DLL4/Notch3/WNT5B axis mediates bidirectional prometastatic crosstalk between melanoma and lymphatic endothelial cells

Despite strong indications that interactions between melanoma and lymphatic vessels actively promote melanoma progression, the molecular mechanisms are not yet completely understood. To characterize molecular factors of this crosstalk, we established human primary lymphatic endothelial cell (LEC) cocultures with human melanoma cell lines. Here, we show that coculture with melanoma cells induced transcriptomic changes in LECs and led to multiple changes in their function. WNT5B, a paracrine signaling molecule upregulated in melanoma cells upon LEC interaction, was found to contribute to the functional changes in LECs. Moreover, WNT5B transcription was regulated by Notch3 in melanoma cells following the coculture with LECs, and Notch3 and WNT5B were coexpressed in melanoma patient primary tumor and metastasis samples. Moreover, melanoma cells derived from LEC coculture escaped efficiently from the primary site to the proximal tumor-draining lymph nodes, which was impaired upon WNT5B depletion. This supported the role of WNT5B in promoting the metastatic potential of melanoma cells through its effects on LECs. Finally, DLL4, a Notch ligand expressed in LECs, was identified as an upstream inducer of the Notch3/WNT5B axis in melanoma. This study elucidated WNT5B as a key molecular factor mediating bidirectional crosstalk between melanoma cells and lymphatic endothelium and promoting melanoma metastasis.

Self-organized and directed branching results in optimal coverage in developing dermal lymphatic networks

Branching morphogenesis is a ubiquitous process that gives rise to high exchange surfaces in the vasculature and epithelial organs. Lymphatic capillaries form branched networks, which play a key role in the circulation of tissue fluid and immune cells. Although mouse models and correlative patient data indicate that the lymphatic capillary density directly correlates with functional output, i.e., tissue fluid drainage and trafficking efficiency of dendritic cells, the mechanisms ensuring efficient tissue coverage remain poorly understood. Here, we use the mouse ear pinna lymphatic vessel network as a model system and combine lineage-tracing, genetic perturbations, whole-organ reconstructions and theoretical modeling to show that the dermal lymphatic capillaries tile space in an optimal, space-filling manner. This coverage is achieved by two complementary mechanisms: initial tissue invasion provides a non-optimal global scaffold via self-organized branching morphogenesis, while VEGF-C dependent side-branching from existing capillaries rapidly optimizes local coverage by directionally targeting low-density regions. With these two ingredients, we show that a minimal biophysical model can reproduce quantitatively whole-network reconstructions, across development and perturbations. Our results show that lymphatic capillary networks can exploit local self-organizing mechanisms to achieve tissue-scale optimization.

Lymphangiogenesis requires Ang2/Tie/PI3K signaling for VEGFR3 cell surface expression

Vascular endothelial growth factor C (VEGF-C) induces lymphangiogenesis via VEGF receptor 3 (VEGFR3), which is encoded by the most frequently mutated gene in human primary lymphedema. Angiopoietins (Angs) and their Tie receptors regulate lymphatic vessel development, and mutations of the ANGPT2 gene were recently found in human primary lymphedema. However, the mechanistic basis of Ang2 activity in lymphangiogenesis is not fully understood. Here, we used gene deletion, blocking Abs, transgene induction, and gene transfer to study how Ang2, its Tie2 receptor, and Tie1 regulate lymphatic vessels. We discovered that VEGF-C–induced Ang2 secretion from lymphatic endothelial cells (LECs) was involved in full Akt activation downstream of phosphoinositide 3 kinase (PI3K). Neonatal deletion of genes encoding the Tie receptors or Ang2 in LECs, or administration of an Ang2-blocking Ab decreased VEGFR3 presentation on LECs and inhibited lymphangiogenesis. A similar effect was observed in LECs upon deletion of the PI3K catalytic p110α subunit or with small-molecule inhibition of a constitutively active PI3K located downstream of Ang2. Deletion of Tie receptors or blockade of Ang2 decreased VEGF-C–induced lymphangiogenesis also in adult mice. Our results reveal an important crosstalk between the VEGF-C and Ang signaling pathways and suggest new avenues for therapeutic manipulation of lymphangiogenesis by targeting Ang2/Tie/PI3K signaling.

