LKB1, the multitasking tumour suppressor kinase

The Tumor Suppressor Kinase LKB1: Metabolic Nexus

Liver kinase B1 (LKB1) is a multitasking tumor suppressor kinase that is implicated in multiple malignancies such as lung, gastrointestinal, pancreatic, and breast. LKB1 was first identified as the gene responsible for Peutz-Jeghers syndrome (PJS) characterized by hamartomatous polyps and oral mucotaneous pigmentation. LKB1 functions to activate AMP-activated protein kinase (AMPK) during energy stress to shift metabolic processes from active anabolic pathways to active catabolic pathways to generate ATP. Genetic loss or inactivation of LKB1 promotes metabolic reprogramming and metabolic adaptations of cancer cells that fuel increased growth and division rates. As a result, LKB1 loss is associated with increased aggressiveness and treatment options for patients with LKB1 mutant tumors are limited. Recently, there has been new insights into the role LKB1 has on metabolic regulation and the identification of potential vulnerabilities in LKB1 mutant tumors. In this review, we discuss the tumor suppressive role of LKB1 and the impact LKB1 loss has on metabolic reprograming in cancer cells, with a focus on lung cancer. We also discuss potential therapeutic avenues to treat malignancies associated with LKB1 loss by targeting aberrant metabolic pathways associated with LKB1 loss.

O‐GlcNAcylation regulates lysophosphatidic acid‐induced cell migration by regulating ERM family proteins

O‐GlcNAcylation of intracellular proteins (O‐GlcNAc) is a post‐translational modification that often competes with phosphorylation in diverse cellular signaling pathways. Recent studies on human malignant tumors have demonstrated that O‐GlcNAc is implicated in cellular features relevant to metastasis. Here, we report that lysophosphatidic acid (LPA)‐induced ovarian cancer cell (OVCAR‐3) migration is regulated by O‐GlcNAc. We found that O‐GlcNAc modification of ERM family proteins, a membrane‐cytoskeletal crosslinker, was inversely correlated with its phosphorylation status. Moreover, the LPA‐induced formation of membrane protrusion structures, as well as the migration of OVCAR‐3 cells, was reduced by the accumulation of O‐GlcNAc. Collectively, these findings suggest that O‐GlcNAc is an essential signaling element controlling ERM family proteins involved in OVCAR‐3 cell migration.

Retinoid X receptor agonists attenuates cardiomyopathy in streptozotocin-induced type 1 diabetes through LKB1-dependent anti-fibrosis effects

Diabetic cardiac fibrosis increases ventricular stiffness and facilitates the occurrence of diastolic dysfunction. Retinoid X receptor (RXR) plays an important role in cardiac development and has been implicated in cardiovascular diseases. In the present study, we investigated the effects of RXR agonist treatment on streptozotocin (STZ)-induced diabetic cardiomyopathy (DCM) and the underlying mechanism. Sprague-Dawley (SD) rats induced by STZ injection were treated with either RXR agonist bexarotene (Bex) or vehicle alone. Echocardiography was performed to determine cardiac structure and function. Cardiac fibroblasts (CFs) were treated with high glucose (HG) with or without the indicated concentration of Bex or the RXR ligand 9-cis-retinoic acid (9-cis-RA). The protein abundance levels were measured along with collagen, body weight (BW), blood biochemical indexes and transforming growth factor-β (TGF-β) levels. The effects of RXRα down-regulation by RXRα small interfering RNA (siRNA) were examined. The results showed that bexarotene treatment resulted in amelioration of left ventricular dysfunction by inhibiting cardiomyocyte apoptosis and myocardial fibrosis. Immunoblot with heart tissue homogenates from diabetic rats revealed that bexarotene activated liver kinase B1 (LKB1) signaling and inhibited p70 ribosomal protein S6 kinase (p70S6K). The increased collagen levels in the heart tissues of DCM rats were reduced by bexarotene treatment. Treatment of CFs with HG resulted in significantly reduced LKB1 activity and increased p70S6K activity. RXRα mediated the antagonism of 9-cis-RA on HG-induced LKB1/p70S6K activation changes in vitro. Our findings suggest that RXR agonist ameliorates STZ-induced DCM by inhibiting myocardial fibrosis via modulation of the LKB1/p70S6K signaling pathway. RXR agonists may serve as novel therapeutic agents for the treatment of DCM.

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.

Truncation- and motif-based pan-cancer analysis reveals tumor-suppressing kinases

A major challenge in cancer genomics is identifying "driver" mutations from the many neutral "passenger" mutations within a given tumor. To identify driver mutations that would otherwise be lost within mutational noise, we filtered genomic data by motifs that are critical for kinase activity. In the first step of our screen, we used data from the Cancer Cell Line Encyclopedia and The Cancer Genome Atlas to identify kinases with truncation mutations occurring within or before the kinase domain. The top 30 tumor-suppressing kinases were aligned, and hotspots for loss-of-function (LOF) mutations were identified on the basis of amino acid conservation and mutational frequency. The functional consequences of new LOF mutations were biochemically validated, and the top 15 hotspot LOF residues were used in a pan-cancer analysis to define the tumor-suppressing kinome. A ranked list revealed MAP2K7, an essential mediator of the c-Jun N-terminal kinase (JNK) pathway, as a candidate tumor suppressor in gastric cancer, despite its mutational frequency falling within the mutational noise for this cancer type. The majority of mutations in MAP2K7 abolished its catalytic activity, and reactivation of the JNK pathway in gastric cancer cells harboring LOF mutations in MAP2K7 or the downstream kinase JNK suppressed clonogenicity and growth in soft agar, demonstrating the functional relevance of inactivating the JNK pathway in gastric cancer. Together, our data highlight a broadly applicable strategy to identify functional cancer driver mutations and define the JNK pathway as tumor-suppressive in gastric cancer.

Live kinase B1 maintains CD34+CD38- AML cell proliferation and self-renewal

Live kinase B1 (LKB1) has been recognized as a tumor suppressor in many human cancers; however, LKB1 maintains self-renewal of hematopoietic stem cells (HSCs). The existence of leukemia stem cells (LSCs) is responsible for drug resistance and leukemia relapse. In acute myeloid leukemia (AML), CD34⁺CD38^- fraction is the most enriched compartment for LSCs. We found that LKB1 was upregulated in CD34⁺CD38^- AML cells. LKB1 downregulation suppressed the long-term proliferation of CD34⁺CD38^- AML cells, induced CD34⁺CD38^- AML cells into G2/M phase, and enhanced the sensitivity of CD34⁺CD38^- AML cells to chemotherapy. Furthermore, LKB1 downregulation in CD34⁺CD38^- AML cells inhibited tumor formation in NOD-SCID mice. Downregulation of LKB1 gene makes LSCs partly loose the characters as stem cells. Gene expression microarray showed that MAPK/ERK pathway was implicated in the regulation of CD34⁺CD38^- AML cell proliferation by LKB1. Together, these findings demonstrate that LKB1 plays an important role in the maintenance of LSCs, which may be responsible for drug resistance and AML relapse.

