Peutz-Jeghers LKB1 mutants fail to activate GSK-3beta, preventing it from inhibiting Wnt signaling

LKB1 biology: assessing the therapeutic relevancy of LKB1 inhibitors

Liver Kinase B1 (LKB1), encoded by Serine-Threonine Kinase 11 (STK11), is a master kinase that regulates cell migration, polarity, proliferation, and metabolism through downstream adenosine monophosphate-activated protein kinase (AMPK) and AMPK-related kinase signalling. Since genetic screens identified STK11 mutations in Peutz-Jeghers Syndrome, STK11 mutants have been implicated in tumourigenesis labelling it as a tumour suppressor. In support of this, several compounds reduce tumour burden through upregulating LKB1 signalling, and LKB1-AMPK agonists are cytotoxic to tumour cells. However, in certain contexts, its role in cancer is paradoxical as LKB1 promotes tumour cell survival by mediating resistance against metabolic and oxidative stressors. LKB1 deficiency has also enhanced the selectivity and cytotoxicity of several cancer therapies. Taken together, there is a need to develop LKB1-specific pharmacological compounds, but prior to developing LKB1 inhibitors, further work is needed to understand LKB1 activity and regulation. However, investigating LKB1 activity is strenuous as cell/tissue type, mutations to the LKB1 signalling pathway, STE-20-related kinase adaptor protein (STRAD) binding, Mouse protein 25-STRAD binding, splicing variants, nucleocytoplasmic shuttling, post-translational modifications, and kinase conformation impact the functional status of LKB1. For these reasons, guidelines to standardize experimental strategies to study LKB1 activity, associate proteins, spliced isoforms, post-translational modifications, and regulation are of upmost importance to the development of LKB1-specific therapies. Therefore, to assess the therapeutic relevancy of LKB1 inhibitors, this review summarizes the importance of LKB1 in cell physiology, highlights contributors to LKB1 activation, and outlines the benefits and risks associated with targeting LKB1.

Hamartomatous polyposis syndrome associated malignancies: Risk, pathogenesis and endoscopic surveillance

Hamartomatous polyposis syndromes (HPS) are a heterogeneous spectrum of diseases that are characterized by diffuse hamartomatous polyps lining the gastrointestinal (GI) tract together with extra-GI manifestations. Classical HPS includes juvenile polyposis syndrome, Peutz-Jeghers syndrome, PTEN hamartoma tumor syndrome and hereditary mixed polyposis syndrome. Patients with HPS have a higher risk of GI and extra-GI malignancies than the general population, although the underlying mechanisms remain unclear and are obviously different from the carcinogenesis of classical adenocarcinoma and colitis-associated malignancy. In this review we aimed to clarify the risks, possible mechanism and endoscopic surveillance of HPS-associated GI malignancies.

Involvement of NADPH Oxidase 1 in Liver Kinase B1-Mediated Effects on Tumor Angiogenesis and Growth

The liver kinase B1 (LKB1) gene is a tumor suppressor with an established role in the control of cell metabolism and oxidative stress. However, whether dis-regulated oxidative stress promotes growth of LKB1-deficient tumors remains substantially unknown. Through in vitro studies, we observed that loss of LKB1 perturbed expression of several genes involved in reactive oxygen species (ROS) homeostasis. In particular, this analysis evidenced strongly up-modulated NADPH oxidase 1 (NOX1) transcript levels in tumor cells lacking LKB1. NOX1 accounted in part for enhanced cytotoxic effects of H₂O₂-induced oxidative stress in A549 LKB1-deficient tumor cells. Notably, genetic and pharmacologic inhibition of NOX1 activity reduced angiogenesis and growth of A549 tumors in mice. These results suggest that NOX1 inhibitors could counteract ROS production and the angiogenic switch in LKB1-deficient tumors.

