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A Confederacy of Kinases: Cdk2 and Cdk4 Conspire to Control Embryonic Cell Proliferation
Molecular Cell 432
Scott B. Selleck1,2 Department of Pediatrics 2 Department of Genetics, Cell Biology and Development The Development Biology Center The University of Minnesota Minneapolis, Minnesota 55455
Coultas, L., Chawengsaksophak, K., and Rossant, J. (2005). Nature 438, 937–945.
1
Hacker, U., Nybakken, K., and Perrimon, N. (2005). Nat. Rev. Mol. Cell Biol. 6, 530–541. Jakobsson, L., Kreuger, J., Holmborn, K., Lundin, L., Eriksson, I., Kjellen, L., and Claesson-Welsh, L. (2006). Dev. Cell 10, 625–634. Kjellen, L. (2003). Biochem. Soc. Trans. 31, 340–342. Lin, X. (2004). Development 131, 6009–6021.
Selected Reading
Robinson, C.J., Mulloy, B., Gallagher, J.T., and Stringer, S.E. (2006). J. Biol. Chem. 281, 1731–1740.
Ashikari-Hada, S., Habuchi, H., Kariya, Y., and Kimata, K. (2005). J. Biol. Chem. 280, 31508–31515.
Ruhrberg, C., Gerhardt, H., Golding, M., Watson, R., Ioannidou, S., Fujisawa, H., Betsholtz, C., and Shima, D.T. (2002). Genes Dev. 16, 2684–2698.
Bellaiche, Y., The, I., and Perrimon, N. (1998). Nature 394, 85–88.
Strigini, M. (2005). J. Neurobiol. 64, 324–333.
Molecular Cell 22, May 19, 2006 ª2006 Elsevier Inc.
DOI 10.1016/j.molcel.2006.05.006
A Confederacy of Kinases: Cdk2 and Cdk4 Conspire to Control Embryonic Cell Proliferation
The mouse embryo is surprisingly resistant to loss of individual cyclins or cyclin-dependent kinases. In a recent issue of Developmental Cell, Berthet et al. (2006) describe collaboration of Cdk2 and Cdk4 in embryogenesis that is revealed only upon their simultaneous loss, resulting in inappropriate activation of the retinoblastoma protein and embryonic lethality.
It is a common conceit among biological scientists that their favorite gene, protein, or pathway is ‘‘key’’ to the cell’s vitality. Those of us working in the cell cycle field are certainly no exception, and thus, recent reports resulting from genetic inactivation of the cell cycle ‘‘core machinery’’ in mice have been accepted with some surprise, a bit of consternation, and a considerable amount of fruitful hypothesizing about the nature of cell cycle control in development and cancer. This state of affairs has been driven by the conclusion that mouse development and much of cell proliferation in vitro proceeds very well in the absence not only of individual cyclins and cyclin-dependent kinases such as cyclin E or Cdk2 but also in animals and cells lacking Cdk4 and Cdk6, all three D type cyclins, both E type cyclins, or Cdk2 and Cdk6 together (Berthet, et al., 2003; Geng, et al., 2003; Kozar, et al., 2004; Malumbres, et al., 2004; Ortega, et al., 2003; Parisi, et al., 2003). It is true many of these latter combinations of gene knockout do not produce viable animals at birth, but the vast number of cell divisions and relatively normal development of most organs into mid- or late embryogenesis suggest that loss of these ‘‘key’’ regulators of proliferation provides a surprisingly minimal barrier to cell proliferation in the early mouse embryo. This persistent development can in part be explained by the remarkable plasticity observed in mouse em-
bryos that is commonly referred to as compensation. Thus, Cdk2 may be dispensable in most proliferating cells as a result of reassignment of Cdc2 as a partner for cyclin E (Aleem, et al., 2005), and the lack of Cdk4 and Cdk6 may be offset by an ability of D cyclins to activate Cdk2 (Malumbres, et al., 2004). Some relief to this assault on the centrality of the cyclins and Cdks to cell proliferation control is provided by the work of Berthet et al. (2006), who now report that loss of Cdk2 and Cdk4 together results in embryonic lethality and severe impairment of embryo cell proliferation in culture despite the very mild effect of each individual deletion. In mouse embryos bearing combined deletion of Cdk2 and Cdk4, which were obtained at low frequency after mitotic recombination of these