Rush hour in AML: news on NPM traffic
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《血液学杂志》
Expanding their pivotal findings on the role of nucleophosmin (NPM) in acute myeloid leukemia (AML), Falini and colleagues elucidate the molecular mechanisms underlying the abnormal cytoplasmic localization of NPM.
Nucleophosmin (NPM) is a multifunctional, highly conserved, and ubiquitously expressed nucleolar phosphoprotein that continuously shuttles between the nucleus and cytoplasm with prominent nucleolar localization.1 As part of a high-molecular-weight complex, NPM physically interacts with a large number of protein partners. It has been implicated in ribosomal protein assembly and transport, and also as a molecular chaperone that prevents protein aggregation in the nucleolus. NPM regulates the stability and transcriptional activity of p53 after different types of stress and acts as a nucleolar binding partner of the alternate-reading-frame protein (ARF).2 In addition, NPM is involved in the initiation of centrosome duplication via cyclin E/cdk2 phosphorylation.
Hypothetical mechanism of altered nucleocytoplasmic traffic of wild-type and mutant NPM. (Left) NPM wild-type (wt) cells show nucleus-restricted NPM positivity. Nuclear export of NPM wild-type (NPMwt) is minimum since it contains only one NES motif available for binding to CRM1 (Exportin 1). Tryptophans 288 and 290 drive NPMwt to nucleolus (Nc), thus acting as a counterforce to NES-dependent NPMwt nuclear export. As a consequence, no NPMwt is detectable in the cytoplasm by immunohistochemistry. (Right) In cells carrying NPM exon 12 mutations, NPM mutants accumulate in the cytoplasm (NPMc+), since they contain 2 NES motifs (1 created at C-terminus by the mutation) available for CRM1 binding. Moreover, they carry mutated tryptophans 288 and 290, which inhibit binding of NPM leukemic mutants to nucleolus (Nc), facilitating their nuclear export. A portion of NPmwt (which escapes NPM mutant recruitment) can bind to nucleoli (not shown). The circled 1 indicates where leptomycin B blocks nuclear export of NPM leukemic mutants by specific inhibition of CRM1. Red squares represent physiologic NES motif; gray squares, new C-terminus NES motif; black ovals, tryptophans 288 and 290; and orange ovals, mutated tryptophans 288 and 290.
NPM has previously been implicated in leukemogenesis and lymphomagenesis, as it was identified as a partner in gene fusions caused by chromosomal translocations. Its role in leukemogenesis has been strengthened by a knock-out mouse model revealing an essential role for NPM in the control of hematopoiesis during embryonic development and for the maintenance of genomic stability.3 In a recent, outstanding study performed by Falini and colleagues, NPM exon 12 mutations were identified as the most frequent gene mutation in adult acute myeloid leukemia (AML) that alters the NPM protein at the C-terminus causing its aberrant cytoplasmic localization.4 It is hypothesized that—analogous to NUP98 gene fusions that affect the nuclear pore complex and its function in nucleocytoplasmic transport—NPM cytoplasmic dislocation is critical for leukemia development.5
In their current study published in this issue of Blood, Falini and coworkers demonstrate that cytoplasmic accumulation of NPM mutants is caused by 2 major alterations that must act in concert: (1) loss of tryptophan residues normally required for NPM binding to the nucleoli and (2) generation of an additional nuclear export signal (NES) motif at the C-terminus by the exon 12 mutation reinforcing the CRM1-dependent nuclear export of the mutated protein. Moreover, NPM leukemic mutants were shown to recruit wild-type NPM from nucleoli to nucleoplasm and cytoplasm through dimerization (see figure).
The findings by Falini et al4 are of biologic and potentially major clinical relevance. One issue for future research will be the identification and characterization of proteins interacting with mutant NPM, a mechanism by which NPM mutants may process their transforming activity. One candidate partner protein is the tumor suppressor ARF, since its interaction with NPM is of critical importance for regulating cell proliferation and apoptosis through the ARF-MDM2-p53 pathway.2,6 Nucleolar sequestration of the NPM-ARF complex appears to be essential for its antioncogenic activity, and one might speculate that cytoplasmic delocalization of NPM results in a displacement of ARF, thereby decreasing its tumor-suppressor activity. Of greatest interest, the altered nucleo-cytoplasmic transport represents a potential target for drug intervention. Leptomycin B is able to block NPM nuclear export by inhibition of CRM1. It can be anticipated that other less toxic drugs or small molecules will be identified as capable of stopping the illegitimate protein transport. A large subset of AML patients may eventually benefit from such targeted therapies.
References
Borer RA, Lehner CF, Eppenberger HM, Nigg EA. Major nucleolar proteins shuttle between nucleus and cytoplasm. Cell. 1989;56: 379-390.
Brady SN, Yu Y, Maggi LB Jr, Weber JD. ARF impedes NPM/B23 shuttling in an Mdm2-sensitive tumor suppressor pathway. Mol Cell Biol. 2004;24: 9327-9338.
Grisendi S, Bernardi R, Rossi M, et al. Role of nucleophosmin in embryonic development and tumorigenesis. Nature. 2005;437: 174-153.
Falini B, Mecucci C, Tiacci E, et al. Cytoplasmic nucleophosmin in acute myelogenous leukemia with a normal karyotype. N Engl J Med. 2005;352: 254-266.
Nakamura T. NUP98 fusion in human leukaemia: dysregulation of the nuclear pore and homeodomain proteins. Int J Hematol. 2005;82: 21-27.