Specific deletion of LKB1/Stk11 in the Müllerian duct mesenchyme drives hyperplasia of the periurethral stroma and tumorigenesis in male mice

Nearly all older men will experience lower urinary tract symptoms associated with benign prostatic hyperplasia (BPH), the etiology of which is not well understood. We have generated Stk11^CKO mice by conditional deletion of the liver kinase B1 (LKB1) tumor suppressor gene, Stk11 (serine threonine kinase 11), in the fetal Müllerian duct mesenchyme (MDM), the caudal remnant of which is thought to be assimilated by the urogenital sinus primordial mesenchyme in males during fetal development. We show that MDM cells contribute to the postnatal stromal cells at the dorsal aspect of the prostatic urethra by lineage tracing. The Stk11^CKO mice develop prostatic hyperplasia with bladder outlet obstruction, most likely because of stromal expansion. The stromal areas from prostates of Stk11^CKO mice, with or without significant expansion, were estrogen receptor positive, which is consistent with both MD mesenchyme-derived cells and the purported importance of estrogen receptors in BPH development and/or progression. In some cases, stromal hyperplasia was admixed with epithelial metaplasia, sometimes with keratin pearls, consistent with squamous cell carcinomas. Mice with conditional deletion of both Stk11 and Pten developed similar features as the Stk11^CKO mice, but at a highly accelerated rate, often within the first few months after birth. Western blot analyses showed that the loss of LKB1 and phosphatase and tensin homolog deleted on chromosome 10 (PTEN) induces activation of the phospho-5' adenosine monophosphate-activated protein kinase and phospho-AKT serine/threonine kinase 1 signaling pathways, as well as increased total and active β-catenin. These results suggest that activation of these signaling pathways can induce hyperplasia of the MD stroma, which could play a significant role in the etiology of human BPH.

GSK-3β controls autophagy by modulating LKB1-AMPK pathway in prostate cancer cells

BACKGROUND

Glycogen synthase kinase 3β (GSK3B, GSK-3β) is a multi-functional protein kinase involved in various cellular processes and its activity elevates after serum deprivation. We have shown that inhibition of GSK-3β activity triggered a profound autophagic response and subsequent necrotic cell death after serum deprivation in prostate cancer cells. In this study, we dissected the mechanisms involved in GSK-3β inhibition-triggered autophagy.

METHODS

Prostate cancer PC-3 and DU145 cells were used in the study. Multiple GSK-3β specific inhibitors were used including small chemicals TDZD8, Tideglusib, TWS119, and peptide L803-mts. Western blot assay coupled with phospho-specific antibodies were used in detecting signal pathway activation. ATP levels were assessed with ATPLite kit and HPLC methods. Autophagy response was determined by evaluating Microtubule-associated proteins 1A/1B light chain 3B (LC3B) processing and p62 protein stability in Western blot assays. Immunofluorescent microscopy was used to detect LKB1 translocation.

RESULTS

Inhibition of GSK-3β activity resulted in a significant decline of cellular ATP production, leading to a significant increase of AMP/ATP ratio, a strong trigger of AMP-activated protein kinase (AMPK) activation in prostate cancer PC-3 cells. In parallel with increased LC-3B biosynthesis and p62 protein reduction, the classical sign of autophagy induction, AMPK was activated after inhibition of GSK-3β activity. Further analysis revealed that Liver kinase B1 (LKB1) but not Calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) is involved in AMPK activation and autophagy induction triggered by GSK-3β inhibition. Meanwhile, GSK-3β inhibition promoted LKB1 translocation from nuclear to cytoplasmic compartment and enhanced LKB1 interaction with its regulatory partners Mouse protein-25 (MO25) and STE20-related adaptor (STRAD).

CONCLUSIONS

In conclusion, our data suggest that GSK-3β plays an important role in controlling autophagy induction by modulating the activation of LKB1-AMPK pathway after serum deprivation.

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

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

LKB1 Loss induces characteristic patterns of gene expression in human tumors associated with NRF2 activation and attenuation of PI3K-AKT

Introduction:

Inactivation of serine/threonine kinase 11 (STK11 or LKB1) is common in lung cancer, and understanding the pathways and phenotypes altered as a consequence will aid the development of targeted therapeutic strategies. Gene and protein expressions in a murine model of v-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (Kras)-mutant lung cancer have been studied to gain insight into the biology of these tumors. However, the molecular consequences of LKB1 loss in human lung cancer have not been fully characterized.