closely linked genes on chromosome 10, only the heart showed dramatic structural alteration, but other organs and the embryo as a whole demonstrated severe size reduction as a result of impaired cell proliferation. Further, early passage mouse embryo fibroblasts (MEFs) derived from these animals proliferated poorly, and not at all by passage four, and instead entered premature senescence. One fundamental conclusion from this work is that either Cdk2 or Cdk4 can provide a significant amount of the stimulus needed for proliferation in embryonic tissues and MEFs, and combined loss shows significant, general loss of proliferative capacity. Berthet et al. (2006) go on to demonstrate that embryos and MEFs lacking Cdk2 and Cdk4 show the signature of ‘‘excess’’ activity of the retinoblastoma protein (pRB), which serves to repress the expression of genes involved in proliferation, notably including the gene encoding Cdc2. Thus, at early passage, the aforementioned compensation by Cdc2 and increased cyclin D1/ Cdk6 complexes may allow proliferation, but later in embryogenesis and after only a few passages of cultured MEFs, Cdc2 and cyclin D1 levels decline sharply, pRB phosphorylation is lost, and cell proliferation ceases. Indeed, the authors show that the pRB-inactivating abilities of the HPV E7 oncoprotein can offset much of this, as can ectopic expression of Cdk2. In sum then, early embryonic cells are very facile at finding an appropriate combination of cyclins and Cdks to inactivate pRB, and
Previews 433
either Cdk4 or Cdk2 greatly facilitates this, but in their combined absence, the cell eventually runs out of options and succumbs to pRB-mediated transcriptional repression. The work of Berthet et al. (2006) restores a degree of faith in the overall importance of cell cycle machinery controlling proper cell proliferation and organism development but still leaves unanswered interesting questions about the nature of the plasticity that allows so much of early embryogenesis to resist the loss of pRB regulation and other putative functions of Cdk2 and Cdk4. For example, why is it that pRB ‘‘wins’’ only fairly late in development by promoting apparent repression of Cdc2? What are the factors that lead to increased accumulation of active pRB in more mature tissues in the absence of Cdk2 and Cdk4 when this appears of little consequence early in development and in MEFs in first passage? One of the most intriguing possibilities here is that cell divisions in the most primitive (or stemlike?) cells rely not on reassortment of available cyclins and Cdks with which we are most familiar but instead use an altogether different and unappreciated mechanism to maintain pRB in a phosphorylated, inactive state that allows the abundant cell divisions needed to efficiently produce the early embryo. Only later, perhaps, does the more familiar Cdk4-Cdk2 paradigm come to the fore to allow precise regulation of cell cycle events associated with the complexities of producing a fully functional, differentiated tissue. Perhaps the most interesting question yet to be addressed in light of this work is one that centers on the potential therapeutic targeting of the cell cycle machinery in diseases like cancer that are marked by alterations in proper cell cycle control. Indeed, if embryonic cells have little need for our favorite Cdks and instead resort to different mechanisms altogether to neutralize pRB and other negative regulators of proliferation, then important therapeutic targets in the ‘‘cancer stem cell,’’ thought by some to coopt the properties of the developmental stem cell, remain to be revealed. Some credence for this possibility was indeed given by studies of Tetsu and McCormick (2003), who startled the cell cycle and cancer community by observing that many cultured cancer cell lines had little if any need for Cdk2. Nevertheless, strong evidence for a specific role for Cdk2 and/or Cdk4 function in cells deprived of serum or stressed by
Molecular Cell 22, May 19, 2006 ª2006 Elsevier Inc.