Inactivation of AMPK Leads to Attenuation of Antigen Presentation and Immune Evasion in Lung Adenocarcinoma

PURPOSE

Mutations in STK11 (LKB1) occur in 17% of lung adenocarcinoma (LUAD) and drive a suppressive (cold) tumor immune microenvironment (TIME) and resistance to immunotherapy. The mechanisms underpinning the establishment and maintenance of a cold TIME in LKB1-mutant LUAD remain poorly understood. In this study, we investigated the role of the LKB1 substrate AMPK in immune evasion in human non-small cell lung cancer (NSCLC) and mouse models and explored the mechanisms involved.

EXPERIMENTAL DESIGN

We addressed the role of AMPK in immune evasion in NSCLC by correlating AMPK phosphorylation and immune-suppressive signatures and by deleting AMPKα1 (Prkaa1) and AMPKα2 (Prkaa2) in a Kras^G12D -driven LUAD. Furthermore, we dissected the molecular mechanisms involved in immune evasion by comparing gene-expression signatures, AMPK activity, and immune infiltration in mouse and human LUAD and gain or loss-of-function experiments with LKB1- or AMPK-deficient cell lines.

RESULTS

Inactivation of both AMPKα1 and AMPKα2 together with Kras activation accelerated tumorigenesis and led to tumors with reduced infiltration of CD8⁺/CD4⁺ T cells and gene signatures associated with a suppressive TIME. These signatures recapitulate those in Lkb1-deleted murine LUAD and in LKB1-deficient human NSCLC. Interestingly, a similar signature is noted in human NSCLC with low AMPK activity. In mechanistic studies, we find that compromised LKB1 and AMPK activity leads to attenuated antigen presentation in both LUAD mouse models and human NSCLC.

CONCLUSIONS

The results provide evidence that the immune evasion noted in LKB1-inactivated lung cancer is due to subsequent inactivation of AMPK and attenuation of antigen presentation.

Shape and Function of Interstitial Chemokine CCL21 Gradients Are Independent of Heparan Sulfates Produced by Lymphatic Endothelium

Gradients of chemokines and growth factors guide migrating cells and morphogenetic processes. Migration of antigen-presenting dendritic cells from the interstitium into the lymphatic system is dependent on chemokine CCL21, which is secreted by endothelial cells of the lymphatic capillary, binds heparan sulfates and forms gradients decaying into the interstitium. Despite the importance of CCL21 gradients, and chemokine gradients in general, the mechanisms of gradient formation are unclear. Studies on fibroblast growth factors have shown that limited diffusion is crucial for gradient formation. Here, we used the mouse dermis as a model tissue to address the necessity of CCL21 anchoring to lymphatic capillary heparan sulfates in the formation of interstitial CCL21 gradients. Surprisingly, the absence of lymphatic endothelial heparan sulfates resulted only in a modest decrease of CCL21 levels at the lymphatic capillaries and did neither affect interstitial CCL21 gradient shape nor dendritic cell migration toward lymphatic capillaries. Thus, heparan sulfates at the level of the lymphatic endothelium are dispensable for the formation of a functional CCL21 gradient.

Lymphatic Vessels in Tumor Dissemination versus Immunotherapy

During the growth of various cancers, primary tumors can escape antitumor immune responses of their host and eventually disseminate into distant organs. Peritumoral lymphatic vessels connect the primary tumor to lymph nodes, facilitating tumor entry into lymph nodes, systemic circulation, and metastasis. Lymph node metastases that occur frequently provide sites of tumor cell spread, whereas tumor antigen transfer into and presentation in tumor-draining lymph nodes induce activation of tumor-specific T-lymphocyte responses that can result in cytolytic targeting of the tumor. Here, we discuss the recently emerged controversial role of the lymphatic vessels in tumor dissemination and cancer immunotherapy.

Lymphatic exosomes promote dendritic cell migration along guidance cues

Inflammation stimulates lymphatic endothelial cells to release exosomes, which accumulate in the perivascular stroma. Brown et al. show that these exosomes promote the directional migration of dendritic cells along guidance cues in complex environments by enhancing dynamic cellular protrusions in a CX3CL1-dependent manner.

