LKB1 Tumor Suppressor Gene Research Articles

Mitochondria-targeted metformin (mitomet) inhibits lung cancer in cellular models and in mice by enhancing the generation of reactive oxygen species.

Lung cancer is the leading cause of cancer-related mortality in the United States. Although some epidemiological studies have shown an inverse relationship between the use of metformin, a widely used antidiabetic drug, and the incidence of lung cancer, the real benefits of the drug are unclear as the efficacy is low and the outcomes are quite heterogeneous. To develop a more potent form of metformin, we synthesized mitochondria-targeted metformin (mitomet) and tested its efficacy in in vitro and in vivo models of lung cancer. Mitomet was cytotoxic to transformed bronchial cells and several non-small cell lung cancer (NSCLC) cell lines but relatively safe to normal bronchial cells, and these effects were mediated mainly via induction of mitochondrial reactive oxygen species. Studies using isogenic A549 cells showed that mitomet was selectively toxic to those cells deficient in the tumor suppressor gene LKB1, which is widely mutated in NSCLC. Mitomet also significantly reduced the multiplicity and size of lung tumors induced by a tobacco smoke carcinogen in mice. Overall, our findings showed that mitomet, which was about 1000 and 100 times more potent than metformin, in killing NSCLC cells and reducing the multiplicity and size of lung tumors in mice, respectively, is a promising candidate for the chemoprevention and treatment of lung cancer, in particular against LKB1-deficient lung cancers which are known to be highly aggressive.

Suppression of m6A mRNA modification by DNA hypermethylated ALKBH5 aggravates the oncological behavior of KRAS mutation/LKB1 loss lung cancer

Oncogenic KRAS mutations combined with the loss of the LKB1 tumor-suppressor gene (KL) are strongly associated with aggressive forms of lung cancer. N6-methyladenosine (m6A) in mRNA is a crucial epigenetic modification that controls cancer self-renewal and progression. However, the regulation and role of m6A modification in this cancer are unclear. We found that decreased m6A levels correlated with the disease progression and poor survival for KL patients. The correlation was mediated by a special increase in ALKBH5 (AlkB family member 5) levels, an m6A demethylase. ALKBH5 gain- or loss-of function could effectively reverse LKB1 regulated cell proliferation, colony formation, and migration of KRAS-mutated lung cancer cells. Mechanistically, LKB1 loss upregulated ALKBH5 expression by DNA hypermethylation of the CTCF-binding motif on the ALKBH5 promoter, which inhibited CTCF binding but enhanced histone modifications, including H3K4me3, H3K9ac, and H3K27ac. This effect could successfully be rescued by LKB1 expression. ALKBH5 demethylation of m6A stabilized oncogenic drivers, such as SOX2, SMAD7, and MYC, through a pathway dependent on YTHDF2, an m6A reader protein. The above findings were confirmed in clinical KRAS-mutated lung cancer patients. We conclude that loss of LKB1 promotes ALKBH5 transcription by a DNA methylation mechanism, reduces m6A modification, and increases the stability of m6A target oncogenes, thus contributing to aggressive phenotypes of KRAS-mutated lung cancer.

Abstract 392: LKB-inactivated lung cancer cells exhibit dependency on INSL4 signaling

