Pancreatic LKB1 deletion leads to acinar polarity defects and cystic neoplasms

LKB1 is a key regulator of energy homeostasis through the activation of AMP-activated protein kinase (AMPK) and is functionally linked to vascular development, cell polarity, and tumor suppression. In humans, germ line LKB1 loss-of-function mutations cause Peutz-Jeghers syndrome (PJS), which is characterized by a predisposition to gastrointestinal neoplasms marked by a high risk of pancreatic cancer. To explore the developmental and physiological functions of Lkb1 in vivo, we examined the impact of conditional Lkb1 deletion in the pancreatic epithelium of the mouse. The Lkb1-deficient pancreas, although grossly normal at birth, demonstrates a defective acinar cell polarity, an abnormal cytoskeletal organization, a loss of tight junctions, and an inactivation of the AMPK/MARK/SAD family kinases. Rapid and progressive postnatal acinar cell degeneration and acinar-to-ductal metaplasia occur, culminating in marked pancreatic insufficiency and the development of pancreatic serous cystadenomas, a tumor type associated with PJS. Lkb1 deficiency also impacts the pancreas endocrine compartment, characterized by smaller and scattered islets and transient alterations in glucose control. These genetic studies provide in vivo evidence of a key role for LKB1 in the establishment of epithelial cell polarity that is vital for pancreatic acinar cell function and viability and for the suppression of neoplasia.     

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.     

Loss of Lkb1 in Adult β Cells Increases β Cell Mass and Enhances Glucose Tolerance in Mice

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

LKB1 and Notch Pathways Interact and Control Biliary Morphogenesis

Background

LKB1 is an evolutionary conserved kinase implicated in a wide range of cellular functions including inhibition of cell proliferation, regulation of cell polarity and metabolism. When Lkb1 is inactivated in the liver, glucose homeostasis is perturbed, cellular polarity is affected and cholestasis develops. Cholestasis occurs as a result from deficient bile duct development, yet how LKB1 impacts on biliary morphogenesis is unknown.

Methodology/Principal Findings

We characterized the phenotype of mice in which deletion of the Lkb1 gene has been specifically targeted to the hepatoblasts. Our results confirmed that lack of LKB1 in the liver results in bile duct paucity leading to cholestasis. Immunostaining analysis at a prenatal stage showed that LKB1 is not required for differentiation of hepatoblasts to cholangiocyte precursors but promotes maturation of the primitive ductal structures to mature bile ducts. This phenotype is similar to that obtained upon inactivation of Notch signaling in the liver. We tested the hypothesis of a functional overlap between the LKB1 and Notch pathways by gene expression profiling of livers deficient in Lkb1 or in the Notch mediator RbpJκ and identified a mutual cross-talk between LKB1 and Notch signaling. In vitro experiments confirmed that Notch activity was deficient upon LKB1 loss.

Conclusion

LKB1 and Notch share a common genetic program in the liver, and regulate bile duct morphogenesis.     

肝激酶B1和造血干细胞内稳态

肝激酶B1(liver kinase B1,LKB1),又名丝氨酸/苏氨酸蛋白激酶11(STK11),是一种蛋白激酶,可磷酸化AMP激活的蛋白激酶和12种其他AMPK相关激酶。LKB1还是一种肿瘤抑制蛋白,生殖细胞LKB1基因突变可引发家族性黑斑息肉综合征,而体细胞突变可造成多种肿瘤发生。小鼠Lkb1的失活可导致造血干细胞(HSC)静息的丧失、快速的HSC消耗、严重的全血细胞减少和最终的致死。Lkb1缺陷的HSC细胞显示出线粒体缺陷、膜电位减少和细胞ATP耗竭。这些结果说明LKB1是一种HSC内稳态和造血过程中的新调节因子。     

