Vascular abnormalities and deregulation of VEGF in Lkb1-deficient mice

Insights from model organisms on the functions of the tumor suppressor protein LKB1: zebrafish chips in

The tumor suppressor LKB1 has emerged as a critical regulator of cell polarity and energy-metabolism. Studies in diverse model organisms continue to unravel the pathways downstream of LKB1; the emerging picture is that the outcomes of LKB1 signaling are mediated by a plethora of tissue-specific and context-dependent effectors.

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.

LKB1 is required for hepatic bile acid transport and canalicular membrane integrity in mice

LKB1 is a 'master' protein kinase implicated in the regulation of metabolism, cell proliferation, cell polarity and tumorigenesis. However, the long-term role of LKB1 in hepatic function is unknown. In the present study, it is shown that hepatic LKB1 plays a key role in liver cellular architecture and metabolism. We report that liver-specific deletion of LKB1 in mice leads to defective canaliculi and bile duct formation, causing impaired bile acid clearance and subsequent accumulation of bile acids in serum and liver. Concomitant with this, it was found that the majority of BSEP (bile salt export pump) was retained in intracellular pools rather than localized to the canalicular membrane in hepatocytes from LLKB1KO (liver-specific Lkb1-knockout) mice. Together, these changes resulted in toxic accumulation of bile salts, reduced liver function and failure to thrive. Additionally, circulating LDL (low-density lipoprotein)-cholesterol and non-esterified cholesterol levels were increased in LLKB1KO mice with an associated alteration in red blood cell morphology and development of hyperbilirubinaemia. These results indicate that LKB1 plays a critical role in bile acid homoeostasis and that lack of LKB1 in the liver results in cholestasis. These findings indicate a novel key role for LKB1 in the development of hepatic morphology and membrane targeting of canalicular proteins.

The serine-threonine kinase LKB1 is essential for survival under energetic stress in zebrafish

Mutations in the serine-threonine kinase (LKB1) lead to a gastrointestinal hamartomatous polyposis disorder with increased predisposition to cancer (Peutz-Jeghers syndrome). LKB1 has many targets, including the AMP-activated protein kinase (AMPK) that is phosphorylated under low-energy conditions. AMPK phosphorylation in turn, affects several processes, including inhibition of the target of rapamycin (TOR) pathway, and leads to proliferation inhibition. To gain insight into how LKB1 mediates its effects during development, we generated zebrafish mutants in the single LKB1 ortholog. We show that in zebrafish lkb1 is dispensable for embryonic survival but becomes essential under conditions of energetic stress. After yolk absorption, lkb1 mutants rapidly exhaust their energy resources and die prematurely from starvation. Notably, intestinal epithelial cells were polarized properly in the lkb1 mutants. We show that attenuation of metabolic rate in lkb1 mutants, either by application of the TOR inhibitor rapamycin or by crossing with von Hippel-Lindau (vhl) mutant fish (in which constitutive hypoxia signaling results in reduced metabolic rate), suppresses key aspects of the lkb1 phenotype. Thus, we demonstrate a critical role for LKB1 in regulating energy homeostasis at the whole-organism level in a vertebrate. Zebrafish models of Lkb1 inactivation could provide a platform for chemical genetic screens to identify compounds that target accelerated metabolism, a key feature of tumor cells.

Dynamic Lkb1-TORC1 signaling as a possible mechanism for regulating the endoderm-intestine transition

The intestinal epithelium arises from undifferentiated endoderm via a developmental program known as the endoderm-intestine transition (EIT). Previously we found that the target of rapamycin complex 1 (TORC1) regulates intestinal growth and differentiation during the EIT in zebrafish. Here we address a possible role for the tumor-suppressor kinase Lkb1 in regulating TORC1 in this context. We find that TORC1 activity is transiently upregulated during the EIT in both zebrafish and mouse. Concomitantly, Lkb1 becomes transiently localized to the nucleus, suggesting that these two phenomena may be linked. Morpholino-mediated knockdown of lkb1 stimulated intestinal growth via upregulation of TORC1, and also induced precocious intestine-specific gene expression in the zebrafish gut epithelium. Knockdown of tsc2, which acts downstream of lkb1, likewise induced early expression of intestine-specific genes. These data suggest that programmed localization of Lkb1 could represent a novel mechanism for regulating the EIT during intestinal development in vertebrates.

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.

Lkb1 regulates cell cycle and energy metabolism in haematopoietic stem cells

Little is known about metabolic regulation in stem cells and how this modulates tissue regeneration or tumour suppression. We studied the Lkb1 tumour suppressor, and its substrate AMPK, kinases that coordinate metabolism with cell growth. Lkb1 deletion caused increased haematopoietic stem cell (HSC) division, rapid HSC depletion, and pancytopenia. HSCs depended more acutely on Lkb1 for cell cycle regulation and survival than many other haematopoietic cells. HSC depletion did not depend on mTOR activation or oxidative stress. Lkb1-deficient HSCs, but not myeloid progenitors, had reduced mitochondrial membrane potential and ATP. AMPK-deficient HSCs showed similar changes in mitochondrial function but remained able to reconstitute irradiated mice. Lkb1-deficient HSCs, but not AMPK-deficient HSCs, exhibited defects in centrosomes and mitotic spindles in culture, and became aneuploid. Lkb1 is therefore required for HSC maintenance through AMPK-dependent and AMPK-independent mechanisms, revealing differences in metabolic and cell cycle regulation between HSCs and some other haematopoietic progenitors.

The LKB1/AMPK polarity pathway

The LKB1 tumor suppressor kinase is an activator of the AMP-activated protein kinase (AMPK), a metabolic gauge that responds to variations of cellular energetic levels by favoring catabolic versus anabolic processes. Recent studies have provided substantial evidence that LKB1 and AMPK control cell polarity from invertebrates to mammals. This review examines how the LKB1-AMPK pathway, in conjunction with other positional signals, converts energy-sensing information into the activation of Myosin II to maintain epithelial-cell architecture but also to complete cell division. This molecular link between polarity and metabolism may constitute an ancient stress-response protective mechanism that was co-opted for tumor suppression during evolution.