Lymphatic endothelial cells (LECs) release extracellular chemokines to guide the migration of dendritic cells. In this study, we report that LECs also release basolateral exosome-rich endothelial vesicles (EEVs) that are secreted in greater numbers in the presence of inflammatory cytokines and accumulate in the perivascular stroma of small lymphatic vessels in human chronic inflammatory diseases. Proteomic analyses of EEV fractions identified >1,700 cargo proteins and revealed a dominant motility-promoting protein signature. In vitro and ex vivo EEV fractions augmented cellular protrusion formation in a CX3CL1/fractalkine-dependent fashion and enhanced the directional migratory response of human dendritic cells along guidance cues. We conclude that perilymphatic LEC exosomes enhance exploratory behavior and thus promote directional migration of CX3CR1-expressing cells in complex tissue environments.

Stromal Lkb1 deficiency leads to gastrointestinal tumorigenesis involving the IL-11-JAK/STAT3 pathway

Germline mutations in the gene encoding tumor suppressor kinase LKB1 lead to gastrointestinal tumorigenesis in Peutz-Jeghers syndrome (PJS) patients and mouse models; however, the cell types and signaling pathways underlying tumor formation are unknown. Here, we demonstrated that mesenchymal progenitor- or stromal fibroblast-specific deletion of Lkb1 results in fully penetrant polyposis in mice. Lineage tracing and immunohistochemical analyses revealed clonal expansion of Lkb1-deficient myofibroblast-like cell foci in the tumor stroma. Loss of Lkb1 in stromal cells was associated with induction of an inflammatory program including IL-11 production and activation of the JAK/STAT3 pathway in tumor epithelia concomitant with proliferation. Importantly, treatment of LKB1-defcient mice with the JAK1/2 inhibitor ruxolitinib dramatically decreased polyposis. These data indicate that IL-11-mediated induction of JAK/STAT3 is critical in gastrointestinal tumorigenesis following Lkb1 mutations and suggest that targeting this pathway has therapeutic potential in Peutz-Jeghers syndrome.

Lymphangiogenesis guidance by paracrine and pericellular factors

This review by Vaahtomeri et al. discusses the mechanisms by which the lymphatic vasculature network is formed, remodeled, and adapted to physiological and pathological challenges. It describes how the lymphatic vasculature network is controlled by an intricate balance of growth factors and biomechanical cues.

Lymphatic vessels are important for tissue fluid homeostasis, lipid absorption, and immune cell trafficking and are involved in the pathogenesis of several human diseases. The mechanisms by which the lymphatic vasculature network is formed, remodeled, and adapted to physiological and pathological challenges are controlled by an intricate balance of growth factor and biomechanical cues. These transduce signals for the readjustment of gene expression and lymphatic endothelial migration, proliferation, and differentiation. In this review, we describe several of these cues and how they are integrated for the generation of functional lymphatic vessel networks.

Dendritic Cells Interpret Haptotactic Chemokine Gradients in a Manner Governed by Signal-to-Noise Ratio and Dependent on GRK6

Navigation of cells along gradients of guidance cues is a determining step in many developmental and immunological processes. Gradients can either be soluble or immobilized to tissues as demonstrated for the haptotactic migration of dendritic cells (DCs) toward higher concentrations of immobilized chemokine CCL21. To elucidate how gradient characteristics govern cellular response patterns, we here introduce an in vitro system allowing to track migratory responses of DCs to precisely controlled immobilized gradients of CCL21. We find that haptotactic sensing depends on the absolute CCL21 concentration and local steepness of the gradient, consistent with a scenario where DC directionality is governed by the signal-to-noise ratio of CCL21 binding to the receptor CCR7. We find that the conditions for optimal DC guidance are perfectly provided by the CCL21 gradients we measure in vivo. Furthermore, we find that CCR7 signal termination by the G-protein-coupled receptor kinase 6 (GRK6) is crucial for haptotactic but dispensable for chemotactic CCL21 gradient sensing in vitro and confirm those observations in vivo. These findings suggest that stable, tissue-bound CCL21 gradients as sustainable "roads" ensure optimal guidance in vivo.