Cell-type-dependent regulation of mTORC1 by REDD1 and the tumor suppressors TSC1/TSC2 and LKB1 in response to hypoxia

mTORC1 is a critical regulator of cell growth that integrates multiple signals and is deregulated in cancer. We previously reported that mTORC1 regulation by hypoxia involves Redd1 and the Tsc1/Tsc2 complex. Here we show that Redd1 induction by hypoxia is tissue dependent and that hypoxia signals are relayed to mTORC1 through different pathways in a tissue-specific manner. In the liver, Redd1 induction is restricted to the centrilobular area, and in primary hepatocytes, mTORC1 inhibition by hypoxia is independent of Redd1. Furthermore, Tsc1/Tsc2 and Arnt (Hif-1β) are similarly dispensable. Hypoxia signaling in hepatocytes involves Lkb1, AMP-activated protein kinase (AMPK), and raptor. Differences in signal relay extend beyond hypoxia and involve AMPK signaling. AMPK activation (using 5-aminoimidazole-4-carboxamide riboside [AICAR]) induces raptor phosphorylation and inhibits mTORC1 in both mouse embryo fibroblasts (MEFs) and hepatocytes, but whereas mTORC1 inhibition is Tsc1/Tsc2 dependent in MEFs, it is independent in hepatocytes. In liver cells, raptor phosphorylation is essential for both AMPK and hypoxia signaling. Thus, context-specific signals are required for raptor phosphorylation-induced mTORC1 inhibition. Our data illustrate a heretofore unappreciated topological complexity in mTORC1 regulation. Interestingly, topological differences in mTORC1 regulation by the tumor suppressor proteins Lkb1 and Tsc1/Tsc2 may underlie their tissue specificity of tumor suppressor action.     

LKB1 Deficiency in Tie2-Cre-expressing Cells Impairs Ischemia-induced Angiogenesis

LKB1 is a tumor suppressor protein whose loss leads to HIF1α-mediated activation of a proangiogenic program in intestinal polyps. LKB1 is also protein kinase regulator of AMP-activated protein kinase (AMPK) signaling, which is essential for endothelial cell responses to tissue ischemia. To discern whether LKB1 signaling is either pro- or antiangiogenic, we investigated ischemia-induced revascularization in mice that were deficient for LKB1 in Tie2-Cre-expressing cells. Whereas homozygous deletion of LKB1 led to embryonic lethality, heterozygous LKB1-knock-out (KO) (Lkb1^flox/+;Tie2^Tg/+) mice were viable. Unchallenged heterozygous LKB1-KO mice displayed normal capillary density, but the revascularization of hind limb following ischemic surgery was significantly impaired as evaluated by laser Doppler flow and capillary density measurements. Reduction of LKB1 in cultured endothelial cells, using either small interfering RNA or an adenovirus expressing nonfunctional kinase-dead LKB1 protein, attenuated endothelial proliferation, migration, and differentiation into network structures on Matrigel that was accompanied by diminished AMPK phosphorylation at Thr-172. Conversely, adenovirus-mediated LKB1 overexpression (Ad-LKB1) augmented network structure formation, and this was associated with elevated AMPK phosphorylation. The augmented differentiation of endothelial cells into network structures induced by Ad-LKB1 was abrogated by the co-transduction of a dominant negative mutant of AMPK. These observations suggest that the LKB1-AMPK signaling axis in endothelial cells is a positive regulator of the revascularization response to tissue ischemia.     

Selective radiosensitization of p53 mutant pancreatic cancer cells by combined inhibition of Chk1 and PARP1

We have recently shown that inhibition of HRR (homologous recombination repair) by Chk1 (checkpoint kinase 1) inhibition radiosensitizes pancreatic cancer cells and others have demonstrated that Chk1 inhibition selectively sensitizes p53 mutant tumor cells. Furthermore, PARP1 [poly (ADP-ribose) polymerase-1] inhibitors dramatically radiosensitize cells with DNA double strand break repair defects. Thus, we hypothesized that inhibition of HRR (mediated by Chk1 via AZD7762) and PARP1 [via olaparib (AZD2281)] would selectively sensitize p53 mutant pancreatic cancer cells to radiation. We also used 2 isogenic p53 cell models to assess the role of p53 status in cancer cells and intestinal epithelial cells to assess overall cancer specificity. DNA damage response and repair were assessed by flow cytometry, γH2AX, and an HRR reporter assay. We found that the combination of AZD7762 and olaparib produced significant radiosensitization in p53 mutant pancreatic cancer cells and in all of the isogenic cancer cell lines. The magnitude of radiosensitization by AZD7762 and olaparib was greater in p53 mutant cells compared with p53 wild type cells. Importantly, normal intestinal epithelial cells were not radiosensitized. The combination of AZD7762 and olaparib caused G₂ checkpoint abrogation, inhibition of HRR, and persistent DNA damage responses. These findings demonstrate that the combination of Chk1 and PARP1 inhibition selectively radiosensitizes p53 mutant pancreatic cancer cells. Furthermore, these studies suggest that inhibition of HRR by Chk1 inhibitors may be a useful strategy for selectively inducing a BRCA1/2 ‘deficient-like’ phenotype in p53 mutant tumor cells, while sparing normal tissue.     