Tumor Suppressor Serine/Threonine Kinase LKB1 Expression, Not Kinase Activity, Increased in the Vascular Smooth Muscle Cells and Neointima in the Rat Carotid Artery Injury Model

BACKGROUND AND OBJECTIVES

Vascular smooth muscle cell (VSMC) proliferation is responsible for the restenosis of previously inserted coronary stents. Angiotensin II (Ang II) is known to regulate VSMC proliferation. LKB1, a serine/threonine kinase, interacts with the p53 pathway and acts as a tumor suppressor.

MATERIALS AND METHODS

We assessed the association of Ang II and the expression of LKB1 in primary cultured murine VSMCs and neointima of the Sprague Dawley rat carotid artery injury model. We created carotid balloon injuries and harvested the injured carotid arteries 14 days after the procedure.

RESULTS

Ang II increased LKB1 expression in a time-dependent manner and peaked at an Ang II concentration of 10(-7) mole/L in VSMCs. In the animal experiment, neointima was markedly increased after balloon injury compared to the control group. Immunohistochemical studies showed that LKB1 expression increased according to neointima thickness. Ang II augmented LKB1 expression after the injury. Western blot analysis of LKB1 with carotid artery lysate revealed the same pattern as LKB1 immunohistochemistry. Increased LKB1 expression started at 5 days after the balloon injury, and peaked at 14 days after the injury. Although LKB1 expression was increased after the injury, LKB1 kinase activity was not increased. Ang II or balloon-injury increased the expression of LKB1 although the LKB1 activity was reduced.

CONCLUSION

Ang II increased LKB1 expression in VSMCs and neointima. These findings were not kinase dependant.

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.

LKB1 haploinsufficiency cooperates with Kras to promote pancreatic cancer through suppression of p21-dependent growth arrest

BACKGROUND & AIMS

Patients carrying germline mutations of LKB1 have an increased risk of pancreatic cancer; however, it is unclear whether down-regulation of LKB1 is an important event in sporadic pancreatic cancer. In this study, we aimed to investigate the impact of LKB1 down-regulation for pancreatic cancer in mouse and human and to elucidate the mechanism by which Lkb1 deregulation contributes to this disease.

METHODS

We first investigated the consequences of Lkb1 deficiency in a genetically modified mouse model of pancreatic cancer, both in terms of disease progression and at the molecular level. To test the relevance of our findings to human pancreatic cancer, we investigated levels of LKB1 and its potential targets in human pancreatic cancer.

RESULTS

We definitively show that Lkb1 haploinsufficiency can cooperate with oncogenic KrasG12D to cause pancreatic ductal adenocarcinoma (PDAC) in the mouse. Mechanistically, this was associated with decreased p53/p21-dependent growth arrest. Haploinsufficiency for p21 (Cdkn1a) also synergizes with KrasG12D to drive PDAC in the mouse. We also found that levels of LKB1 expression were decreased in around 20% of human PDAC and significantly correlated with low levels of p21 and a poor prognosis. Remarkably, all tumors that had low levels of LKB1 had low levels of p21, and these tumors did not express mutant p53.

CONCLUSIONS

We have identified a novel LKB1-p21 axis that suppresses PDAC following Kras mutation in vivo. Down-regulation of LKB1 may therefore serve as an alternative to p53 mutation to drive pancreatic cancer in vivo.

LKB1 deficiency in Tie2-Cre-expressing cells impairs ischemia-induced angiogenesis

LKB1 is a tumor suppressor protein whose loss leads to HIF1alpha-mediated activation of a proangiogenic program in intestinal polyps. LKB1 is also protein kinase regulator of AMP-activated protein kinase (AMPK) signaling, which is essential for endothelial cell responses to tissue ischemia. To discern whether LKB1 signaling is either pro- or antiangiogenic, we investigated ischemia-induced revascularization in mice that were deficient for LKB1 in Tie2-Cre-expressing cells. Whereas homozygous deletion of LKB1 led to embryonic lethality, heterozygous LKB1-knock-out (KO) (Lkb1(flox/+);Tie2(Tg/+)) mice were viable. Unchallenged heterozygous LKB1-KO mice displayed normal capillary density, but the revascularization of hind limb following ischemic surgery was significantly impaired as evaluated by laser Doppler flow and capillary density measurements. Reduction of LKB1 in cultured endothelial cells, using either small interfering RNA or an adenovirus expressing nonfunctional kinase-dead LKB1 protein, attenuated endothelial proliferation, migration, and differentiation into network structures on Matrigel that was accompanied by diminished AMPK phosphorylation at Thr-172. Conversely, adenovirus-mediated LKB1 overexpression (Ad-LKB1) augmented network structure formation, and this was associated with elevated AMPK phosphorylation. The augmented differentiation of endothelial cells into network structures induced by Ad-LKB1 was abrogated by the co-transduction of a dominant negative mutant of AMPK. These observations suggest that the LKB1-AMPK signaling axis in endothelial cells is a positive regulator of the revascularization response to tissue ischemia.

SIRT1 Promotes Proliferation and Prevents Senescence Through Targeting LKB1 in Primary Porcine Aortic Endothelial Cells

Rationale : Endothelial senescence causes endothelial dysfunction, promotes atherogenesis and contributes to age-related vascular disorders. SIRT1 is a conserved NAD + -dependent deacetylase possessing beneficial effects against aging-related diseases, despite that the detailed functional mechanisms are largely uncharacterized.

Objective : The present study is designed to evaluate the protective effects of SIRT1 on endothelial senescence and to elucidate the underlying mechanisms.