Specific Alu elements involved in a significant percentage of copy number variations of the STK11 gene in patients with Peutz-Jeghers syndrome

Peutz–Jeghers syndrome (PJS) is a rare hereditary syndrome characterized by the occurrence of hamartomatous polyps in the gastrointestinal tract, mucocutaneous pigmentation and increased risk of cancer in multiple internal organs. PJS is preconditioned by the manifestation of mutations in the STK11 gene. The majority of detected STK11 changes are small scale mutations, however recent studies showed the significant contribution of medium-sized changes commonly known as copy number variations (CNVs). Here we present a novel 7001 bps deletion of STK11 gene fragment, in which we identified the presence of breakpoints (BPs) within the Alu elements. Comparative meta-analysis with the 80 other CNV cases from 12 publications describing STK11 mutations in patients with PJS revealed the participation of specific Alu elements in all deletions of exons 2–3 so far described. Moreover, we have shown their involvement in the two other CNVs, deletion of exon 2 and deletion of exon 1–3 respectively. Deletion of exons 2–3 of the STK11 gene may prove to be the most recurrent large rearrangement causing PJS. In addition, the sequences present in its BPs may be involved in a formation of a significant percentage of the remaining gene CNVs. This gives a new insight into the conditioning of this rare disease and enables improvements in PJS genetic diagnostics.

Epithelial cell extrusion: Pathways and pathologies

To remove dying or unwanted cells from an epithelium while preserving the barrier function of the layer, epithelia use a unique process called cell extrusion. To extrude, the cell fated to die emits the lipid Sphingosine 1 Phosphate (S1P), which binds the G-protein-coupled receptor Sphingosine 1 Phosphate receptor 2 (S1P₂) in the neighboring cells that activates Rho-mediated contraction of an actomyosin ring circumferentially and basally. This contraction acts to squeeze the cell out apically while drawing together neighboring cells and preventing any gaps to the epithelial barrier. Epithelia can extrude out cells targeted to die by apoptotic stimuli to repair the barrier in the face of death or extrude live cells to promote cell death when epithelial cells become too crowded. Indeed, because epithelial cells naturally turn over by cell death and division at some of the highest rates in the body, epithelia depend on crowding-induced live cell extrusion to preserve constant cell numbers. If extrusion is defective, epithelial cells rapidly lose contact inhibition and form masses. Additionally, because epithelia act as the first line of defense in innate immunity, preservation of this barrier is critical for preventing pathogens from invading the body. Given its role in controlling constant cell numbers and maintaining barrier function, a number of different pathologies can result when extrusion is disrupted. Here, we review mechanisms and signaling pathways that control epithelial extrusion and discuss how defects in these mechanisms can lead to multiple diseases. We also discuss tactics pathogens have devised to hijack the extrusion process to infect and colonize epithelia.

The Prognostic Value of Decreased LKB1 in Solid Tumors: A Meta-Analysis

Background

Liver kinase B1 (LKB1) is a protein kinase that regulates the growth, integrity and polarity of mammalian cells. Recent studies have reported the prognostic value of decreased LKB1 expression in different tumors. However, the results of these studies remain controversial. Therefore, this meta-analysis was performed to more accurately estimate the role of decreased LKB1 in the prognostication of human solid tumors.

Methods

A systematic literature search in the electronic databases PubMed, Embase, Web of Science and CNKI (updated to October 15, 2015) was performed to identify eligible studies. The overall survival (OS), relapse-free survival (RFS), disease-free survival (DFS) and clinicopathological features data were collected from these studies. The hazard ratios (HRs), odds ratios (ORs) and 95% confidence intervals (CIs) were calculated and pooled with a random-effects models using Stata12.0 software.

Results

A total of 14 studies covering 1915 patients with solid tumors were included in this meta-analysis. Decreased LKB1 was associated with poorer OS in both the univariate (HR: 1.86, 95%CI: 1.42–2.42, P<0.001) and multivariate (HR: 1.55, 95%CI: 1.09–2.21, P = 0.015) analyses. A subgroup analysis revealed that the associations between decreased LKB1 and poor OS were significant within the Asian region (HR 2.18, 95%CI: 1.66–2.86, P<0.001) and obvious for lung cancer (HR: 2.16, 95%CI: 1.47–3.18, P<0.001). However, the articles that involved analyses of both RFS and DFS numbered only 3, and no statistically significant correlations of decreased LKB1 with RFS or DFS were observed in this study. Additionally, the pooled odds ratios (ORs) indicated that decreased LKB1 was associated with larger tumor size (OR: 1.60, 95%CI: 1.09–2.36, P = 0.017), lymph node metastasis (OR: 2.41, 95%CI: 1.53–3.78, P<0.001) and a higher TNM stage (OR: 3.35, 95%CI: 2.20–5.09, P<0.001).

Conclusion

These results suggest that decreased LKB1 expression in patients with solid tumors might be related to poor prognosis and serve as a potential predictive marker of poor clinicopathological prognostic factors. Additional studies are required to verify the clinical utility of decreased LKB1 in solid tumors.

AMP-activated protein kinase: structure, function, and role in pathological processes

Recently, AMP-activated protein kinase (AMPK) has emerged as a key regulator of energy balance at cellular and whole-body levels. Due to the involvement in multiple signaling pathways, AMPK efficiently controls ATP-consuming/ATP-generating processes to maintain energy homeostasis under stress conditions. Loss of the kinase activity or attenuation of its expression leads to a variety of metabolic disorders and increases cancer risk. In this review, we discuss recent findings on the structure of AMPK, its activation mechanisms, as well as the consequences of its targets in regulation of metabolism. Particular attention is given to low-molecular-weight compounds that activate or inhibit AMPK; the perspective of therapeutic use of such modulators in treatment of several common diseases is discussed.

Colonic polyps and polyposis syndromes in pediatric patients

PURPOSE OF REVIEW

Gastrointestinal polyps are commonly encountered during childhood and are one of the most common causes of rectal bleeding in this age group. Most polyps are benign and located in the colon, with the most frequent type being juvenile polyps. However, in older pediatric patients, if multiple polyps are present, in patients who have a positive family history, or if polyps are located outside of the colon, either adenomatous polyps or polyps associated with genetic abnormalities are more common.