oncogene introduction in vitro or in vivo supports the idea that cancer cell generation may be uniquely sensitive to loss of one or another of these regulators, despite the flexibility of embryonic cells to counter their loss in development (Berthet, et al., 2003; Geng, et al., 2003; Kozar, et al., 2004; Landis, et al., 2006; Malumbres, et al., 2004; Ortega, et al., 2003; Tsutsui, et al., 1999). The work of Berthet et al. (2006) thus provides us with significant mechanistic insight into the schemes of embryonic cells dedicated at all costs to producing a living organism and guides the way to evaluating the impact of this plasticity on providing the persistent proliferation needed to drive cancerous tumor formation. Philip W. Hinds1 1 Department of Radiation Oncology Molecular Oncology Research Institute Tufts-New England Medical Center Boston, Massachusetts 02115 Selected Reading Aleem, E., Kiyokawa, H., and Kaldis, P. (2005). Nat. Cell Biol. 7, 831– 836. Berthet, C., Aleem, E., Coppola, V., Tessarollo, L., and Kaldis, P. (2003). Curr. Biol. 13, 1775–1785. Berthet, C., Klarmann, K.D., Hilton, M.B., Suh, H.C., Keller, J.R., Kiyokawa, H., and Kaldis, P. (2006). Dev. Cell 10, 563–573. Geng, Y., Yu, Q., Sicinska, E., Das, M., Schneider, J.E., Bhattacharya, S., Rideout, W.M., Bronson, R.T., Gardner, H., and Sicinski, P. (2003). Cell 114, 431–443. Kozar, K., Ciemerych, M.A., Rebel, V.I., Shigematsu, H., Zagozdzon, A., Sicinska, E., Geng, Y., Yu, Q., Bhattacharya, S., Bronson, R.T., et al. (2004). Cell 118, 477–491. Landis, M.W., Pawlyk, B., Li, T., Sicinski, P., and Hinds, P.W. (2006). Cancer Cell 9, 13–22. Malumbres, M., Sotillo, R., Santamaria, D., Galan, J., Cerezo, A., Ortega, S., Dubus, P., and Barbacid, M. (2004). Cell 118, 493–504. Ortega, S., Prieto, I., Odajima, J., Martin, A., Dubus, P., Sotillo, R., Barbero, J.L., Malumbres, M., and Barbacid, M. (2003). Nat. Genet. 35, 25–31. Parisi, T., Beck, A.R., Rougier, N., McNeil, T., Lucian, L., Werb, Z., and Amati, B. (2003). EMBO J. 22, 4794–4803. Tetsu, O., and McCormick, F. (2003). Cancer Cell 3, 233–245. Tsutsui, T., Hesabi, B., Moons, D.S., Pandolfi, P.P., Hansel, K.S., Koff, A., and Kiyokawa, H. (1999). Mol. Cell. Biol. 19, 7011–7019.
DOI 10.1016/j.molcel.2006.05.002
If the Prophet Does Not Come to the Mountain: Dynamics of Signaling Complexes in NF-kB Activation
this complex, to K63-linked polyubiquitin chains attached to RIP1, a receptor-associated adaptor protein (Ea et al., 2006 [in a recent issue of Molecular Cell]; Li et al., 2006; Wu et al., 2006a).
Recruitment of the NF-kB-activating IKK signaling complex to the TNF receptor is shown to be driven by induced binding of NEMO, a regulatory component of
NF-kB transcription factors initiate gene expression response to molecular cues that signify infection, cellular stress, cell-growth stimulation, or developmental switch. Despite the diversity of stimuli and signaling cascades they trigger, in the most widely occurring form of this
Scott B. Selleck1,2 Department of Pediatrics 2 Department of Genetics, Cell Biology and Development The Development Biology Center The University of Minnesota Minneapolis, Minnesota 55455
Coultas, L., Chawengsaksophak, K., and Rossant, J. (2005). Nature 438, 937–945.
1
Hacker, U., Nybakken, K., and Perrimon, N. (2005). Nat. Rev. Mol. Cell Biol. 6, 530–541. Jakobsson, L., Kreuger, J., Holmborn, K., Lundin, L., Eriksson, I., Kjellen, L., and Claesson-Welsh, L. (2006). Dev. Cell 10, 625–634. Kjellen, L. (2003). Biochem. Soc. Trans. 31, 340–342. Lin, X. (2004). Development 131, 6009–6021.
Selected Reading
Robinson, C.J., Mulloy, B., Gallagher, J.T., and Stringer, S.E. (2006). J. Biol. Chem. 281, 1731–1740.
Ashikari-Hada, S., Habuchi, H., Kariya, Y., and Kimata, K. (2005). J. Biol. Chem. 280, 31508–31515.
Ruhrberg, C., Gerhardt, H., Golding, M., Watson, R., Ioannidou, S., Fujisawa, H., Betsholtz, C., and Shima, D.T. (2002). Genes Dev. 16, 2684–2698.
Bellaiche, Y., The, I., and Perrimon, N. (1998). Nature 394, 85–88.
Strigini, M. (2005). J. Neurobiol. 64, 324–333.
Molecular Cell 22, May 19, 2006 ª2006 Elsevier Inc.
DOI 10.1016/j.molcel.2006.05.006
A Confederacy of Kinases: Cdk2 and Cdk4 Conspire to Control Embryonic Cell Proliferation
The mouse embryo is surprisingly resistant to loss of individual cyclins or cyclin-dependent kinases. In a recent issue of Developmental Cell, Berthet et al. (2006) describe collaboration of Cdk2 and Cdk4 in embryogenesis that is revealed only upon their simultaneous loss, resulting in inappropriate activation of the retinoblastoma protein and embryonic lethality.