Methods:

We studied gene expression profiles associated with LKB1 loss in resected lung adenocarcinomas, non–small-cell lung cancer cell lines, and murine tumors. The biological significance of dysregulated genes was interpreted using gene set enrichment and transcription factor analyses and also by integration with somatic mutations and proteomic data.

Results:

Loss of LKB1 is associated with consistent gene expression changes in resected human lung cancers and cell lines that differ substantially from the mouse model. Our analysis implicates novel biological features associated with LKB1 loss, including altered mitochondrial metabolism, activation of the nuclear respiratory factor 2 (NRF2) transcription factor by kelch-like ECH-associated protein 1 (KEAP1) mutations, and attenuation of the phosphatidylinositiol 3-kinase and v-akt murine thymoma viral oncogene homolog (PI3K/AKT) pathway. Furthermore, we derived a 16-gene classifier that accurately predicts LKB1 mutations and loss by nonmutational mechanisms. In vitro, transduction of LKB1 into LKB1-mutant cell lines results in attenuation of this signature.

Conclusion:

Loss of LKB1 defines a subset of lung adenocarcinomas associated with characteristic molecular phenotypes and distinctive gene expression features. Studying these effects may improve our understanding of the biology of these tumors and lead to the identification of targeted treatment strategies.

The tumor suppressor LKB1 antagonizes WNT signaling pathway through modulating GSK3β activity in cell growth of esophageal carcinoma

The tumor suppressor LKB1 gene encodes a serine-threonine kinase that regulates cell proliferation and polarity. Inactivation of LKB1 by mutations in LKB1 or loss of its expression is highly correlated with lung, ovarian, and pancreatic cancers, and WNT/β-catenin pathway is also known to be involved in many human malignancies. However, the relationship between LKB1 and WNT signaling pathway in esophageal carcinoma remains unknown. The expression of LKB1 in 62 cases of esophageal cancer patients was determined by quantitative real-time PCR. It was found that LKB1 mRNA level was significantly lower than the adjacent normal epithelium and that the LKB1 downregulation was correlating with TNM stages. Moreover, the expression of WNT target genes such as Cyclin D1, C-MYC, MMP2, and FZD2 was significantly upregulated in esophageal cancer tissues. LKB1 overexpression in TE10 cells inhibited TOPFlash luciferase reporter activity and WNT target gene expression even in the presence of WNT3A. Conversely, LKB1 knockdown enhanced WNT signaling activity in esophageal cancer cells. It was also found that LKB1 antagonized WNT signaling pathway through interaction with GSK3β to downregulate β-catenin expression level. Functional investigation revealed that LKB1 suppressed the promotion effects of WNT3A on the cell growth of TE10 cells. The LKB1 functions in regulating cell growth and WNT target genes expression were impaired by GSK3β inhibition, suggesting that LKB1 antagonized WNT-induced cell proliferation through enhancement of GSK3β activity. Together, the interaction between LKB1 and GSK3β upregulates GSK3β activity to suppress WNT-induced cell proliferation in esophageal carcinoma cells. Loss of LKB1 expression may result in the deregulation of WNT/β-catenin pathway to promote malignant progression of esophageal cancer.

Knockdown of β-catenin controls both apoptotic and autophagic cell death through LKB1/AMPK signaling in head and neck squamous cell carcinoma cell lines