Selective radiosensitization of p53 mutant pancreatic cancer cells by combined inhibition of Chk1 and PARP1

We have recently shown that inhibition of HRR (homologous recombination repair) by Chk1 (checkpoint kinase 1) inhibition radiosensitizes pancreatic cancer cells, and others have demonstrated that Chk1 inhibition selectively sensitizes p53 mutant tumor cells. Furthermore, PARP1 [poly (ADP-ribose) polymerase-1] inhibitors dramatically radiosensitize cells with DNA double-strand break repair defects. Thus, we hypothesized that inhibition of HRR (mediated by Chk1 via AZD7762) and PARP1 [via olaparib (AZD2281)] would selectively sensitize p53 mutant pancreatic cancer cells to radiation. We also used two isogenic p53 cell models to assess the role of p53 status in cancer cells and intestinal epithelial cells to assess overall cancer specificity. DNA damage response and repair were assessed by flow cytometry, γH2AX and an HRR reporter assay. We found that the combination of AZD7762 and olaparib produced significant radiosensitization in p53 mutant pancreatic cancer cells and in all of the isogenic cancer cell lines. The magnitude of radiosensitization by AZD7762 and olaparib was greater in p53 mutant cells compared with p53 wild-type cells. Importantly, normal intestinal epithelial cells were not radiosensitized. The combination of AZD7762 and olaparib caused G₂ checkpoint abrogation, inhibition of HRR and persistent DNA damage responses. These findings demonstrate that the combination of Chk1 and PARP1 inhibition selectively radiosensitizes p53 mutant pancreatic cancer cells. Furthermore, these studies suggest that inhibition of HRR by Chk1 inhibitors may be a useful strategy for selectively inducing a BRCA1/2 “deficient-like” phenotype in p53 mutant tumor cells, while sparing normal tissue.Key words: pancreatic cancer, Chk1, PARP1, radiosensitization, p53     

Lkb1 and Pten synergise to suppress mTOR-mediated tumorigenesis and epithelial-mesenchymal transition in the mouse bladder

The AKT/PI3K/mTOR pathway is frequently altered in a range of human tumours, including bladder cancer. Here we report the phenotype of mice characterised by deletion of two key players in mTOR regulation, Pten and Lkb1, in a range of tissues including the mouse urothelium. Despite widespread recombination within the range of epithelial tissues, the primary phenotype we observe is the rapid onset of bladder tumorigenesis, with median onset of approximately 100 days. Single deletion of either Pten or Lkb1 had no effect on bladder cell proliferation or tumour formation. However, simultaneous deletion of Lkb1 and Pten led to an upregulation of the mTOR pathway and the hypoxia marker GLUT1, increased bladder epithelial cell proliferation and ultimately tumorigenesis. Bladder tissue also exhibited characteristic features of epithelial-mesenchymal transition, with loss of the epithelial markers E-cadherin and the tight junction protein ZO-1, and increases in the mesenchymal marker vimentin as well as nuclear localization of epithelial-mesenchymal transition (EMT) regulator Snail. We show that these effects were all dependent upon mTOR activity, as rapamycin treatment blocked both EMT and tumorigenesis. Our data therefore establish clear synergy between Lkb1 and Pten in controlling the mTOR pathway within bladder epithelium, and show that loss of this control leads to the disturbance of epithelial structure, EMT and ultimately tumorigenesis.     