Bertwistle D, Sugimoto M, Sherr CJ. Physical and functional interactions of the Arf tumor suppressor protein with nucleophosmin/B23. Mol Cell Biol. 2004;24: 985-996.(Konstanze D?hner, and Har)
Nucleophosmin (NPM) is a multifunctional, highly conserved, and ubiquitously expressed nucleolar phosphoprotein that continuously shuttles between the nucleus and cytoplasm with prominent nucleolar localization.1 As part of a high-molecular-weight complex, NPM physically interacts with a large number of protein partners. It has been implicated in ribosomal protein assembly and transport, and also as a molecular chaperone that prevents protein aggregation in the nucleolus. NPM regulates the stability and transcriptional activity of p53 after different types of stress and acts as a nucleolar binding partner of the alternate-reading-frame protein (ARF).2 In addition, NPM is involved in the initiation of centrosome duplication via cyclin E/cdk2 phosphorylation.
Hypothetical mechanism of altered nucleocytoplasmic traffic of wild-type and mutant NPM. (Left) NPM wild-type (wt) cells show nucleus-restricted NPM positivity. Nuclear export of NPM wild-type (NPMwt) is minimum since it contains only one NES motif available for binding to CRM1 (Exportin 1). Tryptophans 288 and 290 drive NPMwt to nucleolus (Nc), thus acting as a counterforce to NES-dependent NPMwt nuclear export. As a consequence, no NPMwt is detectable in the cytoplasm by immunohistochemistry. (Right) In cells carrying NPM exon 12 mutations, NPM mutants accumulate in the cytoplasm (NPMc+), since they contain 2 NES motifs (1 created at C-terminus by the mutation) available for CRM1 binding. Moreover, they carry mutated tryptophans 288 and 290, which inhibit binding of NPM leukemic mutants to nucleolus (Nc), facilitating their nuclear export. A portion of NPmwt (which escapes NPM mutant recruitment) can bind to nucleoli (not shown). The circled 1 indicates where leptomycin B blocks nuclear export of NPM leukemic mutants by specific inhibition of CRM1. Red squares represent physiologic NES motif; gray squares, new C-terminus NES motif; black ovals, tryptophans 288 and 290; and orange ovals, mutated tryptophans 288 and 290.
NPM has previously been implicated in leukemogenesis and lymphomagenesis, as it was identified as a partner in gene fusions caused by chromosomal translocations. Its role in leukemogenesis has been strengthened by a knock-out mouse model revealing an essential role for NPM in the control of hematopoiesis during embryonic development and for the maintenance of genomic stability.3 In a recent, outstanding study performed by Falini and colleagues, NPM exon 12 mutations were identified as the most frequent gene mutation in adult acute myeloid leukemia (AML) that alters the NPM protein at the C-terminus causing its aberrant cytoplasmic localization.4 It is hypothesized that—analogous to NUP98 gene fusions that affect the nuclear pore complex and its function in nucleocytoplasmic transport—NPM cytoplasmic dislocation is critical for leukemia development.5
In their current study published in this issue of Blood, Falini and coworkers demonstrate that cytoplasmic accumulation of NPM mutants is caused by 2 major alterations that must act in concert: (1) loss of tryptophan residues normally required for NPM binding to the nucleoli and (2) generation of an additional nuclear export signal (NES) motif at the C-terminus by the exon 12 mutation reinforcing the CRM1-dependent nuclear export of the mutated protein. Moreover, NPM leukemic mutants were shown to recruit wild-type NPM from nucleoli to nucleoplasm and cytoplasm through dimerization (see figure).
The findings by Falini et al4 are of biologic and potentially major clinical relevance. One issue for future research will be the identification and characterization of proteins interacting with mutant NPM, a mechanism by which NPM mutants may process their transforming activity. One candidate partner protein is the tumor suppressor ARF, since its interaction with NPM is of critical importance for regulating cell proliferation and apoptosis through the ARF-MDM2-p53 pathway.2,6 Nucleolar sequestration of the NPM-ARF complex appears to be essential for its antioncogenic activity, and one might speculate that cytoplasmic delocalization of NPM results in a displacement of ARF, thereby decreasing its tumor-suppressor activity. Of greatest interest, the altered nucleo-cytoplasmic transport represents a potential target for drug intervention. Leptomycin B is able to block NPM nuclear export by inhibition of CRM1. It can be anticipated that other less toxic drugs or small molecules will be identified as capable of stopping the illegitimate protein transport. A large subset of AML patients may eventually benefit from such targeted therapies.
References
Borer RA, Lehner CF, Eppenberger HM, Nigg EA. Major nucleolar proteins shuttle between nucleus and cytoplasm. Cell. 1989;56: 379-390.
Brady SN, Yu Y, Maggi LB Jr, Weber JD. ARF impedes NPM/B23 shuttling in an Mdm2-sensitive tumor suppressor pathway. Mol Cell Biol. 2004;24: 9327-9338.
Grisendi S, Bernardi R, Rossi M, et al. Role of nucleophosmin in embryonic development and tumorigenesis. Nature. 2005;437: 174-153.
Falini B, Mecucci C, Tiacci E, et al. Cytoplasmic nucleophosmin in acute myelogenous leukemia with a normal karyotype. N Engl J Med. 2005;352: 254-266.
Nakamura T. NUP98 fusion in human leukaemia: dysregulation of the nuclear pore and homeodomain proteins. Int J Hematol. 2005;82: 21-27.
Bertwistle D, Sugimoto M, Sherr CJ. Physical and functional interactions of the Arf tumor suppressor protein with nucleophosmin/B23. Mol Cell Biol. 2004;24: 985-996.(Konstanze D?hner, and Har)