Human iPSC-Derived Neuronal Cells From CTBP1-Mutated Patients Reveal Altered Expression of Neurodevelopmental Gene Networks

A recurrent de novo mutation in the transcriptional corepressor CTBP1 is associated with neurodevelopmental disabilities in children (Beck et al., 2016, 2019; Sommerville et al., 2017). All reported patients harbor a single recurrent de novo heterozygous missense mutation (p.R342W) within the cofactor recruitment domain of CtBP1. To investigate the transcriptional activity of the pathogenic CTBP1 mutant allele in physiologically relevant human cell models, we generated induced pluripotent stem cells (iPSC) from the dermal fibroblasts derived from patients and normal donors. The transcriptional profiles of the iPSC-derived “early” neurons were determined by RNA-sequencing. Comparison of the RNA-seq data of the neurons from patients and normal donors revealed down regulation of gene networks involved in neurodevelopment, synaptic adhesion and anti-viral (interferon) response. Consistent with the altered gene expression patterns, the patient-derived neurons exhibited morphological and electrophysiological abnormalities, and susceptibility to viral infection. Taken together, our studies using iPSC-derived neuron models provide novel insights into the pathological activities of the CTBP1 p.R342W allele.

A pathogenic CtBP1 missense mutation causes altered cofactor binding and transcriptional activity

We previously reported a pathogenic de novo p.R342W mutation in the transcriptional corepressor CTBP1 in four independent patients with neurodevelopmental disabilities [1]. Here, we report the clinical phenotypes of seven additional individuals with the same recurrent de novo CTBP1 mutation. Within this cohort, we identified consistent CtBP1-related phenotypes of intellectual disability, ataxia, hypotonia, and tooth enamel defects present in most patients. The R342W mutation in CtBP1 is located within a region implicated in a high affinity-binding cleft for CtBP-interacting proteins. Unbiased proteomic analysis demonstrated reduced interaction of several chromatin-modifying factors with the CtBP1 W342 mutant. Genome-wide transcriptome analysis in human glioblastoma cell lines expressing -CtBP1 R342 (wt) or W342 mutation revealed changes in the expression profiles of genes controlling multiple cellular processes. Patient-derived dermal fibroblasts were found to be more sensitive to apoptosis during acute glucose deprivation compared to controls. Glucose deprivation strongly activated the BH3-only pro-apoptotic gene NOXA, suggesting a link between enhanced cell death and NOXA expression in patient fibroblasts. Our results suggest that context-dependent relief of transcriptional repression of the CtBP1 mutant W342 allele may contribute to deregulation of apoptosis in target tissues of patients leading to neurodevelopmental phenotypes.

CtBP2 proteome: Role of CtBP in E2F7-mediated repression and cell proliferation

C-terminal binding protein (CtBP) family transcriptional corepressors include CtBP1 and CtBP2. While CtBP1 and CtBP2 share significant amino acid sequence homology, CtBP2 possesses a unique N-terminal domain that is modified by acetylation and contributes to exclusive nuclear localization. Although CtBP1 and CtBP2 are functionally redundant for certain activities during vertebrate development, they also perform unique functions. Previous studies have identified several CtBP1-interacting proteins that included other transcriptional corepressors, DNA-binding repressors and histone modifying enzymatic components such as the histone deacetylases and the histone demethylase LSD-1. Here, we carried out an unbiased proteomic analysis of CtBP2-associated proteins and discovered the association of several components of the CtBP1 proteome as well as novel interactions. The CtBP2 proteome contained components of the NuRD complex and the E2F family member E2F7. E2F7 interacted with the hydrophobic cleft region of CtBP1 and CtBP2 through a prototypical CtBP binding motif, PIDLS. E2F7 repressed E2F1 transcription, inhibited cell proliferation in a CtBP-dependent fashion. Our study identified CtBP as a corepressor of E2F7 and as a regulator of DNA damage response.

Functional similarity between E6 proteins of cutaneous human papillomaviruses and the adenovirus E1A tumor-restraining module

The adenovirus E1A C-terminal region restrains oncogenic transformation through interaction with three distinct cellular protein complexes that include the DYRK1A/1B/HAN11 complex. The E6 proteins of beta-human papillomaviruses (beta-HPVs) also interact with the DYRK1/HAN11 complex. A variant of HPV5 E6 frequently found in epidermodysplasia verruciformis skin lesions interacted less efficiently with DYRK1A/HAN11. The E6 variant and E7 of HPV5 efficiently coimmortalized primary epithelial cells, suggesting that naturally arising variants may contribute potential oncogenic activities of beta-HPV E6 proteins.

Interaction of CtBP with adenovirus E1A suppresses immortalization of primary epithelial cells and enhances virus replication during productive infection

Adenovirus E1A induces cell proliferation, oncogenic transformation and promotes viral replication through interaction with p300/CBP, TRRAP/p400 multi-protein complex and the retinoblastoma (pRb) family proteins through distinct domains in the E1A N-terminal region. The C-terminal region of E1A suppresses E1A/Ras co-transformation and interacts with FOXK1/K2, DYRK1A/1B/HAN11 and CtBP1/2 (CtBP) protein complexes. To specifically dissect the role of CtBP interaction with E1A, we engineered a mutation (DL→AS) within the CtBP-binding motif, PLDLS, and investigated the effect of the mutation on immortalization and Ras cooperative transformation of primary cells and viral replication. Our results suggest that CtBP-E1A interaction suppresses immortalization and Ras co-operative transformation of primary rodent epithelial cells without significantly influencing the tumorigenic activities of transformed cells in immunodeficient and immunocompetent animals. During productive infection, CtBP-E1A interaction enhances viral replication in human cells. Between the two CtBP family proteins, CtBP2 appears to restrict viral replication more than CtBP1 in human cells.

