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LKB1 Kinase: Master and Commander of Metabolism and Polarity
Current Biology, Vol. 14, R383–R385, May 25, 2004, ©2004 Elsevier Ltd. All rights reserved. DOI 10.1016/j.cub.2004.05.012
LKB1 Kinase: Master and Commander of Metabolism and Polarity James Spicer1 and Alan Ashworth2
LKB1, the product of a tumour suppressor gene, is a serine/threonine kinase that coordinates disparate cellular processes. Recent data have revealed novel functions for LKB1, providing new insight into the regulation of cell polarity and energy-generating metabolism.
Like most tumour suppressor genes, LKB1 was identified because inactivating germ-line mutations in the gene are associated with inherited cancer susceptibility. Heterozygous pathogenic LKB1 mutations are found in many patients affected by Peutz-Jeghers syndrome [1,2], a defining hallmark of which is the acquisition of benign intestinal polyps known as hamartomas. Peutz-Jeghers syndrome individuals are highly prone to colorectal and a range of other cancers, which usually display loss of the wild-type LKB1 allele. This gene is also somatically inactivated in some sporadic tumours occurring in individuals without a familial predisposition. LKB1 encodes a protein kinase to which a variety of functions have been ascribed [3], but as yet no individual hypothesis has convincingly explained how loss of LKB1 function contributes to carcinogenesis. Recently, however, the LKB1 protein has been shown to be involved in two biologically critical pathways; dysfunction of both of these pathways may be important in the pathogenesis of Peutz-Jeghers syndrome. First [4], LKB1 has been shown to play a fundamental role in controlling the spatial orientation of structures required to maintain an ordered, polarised epithelium (Figure 1). And second [5–7], LKB1 activity has been suggested to be the elusive master regulator of AMP-dependent kinase (AMPK), which controls the balance of cellular energy consumption and generation (Figure 1). All epithelial surfaces are dependent for their structure and function on planar polarity. By way of paradigm, the intestine is lined by a monolayer of polarised columnar cells, having an apical brush border that provides a large surface area for the transport of electrolytes and nutrients from the gut lumen into the cell. Such polarisation was previously thought to require cell–cell interaction in a confluent monolayer, perhaps through interruption of the diffusion of membrane proteins by tight junctions. Surprisingly, however, it now transpires that a single cell can become polarised simply through the activation of LKB1 [4]. Initial studies of LKB1 were hampered by an apparent lack of kinase activity when the protein was 1Department
of Medical Oncology, Guy’s Hospital, St Thomas’s Street, London SE1 9RT, UK. 2The Breakthrough Breast Cancer Research Centre, Institute of Cancer Research, Fulham Road, London SW3 6JB, UK. E-mail: [email protected]
Dispatch
tested against a wide range of substrates. Recently, however, it has been shown that the pseudokinase STRAD associates with, and activates, LKB1 [8]. This unlocking of the cryptic activity of LKB1 has allowed the controlled expression of kinase activity in gut epithelial cells by manipulating levels of STRAD. This results in the reorganisation of non-polarised cells so that they form asymmetrical apical and basal structures, including a brush border [4]. Polarisation occurs despite the absence of contact with neighbouring cells, and even in cells in suspension. Conversely, depletion of LKB1 in intestinal epithelial cells prevents normal polarisation when the cells form a confluent monolayer. The mechanism by which LKB1 induces polarisation may involve another group of mammalian serine/ threonine kinases, collectively termed the PAR1 family. LKB1 associates with PAR1 [9] and causes its phosphorylation and activation [10,11]. Expression of a dominant-negative PAR1 in mammalian cells growing in a polarised confluent sheet disrupts the planar symmetry of the epithelium, resulting in heaping up of cells [12]. Such a process could easily contribute to the transformed phenotype in cells where PAR1 activity is