Abstract Lung cancer is the leading cause of cancer deaths worldwide. The LKB1 (STK11) tumor suppressor gene is commonly inactivated in non-small cell lung carcinomas (NSCLC), a major form of lung cancer. Currently, no effective treatments are available for LKB1-inactivated lung cancer. Elucidation of critical signaling downstream of LKB1-inactivation has the potential to uncover rational therapeutic targets for this unique and prevalent molecular subset of lung cancers. In this study, we identified INSL4 (insulin-like 4), a member of the insulin/IGF/relaxin superfamily, as a novel downstream target of LKB1-deficiency through gene expression profiling analysis. We observed that INSL4 expression was highly induced through aberrant CRTC-CREB activation subsequent to LKB1 inactivation. INSL4 was highly expressed in human LKB1-deficient NSCLC cells and primary tumors while undetectable in all normal tissues except in the placenta. High INSL4 expression was associated with more advanced lung cancer and worse patient survival. INSL4 depletion inhibited the growth and viability of LKB1-null NSCLC cells in vitro and blocked the growth of lung cancer xenografts. Moreover, expression profiling identified an INSL4-regulated gene program that is associated with the regulation of cell cycle, growth, and survival. In conclusion, LKB1 deficiency induces an autocrine INSL4 signaling that critically supports the growth and survival of lung cancer cells. Therefore, aberrant INSL4 signaling is a promising therapeutic target for LKB1-deficient lung cancers. Citation Format: Rongqiang Yang, Steven W. Li, Zirong Chen, Xin Zhou, Wei Ni, Dongtao A. Fu, Jianrong Lu, Frederic J. Kaye, Lizi Wu. LKB-inactivated lung cancer cells exhibit dependency on INSL4 signaling [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 392.

Role of INSL4 Signaling in Sustaining the Growth and Viability of LKB1-Inactivated Lung Cancer.

The LKB1 tumor suppressor gene is commonly inactivated in non-small cell lung carcinomas (NSCLC), a major form of lung cancer. Targeted therapies for LKB1-inactivated lung cancer are currently unavailable. Identification of critical signaling components downstream of LKB1 inactivation has the potential to uncover rational therapeutic targets. Here we investigated the role of INSL4, a member of the insulin/IGF/relaxin superfamily, in LKB1-inactivated NSCLCs. INSL4 expression was analyzed using global transcriptome profiling, quantitative reverse transcription PCR, western blotting, enzyme-linked immunosorbent assay, and RNA in situ hybridization in human NSCLC cell lines and tumor specimens. INSL4 gene expression and clinical data from The Cancer Genome Atlas lung adenocarcinomas (n = 515) were analyzed using log-rank and Fisher exact tests. INSL4 functions were studied using short hairpin RNA (shRNA) knockdown, overexpression, transcriptome profiling, cell growth, and survival assays in vitro and in vivo. All statistical tests were two-sided. INSL4 was identified as a novel downstream target of LKB1 deficiency and its expression was induced through aberrant CRTC-CREB activation. INSL4 was highly induced in LKB1-deficient NSCLC cells (up to 543-fold) and 9 of 41 primary tumors, although undetectable in all normal tissues except the placenta. Lung adenocarcinomas from The Cancer Genome Atlas with high and low INSL4 expression (with the top 10th percentile as cutoff) showed statistically significant differences for advanced tumor stage (P < .001), lymph node metastasis (P = .001), and tumor size (P = .01). The INSL4-high group showed worse survival than the INSL4-low group (P < .001). Sustained INSL4 expression was required for the growth and viability of LKB1-inactivated NSCLC cells in vitro and in a mouse xenograft model (n = 5 mice per group). Expression profiling revealed INSL4 as a critical regulator of cell cycle, growth, and survival. LKB1 deficiency induces an autocrine INSL4 signaling that critically supports the growth and survival of lung cancer cells. Therefore, aberrant INSL4 signaling is a promising therapeutic target for LKB1-deficient lung cancers.

Utilizing &lt;sup&gt;18&lt;/sup&gt;F-FDG PET/CT Imaging and Quantitative Histology to Measure Dynamic Changes in the Glucose Metabolism in Mouse Models of Lung Cancer