Functional similarity between adenovirus e1b 19k gene and bcl2 oncogene - mutant complementation and suppression of cell-death induced by DNA-damaging agents

Adenovirus mutants defective in a 19 kD protein (19K gene) encoded by the early region E1B induce DNA fragmentation in a manner similar to the internucleosomal DNA fragmentation observed during programmed cell death (apoptosis) induced by a number of chemical, physical and biological stimuli. Zn++ ions effectively suppressed DNA fragmentation induced by a 19K mutant consistent with their inhibition of DNA fragmentation during apoptosis. Since the cellular proto-oncogene Bcl2 has been shown to suppress DNA fragmentation and the resulting programmed cell death, we have carried out a functional complementation analysis to determine whether the E1B 19 kD protein is functionally similar to the Bcl2 protein. Our results indicate that the DNA fragmentation induced by the 19K mutant can be efficiently suppressed by stable expression of human Bcl2 protein. Further, the 19 kD and Bcl2 proteins also function similarly to suppress DNA fragmentation and cell death induced by the DNA damaging agents cisplatin and UV.

Overexpression of BH3-Only Protein BNIP3 Leads to Enhanced Tumor Growth

BCL-2/E1B-19 kDa-interacting protein 3 (BNIP3) is a BH3-only mitochondrial protein. Expression of BNIP3 is strongly stimulated by hypoxia. Up-regulation of BNIP3 has been detected in several human carcinomas including carcinomas of the lung and breast. The significance of BNIP3 overexpression in these cancers is not known. To determine whether BNIP3 plays a role in tumor growth, we generated A549 lung carcinoma cells that overexpressed BNIP3 and examined their ability to form tumors in the mouse xenograft model. All cell lines that overexpressed BNIP3 formed larger tumors compared to the parental or vector-transformed A549 cells. Breast carcinoma cell lines that overexpressed BNIP3 also induced tumors in athymic mice in the absence of hormone administration, while the parental cell line did not. Stable shRNA-mediated knockdown of endogenous BNIP3 severely impaired the tumorigenic activity of A549 cells. The tumor growth-enhancing activity was reduced by deletion of the BH3 domain of BNIP3. Expression of a dominant-negative mutant of BNIP3 lacking the C-terminal transmembrane domain also inhibited the tumorigenic potential of A549 cells. These results suggest that BNIP3 plays a fundamental role in the development of certain solid tumors such as the lung and breast carcinomas.

Adenovirus type 5 E1A and E6 proteins of low-risk cutaneous beta-human papillomaviruses suppress cell transformation through interaction with FOXK1/K2 transcription factors

The adenovirus (Adv) oncoprotein E1A stimulates cell proliferation and inhibits differentiation. These activities are primarily linked to the N-terminal region (exon 1) of E1A, which interacts with multiple cellular protein complexes. The C terminus (exon 2) of E1A antagonizes these processes, mediated in part through interaction with C-terminal binding proteins 1 and 2 (CtBP1/2). To identify additional cellular E1A targets that are involved in the modulation of E1A C-terminus-mediated activities, we undertook tandem affinity purification of E1A-associated proteins. Through mass spectrometric analysis, we identified several known E1A-interacting proteins as well as novel E1A targets, such as the forkhead transcription factors, FOXK1/K2. We identified a Ser/Thr-containing sequence motif in E1A that mediated interaction with FOXK1/K2. We demonstrated that the E6 proteins of two beta-human papillomaviruses (HPV14 and HPV21) associated with epidermodysplasia verruciformis also interacted with FOXK1/K2 through a motif similar to that of E1A. The E1A mutants deficient in interaction with FOXK1/K2 induced enhanced cell proliferation and oncogenic transformation. The hypertransforming activity of the mutant E1A was suppressed by HPV21 E6. An E1A-E6 chimeric protein containing the Ser/Thr domain of the E6 protein in E1A interacted efficiently with FOXK1/K2 and inhibited cell transformation. Our results suggest that targeting FOXK1/K2 may be a common mechanism for certain beta-HPVs and Adv5. E1A exon 2 mutants deficient in interaction with the dual-specificity kinases DYRK1A/1B and their cofactor HAN11 also induced increased cell proliferation and transformation. Our results suggest that the E1A C-terminal region may suppress cell proliferation and oncogenic transformation through interaction with three different cellular protein complexes: FOXK1/K2, DYRK(1A/1B)/HAN11, and CtBP1/2.

BH3-only proteins in apoptosis and beyond: an overview

BH3-only BCL-2 family proteins are effectors of canonical mitochondrial apoptosis. They discharge their pro-apoptotic functions through BH1-3 pro-apoptotic proteins such as BAX and BAK, while their activity is suppressed by BH1-4 anti-apoptotic BCL-2 family members. The precise mechanism by which BH3-only proteins mediate apoptosis remains unresolved. The existing data are consistent with three mutually non-exclusive models (1) displacement of BH1-3 proteins from complexes with BH1-4 proteins; (2) direct interaction with and conformational activation of BH1-3 proteins; and (3) membrane insertion and membrane remodeling. The BH3-only proteins appear to play critical roles in restraining cancer and inflammatory diseases such as rheumatoid arthritis. Molecules that mimic the effect of BH3-only proteins are being used in treatments against these diseases. The cell death activity of a subclass of BH3-only members (BNIP3 and BNIP3L) is linked to cardiomyocyte loss during heart failure. In addition to their established role in apoptosis, several BH3-only members also regulate diverse cellular functions in cell-cycle regulation, DNA repair and metabolism. Several members are implicated in the induction of autophagy and autophagic cell death, possibly through unleashing of the BH3-only autophagic effector Beclin 1 from complexes with BCL-2/BCL-xL. The Chapters included in the current Oncogene Review issues provide in-depth discussions on various aspects of major BH3-only proteins.

Interaction of ZEB and histone deacetylase with the PLDLS-binding cleft region of monomeric C-terminal binding protein 2

Background

Proteins of the C-terminal binding protein (CtBP) family, CtBP1 and CtBP2 are closely related transcriptional regulators that are coded by two different gene loci in the vertebrate genomes. They perform redundant and unique functions during animal development. CtBP proteins mediate their transcriptional function through interaction with various DNA-binding repressors that contain PLDLS-like motifs and chromatin modifying enzymes, such as class I histone deacetylases (HDAC) that do not contain such motifs. The N-terminal region of CtBP1/2 forms a hydrophobic cleft and is involved in interaction with both PLDLS-containing factors and non-PLDLS factors. CtBP proteins function as dimers to mediate transcriptional repression and dimerization is modulated by specific binding to NAD/NADH.

