Marcus, Adam I. PhD; Zhou, Wei PhD
The tumor suppressor LKB1 is mutated in 20 to 30% of non-small cell lung cancers (NSCLCs)1 and ranks as the third highest mutated gene in lung adenocarcinoma after p53 and Ras.1–3 Consequently, LKB1 has moved from a relatively understudied protein to a major player in NSCLC, especially NSCLC metastasis. Similar to other emerging pathways, the molecular details and biologic consequences of LKB1-dependent events have not been fully elucidated. The purpose of this review is to summarize LKB1 function and highlight LKB1-dependent molecular pathways that when compromised could contribute to lung cancer metastasis.
LKB1 AND LUNG METASTASIS
LKB1 is a serine/threonine kinase (formerly known as STK11)4 that contains two nuclear localization sequences, a central kinase domain, and a C-terminal farnesylation motif,5 where the N- and C-terminal noncatalytic regions share no relatedness to other proteins. LKB1 activity is regulated by the pseudokinase STE20-related kinase adaptor alpha and the scaffolding protein mouse protein 25 alpha (MO25) through a phosphorylation-independent mechanism.6,7 The canonical target of LKB1 is the energy regulated AMP-activated protein kinase (AMPK), although LKB1 phosphorylates other AMPK family members such as microtubule-associated protein (MAP)/microtubule affinity-regulating kinases (MARK) 1 to 4 and Snf1-like kinases (NUAK) 1 and 2.8 AMPK itself is a metabolic master regulator that is activated during reduced energy availability or hypoxic stress.9,10 Phosphorylation of the AMPK-α activation loop at Thr172 by LKB1 is essential for AMPK catalytic activity,11,12 and AMPK function is compromised in lkb1−/− mouse embryonic fibroblasts but can be restored after LKB1 reconstitution.13–15
In a seminal publication, LKB1 function was assessed using a mutant k-ras-driven mouse model of lung cancer.16 In this model, LKB1 inactivation alone was insufficient for pulmonary neoplasia, but LKB1 inactivation in mutant k-ras tumors led to adeno, squamous, and large cell carcinomas of the lung. Importantly, these mice also had more frequent metastasis compared with tumors lacking p53 or Ink4a/Arf, and increased tumor burden and larger lesions compared with k-ras mutant-only mice. Although the molecular details underlying these events were not clear, this data supported a role for LKB1 inactivation in the progression and metastasis of K-ras-initiated lung tumors. These findings, along with its high mutation rate thrust LKB1 into the spotlight as an important regulator of lung cancer progression and metastasis. Thus, each of the following three sections summarizes how LKB1 participates in the respective metastasis-related pathway (Figure 1) and how a compromised LKB1 pathway could trigger or promote NSCLC metastasis.
CELL POLARITY AND ENERGY STRESS
In most organs, epithelial cells polarize to form an apical and basal region that provides directional transport of molecules across the epithelial sheet. LKB1 is proposed to be a master regulator of epithelial cell polarity, because LKB1 activation causes cell autonomous polarization, even in the absence of junctional cell-cell contacts.17 LKB1-induced polarization likely occurs through AMPK, because in Drosophila, LKB1-AMPK coordinates epithelial polarity in an energy-dependent manner,18 and in mammalian cells, AMPK regulates tight junction assembly during polarization.19,20 Because a loss of epithelial polarity may serve as a prerequisite for epithelial to mesenchymal transition (EMT)21–24 and subsequent tumor invasion,23,24 compromised LKB1 could trigger aberrant polarity and EMT induction. In fact, LKB1 loss induces EMT in transformed human small airway epithelial cells,25 which raises the unanswered question of whether LKB1 mutant NSCLC patients display EMT and whether this drives metastasis.
Cell polarization is also evident in migrating cells, which generate directional migration by an actin-based lamellipodia. LKB1 is necessary for lung cancer polarization during migration, where LKB1 rapidly translocates to the cellular leading edge in NSCLC cell lines to associate with actin, and regulate active cdc42 (small Rho GTPase)26 through an LKB1-active cdc42-p21-activated kinase (PAK1) complex.27 Loss of LKB1 activity reduces PAK1 and cdc42 activity, presumably resulting in the aberrant cell polarity observed.27 Interestingly, another study in human colon cancer cell lines and mouse embryonic fibroblasts shows that LKB1 represses PAK1 by phosphorylation at a newly described Thr109 site.28 It is not clear why LKB1 seems to both repress and activate PAK1 function,27,28 although it may depend on p53 status28 or whether cells are motile; nevertheless, a dysregulation of PAK1 through defective LKB1 signaling could lead to aberrant polarity and directional migration. Whether AMPK participates in these events is unclear, but AMPK also regulates mammalian cell motility and its loss causes directional migration defects,29 suggesting a potential role for AMPK in energy-dependent regulation of cell motility.
