotal role in regulating apoptosis signaling of tumors, meanwhile, aberrant DAP3 expression is associated with the tumorigenesis and disease progression of various cancers. This review provides an overview of the molecule structure of DAP3 and the discrepant roles played by DAP3 in various types of tumors. Considering the molecular mechanism of DAP3-regulated cancer development, new potential treatment strategies might be developed in the future.

Cancer Risk of Peutz-Jeghers Syndrome and Treatment Experience: A Chinese Medical Center

Peutz-Jeghers syndrome (PJS), also known as hereditary mucocutaneous pigmented gastrointestinal polyposis, is a clinically rare autosomal dominant genetic disease, which falls into the category of hereditary colorectal cancer. There are ∼7,000 new cases of PJS in China every year, and 170,000 PJS patients may survive for a long time in society. PJS polyps are characterized by an early age of onset, difficult diagnosis and treatment, and easy recurrence. With repeated growth, polyps can lead to serious complications such as intestinal obstruction, intussusception, gastrointestinal bleeding, and cancerization, which cause serious clinical problems. Due to repeated hospitalization and endoscopic follow-up, PJS patients and their families suffer from great physical and mental pain and economic burden. With the in-depth understanding of PJS and the development and popularization of endoscopic techniques in the past decade, an integrated treatment modality based on endoscopy plus surgery has gradually become the preferred treatment in most hospitals, which greatly improves the quality of life of PJS patients. However, there is still a lack of effective drug prevention and cure means. In this paper, the current clinical treatment means for PJS polyps were summarized by literature review combined with the treatment experience of our medical center, with a focus on their clinical diagnosis, treatment, and cancer risk.

Detection and analysis of common pathogenic germline mutations in Peutz-Jeghers syndrome

BACKGROUND

Different types of pathogenic mutations may produce different clinical phenotypes, but a correlation between Peutz-Jeghers syndrome (PJS) genotype and clinical phenotype has not been found. Not all patients with PJS have detectable mutations of the STK11/LKB1 gene, what is the genetic basis of clinical phenotypic heterogeneity of PJS? Do PJS cases without STK11/LKB1 mutations have other pathogenic genes? Those are clinical problems that perplex doctors.

AIM

The aim was to investigate the specific gene mutation of PJS, and the correlation between the genotype and clinical phenotype of PJS.

METHODS

A total of 24 patients with PJS admitted to the Air Force Medical Center, PLA (formerly the Air Force General Hospital, PLA) from November 1994 to January 2020 were randomly selected for inclusion in the study. One hundred thirty-nine common hereditary tumor-related genes including STK11/LKB1 were screened and analyzed for pathogenic germline mutations by high-throughput next-generation sequencing (NGS). The mutation status of the genes and their relationship with clinical phenotypes of PJS were explored.

RESULTS

Twenty of the 24 PJS patients in this group (83.3%) had STK11/LKB1 gene mutations, 90% of which were pathogenic mutations, and ten had new mutation sites. Pathogenic mutations in exon 7 of STK11/LKB1 gene were significantly lower than in other exons. Truncation mutations are more common in exons 1 and 4 of STK11/LKB1, and their pathogenicity was significantly higher than that of missense mutations. We also found SLX4 gene mutations in PJS patients.

CONCLUSION

PJS has a relatively complicated genetic background. Changes in the sites responsible for coding functional proteins in exon 1 and exon 4 of STK11/LKB1 may be one of the main causes of PJS. Mutation of the SLX4 gene may be a cause of genetic heterogeneity in PJS.

A novel pathogenic splice site variation in STK11 gene results in Peutz-Jeghers syndrome

Background

Peutz–Jeghers syndrome (PJS) is a rare autosomal dominantly inherited disease resulting in multiple gastrointestinal hamartomatous polyps, mucocutaneous pigmentation, and an increased risk of various types of cancer, and is caused by variations in the serine/threonine protein kinase STK11 (LKB1).

Methods

STK11 gene variations were identified by analyzing STK11 cDNA and genomic DNA. Minigenes carrying the wild‐type and mutant sequences were subjected to in vitro splicing assay to dissect the features of these mutations. The different distribution of wild‐type and mutant protein in cells were tested by Immunofluorescence assays and the functional analysis of the variation were performed using Western blot.

Results

A novel heterozygous splice‐acceptor site variation (c.921‐2 A>C) in intron 7 of the STK11 gene which is co‐segregates with the PJS phenotypes in the proband and all the affected family members and three previously reported variations (c.180C>G, c.580G>A, c.787_790del) were identified in the four families. The c.921‐2 A>C substitution resulted in the inactivation of a splice site and the utilization of a cryptic splice acceptor site surrounding exon 8, generating three different aberrant RNA transcripts, leading to frameshift translation and protein truncation. The results of minigenes indicated that the spliceosome can use a variety of 3’ acceptor site sequences to pair with a given 5’ donor site. The immunofluorescent visualization showed that the distribution of mutant STK11 was different from that of wild‐type STK11, suggesting the mutation may be the causative effect on the dysfunction of the mutant protein. The rescue experiments indicated that the failure of suppressing mTOR phosphorylation by shRNA STK11 could be eliminated by supply of wild‐type STK11 rather than mutant STK11.

Conclusion

We identified a novel heterozygous mutation (c.921‐2 A>C) in the STK11 in a Chinese PJS family. Haploinsufficiency of STK11 might contribute to the pathogenesis of the disease.