Results

In this study, we have investigated the role of dimerization of CtBP2 in recruitment of PLDLS-motif cofactors and non-PLDLS cofactors. Our results indicate that mutations in CtBP2 that interfere with dimerization abolish CtBP2 interaction with most cellular factors, except the PLDLS-motif factor zinc-finger E-box binding homeobox (ZEB) and the non-PLDLS factor HDAC2. Unlike most PLDLS-containing CtBP-binding proteins, ZEB contains three PLDLS-like motifs and all three contribute to the interaction with the CtBP2 monomer. Despite the ability to interact with ZEB and HDAC, the CtBP2 monomer fails to mediate ZEB-dependent transcriptional repression. The lack of repression activity of the CtBP2 monomer is correlated with the competition between ZEB and HDAC for interaction with the CtBP2 monomer.

Conclusion

These results suggest a competition between the canonical PLDLS-motif factors such as E1A and non-PLDLS factor HDAC for interaction with CtBP. They also indicate that the affinity for the CtBP monomer may be determined by the number as well as amino acid sequence compositions of the PLDLS-like motifs. Our results are consistent with a model that the CtBP2 dimer may interact with a PLDLS-containing repressor through one monomer and recruit HDAC and other chromatin modifying enzymes through the second monomer in the CtBP2 dimer.

BAX/BAK-independent mitoptosis during cell death induced by proteasome inhibition?

Proteasome inhibitors induce rapid death of cancer cells. We show that in epithelial cancer cells, such death is associated with dramatic and simultaneous up-regulation of several BH3-only proteins, including BIK, BIM, MCL-1S, NOXA, and PUMA, as well as p53. Elevated levels of these proteins seem to be the result of direct inhibition of their proteasomal degradation, induction of transcription, and active translation. Subsequent cell death is independent of BAX, and probably BAK, and proceeds through the intrinsic mitochondrial apoptosis pathway. We identify the cascade of molecular events responsible for cell death induced by a prototypical proteasome inhibitor, MG132, starting with rapid accumulation of BH3-only proteins in the mitochondria, proceeding through mitochondrial membrane permeabilization and subsequent loss of DeltaPsi(m), and leading to irreversible changes of mitochondrial ultrastructure, degradation of mitochondrial network, and detrimental impairment of crucial mitochondrial functions. Our results also establish a rationale for the broader use of proteasome inhibitors to kill apoptosis-resistant tumor cells that lack functional BAX/BAK proteins.

The transcriptional corepressor CtBP: a foe of multiple tumor suppressors

CtBP1 and CtBP2 are closely related and evolutionarily conserved transcriptional corepressors. There is strong evidence linking CtBPs to tumorigenesis and tumor progression. CtBPs promote epithelial-mesenchymal transition and function as apoptosis antagonists. Also, CtBPs mediate repression of several tumor suppressor genes. Certain tumor suppressors also target CtBPs to restrain their tumor-promoting activity. Down-regulation of CtBPs mediated by some tumor suppressors results in p53-independent apoptosis and reduced tumor cell migration and invasion. The role of CtBPs in modulating the activities of different tumor suppressors is reviewed here. The results discussed here suggest that CtBPs may constitute a novel p53-independent anticancer target.

Inhibition of transcriptional activation and cell proliferation activities of adenovirus E1A by the unique N-terminal domain of CtBP2

The 243-residue E1A protein of adenovirus induces cellular proliferation, at least partly by regulating the transcription of cellular genes. This E1A function requires E1A N-terminal region and conserved regions 1 and 2 (CR1 and CR2), which interact with histone acetyl transferases, p400 chromatin-modifying complex and the Rb family proteins. A PLDLS motif at the E1A C-terminal (CR4) region, interacts with the C-terminal binding proteins (CtBP1 and CtBP2), and antagonizes some E1A functions. In this report, we discovered that the transcriptional activation function of E1A was specifically repressed by a short N-terminal domain unique to CtBP2. The CtBP2-mediated repression of E1A transcriptional activation activity is independent of histone deacetylases, which can be recruited by CtBP1/2 proteins to inhibit transcription. Fusion of the CtBP2 N-terminal 20 residues to the E1A C-terminal region rendered E1A to be inactive in transcriptional activation without interfering with E1A's ability to interact with major cofactors such as pRb, p400 and p300. Substitution of the N-terminal domain of CtBP1 for the CtBP2 domain in E1A-CtBP2 fusion partially restored the transactivation activity of E1A. In a cell-proliferation model utilizing primary baby rat kidney cells and retrovirally expressed E1A, the ability of E1A to induce cellular proliferation was strongly inhibited when the CtBP2 N-terminal region was fused to E1A. These results are consistent with a hypothesis that CtBP2 may inhibit E1A induced cell proliferation by antagonizing the transcriptional activation function controlled by the N-terminal region of E1A.

Role of the PLDLS-binding cleft region of CtBP1 in recruitment of core and auxiliary components of the corepressor complex

C-terminal binding protein (CtBP) family proteins CtBP1 and CtBP2 are highly homologous transcriptional corepressors and are recruited by a large number of transcription factors to mediate sequence-specific transcriptional repression. In addition to DNA-binding repressors, the nuclear protein complex of CtBP1 consists of enzymatic constituents such as histone deacetylases (HDAC1/2), histone methyl transferases (HMTases; G9a and GLP), and the lysine-specific demethylase (LSD1). Additionally, CtBPs also recruit the components of the sumoylation machinery. The CtBPs contain two different unique structural elements, a hydrophobic cleft, with which factors that contain motifs related to the E1A PLDLS motif bind, and a surface groove that binds with factors containing motifs related to the sequence RRTGXPPXL (RRT motif). By structure-based functional dissection of CtBP1, we show that the PLDLS-binding cleft region functions as the primary recruitment center for DNA-binding factors and for the core and auxiliary enzymatic constituents of the CtBP1 corepressor complex. We identify HDAC1/2, CoREST/LSD1, and Ubc9 (E2) as the core constituents of the CtBP1 complex, and these components interact with the PLDLS cleft region through non-PLDLS interactions. Among the CtBP core constituents, HDACs contribute predominantly to the repression activity of CtBP1. The auxiliary components include an HMTase complex (G9a/Wiz/CDYL) and two SUMO E3 ligases, HPC2 and PIAS1. The interaction of auxiliary components with CtBP1 is excluded by PLDLS (E1A)-mediated interactions. Although monomeric CtBP1 is proficient in the recruiting of both core and auxiliary components, NAD(H)-dependent dimerization is required for transcriptional repression. We also provide evidence that CtBP1 functions as a platform for sumoylation of cofactors.