Abstract 712: Mechanisms of stromal Lkb1 loss induced tumorigenesis in mouse models of Peutz-Jeghers syndrome

Germline mutations in tumor suppressor kinase Lkb1 predispose to Peutz-Jeghers syndrome (PJS), with highly penetrant gastrointestinal polyposis and increased cancer risk. PJS polyps display abnormal growth of both stromal and epithelial cells. We have identified clonally expanding fibroblasts as the drivers of tumorigenesis in PJS mouse models by using Fsp1-Cre and Twist2-Cre mice to restrict Lkb1 deletion to stromal cells (unpublished), highlighting the importance of stromal-epithelial signaling in PJS tumorigenesis.

Here, we address the molecular mechanisms of the pathogenesis linked to stromal Lkb1 loss in PJS mouse models. Lkb1 is involved in tissue size control by inhibiting of mTORC1 pathway via AMPK (Shaw et al, Cancer Cell 2004) and by regulating Hippo pathway (Mohseni et al, Nat Cell Biol 2014). First, we investigated the involvement of Lkb1-AMPK-mTORC1 pathway in polyposis. We conditionally deleted both catalytic AMPKá subunits (á1 and á2) in mice using Fsp1-Cre. Surprisingly, Fsp1-Cre-AMPK mice did not develop any tumors by 17 months of age indicating that AMPK is not the critical mediator of Lkb1 tumor suppression in PJS. We next studied the Hippo pathway in polyps of PJS mouse models. Levels of Yap and Taz, the main downstream effectors of Hippo, were elevated in polyps as shown by Western blotting. Immunohistochemical staining revealed profound nuclear Yap/Taz localization indicating transcriptional activity in the stromal compartment throughout the polyps. In contrast, nuclear Yap/Taz staining in epithelial cells was only noted in a restricted polyp base stem cell zone. In addition, we performed RNA-seq analysis of the of Fsp1-Cre;Lkb1flox mouse polyps and noted shared gene expression changes with PJS patient polyps. RNA-seq also revealed significant enrichment of Yap signature, which was validated by qPCR. Finally, we used small intestinal epithelial organoid culture to study the potential of Yap/Taz induced secreted factors to stimulate growth of epithelial cells. We observed that Wnt5a and Epiregulin enhanced epithelial organoid growth, indicating them as candidates mediating the stromal-epithelial signaling and promoting tumorigenesis in PJS polyps.

In conclusion, we show that stromal Lkb1 mutations lead to phenotypes identical to germline Lkb1 heterozygosity in mice and propose increased stromal Yap/Taz activity as a potential mechanism to drive PJS tumorigenes.

Citation Format: Saara Ollila, Kari Vaahtomeri, Iris Wong, Kaisa Laajanen, Tomi P. Mäkelä. Mechanisms of stromal Lkb1 loss induced tumorigenesis in mouse models of Peutz-Jeghers syndrome. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 712.

Abstract 4857: Clonal expansion of Lkb1-deficient stromal cells underlies polyp development in mouse models of Peutz-Jeghers syndrome

While the role of LKB1 mutations in the Peutz-Jeghers polyposis syndrome (PJS) is uncontroversial, the originating cell type remains unclear as Lkb1 mutations in both epithelial cells and stromal smooth muscle cells (SMCs) have been proposed as tumor drivers. Since SMCs do not represent a major fraction of stromal cells in polyps, altered signaling from Lkb1-deficient SMCs to epithelium has been suggested as a possible mechanism for polyposis. Here we investigate the cell type origin and tumor development mechanism of PJS type polyps in mice by targeting Lkb1 in either early mesenchymal progenitors (Twist2-Cre) or in stromal fibroblasts (Fsp1-Cre). Remarkably, both Twist2 (Twist2-Cre;Lkb1flox/+) and Fsp1-driven (Fsp1-Cre;Lkb1flox/+ and Fsp1-Cre;Lkb1flox/flox) Lkb1 deletion led to formation of PJS-type gastrointestinal polyps predominantly in the stomach, as noted before for Lkb1+/- mice. Furthermore, lineage-tracing experiments demonstrated that Lkb1 deletion results in early local expansion of Lkb1-deficient myofibroblast-like alpha smooth muscle actin-expressing cells between gastric glands, and subsequent clonal expansion filling the stroma of the forming polyps. The simultaneous expansion of the adjacent epithelium seems to be secondary to the stromal growth, based on lack of noticeable genetic alterations in the epithelial cells. Immunohistochemical and mRNA analyses demonstrate that the polyps arising from the stromal deletion models are indistinguishable from Lkb1+/- mice and PJS patient polyps. These results indicate that polyps in the Twist2-Cre and Fsp1-Cre mice represent stromal tumors, and suggest that the identified tumorigenic mechanism is shared in PJS syndrome.