Methods and Results : An in vitro senescence model was established by prolonged culture of primary endothelial cells isolated from porcine aorta. The freshly isolated “young” cells gradually underwent senescence during 1 month of repetitive passages. Both mRNA and protein expressions of SIRT1 were progressively decreased. In contrast, the protein levels of LKB1, a serine/threonine kinase and tumor suppressor, and the phosphorylation of its downstream target AMPK(Thr172) were dramatically increased in senescent cells. Overexpression of LKB1 promoted cellular senescence and retarded endothelial proliferation, which could be blocked by increasing SIRT1 levels. Knocking down of SIRT1 induced senescence and elevated the protein levels of LKB1 and phosphorylated AMPK(Thr172). Regardless of the nutritional status, hyperactivation of AMPK was able to induce endothelial senescence. SIRT1 antagonized LKB1-dependent AMPK activation through promoting the deacetylation, ubiquitination and proteasome-mediated degradation of LKB1. The survival signaling of Akt was also found to be modulated by SIRT1 and LKB1, and could cross-regulate AMPK activity.

Conclusions : SIRT1 and LKB1/AMPK are the 2 key sensor systems for regulating endothelial cell survival, proliferation and senescence. The protective activities of SIRT1 may be achieved at least in part by fine tuning the acetylation/deacetylation status and stabilities of LKB1 protein.

STRADalpha deficiency results in aberrant mTORC1 signaling during corticogenesis in humans and mice

Polyhydramnios, megalencephaly, and symptomatic epilepsy syndrome (PMSE) is a rare human autosomal-recessive disorder characterized by abnormal brain development, cognitive disability, and intractable epilepsy. It is caused by homozygous deletions of STE20-related kinase adaptor alpha (STRADA). The underlying pathogenic mechanisms of PMSE and the role of STRADA in cortical development remain unknown. Here, we found that a human PMSE brain exhibits cytomegaly, neuronal heterotopia, and aberrant activation of mammalian target of rapamycin complex 1 (mTORC1) signaling. STRADalpha normally binds and exports the protein kinase LKB1 out of the nucleus, leading to suppression of the mTORC1 pathway. We found that neurons in human PMSE cortex exhibited abnormal nuclear localization of LKB1. To investigate this further, we modeled PMSE in mouse neural progenitor cells (mNPCs) in vitro and in developing mouse cortex in vivo by knocking down STRADalpha expression. STRADalpha-deficient mNPCs were cytomegalic and showed aberrant rapamycin-dependent activation of mTORC1 in association with abnormal nuclear localization of LKB1. Consistent with the observations in human PMSE brain, knockdown of STRADalpha in vivo resulted in cortical malformation, enhanced mTORC1 activation, and abnormal nuclear localization of LKB1. Thus, we suggest that the aberrant nuclear accumulation of LKB1 caused by STRADalpha deficiency contributes to hyperactivation of mTORC1 signaling and disruption of neuronal lamination during corticogenesis, and thereby the neurological features associated with PMSE.

Impaired gastric gland differentiation in Peutz-Jeghers syndrome

Gastrointestinal hamartomatous polyps in the Peutz-Jeghers cancer predisposition syndrome and its mouse model (Lkb1(+/-)) are presumed to contain all cell types native to the site of their occurrence. This study aimed to explore the pathogenesis of Peutz-Jeghers syndrome polyposis by characterizing cell types and differentiation of the epithelium of gastric polyps and predisposed mucosa. Both antral and fundic polyps were characterized by a deficit of pepsinogen C-expressing differentiated gland cells (antral gland, mucopeptic, and chief cells); in large fundic polyps, parietal cells were also absent. Gland cell loss was associated with an increase in precursor neck cells, an expansion of the proliferative zone, and an increase in smooth muscle alpha-actin expressing myofibroblasts in the polyp stroma. Lack of pepsinogen C-positive gland cells identified incipient polyps, and even the unaffected mucosa of young predisposed mice displayed an increase in pepsinogen C negative glands (25%; P = 0045). In addition, in small intestinal polyps, gland cell differentiation was defective, with the absence of Paneth cells. There were no signs of metaplastic differentiation in any of the tissues studied, and both the gastric and small intestinal defects were seen in Lkb1(+/-) mice, as well as polyps from patients with Peutz-Jeghers syndrome. These results identify impaired epithelial differentiation as the earliest pathological sign likely to contribute to tumorigenesis in individuals with inherited Lkb1 mutations.

Skeletal muscle dysfunction in muscle-specific LKB1 knockout mice

Liver kinase B1 (LKB1) is a tumor-suppressing protein that is involved in the regulation of muscle metabolism and growth by phosphorylating and activating AMP-activated protein kinase (AMPK) family members. Here we report the development of a myopathic phenotype in skeletal and cardiac muscle-specific LKB1 knockout (mLKB1-KO) mice. The myopathic phenotype becomes overtly apparent at 30-50 wk of age and is characterized by decreased body weight and a proportional reduction in fast-twitch skeletal muscle weight. The ability to ambulate is compromised with an often complete loss of hindlimb function. Skeletal muscle atrophy is associated with a 50-75% reduction in mammalian target of rapamycin pathway phosphorylation, as well as lower peroxisome proliferator-activated receptor-alpha coactivator-1 content and cAMP response element binding protein phosphorylation (43 and 40% lower in mLKB1-KO mice, respectively). Maximum in situ specific force production is not affected, but fatigue is exaggerated, and relaxation kinetics are slowed in the myopathic mice. The increased fatigue is associated with a 30-78% decrease in mitochondrial protein content, a shift away from type IIA/D toward type IIB muscle fibers, and a tendency (P=0.07) for decreased capillarity in mLKB1-KO muscles. Hearts from myopathic mLKB1-KO mice exhibit grossly dilated atria, suggesting cardiac insufficiency and heart failure, which likely contributes to the phenotype. These findings indicate that LKB1 plays a critical role in the maintenance of both skeletal and cardiac function.