It should be noted that most work on the LKB1-AMPK axis has focused on their cytosolic role, but LKB1 also functions in the nucleus,30–32 and more recently, stress-induced AMPK activity promotes transcription by histone 2B phosphorylation.33 Among the potential AMPK-regulated transcripts, dual specificity phosphatases (DUSPs) may be relevant to metastasis, because the LKB1-AMPK metabolic checkpoint induces DUSP1 and 2 transcription by a p53-dependent mechanism,34 and DUSPs negatively regulate mitogen-activated protein kinase (MAPK) phosphorylation.35 It will be interesting to determine whether AMPK transcriptional regulation of DUSPs or other cancer relevant proteins plays a role in lung cancer metastasis.
CELL DETACHMENT AND ADHESION
Cell detachment and adhesion are necessary for motile cells to interact with the microenvironment and generate a force to move.36 Primary and metastatic de novo lung cancers from mutant k-ras/lkb1 tumors show defects in cell adhesion, whereby src is activated, and focal adhesion is impaired.37 Specifically, focal adhesion kinase (FAK) phosphorylation is increased in LKB1 mutant tumors, and this correlates with increased invasion and migration. FAK is a cell-adhesion protein that signals through integrins and in some cases growth factor receptors to relay cues from the extracellular matrix (ECM) through the plasma membrane and into the cytoplasm.38 There it acts as a signaling node at adhesion sites to promote cytoskeletal reorganization, adhesion, migration, and survival.38,39 Thus, the increased metastatic potential of mutant k-ras/lkb1 tumors could be due to stronger adhesion to the ECM, which may increase the likelihood of single cells to successfully escape the primary tumor and navigate through the microenvironment.
Interestingly, Zagorska et al.40 suggested a potentially different mechanism for LKB1-mediated adhesion by an LKB1-NUAK1 pathway. In this case, LKB1-NUAK1 regulates cell detachment and adhesion through myosin light chain 2 and myosin phosphatase, whereby inhibition of LKB1-NUAK1 pathway impaired cell detachment and increased adhesion. This discovery is equally as exciting, and it remains to be seen whether these two LKB1-dependent adhesion pathways are linked potentially through a FAK-src-myosin light chain kinase pathway.41 In both cases, one can envision a scenario whereby LKB1 mutant tumor cells are abnormally adherent, thereby providing a mechanism for escaping cells to firmly attach to the ECM during invasion.
Anoikis is a form of apoptosis that is triggered by poor contact between the cell and the ECM. Cancer cells can become resistant to anoikis and consequently display anchorage-independent growth.42,43 LKB1 participates in p53-dependent anoikis through the salt-inducible kinase (SIK1), an AMPK family member.8,44 SIK1 was required for LKB1 to promote p53-dependent anoikis and suppress anchorage-independent growth and invasion. SIK1 loss promoted metastatic spread and survival of cells as micrometastases in the lungs. Loss of LKB1, p53, or SIK1 resulted in anoikis resistance and, hence, survival, despite being unattached to the ECM. Thus, when taken in combination with the increased adhesion observed in LKB1 mutant cells,37,40 LKB1 loss could provide cells not only the ability to adhere to the ECM during invasion but also the ability to survive when unattached.
A logical next step is to determine whether LKB1 mutational status can be used as a predictive marker of metastatic disease. To our knowledge, a large-scale clinical study in NSCLC testing this hypothesis has not been done. Furthermore, because LKB1 signaling negatively regulates tumor metastasis, activators of LKB1-dependent signaling may have clinical utility. The best characterized activator of LKB1/AMPK signaling is metformin, an antidiabetic drug. Several epidemiological studies show decreased cancer incidence in metformin-treated patients, and preclinical data indicated that metformin has direct antitumor effects. These works has been reviewed extensively by others.45–47 Metformin, however, requires LKB1 to activate AMPK function, thus for tumors with LKB1 inactivation, phosphatidylinositol ether lipid analogues were recently developed that can activate AMPK in LKB1-mutant NSCLC cells.48 Therefore, these agents in particular could be used in LKB1 mutant patients to “rescue” LKB1 defects.
Taken together, LKB1 oversees several metastasis-related pathways discussed in this review including cell adhesion, polarity, and anoikis. Many of these motility pathways are historically linked and share common signaling molecules such as FAK, myosin, and cdc42 (Figure 1). Precisely, how LKB1 regulates these pathways and how these pathways interact are likely to be topics of interest over the next few years. Furthermore, it is unclear whether all LKB1 mutations observed in patients or cell lines result in similar phenotypes or whether certain mutations induce pathway-specific phenotypes. In either case, a systematic evaluation of LKB1 mutations and their effects on NSCLC invasion and metastasis is warranted. Ultimately, an understanding of LKB1 function and how a compromised LKB1 pathway impacts metastasis could reveal new opportunities for predicting and controlling NSCLC metastasis.