BH3-only protein BIK induces caspase-independent cell death with autophagic features in Bcl-2 null cells

The BH3-only protein BIK normally induces apoptotic cell death. Here, we have investigated the role of BCL-2 in BIK-induced cell death using Bcl-2+/+ and Bcl-2-/- mouse embryo fibroblasts. Ectopic expression of BIK in Bcl-2-/- cells resulted in enhanced cell death compared to Bcl-2+/+ cells. In these cells, while caspase-8 was activated, there was no significant activation of caspase-9 and 3. There was no detectable mitochondrial to cytosolic release of cytochrome-c. However, there was significant redistribution of AIF from mitochondria to the nucleus. The extent of BIK-induced cell death was augmented by treatment with the pancaspase inhibitor, zVAD-fmk. The Bcl-2 null cells expressing BIK exhibited autophagic features such as cytosolic vacuoles, punctate distribution of LC3 and enhanced expression of Beclin-1. The survival of BIK-expressing Bcl-2-/- cells was enhanced in the presence of PI3 kinase inhibitors 3-methyladenine and Wortmannin and also by depletion of Atg5 and Beclin-1. Death of BIK-expressing Bcl-2-/- cells treated with zVAD-fmk was increased under caspase-8 depletion. Our results suggest enhanced expression of BIK in the Bcl-2 deficient cells leads to cell death with autophagic features and the extent of such cell death could be increased by inhibition of caspases.

Evidence for involvement of BH3-only proapoptotic members in adenovirus-induced apoptosis

Mammalian cells infected with human adenoviruses (Ads) undergo an apoptotic response as a result of expression of the viral E1A proteins, and this process is suppressed by the viral E1B-19K protein. The intermediary steps in the Ad-induced apoptosis pathway are not fully resolved. The apical step in the canonical mammalian apoptosis pathway involves functional activation of one or more of the BH3-only BCL-2 family proapoptotic proteins. Previous reports have suggested that Ad-induced apoptosis may be initiated at checkpoints downstream of the BH3-only proteins. Here, we undertook genetic and biochemical studies to determine the roles of BH3-only proteins in Ad-induced apoptosis. We examined the activities of the cellular antiapoptosis protein BCL-xL and its mutants expressed from the E1B region of the Ad5 genome. Our results showed efficient suppression of Ad-induced apoptosis by a BCL-xL mutant (mt1) deficient in interaction with multidomain proapoptotic proteins BAX and BAK but proficient in interaction with BH3-only proteins, suggesting a role for BH3-only proteins in the initiation of Ad-induced apoptosis. Further, the antiapoptotic activity of BCL-xL mt1 in Ad-infected cells was observed in spite of BAK activation as a consequence of MCL-1 degradation. Analysis of the mRNA levels of various BH3-only members by reverse transcription-PCR revealed prominent activation of the Bik gene. Further, the BIK protein was also modified into an apoptotically enhanced phosphorylated form during the viral infection. In addition to BIK, enhanced level of BIM was observed in Ad-infected cells. Between the two major E1A proteins coded by the 12S and 13S mRNAs, the 13S product appeared to contribute to the activation of these BH3-only members and apoptosis during viral infection. Depletion of BIK by the use of small interfering RNA reduced the level of Ad-induced apoptosis. Our results are consistent with a model that activation of the BH3-only members may initiate Ad-induced apoptosis.

PLDLS-dependent interaction of E1A with CtBP: regulation of CtBP nuclear localization and transcriptional functions

C-terminal binding proteins (CtBPs) are cellular corepressors that are targeted by adenovirus E1A. A conserved motif of E1A (PLDLS) interacts with an N-terminal hydrophobic cleft of CtBPs. Many cellular cofactors also interact with CtBPs through PLDLS-like motifs. E1A interaction with CtBP2 changed the composition of the CtBP2 protein complex and enhanced CtBP2 acetylation. We have identified a mutant of CtBP2 (M48A) that fails to interact with cellular cofactors while interacting normally with E1A. Other cleft mutations in CtBP2 affected interaction of both cellular cofactors and E1A. The M48A mutant did not repress the cellular E-cadherin promoter but inhibited transactivation mediated by the E1A N-terminal region through interaction with the E1A PLDLS motif. In vitro, E1A enhanced CtBP2 acetylation by p300 via a mechanism involving dissociation of acetylated CtBP2 from p300. E1A enhanced nuclear localization of CtBP1 as well as a cytoplasmically localized acetylation-deficient mutant of CtBP2 (3KR-CtBP2) through PLDLS-dependent interaction. Chromatin immunoprecipitation assays revealed presence of CtBP2 on E-cadherin and c-fos promoters. While E1A did not significantly alter targeting of CtBP2 to the E-cadherin and c-fos promoters, it dramatically enhanced promoter targeting of 3KR-CtBP2. Our results raise a possibility that E1A may gain access to cellular promoters through PLDLS-dependent interaction with CtBPs.