impaired by loss of functional LKB1. The interplay between LKB1 and PAR1 has also been documented in non-mammalian systems. The equivalent of LKB1 in the nematode worm Caenorhabditis elegans was classified within the ‘partitioning defective’ (Par) group of genes, which are implicated in the regulation of asymmetric cell division in the early embryo; Par4 appears to be the worm LKB1 orthologue. The phenotypic effects of inactivating Par4/LKB1 or Par1 are similar, suggesting that the two protein products of these genes cooperate in a kinase cascade in this organism too [13]. Similarly, Drosophila PAR1 functions to establish polarity of the fly egg chamber and embryo, and the consequence of loss of LKB1 activity is disruption of the normal polarised cell morphology in epithelial follicle cells [14]. Furthermore, overexpression of LKB1 can partially rescue the par1 mutant phenotype. These results, in worms and flies further re-inforce the connection between the LKB1 and PAR1 kinases in the regulation of cell polarity. How does the LKB1/PAR1 cascade interact with the cellular machinery to generate polarity? Cell polarisation requires asymmetry in the localisation of structural components including microtubules, and it may be that these structures are targeted by LKB1, via PAR1. In support of this, two members of the mammalian PAR1 family were initially characterised in a screen for kinases able to phosphorylate microtubuleassociated proteins [15], an event which destabilises microtubules and leads to their dissociation into tubulin subunits. The coordinated dissociation and reassociation of tubulin is essential to asymmetric microtubular function in a number of processes including mitotic spindle formation. As well as effects on cellular polarity, downregulation of proliferative signalling may
Dispatch R384
A
B Apical STRAD
STRAD
LKB1 Metformin
PAR1
P
PAR1
P
AMPK
AMPK
ATP ATP
? Cdc42/ microtubules TSC2
TSC2
AMP
AMP
Polarisation Rapamycin
? Cdc42/ microtubules
mTOR
Protein synthesis inhibited
Loss of polarity
mTOR
Protein synthesis activated
Basal Current Biology
Figure 1. LKB1 kinase activity coordinates diverse cellular processes. (A) Both AMPK and PAR1 are phosphorylated and activated by LKB1. LKB1-directed PAR1 activation promotes correct polarisation, including brush border formation in an intestinal epithelial cell. In parallel, AMPK redirects metabolic priorities towards the generation of ATP, and downregulates anabolic processes, including protein synthesis. Metformin is an activator of AMPK used in the treatment of diabetes. Rapamycin is an inhibitor of mTOR. (B) Absence of LKB1 results in failure of PAR1 activation and loss of cell polarity. In addition, failure of AMPK activation releases inhibition of mTOR, switching on anabolic pathways required for cell division.
be a consequence of PAR1 acting downstream from LKB1. PAR1 is known to phosphorylate and activate Dishevelled [16], a key component of the Wnt pathway that is upregulated in most colorectal cancers. So LKB1 might regulate Wnt signalling through its interaction with PAR1, and some recent observations support this conjecture [10,17]. There is increasing evidence that LKB1 is a master kinase which, when associated with STRAD, can phosphorylate a number of mammalian kinases related in sequence to PAR1 [11]. Of particular importance is the demonstration, by several groups [5–7], that LKB1 is an activating upstream kinase for AMPK. AMPK plays a key role in controlling the energy charge of the cell, and an upstream AMPK kinase has long been sought. LKB1 and STRAD co-purify with a cellular extract that phosphorylates and dramatically activates AMPK in vitro [5]. This unexpected result has been convincingly confirmed in cells; agents such as H2O2 activate AMPK in wild-type mouse embryonic fibroblasts, but not in cells lacking LKB1 [5–7]. Phosphorylation and activation of AMPK redirects cell metabolism towards generation of ATP, and away from energy-requiring macromolecular synthesis such as that required for cell division [18]. Therefore cells in which LKB1 activity has been lost may enjoy a proliferative advantage (Figure 1). The mechanisms by which neoplastic cells activate catabolic pathways to