Rapid Gut Adaptation to Preterm Birth Involves Feeding-Related DNA Methylation Reprogramming of Intestinal Genes in Pigs

Following preterm birth, the immature gut function and immunology must rapidly adapt to cope with bacterial colonization and enteral milk feeding. We hypothesized that intestinal epigenetic changes are involved in the gut response to preterm birth and the first feeding. Using piglets as models for infants, preterm, and term pigs were fed total parenteral nutrition (TPN) or partial enteral feeding for 5 days, followed by exclusive enteral feeding with bovine milk until day 26 (weaning age). Intestinal structure, function, microbiome, DNA methylome, and gene expressions were compared between preterm and term pigs on days 0, 5, and 26 (n = 8 in each group). At birth, the intestine of preterm pigs showed villus atrophy and global hypermethylation, affecting genes related to the Wnt signaling pathway. Hypermethylation-associated lowered expression of lipopolysaccharide-binding protein and genes related to the Toll-like receptor 4 pathway were evident during the first 5 days of life, but most early methylation differences disappeared by day 26. Regardless, sucrase and maltase activities (adult-type brush border enzymes) remained reduced, and the gut microbiota altered (fewer Akkermansia, more Lachnoclostridia and Lactobacilli) until day 26 in preterm pigs. During the 0- to 5-day period, many new preterm–term methylation differences appeared, but mainly when no enteral feed was provided (TPN feeding). These methylation differences affected intestinal genes related to cell metabolism, including increased GCK (glucokinase) expression via promoter hypomethylation. In conclusion, the immature intestine has a remarkable capacity to adapt its gene methylation and expression after preterm birth, and only few preterm-related defects persisted until weaning. Early enteral feeding may be important to stimulate the methylation reprogramming of intestinal genes, allowing rapid intestinal adaptation to preterm birth.

An exploration of genotype-phenotype link between Peutz-Jeghers syndrome and STK11: a review

Peutz-Jeghers Syndrome (PJS) is an autosomal dominant hereditary polyposis syndrome. Clinical features include hamartomatous polyps, mucocutaneous pigmentation and an increased predisposition towards developing malignancy. Variants in STK11, a tumour suppressor gene, located on Chromosome 19, predispose to PJS. Peutz-Jeghers Syndrome is associated with increased rates of malignancy, particularly gastrointestinal. However, PJS is also associated with increased gynaecological, testicular and thyroid papillary malignancy. Truncating variants in STK11 are thought to predispose to a more severe phenotype. Phenotype severity is based on earlier onset of gastrointestinal pathology arising from the polyps, such as intussusception or earlier onset malignancy. Missense variants are generally considered less severe than truncating variants. There remain a large number of variants of undetermined significance. Studies have attempted to correlate the location of variants with impact on protein structure and overall severity of the PJS phenotype. The results from these cohort studies have consistently found a non-random distribution of variants. Nevertheless, a consensus on phenotype severity based on variant location is yet to be established. A centralised database that collates all known variants would facilitate the interpretation of these variants, best under the governance of an international disease-specific organisation (InSiGHT). In particular, it could help explore the significance of variants based on their type or location. Understanding the genotype-phenotype link between STK11 variants and PJS could allow more personalised care for PJS patients and their families via appropriate risk stratification and personalised and targeted cancer screening.

Orange is the new black: Kinases are the new master regulators of tumor suppression

For many decades, kinases have predominantly been characterized as oncogenes and drivers of tumorigenesis, because activating mutations in kinases occur in cancer with high frequency. The oncogenic functions of kinases relate to their roles as growth factor receptors and as critical mediators of mitogen-activated pathways. Indeed, some of the most promising cancer therapeutic agents are kinase inhibitors. However, cancer genomics studies, especially screens that utilize high-throughput identification of loss-of-function somatic mutations, are beginning to shed light on a widespread role for kinases as tumor suppressors. The initial characterization of tumor-suppressing kinases- in particular members of the protein kinase C (PKC) family, MKK4 of the mitogen-activated protein kinase kinase family, and DAPK3 of the death-associated protein kinase family- laid the foundation for bioinformatic approaches that enable the identification of other tumor-suppressing kinases. In this review, we discuss the important role that kinases play as tumor suppressors, using several examples to illustrate the history of their discovery and highlight the modern approaches that presently aid in the identification of tumor-suppressing kinases. © 2018 IUBMB Life, 71(6):738-748, 2019.

A pan-cancer analysis of driver gene mutations, DNA methylation and gene expressions reveals that chromatin remodeling is a major mechanism inducing global changes in cancer epigenomes

Background

Recent large-scale cancer sequencing studies have discovered many novel cancer driver genes (CDGs) in human cancers. Some studies also suggest that CDG mutations contribute to cancer-associated epigenomic and transcriptomic alterations across many cancer types. Here we aim to improve our understanding of the connections between CDG mutations and altered cancer cell epigenomes and transcriptomes on pan-cancer level and how these connections contribute to the known association between epigenome and transcriptome.

Method

Using multi-omics data including somatic mutation, DNA methylation, and gene expression data of 20 cancer types from The Cancer Genome Atlas (TCGA) project, we conducted a pan-cancer analysis to identify CDGs, when mutated, have strong associations with genome-wide methylation or expression changes across cancer types, which we refer as methylation driver genes (MDGs) or expression driver genes (EDGs), respectively.

Results

We identified 32 MDGs, among which, eight are known chromatin modification or remodeling genes. Many of the remaining 24 MDGs are connected to chromatin regulators through either regulating their transcription or physically interacting with them as potential co-factors. We identified 29 EDGs, 26 of which are also MDGs. Further investigation on target genes’ promoters methylation and expression alteration patterns of these 26 overlapping driver genes shows that hyper-methylation of target genes’ promoters are significantly associated with down-regulation of the same target genes and hypo-methylation of target genes’ promoters are significantly associated with up-regulation of the same target genes.

Conclusion

This finding suggests a pivotal role for genetically driven changes in chromatin remodeling in shaping DNA methylation and gene expression patterns during tumor development.

Electronic supplementary material

The online version of this article (10.1186/s12920-018-0425-z) contains supplementary material, which is available to authorized users.

Intestinal Epithelial Cell-Derived LKB1 Suppresses Colitogenic Microbiota

Dysregulation of the immune barrier function of the intestinal epithelium can often result in dysbiosis. In this study we report a novel role of intestinal epithelial cell (IEC)-derived liver kinase B1 (LKB1) in suppressing colitogenic microbiota. IEC-specific deletion of LKB1 (LKB1^ΔIEC) resulted in an increased susceptibility to dextran sodium sulfate (DSS)-induced colitis and a definitive shift in the composition of the microbial population in the mouse intestine. Importantly, transfer of the microbiota from LKB1^ΔIEC mice was sufficient to confer increased susceptibility to DSS-induced colitis in wild-type recipient mice. Collectively, the data indicate that LKB1 deficiency in intestinal epithelial cells nurtures the outgrowth of colitogenic bacteria in the commensal community. In addition, LKB1 deficiency in the intestinal epithelium reduced the production of IL-18 and antimicrobial peptides in the colon. Administration of exogenous IL-18 restored the expression of antimicrobial peptides, corrected the outgrowth of several bacterial genera, and rescued the LKB1^ΔIEC mice from increased sensitivity to DSS challenge. Taken together, our study reveals an important function of LKB1 in IECs for suppressing colitogenic microbiota by IL-18 expression.

