The three human homologues of Aurora kinases (A, B and C) are essential for proper execution of various mitotic events and are important for maintaining genomic integrity. Aurora-A is mainly localized at spindle poles and the mitotic spindle during mitosis, where it regulates the functions of centrosomes, spindles and kinetochores required for proper mitotic progression. Recent studies have revealed that Aurora-A is frequently overexpressed in various cancer cells, indicating its involvement in tumorigenesis. What are the normal physiological roles of Aurora-A, how are these regulated and how might the enzyme function during tumorigenesis?

Aurora family kinases contribute to regulation of mitosis. Using RNA interference in synchronized HeLa cells, we now show that Aurora-A is required for mitotic entry. We found that initial activation of Aurora-A in late G2 phase of the cell cycle is essential for recruitment of the cyclin B1-Cdk1 complex to centrosomes, where it becomes activated and commits cells to mitosis. A two-hybrid screen identified the LIM protein Ajuba as an Aurora-A binding protein. Ajuba and Aurora-A interact in mitotic cells and become phosphorylated as they do so. In vitro analyses revealed that Ajuba induces the autophosphorylation and consequent activation of Aurora-A. Depletion of Ajuba prevented activation of Aurora-A at centrosomes in late G2 phase and inhibited mitotic entry. Overall, our data suggest that Ajuba is an essential activator of Aurora-A in mitotic commitment.

Decades of research in molecular oncology have brought about promising new therapies that are designed to target specific molecules that promote tumor growth and survival. The epidermal growth factor receptor (EGFR) is one of the first identified important targets of these novel antitumor agents. Approximately half of cases of triple-negative breast cancer (TNBC) and inflammatory breast cancer (IBC) overexpress EGFR. Thus, EGFR inhibitors for treatment of breast cancer have been evaluated in several studies. However, results so far have been disappointing. One of the reasons for these unexpected results is the lack of biomarkers for predicting which patients are most likely to respond to EGFR inhibitors. Recent studies have shown that EGFR and its downstream pathway regulate epithelial-mesenchymal transition, migration, and tumor invasion and that high EGFR expression is an independent predictor of poor prognosis in IBC. Further, recent studies have shown that targeting EGFR enhances the chemosensitivity of TNBC cells by rewiring apoptotic signaling networks in TNBC. These studies indicate that EGFR-targeted therapy might have a promising role in TNBC and IBC. Further studies of the role of EGFR in TNBC and IBC are needed to better understand the best way to use EGFR-targeted therapy—e.g., as a chemosensitizer or to prevent metastases—to treat these aggressive diseases.

The cyclic dinucleotide c-di-GMP synthesized by the diadenylate cyclase domain was recently discovered as a messenger molecule for signaling DNA breaks in Bacillus subtilis. By searching bacterial genomes, we identified a family of DHH/ DHHA1 domain proteins (COG3387) that co-occur with a subset of the diadenylate cyclase domain proteins. Here we report that the B. subtilis protein YybT, a member of the COG3387 family proteins, exhibits phosphodiesterase activity toward cyclic dinucleotides. The DHH/DHHA1 domain hydrolyzes c-di-AMP and c-di-GMP to generate the linear dinucleotides 5-pApA and 5-pGpG. The data suggest that c-di-AMP could be the physiological substrate for YybT given the physiologically relevant Michaelis-Menten constant (K m ) and the presence of YybT family proteins in the bacteria lacking c-di-GMP signaling network. The bacterial regulator ppGpp was found to be a strong competitive inhibitor of the DHH/DHHA1 domain, suggesting that YybT is under tight control during stringent response. In addition, the atypical GGDEF domain of YybT exhibits unexpected ATPase activity, distinct from the common diguanylate cyclase activity for GGDEF domains. We further demonstrate the participation of YybT in DNA damage and acid resistance by characterizing the phenotypes of the ⌬yybT mutant. The novel enzymatic activity and stress resistance together point toward a role for YybT in stress signaling and response.The cyclic dinucleotide c-di-GMP 2 has been firmly established as a major bacterial messenger molecule in recent years, with the cellular level of c-di-GMP regulated by diguanylate cyclase and phosphodiesterase domain proteins (1-3). In contrast, the existence of the structurally similar 3Ј,5Ј-cyclic diguanylate (c-di-AMP) in living cells was unknown until the recent serendipitous discovery of the dinucleotide bound by the DisA protein from Bacillus subtilis (4,5). It was found that c-di-AMP was synthesized by the diadenylate cyclase (DAC) domain of DisA via condensation of two ATP molecules. Witte et al. (5) suggested that c-di-AMP is involved in signaling DNA damage considering that the DNA integrity scanning protein DisA scouts the chromosome for DNA double-stranded breaks. Subsequent genomic mining revealed that the DAC domain proteins are widespread in bacteria and archaea, with many of them associated with putative sensor domains (6). The wide occurrence and domain architecture of the DAC domain proteins hinted that c-di-AMP may be another hidden nucleotide messenger mediating various cellular functions and phenotypes.Currently, there is no report of c-di-AMP degrading or exporting proteins for controlling cellular c-di-AMP level. To identify potential c-di-AMP degrading proteins, we searched bacterial genomes for phosphodiesterase or phosphoesterase proteins that co-occur with the DAC domain-containing proteins (6). We found that a group of multidomain proteins seems to co-occur with a subset of the DAC domain proteins, which include the homologs of YojJ and YbbP from B. subtilis. This group of pr...

Tumorigenesis is a consequence of loss of tumor suppressors and activation of oncogenes. Expression of the mitotic checkpoint protein Chfr is lost in 20-50% of primary tumors and tumor cell lines. To explore whether downregulation of Chfr contributes directly to tumorigenesis, we generated Chfr knockout mice. Chfr-deficient mice are cancer-prone, develop spontaneous tumors and have increased skin tumor incidence after treatment with dimethylbenz(a)anthracene. Chfr deficiency leads to chromosomal instability in embryonic fibroblasts and regulates the mitotic kinase Aurora A, which is frequently upregulated in a variety of tumors. Chfr physically interacts with Aurora A and ubiquitinates Aurora A both in vitro and in vivo. Collectively, our data suggest that Chfr is a tumor suppressor and ensures chromosomal stability by controlling the expression levels of key mitotic proteins such as Aurora A.