The Wnt/β-catenin pathway regulates the viability and radiosensitivity of head and neck squamous cancer cells (HNSCC). Increased β-catenin predisposes HNSCC patients to poor prognosis and survival. This study was conducted to determine the mechanism by which β-catenin regulates the viability of HNSCC. AMC-HN-3, -HN-8, UM-SCC-38, and -SCC-47 cells, which were established from human head and neck cancer specimens, and underwent cell death following β-catenin silencing. β-Catenin silencing significantly induced G1 arrest and increased the expression of Bax and active caspase-3, which demonstrates the sequential activation of apoptotic cascades following treatment of HNSCC with targeted siRNA. Intriguingly, β-catenin silencing also induced autophagy. Here, we confirm that the number of autophagic vacuoles and the expression of type II light chain 3 were increased in cells that were treated with β-catenin siRNA. These cell death modes are most likely due to the activation of LKB1-dependent AMPK following β-catenin silencing. The activated LKB1/AMPK pathway in AMC-HN-3 cells caused G1 arrest by phosphorylating p53 and suppressing mTOR signaling. In addition, treating AMC-HN-3 cells with LKB1 siRNA preserved cell viability against β-catenin silencing-induced cytotoxicity. Taken together, these results imply that following β-catenin silencing, HNSCC undergo both apoptotic and autophagic cell death that are under the control of LKB1/AMPK. To the best of our knowledge, these results suggest for the first time that novel crosstalk between β-catenin and the LKB1/AMPK pathway regulates the viability of HNSCC. This study thus presents new insights into our understanding of the cellular and molecular mechanisms involved in β-catenin silencing-induced cell death.

Mucosal pigmentation caused by imatinib: report of three cases

Imatinib mesylate (STI-571, Gleevec(®)), a tyrosine kinase inhibitor, is a first-line medication for treating chronic myeloid leukemia (CML). Clinical studies revealed very good hematological responses without significant side effects. However, imatinib may lead to mucosal pigmentation. Three patients, two males aged 64 and 53 and one female aged 29 presented with a painless, diffuse, grey-blue pigmentation of the mucosa of the hard palate. Both male patients had a history of CML and had been on imatinib for 4 and 10 years, respectively. The female patient had been on imatinib for 4 years for pelvic fibromatosis. Histopathologically, deposition of fine, dark-brown, spherical granules was noted within the connective tissue. There was no inflammation or hemorrhage, and no melanosis or melanocytic hyperplasia in the epithelium. The granules stained positively for both Fontana-Masson and Prussian blue stains. Imatinib-induced pigmentation is similar to that caused by other medications such as minocycline and anti-malarial medications, namely the deposition of a drug metabolite containing melanin and iron.

Endopancreatic bile duct cholangiocarcinoma in a patient with peutz-jeghers syndrome

Peutz-Jeghers syndrome is a rare autosomal dominant inherited disease characterized by a special type of hamartomatous gastrointestinal polyps combined with mucocutaneous melanin pigmentations. Patients with the syndrome have a high risk of developing neoplasia, with colon, small bowel, and stomach being the most common gastrointestinal sites. Herein, we present the occurrence of a rare tumor in patients with Peutz-Jeghers syndrome; a cholangiocarcinoma of the endopancreatic bile duct. A minireview is also presented. It can be concluded that cholangiocarcinoma remains a possible diagnosis in PJS patients, as in others that present with biliary obstruction. PJS patients may be at higher risk than others in view of their propensity for malignancy.

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

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

Involvement of LKB1 in epithelial-mesenchymal transition (EMT) of human lung cancer cells

Epithelial-mesenchymal transition (EMT) is a critical phenotypic alteration of cancer cells that triggers invasion and metastasis. Lung cancer cells often show mesenchymal phenotypes; however, a causative genetic alteration for the induction of EMT in lung cancer cells remains unknown. Recent studies have shown that the LKB1 gene is mutated in up to one-third of lung adenocarcinomas. Therefore, to pursue the possible involvement of LKB1 inactivation in the induction of EMT in lung carcinogenesis, we generated immortalized lung epithelial cells and lung adenocarcinoma cells with stable or transient LKB1 knockdown. LKB1 knockdown increased cell motility and invasiveness, and induced the expression of several mesenchymal marker proteins accompanied by the expression of ZEB1, a transcriptional repressor for E-cadherin and an EMT inducer. In agreement with the recent findings, expression of miR-200a/c was inversely correlated with that of ZEB1 in LKB1 knockdown clones with mesenchymal phenotype. Furthermore, transient knockdown of LKB1 induced ZEB1 mRNA and increased cell motility, and this motility was suppressed by ZEB1 repression. These results strongly indicate that LKB1 inactivation triggers EMT in lung cancer cells through the induction of ZEB1.