Genetic and epigenetic profiling of a solitary Peutz-Jeghers colon polyp

Colon polyps represent precursor lesions of colon cancers and their malignant potential varies according to histological subtype. A rare subtype of colon polyps is the Peutz–Jeghers (PJ) polyp. PJ polyps mostly occur in the context of Peutz–Jeghers syndrome, which is characterized by the development of multiple polyps in the intestinal tract and hyperpigmentation of oral mucosa and lips. Peutz–Jeghers is an autosomal dominant disorder caused by pathogenic variants of the serine threonine kinase STK11. PJ polyps very rarely occur outside of the syndrome and are then referred to as solitary PJ polyps. Contrary to the situation in Peutz–Jeghers, the genetic basis and the malignant potential of solitary PJ polyps are currently unknown. Here we describe a detailed and comprehensive genetic profile of a solitary PJ polyp. Pathological examination revealed a high tissue homogeneity with >80% epithelial cells. Whole-genome sequencing failed to identify any clonal mutations but demonstrated a significant number of subclonal mutations. No somatic or germline mutations were found at the STK11 locus, suggesting that solitary PJ polyps are genetically distinct from Peutz–Jeghers polyps. In addition, methylome analysis revealed global hypomethylation and CpG island hypermethylation, two features that have been described as hallmarks of the colorectal cancer epigenome. These results provide an example of a premalignant lesion that is defined by epigenetic, rather than genetic changes. Furthermore, our findings support the notion that solitary PJ polyps constitute neoplastic tissue with malignant potential that should be removed for cancer prevention.

Structure of the complex of phosphorylated liver kinase B1 and 14-3-3ζ

The serine/threonine protein kinase liver kinase B1 (LKB1) is a tumour suppressor and plays important roles in development and metabolism. It phosphorylates AMPK and AMPK-related kinases to regulate multiple physiological processes. Mutations in LKB1 often occur in multiple cancers. LKB1 can be suppressed by 14-3-3 proteins in a phosphorylation-dependent manner. Previously, the structure of a 14-3-3ζ-LKB1 fusion protein has been reported, revealing a phosphorylation-independent binding mode of LKB1 to 14-3-3 proteins. Here, the crystal structure of phosphorylated LKB1 peptide in complex with 14-3-3ζ was solved, which provides a structural basis for the phosphorylation-dependent recognition of LKB1 by 14-3-3 proteins.

AMPK and HIF signaling pathways regulate both longevity and cancer growth: the good news and the bad news about survival mechanisms

The AMP-activated protein kinase (AMPK) and hypoxia-inducible factor (HIF) signaling pathways are evolutionarily-conserved survival mechanisms responding to two fundamental stresses, energy deficiency and/or oxygen deprivation. The AMPK and HIF pathways regulate the function of a survival network with several transcription factors, e.g. FOXO, NF-κB, NRF2, and p53, as well as with protein kinases and other factors, such as mTOR, ULK1, HDAC5, and SIRT1. Given that AMPK and HIF activation can enhance not only healthspan and lifespan but also cancer growth in a context-dependent manner; it seems that cancer cells can hijack certain survival factors to maintain their growth in harsh conditions. AMPK activation improves energy metabolism, stimulates autophagy, and inhibits inflammation, whereas HIF-1α increases angiogenesis and helps cells to adapt to severe conditions. First we will review how AMPK and HIF signaling mechanisms control the function of an integrated survival network which is able not only to improve the regulation of longevity but also support the progression of tumorigenesis. We will also describe distinct crossroads between the regulation of longevity and cancer, e.g. specific regulation through the AMPKα and HIF-α isoforms, the Warburg effect, mitochondrial dynamics, and cellular senescence.

Identification of a Tumor Specific, Active-Site Mutation in Casein Kinase 1α by Chemical Proteomics

We describe the identification of a novel, tumor-specific missense mutation in the active site of casein kinase 1α (CSNK1A1) using activity-based proteomics. Matched normal and tumor colon samples were analyzed using an ATP acyl phosphate probe in a kinase-targeted LC-MS2 platform. An anomaly in the active-site peptide from CSNK1A1 was observed in a tumor sample that was consistent with an altered catalytic aspartic acid. Expression and analysis of the suspected mutant verified the presence of asparagine in the probe-labeled, active-site peptide for CSNK1A1. Genomic sequencing of the colon tumor samples confirmed the presence of a missense mutation in the catalytic aspartic acid of CSNK1A1 (GAC→AAC). To our knowledge, the D163N mutation in CSNK1A1 is a newly defined mutation to the conserved, catalytic aspartic acid of a protein kinase and the first missense mutation identified using activity-based proteomics. The tumorigenic potential of this mutation remains to be determined.

Characterization of the STK11 splicing variant as a normal splicing isomer in a patient with Peutz-Jeghers syndrome harboring genomic deletion of the STK11 gene

We report a STK11 splicing variant comprising a 131-bp insertion that is derived from intron 1, which has previously been reported to possess potent pathogenicity. The same variant was detected in a Peutz–Jeghers syndrome patient harboring a genomic deletion in the vicinity of exon 1 of the STK11 gene, which indicated that this variant was derived from the wild-type allele. We also found the same variant in other normal subjects. This variant corresponds to the predicted transcript variant of STK11 (XM_011528209), which is derived from the genomic sequence of Chr19 (NT_011295.12). Therefore, we concluded that the splicing variant was not pathogenic.

Molecular mechanisms of LKB1 induced cell cycle arrest

LKB1 is a serine/threonine protein kinase mutated in patients with Peutz-Jeghers syndrome. Biallelic inactivation of LKB1 is present in up to 30% of cases of non-small cell lung cancer (NSCLC). As a tumor suppressor, LKB1 functions in arresting the cell cycle and inhibiting cell growth. LKB1 leads to induction of p21/WAF1 expression in a p53-dependent mechanism, which is mediated by cytoplasmic LKB1 initiating negative regulation of cell growth or nuclear LKB1 directly involved in transcriptional regulation of p21/WAF1. Alternatively, p53 and p21/WAF1-independent mechanism of regulating cell cycle by LKB1 is also reported.