Transcriptional regulation by C-terminal binding proteins

C-terminal binding protein family members function predominantly as transcriptional corepressors in association with sequence specific DNA-binding transcriptional repressors. The vertebrates have two CtBP genes while the invertebrates contain a single gene. Genetic studies indicate that the CtBP genes play pivotal roles in animal development. The vertebrate C-terminal binding proteins (CtBP1 and CtBP2) are highly related and are functionally redundant for certain developmental processes and non-redundant for others. The animal C-terminal binding proteins exhibit structural and functional similarity to d-isomer-specific 2-hydroxy acid dehydrogenases (D2-HDH). They function as dimers, recruiting transcriptional regulators through two protein-binding interfaces in each monomer. The corepressor complex of CtBP1 contains enzymatic constituents that mediate coordinated histone modification by deacetylation and methylation of histone H3-Lysine 9 and demethylation of histone H3-Lysine 4. CtBP also recruits the small ubiquitin-related modifier (SUMO) conjugating E2 enzyme UBC9 and a SUMO E3 ligase (HPC2), suggesting that CtBP-mediated transcriptional regulation may also involve SUMOylation of transcription factors. In addition to gene-specific transcriptional repression, CtBP1 appears to antagonize the activity of the global transcriptional coactivators, p300/CBP. Genetic evidence also suggests that the fly CtBP (dCtBP) and the vertebrate CtBP2 might activate transcription in a context-dependent manner. The transcriptional regulatory activity of CtBP is modulated by the nuclear NADH/NAD+ ratio and hence appears to be influenced by the metabolic status of the cell. The nuclear dinucleotide ratio may differentially influence the repression activities of factors that recruit CtBP through PLDLS-like motifs and those through non-PLDLS-motifs.

Changes in C-terminal binding protein 2 (CtBP2) corepressor complex induced by E1A and modulation of E1A transcriptional activity by CtBP2

The N-terminal region of adenovirus E1A interacts with histone acetyl transferases (HATs) such as p300, P/CAF, and GCN5. The C-terminal region interacts with the transcriptional corepressors CtBP1 and CtBP2. The functional significance of co-recruitment of HATs and CtBPs by E1A is not well understood. In this study, we have shown that E1A enhanced acetylation of CtBP2 by recruitment of p300 to the CtBP2 complex. Additionally, E1A also displaced the histone methyltransferase G9a and the E-box repressor ZEB from the CtBP2 complex through the C-terminal CtBP-binding domain. A transcriptional activation function encoded by the E1A N-terminal region was efficiently inhibited by CtBP2 but not by a mutant with an N-terminal deletion or by a mutant deficient in interaction with E1A. Two isoforms of CtBP1 (CtBP1-L and CtBP1-S) poorly inhibited transcriptional activity of the E1A N-terminal region. Thus, the N-terminal domain of CtBP2 may contribute a unique transcriptional regulatory activity of CtBP2. Our results provide new insights by which CtBP might modulate the biochemical activities of E1A.

Bak functionally complements for loss of Bax during p14ARF-induced mitochondrial apoptosis in human cancer cells

In contrast to the initial notion that the biological activity of p14(ARF) strictly depends on a functional mdm-2/p53 signaling axis, we recently demonstrated that p14(ARF) mediates apoptosis in a p53/Bax-independent manner. Here, we show that p14(ARF) induces breakdown of the mitochondrial membrane potential and cytochrome c release before triggering caspase-9- and caspase-3/7-like activities in p53/Bax-deficient DU145 prostate cancer cells expressing wild-type Bak. Re-expression of Bax in these cells failed to further enhance p14(ARF)-induced apoptosis, suggesting that p14(ARF)-induced apoptosis primarily depends on Bak but not Bax in these cells. To further define the role of Bak and Bax in p14(ARF)-induced mitochondrial apoptosis, we employed short interference RNA for the knockdown of bak in isogeneic, p53 wild-type HCT116 colon cancer cells either proficient or deficient for Bax. There, combined loss of Bax and Bak attenuated p14(ARF)-induced apoptosis whereas single loss of Bax or Bak was only marginally effective, as in the case of DU145. Notably, HCT116 cells deficient for Bax and Bak failed to release cytochrome c and showed attenuated activation of caspase-9 (LEHDase) and caspase-3/caspase-7 (DEVDase) upon p14(ARF) expression. These data indicate that p14(ARF) triggers apoptosis via a Bax/Bak-dependent pathway in p53-proficient HCT116, whereas Bax is dispensable in p53-deficient DU145 cells. Nevertheless, a substantial proportion of p14(ARF)-induced cell death proceeds in a Bax/Bak-independent manner. This is also the case for inhibition of clonogenic growth that occurs, at least in part, through an entirely Bax/Bak-independent mechanism.

Genetic identification of adenovirus type 5 genes that influence viral spread

The mechanisms that control cell-to-cell spread of human adenoviruses (Ad) are not well understood. Two early viral proteins, E1B-19K and E3-ADP, appear to have opposing effects since viral mutants that are individually deficient in E1B-19K produce large plaques (G. Chinnadurai, Cell 33:759-766, 1983), while mutants deficient in E3-ADP produce small plaques (A. E. Tollefson et al., J. Virol. 70:2296-2306, 1996) on infected cell monolayers. We have used a genetic strategy to identify different viral genes that influence adenovirus type 5 (Ad5) spread in an epithelial cancer cell line. An Ad5 mutant (dl327; lacking most of the E3 region) with the restricted-spread (small-plaque) phenotype was randomly mutagenized with UV, and 27 large-plaque (lp) mutants were isolated. A combination of analyses of viral proteins and genomic DNA sequences have indicated that 23 mutants contained lesions in the E1B region affecting either 19K or both 19K and 55K proteins. Four other lp mutants contained lesions in early regions E1A and E4, in the early L1 region that codes for the i-leader protein, and in late regions that code for the viral structural proteins, penton base, and fiber. Our results suggest that the requirement of E3-ADP for Ad spread could be readily compensated for by abrogation of the functions of E1B-19K and provide genetic evidence that these two viral proteins influence viral spread in opposing manners. In addition to E1B and E3 proteins, other early and late proteins that regulate viral replication and infectivity also influence lateral viral spread. Our studies have identified novel mutations that could be exploited in designing efficient oncolytic Ad vectors.