Immunohistological evidence for Wnt-signaling activation in Peutz-Jeghers polyposis

OBJECTIVE

Molecular pathogenesis of gastrointestinal polyposis in Peutz-Jegher's syndrome (PJS) has been linked to the loss-of-function mutation of LKB1. Recent functional genetic studies have pointed out that LKB1 plays a physiological role in controlling the Wnt-signaling pathway and activation of the pathway as a consequence of LKB1 haploinsufficiency might be responsible for the development of harmatomatous polyps. This study aimed to look for immunohistochemical evidence of Wnt-signaling activation in PJS polyps.

METHOD

Beta-catenin immunohistochemistry patterns were evaluated in gastrointestinal polyps from five cases of PJS. All patients were also evaluated for germline mutations of LKB1 and somatic mutations of beta-catenin in the polyps.

RESULTS

Four of the five cases had germline mutations of LKB1, including two novel mutations, a one-base insertion at codon 53 and a large deletion encompassing exon 3 (codon 136-155). PJS polyps from all patients showed generalized membrane and cytoplasmic localizations of beta-catenin along the mucosal endothelium. Polyps from two cases with LKB1 mutations revealed moderate-intensity nuclear staining in approximately 20 and 70% of the polyps.

CONCLUSION

The study offers additional evidence of Wnt-signaling activation in PJS polyp development at the tissue level, although the degree of up-regulation was not as high as has been found in Wnt-associated neoplasms.

Involvement of SIK2/TORC2 signaling cascade in the regulation of insulin-induced PGC-1alpha and UCP-1 gene expression in brown adipocytes

Salt-inducible kinase 2 (SIK2) is expressed abundantly in adipose tissues and represses cAMP-response element-binding protein (CREB)-mediated gene expression by phosphorylating the coactivator transducer of regulated CREB activity (TORC2). Phosphorylation at Ser(587) of SIK2 diminishes its TORC2 phosphorylation activity. In 3T3-L1 white adipocytes, SIK2 downregulates lipogenic gene in response to nutritional stresses. To investigate the impact of SIK2 on the function of brown adipose tissue (BAT), we used T37i brown adipocytes, mice with diet-induced obesity, and SIK2 mutant (S587A) transgenic mice. When T37i adipocytes were treated with insulin, the levels of peroxisome proliferator-activated receptor-coactivator-1alpha (PGC-1alpha) and uncoupling protein-1 (UCP-1) mRNA were increased, and the induction was inhibited by overexpression of SIK2 (S587A) mutant or dominant-negative CREB. Insulin enhanced SIK2 phosphorylation at Ser(587), which was accompanied by decrease in phospho-TORC2. Similarly, the decrease in the level of SIK2 phosphorylation at Ser(587) was observed in the BAT of mice with diet-induced obesity, which was negatively correlated with TORC2 phosphorylation. To confirm the negative correlation between SIK2 phosphorylation at Ser(587) and TORC2 phosphorylation in BAT, SIK2 mutant (S587A) was overexpressed in adipose tissues by using the adipocyte fatty acid-binding protein 2 promoter. The expression of recombinant SIK2 (S587A) was restricted to BAT, and the levels of phospho-TORC2 were elevated in BAT of transgenic mice. Male transgenic mice developed high-fat diet-induced obesity, and their BAT expressed low levels of PGC-1alpha and UCP-1 mRNA, suggesting that SIK2-TORC2 cascade may be important for the regulation of PGC-1alpha and UCP-1 gene expression in insulin signaling in BAT.

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.

Importance of autophosphorylation at Ser186 in the A-loop of salt inducible kinase 1 for its sustained kinase activity