Metabolic stress regulates cytoskeletal dynamics and metastasis of cancer cells

Metabolic reprogramming is an important driver of tumor progression; however, the metabolic regulators of tumor cell motility and metastasis are not understood. Here, we show that tumors maintain energy production under nutrient deprivation through the function of HSP90 chaperones compartmentalized in mitochondria. Using cancer cell lines, we found that mitochondrial HSP90 proteins, including tumor necrosis factor receptor-associated protein-1 (TRAP-1), dampen the activation of the nutrient-sensing AMPK and its substrate UNC-51-like kinase (ULK1), preserve cytoskeletal dynamics, and release the cell motility effector focal adhesion kinase (FAK) from inhibition by the autophagy initiator FIP200. In turn, this results in enhanced tumor cell invasion in low nutrients and metastatic dissemination to bone or liver in disease models in mice. Moreover, we found that phosphorylated ULK1 levels were correlated with shortened overall survival in patients with non-small cell lung cancer. These results demonstrate that mitochondrial HSP90 chaperones, including TRAP-1, overcome metabolic stress and promote tumor cell metastasis by limiting the activation of the nutrient sensor AMPK and preventing autophagy.

Integrin-binding protein nischarin interacts with tumor suppressor liver kinase B1 (LKB1) to regulate cell migration of breast epithelial cells

Biallelic inactivation of LKB1, a serine/threonine kinase, has been detected in 30% of lung adenocarcinomas, and inhibition of breast tumor growth has been demonstrated. We have identified the tumor suppressor, Nischarin, as a novel binding partner of LKB1. Our mapping analysis shows that the N terminus of Nischarin interacts with amino acids 44-436 of LKB1. Time lapse microscopy and Transwell migration data show that the absence of both Nischarin and LKB1 from an invasive breast cancer cell line (MDA-MB-231) enhances migration as measured by increased distance and speed of migrating cells. Our data suggest that this is a result of elevated PAK1 and LIMK1 phosphorylation. Moreover, the absence of Nischarin and LKB1 increased tumor growth in vivo. Consistent with this, the percentage of S phase cells was increased, as demonstrated by flow cytometry and enhanced cyclin D1. The absence of Nischarin and LKB1 also led to a dramatic increase in the formation of lung metastases. Our studies, for the first time, demonstrate functional interaction between LKB1 and Nischarin to inhibit cell migration and breast tumor progression. Mechanistically, we show that these two proteins together regulate PAK-LIMK-Cofilin and cyclin D1/CDK4 pathways.

One novel deletion and one splicing mutation of the LKB1 gene in two Chinese patients with Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is an uncommon autosomal dominant inherited disease, characterized by the occurrence of gastrointestinal hamartomatous polyps and pigmentation of the lips, buccal mucosa, and digits. Patients with PJS have a significant risk for developing tumors in multiple organs. Germline mutation of the LKB1 gene, which encodes a serine/threonine kinase that acts as a tumor suppressor, has been identified as a cause of PJS. The current study included two Chinese PJS probands and their available family members, as well as 200 unrelated healthy controls for comparison. Genomic DNA was extracted from the peripheral blood of these subjects. The nine coding exons and flanking introns of the LKB1 gene in the two probands and their family members were amplified by polymerase chain reaction (PCR) and then directly sequenced. Mutations identified in the patients were checked in the 200 healthy controls by PCR and denaturing high-performance liquid chromatography. Total RNA was extracted from the patient who was found to have a dubious splice site mutation and his available family members. Reverse transcription PCR was performed to identify the abnormal splicing caused by the splice site mutation. Two types of mutations were detected in the two PJS families. One type was a previously unreported 30-base-pair deletion in exon 4, and the other was an intron mutation that affected splicing. None of the 200 controls had either of these two types of mutations. The results provide support that mutation of the LKB1 gene is a cause of PJS, and expand the spectrum of LKB1 gene mutations.

AMPKα2 Suppresses Murine Embryonic Fibroblast Transformation and Tumorigenesis

AMP-activated kinase (AMPK) is a key metabolic sensor and stress signaling kinase. AMPK activity is known to suppress anabolic processes such as protein and lipid biosynthesis and promote energy-producing pathways including fatty acid oxidation, resulting in increased cellular energy. In addition, AMPK localizes to centrosomes during cell division, plays a role in cellular polarization, and directly targets p53, affecting apoptosis. Two distinct catalytic AMPKα isoforms exist: α1 and α2. Multiple reports indicate that both common and distinct functions exist for each of the 2 α isoforms. AMPK activation has been shown to repress tumor growth, and it has been suggested that AMPK may function as a metabolic tumor suppressor. To evaluate the potential role of each of the AMPKα isoforms in modulating cellular transformation, susceptibility to Ras-induced transformation was evaluated in normal murine embryonic fibroblasts (MEFs) obtained from genetically deleted AMPKα1- or AMPKα2-null mice. This study demonstrated that while AMPKα1 is the dominant AMPK isoform expressed in MEFs, only the AMPKα2-null MEFs displayed increased susceptibility to H-RasV12 transformation in vitro and tumorigenesis in vivo. Conversely, AMPKα1-null MEFs, which demonstrated compensation with increased expression of AMPKα2, displayed minimal transformation susceptibility, decreased cell survival, decreased cell proliferation, and increased apoptosis. Finally, this study demonstrates that AMPKα2 was selectively responsible for targeting p53, thus contributing to the suppression of transformation and tumorigenic mechanisms.

Cross-talk between AMPK and mTOR in regulating energy balance

Energy balance is maintained by a complex homeostatic system involving some signaling pathways and "nutrient sensors" in multiple tissues and organs. Any defect associated with the pathways can lead to metabolic disorders including obesity, type 2 diabetes, and the metabolic syndrome. The 5'-adenosine monophosphate-activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) appear to play a significant role in the intermediary metabolism of these diseases. AMPK is involved in the fundamental regulation of energy balance at the whole body level by responding to hormonal and nutrient signals in the central nervous system and peripheral tissues that modulate food intake and energy expenditure. Mammalian target of rapamycin (mTOR),is one of the downstream targets of AMPK functions as an intracellular nutrient sensor to control protein synthesis, cell growth, and metabolism. Recent research demonstrated the possible interplay between mTOR and AMPK signaling pathways. In this review, we will present current knowledge of AMPK and mTOR pathways in regulating energy balance and demonstrate the convergence between these two pathways.

