Phosphorylation of p53 at Ser 46 was shown to regulate p53 apoptotic activity. Here we demonstrate that homeodomain-interacting protein kinase-2 (HIPK2), a member of a novel family of nuclear serine/threonine kinases, binds to and activates p53 by directly phosphorylating it at Ser 46. HIPK2 localizes with p53 and PML-3 into the nuclear bodies and is activated after irradiation with ultraviolet. Antisense inhibition of HIPK2 expression reduces the ultraviolet-induced apoptosis. Furthermore, HIPK2 and p53 cooperate in the activation of p53-dependent transcription and apoptotic pathways. These data define a new functional interaction between p53 and HIPK2 that results in the targeted subcellular localization of p53 and initiation of apoptosis.

Expression of the Y-box protein YB-1 is increased in proliferating normal and cancer cells, but its role in cell proliferation and cell cycle progression is unclear. We have identified a cell cycle-dependent relocalization of YB-1 from the cytoplasm to the nucleus at the G 1 /S phase transition and demonstrate that both the charged zipper and the cold shock domain are involved in regulating this process. Using cell lines that constitutively overexpress YB-1, we show that nuclear accumulation of YB-1 is associated with increased cyclin A and cyclin B1 mRNA and protein expression. We provide evidence that deregulated YB-1 expression is linked to adhesion-independent cell proliferation through the induction of cyclin A. Thus, we have identified YB-1 as a cell cycle stage-specific transcription factor important for cell proliferation.

During normal cell cycles, the function of mitotic cyclin-cdk1 complexes, as well as of cdc25C phosphatase, is required for G 2 phase progression. Accordingly, the G 2 arrest induced by DNA damage is associated with a down-regulation of mitotic cyclins, cdk1, and cdc25C phosphatase expression. We found that the promoter activity of these genes is repressed in the G 2 arrest induced by DNA damage. We asked whether the CCAATbinding NF-Y modulates mitotic cyclins, cdk1, and cdc25C gene transcription during this type of G 2 arrest. In our experimental conditions, the integrity of the CCAAT boxes of cyclin B1, cyclin B2, and cdc25C promoters, as well as the presence of a functional NF-Y complex, is strictly required for the transcriptional inhibition of these promoters. Furthermore, a dominantnegative p53 protein, impairing doxorubicin-induced G 2 arrest, prevents transcriptional down-regulation of the mitotic cyclins, cdk1, and cdc25C genes. We conclude that, as already demonstrated for cdk1, NF-Y mediates the transcriptional inhibition of the mitotic cyclins and the cdc25C genes during p53-dependent G 2 arrest induced by DNA damage. These data suggest a transcriptional regulatory role of NF-Y in the G 2 checkpoint after DNA damage.

Cyclin B2 is a regulator of p34cdc2 kinase, involved in G2/M progression of the cell cycle, whose gene is strictly regulated at the transcriptional level in cycling cells. The mouse promoter was cloned and three conserved CCAAT boxes were found. In this study, we analysed the mechanisms leading to activation of the cyclin B2 CCAAT boxes: a combination of (i) genomic footprinting, (ii) transfections with single, double and triple mutants, (iii) EMSAs with nuclear extracts, antibodies and NF-Y recombinant proteins and (iv) transfections with an NF-YA dominant negative mutant established the positive role of the three CCAAT sequences and proved that NF-Y plays a crucial role in their activation. NF-Y, an ubiquitous trimer containing histone fold subunits, activates several other promoters regulated during the cell cycle. To analyse the levels of NF-Y subunits in the dierent phases of the cycle, we separated MEL cells by elutriation, obtaining fractions 480% pure. The mRNA and protein levels of the histone-fold containing NF-YB and NF-YC were invariant, whereas the NF-YA protein, but not its mRNA, was maximal in mid-S and decreased in G2/M. EMSA con®rmed that the CCAAT-binding activity followed the amount of NF-YA, indicating that this subunit is limiting within the NF-Y complex, and suggesting that post-transcriptional mechanisms regulate NF-YA levels. Our results support a model whereby ®ne tuning of this activator is important for phase-speci®c transcription of CCAATcontaining promoters.