Autophosphorylation is an important mechanism by which protein kinases regulate their own biological activities. Salt inducible kinase 1 (SIK1) is a regulator in the feedback cascades of cAMP-mediated gene expression, while its kinase domain also features autophosphorylation activity. We provide evidence that Ser186 in the activation loop is the site of autophosphorylation and essential for the kinase activity. Ser186 is located at the +4 position of the critical Thr residue Thr182, which is phosphorylated by upstream kinases such as LKB1. The relationship between phosphorylation at Ser186 and at Thr182 in COS-7 cells indicates that the former is a prerequisite for the latter. Glycogen synthase kinase-3beta (GSK-3beta) phosphorylates Ser/Thr residues located at the fourth position ahead of the pre-phosphorylated Ser/Thr residues, and inhibitors of GSK-3beta reduce the phosphorylation at Thr182. The results of an in vitro reconstitution assay also indicate that GSK-3beta could be the SIK1 kinase. However, overexpression and knockdown of GSK-3beta in LKB1-defective HeLa cells suggests that GSK-3beta alone may not be able to phosphorylate or activate SIK1, indicating that LKB1 may play a crucial role by phosphorylating SIK1 at Thr182, possibly as an initiator of the autophosphorylation cascade, and GSK-3beta may phosphorylate SIK1 at Thr182 by recognizing the priming-autophosphorylation at Ser186 in cultured cells. This may also be the case for the other isoform SIK2, but not for SIK3.

Lkb1 is required for TGFbeta-mediated myofibroblast differentiation

Inactivating mutations of the tumor-suppressor kinase gene LKB1 underlie Peutz-Jeghers syndrome (PJS), which is characterized by gastrointestinal hamartomatous polyps with a prominent smooth-muscle and stromal component. Recently, it was noted that PJS-type polyps develop in mice in which Lkb1 deletion is restricted to SM22-expressing mesenchymal cells. Here, we investigated the stromal functions of Lkb1, which possibly underlie tumor suppression. Ablation of Lkb1 in primary mouse embryo fibroblasts (MEFs) leads to attenuated Smad activation and TGFbeta-dependent transcription. Also, myofibroblast differentiation of Lkb1(-/-) MEFs is defective, resulting in a markedly decreased formation of alpha-smooth muscle actin (SMA)-positive stress fibers and reduced contractility. The myofibroblast differentiation defect was not associated with altered serum response factor (SRF) activity and was rescued by exogenous TGFbeta, indicating that inactivation of Lkb1 leads to defects in myofibroblast differentiation through attenuated TGFbeta signaling. These results suggest that tumorigenesis by Lkb1-deficient SM22-positive cells involves defective myogenic differentiation.

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.

Recent advances in colorectal cancer genetics and diagnostics

Colorectal cancer (CRC) is one of the most prevalent cancers and leading cause of cancer mortality worldwide. It is also one of the most curable cancers if detected early. This review classifies the diverse disease subtypes using various parameters including phenotypes of the polyps and describes how recent advances in genetics have impacted on disease diagnostics. For familial syndromes, the discovery of initiating mutations in the germline made personalized medicine a reality. A model linking the main tumorigenesis (Wnt/TGF-beta-BMP/LKB-1/PI3K-AKT) pathways and a strategy for gene testing are proposed. For sporadic CRC, high throughput technology has enabled the discovery of susceptibility loci that increased CRC risk. The ramifications of screening the population for susceptibility loci are discussed.

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.

Epigenetic alteration of Wnt pathway antagonists in progressive glandular neoplasia of the lung

BACKGROUND

Atypical adenomatous hyperplasia (AAH) is now recognized as a precursor lesion from which lung adenocarcinomas arise and thus represents an ideal target for studying the early genetic and epigenetic alterations associated with lung tumorigenesis such as alterations of the Wnt pathway.

METHODS

We assessed the level of Wnt signaling activity in lung cancer cell lines by determining the level of active beta-catenin and determined the level of expression of Wnt antagonists APC, DKK1, DKK3, LKB1, SFRP1, 2, 4, 5, WIF1 and RUNX3 using reverse transcription-polymerase chain reaction. Using multiplex nested methylation-specific polymerase chain reaction, we analyzed promoter region methylation of these genes in resected lung tissue in the histopathologic sequence of glandular neoplasia (normal lung parenchyma, low-grade and high-grade AAH, adenocarcinoma).

RESULTS

The majority of non-small cell lung cancer cell lines (11 of 16, 69%) have evidence of active Wnt signaling and silencing of Wnt antagonists correlated with promoter hypermethylation. Promoter region methylation of Wnt antagonists was common in primary lung adenocarcinoma and there was a significant increase in the frequency of methylation for Wnt antagonist genes and the number of genes methylated with each stage of tumorigenesis (test for rend P <or= 0.01). Additionally, odds ratios for promoter hypermethylation of individual or multiple Wnt antagonist genes and adenocarcinomas were statistically significantly elevated and ranged between 3.64 and 48.17.