AMPK induces vascular smooth muscle cell senescence via LKB1 dependent pathway

Vascular cells have a limited lifespan with limited cell proliferation and undergo cellular senescence. The functional changes associated with cellular senescence are thought to contribute to age-related vascular disorders. AMP-activated protein kinase (AMPK) has been discussed in terms of beneficial or harmful effects for aging-related diseases. However, the detailed functional mechanisms of AMPK are largely unclear. An aging model was established by stimulating vascular smooth muscle cell (VSMC) with adriamycin. Adriamycin progressively increased the mRNA and protein expressions of AMPK. The phosphorylation levels of LKB1 and acetyl-CoA carboxylase (ACC), the upstream and downstream of AMPK, were dramatically increased by adriamycin stimulation. The expressions of p53 and p21, which contribute to vascular senescence, were also increased. Inhibition of AMPK diminished senescence-associated β-galactosidase (SA-β-gal) staining, and restored VSMC proliferation. Cytosolic translocation of LKB1 by adriamycin could be a mechanism for AMPK activation in senescence. Furthermore, p53 siRNA and p21 siRNA transfection attenuated adriamycin-induced SA-β-gal staining. These results suggest that LKB1 dependent AMPK activation elicits VSMC senescence and p53-p21 pathway is a mediator of LKB1/AMPK-induced senescence.

LKB1 mutations frequently detected in mucinous bronchioloalveolar carcinoma

OBJECTIVE

LKB1 mutations are common in patients with Peutz-Jeghers syndrome, which is characterized by mucocutaneous pigmentation, intestinal polyps and a high incidence of cancers at variable sites. This study investigated the status of the LKB1 gene in mucinous bronchioloalveolar carcinoma with or without Peutz-Jeghers syndrome.

METHODS

Three mucinous bronchioloalveolar carcinoma tumors from two Peutz-Jeghers syndrome patients and seven tumors from sporadic mucinous bronchioloalveolar carcinoma patients were collected by surgery between 2002 and 2008, and high molecular weight genomic DNA was extracted from them. The nucleotide sequences in exons 1-9 of LKB1 were determined by genomic polymerase chain reaction-direct sequencing. The loss of heterozygosity was analyzed by high-resolution fluorescent microsatellite analysis using two microsatellite markers that encompass the LKB1 locus, D19S886 and D19S565. The mutations of KRAS, EGFR and p53 were also evaluated.

RESULTS

The germline mutation of LKB1 in the Peutz-Jeghers syndrome patients was identified as G215D by analyzing genomic DNA from normal lung tissue specimens. Furthermore, two of the three mucinous bronchioloalveolar carcinomas from these Peutz-Jeghers syndrome patients exhibited additional somatic mutations. On the other hand, four of seven sporadic 'non-Peutz-Jeghers syndrome' mucinous bronchioloalveolar carcinomas had LKB1 mutations. Loss of heterozygosity analyses revealed allelic loss in two tumors with LKB1 mutations. As a result, 70% of the mucinous bronchioloalveolar carcinomas exhibited LKB1 mutations. KRAS, EGFR and p53 mutations were mutually exclusive and observed in four, two and one tumors, respectively. Among them, five mutations occurred concomitantly with LKB1 mutations.

CONCLUSIONS

The relatively high frequency of LKB1 mutations in mucinous bronchioloalveolar carcinoma patients may therefore suggest its involvement in lung carcinogenesis, at least in mucinous bronchioloalveolar carcinoma.

Liver kinase B1 (LKB1) in the pathogenesis of epithelial cancers

LKB1 acts as a master kinase, with its major protein targets being the family of AMPKs. Through activation of multiple signaling pathways, LKB1's main physiologic functions involve regulating cellular growth, metabolism, and polarity. Germline mutations in LKB1 result in Peutz-Jeghers Syndrome, a rare cancer susceptibility syndrome. In addition, multiple LKB1 mutations have been identified in sporadic cancers, especially those of the lung. Recent studies from a variety of murine models have helped characterize LKB1's role in the pathogenesis of epithelial cancers. In some tumor types, LKB1 might function chiefly to suppress cell growth or invasion, while in other cases, it may serve to prevent metastasis. Moreover, molecular signatures of individual tumors likely influence LKB1's operational role, as multiple studies have shown that LKB1 can synergize with other tumor suppressors and/or oncogenes to accelerate tumorigenesis. To date, LKB1 has been considered mainly a tumor suppressor; however, some studies have suggested its potential oncogenic role, mainly through the suppression of apoptosis. In short, LKB1 is a tissue and context-specific kinase. This review aims to summarize our current understanding of its role in the pathogenesis of epithelial cancers.

The long and winding road to rational treatment of cancer associated with LKB1/AMPK/TSC/mTORC1 signaling

The liver kinase B1 (LKB1)/adenosine mono-phosphate-activated protein kinase (AMPK)/tuberous sclerosis complex (TSC)/mammalian target of rapamycin (mTOR) complex (mTORC1) cassette constitutes a canonical signaling pathway that integrates information on the metabolic and nutrient status and translates this into regulation of cell growth. Alterations in this pathway are associated with a wide variety of cancers and hereditary hamartoma syndromes, diseases in which hyperactivation of mTORC1 has been described. Specific mTORC1 inhibitors have been developed for clinical use, and these drugs have been anticipated to provide efficient treatment for these diseases. In the present review, we provide an overview of the metabolic LKB1/AMPK/TSC/mTORC1 pathway, describe how its aberrant signaling associates with cancer development, and indicate the difficulties encountered when biochemical data are extrapolated to provide avenues for rational treatment of disease when targeting this signaling pathway. A careful examination of preclinical and clinical studies performed with rapamycin or derivatives thereof shows that although results are encouraging, we are only half way in the long and winding road to design rationale treatment targeted at the LKB1/AMPK/TSC/mTORC1 pathway. Inherited cancer syndromes associated with this pathway such as the Peutz-Jeghers syndrome and TSC, provide perfect models to study the relationship between genetics and disease phenotype, and to delineate the complexities that underlie translation of biochemical and genetical information to clinical management, and thus provide important clues for devising novel rational medicine for cancerous diseases in general.