A hallmark of advanced tumors is a switch to aerobic glycolysis that is readily measured by [18F]-2-fluoro-2-deoxy-D-glucose positron emission tomography (18F-FDG PET) imaging. Co-mutations in the KRAS proto-oncogene and the LKB1 tumor suppressor gene are frequent events in lung cancer that drive hypermetabolic, glycolytic tumor growth. A critical pathway regulating the growth and metabolism of these tumors is the mechanistic target of the rapamycin (mTOR) pathway, which can be effectively targeted using selective catalytic mTOR kinase inhibitors. The mTOR inhibitor MLN0128 suppresses glycolysis in mice bearing tumors with Kras and Lkb1 co-mutations, referred to as KL mice. The therapy response in KL mice is first measured by 18F-FDG PET and computed tomography (CT) imaging before and after the delivery of MLN0128. By utilizing 18F-FDG PET/CT, researchers are able to measure dynamic changes in the glucose metabolism in genetically engineered mouse models (GEMMs) of lung cancer following a therapeutic intervention with targeted therapies. This is followed by ex vivo autoradiography and a quantitative immunohistochemical (qIHC) analysis using morphometric software. The use of qIHC enables the detection and quantification of distinct changes in the biomarker profiles following treatment as well as the characterization of distinct tumor pathologies. The coupling of PET imaging to quantitative histology is an effective strategy to identify metabolic and therapeutic responses in vivo in mouse models of disease.

Utilizing 18F-FDG PET/CT Imaging and Quantitative Histology to Measure Dynamic Changes in the Glucose Metabolism in Mouse Models of Lung Cancer.

A hallmark of advanced tumors is a switch to aerobic glycolysis that is readily measured by [18F]-2-fluoro-2-deoxy-D-glucose positron emission tomography (18F-FDG PET) imaging. Co-mutations in the KRAS proto-oncogene and the LKB1 tumor suppressor gene are frequent events in lung cancer that drive hypermetabolic, glycolytic tumor growth. A critical pathway regulating the growth and metabolism of these tumors is the mechanistic target of the rapamycin (mTOR) pathway, which can be effectively targeted using selective catalytic mTOR kinase inhibitors. The mTOR inhibitor MLN0128 suppresses glycolysis in mice bearing tumors with Kras and Lkb1 co-mutations, referred to as KL mice. The therapy response in KL mice is first measured by 18F-FDG PET and computed tomography (CT) imaging before and after the delivery of MLN0128. By utilizing 18F-FDG PET/CT, researchers are able to measure dynamic changes in the glucose metabolism in genetically engineered mouse models (GEMMs) of lung cancer following a therapeutic intervention with targeted therapies. This is followed by ex vivo autoradiography and a quantitative immunohistochemical (qIHC) analysis using morphometric software. The use of qIHC enables the detection and quantification of distinct changes in the biomarker profiles following treatment as well as the characterization of distinct tumor pathologies. The coupling of PET imaging to quantitative histology is an effective strategy to identify metabolic and therapeutic responses in vivo in mouse models of disease.

MiR-93 Promotes Tumorigenesis and Metastasis of Non-Small Cell Lung Cancer Cells by Activating the PI3K/Akt Pathway via Inhibition of LKB1/PTEN/CDKN1A.

Background: Non-small cell lung cancer (NSCLC) accounts for approximately 85% of clinical lung cancer cases. MicroRNA-93 (miR-93) is an oncomiR in many types of human cancer, exerting pivotal effects in the development and progression of malignancies, including NSCLC. However, the mechanism underlying miR-93 involvement in NSCLC is unknown. Our purpose was to reveal and explain this mechanism, with the goal of contributing to the development of new diagnostic biomarkers and individualized therapeutic tools.Methods: The expression of miR-93 was determined in NSCLC cell lines A549, H1975, and H1299. The cells were transfected with control plasmids (Mock group), miR-93 overexpression plasmids (miR-93 Up group), or miR-93 inhibitor plasmids (miR-93 Down group) to generate stable miR-93-overexpressing or -depleted cells. The effects of miR-93 on proliferation, migration, and invasion of these cells were determined. The in vivo effects of miR-93 on tumor metastasis were determined in an NSCLC xenograft mouse model. The molecular mechanisms underlying these effects were investigated via dual luciferase reporter assay and western blotting.Results: MiR-93 expression levels were significantly greater in the NSCLC cell lines than in normal lung epithelial cells. Cell proliferation, migration, and invasion were significantly stimulated by miR-93 upregulation (all P<0.05) and inhibited by miR-93 downregulation. Dual luciferase reporter assay demonstrated that miR-93 directly bound with the 3′-untranslated region of the tumor suppressor gene LKB1. Western blotting analysis indicated that miR-93 activated the PI3K/Akt pathway by inhibiting LKB1, PTEN, and p21. Increased expression of miR-93 induced significant hepatic metastasis of lung cancer in the xenograft mouse model.Conclusion: Overexpression of miR-93 facilitates tumorigenesis and metastasis of NSCLC. These findings provide novel insight into the mechanism of miR-93 involvement in NSCLC, suggesting that miR-93 may serve as a potential therapeutic target.