Acetylation by p300 Regulates Nuclear Localization and Function of the Transcriptional Corepressor CtBP2

CtBP family members, CtBP1 and CtBP2, are unique transcriptional regulators that adapt a metabolic enzyme fold, and their activities are regulated by NAD(H)-binding. CtBP1 is both cytoplasmic and nuclear, and its subcellular localization is regulated by sumoylation, phosphorylation, and binding to a PDZ protein. In contrast, we showed that CtBP2 is exclusively nuclear. CtBP1 and CtBP2 are highly similar, but differ at the N-terminal 20 amino acid region. Substitution of the N-terminal domain of CtBP1 with the corresponding CtBP2 domain confers a dominant nuclear localization pattern to CtBP1. The N-terminal domain of CtBP2 contains three Lys residues. Our results show that these Lys residues are acetylated by the nuclear acetylase p300. Although all three Lys residues of CtBP2 (Lys-6, Lys-8, and Lys-10) appear to be acetylated, acetylation of Lys-10 is critical for nuclear localization. CtBP2 with a single amino acid substitution at Lys-10 (K10R) is predominantly localized in the cytoplasm. The cytoplasmic localization of the K10R mutant is correlated with enhanced nuclear export that is inhibited by leptomycin B. Furthermore, lack of acetylation at Lys-10 renders CtBP2 to be more efficient in repression of the E-cadherin promoter. Our studies have revealed the important roles of acetylation in regulating subcellular localization and transcriptional activity of CtBP2.

CtIP, a candidate tumor susceptibility gene is a team player with luminaries

CtIP is a nuclear protein conserved among vertebrates that was discovered as a cofactor of the transcriptional corepressor CtBP. CtIP also interacts with the tumor suppressors such as BRCA1 and the pRb family members through binding sites that are frequently mutated in human cancers. CtIP is a target for BRCA1-dependent phosphorylation by the ATM kinase induced by DNA double strand breakage. CtIP plays a role in DNA-damage-induced cell cycle checkpoint control at the G2/M transition. Homozygous inactivation of the Ctip gene causes very early embryonic lethality during mouse development. The Ctip(-/-) embryo cells are arrested in G1 and do not enter S phase. Depletion of Ctip in established mouse embryo fibroblasts arrests cells in G1 and results in an accumulation of hypophosphorylated Rb and the Cdk inhibitor p21, suggesting that CtIP is also a critical regulator of G1/S transition of the cell cycle. The Ctip gene contains a mononucleotide (A9) repeat and one of the alleles is mutated at a high frequency in colon cancers with microsatellite instability. The Ctip(+/-) mice develop multiple types of tumors suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Among the various tumor types observed in Ctip(+/-) heterozygous mice, large lymphomas are prevalent. Recent studies raise the possibility that Ctip may itself be a tumor susceptibility gene and suggest that it might be important for the activities of tumor suppressors BRCA1, pRb family proteins and Ikaros family members.

Mitochondrial localization of p53 during adenovirus infection and regulation of its activity by E1B-19K

Recent results have revealed that the p53 tumor suppressor protein possesses a direct transcription-independent apoptotic activity. During apoptosis induced by genotoxic stress, a small fraction of p53 is targeted to mitochondria where it initiates apoptosis by causing mitochondrial dysfunction. In adenovirus-infected cells, the expression of E1A protein enhances the accumulation of p53 during early phases of infection and during late times after infection, it is targeted for degradation by the combined action of E1B-55K and E4-orf6 proteins. The functional significance of E1A-mediated accumulation of p53 during early phases of viral replication is not known. Our studies with isogenic epithelial cell lines that differ only on the status of p53 indicate that Ad infection induces apoptosis by p53-dependent and -independent pathways and both pathways are suppressed by E1B-19K. We show that during early phase of Ad infection, a fraction of p53 is targeted to the mitochondria. In virus infected cells, a large fraction of the viral antiapoptosis protein E1B-19K is also localized in mitochondria during early and late phases of infection. Coimmunoprecipitation analysis has revealed that p53 and E1B-19K form a complex in mitochondria. The interaction of 19K involves two noncontiguous regions located around amino-acid residues 14-15 and 123-124. On p53, the mutations within the DNA-binding domain reduce interaction with E1B-19K. Our studies also suggest that 19K may additionally complex with the multidomain mitochondrial proapoptotic protein BAK, thereby reducing the level of p53 interaction with BAK. We suggest that p53-induced apoptosis may be important for efficient cell lysis and viral spread and that E1B-19K may neutralize the apoptotic activity of p53 at multiple levels.

Modulation of oncogenic transformation by the human adenovirus E1A C-terminal region

The E1A oncogene of human adenoviruses cooperates with other viral and cellular oncogenes in oncogenic transformation of primary and established cells. The N-terminal half of E1A proteins that form specific protein complexes with pRb family and p300/CBP transcriptional regulators is essential for the transforming activities of E1A. Although the C-terminal half of E1A is dispensable for the transforming activities, it negatively modulates the oncogenic activities of the N-terminal region. Mutants of E1A lacking the C-terminal half or a short C-terminal region exhibit a hyper-transforming phenotype in cooperative transformation assays with the activated ras oncogene. The E1A C-terminal region implicated in the oncogenesis-restraining activity interacts with a 48-kDa cellular phosphoprotein, CtBP, that functions as a transcriptional corepressor. It appears that the C-terminal region of E1A may suppress E1A-mediated oncogenic transformation by a dual mechanism of relieving repression cellular genes by CtBP, and also by antagonizing the oncogenic activities of the N-terminal half of E1A.

Pro-apoptotic activity of transiently expressed BCL-2 occurs independent of BAX and BAK

BCL-2 suppresses apoptosis induced by a wide variety of stimuli in multiple cell types. Most of the in vitro studies that have examined the activity of BCL-2 have employed stable cell lines that ectopically express BCL-2. We have reported that BCL-2 is expressed at high levels in the absence of the 5'- and 3'-UTRs of the Bcl-2 gene and transient high level of expression results in potent cell death (Uhlmann et al., [1998]: JBC 278:17926-17932). Expression of BCL-2 under the transcriptional control of the cognate 5'- and 3'-UTRs express lower levels of BCL-2 and does not cause cell death. Our present results suggest that in contrast to BCL-2, transient expression of BCL-xL does not induce cell death and coexpression of BCL-xL with the pro-apoptotic BCL-2 does not suppress cell death. The pro-apoptotic activity of BCL-2 appears to involve activation of the cytochrome c/caspase 9/caspase 3 pathway. Elevated levels of BCL-2 expression results in N-terminal cleavage of BCL-2 at a novel site different from a previously identified caspase cleavage site at Asp 34 by a non-caspase protease. Transient expression of a BCL-2 mutant lacking aa 51-85 within the loop region induces efficient cell death and N-terminal cleavage of BCL-2 while a different deletion mutant lacking aa 30-91 induces reduced levels of cell death in the absence of BCL-2 cleavage suggesting that N-terminal processing of BCL-2 may be an amplification event in BCL-2-mediated cell death. Overexpression of BCL-2 in a Bax-null human colon cancer cell line (HCT116Bax-/-) induces efficient cell death. The pro-apoptotic activity of BCL-2 is also observed in a Bax-null cells in which BAK expression is inhibited by stable RNAi expression. Our results suggest that BCL-2 contains an intrinsic pro-apoptotic activity and can induce apoptosis independent of BAX and BAK under specific conditions.