High prevalence of germline STK11 mutations in Hungarian Peutz-Jeghers Syndrome patients

BACKGROUND

Peutz-Jeghers syndrome (PJS) is a rare autosomal dominantly inherited disease characterized by gastrointestinal hamartomatous polyposis and mucocutaneous pigmentation. The genetic predisposition for PJS has been shown to be associated with germline mutations in the STK11/LKB1 tumor suppressor gene. The aim of the present study was to characterize Hungarian PJS patients with respect to germline mutation in STK11/LKB1 and their association to disease phenotype.

METHODS

Mutation screening of 21 patients from 13 PJS families were performed using direct DNA sequencing and multiplex ligation-dependent probe amplification (MLPA). Comparative semi-quantitative sequencing was applied to investigate the mRNA-level effects of nonsense and splice-affecting mutations.

RESULTS

Thirteen different pathogenic mutations in STK11, including a high frequency of large genomic deletions (38%, 5/13), were identified in the 13 unrelated families studied. One of these deletions also affects two neighboring genes (SBNO2 and GPX4), located upstream of STK11, with a possible modifier effect. The majority of the point mutations (88%, 7/8) can be considered novel. Quantification of the STK11 transcript at the mRNA-level revealed that the expression of alleles carrying a nonsense or frameshift mutation was reduced to 30-70% of that of the wild type allele. Mutations affecting splice-sites around exon 2 displayed an mRNA processing pattern indicative of co-regulated splicing of exons 2 and 3.

CONCLUSIONS

A combination of sensitive techniques may assure a high (100%) STK11 mutation detection frequency in PJS families. Characterization of mutations at mRNA level may give a deeper insight into the molecular consequences of the pathogenic mutations than predictions made solely at the genomic level.

p53 regulation of the IGF-1/AKT/mTOR pathways and the endosomal compartment

In response to various stress signals, which introduce infidelity into the processes of cell growth and division, p53 initiates cell-cycle arrest, apoptosis, or senescence to maintain fidelity throughout the cell cycle. Although these functions are traditionally thought of as the major functions of the p53 protein for tumor suppression, recent studies have revealed some additional novel functions of the p53 pathway. These include the down-regulation of two central cell-growth pathways, the IGF/AKT-1 and mTOR pathways, and the up-regulation of the activities of the endosomal compartment. The IGF-1/AKT and mTOR pathways are two evolutionarily conserved pathways that play critical roles in regulation of cell proliferation, survival, and energy metabolism. In response to stress, p53 transcribes a group of critical negative regulators in these two pathways, including IGF-BP3, PTEN, TSC2, AMPK beta1, and Sestrin1/2, which leads to the reduction in the activities of these two pathways. Furthermore, p53 transcribes several critical genes regulating the endosomal compartment, including TSAP6, Chmp4C, Caveolin-1, and DRAM, and increases exosome secretion, the rate of endosomal removal of growth factor receptors (e.g., EGFR) from cell surface, and enhances autophagy. These activities all function to slow down cell growth and division, conserve and recycle cellular resources, communicate with adjacent cells and dendritic cells of the immune system, and inform other tissues of the stress signals. This coordinated regulation of IGF-1/AKT/mTOR pathways and the endosomal compartment by the p53 pathway integrates the molecular, cellular, and systemic levels of activities and prevents the accumulations of errors in response to stress and restores cellular homeostasis after stress.

Lactic acidosis triggers starvation response with paradoxical induction of TXNIP through MondoA

Although lactic acidosis is a prominent feature of solid tumors, we still have limited understanding of the mechanisms by which lactic acidosis influences metabolic phenotypes of cancer cells. We compared global transcriptional responses of breast cancer cells in response to three distinct tumor microenvironmental stresses: lactic acidosis, glucose deprivation, and hypoxia. We found that lactic acidosis and glucose deprivation trigger highly similar transcriptional responses, each inducing features of starvation response. In contrast to their comparable effects on gene expression, lactic acidosis and glucose deprivation have opposing effects on glucose uptake. This divergence of metabolic responses in the context of highly similar transcriptional responses allows the identification of a small subset of genes that are regulated in opposite directions by these two conditions. Among these selected genes, TXNIP and its paralogue ARRDC4 are both induced under lactic acidosis and repressed with glucose deprivation. This induction of TXNIP under lactic acidosis is caused by the activation of the glucose-sensing helix-loop-helix transcriptional complex MondoA:Mlx, which is usually triggered upon glucose exposure. Therefore, the upregulation of TXNIP significantly contributes to inhibition of tumor glycolytic phenotypes under lactic acidosis. Expression levels of TXNIP and ARRDC4 in human cancers are also highly correlated with predicted lactic acidosis pathway activities and associated with favorable clinical outcomes. Lactic acidosis triggers features of starvation response while activating the glucose-sensing MondoA-TXNIP pathways and contributing to the “anti-Warburg” metabolic effects and anti-tumor properties of cancer cells. These results stem from integrative analysis of transcriptome and metabolic response data under various tumor microenvironmental stresses and open new paths to explore how these stresses influence phenotypic and metabolic adaptations in human cancers.

Solid tumors usually have many differences in their chemical environments, such as low oxygen, depletion of glucose, high acidity (low pH), and accumulation of lactate, from normal tissues. These changes are usually called tumor microenvironmental stresses. In this study, we have used microarrays to compare the transcriptional response and metabolic adaptation in response to these different stresses seen in the tumor microenvironments. Through these comparisons, we have found that lactic acidosis triggers a starvation response, highly similar to glucose deprivation, even in the presence of abundant nutrients and oxygen. Even the cells seem to be starved; cells under lactic acidosis have decreased glucose uptake. We found this unexpected biological behavior was due to the paradoxical induction of a glucose-sensing Mondo-TXNIP pathway. The activation of this novel anti-tumor pathway under lactic acidosis contributes to the anti-Warburg effect and the restriction of cell growth in tumorigenesis by limiting nutrient availability and its inactivation may be required for tumor progression under these microenvironmental stresses.