fuel cell division have been little studied. Although there are precedents for the switching on of ‘housekeeping’ functions, such as tRNA transcription, in the transformed phenotype [19], this link between a tumour suppressor gene and such a central metabolic process is an exciting development. The control of protein synthesis by AMPK occurs, at least in part, via mammalian target of rapamycin (mTOR), a kinase which is inhibited following phosphorylation of TCS2 (also known as tuberin) by AMPK [20]. Through this pathway, loss of LKB1 activity in hamartomas would release the inhibition of protein translation facilitating cell growth and proliferation (Figure 1). Provocatively, germ-line TSC2 mutations are found in tuberous sclerosis, another inherited syndrome having hamartomatous polyps as part of the phenotype. Therefore mis-regulation of the AMPK–mTOR–TSC2 pathway might provide an explanation for the similarities in the histology of the benign hamartomas seen in both Peutz-Jeghers syndrome and tuberous sclerosis. These insights into LKB1 function suggest future new avenues for anti-cancer therapy, either through restoration of appropriate cell polarisation in cells that have lost functional LKB1, or by the modification of AMPK activity. Metformin, the most widely prescribed oral hypoglycaemic used in the treatment of diabetes, is an activator of AMPK; metformin-related drugs might therefore find application as anti-proliferative
Current Biology R385
agents in certain cancers. Furthermore, inhibitors of mTOR are already in clinical development. Conversely, modification of the activity of the LKB1 may prove useful in the treatment of diabetes. References 1. Jenne, D.E., Reimann, H., Nezu, J., Friedel, W., Loff, S., Jeschke, R., Muller, O., Back, W., and Zimmer, M. (1998). Peutz-Jeghers syndrome is caused by mutations in a novel serine threonine kinase. Nat. Genet. 18, 38-43. 2. Hemminki, A., Markie, D., Tomlinson, I., Avizienyte, E., Roth, S., Loukola, A., Bignell, G., Warren, W., Aminoff, M., Hoglund, P., et al. (1998). A serine/threonine kinase gene defective in Peutz-Jeghers syndrome. Nature 391, 184-187. 3. Boudeau, J., Sapkota, G., and Alessi, D.R. (2003). LKB1, a protein kinase regulating cell proliferation and polarity. FEBS Lett. 546, 159165. 4. Baas, A.F., Kuipers, J., van der Wel, N.N., Batlle, E., Koerten, H.K., Peters, P.J., and Clevers, H. (2004). Complete polarization of single intestinal epithelial cell upon activation of LKB1 by STRAD. Cell 116, 457-466. 5. Hawley, S.A., Boudeau, J., Reid, J.L., Mustard, K.J., Udd, L., Makela, T.P., Alessi, D.R., and Hardie, D.G. (2003). Complexes between the LKB1 tumor suppressor, STRADalpha/beta and MO25alpha/beta are upstream kinases in the AMP-activated protein kinase cascade. J. Biol. 2, 28. 6. Woods, A., Johnstone, S.R., Dickerson, K., Leiper, F.C., Fryer, L.G., Neumann, D., Schlattner, U., Wallimann, T., Carlson, M., and Carling, D. (2003). LKB1 is the upstream kinase in the AMP-activated protein kinase cascade. Curr. Biol. 13, 2004-2008. 7. Shaw, R.J., Kosmatka, M., Bardeesy, N., Hurley, R.L., Witters, L.A., DePinho, R.A., and Cantley, L.T. (2004). The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress. Proc. Natl. Acad. Sci. USA 101, 3329-3335. 8. Baas, A.F., Boudeau, J., Sapkota, G.P., Smit, L., Medema, R., Morrice, N.A., Alessi, D.R., and Clevers, H.C. (2003). Activation of the tumour suppressor kinase LKB1 by the STE20-like pseudokinase STRAD. EMBO J. 22, 3062-3072. 9. Brajenovic, M., Joberty, G., Kuster, B., Bouwmeester, T., and Drewes, G. (2003). Comprehensive proteomic analysis of human Par protein complexes reveals an interconnected protein network. J. Biol. Chem. 279, 12804-12811. 10. Spicer, J., Rayter, S., Young, N., Elliott, R., Ashworth, A., and Smith, D. (2003). Regulation of the Wnt signalling component PAR1A by the Peutz-Jeghers syndrome kinase LKB1. Oncogene 22, 47524756. 11. Lizcano, J.M., Goransson, O., Toth, R., Deak, M., Morrice, N.A., Boudeau, J., Hawley, S.A., Udd, L., Makela, T.P., Hardie, D.G., and Alessi, D.R. (2004). LKB1 is a master kinase that activates 13 kinases of the AMPK subfamily, including MARK/PAR-1. EMBO J. 23, 833-843. 