Multifaceted roles of CCAR family proteins in the DNA damage response and cancer

The cell cycle apoptosis regulator (CCAR) family of proteins consists of two proteins, CCAR1 and CCAR2, that play a variety of roles in cellular physiology and pathology. These multidomain proteins are able to perform multiple interactions and functions, playing roles in processes such as stress responses, metabolism, and the DNA damage response. The evolutionary conservation of CCAR family proteins allows their study in model organisms such as Caenorhabditis elegans, where a role for CCAR in aging was revealed. This review particularly highlights the multifaceted roles of CCAR family proteins and their implications in the DNA damage response and in cancer biology.

The putative oncoprotein Bcl-3 induces cyclin D1 to stimulate G(1) transition

Bcl-3 is a distinctive member of the IkappaB family of NF-kappaB inhibitors because it can function to coactivate transcription. A potential involvement of Bcl-3 in oncogenesis is highlighted by the fact that it was cloned due to its location at a breakpoint junction in some cases of human B-cell chronic lymphocytic leukemia and that it is highly expressed in human breast tumor tissue. To analyze the effects of Bcl-3 dysregulation in breast epithelial cells, we created stable immortalized human breast epithelial cell lines either expressing Bcl-3 or carrying the corresponding vector control plasmid. Analysis of the Bcl-3-expressing cells suggests that these cells have a shortened G(1) phase of the cell cycle as well as a significant increase in hyperphosphorylation of the retinoblastoma protein. Additionally, the cyclin D1 gene was found to be highly expressed in these cells. Upon further analysis, Bcl-3, acting as a coactivator with NF-kappaB p52 homodimers, was demonstrated to directly activate the cyclin D1 promoter through an NF-kappaB binding site. Therefore, our results demonstrate that dysregulated expression of Bcl-3 potentiates the G(1) transition of the cell cycle by stimulating the transcription of the cyclin D1 gene in human breast epithelial cells.

The MHC-specific enhanceosome and its role in MHC class I and beta(2)-microglobulin gene transactivation

The promoter regions of MHC class I and beta(2)-microglobulin (beta(2)m) genes possess a regulatory module consisting of S, X, and Y boxes, which is shared by MHC class II and its accessory genes. In this study we show that, similar to MHC class II, the SXY module in MHC class I and beta(2)m promoters is cooperatively bound by a multiprotein complex containing regulatory factor X, CREB/activating transcription factor, and nuclear factor Y. Together with the coactivator class II transactivator this multiprotein complex drives transactivation of these genes. In contrast to MHC class II, the multiprotein complex has an additional function in the constitutive transactivation of MHC class I and beta(2)m genes. The requirement for all transcription factors in the complex and correct spacing of the binding sites within the SXY regulatory module for complex formation and functioning of this multiprotein complex strongly suggests that this complex can be regarded as a bona fide enhanceosome. The general coactivators CREB binding protein, p300, general control nonderepressible-5, and p300/CREB binding protein-associated factor exert an ancillary function in MHC class I and beta(2)m transactivation, but exclusively through the class II transactivator component of this enhanceosome. Thus, the SXY module is the basis for a specific enhanceosome important for the constitutive and inducible transactivation of MHC class I and beta(2)m genes.

The p65 (RelA) subunit of NF-kappaB interacts with the histone deacetylase (HDAC) corepressors HDAC1 and HDAC2 to negatively regulate gene expression

Regulation of NF-kappaB transactivation function is controlled at several levels, including interactions with coactivator proteins. Here we show that the transactivation function of NF-kappaB is also regulated through interaction of the p65 (RelA) subunit with histone deacetylase (HDAC) corepressor proteins. Our results show that inhibition of HDAC activity with trichostatin A (TSA) results in an increase in both basal and induced expression of an integrated NF-kappaB-dependent reporter gene. Chromatin immunoprecipitation (ChIP) assays show that TSA treatment causes hyperacetylation of the wild-type integrated NF-kappaB-dependent reporter but not of a mutant version in which the NF-kappaB binding sites were mutated. Expression of HDAC1 and HDAC2 repressed tumor necrosis factor (TNF)-induced NF-kappaB-dependent gene expression. Consistent with this, we show that HDAC1 and HDAC2 target NF-kappaB through a direct association of HDAC1 with the Rel homology domain of p65. HDAC2 does not interact with NF-kappaB directly but can regulate NF-kappaB activity through its association with HDAC1. Finally, we show that inhibition of HDAC activity with TSA causes an increase in both basal and TNF-induced expression of the NF-kappaB-regulated interleukin-8 (IL-8) gene. Similar to the wild-type integrated NF-kappaB-dependent reporter, ChIP assays showed that TSA treatment resulted in hyperacetylation of the IL-8 promoter. These data indicate that the transactivation function of NF-kappaB is regulated in part through its association with HDAC corepressor proteins. Moreover, it suggests that the association of NF-kappaB with the HDAC1 and HDAC2 corepressor proteins functions to repress expression of NF-kappaB-regulated genes as well as to control the induced level of expression of these genes.