Medical management update: Peutz Jeghers syndrome

Peutz Jeghers syndrome (PJS) is an autosomal dominant disease characterized by hamartomatous polyposis and distinct mucocutaneous pigmentation. PJS is associated with an increased risk for several cancers and other complications such as small intestine intussusception, short bowel syndrome, and anemia. Medical management mainly consists of treatment of the polyps and surveillance. This medical management update will review clinical concepts, therapeutic advances, and emphasize features of PJS important to the oral health care provider.

The serine/threonine kinase LKB1 controls thymocyte survival through regulation of AMPK activation and Bcl-XL expression

LKB1 is a serine/threonine kinase that directly activates the energy sensor AMP-activated protein kinase (AMPK) in response to bioenergetic stress, and mainly acts as a tumor suppressor that controls cell polarity and proliferation. Although LKB1 is expressed in multiple tissues including the thymus and the spleen, its roles in T-cell development and function remain unknown. Here, we show that T-cell-specific deletion of LKB1 resulted in reduced survival of double-positive (DP) thymocytes and impaired generation of both CD4 and CD8 single-positive thymocytes. Disruption of LKB1 not only prevented the activation of AMPK but also impaired the expression of anti-apoptotic protein Bcl-XL. Importantly, ectopic expression of either Bcl-XL or the constitutively active AMPK mutant significantly rescued DP thymocytes from LKB1 deficiency-induced cell death. Moreover, ectopic expression of the constitutively active AMPK mutant was found to restore the expression of Bcl-XL in LKB1-deficient DP thymocytes. These findings identify LKB1 as a critical factor for the survival of DP thymocytes through regulation of AMPK activation and Bcl-XL expression.

Novel mutations in the STK11 gene in Thai patients with Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS), a rare autosomal dominant inherited disorder, is characterized by hamartomatous gastrointestinal polyps and mucocutaneous pigmentation. Patients with this syndrome have a predisposition to a variety of cancers in multiple organs. Mutations in the serine/threonine kinase 11 (STK11) gene have been identified as a major cause of PJS. Here we present the clinical and molecular findings of two unrelated Thai individuals with PJS. Mutation analysis by Polymerase Chain Reaction-sequencing of the entire coding region of STK11 revealed two potentially pathogenic mutations. One harbored a single nucleotide deletion (c.182delG) in exon 1 resulting in a frameshift leading to premature termination at codon 63 (p.Gly61AlafsX63). The other carried an in-frame 9-base-pair (bp) deletion in exon 7, c.907_915del9 (p.Ile303_Gln305del). Both deletions were de novo and have never been previously described. This study has expanded the genotypic spectrum of the STK11 gene.

Regulation of protein synthesis by ionizing radiation

Ionizing radiation (IR) is a physiologically important stress to which cells respond by the activation of multiple signaling pathways. Using a panel of immortalized and transformed breast epithelial cell lines, we demonstrate that IR regulation of protein synthesis occurs in nontransformed cells and is lost with transformation. In nontransformed cells, IR rapidly activates the MAP kinases ERK1/2, resulting in an early transient increase in cap-dependent mRNA translation that involves mTOR and is radioprotective, enhancing the translation of a subset of mRNAs encoding proteins involved in DNA repair and cell survival. Following a transient increase in translation, IR-sensitive (nontransformed) cells inhibit cap-dependent protein synthesis through a mechanism that involves activation of p53, induction of Sestrin 1 and 2 genes, and stimulation of AMP kinase, inhibiting mTOR and hypophosphorylating 4E-BP1. IR is shown to block proteasome-mediated decay of 4E-BP1, increasing its abundance and the sequestration of eIF4E. The IR signal that impairs mTOR-dependent protein synthesis at late times is assembly of the DNA damage response machinery, consisting of Mre11, Rad50, and NBS1 (MRN); activation of the MRN complex kinase ATM; and p53. These results link genotoxic signaling from the DNA damage response complex to the control of protein synthesis.

Enhancing mTOR-targeted cancer therapy

BACKGROUND

The mammalian target of rapamycin (mTOR) has emerged as an attractive cancer therapeutic target. Accordingly, several mTOR inhibitors (e.g., rapamycin and its analogs; rapalogs) are currently being tested in many cancer clinical trials. Despite the encouraging results showing that some rapalogs improved overall survival among patients with metastatic renal-cell carcinoma, the single-agent activity of rapalogs in most other tumor-types has been modest, at best.

OBJECTIVE

To review the current understanding of the mTOR axis and discuss potential strategies to enhance mTOR-targeted cancer therapy.

METHODS

Preclinical and clinical data in peer-reviewed reports on the novel biological and therapeutic parts of the mTOR axis are discussed.

CONCLUSION

The mTOR axis involves complex regulatory networks. Inhibition of the mTOR axis with a rapalog induces feedback activation of several survival signaling pathways such as Akt activation, which, in turn, blunt rapalogs' anticancer efficacy. Thus, blockage or prevention of the activation of these survival signaling pathways may enhance mTOR-targeted cancer therapy.

Adenosine Monophosphate-Activated Protein Kinase (AMPK) as a New Target for Antidiabetic Drugs: A Review on Metabolic, Pharmacological and Chemical Considerations

In view of the epidemic nature of type 2 diabetes and the substantial rate of failure of current oral antidiabetic drugs the quest for new therapeutics is intensive. The adenosine monophosphate-activated protein kinase (AMPK) is an important regulatory protein for cellular energy balance and is considered a master switch of glucose and lipid metabolism in various organs, especially in skeletal muscle and liver. In skeletal muscles, AMPK stimulates glucose transport and fatty acid oxidation. In the liver, it augments fatty acid oxidation and decreases glucose output, cholesterol and triglyceride synthesis. These metabolic effects induced by AMPK are associated with lowering blood glucose levels in hyperglycemic individuals. Two classes of oral antihyperglycemic drugs (biguanidines and thiazolidinediones) have been shown to exert some of their therapeutic effects by directly or indirectly activating AMPK. However, side effects and an acquired resistance to these drugs emphasize the need for the development of novel and efficacious AMPK activators. We have recently discovered a new class of hydrophobic D-xylose derivatives that activates AMPK in skeletal muscles in a non insulin-dependent manner. One of these derivatives (2,4;3,5-dibenzylidene-D-xylose-diethyl-dithioacetal) stimulates the rate of hexose transport in skeletal muscle cells by increasing the abundance of glucose transporter-4 (GLUT-4) in the plasma membrane through activation of AMPK. This compound reduces blood glucose levels in diabetic mice and therefore offers a novel strategy of therapeutic intervention strategy in type 2 diabetes. The present review describes various classes of chemically-related compounds that activate AMPK by direct or indirect interactions and discusses their potential for candidate antihyperglycemic drug development.