LKB1 Suppresses p21-activated Kinase-1 (PAK1) by Phosphorylation of Thr109 in the p21-binding Domain

The serine/threonine protein kinase LKB1 is a tumor suppressor gene mutated in Peutz-Jeghers syndrome patients. The mutations are found also in several types of sporadic cancer. Although LKB1 is implicated in suppression of cell growth and metastasis, the detailed mechanisms have not yet been elucidated. In this study, we investigated the effect of LKB1 on cell motility, whose acquisition occurs in early metastasis. The knockdown of LKB1 enhanced cell migration and PAK1 activity in human colon cancer HCT116 cells, whereas forced expression of LKB1 in Lkb1-null mouse embryonic fibroblasts suppressed PAK1 activity and PAK1-mediated cell migration simultaneously. Notably, LKB1 directly phosphorylated PAK1 at Thr¹⁰⁹ in the p21-binding domain in vitro. The phosphomimetic T109E mutant showed significantly lower protein kinase activity than wild-type PAK1, suggesting that the phosphorylation at Thr¹⁰⁹ by LKB1 was responsible for suppression of PAK1. Consistently, the nonphosphorylatable T109A mutant was resistant to suppression by LKB1. Furthermore, we found that PAK1 was activated in the hepatocellular carcinomas and the precancerous liver lesions of Lkb1(+/−) mice. Taken together, these results suggest that PAK1 is a direct downstream target of LKB1 and plays an essential role in LKB1-induced suppression of cell migration.     

LKB1 and AMPK maintain epithelial cell polarity under energetic stress

LKB1 is mutated in both familial and spontaneous tumors, and acts as a master kinase that activates the PAR-1 polarity kinase and the adenosine 5'monophosphate-activated kinase (AMPK). This has led to the hypothesis that LKB1 acts as a tumor suppressor because it is required to maintain cell polarity and growth control through PAR-1 and AMPK, respectively. However, the genetic analysis of LKB1-AMPK signaling in vertebrates has been complicated by the existence of multiple redundant AMPK subunits. We describe the identification of mutations in the single Drosophila melanogaster AMPK catalytic subunit AMPKalpha. Surprisingly, ampkalpha mutant epithelial cells lose their polarity and overproliferate under energetic stress. LKB1 is required in vivo for AMPK activation, and lkb1 mutations cause similar energetic stress-dependent phenotypes to ampkalpha mutations. Furthermore, lkb1 phenotypes are rescued by a phosphomimetic version of AMPKalpha. Thus, LKB1 signals through AMPK to coordinate epithelial polarity and proliferation with cellular energy status, and this might underlie the tumor suppressor function of LKB1.     

Stimulation of glucose transport by AMP-activated protein kinase via activation of p38 mitogen-activated protein kinase

Activation of AMP-activated protein kinase (AMPK) has been recently demonstrated to be associated with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR)-stimulated glucose transport mediated by both GLUT1 and GLUT4 transporters. However, signaling events upstream and downstream of AMPK are unknown. Here we report that 1) p38 mitogen-activated protein kinase (MAPK) and mitogen-activated protein kinase kinase 3 (MKK3) were activated by AICAR in Clone 9 cells, which express only the GLUT1 transporters, and 2) activation of p38 was required for AICAR-stimulated glucose transport since treatment of the cells with p38 inhibitor SB203580 or overexpression of dominant negative p38 mutant inhibited glucose transport. Moreover, we found that overexpression of the constitutively active form of AMPK mutant also resulted in a significant activation of p38, and inhibition of p38 activity by SB203580 did not affect AICAR-stimulated activation of AMPK. These findings demonstrate that AICAR-stimulated activation of p38 is indeed mediated by AMPK, and the p38 MAPK cascade is downstream of AMPK in the signaling pathway of AICAR-stimulated glucose transport in Clone 9 cells.     

Involvement of AMP-activated protein kinase and p38 mitogen-activated protein kinase in 8-Cl-cAMP-induced growth inhibition