Tumor necrosis factor alpha-induced phosphorylation of RelA/p65 on Ser529 is controlled by casein kinase II

Nuclear factor kappaB (NF-kappaB)/Rel transcription factors are key regulators of a variety of genes involved in immune and inflammatory responses, growth, differentiation, apoptosis, and development. In unstimulated cells, NF-kappaB/Rel proteins are sequestered in the cytoplasm by IkappaB inhibitor proteins. Many extracellular stimuli, such as tumor necrosis factor alpha (TNFalpha), cause rapid phosphorylation of IkappaB at N-terminal serine residues leading to ubiquitination and degradation of the inhibitor. Subsequently, NF-kappaB proteins translocate to the nucleus and activate gene expression through kappaB response elements. TNFalpha, as well as certain other stimuli, also induces the phosphorylation of the NF-kappaB proteins. Previously, we have shown that TNFalpha induces RelA/p65 phosphorylation at serine 529 and that this inducible phosphorylation increases NF-kappaB transcriptional activity on an exogenously supplied reporter (). In this report, we demonstrate that casein kinase II (CKII) interacts with p65 in vivo and can phosphorylate p65 at serine 529 in vitro. A CKII inhibitor (PD144795) inhibited TNFalpha-induced p65 phosphorylation in vivo. Furthermore, our results indicate that the association between IkappaBalpha and p65 inhibits p65 phosphorylation by CKII and that degradation of IkappaBalpha allows CKII to phosphorylate p65 to increase NF-kappaB transactivation potential. These data may explain the ability of CKII to modulate cell growth and demonstrate a mechanism whereby CKII can function in an inducible manner.

Orientation and positional mapping of the subunits of the multicomponent transcription factors RFX and X2BP to the major histocompatibility complex class II transcriptional enhancer

Major histocompatibility complex class II genes contain a common complex enhancer that allows for their coordinate regulation. The X box element of the enhancer cooperatively binds the multisubunit transcription factors RFX and X2BP. RFX is an essential class II transcription factor and contains three distinct proteins: RFX5, RFX-B/Ank and RFXAP. X2BP, a CREB/ATF family transcription factor, most likely binds as a homodimer. A site-specific protein-DNA photocrosslinking assay was used to investigate the interactions of the subunits of RFX and X2BP with X box DNA. Two of the RFX subunits, RFX5 and RFX-B/Ank, were found to bind defined sites within the X1 half of the X box. The third RFX subunit, RFXAP, made extensive X1 box contacts. The subunits of X2BP made contacts with the edges of the X2 half of the X box in a manner consistent with other bZIP transcription factor contact patterns. The resulting map provides specific base pair contacts and subunit orientation with respect to the DNA sequence of the RFX-X2BP-X box complex. Our results suggest possible stoichiometry of the RFX subunits and potential interaction between RFX-B/Ank and RFXAP with one of the subunits of X2BP.

HLA-DMA and HLA-DMB gene expression functions through the conserved S-X-Y region

The MHC class II homologous proteins HLA-DMA and HLA-DMB function in the loading of peptides onto class II molecules. Like the class II genes, the HLA-DM genes contain upstream regulatory sequences similar to the S-X-Y regulatory region as well as additional putative regulatory sites. To determine whether the DM genes are regulated in a similar manner as class II genes, a series of in vivo and in vitro analyses was performed. Deletion analysis showed that expression from the DM promoters is dependent on the conserved S-X-Y region. The class II-specific transcription factors RFX and CIITA are also required for expression, as cell lines deficient in these factors failed to allow transcription from the DM promoters. In addition, in vivo footprint analysis showed the putative X and Y boxes to be occupied by transcription factors in wild-type B cells, but not in RFX-deficient B cells. In astrocytes, IFN-gamma treatment induced increased occupancy of these sites. None of the other putative regulatory sites was occupied in vivo, indicating that they may not be functional. Finally, gel shift analysis showed synergistic complex formation between proteins that bind to the putative X boxes of the DM genes, as is found for the DRA gene. Therefore, the DM genes share a common mechanism of regulation with the class II genes.

HLA-DMA and HLA-DMB gene expression functions through the conserved S-X-Y region

The MHC class II homologous proteins HLA-DMA and HLA-DMB function in the loading of peptides onto class II molecules. Like the class II genes, the HLA-DM genes contain upstream regulatory sequences similar to the S-X-Y regulatory region as well as additional putative regulatory sites. To determine whether the DM genes are regulated in a similar manner as class II genes, a series of in vivo and in vitro analyses was performed. Deletion analysis showed that expression from the DM promoters is dependent on the conserved S-X-Y region. The class II-specific transcription factors RFX and CIITA are also required for expression, as cell lines deficient in these factors failed to allow transcription from the DM promoters. In addition, in vivo footprint analysis showed the putative X and Y boxes to be occupied by transcription factors in wild-type B cells, but not in RFX-deficient B cells. In astrocytes, IFN-gamma treatment induced increased occupancy of these sites. None of the other putative regulatory sites was occupied in vivo, indicating that they may not be functional. Finally, gel shift analysis showed synergistic complex formation between proteins that bind to the putative X boxes of the DM genes, as is found for the DRA gene. Therefore, the DM genes share a common mechanism of regulation with the class II genes.

Activation of class II MHC genes requires both the X box region and the class II transactivator (CIITA)

CIITA, a gene that can complement a transcriptional mutation of the major histocompatibility complex (MHC) class II genes, was tested for its ability to function as a coactivator, CIITA cDNA clones isolated showed alternative RNA splicing, but only one splice site combination was able to restore class II MHC gene expression. DNA-mediated transfection experiments showed that CIITA directs its activity through the X box element; the presence of CIITA leads to the formation of a higher order complex at the X box region; and CIITA contains a potent activation domain. These findings support the hypothesis that CIITA directly interacts with the MHC class II-specific transcription factors and is required for expression.