12. Bohm, H., Brinkmann, V., Drab, M., Henske, A., and Kurzchalia, T.V. (1997). Mammalian homologues of C. elegans PAR-1 are asymmetrically localized in epithelial cells and may influence their polarity. Curr. Biol. 7, 603-606. 13. Watts, J.L., Morton, D.G., Bestman, J., and Kemphues, K.J. (2000). The C. elegans par-4 gene encodes a putative serine-threonine kinase required for establishing embryonic asymmetry. Development 127, 1467-1475. 14. Martin, S.G., and St Johnston, D. (2003). A role for Drosophila LKB1 in anterior-posterior axis formation and epithelial polarity. Nature 421, 379-384. 15. Drewes, G., Ebneth, A., Preuss, U., Mandelkow, E.M., and Mandelkow, E. (1997). MARK, a novel family of protein kinases that phosphorylate microtubule- associated proteins and trigger microtubule disruption. Cell 89, 297-308. 16. Sun, T.Q., Lu, B., Feng, J.J., Reinhard, C., Jan, Y.N., Fantl, W.J., and Williams, L.T. (2001). PAR-1 is a Dishevelled-associated kinase and a positive regulator of Wnt signalling. Nat. Cell Biol. 3, 628-636. 17. Ossipova, O., Bardeesy, N., DePinho, R.A., and Green, J.B. (2003). LKB1 (XEEK1) regulates Wnt signalling in vertebrate development. Nat. Cell Biol. 5, 889-894. 18. Hardie, D.G., Carling, D., and Carlson, M. (1998). The AMPactivated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell? Annu. Rev. Biochem. 67, 821-855. 19. Gomez-Roman, N., Grandori, C., Eisenman, R.N. and White R.J. (2003). Direct activation of RNA Polymerase III transcription by cmyc. Nature 421, 290-294. 20. Inoki, K., Zhu, T., and Guan, K.L. (2003). TSC2 mediates cellular energy response to control cell growth and survival. Cell 115, 577590.
LKB1 Kinase: Master and Commander of Metabolism and Polarity James Spicer1 and Alan Ashworth2
LKB1, the product of a tumour suppressor gene, is a serine/threonine kinase that coordinates disparate cellular processes. Recent data have revealed novel functions for LKB1, providing new insight into the regulation of cell polarity and energy-generating metabolism.
Like most tumour suppressor genes, LKB1 was identified because inactivating germ-line mutations in the gene are associated with inherited cancer susceptibility. Heterozygous pathogenic LKB1 mutations are found in many patients affected by Peutz-Jeghers syndrome [1,2], a defining hallmark of which is the acquisition of benign intestinal polyps known as hamartomas. Peutz-Jeghers syndrome individuals are highly prone to colorectal and a range of other cancers, which usually display loss of the wild-type LKB1 allele. This gene is also somatically inactivated in some sporadic tumours occurring in individuals without a familial predisposition. LKB1 encodes a protein kinase to which a variety of functions have been ascribed [3], but as yet no individual hypothesis has convincingly explained how loss of LKB1 function contributes to carcinogenesis. Recently, however, the LKB1 protein has been shown to be involved in two biologically critical pathways; dysfunction of both of these pathways may be important in the pathogenesis of Peutz-Jeghers syndrome. First [4], LKB1 has been shown to play a fundamental role in controlling the spatial orientation of structures required to maintain an ordered, polarised epithelium (Figure 1). And second [5–7], LKB1 activity has been suggested to be the elusive master regulator of AMP-dependent kinase (AMPK), which controls the balance of cellular energy consumption and generation (Figure 1). All epithelial surfaces are dependent for their structure and function on planar polarity. By way of paradigm, the intestine is lined by a monolayer of polarised columnar cells, having an apical brush border that provides a large surface area for the transport of electrolytes and nutrients from the gut lumen into the cell. Such polarisation was previously thought to require cell–cell interaction in a confluent monolayer, perhaps through interruption of the diffusion of membrane proteins by tight junctions. Surprisingly, however, it now transpires that a single cell can become polarised simply through the activation of LKB1 [4]. Initial studies of LKB1 were hampered by an apparent lack of kinase activity when the protein was 1Department