The observation that cyclin B1 protein and mRNAs are down-regulated in terminally dierentiated (TD) C2C12 cells, suggested us to investigate the transcriptional regulation of the cyclin B1 gene in these cells. Transfections of cyclin B1 promoter constructs indicate that two CCAAT boxes support cyclin B1 promoter activity in proliferating cells. EMSAs demonstrate that both CCAAT boxes are recognized by the trimeric NF-Y complex in proliferating but not in TD cells. Transfecting a dominant-negative mutant of NF-YA we provide evidence that NF-Y is required for maximal promoter activity. Addition of recombinant NF-YA to TD C2C12 nuclear extracts restores binding activity in vitro, thus indicating that the loss of NF-YA in TD cells is responsible for the lack of the NF-Y binding to the CCAAT boxes. Consistent with this, we found that the NF-YA protein is absent in TD C2C12 cells. In conclusion, our data demonstrate that NF-Y is required for cyclin B1 promoter activity. We also demonstrate that cyclin B1 expression is regulated at the transcriptional level in TD C2C12 cells and that the switch-o of cyclin B1 promoter activity in dierentiated cells depends upon the loss of a functional NF-Y complex. In particular the loss of NF-YA protein is most likely responsible for its inactivation.

Previously published online as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/abstract.php?id=654 KEY WORDSCCAAT, NF-Y, histone fold ACKNOWLEDGEMENTSWe wish to thank Y. Chen for gift of NF-YCb vector, E. Lalli for gift of Leptomycin B and C. Vitale for his help with confocal images. R.M. is supported by grants from MIUR-COFIN and AIRC. Report Cell Cycle Regulation of NF-YC Nuclear LocalizationABSTRACT NF-Y is a trimeric activator with histone fold-HFM-subunits that binds to the CCAAT-box and is required for a majority of cell cycle promoters, often in conjuction with E2Fs. In vivo binding of NF-Y is dynamic during the cell cycle and correlates with gene activation. We performed immunofluorescence studies on endogenous, GFP-and Flag-tagged overexpressed NF-Y subunits. NF-YA, NF-YB are nuclear proteins. Unexpectedly, NF-YC localizes both in cytoplamatic and nuclear compartments and its nuclear localization is determined by the interaction with its heterodimerization partner NF-YB. Most importantly, compartmentalization is regulated during the cell cycle of serum restimulated NIH3T3 cells, accumulating in the nucleus at the onset of S phase. These data point to the control of HFM heterodimerization as an important layer of NF-Y regulation during cell cycle progression.

NF-Y is composed of three subunits, NF-YA, NF-YB, and NF-YC, all required for DNA binding. All subunits are expressed in proliferating skeletal muscle cells, whereas NF-YA alone is undetectable in terminally differentiated cells in vitro. By immunohistochemistry, we show that the NF-YA protein is not expressed in the nuclei of skeletal and cardiac muscle cells in vivo. By chromatin immunoprecipitation experiments, we demonstrate herein that NF-Y does not bind to the CCAAT boxes of target promoters in differentiated muscle cells. Consistent with this, the activity of these promoters is down-regulated in differentiated muscle cells. Finally, forced expression of the NF-YA protein in cells committed to differentiate leads to an impairment in the down-regulation of cyclin A, cyclin B1, and cdk1 expression and is accompanied by a delay in myogenin expression. Thus, our results indicate that the suppression of NF-Y function is of crucial importance for the inhibition of several cell cycle genes and the induction of the early muscle-specific program in postmitotic muscle cells.

Background: Nitric oxide (NO) regulates class I and IIa histone deacetylase (HDAC) function. NO production is regulated by class III HDACs (sirtuins). Results: NO functions as a bridging molecule between class I and sirtuins (SIRTs). Conclusion:The SIRT-NO-class I HDAC axis provides key signals during wound repair. Significance: Modulation of HDAC activity may play an important role in tissue regeneration.