Citation Format: Saara Ollila, Kaisa Laajanen, Iris Wong, Kari Vaahtomeri, Tomi P. Mäkelä. Clonal expansion of Lkb1-deficient stromal cells underlies polyp development in mouse models of Peutz-Jeghers syndrome. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4857. doi:10.1158/1538-7445.AM2014-4857

Abstract C55: Investigating mechanisms of gastrointestinal polyposis and osteoblastic tumor formation induced by mesenchymal Lkb1 deficiency

Lkb1 has emerged as an important tumor suppressor gene in both sporadic and hereditary cancer. Inactivating germline mutations in the LKB1 gene underlie familial Peutz-Jeghers Syndrome (PJS), a cancer-prone disorder characterized by predisposition to gastrointestinal polyposis. Targeted disruption of Lkb1 in mesenchymal smooth muscle cells was sufficient to recapitulate PJS-type polyposis in mice (Katajisto et al, 2008) and Lkb1 heterozygous mice develop osteoblastic tumors (Robinson et al, 2008). To investigate the mechanism(s) by which Lkb1 loss in mesenchymal tissues drives tumorigenesis, we deleted Lkb1 using two mesenchymal deletors (Fsp1-Cre and Twist2-Cre) expressed in mesenchymal progenitors. Heterozygous deletion of Lkb1 with either Fsp1-Cre (Lkb1flox/+;Fsp1-Cre) or Twist2-Cre (Lkb1flox/+;Twist2-Cre) results in both polyposis and osteoblastic tumor. Homozygous Lkb1 deletion using Fsp1-Cre (Lkb1flox/flox;Fsp1-Cre) leads to rapid polyposis and osteoblastic tumors with complete penetrance, indicating that complete loss of Lkb1 is advantageous for tumorigenesis. AMPK, a key substrate of Lkb1 connected to several other tumor suppressors, has been proposed to mediate Lkb1 tumor suppressive function. However, homozygous deletion of both AMPKα1 and AMPKα2 using Fsp1-Cre does not lead to tumorigenesis. These results implicate other Lkb1 substrate kinases in tumor suppression and suggest Lkb1 loss in a common progenitor underlies mesenchymal tumorigenesis.

Katajisto P, et al., (2008) LKB1 signaling in mesenchymal cells required for suppression of gastrointestinal polyposis. Nat Genet, 40(4): p. 455-9

Robinson J, et al., (2008) Osteogenic tumours in Lkb1-deficient mice. Exp Mol Pathol, 85(3): p. 223-6

Citation Format: Kaisa Laajanen, Saara Ollila, Iris PL Wong, Kari Vaahtomeri, Gustavo Leone, Benoit Viollet, Makela P. Tomi. Investigating mechanisms of gastrointestinal polyposis and osteoblastic tumor formation induced by mesenchymal Lkb1 deficiency. [abstract]. In: Proceedings of the Third AACR International Conference on Frontiers in Basic Cancer Research; Sep 18-22, 2013; National Harbor, MD. Philadelphia (PA): AACR; Cancer Res 2013;73(19 Suppl):Abstract nr C55.

An association between NUAK2 and MRIP reveals a novel mechanism for regulation of actin stress fibers

