Small-molecule kinase inhibitors provide insight into Mps1 cell cycle function.

Mps1, a dual-specificity kinase, is required for the proper functioning of the spindle assembly checkpoint and for the maintenance of chromosomal stability. As Mps1 function has been implicated in numerous phases of the cell cycle, the development of a potent, selective small-molecule inhibitor of Mps1 should facilitate dissection of Mps1-related biology. We describe the cellular effects and Mps1 cocrystal structures of new, selective small-molecule inhibitors of Mps1. Consistent with RNAi studies, chemical inhibition of Mps1 leads to defects in Mad1 and Mad2 establishment at unattached kinetochores, decreased Aurora B kinase activity, premature mitotic exit and gross aneuploidy, without any evidence of centrosome duplication defects. However, in U2OS cells having extra centrosomes (an abnormality found in some cancers), Mps1 inhibition increases the frequency of multipolar mitoses. Lastly, Mps1 inhibitor treatment resulted in a decrease in cancer cell viability.

Inhibition of apoptosis by MAD1 is mediated by repression of the PTEN tumor suppressor gene.

The MYC/MAX/MAD network of transcriptional regulators controls distinct aspects of cell physiology, including cell proliferation and apoptosis. Within the network MAD proteins antagonize the functions of MYC oncoproteins, and the latter are deregulated in the majority of human cancers. While MYC sensitizes cells to proapoptotic signals, the transcriptional repressor MAD1 inhibits apoptosis in response to a broad range of stimuli, including oncoproteins. The molecular targets of MAD1 that mediate inhibition of apoptosis are not known. Here we describe the phosphatase and tensin homologue deleted on chromosome ten (PTEN) tumor suppressor gene as a target of MAD1. By binding to the proximal promoter region, MAD1 downregulated PTEN expression. PTEN functions as a lipid phosphatase that regulates the phosphatidylinositol 3-kinase/AKT pathway. Indeed MAD1-dependent repression of PTEN led to activation of AKT and subsequent stimulation of the antiapoptotic NF-kappaB pathway. Interfering with AKT function affected the control of Fas-induced apoptosis by MAD1. In addition, knockdown of PTEN using small interfering RNA (siRNA) or the lack of PTEN rendered cells insensitive to inhibition of apoptosis by MAD1. These findings identify the PTEN gene as a target of the MYC-antagonist MAD1 and provide a molecular framework critical for the ability of MAD1 to inhibit apoptosis.

The protein kinase DYRK1A phosphorylates the splicing factor SF3b1/SAP155 at Thr434, a novel in vivo phosphorylation site.

BACKGROUND: The U2 small nuclear ribonucleoprotein particle (snRNP) component SF3b1/SAP155 is the only spliceosomal protein known to be phosphorylated concomitant with splicing catalysis. DYRK1A is a nuclear protein kinase that has been localized to the splicing factor compartment. Here we describe the identification of DYRK1A as a protein kinase that phosphorylates SF3b1 in vitro and in cultivated cells. RESULTS: Overexpression of DYRK1A caused a markedly increased phosphorylation of SF3b1 in COS-7 cells as assessed by Western blotting with an antibody specific for phosphorylated Thr-Pro dipeptide motifs. Phosphopeptide mapping of metabolically labelled SF3b1 showed that the majority of the in vivo-phosphopeptides corresponded to sites also phosphorylated by DYRK1A in vitro. Phosphorylation with cyclin E/CDK2, a kinase previously reported to phosphorylate SF3b1, generated a completely different pattern of phosphopeptides. By mass spectrometry and mutational analysis of SF3b1, Thr434 was identified as the major phosphorylation site for DYRK1A. Overexpression of DYRK1A or the related kinase, DYRK1B, resulted in an enhanced phosphorylation of Thr434 in endogenous SF3b1 in COS-7 cells. Downregulation of DYRK1A in HEK293 cells or in HepG2 cells by RNA interference reduced the phosphorylation of Thr434 in SF3b1. CONCLUSION: The present data show that the splicing factor SF3b1 is a substrate of the protein kinase DYRK1A and suggest that DYRK1A may be involved in the regulation of pre mRNA-splicing.

The ITK-SYK fusion oncogene induces a T-cell lymphoproliferative disease in mice mimicking human disease.

