B-Myb switches from Cyclin/Cdk-dependent to Jnk- and p38 kinase-dependent phosphorylation and associates with SC35 bodies after UV stress

Intramolecular interaction of B-MYB is regulated through Ser-577 phosphorylation

The transcription factor B-MYB is an important regulator of cell cycle-related processes that is activated by step-wise phosphorylation of multiple sites by cyclin-dependent kinases (CDKs) and conformational changes induced by the peptidylprolyl cis/trans isomerase Pin1. Here, we show that a conserved amino acid sequence around Ser-577 in the C-terminal part of B-MYB is able to interact with the B-MYB DNA-binding domain. Phosphorylation of Ser-577 disrupts this interaction and is regulated by the interplay of CDKs and the phosphatase CDC14B. Deletion of sequences surrounding Ser-577 hyperactivates the transactivation potential of B-MYB, decreases its proteolytic stability, and causes cell cycle defects. Overall, we show for the first time that B-MYB can undergo an intramolecular interaction that is controlled by the phosphorylation state of Ser-577.

Characterization of the zinc finger proteins ZMYM2 and ZMYM4 as novel B-MYB binding proteins

B-MYB, a highly conserved member of the MYB transcription factor family, is expressed ubiquitously in proliferating cells and plays key roles in important cell cycle-related processes, such as control of G2/M-phase transcription, cytokinesis, G1/S-phase progression and DNA-damage reponse. Deregulation of B-MYB function is characteristic of several types of tumor cells, underlining its oncogenic potential. To gain a better understanding of the functions of B-MYB we have employed affinity purification coupled to mass spectrometry to discover novel B-MYB interacting proteins. Here we have identified the zinc-finger proteins ZMYM2 and ZMYM4 as novel B-MYB binding proteins. ZMYM4 is a poorly studied protein whose initial characterization reported here shows that it is highly SUMOylated and that its interaction with B-MYB is stimulated upon induction of DNA damage. Unlike knockdown of B-MYB, which causes G2/M arrest and defective cytokinesis in HEK293 cells, knockdown of ZMYM2 or ZMYM4 have no obvious effects on the cell cycle of these cells. By contrast, knockdown of ZMYM2 strongly impaired the G1/S-phase progression of HepG2 cells, suggesting that ZMYM2, like B-MYB, is required for entry into S-phase in these cells. Overall, our work identifies two novel B-MYB binding partners with possible functions in the DNA-damage response and the G1/S-transition.

Activation of the oncogenic transcription factor B-Myb via multisite phosphorylation and prolyl cis/trans isomerization

The oncogenic transcription factor B-Myb is an essential regulator of late cell cycle genes whose activation by phosphorylation is still poorly understood. We describe a stepwise phosphorylation mechanism of B-Myb, which involves sequential phosphorylations mediated by cyclin-dependent kinase (Cdk) and Polo-like kinase 1 (Plk1) and Pin1-facilitated peptidyl-prolyl cis/trans isomerization. Our data suggest a model in which initial Cdk-dependent phosphorylation of B-Myb enables subsequent Pin1 binding and Pin1-induced conformational changes of B-Myb. This, in turn, initiates further phosphorylation of Cdk-phosphosites, enabling Plk1 docking and subsequent Plk1-mediated phosphorylation of B-Myb to finally allow B-Myb to stimulate transcription of late cell cycle genes. Our observations reveal novel mechanistic hierarchies of B-Myb phosphorylation and activation and uncover regulatory principles that might also apply to other Myb family members. Strikingly, overexpression of B-Myb and of factors mediating its activation strongly correlates with adverse prognoses for tumor patients, emphasizing B-Myb's role in tumorigenesis.