RECENT FINDINGS

Imaging techniques such as ultrasound and computed tomographic colonoscopy have recently been utilized to identify simple juvenile colonic polyps in children with rectal bleeding in whom there is a high index of suspicion. Colonoscopy with polypectomy is still required for histologic evaluation and resection of the polyp. There have been significant advances in genetic testing and management of hereditary gastrointestinal cancer syndromes with onset in childhood or adolescence that may ultimately reduce long-term morbidity and mortality. In addition to enhanced gastrointestinal and extraintestinal malignancy screening for affected individuals, specific gene mutations within a given condition such as adenomatous polyposis coli may predict clinical course and timing of specific interventions such as colectomy. In other conditions such as phosphatase and tensin homolog hamartoma tumor syndrome, phenotype may not be predicted by genotype.

SUMMARY

Pediatricians, pediatric gastroenterologists, and adult gastroenterologists caring for children should understand how to differentiate benign polyps in the pediatric age group from those associated with a higher risk of complications including recurrence risk and risk of development of intestinal or extraintestinal malignancy. Recent advances in genetic testing, as well as development of consensus guidelines, are key in the identification, screening, and follow-up of children and adolescents with polyposis syndromes.

[Hereditary predisposition to cancers of the digestive tract, breast, gynecological and gonadal: focus on the Peutz-Jeghers]

Peutz-Jeghers syndrome (PJS) is a rare autosomal dominant disease due to mutations in the tumor suppressor gene STK11. PJS is characterized by periorificial hyperpigmented macules (lentiginosis) and hamartomatous polyposis. Polyps can be located anywhere in the gastrointestinal tract, but are preferably observed in the small bowel (70-90%), the colon (50%) and the stomach (25%). They tend to be cancerous in a particular sequence hamartoma-dysplasia-cancer. The diagnosis is often made in the first or second decade following the appearance of lentigines or upon the occurrence of complications due to polyps (obstruction, intussusception, occult bleeding responsible for anemia). Furthermore PJS is associated with a significant increase in cancer risk (relative risk of 89% over the life according to the most recent series). Digestive cancers are the more frequent with cumulative incidences of 55% for gastro-intestinal cancer (39% for colorectal cancer, 13% for small bowel cancer and between 11 and 36% for pancreatic cancer, respectively). There is also an increased risk of non digestive cancers. In particular the risk of breast cancer is similar to that of patients carrying deleterious BRCA1 or BRCA2 mutations (cumulative incidence of 45%). Gynecological and gonadal tumors are frequent as well and can be more (adenoma malignum) or less aggressive (ovarian sex cord tumors with annular tubules and testicular tumors with calcified Sertoli cells). Finally the frequency of lung cancer is moderately increased. Recommendations for screening and management based on retrospective series in the literature have led to various strategies. The aim of this paper is to summarize the clinical and molecular diagnostic criteria of PJS as well as recommendations on screening strategies, management and monitoring.

Gastric-type endocervical glandular neoplasms associated with aberrant p16 expression and K-RAS gene mutation in Peutz-Jeghers syndrome

In this report, unique endocervical glandular lesions exhibiting gastric differentiation were examined in a patient with Peutz-Jeghers syndrome. The result of the human papillomavirus (HPV) in situ hybridization (ISH) for the hysterectomy specimens was negative, but they demonstrated a papillary mucinous adenocarcinoma at the proximal endocervix continuous to atypical lobular endocervical glandular hyperplasia. Both contained MUC6-positive neutral mucin in cytoplasm, and showed different immunoreactivity to p16, Ki-67, and p53. Moreover, they harbored the identical K-RAS gene mutation suggesting that there was a common origin. Somatic K-RAS mutation and defective function of p16 may have been involved in the tumorigenesis of these unusual mucinous neoplasms.

Retinoid X receptor agonists inhibit hypertension-induced myocardial hypertrophy by modulating LKB1/AMPK/p70S6K signaling pathway

BACKGROUND

Retinoid X receptor (RXR) has been demonstrated to play an important role in cardiac development and has been implicated in cardiovascular diseases. This study aimed to examine the effects of RXRα agonist bexarotene on pathological left ventricular hypertrophy (LVH) in a spontaneously hypertensive rat (SHR) model and the underlying mechanism.

METHODS

WKY rats served as controls. SHRs were randomized into 3 groups at the age of 4 weeks and were treated (once daily for 12 weeks) with either bexarotene (30 or 100mg/kg body weight) or vehicle alone. Echocardiography was performed to determine cardiac structure and function. Neonatal cardiomyocytes were treated with AngII (10(-7) mmol/L) with or without the indicated concentration of RXRα ligand 9-cis-RA. The protein abundances of β-actin, RXRα, LKB1, phospho-LKB1, AMPK, phospho-AMPK, P70S6K, phospho-P70S6K, ACE, and AT1 receptor were measured along with blood pressure, body weight and angiotensin II (Ang II) levels. The effects of LKB1 downregulation by LKB1 small, interfering RNA were examined.

RESULTS

Treatment of SHRs with bexarotene resulted in significant inhibition of LVH without eliminating hypertension. Immunoblot with heart tissue homogenates from SHRs revealed that bexarotene activated the LKB1/AMPK signaling pathway and inhibited p70S6K. However, the increased Ang II levels in SHR serum and heart tissue were not reduced by bexarotene treatment. Treatment of cardiomyocytes with Ang II resulted in significantly reduced LKB1/AMPK activity and increased p70S6K activity. 9-cis-RA antagonized Ang II-induced LKB1/AMPK and p70S6K activation changes in vitro.

CONCLUSIONS

RXR agonists prevent the inhibition of the LKB1/AMPK/p70S6K pathway and regulate protein synthesis to reduce LVH. This antihypertrophic effect of bexarotene is independent of blood pressure.

Molecular biology of lung cancer

Lung cancers are characterised by abundant genetic diversity with relatively few recurrent mutations occurring at high frequency. However, the genetic alterations often affect a common group of oncogenic signalling pathways. There have been vast improvements in our understanding of the molecular biology that underpins lung cancer in recent years and this has led to a revolution in the diagnosis and treatment of lung adenocarcinomas (ADC) based on the genotype of an individual's tumour. New technologies are identifying key and potentially targetable genetic aberrations not only in adenocarcinoma but also in squamous cell carcinoma (SCC) of the lung. Lung cancer mutations have been identified in v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (KRAS), epidermal growth factor receptor (EGFR), BRAF and the parallel phosphatidylinositol 3-kinase (PI3K) pathway oncogenes and more recently in MEK and HER2 while structural rearrangements in ALK, ROS1 and possibly rearranged during transfection (RET) provide new therapeutic targets. Amplification is another mechanism of activation of oncogenes such as MET in adenocarcinoma, fibroblastgrowth factor receptor 1 (FGFR1) and discoidin domain receptor 2 (DDR2) in SCC. Intriguingly, many of these genetic alternations are associated with smoking status and with particular racial and gender differences, which may provide insight into the mechanisms of carcinogenesis and role of host factors in lung cancer development and progression. The role of tumour suppressor genes is increasingly recognised with aberrations reported in TP53, PTEN, RB1, LKB11 and p16/CDKN2A. Identification of biologically significant genetic alterations in lung cancer that lead to activation of oncogenes and inactivation of tumour suppressor genes has the potential to provide further therapeutic opportunities. It is hoped that these discoveries may make a major contribution to improving outcome for patients with this poor prognosis disease.