Peripheral T-cell lymphomas (PTCL) constitute a major treatment problem with high mortality rates due to the minimal effectiveness of conventional chemotherapy. Recent findings identified ITK-SYK as the first recurrent translocation in 17% of unspecified PTCLs and showed the overexpression of SYK in more than 90% of PTCLs. Here, we show that the expression of ITK-SYK in the bone marrow of BALB/c mice causes a T-cell lymphoproliferative disease in all transplanted mice within 8 weeks after transplantation. The disease was characterized by the infiltration of spleen, lymph nodes, bone marrow, and skin with CD3+CD4+CD8- and CD3+CD4-CD8- ITK-SYK-positive T-cells accompanied by a systemic inflammatory reaction with upregulation of interleukin 5 and INF-gamma. ITK-SYK-positive T-cells showed enhanced apoptosis resistance and INF-gamma production in vitro. The disease was serially transplantable, inducing clonal T-cell expansion in secondary recipients. The action of ITK-SYK in vivo was dependent on SYK kinase activity and disease development could be inhibited by the treatment of mice with SYK inhibitors. Interestingly, the translocation of ITK-SYK from the membrane to the cytoplasm, using a point mutation in the pleckstrin homology domain (ITK-SYK R29C), did not abolish, but rather, enhanced disease development in transplanted mice. CBL binding was strongly enhanced in membrane-associated ITK-SYK E42K and was causative for delayed disease development. Our results show that ITK-SYK causes a T-cell lymphoproliferative disease in mice, supporting its role in T-cell lymphoma development in humans. Therefore, pharmacologic inhibition of SYK in patients with U-PTCLs carrying the ITK-SYK fusion protein might be an effective treatment strategy.

A TRAIL receptor-dependent synthetic lethal relationship between MYC activation and GSK3beta/FBW7 loss of function.

The MYC protooncogene is frequently deregulated in human cancers. Here, by screening a kinase-directed library of small inhibitory RNAs, we identify glycogen synthase kinase 3beta (GSK3beta) as a gene whose inactivation potentiates TNF-related apoptosis-inducing ligand death receptor-mediated apoptosis specifically in MYC-overexpressing cells. Small inhibitory RNA-induced silencing of GSK3beta prevents phosphorylation of MYC on T58, thereby inhibiting recognition of MYC by the E3 ubiquitin ligase component FBW7. Attenuating the GSK3beta-FBW7 axis results in stabilization of MYC, up-regulation of surface levels of the TNF-related apoptosis-inducing ligand death receptor 5, and potentiation of death receptor 5-induced apoptosis in vitro and in vivo. These results identify GSK3beta and FBW7 as potential cancer therapeutic targets and MYC as a critical substrate in the GSK3beta survival-signaling pathway. The results also demonstrate paradoxically that MYC-expressing tumors might be treatable by drug combinations that increase rather than decrease MYC oncoprotein function.

Mad1 function in cell proliferation and transcriptional repression is antagonized by cyclin E/CDK2.

The transcription factors of the Myc/Max/Mad network play essential roles in the regulation of cellular behavior. Mad1 inhibits cell proliferation by recruiting an mSin3-corepressor complex that contains histone deacetylase activity. Here we demonstrate that Mad1 is a potent inhibitor of the G(1) to S phase transition, a function that requires Mad1 to heterodimerize with Max and to bind to the corepressor complex. Cyclin E/CDK2, but not cyclin D and cyclin A complexes, fully restored S phase progression. In addition inhibition of colony formation and gene repression by Mad1 were also efficiently antagonized by cyclin E/CDK2. This was the result of cyclin E/CDK2 interfering with the interaction of Mad1 with HDAC1 and reducing HDAC activity. Our findings define a novel interplay between the cell cycle regulator cyclin E/CDK2 and Mad1 and its associated repressor complex and suggests an additional mechanism how cyclin E/CDK2 affects the G(1) to S phase transition.

Stimulation of c-MYC transcriptional activity and acetylation by recruitment of the cofactor CBP.

The c-MYC oncoprotein regulates various aspects of cell behaviour by modulating gene expression. Here, we report the identification of the cAMP-response-element-binding protein (CBP) as a novel c-MYC binding partner. The two proteins interact both in vitro and in cells, and CBP binds to the carboxy-terminal region of c-MYC. Importantly, CBP, as well as p300, is associated with E-box-containing promoter regions of genes that are regulated by c-MYC. Furthermore, c-MYC and CBP/p300 function synergistically in the activation of reporter-gene constructs. Thus, CBP and p300 function as positive cofactors for c-MYC. In addition, c-MYC is acetylated in cells. This modification does not require MYC box II, suggesting that it is independent of TRRAP complexes. Instead, CBP acetylates c-MYC in vitro, and co-expression of CBP with c-MYC stimulates in vivo acetylation. Functionally, this results in a decrease in ubiquitination and stabilization of c-MYC proteins. Thus, CBP and p300 are novel functional binding partners of c-MYC.