Serine Metabolism Links Tumor Suppression to the Epigenetic Landscape

Intermediary metabolism provides substrates that shape epigenetic status, but whether this interaction can be responsible for oncogenesis is largely unknown. In a recent issue of Nature, Kottakis etal. (2016) now show that the common tumor suppressor gene LKB1 can function by mediating this connection through an LKB1/AMPK/mTOR signaling axis.

CAMP/CREB-regulated LINC00473 marks LKB1-inactivated lung cancer and mediates tumor growth.

The LKB1 tumor suppressor gene is frequently mutated and inactivated in non-small cell lung cancer (NSCLC). Loss of LKB1 promotes cancer progression and influences therapeutic responses in preclinical studies; however, specific targeted therapies for lung cancer with LKB1 inactivation are currently unavailable. Here, we have identified a long noncoding RNA (lncRNA) signature that is associated with the loss of LKB1 function. We discovered that LINC00473 is consistently the most highly induced gene in LKB1-inactivated human primary NSCLC samples and derived cell lines. Elevated LINC00473 expression correlated with poor prognosis, and sustained LINC00473 expression was required for the growth and survival of LKB1-inactivated NSCLC cells. Mechanistically, LINC00473 was induced by LKB1 inactivation and subsequent cyclic AMP-responsive element-binding protein (CREB)/CREB-regulated transcription coactivator (CRTC) activation. We determined that LINC00473 is a nuclear lncRNA and interacts with NONO, a component of the cAMP signaling pathway, thereby facilitating CRTC/CREB-mediated transcription. Collectively, our study demonstrates that LINC00473 expression potentially serves as a robust biomarker for tumor LKB1 functional status that can be integrated into clinical trials for patient selection and treatment evaluation, and implicates LINC00473 as a therapeutic target for LKB1-inactivated NSCLC.

Par6 family proteins in cancer.

The regulatory networks of cell polarization and polarity effector proteins have been subjects of feverish interest in the field of cell and developmental biology but many advances in the polarity research may have flown unnoticed past the radar of mainstream cancer biologist. However, recent findings suggesting important cell cycle gatekeeping functions for polarity proteins may change that. For example, PAR6 proteins interact with classical cancer driver signaling pathways, including MAPK and PI3K and moreover, PAR6 (PARD6) genes are frequently altered in various cancers. Mammalian genomes harbor three different PARD6 genes. Recent studies in breast cancer have suggested that different PARD6 genes are not only important players but may play even opposite roles during tumorigenesis.

A novel LKB1 isoform enhances AMPK metabolic activity and displays oncogenic properties.