Supported by an American Cancer Society Research Scholar Grant (RSG-08-035-01-CSM, to A.I.M.), a National Lung Cancer Partnership (to A.I.M.), and by a Lung Cancer Program Project (1PO1 CA116676, to A.I.M. and W.Z.).
Both A.I.M. and W.Z. are Georgia Cancer Coalition Distinguished Scholars.
1. Carretero J, Medina PP, Pio R, et al. Novel and natural knockout lung cancer cell lines for the LKB1/STK11 tumor suppressor gene. Oncogene 2004;23:4037–4040.
2. Ding L, Getz G, Wheeler DA, et al. Somatic mutations affect key pathways in lung adenocarcinoma. Nature 2008;455:1069–1075.
3. Sanchez-Cespedes M, Parrella P, Esteller M, et al. Inactivation of LKB1/STK11 is a common event in adenocarcinomas of the lung. Cancer Res 2002;62:3659–3662.
4. Jenne DE, Reimann H, Nezu J, et al. Peutz-Jeghers syndrome is caused by mutations in a novel serine threonine kinase. Nat Genet 1998;18:38–43.
5. Alessi DR, Sakamoto K, Bayascas JR. LKB1-dependent signaling pathways. Annu Rev Biochem 2006;75:137–163.
6. Boudeau J, Scott JW, Resta N, et al. Analysis of the LKB1-STRAD-MO25 complex. J Cell Sci 2004;117:6365–6375.
7. Zeqiraj E, Filippi BM, Deak M, et al. Structure of the LKB1-STRAD-MO25 complex reveals an allosteric mechanism of kinase activation. Science 2009;326:1707–1711.
8. Lizcano JM, Goransson O, Toth R, et al. LKB1 is a master kinase that activates 13 kinases of the AMPK subfamily, including MARK/PAR-1. EMBO J 2004;23:833–843.
9. Hardie DG, Carling D, Carlson M. The AMP-activated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell? Annu Rev Biochem 1998;67:821–855.
10. Mu J, Brozinick JT Jr, Valladares O, et al. A role for AMP-activated protein kinase in contraction- and hypoxia-regulated glucose transport in skeletal muscle. Mol Cell 2001;7:1085–1094.
11. Hawley SA, Davison M, Woods A, et al. Characterization of the AMP-activated protein kinase kinase from rat liver and identification of threonine 172 as the major site at which it phosphorylates AMP-activated protein kinase. J Biol Chem 1996;271:27879–27887.
12. Stein SC, Woods A, Jones NA, et al. The regulation of AMP-activated protein kinase by phosphorylation. Biochem J 2000;345:437–443.
13. Hawley SA, Boudeau J, Reid JL, et al. Complexes between the LKB1 tumor suppressor, STRAD alpha/beta and MO25 alpha/beta are upstream kinases in the AMP-activated protein kinase cascade. J Biol 2003;2:28.
14. Moller DE. New drug targets for type 2 diabetes and the metabolic syndrome. Nature 2001;414:821–827.
15. Shaw RJ, Kosmatka M, Bardeesy N, et al. The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress. Proc Natl Acad Sci USA 2004;101:3329–3335.
16. Ji H, Ramsey MR, Hayes DN, et al. LKB1 modulates lung cancer differentiation and metastasis. Nature 2007;448:807–810.
17. Baas AF, Kuipers J, van der Wel NN, et al. Complete polarization of single intestinal epithelial cells upon activation of LKB1 by STRAD. Cell 2004;116:457–466.
18. Mirouse V, Swick LL, Kazgan N, et al. LKB1 and AMPK maintain epithelial cell polarity under energetic stress. J Cell Biol 2007;177:387–392.
19. Zhang L, Li J, Young LH, et al. AMP-activated protein kinase regulates the assembly of epithelial tight junctions. Proc Natl Acad Sci USA 2006;103:17272–17277.
20. Zheng B, Cantley LC. Regulation of epithelial tight junction assembly and disassembly by AMP-activated protein kinase. Proc Natl Acad Sci USA 2007;104:819–822.
21. Lillie FR. The Development of the Chick. New York: H. Holt and company, 1908.
22. Huber MA, Kraut N, Beug H. Molecular requirements for epithelial-mesenchymal transition during tumor progression. Curr Opin Cell Biol 2005;17:548–558.
23. Thiery JP. Epithelial-mesenchymal transitions in development and pathologies. Curr Opin Cell Biol 2003;15:740–746.
24. Thiery JP. Epithelial-mesenchymal transitions in tumour progression. Nat Rev Cancer 2002;2:442–454.
25. Roy BC, Kohno T, Iwakawa R, et al. Involvement of LKB1 in epithelial-mesenchymal transition (EMT) of human lung cancer cells. Lung Cancer. In press.