It is a common conceit among biological scientists that their favorite gene, protein, or pathway is ‘‘key’’ to the cell’s vitality. Those of us working in the cell cycle field are certainly no exception, and thus, recent reports resulting from genetic inactivation of the cell cycle ‘‘core machinery’’ in mice have been accepted with some surprise, a bit of consternation, and a considerable amount of fruitful hypothesizing about the nature of cell cycle control in development and cancer. This state of affairs has been driven by the conclusion that mouse development and much of cell proliferation in vitro proceeds very well in the absence not only of individual cyclins and cyclin-dependent kinases such as cyclin E or Cdk2 but also in animals and cells lacking Cdk4 and Cdk6, all three D type cyclins, both E type cyclins, or Cdk2 and Cdk6 together (Berthet, et al., 2003; Geng, et al., 2003; Kozar, et al., 2004; Malumbres, et al., 2004; Ortega, et al., 2003; Parisi, et al., 2003). It is true many of these latter combinations of gene knockout do not produce viable animals at birth, but the vast number of cell divisions and relatively normal development of most organs into mid- or late embryogenesis suggest that loss of these ‘‘key’’ regulators of proliferation provides a surprisingly minimal barrier to cell proliferation in the early mouse embryo. This persistent development can in part be explained by the remarkable plasticity observed in mouse em-
bryos that is commonly referred to as compensation. Thus, Cdk2 may be dispensable in most proliferating cells as a result of reassignment of Cdc2 as a partner for cyclin E (Aleem, et al., 2005), and the lack of Cdk4 and Cdk6 may be offset by an ability of D cyclins to activate Cdk2 (Malumbres, et al., 2004). Some relief to this assault on the centrality of the cyclins and Cdks to cell proliferation control is provided by the work of Berthet et al. (2006), who now report that loss of Cdk2 and Cdk4 together results in embryonic lethality and severe impairment of embryo cell proliferation in culture despite the very mild effect of each individual deletion. In mouse embryos bearing combined deletion of Cdk2 and Cdk4, which were obtained at low frequency after mitotic recombination of these closely linked genes on chromosome 10, only the heart showed dramatic structural alteration, but other organs and the embryo as a whole demonstrated severe size reduction as a result of impaired cell proliferation. Further, early passage mouse embryo fibroblasts (MEFs) derived from these animals proliferated poorly, and not at all by passage four, and instead entered premature senescence. One fundamental conclusion from this work is that either Cdk2 or Cdk4 can provide a significant amount of the stimulus needed for proliferation in embryonic tissues and MEFs, and combined loss shows significant, general loss of proliferative capacity. Berthet et al. (2006) go on to demonstrate that embryos and MEFs lacking Cdk2 and Cdk4 show the signature of ‘‘excess’’ activity of the retinoblastoma protein (pRB), which serves to repress the expression of genes involved in proliferation, notably including the gene encoding Cdc2. Thus, at early passage, the aforementioned compensation by Cdc2 and increased cyclin D1/ Cdk6 complexes may allow proliferation, but later in embryogenesis and after only a few passages of cultured MEFs, Cdc2 and cyclin D1 levels decline sharply, pRB phosphorylation is lost, and cell proliferation ceases. Indeed, the authors show that the pRB-inactivating abilities of the HPV E7 oncoprotein can offset much of this, as can ectopic expression of Cdk2. In sum then, early embryonic cells are very facile at finding an appropriate combination of cyclins and Cdks to inactivate pRB, and
Previews 433
either Cdk4 or Cdk2 greatly facilitates this, but in their combined absence, the cell eventually runs out of options and succumbs to pRB-mediated transcriptional repression. The work of Berthet et al. (2006) restores a degree of faith in the overall importance of cell cycle machinery controlling proper cell proliferation and organism development but still leaves unanswered interesting questions about the nature of the plasticity that allows so much of early embryogenesis to resist the loss of pRB regulation and other putative functions of Cdk2 and Cdk4. For example, why is it that pRB ‘‘wins’’ only fairly late in development by promoting apparent repression of Cdc2? What are the factors that lead to increased accumulation of active pRB in more mature tissues in the absence of Cdk2 and Cdk4 when this appears of little consequence early in development and in MEFs in first passage? One of the most intriguing possibilities here is that cell divisions in the most primitive (or stemlike?) cells rely not on reassortment of available cyclins and Cdks with which we are most familiar but instead use an altogether different and unappreciated mechanism to maintain pRB in a phosphorylated, inactive state that allows the abundant cell divisions needed to efficiently produce the early embryo. Only later, perhaps, does the more familiar Cdk4-Cdk2 paradigm come to the fore to allow precise regulation of cell cycle events associated with the complexities of producing a fully functional, differentiated tissue. Perhaps the most interesting question yet to be addressed in light of this work is one that centers on the potential therapeutic targeting of the cell cycle machinery in diseases like cancer that are marked by alterations in proper cell cycle control. Indeed, if embryonic cells have little need for our favorite Cdks and instead resort to different mechanisms altogether to neutralize pRB and other negative regulators of proliferation, then important therapeutic targets in the ‘‘cancer stem cell,’’ thought by some to coopt the properties of the developmental stem cell, remain to be revealed. Some credence for this possibility was indeed given by studies of Tetsu and McCormick (2003), who startled the cell cycle and cancer community by observing that many cultured cancer cell lines had little if any need for Cdk2. Nevertheless, strong evidence for a specific role for Cdk2 and/or Cdk4 function in cells deprived of serum or stressed by
Molecular Cell 22, May 19, 2006 ª2006 Elsevier Inc.