of Medical Oncology, Guy’s Hospital, St Thomas’s Street, London SE1 9RT, UK. 2The Breakthrough Breast Cancer Research Centre, Institute of Cancer Research, Fulham Road, London SW3 6JB, UK. E-mail: [email protected]
Dispatch
tested against a wide range of substrates. Recently, however, it has been shown that the pseudokinase STRAD associates with, and activates, LKB1 [8]. This unlocking of the cryptic activity of LKB1 has allowed the controlled expression of kinase activity in gut epithelial cells by manipulating levels of STRAD. This results in the reorganisation of non-polarised cells so that they form asymmetrical apical and basal structures, including a brush border [4]. Polarisation occurs despite the absence of contact with neighbouring cells, and even in cells in suspension. Conversely, depletion of LKB1 in intestinal epithelial cells prevents normal polarisation when the cells form a confluent monolayer. The mechanism by which LKB1 induces polarisation may involve another group of mammalian serine/ threonine kinases, collectively termed the PAR1 family. LKB1 associates with PAR1 [9] and causes its phosphorylation and activation [10,11]. Expression of a dominant-negative PAR1 in mammalian cells growing in a polarised confluent sheet disrupts the planar symmetry of the epithelium, resulting in heaping up of cells [12]. Such a process could easily contribute to the transformed phenotype in cells where PAR1 activity is impaired by loss of functional LKB1. The interplay between LKB1 and PAR1 has also been documented in non-mammalian systems. The equivalent of LKB1 in the nematode worm Caenorhabditis elegans was classified within the ‘partitioning defective’ (Par) group of genes, which are implicated in the regulation of asymmetric cell division in the early embryo; Par4 appears to be the worm LKB1 orthologue. The phenotypic effects of inactivating Par4/LKB1 or Par1 are similar, suggesting that the two protein products of these genes cooperate in a kinase cascade in this organism too [13]. Similarly, Drosophila PAR1 functions to establish polarity of the fly egg chamber and embryo, and the consequence of loss of LKB1 activity is disruption of the normal polarised cell morphology in epithelial follicle cells [14]. Furthermore, overexpression of LKB1 can partially rescue the par1 mutant phenotype. These results, in worms and flies further re-inforce the connection between the LKB1 and PAR1 kinases in the regulation of cell polarity. How does the LKB1/PAR1 cascade interact with the cellular machinery to generate polarity? Cell polarisation requires asymmetry in the localisation of structural components including microtubules, and it may be that these structures are targeted by LKB1, via PAR1. In support of this, two members of the mammalian PAR1 family were initially characterised in a screen for kinases able to phosphorylate microtubuleassociated proteins [15], an event which destabilises microtubules and leads to their dissociation into tubulin subunits. The coordinated dissociation and reassociation of tubulin is essential to asymmetric microtubular function in a number of processes including mitotic spindle formation. As well as effects on cellular polarity, downregulation of proliferative signalling may
Dispatch R384
A
B Apical STRAD
STRAD
LKB1 Metformin
PAR1
P
PAR1
P
AMPK
AMPK
ATP ATP
? Cdc42/ microtubules TSC2
TSC2
AMP
AMP
Polarisation Rapamycin
? Cdc42/ microtubules
mTOR
Protein synthesis inhibited
Loss of polarity
mTOR
Protein synthesis activated
Basal Current Biology
Figure 1. LKB1 kinase activity coordinates diverse cellular processes. (A) Both AMPK and PAR1 are phosphorylated and activated by LKB1. LKB1-directed PAR1 activation promotes correct polarisation, including brush border formation in an intestinal epithelial cell. In parallel, AMPK redirects metabolic priorities towards the generation of ATP, and downregulates anabolic processes, including protein synthesis. Metformin is an activator of AMPK used in the treatment of diabetes. Rapamycin is an inhibitor of mTOR. (B) Absence of LKB1 results in failure of PAR1 activation and loss of cell polarity. In addition, failure of AMPK activation releases inhibition of mTOR, switching on anabolic pathways required for cell division.