The molecular mechanisms that underlie the tumor suppressor function of LKB1

Germline mutations of the LKB1 tumor suppressor gene result in Peutz-Jeghers syndrome (PJS) characterized by intestinal hamartomas and increased incidence of epithelial cancers. Inactivating mutations in LKB1 have also been found in certain sporadic human cancers and with particularly high frequency in lung cancer. LKB1 has now been demonstrated to play a crucial role in pulmonary tumorigenesis, controlling initiation, differentiation, and metastasis. Recent evidences showed that LKB1 is a multitasking kinase, with great potential in orchestrating cell activity. Thus far, LKB1 has been found to play a role in cell polarity, energy metabolism, apoptosis, cell cycle arrest, and cell proliferation, all of which may require the tumor suppressor function of this kinase and/or its catalytic activity. This review focuses on remarkable recent findings concerning the molecular mechanism by which the LKB1 protein kinase operates as a tumor suppressor and discusses the rational treatment strategies to individuals suffering from PJS and other common disorders related to LKB1 signaling.

Lkb1 deficiency alters goblet and paneth cell differentiation in the small intestine

The Lkb1 tumour suppressor is a multitasking kinase participating in a range of physiological processes. We have determined the impact of Lkb1 deficiency on intestinal homeostasis, particularly focussing on secretory cell differentiation and development since we observe strong expression of Lkb1 in normal small intestine Paneth and goblet cells. We crossed mice bearing an Lkb1 allele flanked with LoxP sites with those carrying a Cyp1a1-specific inducible Cre recombinase. Lkb1 was efficiently deleted from the epithelial cells of the mouse intestine after intraperitoneal injection of the inducing agent beta-naphthoflavone. Bi-allelic loss of Lkb1 led to the perturbed development of Paneth and goblet cell lineages. These changes were characterised by the lack of Delta ligand expression in Lkb1-deficient secretory cells and a significant increase in the levels of the downstream Notch signalling effector Hes5 but not Hes1. Our data show that Lkb1 is required for the normal differentiation of secretory cell lineages within the intestine, and that Lkb1 deficiency modulates Notch signalling modulation in post-mitotic cells.

Homeostatic functions of the p53 tumor suppressor: regulation of energy metabolism and antioxidant defense

The p53 tumor suppressor plays pivotal role in the organism by supervising strict compliance of individual cells to needs of the whole organisms. It has been widely accepted that p53 acts in response to stresses and abnormalities in cell physiology by mobilizing the repair processes or by removing the diseased cells through initiating the cell death programs. Recent studies, however, indicate that even under normal physiological conditions certain activities of p53 participate in homeostatic regulation of metabolic processes and that these activities are important for prevention of cancer. These novel functions of p53 help to align metabolic processes with the proliferation and energy status, to maintain optimal mode of glucose metabolism and to boost the energy efficient mitochondrial respiration in response to ATP deficiency. Additional activities of p53 in non-stressed cells tune up the antioxidant defense mechanisms reducing the probability of mutations caused by DNA oxidation under conditions of daily stresses. The deficiency in the p53-mediated regulation of glycolysis and mitochondrial respiration greatly accounts for the deficient respiration of the predominance of aerobic glycolysis in cancer cells (the Warburg effect), while the deficiency in the p53-modulated antioxidant defense mechanisms contributes to mutagenesis and additionally boosts the carcinogenesis process.

LKB1 induces apical trafficking of Silnoon, a monocarboxylate transporter, in Drosophila melanogaster

Silnoon (Sln) is a monocarboxylate transporter (MCT) that mediates active transport of metabolic monocarboxylates such as butyrate and lactate. Here, we identify Sln as a novel LKB1-interacting protein using Drosophila melanogaster genetic modifier screening. Sln expression does not affect cell cycle progression or cell size but specifically enhances LKB1-dependent apoptosis and tissue size reduction. Conversely, down-regulation of Sln suppresses LKB1-dependent apoptosis, implicating Sln as a downstream mediator of LKB1. The kinase activity of LKB1 induces apical trafficking of Sln in polarized cells, and LKB1-dependent Sln trafficking is crucial for triggering apoptosis induced by extracellular butyrate. Given that LKB1 functions to control both epithelial polarity and cell death, we propose Sln is an important downstream target of LKB1.

LKB1 is necessary for Akt-mediated phosphorylation of proapoptotic proteins

LKB1 plays the role of tumor suppressor, opposite to Akt, by negatively regulating mammalian target of rapamycin through the activation of AMP-activated protein kinase and TSC signaling. We have discovered a novel, potentially oncogenic role for LKB1 as a supporter of Akt-mediated phosphorylation of proapoptotic proteins. We found that Akt activation led to increased phosphorylation of FoxO3a at Thr(32) in LKB1 wild-type cells but not in LKB1-null cells. Depletion of LKB1 in the cells with wild-type LKB1 resulted in attenuation of that phosphorylation of FoxO3a by activated Akt, whereas the restoration of LKB1 function in LKB1-null cells reestablished Akt-mediated FoxO3a phosphorylation. On expanding our analysis to other Akt targets, using isogenic LKB1 knockdown cell line pairs and a phospho-specific antibody microarray, we observed that there was a requirement for LKB1 in the phosphorylation of other Akt downstream targets, including Ask1 (Ser(83)), Bad (Ser(136)), FoxO1 (Ser(319)), FoxO4 (Ser(197)), and glycogen synthase kinase 3beta (GSK3beta; Ser(9)). Because the phosphorylation of these sites by Akt suppresses apoptosis, the requirement of LKB1 suggests that LKB1 may have an antiapoptotic role in tumor cells with constitutively active Akt. Indeed, we found that the suppression of LKB1 expression led to apoptosis in three cell lines in which Akt is constitutively active but not in two cell lines without Akt activation. This observation may explain the lack of LKB1 somatic mutations in brain, breast, and colon cancers, where Akt is frequently activated due to mutations in phosphatidylinositol 3-kinase, PTEN, or Akt itself.