ABSTRACT
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.
Subject(s)
Fibroblasts/metabolism , Gene Expression Regulation , Protein Serine-Threonine Kinases/genetics , Protein Serine-Threonine Kinases/physiology , Transforming Growth Factor beta/metabolism , AMP-Activated Protein Kinases , Actins/metabolism , Animals , Cell Differentiation , Gene Deletion , Mice , Mice, Transgenic , Models, Biological , Muscle Contraction , Muscles/metabolism , Protein Serine-Threonine Kinases/metabolism , Serum Response Factor/metabolism , Smad Proteins/metabolismABSTRACT
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.
Subject(s)
Endothelial Cells/metabolism , Muscle, Smooth, Vascular/blood supply , Muscle, Smooth, Vascular/metabolism , Myocytes, Smooth Muscle/metabolism , Protein Serine-Threonine Kinases/metabolism , Transforming Growth Factor beta1/metabolism , AMP-Activated Protein Kinases , Animals , Embryo, Mammalian/blood supply , Embryo, Mammalian/embryology , Embryo, Mammalian/metabolism , Endothelium/embryology , Endothelium/metabolism , Enzyme Activation , Female , Gene Expression Regulation, Developmental , Male , Mice , Mice, Knockout , Muscle, Smooth, Vascular/embryology , Protein Serine-Threonine Kinases/deficiency , Protein Serine-Threonine Kinases/genetics , Signal Transduction/drug effects , Tissue Culture Techniques , Transforming Growth Factor beta1/pharmacologyABSTRACT
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.
Subject(s)
Protein Serine-Threonine Kinases/physiology , Tumor Suppressor Proteins/physiology , AMP-Activated Protein Kinase Kinases , Adenocarcinoma/genetics , Animals , Cell Polarity/physiology , Cell Proliferation/drug effects , Cyclooxygenase 2 Inhibitors/therapeutic use , Humans , Lung Neoplasms/genetics , Mice , Peutz-Jeghers Syndrome/genetics , Peutz-Jeghers Syndrome/physiopathology , Peutz-Jeghers Syndrome/therapy , Protein Kinases/physiology , Signal TransductionABSTRACT
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.