be a consequence of PAR1 acting downstream from LKB1. PAR1 is known to phosphorylate and activate Dishevelled [16], a key component of the Wnt pathway that is upregulated in most colorectal cancers. So LKB1 might regulate Wnt signalling through its interaction with PAR1, and some recent observations support this conjecture [10,17]. There is increasing evidence that LKB1 is a master kinase which, when associated with STRAD, can phosphorylate a number of mammalian kinases related in sequence to PAR1 [11]. Of particular importance is the demonstration, by several groups [5–7], that LKB1 is an activating upstream kinase for AMPK. AMPK plays a key role in controlling the energy charge of the cell, and an upstream AMPK kinase has long been sought. LKB1 and STRAD co-purify with a cellular extract that phosphorylates and dramatically activates AMPK in vitro [5]. This unexpected result has been convincingly confirmed in cells; agents such as H2O2 activate AMPK in wild-type mouse embryonic fibroblasts, but not in cells lacking LKB1 [5–7]. Phosphorylation and activation of AMPK redirects cell metabolism towards generation of ATP, and away from energy-requiring macromolecular synthesis such as that required for cell division [18]. Therefore cells in which LKB1 activity has been lost may enjoy a proliferative advantage (Figure 1). The mechanisms by which neoplastic cells activate catabolic pathways to
fuel cell division have been little studied. Although there are precedents for the switching on of ‘housekeeping’ functions, such as tRNA transcription, in the transformed phenotype [19], this link between a tumour suppressor gene and such a central metabolic process is an exciting development. The control of protein synthesis by AMPK occurs, at least in part, via mammalian target of rapamycin (mTOR), a kinase which is inhibited following phosphorylation of TCS2 (also known as tuberin) by AMPK [20]. Through this pathway, loss of LKB1 activity in hamartomas would release the inhibition of protein translation facilitating cell growth and proliferation (Figure 1). Provocatively, germ-line TSC2 mutations are found in tuberous sclerosis, another inherited syndrome having hamartomatous polyps as part of the phenotype. Therefore mis-regulation of the AMPK–mTOR–TSC2 pathway might provide an explanation for the similarities in the histology of the benign hamartomas seen in both Peutz-Jeghers syndrome and tuberous sclerosis. These insights into LKB1 function suggest future new avenues for anti-cancer therapy, either through restoration of appropriate cell polarisation in cells that have lost functional LKB1, or by the modification of AMPK activity. Metformin, the most widely prescribed oral hypoglycaemic used in the treatment of diabetes, is an activator of AMPK; metformin-related drugs might therefore find application as anti-proliferative
Current Biology R385
agents in certain cancers. Furthermore, inhibitors of mTOR are already in clinical development. Conversely, modification of the activity of the LKB1 may prove useful in the treatment of diabetes. References 1. Jenne, D.E., Reimann, H., Nezu, J., Friedel, W., Loff, S., Jeschke, R., Muller, O., Back, W., and Zimmer, M. (1998). Peutz-Jeghers syndrome is caused by mutations in a novel serine threonine kinase. Nat. Genet. 18, 38-43. 2. Hemminki, A., Markie, D., Tomlinson, I., Avizienyte, E., Roth, S., Loukola, A., Bignell, G., Warren, W., Aminoff, M., Hoglund, P., et al. (1998). A serine/threonine kinase gene defective in Peutz-Jeghers syndrome. Nature 391, 184-187. 3. Boudeau, J., Sapkota, G., and Alessi, D.R. (2003). LKB1, a protein kinase regulating cell proliferation and polarity. FEBS Lett. 546, 159165. 