Somatic mutations of STK11 gene in human papillomavirus positive and negative penile cancer

Background

Human papillomavirus (HPV) infection accounts for about 40-50% of all cases of penile carcinoma suggesting that other factors, including host genetic status, are involved in neoplastic transformation. In this perspective, STK11 gene, which has been found frequently mutated in HPV-related cervical carcinoma, has been analyzed in HPV-positive and HPV-negative invasive penile cancers to establish its mutational status and the possible correlation of HPV infection with specific genetic alterations.

Methods

Genomic DNAs extracted from 26 cases of penile squamous cell carcinoma were analyzed for genetic alterations in the exons 1 to 9 of STK11 gene by quantitative real-time PCR. Ratios of potentially deleted and non-deleted exons were indicative of specific loss of STK11 coding regions. DNA samples of 5 cancer cases were subjected to standard PCR amplification of STK11 exons 1 to 9 and analyzed for somatic mutations by direct nucleotide sequencing analysis.

Results

Heterozygous deletions of STK11 exon 1 and 2 were identified in 2 out of 14 HPV-positive (14.3%) and 1 out of 12 HPV-negative cases (8.3%). Complete nucleotide sequencing analysis of exons 1 to 9 showed a single nucleotide change upstream the exon 2 coding region in 1 out of 5 penile carcinoma samples.

Conclusions

The present results suggest that single nucleotide mutations and/or deletions of STK11 gene are rare events in penile cancer. Moreover, no significant association was observed between STK11 alterations and HPV infection in these tumors.

Genetic and biochemical alterations in non-small cell lung cancer

Despite significant advances in the detection and treatment of lung cancer, it causes the highest number of cancer-related mortality. Recent advances in the detection of genetic alterations in patient samples along with physiologically relevant animal models has yielded a new understanding of the molecular etiology of lung cancer. This has facilitated the development of potent and specific targeted therapies, based on the genetic and biochemical alterations present in the tumor, especially non-small-cell lung cancer (NSCLC). It is now clear that heterogeneous cell signaling pathways are disrupted to promote NSCLC, including mutations in critical growth regulatory proteins (K-Ras, EGFR, B-RAF, MEK-1, HER2, MET, EML-4-ALK, KIF5B-RET, and NKX2.1) and inactivation of growth inhibitory pathways (TP53, PTEN, p16, and LKB-1). How these pathways differ between smokers and non-smokers is also important for clinical treatment strategies and development of targeted therapies. This paper describes these molecular targets in NSCLC, and describes the biological significance of each mutation and their potential to act as a therapeutic target.

Liver kinase B1 expression (LKB1) is repressed by estrogen receptor alpha (ERα) in MCF-7 human breast cancer cells

BACKGROUND

Liver kinase 1 (LKB1) is emerging as a multifunctional protein, acting as a key metabolic enzyme, regulator of cell polarity, and transcription factor. Altered LKB1 expression has been linked with various cancers and may be a potential prognostic marker. While the functional role of LKB1 continues to undergo intensive investigation, the molecular mechanisms that regulate its expression remain to be defined more clearly. Recent reports have established a possible link between estrogen receptor alpha (ERα) signaling and LKB1 in MCF-7 human breast cancer cells. The current study aimed to investigate whether LKB1 is transcriptionally regulated by ERα in MCF-7 cells.

METHODS

siRNA transfections were used to transiently knock down LKB1 and ERα. LKB1 and ERα mRNA and protein levels were evaluated by real-time PCR and Western blotting, respectively. An approximately 3 kilobase pair human LKB1 promoter construct and various truncations were generated, transfected into MCF-7 cells, and luciferase reporter assays were performed. Cells were also treated with various doses of 17-β-estradiol (E2) to evaluate the effect on LKB1 and ERα mRNA levels.

RESULTS

LKB1 mRNA and protein levels were significantly lower in ERα-positive MCF-7 compared to ERα-negative MDA-MB-231 breast cancer cells, suggesting that ERα may act as a repressor. siRNA-mediated knock-down of ERα in MCF-7 cells significantly increased LKB1 promoter activity and expression at the mRNA and protein levels, and computational analysis revealed the presence of several putative estrogen response element (ERE) DNA binding sites in the LKB1 promoter region. In addition, treatment with E2 led to an increase in LKB1 expression, concomitant with decreased expression of ERα in MCF-7 cells. The E2-mediated increase was abrogated by pretreatment with actinomycin D, supporting that the observed changes in LKB1 levels were transcriptionally regulated.

CONCLUSIONS

ERα repressively modulates the expression of LKB1 at the transcriptional level. Targeting the expression of LKB1 by modulating ERα signaling may provide a potential approach to further evaluate its function in ERα-positive breast cancers.

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.

Elevation of WNT5A expression in polyp formation in Lkb1+/- mice and Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is a rare, inherited disease caused by germline mutation of the LKB1 gene. Patients with PJS develop characteristic polyps in the digestive tract and carry an elevated risk of cancers in multiple organs, including the intestinal tract. While LKB1 is capable of phosphorylating AMPK and regulates the mTOR pathway, it is also known to be a multitasking protein that can influence other cellular processes, including cell polarity. We hypothesized that there may be other biological pathways directly or indirectly affected by the loss of LKB1 in PJS and aimed to investigate this possibility through transcriptional profiling of polyps harvested from an Lkb1(+/-) mouse model of PJS and from PJS patients. We identified alterations in the mRNA level of a wide range of genes, including some that are involved in Wnt signalling (Wnt5a, Wif1, Dixdc1, Wnt11, Ccnd1, and Ccnd2), although we did not observe nuclear localization of β-catenin in over 93 human PJS intestinal polyps or in 24 gastric polyps from Lkb1(+/-) mice. Among these genes, WNT5A, a non-canonical and non-transforming Wnt, is consistently up-regulated in both Lkb1(+/-) mice and human PJS polyps at a high level. We performed in situ hybridization to further define the spatial expression pattern of WNT5A and observed a strong signal in the stroma of mouse and human polyps compared to no or very low expression in the mucosa. Our findings indicate that WNT5A plays an important role in PJS polyposis.