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.

Fishing for intestinal cancer models: unraveling gastrointestinal homeostasis and tumorigenesis in zebrafish

Zebrafish has proven to be a highly versatile model for comprehensive studies of gene function in development. Given that the molecular pathways involved in epithelial carcinogenesis appear to be conserved across vertebrates, zebrafish is now considered as a valid model to study tumor biology. Development and homeostasis in multicellular organisms are dependent on a complex interplay between cell proliferation, migration, differentiation, and cell death. The Wnt signaling pathway is a major signaling pathway during embryonic development and is the key regulator of self-renewal homeostasis in several adult tissues. A large body of knowledge on adult stem-cell biology has arisen from the study of the intestinal epithelium over the past 20 years. The Wnt pathway has appeared as its principal regulator of homeostatic self-renewal. Moreover, most cancers of the intestine are caused by activating mutations in the Wnt pathway. Recently, zebrafish models have been developed to study Wnt pathway-induced cancer. An appealing avenue for cancer research in zebrafish is large-scale screens to identify chemotherapeutic and chemopreventive agents in conjunction with the in vivo imaging approaches that zebrafish affords.

Conditional deletion of the Lkb1 gene in the mouse mammary gland induces tumour formation

Heterozygous germline mutations in the LKB1 (STK11) gene cause Peutz-Jeghers syndrome (PJS), an autosomal dominant disorder characterized by hamartomatous polyposis of the gastrointestinal tract and an increased risk of colorectal, breast, and other cancers. To model the role of LKB1 mutation in mammary tumourigenesis, we have used a conditional gene targeting strategy to generate a mouse in which exons encoding the kinase domain of Lkb1 were deleted specifically in the mammary gland. Mammary gland tumours developed in these mice with a latency of 46-85 weeks and occurred in the thoracic or inguinal glands. These tumours were grade 2 invasive ductal carcinomas or solid papillary carcinomas with histological features similar to those described in breast cancers arising in patients with PJS. This mouse model of Lkb1 deficiency provides a potentially useful tool to investigate the role of Lkb1 in tumourigenesis and to guide the development of therapeutic approaches.

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 is essential for the proliferation of T-cell progenitors and mature peripheral T cells

The serine/threonine kinase LKB1 has a conserved role in Drosophila and nematodes to co-ordinate cell metabolism. During T lymphocyte development in the thymus, progenitors need to synchronize increased metabolism with the onset of proliferation and differentiation to ensure that they can meet the energy requirements for development. The present study explores the role of LKB1 in this process and shows that loss of LKB1 prevents thymocyte differentiation and the production of peripheral T lymphocytes. We find that LKB1 is required for several key metabolic processes in T-cell progenitors. For example, LKB1 controls expression of CD98, a key subunit of the L-system aa transporter and is also required for the pre-TCR to induce and sustain the regulated phosphorylation of the ribosomal S6 subunit, a key regulator of protein synthesis. In the absence of LKB1 TCR-beta-selected thymocytes failed to proliferate and did not survive. LBK1 was also required for survival and proliferation of peripheral T cells. These data thus reveal a conserved and essential role for LKB1 in the proliferative responses of both thymocytes and mature T cells.

No association between promoter polymorphism of STK11 gene and lung cancer risk in the Korean population

PURPOSE

Serine-threonine kinase11 (STK11) was originally identified in 1997 as the causative mutation that's responsible for Peutz-Jeghers Syndrome (PJS). Several recent studies have reported that the STK11 gene is an important human tumor suppressor gene in lung cancer. We evaluated the associations between the polymorphisms of the STK11 promoter region and the risk of lung cancer in 901 Koreans.

MATERIALS AND METHODS

By direct sequencing, we first discovered three novel polymorphisms (-1,795 T>C, -981 C>T and -160 G>T) and four known polymorphisms (-1,580 C>T, -1,494 A>C, -881 A>G and -458 G>C) of the STK11 promoter region in 24 blood samples of 24 Korean lung cancer patients. Further genotype analyses were then performed on 443 lung cancer patients and 458 controls.

RESULTS

We discovered three novel polymorphisms and we identified four known polymorphisms of the STK11 promoter region in a Korean population. Statistical analyses revealed that the genotypes and haplotypes in the STK11 gene were not significantly associated with the risk of lung cancer in a Korean population.

CONCLUSION

This is the first study that's focused on the association of STK11 promoter polymorphisms and the risk of lung cancer in a Korean population. To evaluate the role of the STK11 gene for the risk of lung cancer, the genotypes of the STK11 promoter region (-1,795 T>C, -1,494 A>C and -160 G>T) were determined in 901 Koreans, yet the result revealed no significant difference between the lung cancer patients and the controls. These results suggest that the three promoter polymorphisms we studied are not important risk factors for the susceptibility to lung cancer in Koreans.

The cyclin-dependent kinase inhibitor p21 is a crucial target for histone deacetylase 1 as a regulator of cellular proliferation

Histone deacetylases (HDACs) are chromatin-modifying enzymes that are involved in the regulation of proliferation, differentiation and development. HDAC inhibitors induce cell cycle arrest, differentiation, or apoptosis in tumor cells and are therefore promising antitumor agents. Numerous genes were found to be deregulated upon HDAC inhibitor treatment; however, the relevant target enzymes are still unidentified. HDAC1 is required for mouse development and unrestricted proliferation of embryonic stem cells. We show here that HDAC1 reversibly regulates cellular proliferation and represses the cyclin-dependent kinase inhibitor p21 in embryonic stem cells. Disruption of the p21 gene rescues the proliferation phenotype of HDAC1(-/-) embryonic stem cells but not the embryonic lethality of HDAC1(-/-) mice. In the absence of HDAC1, mouse embryonic fibroblasts scarcely undergo spontaneous immortalization and display increased p21 expression. Chromatin immunoprecipitation assays demonstrate a direct regulation of the p21 gene by HDAC1 in mouse embryonic fibroblasts. Transformation with simian virus 40 large T antigen or ablation of p21 restores normal immortalization of primary HDAC1(-/-) fibroblasts. Our data demonstrate that repression of the p21 gene is crucial for HDAC1-mediated control of proliferation and immortalization. HDAC1 might therefore be one of the relevant targets for HDAC inhibitors as anticancer drugs.