4. Baas, A.F., Kuipers, J., van der Wel, N.N., Batlle, E., Koerten, H.K., Peters, P.J., and Clevers, H. (2004). Complete polarization of single intestinal epithelial cell upon activation of LKB1 by STRAD. Cell 116, 457-466. 5. Hawley, S.A., Boudeau, J., Reid, J.L., Mustard, K.J., Udd, L., Makela, T.P., Alessi, D.R., and Hardie, D.G. (2003). Complexes between the LKB1 tumor suppressor, STRADalpha/beta and MO25alpha/beta are upstream kinases in the AMP-activated protein kinase cascade. J. Biol. 2, 28. 6. Woods, A., Johnstone, S.R., Dickerson, K., Leiper, F.C., Fryer, L.G., Neumann, D., Schlattner, U., Wallimann, T., Carlson, M., and Carling, D. (2003). LKB1 is the upstream kinase in the AMP-activated protein kinase cascade. Curr. Biol. 13, 2004-2008. 7. Shaw, R.J., Kosmatka, M., Bardeesy, N., Hurley, R.L., Witters, L.A., DePinho, R.A., and Cantley, L.T. (2004). The tumor suppressor LKB1 kinase directly activates AMP-activated kinase and regulates apoptosis in response to energy stress. Proc. Natl. Acad. Sci. USA 101, 3329-3335. 8. Baas, A.F., Boudeau, J., Sapkota, G.P., Smit, L., Medema, R., Morrice, N.A., Alessi, D.R., and Clevers, H.C. (2003). Activation of the tumour suppressor kinase LKB1 by the STE20-like pseudokinase STRAD. EMBO J. 22, 3062-3072. 9. Brajenovic, M., Joberty, G., Kuster, B., Bouwmeester, T., and Drewes, G. (2003). Comprehensive proteomic analysis of human Par protein complexes reveals an interconnected protein network. J. Biol. Chem. 279, 12804-12811. 10. Spicer, J., Rayter, S., Young, N., Elliott, R., Ashworth, A., and Smith, D. (2003). Regulation of the Wnt signalling component PAR1A by the Peutz-Jeghers syndrome kinase LKB1. Oncogene 22, 47524756. 11. Lizcano, J.M., Goransson, O., Toth, R., Deak, M., Morrice, N.A., Boudeau, J., Hawley, S.A., Udd, L., Makela, T.P., Hardie, D.G., and Alessi, D.R. (2004). LKB1 is a master kinase that activates 13 kinases of the AMPK subfamily, including MARK/PAR-1. EMBO J. 23, 833-843. 12. Bohm, H., Brinkmann, V., Drab, M., Henske, A., and Kurzchalia, T.V. (1997). Mammalian homologues of C. elegans PAR-1 are asymmetrically localized in epithelial cells and may influence their polarity. Curr. Biol. 7, 603-606. 13. Watts, J.L., Morton, D.G., Bestman, J., and Kemphues, K.J. (2000). The C. elegans par-4 gene encodes a putative serine-threonine kinase required for establishing embryonic asymmetry. Development 127, 1467-1475. 14. Martin, S.G., and St Johnston, D. (2003). A role for Drosophila LKB1 in anterior-posterior axis formation and epithelial polarity. Nature 421, 379-384. 15. Drewes, G., Ebneth, A., Preuss, U., Mandelkow, E.M., and Mandelkow, E. (1997). MARK, a novel family of protein kinases that phosphorylate microtubule- associated proteins and trigger microtubule disruption. Cell 89, 297-308. 16. Sun, T.Q., Lu, B., Feng, J.J., Reinhard, C., Jan, Y.N., Fantl, W.J., and Williams, L.T. (2001). PAR-1 is a Dishevelled-associated kinase and a positive regulator of Wnt signalling. Nat. Cell Biol. 3, 628-636. 17. Ossipova, O., Bardeesy, N., DePinho, R.A., and Green, J.B. (2003). LKB1 (XEEK1) regulates Wnt signalling in vertebrate development. Nat. Cell Biol. 5, 889-894. 18. Hardie, D.G., Carling, D., and Carlson, M. (1998). The AMPactivated/SNF1 protein kinase subfamily: metabolic sensors of the eukaryotic cell? Annu. Rev. Biochem. 67, 821-855. 19. Gomez-Roman, N., Grandori, C., Eisenman, R.N. and White R.J. (2003). Direct activation of RNA Polymerase III transcription by cmyc. Nature 421, 290-294. 20. Inoki, K., Zhu, T., and Guan, K.L. (2003). TSC2 mediates cellular energy response to control cell growth and survival. Cell 115, 577590.