Monoclonal antibodies against Xenopus greatwall kinase

The ATM-E6AP-MASTL axis mediates DNA damage checkpoint recovery

Checkpoint activation after DNA damage causes a transient cell cycle arrest by suppressing cyclin-dependent kinases (CDKs). However, it remains largely elusive how cell cycle recovery is initiated after DNA damage. In this study, we discovered the upregulated protein level of MASTL kinase hours after DNA damage. MASTL promotes cell cycle progression by preventing PP2A/B55-catalyzed dephosphorylation of CDK substrates. DNA damage-induced MASTL upregulation was caused by decreased protein degradation, and was unique among mitotic kinases. We identified E6AP as the E3 ubiquitin ligase that mediated MASTL degradation. MASTL degradation was inhibited upon DNA damage as a result of the dissociation of E6AP from MASTL. E6AP depletion reduced DNA damage signaling, and promoted cell cycle recovery from the DNA damage checkpoint, in a MASTL-dependent manner. Furthermore, we found that E6AP was phosphorylated at Ser-218 by ATM after DNA damage and that this phosphorylation was required for its dissociation from MASTL, the stabilization of MASTL, and the timely recovery of cell cycle progression. Together, our data revealed that ATM/ATR-dependent signaling, while activating the DNA damage checkpoint, also initiates cell cycle recovery from the arrest. Consequently, this results in a timer-like mechanism that ensures the transient nature of the DNA damage checkpoint.

The ATM-E6AP-MASTL axis mediates DNA damage checkpoint recovery

Checkpoint activation after DNA damage causes a transient cell cycle arrest by suppressing CDKs. However, it remains largely elusive how cell cycle recovery is initiated after DNA damage. In this study, we discovered the upregulated protein level of MASTL kinase hours after DNA damage. MASTL promotes cell cycle progression by preventing PP2A/B55-catalyzed dephosphorylation of CDK substrates. DNA damage-induced MASTL upregulation was caused by decreased protein degradation, and was unique among mitotic kinases. We identified E6AP as the E3 ubiquitin ligase that mediated MASTL degradation. MASTL degradation was inhibited upon DNA damage as a result of the dissociation of E6AP from MASTL. E6AP depletion reduced DNA damage signaling, and promoted cell cycle recovery from the DNA damage checkpoint, in a MASTL-dependent manner. Furthermore, we found that E6AP was phosphorylated at Ser-218 by ATM after DNA damage and that this phosphorylation was required for its dissociation from MASTL, the stabilization of MASTL, and the timely recovery of cell cycle progression. Together, our data revealed that ATM/ATR-dependent signaling, while activating the DNA damage checkpoint, also initiates cell cycle recovery from the arrest. Consequently, this results in a timer-like mechanism that ensures the transient nature of the DNA damage checkpoint.

Kinase-Independent Functions of MASTL in Cancer: A New Perspective on MASTL Targeting

Microtubule-associated serine/threonine kinase-like (MASTL; Greatwall) is a well-characterized kinase, whose catalytic role has been extensively studied in relation to cell-cycle acceleration. Importantly, MASTL has been implicated to play a substantial role in cancer progression and subsequent studies have shown that MASTL is a significant regulator of the cellular actomyosin cytoskeleton. Several kinases have non-catalytic properties, which are essential or even sufficient for their functions. Likewise, MASTL functions have been attributed both to kinase-dependent phosphorylation of downstream substrates, but also to kinase-independent regulation of the actomyosin contractile machinery. In this review, we aimed to highlight the catalytic and non-catalytic roles of MASTL in proliferation, migration, and invasion. Further, we discussed the implications of this dual role for therapeutic design.

Cell cycle-dependent regulation of Greatwall kinase by protein phosphatase 1 and regulatory subunit 3B

Greatwall (Gwl) kinase plays an essential role in the regulation of mitotic entry and progression. Mitotic activation of Gwl requires both cyclin-dependent kinase 1 (CDK1)-dependent phosphorylation and its autophosphorylation at an evolutionarily conserved serine residue near the carboxyl terminus (Ser-883 in Xenopus). In this study we show that Gwl associates with protein phosphatase 1 (PP1), particularly PP1γ, which mediates the dephosphorylation of Gwl Ser-883. Consistent with the mitotic activation of Gwl, its association with PP1 is disrupted in mitotic cells and egg extracts. During mitotic exit, PP1-dependent dephosphorylation of Gwl Ser-883 occurs prior to dephosphorylation of other mitotic substrates; replacing endogenous Gwl with a phosphomimetic S883E mutant blocks mitotic exit. Moreover, we identified PP1 regulatory subunit 3B (PPP1R3B) as a targeting subunit that can direct PP1 activity toward Gwl. PPP1R3B bridges PP1 and Gwl association and promotes Gwl Ser-883 dephosphorylation. Consistent with the cell cycle-dependent association of Gwl and PP1, Gwl and PPP1R3B dissociate in M phase. Interestingly, up-regulation of PPP1R3B facilitates mitotic exit and blocks mitotic entry. Thus, our study suggests PPP1R3B as a new cell cycle regulator that functions by governing Gwl dephosphorylation.