CCL2 is a negative regulator of AMP-activated protein kinase to sustain mTOR complex-1 activation, survivin expression, and cell survival in human prostate cancer PC3 cells

CCL2 is a cytokine prevalent in the prostate cancer tumor microenvironment. Recently, we reported that CCL2 induces the mammalian target of rapamycin (mTOR) pathway to promote prostate cancer PC3 cell survival; however, the mechanism used by CCL2 to maintain mTOR complex-1 (mTORC1) activation requires clarification. This study demonstrates that upon serum starvation, CCL2 functions as a negative regulator of AMP-activated protein kinase (AMPK) by decreasing phosphorylation at its major regulatory site (Thr(172)) in PC3, DU145, and C4-2B prostate cancer cells. The CCL2-mediated AMPK regulation decreased raptor phosphorylation (Ser(792)) resulting in hyperactivation of mTORC1. D942, a pharmacological activator of AMPK, stunted CCL2-induced mTORC1 activity, survivin expression, and cell survival without significantly affecting Akt activity. CCL2, however, conferred some resistance to the lethal effect of D942 compared with untreated cells. By using Akt-specific inhibitor X, it was shown that Akt inactivation did not cause an increase in AMPK phosphorylation in CCL2-stimulated cells, suggesting that CCL2-mediated negative regulation of AMPK is independent of Akt. Furthermore, bisindolylmaleimide-V, a specific inhibitor of p70(S6K), stunted survivin expression and induced cell death in CCL2-treated PC3. Altogether, these findings suggest that CCL2 hyperactivates mTORC1 through simultaneous regulation of both AMPK and Akt pathways and reveals a new network that promotes prostate cancer: CCL2-AMPK-mTORC1-survivin.

A De Novo mutation of STK11 gene in a Chinese patient with Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is an autosomal-dominant inherited disorder characterized by mucocutaneous pigmentation, hamartomatous polyposis of the gastrointestinal tract, and an increased risk for the development of both gastrointestinal and extraintestinal malignancies. Germline mutation of the STK11 gene, which encodes a serine-threonine kinase, is responsible for PJS. We collected blood samples from a Chinese PJS family consisting of a total of four individuals (one male and three females) including one PJS patient. The whole coding region of STK11 was amplified by polymerase chain reaction and products analyzed by direct sequencing. Molecular analysis of the STK11 gene in this case of PJS revealed a substitution of thymine 217 for adenine (C.217T > A) in exon 1, resulting in a change of codon 73 from cysteine to serine (C73S). The point mutation was not found in normal individuals in this PJS family or in 100 control individuals. The results presented here enlarge the spectrum of mutations of the STK11 gene by identifying a de novo mutation in a PJS patient and further support the hypothesis that STK11 mutations are disease-causing mutations for PJS.

Hereditary hamartomatous polyposis syndromes: understanding the disease risks as children reach adulthood

Hamartomatous polyposis syndromes are a rare group of hereditary autosomal dominant disorders that comprise less than 1% of all hereditary colorectal cancers. Hamartomatous polyps, in and of themselves, are benign entities; however, these hamartomatous polyposis syndromes have a malignant potential for the development of colorectal cancer as well as extracolonic cancers. Early detection and proper surveillance are vital to minimizing the risk of carcinoma. This article provides a critical review of the clinical presentation, pathology, genetics, and screening and surveillance guidelines of juvenile polyposis syndrome, PTEN hamartoma tumor syndrome, and Peutz-Jeghers syndrome.

Cardiac-specific deletion of LKB1 leads to hypertrophy and dysfunction

LKB1 encodes a serine/threonine kinase, which functions upstream of the AMP-activated protein kinase (AMPK) superfamily. To clarify the role of LKB1 in heart, we generated and characterized cardiac myocyte-specific LKB1 knock-out (KO) mice using alpha-myosin heavy chain-Cre deletor strain. LKB1-KO mice displayed biatrial enlargement with atrial fibrillation and cardiac dysfunction at 4 weeks of age. Left ventricular hypertrophy was observed in LKB1-KO mice at 12 weeks but not 4 weeks of age. Collagen I and III mRNA expression was elevated in atria at 4 weeks, and atrial fibrosis was seen at 12 weeks. LKB1-KO mice displayed cardiac dysfunction and atrial fibrillation and died within 6 months of age. Indicative of a prohypertrophic environment, the phosphorylation of AMPK and eEF2 was reduced, whereas mammalian target of rapamycin (mTOR) phosphorylation and p70S6 kinase phosphorylation were increased in both the atria and ventricles of LKB1-deficient mice. Consistent with vascular endothelial growth factor mRNA and protein levels being significantly reduced in LKB1-KO mice, these mice also exhibited a reduction in capillary density of both atria and ventricles. In cultured cardiac myocytes, LKB1 silencing induced hypertrophy, which was ameliorated by the expression of a constitutively active form AMPK or by treatment with the inhibitor of mTOR, rapamycin. These findings indicate that LKB1 signaling in cardiac myocytes is essential for normal development of the atria and ventricles. Cardiac hypertrophy and dysfunction in LKB1-deficient hearts are associated with alterations in AMPK and mTOR/p70S6 kinase/eEF2 signaling and with a reduction in vascular endothelial growth factor expression and vessel rarefaction.

LKB1 regulates polarity remodeling and adherens junction formation in the Drosophila eye

The serine-threonine kinase LKB1 regulates cell polarity from Caenorhabditis elegans to man. Loss of lkb1 leads to a cancer predisposition, known as Peutz-Jeghers Syndrome. Biochemical analysis indicates that LKB1 can phosphorylate and activate a family of AMPK- like kinases, however, the precise contribution of these kinases to the establishment and maintenance of cell polarity is still unclear. Recent studies propose that LKB1 acts primarily through the AMP kinase to establish and/or maintain cell polarity. To determine whether this simple model of how LKB1 regulates cell polarity has relevance to complex tissues, we examined lkb1 mutants in the Drosophila eye. We show that adherens junctions expand and apical, junctional, and basolateral domains mix in lkb1 mutants. Surprisingly, we find LKB1 does not act primarily through AMPK to regulate cell polarity in the retina. Unlike lkb1 mutants, ampk retinas do not show elongated rhabdomeres or expansion of apical and junctional markers into the basolateral domain. In addition, nutrient deprivation does not reveal a more dramatic polarity phenotype in lkb1 photoreceptors. These data suggest that AMPK is not the primary target of LKB1 during eye development. Instead, we find that a number of other AMPK-like kinase, such as SIK, NUAK, Par-1, KP78a, and KP78b show phenotypes similar to weak lkb1 loss of function in the eye. These data suggest that in complex tissues, LKB1 acts on an array of targets to regulate cell polarity.