8-Cl-cAMP (8-chloro-cyclic AMP), which induces differentiation, growth inhibition and apoptosis in various cancer cells, has been investigated as a putative anti-cancer drug. Although we reported that 8-Cl-cAMP induces growth inhibition via p38 mitogen-activated protein kinase (MAPK) and a metabolite of 8-Cl-cAMP, 8-Cl-adenosine mediates this process, the action mechanism of 8-Cl-cAMP is still uncertain. In this study, it was found that 8-Cl-cAMP-induced growth inhibition is mediated by AMP-activated protein kinase (AMPK). 8-Cl-cAMP was shown to activate AMPK, which was also dependent on the metabolic degradation of 8-Cl-cAMP. A potent agonist of AMPK, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) could also induce growth inhibition and apoptosis. To further delineate the role of AMPK in 8-Cl-cAMP-induced growth inhibition and apoptosis, we used two approaches: pharmacological inhibition of the enzyme with compound C and expression of a dominant negative mutant (a kinase-dead form of AMPKalpha2, KD-AMPK). AICAR was able to activate p38 MAPK and pre-treatment with AMPK inhibitor or expression of KD-AMPK blocked this p38 MAPK activation. Cell growth inhibition was also attenuated. Furthermore, p38 MAPK inhibitor attenuated 8-Cl-cAMP- or AICAR-induced growth inhibition but had no effect on AMPK activation. These results demonstrate that 8-Cl-cAMP induced growth inhibition through AMPK activation and p38 MAPK acts downstream of AMPK in this signaling pathway.     

AMPK activation by ASP4132 inhibits non-small cell lung cancer cell growth

Activation of adenosine monophosphate-activated protein kinase (AMPK) is able to produce significant anti-non-small cell lung cancer (NSCLC) cell activity. ASP4132 is an orally active and highly effective AMPK activator. The current study tested its activity against NSCLC cells. In primary NSCLC cells and established cell lines (A549 and NCI-H1944) ASP4132 potently inhibited cell growth, proliferation and cell cycle progression as well as cell migration and invasion. Robust apoptosis activation was detected in ASP4132-treated NSCLC cells. Furthermore, ASP4132 treatment in NSCLC cells induced programmed necrosis, causing mitochondrial p53-cyclophilin D (CyPD)-adenine nucleotide translocase 1 (ANT1) association, mitochondrial depolarization and medium lactate dehydrogenase release. In NSCLC cells ASP4132 activated AMPK signaling, induced AMPKα1-ACC phosphorylation and increased AMPK activity. Furthermore, AMPK downstream events, including mTORC1 inhibition, receptor tyrosine kinases (PDGFRα and EGFR) degradation, Akt inhibition and autophagy induction, were detected in ASP4132-treated NSCLC cells. Importantly, AMPK inactivation by AMPKα1 shRNA, knockout (using CRISPR/Cas9 strategy) or dominant negative mutation (T172A) almost reversed ASP4132-induced anti-NSCLC cell activity. Conversely, a constitutively active AMPKα1 (T172D) mimicked and abolished ASP4132-induced actions in NSCLC cells. In vivo, oral administration of a single dose of ASP4132 largely inhibited NSCLC xenograft growth in SCID mice. AMPK activation, mTORC1 inhibition and EGFR-PDGFRα degradation as well as Akt inhibition and autophagy induction were detected in ASP4132-treated NSCLC xenograft tumor tissues. Together, activation of AMPK by ASP4132 potently inhibits NSCLC cell growth in vitro and in vivo.Subject terms: Targeted therapies, Non-small-cell lung cancer     

Opinion: alternative views of AMP-activated protein kinase

Genes most closely related to adenosine monophosphate (AMP)-activated protein kinase, including SAD kinases and Par-1 regulate cell polarity, although AMP-activated protein kinase (AMPK) modulates cellular energy status. LKB1 (Par-4) is required for normal activation of AMPK in the liver and also regulates cell polarity. AMPK is proposed to inhibit energy consuming activity while initiating energy producing activity during energy limitation. Demonstration that metformin, a common drug for Type 2 diabetes, requires LKB1 for full therapeutic benefit has increased interest in AMPK signaling. Despite the potential importance of AMPK signaling for diabetes, metabolic syndrome and even cancer, the developmental processes regulated by AMPK in genetically mutant animals require further elucidation. Mouse conditional null mutants for AMPK activity will allow genetic elucidation of AMPK function in vivo. This perspective focuses on sequence and structural moieties of AMPK and genetic analysis of AMPK mutations. Interestingly, the predicted protein structure of the carboxy-terminus of AMPKα resembles the carboxy-terminal KA-1 domain of MARK3, a Par-1 orthologue.     