The LKB1 tumor suppressor gene encodes a master kinase that coordinates the regulation of energetic metabolism and cell polarity. We now report the identification of a novel isoform of LKB1 (named ΔN-LKB1) that is generated through alternative transcription and internal initiation of translation of the LKB1 mRNA. The ΔN-LKB1 protein lacks the N-terminal region and a portion of the kinase domain. Although ΔN-LKB1 is catalytically inactive, it potentiates the stimulating effect of LKB1 on the AMP-activated protein kinase (AMPK) metabolic sensor through a direct interaction with the regulatory autoinhibitory domain of AMPK. In contrast, ΔN-LKB1 negatively interferes with the LKB1 polarizing activity. Finally, combining in vitro and in vivo approaches, we showed that ΔN-LKB1 has an intrinsic oncogenic property. ΔN-LKB1 is expressed solely in the lung cancer cell line, NCI-H460. Silencing of ΔN-LKB1 decreased the survival of NCI-H460 cells and inhibited their tumorigenicity when engrafted in nude mice. In conclusion, we have identified a novel LKB1 isoform that enhances the LKB1-controlled AMPK metabolic activity but inhibits LKB1-induced polarizing activity. Both the LKB1 tumor suppressor gene and the oncogene ΔN-LKB1 are expressed from the same locus and this may account for some of the paradoxical effects of LKB1 during tumorigenesis.

The CRTC1-NEDD9 Signaling Axis Mediates Lung Cancer Progression Caused by LKB1 Loss

Somatic mutation of the tumor suppressor gene LKB1 occurs frequently in lung cancer where it causes tumor progression and metastasis, but the underlying mechanisms remain mainly unknown. Here, we show that the oncogene NEDD9 is an important downstream mediator of lung cancer progression evoked by LKB1 loss. In de novo mouse models, RNAi-mediated silencing of Nedd9 inhibited lung tumor progression, whereas ectopic NEDD9 expression accelerated this process. Mechanistically, LKB1 negatively regulated NEDD9 transcription by promoting cytosolic translocation of CRTC1 from the nucleus. Notably, ectopic expression of either NEDD9 or CRTC1 partially reversed the inhibitory function of LKB1 on metastasis of lung cancer cells. In clinical specimens, elevated expression of NEDD9 was associated with malignant progression and metastasis. Collectively, our results decipher the mechanism through which LKB1 deficiency promotes lung cancer progression and metastasis, and provide a mechanistic rationale for therapeutic attack of these processes.

Abstract 153: Honokiol, a promising small molecular weight natural agent, activates AMP-activated protein kinase in breast cancer cells via LKB1-dependent pathway and inhibits breast carcinogenesis

Abstract Introduction: Research over the past few years have established LKB1-AMPK pathway as an important metabolic checkpoint interconnecting the signaling pathways controlling metabolism and cancer cell growth. Thus, it is of major interest to find effective therapeutic strategies to manipulate LKB1-AMPK axis to inhibit cancer growth. The importance of active constitutive agents in natural products has become increasingly apparent owing to their potential cancer preventive as well as therapeutic properties. Honokiol, a small-molecule polyphenol isolated from magnolia species, is widely known for its therapeutic potential as an anti-inflammatory, anti-thrombosis, anti-oxidant agent and more recently for its protective function in carcinogenesis. In the present study, we sought to examine the effectiveness of honokiol in modulating LKB1-AMPK axis, preventing mammary tumorigenesis and inhibiting metastasis. Methods: Clonogenicity, 3D-colony formation, matrigel invasion, scratch-migration and spheroid-migration assays were used to examine breast cancer cell growth and metastatic potential upon honokiol treatment. Western blot and immunofluorescence analysis were used to examine activation of LKB1-AMPK axis. Functional importance of AMPK and LKB1 was examined by using AMPK-null and AMPK-WT immortalized mouse embryonic fibroblasts (MEFs) and isogenic LKB1-knockdown cell line pairs. Breast cancer xenografts, immunohistochemical and western blot analysis of tumors were used. Results: Analysis of the underlying molecular mechanisms revealed that honokiol treatment increases AMP-activated protein kinase (AMPK) phosphorylation and activity as evident by increased phosphorylation of downstream target of AMPK, acetyl-coenzyme A carboxylase (ACC) and inhibition of phosphorylation of p70S6kinase (pS6K) and eukaryotic translation initiation factor 4E binding protein 1 (4EBP1). Using AMPK-null and AMPK-WT (MEFs); we found that AMPK is required for honokiol-mediated modulation of pACC-pS6K. Intriguingly, we discovered that honokiol treatment increased the expression and cytoplasmic translocation of tumor suppressor gene LKB1 in breast cancer cells. LKB1 knockdown inhibited honokiol-mediated activation of AMPK and more importantly, inhibition of migration and invasion of breast cancer cells. Furthermore, honokiol treatment resulted in inhibition of breast tumorigenesis in vivo. Analysis of tumors showed significant increase in the levels of cytoplasmic LKB1 and phospho-AMPK in honokiol-treated tumors. Conclusions: Taken together, these data providing first in vitro and in vivo evidence of the efficacy of honokiol in activating LKB1-AMPK axis resulting in effective inhibition of mammary tumorigenesis and metastasis provide impetus to determine its activity in clinical setting. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 153. doi:1538-7445.AM2012-153