Actin stress fiber assembly and contractility in nonmuscle motile cells requires phosphorylation of myosin regulatory light chain (MLC). Dephosphorylation and disassembly are mediated by MLC phosphatase, which is targeted to actin fibers by the association of its regulatory subunit MYPT1 with myosin phosphatase Rho-interacting protein (MRIP). In the present study, we identify the kinase NUAK2 as a second protein targeted by MRIP to actin fibers. Association of NUAK2 with MRIP increases MLC phosphorylation and promotes formation of stress fibers. This activity does not require the kinase activity of NUAK2 but is dependent on both MRIP and MYPT1, indicating that the NUAK2-MRIP association inhibits fiber disassembly and MYPT1-mediated MLC dephosphorylation. NUAK2 levels are strongly induced by stimuli increasing actomyosin fiber formation, and NUAK2 is required for fiber maintenance in exponentially growing cells, implicating NUAK2 in a positive-feedback loop regulating actin stress fibers independently of the MLC kinase Rho-associated protein kinase (ROCK). The identified MRIP-NUAK2 association reveals a novel mechanism for the maintenance of actin stress fibers through counteracting MYPT1 and, together with recent results, implicates the NUAK proteins as important regulators of the MLC phosphatase acting in both a kinase-dependent and kinase-independent manner.

Non-CDK-bound p27 (p27(NCDK)) is a marker for cell stress and is regulated through the Akt/PKB and AMPK-kinase pathways

p27Kip1 (p27) tumour suppressor protein is regulated by multiple mechanisms including its turnover, localization and complex formation with its key targets, cyclin-dependent kinases (CDK) and cyclins. We have earlier shown that p27 exists in cells in a form that lacks cyclin/CDK interactions (hence non-CDK, p27(NCDK)) but the nature of p27(NCDK) has remained unresolved. Here we demonstrate that the epitope recognized by the p27(NCDK)-specific antibody resides in the p27 CDK-interaction domain and that p27(NCDK) is regulated by the balance of CDK inhibitors and cyclin-CDK complexes. We find that signalling by cellular growth promoting pathways, like phosphoinositol 3-kinase (PI3K) and specifically Akt/PKB kinase, inversely correlates with p27(NCDK) levels whereas total p27 levels are unaffected. p27(NCDK), but not total p27, is increased by cellular perturbations such as hyperosmotic and metabolic stress and activation of AMP-activated protein kinase (AMPK). By using AMPK catalytic subunit proficient and deficient cells we further demonstrate that the AMPK pathway governs p27(NCDK) responses to metabolic stress and PI3K inhibition. These results indicate that p27(NCDK) is a sensitive marker for both cell stress and proliferation over and above p27 and is regulated by Akt/PKB and AMPK pathways.

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.

The LKB1 tumor suppressor kinase in human disease

Inactivating germline mutations in the LKB1 gene underlie Peutz-Jeghers syndrome characterized by hamartomatous polyps and an elevated risk for cancer. Recent studies suggest the involvement of LKB1 also in more common human disorders including diabetes and in a significant fraction of lung adenocarcinomas. These observations have increased the interest towards signaling pathways of this tumor suppressor kinase. The recent breakthroughs in understanding the molecular functions of the LKB1 indicate its contribution as a regulator of cell polarity, energy metabolism and cell proliferation. Here we review how the substrates and cellular functions of LKB1 may be linked to Peutz-Jeghers syndrome and other diseases, and discuss how some of the molecular changes associated with altered LKB1 signaling might be used in therapeutic approaches.

Growth arrest by the LKB1 tumor suppressor: induction of p21(WAF1/CIP1)

Germline mutations of the LKB1 tumor suppressor gene lead to Peutz-Jeghers syndrome (PJS), with a predisposition to cancer. LKB1 encodes for a nuclear and cytoplasmic serine/threonine kinase, which is inactivated by mutations observed in PJS patients. Restoring LKB1 activity into cancer cell lines defective for its expression results in a G(1) cell cycle arrest. Here we have investigated molecular mechanisms leading to this arrest. Reintroduced active LKB1 was cytoplasmic and nuclear, whereas most kinase-defective PJS mutants of LKB1 localized predominantly to the nucleus. Moreover, when LKB1 was forced to remain cytoplasmic through disruption of the nuclear localization signal, it retained full growth suppression activity in a kinase-dependent manner. LKB1-mediated G(1) arrest was found to be bypassed by co-expression of the G(1) cyclins cyclin D1 and cyclin E. In addition, the protein levels of the CDK inhibitor p21(WAF1/CIP1) and p21 promoter activity were specifically upregulated in LKB1-transfected cells. Both the growth arrest and the induction of the p21 promoter were found to be p53-dependent. These results suggest that growth suppression by LKB1 is mediated through signaling of cytoplasmic LKB1 to induce p21 through a p53-dependent mechanism.