Regulation of AMP-activated protein kinase by multisite phosphorylation in response to agents that elevate cellular cAMP

The AMP-activated protein kinase (AMPK) and cAMP signaling systems are both key regulators of cellular metabolism. In this study, we show that AMPK activity is attenuated in response to cAMP-elevating agents through modulation of at least two of its alpha subunit phosphorylation sites, viz. alpha-Thr(172) and alpha1-Ser(485)/alpha2-Ser(491), in the clonal beta-cell line INS-1 as well as in mouse embryonic fibroblasts and COS cells. Forskolin, isobutylmethylxanthine, and the glucose-dependent insulinotropic peptide inhibited AMPK activity and reduced phosphorylation of the activation loop alpha-Thr(172) via inhibition of calcium/calmodulin-dependent protein kinase kinase-alpha and -beta, but not LKB1. These agents also enhanced phosphorylation of alpha-Ser(485/491) by the cAMP-dependent protein kinase. AMPK alpha-Ser(485/491) phosphorylation was necessary but not sufficient for inhibition of AMPK activity in response to forskolin/isobutylmethylxanthine. We show that AMPK alpha-Ser(485/491) can be a site for autophosphorylation, which may play a role in limiting AMPK activation in response to energy depletion or other regulators. Thus, our findings not only demonstrate cross-talk between the cAMP/cAMP-dependent protein kinase and AMPK signaling modules, but also describe a novel mechanism by which multisite phosphorylation of AMPK contributes to regulation of its enzyme activity.     

AMPK/LKB1 Signaling in Epithelial Cell Polarity and Cell Division

Cells must coordinate diverse processes including cell division, cell migration, and cell polarity with the cell’s metabolic status. How single molecules coordinate these seemingly distinct cell biological events remains relatively unexplored. AMP-activated protein kinase (AMPK) sits at a unique position as a proposed energy sensor that can interface with diverse signaling molecules ranging from LKB1 to mammalian target of rapamycin (mTOR), affecting processes from ribosomal biogenesis to actin regulation. Determining biologically relevant direct kinase targets remains challenging. Alternatively, one can genetically inactivate a kinase and subsequently characterize cellular and whole animal phenotypes without the kinase’s activity. Recent genetic studies inactivating AMPK activity in Drosophila indicate unanticipated roles for AMPK as a regulator of epithelial polarity, consistent with known roles of an upstream activator, LKB1 as a PAR (partioning defective) mutant in Caenorhabditis elegans and polarity regulator. Additional genetic analyses demonstrate that both AMPK and LKB1 function are required for faithful chromosomal segregation during mitosis. At least some of these apparently divergent phenotypes may be mediated through myosin regulatory light chain, and presumably the acto-myosin complex, which can affect both polarity and cell division. Chromosomal integrity defects could also be consistent with LKB1’s role as a known human tumor suppressor gene. Elucidating the molecular players that interface with AMPK and their potential energy dependent regulation remains an important challenge to fully understand AMPK signaling.     

Sgt1 acts via an LKB1/AMPK pathway to establish cortical polarity in larval neuroblasts

Drosophila neuroblasts are a model system for studying stem cell self-renewal and the establishment of cortical polarity. Larval neuroblasts generate a large apical self-renewing neuroblast, and a small basal cell that differentiates. We performed a genetic screen to identify regulators of neuroblast self-renewal, and identified a mutation in sgt1 (suppressor-of-G2-allele-of-skp1) that had fewer neuroblasts. We found that sgt1 neuroblasts have two polarity phenotypes: failure to establish apical cortical polarity at prophase, and lack of cortical Scribble localization throughout the cell cycle. Apical cortical polarity was partially restored at metaphase by a microtubule-induced cortical polarity pathway. Double mutants lacking Sgt1 and Pins (a microtubule-induced polarity pathway component) resulted in neuroblasts without detectable cortical polarity and formation of "neuroblast tumors." Mutants in hsp83 (encoding the predicted Sgt1-binding protein Hsp90), LKB1, or AMPKα all show similar prophase apical cortical polarity defects (but no Scribble phenotype), and activated AMPKα rescued the sgt1 mutant phenotype. We propose that an Sgt1/Hsp90-LKB1-AMPK pathway acts redundantly with a microtubule-induced polarity pathway to generate neuroblast cortical polarity, and the absence of neuroblast cortical polarity can produce neuroblast tumors.