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.

Advances in research on hereditary colorectal neoplasms

Colorectal cancer is one of the most common digestive malignant tumors in China. Its incidence and mortality rates rank top among all malignant tumors. Previous studies show that nearly one-third of colorectal cancers are associated with hereditary colorectal tumors. Hereditary colorectal tumors are mainly divided into two categories: hereditary nonpolyposis colorectal cancer and hereditary colorectal polyposis. In this article, we will review the recent advances in research on hereditary colorectal neoplasms.

Oral rapamycin reduces tumour burden and vascularization in Lkb1(+/-) mice

Patients with Peutz-Jeghers syndrome (PJS) are affected by hamartomatous intestinal polyposis and increased risk of cancers in multiple organs caused by germline mutations in the tumour suppressor gene LKB1. Murine models that recapitulate aspects of PJS have been created. Here we examine the therapeutic effect of rapamycin, a macrolide with anti-tumourigenic and anti-angiogenic properties, in reducing tumour incidence in a large cohort of Lkb1(+/-) mice. To study the influence of early intervention, the animals were dosed with rapamycin from the age of 8 weeks, well before the onset of polyposis. These mice continued to receive the drug, which was well tolerated, throughout their lives. At sacrifice, we observed a reduction in gastric tumour burden in the rapamycin-treated mice (p = 0.0001) compared with age- and sex-matched controls. Treated animals also have a lower number of polyps per mouse than controls. In the polyps from the treated mice, phosphorylation of ribosomal p70 S6 kinase was maintained, while the phosphorylation of AKT at serine-473 was elevated, suggesting that mTORC1 function is maintained at this dosage. Despite this, a significant reduction in microvessel density was seen in polyps from the rapamycin-treated mice compared to those from the control mice (p = 5 x 10(-5)), suggesting that the anti-angiogenic effect of rapamycin played a role in polyp reduction. Overall, we demonstrated that prolonged oral administration of rapamycin from an early age is effective in lowering tumour burden in the Lkb1(+/-) mice without evident side effects.

Loss of Lkb1 in adult beta cells increases beta cell mass and enhances glucose tolerance in mice

The Lkb1 tumor suppressor exerts its biological effects through phosphorylation and consequent activation of the AMP kinase (AMPK) family. Extensive genetic and biochemical evidence supports a role for Lkb1 in cell cycle arrest, establishment of cell polarity, and cellular energy metabolism. However, the role of Lkb1 and the AMPK family in beta cell function in vivo has not been established. We generated conditional knockout mice with a deletion of the Lkb1 gene in the beta cell compartment of pancreatic islets; these mice display improved glucose tolerance and protection against diet-induced hyperglycemia. Lkb1(-/-) beta cells are hypertrophic because of elevated mTOR activity; they also proliferate more and secrete more insulin in response to glucose. These data indicate that inhibiting Lkb1 activity in beta cells may facilitate beta cell expansion and glucose tolerance in vivo.

LKB1 and AMPK family signaling: the intimate link between cell polarity and energy metabolism

Research on the LKB1 tumor suppressor protein mutated in cancer-prone Peutz-Jeghers patients has continued at a feverish pace following exciting developments linking energy metabolism and cancer development. This review summarizes the current state of research on the LKB1 tumor suppressor. The weight of the evidence currently indicates an evolutionary conserved role for the protein in the regulation of various aspects of cellular polarity and energy metabolism. We focus on studies examining the concept that both cellular polarity and energy metabolism are regulated through the conserved LKB1-AMPK signal transduction pathway. Recent studies from a variety of model organisms have given new insight into the mechanism of polyp development and cancer formation in Peutz-Jeghers patients and the role of LKB1 mutation in sporadic tumorigenesis. Conditional LKB1 mouse models have outlined a tissue-dependent context for pathway activation and suggest that LKB1 may affect different AMPK isoforms independently. Elucidation of the molecular mechanism responsible for Peutz-Jeghers syndrome will undoubtedly reveal important insight into cancer development in the larger population.

Endothelial cell migration and vascular endothelial growth factor expression are the result of loss of breast tissue polarity

Recruiting a new blood supply is a rate-limiting step in tumor progression. In a three-dimensional model of breast carcinogenesis, disorganized, proliferative transformed breast epithelial cells express significantly higher expression of angiogenic genes compared with their polarized, growth-arrested nonmalignant counterparts. Elevated vascular endothelial growth factor (VEGF) secretion by malignant cells enhanced recruitment of endothelial cells (EC) in heterotypic cocultures. Significantly, phenotypic reversion of malignant cells via reexpression of HoxD10, which is lost in malignant progression, significantly attenuated VEGF expression in a hypoxia-inducible factor 1alpha-independent fashion and reduced EC migration. This was due primarily to restoring polarity: forced proliferation of polarized, nonmalignant cells did not induce VEGF expression and EC recruitment, whereas disrupting the architecture of growth-arrested, reverted cells did. These data show that disrupting cytostructure activates the angiogenic switch even in the absence of proliferation and/or hypoxia and restoring organization of malignant clusters reduces VEGF expression and EC activation to levels found in quiescent nonmalignant epithelium. These data confirm the importance of tissue architecture and polarity in malignant progression.