CONCLUSION

These results show that gene silencing of Wnt antagonists by promoter hypermethylation occurs during the earliest stages of glandular neoplasia of the lung and accumulates with progression toward malignancy.

Regulation of CREB-mediated gene expression by salt inducible kinase

Salt inducible kinase (SIK) was identified as a molecule induced in the adrenal glands of rats fed with a high-salt diet. A major downstream of SIK is regulation of camp-responsive element (CRE)-dependent gene expression. SIK represses the activity of CRE-binding protein (CREB) by phosphorylating a CREB-specific co-activator transducer of regulated CREB activity (TORC). When TORC is dephosphorylated it activates CREB in a CREB-phosphorylation independent manner. The importance of the dephosphorylation of TORC has been suggested by the fact that a kinase inhibitor staurosporine induces dephosphorylation of TORC and upregulates the gene expression of CYP11A, CYP11B1, CYP11B2 and StAR in adrenocortical cells. The identification of SIK caused a stir in the field of CREB studies and led to disclosure of cascades hidden behind the classical mechanism for CREB activity.

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

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

Peutz-Jeghers syndrome and screening for pancreatic cancer

BACKGROUND

Cancer risk, including pancreatic, is high in those with Peutz-Jeghers syndrome (PJS). It has been suggested that such patients should undergo screening for pancreatic cancer.

METHODS

The risk of pancreatic cancer in PJS, pancreatic screening and potential screening strategies were reviewed. Cost-effectiveness was assessed according to American Gastroenterology Association guidelines and a risk stratification model proposed by the European Registry of Hereditary Pancreatitis and Familial Pancreatic Cancer.

RESULTS

The risk of pancreatic cancer is increased in PJS but screening would cost over US 35,000 dollars per life saved. Risk stratification reduces cost by 100,000 dollars and costs fall to 50,000 dollars per life saved if deaths from other forms of cancer are avoided.

CONCLUSION

Screening should be performed only on a research basis to evaluate the benefit and cost-effectiveness in high-risk groups.

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

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

The tumor suppressor LKB1 induces p21 expression in collaboration with LMO4, GATA-6, and Ldb1

The LKB1/STK11 serine/threonine kinase is mutated in Peutz-Jeghers syndrome and various sporadic cancers such as lung adenocarcinoma. We show here that LKB1 forms a complex with LMO4, GATA-6, and Ldb1, and enhances GATA-mediated transactivation in a kinase-dependent manner. We further demonstrate that LKB1 has the potential to induce p21 expression in collaboration with LMO4, GATA-6, and Ldb1 through the p53-independent mechanism. Our findings suggest that LKB1 regulates GATA-mediated gene expression and that this activity of LKB1 may be important for its tumor suppressor function.

The inherited genetics of pancreatic cancer and prospects for secondary screening

It is estimated that pancreatic cancer has a familial component in approximately 5-10% of cases. Some of these cases are part of a defined cancer syndrome with a known gene mutation but in the remaining the causative gene remains unknown. In recent years, a better understanding of the molecular events that occur in the progression model of pancreatic cancer has lead to the development of secondary screening programmes with the aim of identifying early precursor lesions or pre-invasive cancer at a stage amenable to curative resection. High-risk groups who have an inherited predisposition for pancreatic cancer form the ideal group to study in developing a robust screening programme. Multimodality screening using computed tomography and endoluminal ultrasound in combination with molecular analysis of pancreatic juice are proving promising as diagnostics tools or at least serving as predictors of risk over a defined period.

Polarity proteins in axon specification and synaptogenesis

The neuron is a prime example of a highly polarized cell. It is becoming clear that conserved protein complexes, which have been shown to regulate polarity in such diverse systems as the C. elegans zygote and mammalian epithelia, are also required for neuronal polarization. This review considers the role of these polarity proteins in axon specification and synaptogenesis.