A nuclear kinesin-like protein interacts with and stimulates the activity of the leucine-rich nuclear export signal of the human immunodeficiency virus type 1 rev protein

The Rev protein of human immunodeficiency virus type 1 (HIV-1) is essential for the nucleocytoplasmic transport of unspliced and partially spliced HIV mRNAs containing the Rev response element (RRE). In a yeast two-hybrid screen of a HeLa cell-derived cDNA expression library for human factors interacting with the Rev leucine-rich nuclear export sequence (NES), we identified a kinesin-like protein, REBP (Rev/Rex effector binding protein), highly homologous to Kid, the carboxy-terminal 75-residue region of which interacts specifically with the NESs of HIV-1 Rev, human T-cell leukemia virus type 1 Rex, and equine infectious anemia virus Rev but not with functionally inactive mutants thereof. REBP is a nuclear protein that colocalizes with Rev in the nucleoplasm and nuclear periphery of transfected cells. Specific, albeit weak, interaction between REBP and Rev could be demonstrated in coimmunoprecipitation assays in BSC-40 cells. REBP can modestly enhance Rev-dependent RRE-linked reporter gene expression both independently and in cooperation with the nucleoporin cofactor Rab/hRIP. Thus, REBP displays the characteristics expected of an authentic mediator of Rev NES function and may play a role in RRE RNA transport during HIV infection.

Association of class I histone deacetylases with transcriptional corepressor CtBP

The C-terminal binding protein (CtBP) family proteins are transcriptional regulators that are conserved from worm to human. They function as corepressors of a wide array of DNA-binding transcriptional repressors. The mammalian CtBPs appear to mediate transcriptional repression in a histone deacetylase (HDAC)-dependent or -independent manner, depending on the context of the promoter. To identify the components of the CtBP corepressor complex, we isolated CtBP-containing protein complexes from the nuclear extracts prepared from HeLa cells infected with adenovirus vectors that expresses hCtBP1. Western blot analysis of these complexes suggests that hCtBP1 associates with class I HDACs, HDAC-1, HDAC-2 and HDAC-3. Some of these HDACs also interact with the Drosophila CtBP homolog, dCtBP. The CtBP protein complex exhibits significant HDAC activity in vitro suggesting that association of CtBP with HDACs may be functionally relevant.

Nicotinamide adenine dinucleotide stimulates oligomerization, interaction with adenovirus E1A and an intrinsic dehydrogenase activity of CtBP

The C-terminal region of adenovirus E1A interacts with the transcriptional corepressor, CtBP. The mechanism of transcriptional regulation by CtBP is not known. CtBP shares a significant homology with NAD(+)-dependent D2-hydroxy acid dehydrogenases. CtBP binds to NAD(+) and NADH. Both forms of the dinucleotide stimulate oligomerization of native CtBP and enhance complex formation with E1A. CtBP also has a slow dehydrogenase activity. Interaction of CtBP with E1A reduces the dehydrogenase activity. Our results raise the possibility that the oxidation/reduction reactions of CtBP may regulate transcription. Thus, CtBP is a unique transcriptional regulator with an enzymatic activity similar to metabolic dehydrogenases. The levels of intracellular nicotinamide adenine dinucleotide may modulate transcriptional activity of CtBP.

CtBP family proteins: more than transcriptional corepressors

CtBP family proteins predominantly function as transcriptional corepressors. Studies with mutant mouse suggest that the two mouse genes, Ctbp1 and Ctbp2, play unique and redundant gene regulatory roles during development.1 Ctbp1-deficient mice are viable, but are small and die early, while Ctbp2 deficiency leads to embryonic lethality. Ctbp2-null mutation causes defects in axial patterning, heart morphogenesis and neural development. The Ctbp2 mutant phenotype is more severe in the absence of Ctbp1. The studies with Ctbp2 mutant embryos suggest that CtBP can also activate transcription. A plant CtBP homolog, Angustifolia (AN), has recently been identified.2,3 AN controls polar elongation of leaf cells via the microtubule cytoskeleton. Microarray analysis suggests that AN also functions as a transcriptional repressor. Thus, the CtBP family proteins control cellular processes by serving as transcriptional activators and regulators of the cytoskeleton as well as transcriptional corepressors.

Sox9 transactivation and testicular expression of a novel human gene, KIAA0800

The Sry and Sox9 sex-determination factors initiate and promote testis differentiation by gene transactivation through similar promoter elements. However, knowledge is limited concerning what genes are regulated by Sry/Sox9 in the testis. Identification and characterization of Sry/Sox9-regulated genes are critical for understanding sexual differentiation. We now demonstrate that a novel human gene, KIAA0800, is preferentially expressed in the testis and is transactivated by Sox9. The KIAA0800 promoter is repressed by an upstream element involving a polyT track and two Alu repeats. Two specific Sox9-bindings sites have been identified in the KIAA0800 promoter by using DNaseI footprinting assays and gel electrophoretic mobility shift assays. Sox9 transactivation of the KIAA0800 promoter appears to be exerted mainly through the relief of promoter repression. Genes homologous to the human KIAA0800 exist in organisms with differentiated sex tissues including mouse, Drosophila, and C. elegans, but not in unicellular organisms, including yeast and bacteria. Further, our recent sequence analysis shows that KIAA0800 protein is 97% identical between human and mouse. Thus, KIAA0800 is a novel Sox9-activated gene that is evolutionarily conserved and potentially involved in sexual differentiation.