LKB1 catalytic activity contributes to estrogen receptor alpha signaling

The tumor suppressor serine-threonine kinase LKB1 is mutated in Peutz-Jeghers syndrome (PJS) and in epithelial cancers, including hormone-sensitive organs such as breast, ovaries, testes, and prostate. Clinical studies in breast cancer patients show low LKB1 expression is related to poor prognosis, whereas in PJS, the risk of breast cancer is similar to the risk from germline mutations in breast cancer (BRCA) 1/BRCA2. In this study, we investigate the role of LKB1 in estrogen receptor alpha (ERalpha) signaling. We demonstrate for the first time that LKB1 binds to ERalpha in the cell nucleus in which it is recruited to the promoter of ERalpha-responsive genes. Furthermore, LKB1 catalytic activity enhances ERalpha transactivation compared with LKB1 catalytically deficient mutants. The significance of our discovery is that we demonstrate for the first time a novel functional link between LKB1 and ERalpha. Our discovery places LKB1 in a coactivator role for ERalpha signaling, broadening the scientific scope of this tumor suppressor kinase and laying the groundwork for the use of LKB1 as a target for the development of new therapies against breast cancer.

Uncoupling of the LKB1-AMPKalpha energy sensor pathway by growth factors and oncogenic BRAF

Background

Understanding the biochemical mechanisms contributing to melanoma development and progression is critical for therapeutical intervention. LKB1 is a multi-task Ser/Thr kinase that phosphorylates AMPK controlling cell growth and apoptosis under metabolic stress conditions. Additionally, LKB1^Ser428 becomes phosphorylated in a RAS-Erk1/2-p90^RSK pathway dependent manner. However, the connection between the RAS pathway and LKB1 is mostly unknown.

Methodology/Principal Findings

Using the UV induced HGF transgenic mouse melanoma model to investigate the interplay among HGF signaling, RAS pathway and PI3K pathway in melanoma, we identified LKB1 as a protein directly modified by HGF induced signaling. A variety of molecular techniques and tissue culture revealed that LKB1^Ser428 (Ser431 in the mouse) is constitutively phosphorylated in BRAF^V600E mutant melanoma cell lines and spontaneous mouse tumors with high RAS pathway activity. Interestingly, BRAF^V600E mutant melanoma cells showed a very limited response to metabolic stress mediated by the LKB1-AMPK-mTOR pathway. Here we show for the first time that RAS pathway activation including BRAF^V600E mutation promotes the uncoupling of AMPK from LKB1 by a mechanism that appears to be independent of LKB1^Ser428 phosphorylation. Notably, the inhibition of the RAS pathway in BRAF^V600E mutant melanoma cells recovered the complex formation and rescued the LKB1-AMPKα metabolic stress-induced response, increasing apoptosis in cooperation with the pro-apoptotic proteins Bad and Bim, and the down-regulation of Mcl-1.

Conclusions/Significance

These data demonstrate that growth factor treatment and in particular oncogenic BRAF^V600E induces the uncoupling of LKB1-AMPKα complexes providing at the same time a possible mechanism in cell proliferation that engages cell growth and cell division in response to mitogenic stimuli and resistance to low energy conditions in tumor cells. Importantly, this mechanism reveals a new level for therapeutical intervention particularly relevant in tumors harboring a deregulated RAS-Erk1/2 pathway.

Hamartomatous polyposis syndromes

Since the histologic description of the hamartomatous polyp in 1957 by Horrilleno and colleagues, descriptions have appeared of several different syndromes with the propensity to develop these polyps in the upper and lower gastrointestinal tracts. These syndromes include juvenile polyposis, Peutz-Jeghers syndrome, hereditary mixed polyposis syndrome, and the phosphatase and tensin homolog gene (PTEN) hamartoma tumor syndromes (Cowden and Bannayan-Riley-Ruvalcaba syndromes), which are autosomal-dominantly inherited, and Cronkhite-Canada syndrome, which is acquired. This article reviews the clinical aspects, the molecular pathogenesis, the affected organ systems, the risks of cancer, and the management of these hamartomatous polyposis syndromes. Although the incidence of these syndromes is low, it is important for clinicians to recognize these disorders to prevent morbidity and mortality in these patients, and to perform presymptomatic testing in patients at risk.

Lkb1 deficiency causes prostate neoplasia in the mouse

Mutation of LKB1 is the key molecular event underlying Peutz-Jeghers syndrome, a dominantly inherited condition characterized by a predisposition to a range of malignancies, including those of the reproductive system. We report here the use of a Cre-LoxP strategy to directly address the role of Lkb1 in prostate neoplasia. Recombination of a LoxP-flanked Lkb1 allele within all four murine prostate lobes was mediated by spontaneous activation of a p450 CYP1A1-driven Cre recombinase transgene (termed AhCre). Homozygous mutation of Lkb1 in males expressing AhCre reduced longevity, with 100% manifesting atypical hyperplasia and 83% developing prostate intraepithelial neoplasia (PIN) of the anterior prostate within 2 to 4 months. We also observed focal hyperplasia of the dorsolateral and ventral lobes (61% and 56% incidence, respectively), bulbourethral gland cysts associated with atypical hyperplasia (100% incidence), hyperplasia of the urethra (39% incidence), and seminal vesicle squamous metaplasia (11% incidence). PIN foci overexpressed nuclear beta-catenin, p-Gsk3 beta, and downstream Wnt targets. Immunohistochemical analysis of foci also showed a reduction in Pten activation and up-regulation of both p-PDK1 (an AMPK kinase) and phosphorylated Akt. Our data are therefore consistent with deregulation of Wnt and phosphoinositide 3-kinase/Akt signaling cascades after loss of Lkb1 function. For the first time, this model establishes a link between the tumor suppressor Lkb1 and prostate neoplasia, highlighting a tumor suppressive role within the mouse and raising the possibility of a similar association in the human.