Tumor suppressor function of Liver kinase B1 (Lkb1) is linked to regulation of epithelial integrity

Although loss of epithelial integrity is a hallmark of advanced cancer, it remains poorly understood whether genetic alterations corrupting this integrity causally facilitate tumorigenesis. We show that conditional deletion of tumor suppressor gene Lkb1 (Par-4) in the mammary gland compromises epithelial integrity manifested by mislocalization of cell polarity markers, lateralization of tight junctions, deterioration of desmosomes and basement membrane (BM), and hyperbranching of the mammary ductal tree. We identify the desmosomal BM remodelling serine protease Hepsin as a key factor mediating Lkb1 loss-induced structural alterations in mammary epithelium and BM fragmentation. Although loss of Lkb1 alone does not promote mammary tumorigenesis, combination of Lkb1 deficiency with oncogenic c-Myc leads to dramatic acceleration in tumor formation. The results coupling Lkb1 loss-mediated epithelial integrity defects to mislocalization of serine protease Hepsin and to oncogenic synergy with c-Myc imply that Lkb1 loss facilitates oncogenic proliferation by releasing epithelial cells from structural BM boundaries.

The tumor suppressor kinase LKB1: lessons from mouse models

Mutations in the tumor suppressor gene LKB1 are important in hereditary Peutz-Jeghers syndrome, as well as in sporadic cancers including lung and cervical cancer. LKB1 is a kinase-activating kinase, and a number of LKB1-dependent phosphorylation cascades regulate fundamental cellular and organismal processes in at least metabolism, polarity, cytoskeleton organization, and proliferation. Conditional targeting approaches are beginning to demonstrate the relevance and specificity of these signaling pathways in development and homeostasis of multiple organs. More than one of the pathways also appear to contribute to tumor growth following Lkb1 deficiencies based on a number of mouse tumor models. Lkb1-dependent activation of AMPK and subsequent inactivation of mammalian target of rapamycin signaling are implicated in several of the models, and other less well characterized pathways are also involved. Conditional targeting studies of Lkb1 also point an important role of LKB1 in epithelial-mesenchymal interactions, significantly expanding knowledge on the relevance of LKB1 in human disease.

Abstract B103: Integrative genomic and proteomic analyses identify novel targets for Lkb1 deficient metastatic lung tumors