oncogene introduction in vitro or in vivo supports the idea that cancer cell generation may be uniquely sensitive to loss of one or another of these regulators, despite the flexibility of embryonic cells to counter their loss in development (Berthet, et al., 2003; Geng, et al., 2003; Kozar, et al., 2004; Landis, et al., 2006; Malumbres, et al., 2004; Ortega, et al., 2003; Tsutsui, et al., 1999). The work of Berthet et al. (2006) thus provides us with significant mechanistic insight into the schemes of embryonic cells dedicated at all costs to producing a living organism and guides the way to evaluating the impact of this plasticity on providing the persistent proliferation needed to drive cancerous tumor formation. Philip W. Hinds1 1 Department of Radiation Oncology Molecular Oncology Research Institute Tufts-New England Medical Center Boston, Massachusetts 02115 Selected Reading Aleem, E., Kiyokawa, H., and Kaldis, P. (2005). Nat. Cell Biol. 7, 831– 836. Berthet, C., Aleem, E., Coppola, V., Tessarollo, L., and Kaldis, P. (2003). Curr. Biol. 13, 1775–1785. Berthet, C., Klarmann, K.D., Hilton, M.B., Suh, H.C., Keller, J.R., Kiyokawa, H., and Kaldis, P. (2006). Dev. Cell 10, 563–573. Geng, Y., Yu, Q., Sicinska, E., Das, M., Schneider, J.E., Bhattacharya, S., Rideout, W.M., Bronson, R.T., Gardner, H., and Sicinski, P. (2003). Cell 114, 431–443. Kozar, K., Ciemerych, M.A., Rebel, V.I., Shigematsu, H., Zagozdzon, A., Sicinska, E., Geng, Y., Yu, Q., Bhattacharya, S., Bronson, R.T., et al. (2004). Cell 118, 477–491. Landis, M.W., Pawlyk, B., Li, T., Sicinski, P., and Hinds, P.W. (2006). Cancer Cell 9, 13–22. Malumbres, M., Sotillo, R., Santamaria, D., Galan, J., Cerezo, A., Ortega, S., Dubus, P., and Barbacid, M. (2004). Cell 118, 493–504. Ortega, S., Prieto, I., Odajima, J., Martin, A., Dubus, P., Sotillo, R., Barbero, J.L., Malumbres, M., and Barbacid, M. (2003). Nat. Genet. 35, 25–31. Parisi, T., Beck, A.R., Rougier, N., McNeil, T., Lucian, L., Werb, Z., and Amati, B. (2003). EMBO J. 22, 4794–4803. Tetsu, O., and McCormick, F. (2003). Cancer Cell 3, 233–245. Tsutsui, T., Hesabi, B., Moons, D.S., Pandolfi, P.P., Hansel, K.S., Koff, A., and Kiyokawa, H. (1999). Mol. Cell. Biol. 19, 7011–7019.
DOI 10.1016/j.molcel.2006.05.002
If the Prophet Does Not Come to the Mountain: Dynamics of Signaling Complexes in NF-kB Activation
this complex, to K63-linked polyubiquitin chains attached to RIP1, a receptor-associated adaptor protein (Ea et al., 2006 [in a recent issue of Molecular Cell]; Li et al., 2006; Wu et al., 2006a).
Recruitment of the NF-kB-activating IKK signaling complex to the TNF receptor is shown to be driven by induced binding of NEMO, a regulatory component of
NF-kB transcription factors initiate gene expression response to molecular cues that signify infection, cellular stress, cell-growth stimulation, or developmental switch. Despite the diversity of stimuli and signaling cascades they trigger, in the most widely occurring form of this