Role of the PI3K/Akt, mTOR, and STK11/LKB1 pathways in the tumorigenesis of sclerosing hemangioma of the lung

Although the histogenesis of sclerosing hemangioma (SH) of the lung is now thought to be respiratory epithelial in origin, the genetic abnormalities that mediate its development are not known. Because pathophysiology of several syndromes associated with benign tumors may converge on the tuberous sclerosis complex (TSC), serine/threonine kinase 11 (STK11), and mammalian target of rapamycin (mTOR) pathways, the purpose of the present paper was to investigate their roles in the development of SH. Semiquantitative immunohistochemical analysis was done to assess the expression of phospho-mTOR, phospho-S6 ribosomal protein, phosphatase and tensin homolog deleted on chromosome 10 (PTEN), phospho-Akt, STK11, tuberin, hamartin, vascular endothelial growth factor (VEGF), and hypoxia-inducible factor-1alpha (HIF-1alpha) in 19 cases of typical SH. To determine whether genetic alteration of STK11 is involved in the development of SH, all encoding exons of STK11 were analyzed by polymerase chain reaction (PCR) amplification and direct sequencing of genomic DNA of six specimens. The six specimens were also investigated for whether promoter hypermethylation exists as an alternative inactivating mechanism for STK11. All specimens showed moderate to marked reaction to phospho-S6 ribosomal protein and PTEN; 16 specimens (84%) showed slight to moderate reaction to phospho-mTOR, negative reaction to STK11, and slight to moderate reaction to hamartin; 11 (58%) showed slight to moderate reaction to phospho-Akt; 18 (95%) showed slight to moderate reaction to tuberin and positive reaction for HIF-1alpha; and 17 (90%) showed moderate reaction to VEGF. No somatic mutation of STK11 was found and the six specimens were unmethylated in the promoter region. These data imply that aberrant mTOR signaling may play a role in the development of SH, and its vascular nature may be due partially to high levels of VEGF caused by dysregulation of mTOR signaling.

LKB1 and AMPK in cell polarity and division

LKB1 and AMP-activated protein kinase (AMPK) are serine-threonine kinases implicated in key cellular pathways, including polarity establishment and energy sensing, respectively. Recent in vivo analyses in Drosophila have demonstrated vital roles for both AMPK and LKB1--in part through the myosin regulatory light chain--in cell polarity and cell division. Evidence from mammalian experiments also supports non-metabolic functions for LKB1 and AMPK. This review examines unanticipated AMPK functions for initiating and maintaining cell polarity and completing normal cell division. The ability of AMPK to sense energy status might be coupled with fundamental cell biological functions.

[Peutz-Jeghers' syndrome with malignant development in a hamartomatous polyp: report of one case and review of the literature]

The malignant potential of hamartomatous polyps in Peutz-Jeghers' (PPJ) syndrome has been debated. Although it is a very rare event, these polyps can become malignant, as demonstrated by this report. One case of colonic adenocarcinoma associated with Peutz-Jeghers' syndrome is described in a 62-year-old woman. The patient had colonic carcinoma which developed in a hamartomatous polyp. The malignant development of this colonic hamartomatous polyp arising in Peutz-Jeghers' syndrome was pathologically confirmed at surgery. This case also shows a sequence of hamartoma-dysplasia-carcinoma in a hamartomatous polyp without adenomatous changes. This suggests that hamartomatous polyps in Peutz-Jeghers' syndrome may develop into adenocarcinoma and may be a precursor of gastrointestinal carcinomas. STK 11 is a tumor suppressor gene regulating the development of hamartomas, and this somatic mutation promotes gastrointestinal cancer at later stages in Peutz-Jeghers' syndrome.

High-level expression of functional tumor suppressor LKB1 in Escherichia coli

The human LKB1 tumor suppressor has been implicated as an important regulator of many cellular processes and signaling pathways, indicating that it could be a good candidate for anticancer drugs. The failure of its obtain high-level expression has been a major obstacle to study its protein structure and function in vitro. Here, we describe the high-level expression of human LKB1 in Escherichia coli and show its kinase activity and anticancer effects on a tumor cell line. The gene encoding LKB1 was optimized by replacing rare codons with codons frequently used in E. coli and synthesized with overlapping primers. The recombinant His-LKB1 was expressed in hosts BL21(DE3) (BL) and Rosetta-gami(DE3)pLysS (RG). His-LKB1 from BL was present mainly as inclusion body. The soluble His-LKB1 from RG accounted for 34.1% of total proteins and the yield of purified His-LKB1 was approximately 92 microg/ml. Purified His-LKB1 protein from both hosts was functionally active, as shown by reversible autophosphorylation and kinase activity in the absence of any other associated kinase. The growth inhibitory ratio of the purified BL-derived and RG-derived His-LKB1 on hepatic carcinoma SMMC-7721 cells was 24.97% and 45.68%, respectively, and both could produce significant cell-cycle arrest.

LKB1 catalytically deficient mutants enhance cyclin D1 expression

Mutations in the serine-threonine tumor-suppressor kinase LKB1 are responsible for Peutz-Jeghers syndrome, characterized by hamartomatous proliferation and an increased risk of developing cancer. Mutations in lkb1 have also been identified in sporadic cancers, suggesting a wider role for LKB1 in cancer that is not limited to hamartomatous polyposis syndromes. Here, we show that LKB1 catalytically deficient mutants, when introduced into DLD1p21-/-p53-/- colorectal cancer cells, allowed for progression of cells through to S phase of cell cycle and elicited the expression of Rb, cyclin E, and cyclin A2 whereas the introduction of LKB1 lead to G1 cell cycle arrest independent of p21(WAF/CIP1) and/or p53 expression. Furthermore, we show that LKB1 catalytically deficient mutants activate the expression of cyclin D1 through recruitment to response elements within the promoter of the oncogene. In addition to compromising the tumor-suppressor function of LKB1, our findings highlight an emerging role for LKB1 catalytically deficient mutants, a gain of oncogenic properties.

Association of LKB1 with a WD-repeat protein WDR6 is implicated in cell growth arrest and p27Kip1 induction

Germline mutations of the serine/threonine kinase LKB1 (also known as STK11) lead to Peutz-Jeghers syndrome (PJS) that is associated with increased incidence of malignant cancers. However, the tumor suppressor function of LKB1 has not been fully elucidated. We applied yeast two-hybrid screening and identified that a novel WD-repeat protein WDR6 was able to interact with LKB1. Immunofluorescence staining revealed that WDR6 was localized in cytoplasm, similar to the localization of LKB1. Expression of LKB1 was able to inhibit colony formation of Hela cells. Interestingly, coexpression of WDR6 with LKB1 enhanced the inhibitory effect of LKB1 on Hela cell proliferation. Consistently, WDR6 was able to synergize with LKB1 in cell cycle G1 arrest in Hela cells. Coexpression of WDR6 and LKB1 was able to induce a cyclin-dependent kinase (CDK) inhibitor p27(Kip1). Furthermore, the stimulatory effect of LKB1 on p27(Kip1) promoter activity was significantly elevated by coexpression with WDR6. Collectively, these results provided initial evidence that WDR6 is implicated in the cell growth inhibitory pathway of LKB1 via regulation of p27(Kip1).