26. Etienne-Manneville S. Cdc42—the centre of polarity. J Cell Sci 2004;117:1291–1300.
27. Zhang S, Schafer-Hales K, Khuri FR, et al. The tumor suppressor LKB1 regulates lung cancer cell polarity by mediating cdc42 recruitment and activity. Cancer Res 2008;68:740–748.
28. Deguchi A, Miyoshi H, Kojima Y, et al. LKB1 suppresses p21-activated kinase-1 (PAK1) by phosphorylation of Thr109 in the p21-binding domain. J Biol Chem 2010;285:18283–18290.
29. Nakano A, Kato H, Watanabe T, et al. AMPK controls the speed of microtubule polymerization and directional cell migration through CLIP-170 phosphorylation. Nat Cell Biol 2010;12:583–590.
30. Marignani PA, Kanai F, Carpenter CL. LKB1 associates with Brg1 and is necessary for Brg1-induced growth arrest. J Biol Chem 2001;276:32415–32418.
31. Tiainen M, Vaahtomeri K, Ylikorkala A, et al. Growth arrest by the LKB1 tumor suppressor: induction of p21(WAF1/CIP1). Hum Mol Genet 2002;11:1497–1504.
32. Upadhyay S, Liu C, Chatterjee A, et al. LKB1/STK11 suppresses cyclooxygenase-2 induction and cellular invasion through PEA3 in lung cancer. Cancer Res 2006;66:7870–7879.
33. Bungard D, Fuerth BJ, Zeng PY, et al. Signaling kinase AMPK activates stress-promoted transcription via histone H2B phosphorylation. Science 2010;329:1201–1205.
34. Kim MJ, Park IJ, Yun H, et al. AMP-activated protein kinase antagonizes pro-apoptotic extracellular signal-regulated kinase activation by inducing dual-specificity protein phosphatases in response to glucose deprivation in HCT116 carcinoma. J Biol Chem 2010;285:14617–14627.
35. Keyse SM. Dual-specificity MAP kinase phosphatases (MKPs) and cancer. Cancer Metastasis Rev 2008;27:253–261.
36. Mitra SK, Hanson DA, Schlaepfer DD. Focal adhesion kinase: in command and control of cell motility. Nat Rev Mol Cell Biol 2005;6:56–68.
37. Carretero J, Shimamura T, Rikova K, et al. Integrative genomic and proteomic analyses identify targets for Lkb1-deficient metastatic lung tumors. Cancer Cell 2010;17:547–559.
38. Schaller MD, Borgman CA, Cobb BS, et al. pp125FAK a structurally distinctive protein-tyrosine kinase associated with focal adhesions. Proc Natl Acad Sci USA 1992;89:5192–5196.
39. Tilghman RW, Slack-Davis JK, Sergina N, et al. Focal adhesion kinase is required for the spatial organization of the leading edge in migrating cells. J Cell Sci 2005;118:2613–2623.
40. Zagorska A, Deak M, Campbell DG, et al. New roles for the LKB1-NUAK pathway in controlling myosin phosphatase complexes and cell adhesion. Sci Signal 2010;3:ra25.
41. Webb DJ, Donais K, Whitmore LA, et al. FAK-Src signalling through paxillin, ERK and MLCK regulates adhesion disassembly. Nat Cell Biol 2004;6:154–161.
42. Rennebeck G, Martelli M, Kyprianou N. Anoikis and survival connections in the tumor microenvironment: is there a role in prostate cancer metastasis? Cancer Res 2005;65:11230–11235.
43. Frisch SM, Screaton RA. Anoikis mechanisms. Curr Opin Cell Biol 2001;13:555–562.
44. Cheng H, Liu P, Wang ZC, et al. SIK1 couples LKB1 to p53-dependent anoikis and suppresses metastasis. Sci Signal 2009;2:ra35.
45. Ben Sahra I, Le Marchand-Brustel Y, Tanti JF, et al. Metformin in cancer therapy: a new perspective for an old antidiabetic drug? Mol Cancer Ther 9:1092–1099.
46. Gonzalez-Angulo AM, Meric-Bernstam F. Metformin: a therapeutic opportunity in breast cancer. Clin Cancer Res 2010;16:1695–1700.
47. Hadad SM, Fleming S, Thompson AM. Targeting AMPK: a new therapeutic opportunity in breast cancer. Crit Rev Oncol Hematol 2008;67:1–7.
48. Memmott RM, Gills JJ, Hollingshead M, et al. Phosphatidylinositol ether lipid analogues induce AMP-activated protein kinase-dependent death in LKB1-mutant non small cell lung cancer cells. Cancer Res 2008;68:580–588.