Cefradine blocks solar-ultraviolet induced skin inflammation through direct inhibition of T-LAK cell-originated protein kinase

Skin inflammation, and skin cancer induced by excessive solar ultraviolet (SUV) is a great threat to human health. SUV induced skin inflammation through activating p38 mitogen-activated protein kinase (p38) and c-Jun N-termeinal kinases (JNKs). T-LAK cell-originated protein kinase (TOPK) plays an important role in this process. Herein, the clinical data showed TOPK, phospho-p38, phospho-JNKs were highly expressed in human solar dermatitis. Ex vivo studies showed that SUV induced the phosphorylation of p38 and JNKs in HaCat and JB6 cells in a dose and time dependent manner. Molecule docking model indicated cefradine, an FDA-approved cephalosporin antibiotic, directly binds with TOPK. The result of in vitro binding assay verified cefradine can directly bind with TOPK. In vitro kinase results showed cefradine can inhibit TOPK activity. Ex vivo studies further showed cefradine inhibited SUV-induced the phosphorylation level of p38, JNKs and H2AX through inhibiting TOPK activity in a dose and time dependent manner, and cefradine inhibited the secretion of IL6 and TNF-α in HaCat and JB6 cells. In vivo studies showed that cefradine down-regulated SUV-induced the phosphorylation of p38, JNKs and H2AX and inhibited the secretion of IL6 and TNF-α in Babl/c mice. These results indicated that cefradine can inhibit SUV-induced skin inflammation by blocking TOPK signaling pathway, and TOPK is an effective target for suppressing inflammation induced by SUV irradiation.

Interplay with the Mre11-Rad50-Nbs1 complex and phosphorylation by GSK3β implicate human B-Myb in DNA-damage signaling

B-Myb, a highly conserved member of the Myb transcription factor family, is expressed ubiquitously in proliferating cells and controls the cell cycle dependent transcription of G2/M-phase genes. Deregulation of B-Myb has been implicated in oncogenesis and loss of genomic stability. We have identified B-Myb as a novel interaction partner of the Mre11-Rad50-Nbs1 (MRN) complex, a key player in the repair of DNA double strand breaks. We show that B-Myb directly interacts with the Nbs1 subunit of the MRN complex and is recruited transiently to DNA-damage sites. In response to DNA-damage B-Myb is phosphorylated by protein kinase GSK3β and released from the MRN complex. A B-Myb mutant that cannot be phosphorylated by GSK3β disturbs the regulation of pro-mitotic B-Myb target genes and leads to inappropriate mitotic entry in response to DNA-damage. Overall, our work suggests a novel function of B-Myb in the cellular DNA-damage signalling.

PDIP38 is translocated to the spliceosomes/nuclear speckles in response to UV-induced DNA damage and is required for UV-induced alternative splicing of MDM2

PDIP38 (polymerase delta interacting protein 38) was originally discovered as a protein that interacts with DNA polymerase δ and PCNA. PDIP38 is present in multiple intracellular locations and is a multifunctional protein that has been implicated in several diverse cellular functions. We investigated the nuclear localization of PDIP38 in order to gain insights to its response to UV damage. PDIP38 was found to form distinct nuclear foci in response to UV irradiation in several cell lines, including HeLa S3 and A549 cells. However, these foci were not those associated with UV repair foci. Using various markers for different nuclear subcompartments, the UV-induced PDIP38 foci were identified as spliceosomes/nuclear speckles, the storage and assembly sites for mRNA splicing factors. To assess the role of PDIP38 in the regulation of splicing events, the effects of PDIP38 depletion on the UV-induced alternate splicing of MDM2 transcripts were examined by nested RT-PCR. Alternatively spliced MDM2 products were induced by UV treatment but were greatly reduced in cells expressing shRNA targeting PDIP38. These findings indicate that upon UV-induced DNA damage, PDIP38 is translocated to spliceosomes and contributes to the UV-induced alternative splicing of MDM2 transcripts. Similar results were obtained when cells were subjected to transcriptional stresses with actinomycin D or α-amanitin. Taken together, these studies show that PDIP38 is a protein regulated in a dynamic manner in response to genotoxic stress, as evidenced by its translocation to the spliceosomes. Moreover, PDIP38 is required for the induction of the alternative splicing of MDM2 in response to UV irradiation.