The LKB1-AMPK pathway: metabolism and growth control in tumour suppression

In the past decade, studies of the human tumour suppressor LKB1 have uncovered a novel signalling pathway that links cell metabolism to growth control and cell polarity. LKB1 encodes a serine-threonine kinase that directly phosphorylates and activates AMPK, a central metabolic sensor. AMPK regulates lipid, cholesterol and glucose metabolism in specialized metabolic tissues, such as liver, muscle and adipose tissue. This function has made AMPK a key therapeutic target in patients with diabetes. The connection of AMPK with several tumour suppressors suggests that therapeutic manipulation of this pathway using established diabetes drugs warrants further investigation in patients with cancer.

Mutations in STK11 gene in Czech Peutz-Jeghers patients

Background

Peutz-Jeghers syndrome (PJS) is an autosomal dominant hereditary disease characterized by mucocutaneous pigmentation and gastrointestinal hamartomatous polyposis. The germline mutations in the serine/threonine kinase 11 (STK11) gene have been shown to be associated with the disease. Individuals with PJS are at increased risk for development of various neoplasms. The aim of the present study was to characterize the genotype and phenotype of Czech patients with PJS.

Methods

We examined genomic DNA of 8 individuals from five Czech families by sequencing analysis of STK11 gene, covering its promotor region, the entire coding region and the splice-site boundaries, and by multiplex ligation-dependent probe amplification (MLPA) assay designed for the identification of large exonic deletions or duplications of STK11 gene.

Results

We found pathogenic mutations in STK11 gene in two families fulfilling the diagnostic criteria of PJS and in one of three sporadic cases not complying with the criteria. The patient with the frameshift mutation in STK11 gene developed aggressive gastric cancer. No other studied proband has developed a carcinoma so far.

Conclusion

Our results showed that a germline mutation of STK11 gene can be found not only in probands fulfilling the PJS diagnostic criteria, but also in some sporadic cases not complying with the criteria. Moreover, we observed a new case of aggressive gastric cancer in a young patient with a frameshift mutation of STK11 gene.

Identification of the serine 307 of LKB1 as a novel phosphorylation site essential for its nucleocytoplasmic transport and endothelial cell angiogenesis

LKB1, a master kinase that controls at least 13 downstream protein kinases including the AMP-activated protein kinase (AMPK), resides mainly in the nucleus. A key step in LKB1 activation is its export from the nucleus to the cytoplasm. Here, we identified S307 of LKB1 as a putative novel phosphorylation site which is essential for its nucleocytoplasmic transport. In a cell-free system, recombinant PKC-zeta phosphorylates LKB1 at S307. AMPK-activating agents stimulate PKC-zeta activity and LKB1 phosphorylation at S307 in endothelial cells, hepatocytes, skeletal muscle cells, and vascular smooth muscle cells. Like the kinase-dead LKB1 D194A mutant (mutation of Asp194 to Ala), the constitutively nucleus-localized LKB1 SL26 mutant and the LKB1 S307A mutant (Ser307 to Ala) exhibit a decreased association with STRAD alpha. Interestingly, the PKC-zeta consensus sequence surrounding LKB1 S307 is disrupted in the LKB1 SL26 mutant, thus providing a likely molecular explanation for this mutation causing LKB1 dysfunction. In addition, LKB1 nucleocytoplasmic transport and AMPK activation in response to peroxynitrite are markedly reduced by pharmacological inhibition of CRM1, which normally facilitates nuclear export of LKB1-STRAD complexes. In comparison to the LKB1 wild type, the S307A mutant complexes show reduced association with CRM1. Finally, adenoviral overexpression of wild-type LKB1 suppresses, while the LKB1 S307A mutant increases, tube formation and hydrogen peroxide-enhanced apoptosis in cultured endothelial cells. Taken together, our results suggest that, in multiple cell types the signaling pathways engaged by several physiological stimuli converge upon PKC-zeta-dependent LKB1 phosphorylation at S307, which directs the nucleocytoplasmic transport of LKB1 and consequent AMPK activation.

mTOR and HIF-1alpha-mediated tumor metabolism in an LKB1 mouse model of Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is a familial cancer disorder due to inherited loss of function mutations in the LKB1/ STK11 serine/threonine kinase. PJS patients develop gastrointestinal hamartomas with 100% penetrance often in the second decade of life, and demonstrate an increased predisposition toward the development of a number of additional malignancies. Among mitogenic signaling pathways, the mammalian-target of rapamycin complex 1 (mTORC1) pathway is hyperactivated in tissues and tumors derived from LKB1-deficient mice. Consistent with a central role for mTORC1 in these tumors, rapamycin as a single agent results in a dramatic suppression of preexisting GI polyps in LKB1+/- mice. However, the key targets of mTORC1 in LKB1-deficient tumors remain unknown. We demonstrate here that these polyps, and LKB1- and AMPK-deficient mouse embryonic fibroblasts, show dramatic up-regulation of the HIF-1alpha transcription factor and its downstream transcriptional targets in an rapamycin-suppressible manner. The HIF-1alpha targets hexokinase II and Glut1 are up-regulated in these polyps, and using FDG-PET, we demonstrate that LKB1+/- mice show increased glucose utilization in focal regions of their GI tract corresponding to these gastrointestinal hamartomas. Importantly, we demonstrate that polyps from human Peutz-Jeghers patients similarly exhibit up-regulated mTORC1 signaling, HIF-1alpha, and GLUT1 levels. Furthermore, like HIF-1alpha and its target genes, the FDG-PET signal in the GI tract of these mice is abolished by rapamycin treatment. These findings suggest a number of therapeutic modalities for the treatment and detection of hamartomas in PJS patients, and potential for the screening and treatment of the 30% of sporadic human lung cancers bearing LKB1 mutations.