N-Myc overexpression increases cisplatin resistance in neuroblastoma via deregulation of mitochondrial dynamics

N-Myc is a global transcription factor that regulates the expression of genes involved in a number of essential cellular processes including: ribosome biogenesis, cell cycle and apoptosis. Upon deregulation, N-Myc can drive pathologic expression of many of these genes, which ultimately defines its oncogenic potential. Overexpression of N-Myc has been demonstrated to contribute to tumorigenesis, most notably for the pediatric tumor, neuroblastoma. Herein, we provide evidence that deregulated N-Myc alters the expression of proteins involved in mitochondrial dynamics. We found that N-Myc overexpression leads to increased fusion of the mitochondrial reticulum secondary to changes in protein expression due to aberrant transcriptional and post-translational regulation. We believe the structural changes in the mitochondrial network in response to N-Myc amplification in neuroblastoma contributes to two important aspects of tumor development and maintenance—bioenergetic alterations and apoptotic resistance. Specifically, we found that N-Myc overexpressing cells are resistant to programmed cell death in response to exposure to low doses of cisplatin, and demonstrated that this was dependent on increased mitochondrial fusion. We speculate that these changes in mitochondrial structure and function may contribute significantly to the aggressive clinical ph9enotype of N-Myc amplified neuroblastoma.

Regulation of Greatwall kinase by protein stabilization and nuclear localization

Greatwall (Gwl) functions as an essential mitotic kinase by antagonizing protein phosphatase 2A. In this study we identified Hsp90, Cdc37 and members of the importin α and β families as the major binding partners of Gwl. Both Hsp90/Cdc37 chaperone and importin complexes associated with the N-terminal kinase domain of Gwl, whereas an intact glycine-rich loop at the N-terminus of Gwl was essential for binding of Hsp90/Cdc37 but not importins. We found that Hsp90 inhibition led to destabilization of Gwl, a mechanism that may partially contribute to the emerging role of Hsp90 in cell cycle progression and the anti-proliferative potential of Hsp90 inhibition. Moreover, in agreement with its importin association, Gwl exhibited nuclear localization in interphase Xenopus S3 cells, and dynamic nucleocytoplasmic distribution during mitosis. We identified KR456/457 as the locus of importin binding and the functional NLS of Gwl. Mutation of this site resulted in exclusion of Gwl from the nucleus. Finally, we showed that the Gwl nuclear localization is indispensable for the biochemical function of Gwl in promoting mitotic entry.

Deficient DNA damage signaling leads to chemoresistance to cisplatin in oral cancer

Activation of the cellular DNA damage response (DDR) is an important determinant of cell sensitivity to cisplatin and other chemotherapeutic drugs that eliminate tumor cells through induction of DNA damage. It is therefore important to investigate whether alterations of the DNA damage-signaling pathway confer chemoresistance in cancer cells and whether pharmacologic manipulation of the DDR pathway can resensitize these cells to cancer therapy. In a panel of oral/laryngeal squamous cell carcinoma (SCC) cell lines, we observed deficiencies in DNA damage signaling in correlation with cisplatin resistance, but not with DNA repair. These deficiencies are consistent with reduced expression of components of the ataxia telangiectasia mutated (ATM)-dependent signaling pathway and, in particular, strong upregulation of Wip1, a negative regulator of the ATM pathway. Wip1 knockdown or inhibition enhanced DNA damage signaling and resensitized oral SCC cells to cisplatin. In contrast to the previously reported involvement of Wip1 in cancer, Wip1 upregulation and function in these SCC cells is independent of p53. Finally, using xenograft tumor models, we showed that Wip1 upregulation promotes tumorigenesis and its inhibition improves the tumor response to cisplatin. Thus, this study reveals that chemoresistance in oral SCCs is partially attributed to deficiencies in DNA damage signaling, and Wip1 is an effective drug target for enhanced cancer therapy.

Greatwall and Polo-like kinase 1 coordinate to promote checkpoint recovery

Checkpoint recovery upon completion of DNA repair allows the cell to return to normal cell cycle progression and is thus a crucial process that determines cell fate after DNA damage. We previously studied this process in Xenopus egg extracts and established Greatwall (Gwl) as an important regulator. Here we show that preactivated Gwl kinase can promote checkpoint recovery independently of cyclin-dependent kinase 1 (Cdk1) or Plx1 (Xenopus polo-like kinase 1), whereas depletion of Gwl from extracts exhibits no synergy with that of Plx1 in delaying checkpoint recovery, suggesting a distinct but related relationship between Gwl and Plx1. In further revealing their functional relationship, we found mutual dependence for activation of Gwl and Plx1 during checkpoint recovery, as well as their direct association. We characterized the protein association in detail and recapitulated it in vitro with purified proteins, which suggests direct interaction. Interestingly, Gwl interaction with Plx1 and its phosphorylation by Plx1 both increase at the stage of checkpoint recovery. More importantly, Plx1-mediated phosphorylation renders Gwl more efficient in promoting checkpoint recovery, suggesting a functional involvement of such regulation in the recovery process. Finally, we report an indirect regulatory mechanism involving Aurora A that may account for Gwl-dependent regulation of Plx1 during checkpoint recovery. Our results thus reveal novel mechanisms underlying the involvement of Gwl in checkpoint recovery, in particular, its functional relationship with Plx1, a well characterized regulator of checkpoint recovery. Coordinated interplays between Plx1 and Gwl are required for reactivation of these kinases from the G(2)/M DNA damage checkpoint and efficient checkpoint recovery.