Abstract Lung cancer is the leading cause of cancer related mortality worldwide. Gene expression profiling of primary human lung tumors has successfully led to the identification of aberrant signalling pathways involved in the initiation of non-small cell lung cancer (NSCLC). However, advances in defining the pathways deregulated in lung cancer progression and metastases have been limited. The process of tumor metastasis is a complex, multistage process that involves both metastasis initiation functions, such as invasion and angiogenesis, as well as metastasis progression functions such as extravasation and survival. In mice, lung-specific deletion of the Lkb1 tumor suppressor gene and activation of mutant Kras results in lung tumors with a high penetrance of lymph node and distant metastases. However, which of the several candidate pathways are responsible for the pro-metastatic effects of Lkb1 loss has yet to be defined. To identify altered signal transduction pathways involved in the progression and metastases of Lkb1 deficient lung tumors we have integrated genomic and phosphoproteomic signatures of primary and metastastic, Lkb1 deficient lung tumors to identify differentially activated signal transduction pathways. These studies revealed a number of signatures associated with the process of metastasis such as EMT, stem cell phenotype, growth factor signaling and focal adhesion. Moreover, several tyrosine kinases, including the non-receptor tyrosine kinase Src and the focal adhesion kinase, are activated in Lkb1 deficient primary and metastatic lung tumors, resulting in increased motility and Src-dependent adhesion of LKB1 deficient cells. Collectively, these results point towards a mechanism behind the increased propensity for metastases seen in Lkb1 deficient lung tumors and identify molecularly targetable pathways for the treatment of LKB1 deficient NSCLC in humans. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B103.

LKB1 is required for adiponectin-mediated modulation of AMPK–S6K axis and inhibition of migration and invasion of breast cancer cells

Adiponectin is widely known as an adipocytokine with therapeutic potential for its markedly protective function in the pathogenesis of obesity-related disorders, metabolic syndrome, systemic insulin resistance, cardiovascular disease and more recently carcinogenesis. In the present study, we show that adiponectin inhibits adhesion, invasion and migration of breast cancer cells. Further analysis of the underlying molecular mechanisms revealed that adiponectin treatment increased AMP-activated protein kinase (AMPK) phosphorylation and activity as evident by increased phosphorylation of downstream target of AMPK, acetyl-coenzyme A carboxylase and inhibition of p70S6 kinase (S6K). Intriguingly, we discovered that adiponectin treatment increases the expression of tumor suppressor gene LKB1 in breast cancer cells. Overexpression of LKB1 in breast cancer cells further increased adiponectin-mediated phosphorylation of AMPK. Using isogenic LKB1 knockdown cell line pair, we found that LKB1 is required for adiponectin-mediated modulation of AMPK-S6K axis and more importantly, inhibition of adhesion, migration and invasion of breast cancer cells. Taken together these data present a novel mechanism involving specific upregulation of tumor suppressor gene LKB1 by which adiponectin inhibits adhesion, invasion and migration of breast cancer cells. Our findings indicate the possibility of using adiponectin analogues to inhibit invasion and migration of breast cancer cells.

The molecular mechanisms that underlie the tumor suppressor function of LKB1

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

LKB1; 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.

Loss of Lkb1 Provokes Highly Invasive Endometrial Adenocarcinomas

Mutations in the LKB1 tumor suppressor gene result in the Peutz-Jeghers syndrome, an autosomal dominant condition characterized by hamartomatous polyps of the gastrointestinal tract and a dramatically increased risk of epithelial malignancies at other sites, including the female reproductive tract. Here we show that female mice heterozygous for a null Lkb1 allele spontaneously develop highly invasive endometrial adenocarcinomas. To prove that these lesions were indeed due to Lkb1 inactivation, we introduced an adenoviral Cre vector into the uterine lumen of mice harboring a conditional allele of Lkb1. This endometrial-specific deletion of the Lkb1 gene provoked highly invasive and sometimes metastatic endometrial adenocarcinomas closely resembling those observed in Lkb1 heterozygotes. Tumors were extremely well differentiated and histopathologically distinctive and exhibited alterations in AMP-dependent kinase signaling. Although Lkb1 has been implicated in the establishment of cell polarity, and loss of polarity defines most endometrial cancers, Lkb1-driven endometrial cancers paradoxically exhibit (given their highly invasive phenotype) normal cell polarity and apical differentiation. In human endometrial cancers, Lkb1 expression was inversely correlated with tumor grade and stage, arguing that Lkb1 inactivation or down-regulation also contributes to endometrial cancer progression in women. This study shows that Lkb1 plays an important role in the malignant transformation of endometrium and that Lkb1 loss promotes a highly invasive phenotype.