The construction of siRNA plasmid targeting mouse HIF-1alpha and in vitro study of its inhibition effect

OBJECTIVE

To construct effective RNA-interference plasmids targeting mouse HIF-1alpha gene and testify their effects and specificities in interfering HIF-1alpha expression.

METHODS

Three RNA-interference plasmids targeting mouse HIF-1alpha gene, pBS/U6/HIF-1alpha-siRNAI~III, were constructed and identified using double digestion method in the present study. RT-PCR, immunostaining and western blotting were employed to detect the expression alterations of HIF-1alpha in 293T cells following transfections of the three plasmids, respectively. The interference effect of pBS/U6/HIF1alphai-II in SH-SY5Y cell line was further investigated.

RESULTS

All the three RNA-interference plasmids, especially pBS/U6/HIF1alphai-II, showed significant inhibition in HIF-1alpha expression in 293T cell line. pBS/U6/HIF1alphai-II could also inhibit HIF-1alpha expression in SH-SY5Y cell line, in a dose-dependent way.

CONCLUSION

Plasmid pBS/U6/HIF1alphai-II constructed in our study can effectively and specifically inhibit HIF-1alpha expression, and its role in neural tube development and dysfunction will be further investigated. Construct of pBS/U6/HIF1alphai-II plasmid will provide a useful tool to study the role of HIF-1 pathway in embryogenesis, oncogenesis and ischemia development.

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.

AMP-activated protein kinase and cancer

AMP-activated protein kinase (AMPK) is a cellular energy sensor that is conserved in eukaryotes. Elevated AMP/ATP ratio activates AMPK, which inhibits energy-consuming processes and activates energy-producing processes to restore the energy homeostasis inside the cell. AMPK activators, metformin and thiazolidinediones, are used for the treatment of type II diabetes. Recently, reports have indicated that AMPK may also be a beneficial target for cancer treatment. Cancer cells have characteristic metabolic changes different from normal cells and, being a key metabolic regulator, AMPK may regulate the switch. AMPK may act to inhibit tumorigenesis through regulation of cell growth, cell proliferation, autophagy, stress responses and cell polarity.

BRG1 and LKB1: tales of two tumor suppressor genes on chromosome 19p and lung cancer

Losses of heterozygosity (LOH) of the short arm of chromosome 19 are frequent in lung cancer, suggesting that one or more tumor suppressor genes are present in this region. The LKB1 gene, also called STK11, is somatically inactivated through point mutations and large deletions in lung tumors, demonstrating that LKB1 is a target of the LOH of this chromosomal arm. Data from several independent groups have provided information about the profiles of lung tumors with LKB1 inactivation and it is generally agreed that this alteration strongly predominates in non-small cell lung cancer, in particular adenocarcinomas, in smokers. The LKB1 protein has serine-threonine kinase activity and is involved in the regulation of the cell energetic checkpoint through the phosphorylation and activation of adenosine monophosphate-dependent kinase (AMPK). LKB1 is also involved in other processes such as cell polarization, probably through substrates other than AMPK. Interestingly, another gene on chromosome 19p, BRG1, encoding a component of the SWI/SNF chromatin-remodeling complex, has emerged as a tumor suppressor gene that is altered in lung tumors. Similar to LKB1, BRG1 is somatically inactivated by point mutations or large deletions in lung tumors featuring LOH of chromosome 19p. These observations suggest an important role for BRG1 in lung cancer and highlight the need to further our understanding of the function of Brahma/SWI2-related gene 1 (BRG1) in cancer. Finally, simultaneous mutations at LKB1 and BRG1 are common in lung cancer cells, which exemplifies how a single event, LOH of chromosome 19p in this instance, targets two different tumor suppressors.

LKB1; linking cell structure and tumor suppression

Germ line mutations in the LKB1 tumor suppressor gene are associated with the Peutz-Jeghers polyposis and cancer syndrome. Somatic mutations in Lkb1 are observed in sporadic pulmonary, pancreatic and biliary cancers and melanomas. The LKB1 serine-threonine kinase functionally and biochemically links control of cellular structure and energy utilization through activation of the AMPK family of kinases. Lkb1 regulates cell polarity through downstream kinases including AMPKs, MARKs and BRSKs, and nutrient utilization and cellular metabolism through the AMPK-mTOR pathway. LKB1 has been shown to affect normal chromosomal segregation, TGF-beta signaling in the mesenchyme and WNT and p53 activity. Although each of the LKB1-dependent processes and downstream pathways have been individually delineated through work across a range of experimental systems, how they relate to Lkb1's role as a tumor suppressor remains to be fully explored and elucidated. The recent development of mouse cancer models harboring engineered mutations in Lkb1 have offered insights into how LKB1 may be functioning to restrain tumorigenesis and how its role as a master regulator of polarity and metabolism could contribute to its tumor suppressor function.

LKB1 in endothelial cells is required for angiogenesis and TGFbeta-mediated vascular smooth muscle cell recruitment

Inactivation of the tumor suppressor kinase Lkb1 in mice leads to vascular defects and midgestational lethality at embryonic day 9-11 (E9-E11). Here, we have used conditional targeting to investigate the defects underlying the Lkb1(-/-) phenotype. Endothelium-restricted deletion of Lkb1 led to embryonic death at E12.5 with a loss of vascular smooth muscle cells (vSMCs) and vascular disruption. Transforming growth factor beta (TGFbeta) pathway activity was reduced in Lkb1-deficient endothelial cells (ECs), and TGFbeta signaling from Lkb1(-/-) ECs to adjacent mesenchyme was defective, noted as reduced SMAD2 phosphorylation. The addition of TGFbeta to mutant yolk sac explants rescued the loss of vSMCs, as evidenced by smooth muscle alpha actin (SMA) expression. These results reveal an essential function for endothelial Lkb1 in TGFbeta-mediated vSMC recruitment during angiogenesis.