Asymmetric Stem Cell Division in Development and Cancer

Asymmetric stem cell division leads to another stem cell via self-renewal, and a second cell type which can be either a differentiating progenitor or a postmitotic cell. The regulation of this balanced process is mainly achieved by polarization of the stem cell along its apical-basal axis and the basal localization and asymmetric segregation of cell fate determinants solely to the differentiating cell. It has long been speculated that disturbance of this process can induce a cancer-like state. Recent molecular genetic evidence in Drosophila melanogaster suggests that impaired polarity formation in neuroblast stem cells results in symmetric stem cell divisions, whereas defects in progenitor cell differentiation leads to mutant cells that are unable to differentiate but rather continue to proliferate. In both cases, the net result is unrestrained self-renewal of mutant stem cells, eventually leading to hyperproliferation and malignant neoplastic tissue formation. Thus, deregulated stem cells can play a pivotal role in Drosophila tumor formation. Moreover, recent evidence suggests that so-called cancer stem cells may drive the growth and metastasis of human tumors too. Indeed, cancer stem cells have already been identified in leukemia, and in solid tumors of the breast and brain. In addition, inappropriate activation of pathways promoting the self-renewal of somatic stem cells including defects in asymmetric cell division has been shown to cause neoplastic proliferation and cancer formation. Taken together, these data indicate that evolutionary conserved mechanisms regulate stem and progenitor cell self-renewal and tumor suppression via asymmetric cell division control.

5'-AMP-activated protein kinase (AMPK) is induced by low-oxygen and glucose deprivation conditions found in solid-tumor microenvironments

Low oxygen gradients (hypoxia and anoxia) are important determinants of pathological conditions under which the tissue blood supply is deficient or defective, such as in solid tumors. We have been investigating the relationship between the activation of hypoxia-inducible factor 1 (HIF-1), the primary transcriptional regulator of the mammalian response to hypoxia, and 5'-AMP-activated protein kinase (AMPK), another regulatory system important for controlling cellular energy metabolism. In the present study, we used mouse embryo fibroblasts nullizygous for HIF-1alpha or AMPK expression to show that AMPK is rapidly activated in vitro by both physiological and pathophysiological low-oxygen conditions, independently of HIF-1 activity. These findings imply that HIF-1 and AMPK are components of a concerted cellular response to maintain energy homeostasis in low-oxygen or ischemic-tissue microenvironments. Finally, we used transformed derivatives of wild-type and HIF-1alpha- or AMPKalpha-null mouse embryo fibroblasts to determine whether AMPK is activated in vivo. We obtained evidence that AMPK is activated in authentic hypoxic tumor microenvironments and that this activity overlaps with regions of hypoxia detected by a chemical probe. We also showed that AMPK is important for the growth of this tumor model.

Role of insulin, adipocyte hormones, and nutrient-sensing pathways in regulating fuel metabolism and energy homeostasis: a nutritional perspective of diabetes, obesity, and cancer

Traditionally, nutrients such as glucose and amino acids have been viewed as substrates for the generation of high-energy molecules and as precursors for the biosynthesis of macromolecules. However, it is now apparent that nutrients also function as signaling molecules in functionally diverse signal transduction pathways. Glucose and amino acids trigger signaling cascades that regulate various aspects of fuel and energy metabolism and control the growth, proliferation, and survival of cells. Here, we provide a functional and regulatory overview of three well-established nutrient signaling pathways-the hexosamine signaling pathway, the mTOR (mammalian target of rapamycin) signaling pathway, and the adenosine monophosphate-activated protein kinase (AMPK) signaling pathway. Nutrient signaling pathways are interconnected, coupled to insulin signaling, and linked to the release of metabolic hormones from adipose tissue. Thus, nutrient signaling pathways do not function in isolation. Rather, they appear to serve as components of a larger "metabolic regulatory network" that controls fuel and energy metabolism (at the cell, tissue, and whole-body levels) and links nutrient availability with cell growth and proliferation. Understanding the diverse roles of nutrients and delineating nutrient signaling pathways should facilitate drug discovery research and the search for novel therapeutic compounds to prevent and treat various human diseases such as diabetes, obesity, and cancer.

LKB1 exonic and whole gene deletions are a common cause of Peutz-Jeghers syndrome

BACKGROUND

LKB1/STK11 germline mutations cause Peutz-Jeghers syndrome (PJS). The existence of a second PJS locus is controversial, the evidence in its favour being families unlinked to LKB1 and the low frequency of LKB1 mutations found using conventional methods in several studies. Exonic and whole gene deletion or duplication events cannot be detected by routine mutation screening methods.

OBJECTIVE

To seek evidence for LKB1 germline deletions or duplications by screening patients meeting clinical criteria for PJS but without detected mutations on conventional screening.

METHODS

From an original cohort of 76 patients, 48 were found to have a germline mutation by direct sequencing; the remaining 28 were examined using multiplex ligation dependent probe amplification (MLPA) analysis to detect LKB1 copy number changes.

RESULTS

Deletions were found in 11 of the 28 patients (39%)--that is, 14% of all PJS patients (11/76). Five patients had whole gene deletions, two had the promoter and exon 1 deleted, and in one patient exon 8 was deleted. Other deletions events involved: loss of exons 2-10; deletion of the promoter and exons 1-3; and loss of part of the promoter. No duplications were detected. Nine samples with deletions were sequenced at reported single nucleotide polymorphisms to exclude heterozygosity; homozygosity was found in all cases. No MLPA copy number changes were detected in 22 healthy individuals.

CONCLUSIONS

These results lessen the possibility of a second PJS locus, as the detection rate of germline mutations in PJS patients was about 80% (59/76). It is suggested that MLPA, or a suitable alternative, should be used for routine genetic testing of PJS patients in clinical practice.

Mutations in the humanLKB1/STK11gene

The human LKB gene (official HUGO symbol, STK11) encodes a serine/threonine protein kinase that is defective in patients with Peutz-Jeghers syndrome (PJS). PJS is an autosomal dominantly inherited syndrome characterized by hamartomatous polyposis of the gastrointestinal tract and mucocutaneous pigmentation. To date, 145 different germline LKB1 mutations have been reported. The majority of the mutations lead to a truncated protein product. One mutational hotspot has been observed. A 1-bp deletion and a 1-bp insertion at the mononucleotide repeat (C6 repeat, c.837-c.842) between the codons 279-281 have been found in six and seven unrelated PJS families, respectively. However, these mutations account only for approximately 7% of all mutations identified in the PJS families (13/193). A review of the literature provides a total of 40 different somatic LKB1 mutations in 41 sporadic tumors and seven cancer cell lines. Mutations occur particularly in lung and colorectal cancer. Most of the somatic LKB1 mutations result in truncation of the protein. A mutational hotspot seems to be a C6 repeat accounting for 12.5% of all somatic mutations (6/48). These results are concordant with the germline mutation spectrum. However, the proportion of the missense mutations seems to be higher among the somatic mutations (45%) than among the germline mutations (21%), and only seven of the mutations are exactly the same in both of the mutation types.