Osteogenic tumours in Lkb1-deficient mice

Germline mutation in LKB1 is the cause of Peutz-Jeghers Syndrome in humans, a rare disorder predisposing to cancer and multiple gastrointestinal hamartomous polyps. Mice harboring a germline inactivating Lkb1 mutation develop similar gastrointestinal polyps and liver neoplasia. We observed paralysis in approximately 2% of Lkb1(+/-) mice on two genetic backgrounds, C57BL/6J and 129/sv, at around 300 days of age. Stepped serial sectioning of the whole spinal column found multiple osteogenic tumours that were lobulated, showed osteoid formation and had an infiltrative growth pattern, which extended into the surrounding muscle. Osteogenic tumours were also present in asymptomatic Lkb1(+/-) mice (n=12) in the lateral spinous processes, spinous vertebral bodies and the bodies of sacral tail vertebrae. Although asymptomatic, the proliferation in several mice caused a narrowing and compression of the spinal canal. The long bones of Lkb1(+/-) mice had osteoblastosis within the femur and tibia indicating that the process is multi-focal; bone remodelling was accompanied by angiogenesis. No wild type Lkb1(+/+) siblings (n=12) showed aberrant osteoblastosis or bone remodelling. This is the first report of multifocal osteoblastic tumours in Lkb1(+/-) mice and our observations indicate that Lkb1, like Pten, may have a distinct role in controlling osteoblast proliferation in the mouse.

Suppression of Peutz-Jeghers polyposis by targeting mammalian target of rapamycin signaling

PURPOSE

Peutz-Jeghers syndrome (PJS) is a unique disorder characterized by the development of hamartomas in the gastrointestinal tract as well as increased risks for variety of malignancies. Germ-line mutations of LKB1 cause PJS. We have generated Lkb1+/- mice, which model human PJS. Rapamycin and its analogues are promising preventive and therapeutic agents that specifically inhibit signaling from mammalian target of rapamycin (mTOR). Hyperactivation of mTOR signaling has been associated with PJS. The objective of the study is to investigate the efficacy of mTOR inhibition in suppressing Peutz-Jeghers polyposis in Lkb1+/- mice.

EXPERIMENTAL DESIGN

We initiated a trial of rapamycin in Lkb1+/- mice at 9 months of age (after the onset of polyposis) at the dose of 2 mg/kg/d for a 2-month period. We assessed the efficacy of rapamycin by measuring polyp sizes and tumor burden. To examine the effect of rapamycin on mTOR signaling, phosphorylation levels of S6 were evaluated by immunostaining.

RESULTS

We observed a significant decrease in mean tumor burden (Student's t test, P = 0.023) as well as total tumor burden in rapamycin-treated group compared with control group. Comparison of the polyp size observed in both rapamycin-treated and control groups showed that rapamycin efficiently decreased the tumor burden of large polyps (> 8 mm). This inhibition of rapamycin was associated with a decrease in phosphorylated S6 levels in the polyps.

CONCLUSIONS

Rapamycin effectively suppresses Peutz-Jeghers polyposis in a mouse model, suggesting that rapamycin or its analogues may represent a new targeted therapy for the treatment of PJS.

In vivo genome-wide expression study on human circulating B cells suggests a novel ESR1 and MAPK3 network for postmenopausal osteoporosis

INTRODUCTION

Osteoporosis is characterized by low BMD. Studies have shown that B cells may participate in osteoclastogenesis through expression of osteoclast-related factors, such as RANKL, transforming growth factor beta (TGFB), and osteoprotegerin (OPG). However, the in vivo significance of B cells in human bone metabolism and osteoporosis is still largely unknown, particularly at the systematic gene expression level.

MATERIALS AND METHODS

In this study, Affymetrix HG-U133A GeneChip arrays were used to identify genes differentially expressed in B cells between 10 low and 10 high BMD postmenopausal women. Significance of differential expression was tested by t-test and adjusted for multiple testing with the Benjamini and Hochberg (BH) procedure (adjusted p </= 0.05).

RESULTS

Twenty-nine genes were downregulated in the low versus high BMD group. These genes were further analyzed using Ingenuity Pathways Analysis (Ingenuity Systems). A network involving estrogen receptor 1 (ESR1) and mitogen activated protein kinase 3 (MAPK3) was identified. Real-time RT-PCR confirmed differential expression of eight genes, including ESR1, MAPK3, methyl CpG binding protein 2 (MECP2), proline-serine-threonine phosphatase interacting protein 1 (PSTPIP1), Scr-like-adaptor (SLA), serine/threonine kinase 11 (STK11), WNK lysine-deficient protein kinase 1 (WNK1), and zinc finger protein 446 (ZNF446).

CONCLUSIONS

This is the first in vivo genome-wide expression study on human B cells in relation to osteoporosis. Our results highlight the significance of B cells in the etiology of osteoporosis and suggest a novel mechanism for postmenopausal osteoporosis (i.e., that downregulation of ESR1 and MAPK3 in B cells regulates secretion of factors, leading to increased osteoclastogenesis or decreased osteoblastogenesis).

Cell cycle-dependent ciliogenesis and cancer

In mammals, most cell types have primary cilia, protruding structures involved in sensing mechanical and chemical signals from the extracellular environment that act as major communication hubs for signaling controlling cell differentiation and polarity. The list of clinical disorders associated with ciliary dysfunction has expanded from polycystic kidney disease to include many others. Transformed cells commonly lack cilia, but whether this lack is cause or consequence of transformation is not well understood. Here we discuss work addressing recently identified actions of the cancer-promoting proteins Aurora A and HEF1/NEDD9/CAS-L at cilia. Together with older studies, this work suggests that loss of cilia in cancer may contribute to the insensitivity of cancer cells to environmental repressive signals, based in part on derangement of cell cycle checkpoints governed by cilia and centrosomes.

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.