Requirement of BAX for efficient adenovirus-induced apoptosis

Infection of human epithelial cells with adenoviruses induces an apoptosis paradigm that is efficiently suppressed by the expression of viral E1B-19K protein, which is a functional homolog of the cellular antiapoptosis protein BCL-2. The mechanisms of adenovirus (Ad)-induced apoptosis appear to involve the cellular BCL-2 family proapoptotic proteins. Recent genetic studies with fibroblasts derived from mutant mouse embryos indicate that a class of the BCL-2 family proapoptotic proteins (designated BH-123 or multidomain proteins) such as BAX and BAK constitutes an essential component of the core apoptosis machinery in animal cells. We have examined the role of BAX in Ad-induced apoptosis in human epithelial cells using two colon cancer cell lines, HCT116Bax (Bax(+/-)) and HCT116BaxKO (Bax(-/-)) (L. Zhang, J. Yu, B. H. Park, K. W. Kinzler, and B. Vogelstein, Science 290:989-992, 2000). Infection of Bax(+/-) cells with an Ad type 2 mutant (dl250) defective in expression of the E1B-19K protein resulted in enhanced cytopathic effect, large plaques on cell monolayers, fragmentation of cellular DNA, and enhanced cell death. These mutant phenotypes were not efficiently expressed in Bax(-/-) cells, suggesting that BAX is essential for Ad-induced apoptosis. Infection of Bax(+/-) cells with dl250 induced increased levels of an N-terminally processed form of BAX. Cells infected with the 19K mutant also contained enhanced levels of truncated BAX in membrane-inserted form. Our results suggest that at least a part of the mechanism utilized by E1B-19K to suppress apoptosis during Ad infection may involve modulation of the activities of BAX.

Critical requirement of BAX for manifestation of apoptosis induced by multiple stimuli in human epithelial cancer cells

Studies with mouse embryo fibroblasts deficient for the BCL-2 family multidomain proapoptotic proteins BAX and BAK have revealed that both of these proteins are essential for apoptosis induced by multiple stimuli, suggesting that these proapoptotic proteins are functionally overlapping in these cells [M. C. Wei et al., Science (Wash. DC), 292: 727-730, 2001; W. X. Zong et al., Genes Dev., 15: 1481-1486, 2001]. We have determined the effect of several different apoptotic stimuli in a Bax-deficient human epithelial cancer cell line (HCT116BaxKO). We show that this cell line expresses functional BAK protein and is defective in manifestation of apoptosis induced by the BH3-only proteins BIK and BID as well as extrinsic stimuli that engage the death receptors, tumor necrosis factor receptor, tumor necrosis factor-related apoptosis-inducing ligand receptor, and Fas. In addition, this cell line is deficient for apoptosis induced by cytotoxic agents such as UV, staurosporine, and thapsigargin that induce either mitochondrial or endoplasmic reticulum stress. Our results suggest that BAX plays a critical role in the manifestation of apoptosis paradigms induced by multiple stimuli in human epithelial cancer cells. Our results also suggest that the integrity of BAX may have important consequences in the progression of epithelial tumors and in determining the outcome of chemotherapeutic regimens of such tumors.

CtBP, an unconventional transcriptional corepressor in development and oncogenesis

CtBP family proteins are conserved among vertebrates and invertebrates and function as transcriptional corepressors. They repress transcription in a histone deacetylase-dependent or -independent manner. CtBPs play important roles during development and oncogenesis. In this review, their unusual properties, the mechanisms of transcriptional repression, regulation, and their biological functions are discussed.

The pro-apoptotic protein, Bik, exhibits potent antitumor activity that is dependent on its BH3 domain

The Bcl-2 homology 3 (BH3) domain is present in most members of the Bcl-2 protein family and is required to confer the death-inducing properties of pro-apoptotic members, including Bax, Bak, Bad, and Bik, in cell-based assay systems. To determine whether the BH3 domain possesses a similar role in tumor tissues in vivo, we overexpressed the wild-type Bik protein and its BH3-deleted counterpart, using adenoviral technology, in chemoresistant human tumor prostate (PC-3) and colon (HT-29) cell lines growing in vitro and in vivo. Bik caused apoptosis in both PC-3 and HT-29 cells in vitro by inducing the release of cytochrome c from mitochondria to cytoplasm, resulting in the catalytic activation of caspases 9, 7, and 3 and cleavage of poly(ADP-ribose) polymerase and DNA fragmentation. When the BH3 domain was deleted from the Bik protein, no effect on mitochondrial activity or cell morphology could be observed. Furthermore, intratumoral injection of an adenovirus vector expressing the Bik gene, but not the deleted BH3 Bik gene, suppressed the growth of PC-3 and HT-29 xenografts established in nude mice. Histological examination of tumors from mice treated with the wild-type Bik adenoviral construct demonstrated cellular debris, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling positive staining, and morphological changes associated with apoptosis. In contrast, tissue sections obtained from tumors treated with the BH3-deleted Bik adenoviral construct showed no evidence of apoptosis. Thus, our results suggest that the BH3 domain is required for the antitumor activity of the Bik protein and provides a novel therapeutic approach for cancer therapy.

Proteasomes modulate balance among proapoptotic and antiapoptotic Bcl-2 family members and compromise functioning of the electron transport chain in leukemic cells

The mechanism underlying apoptosis induced by proteasome inhibition in leukemic Jurkat and Namalwa cells was investigated in this study. The proteasome inhibitor lactacystin differentially regulated the protein levels of proapoptotic Bcl-2 family members and Bik was accumulated at the mitochondria. Bik overexpression sufficed to induce apoptosis in these cells. Detailed examination along the respiration chain showed that lactacystin compromised a step after complex III, and exogenous cytochrome c could overcome this compromise. Probably as a result, the succinate-stimulated generation of mitochondrial membrane potential was significantly diminished. Bcl-x(L) interacted with Bik in the cells, and Bcl-x(L) overexpression prevented cytochrome c leakage out of the mitochondria, corrected the mitochondrial membrane potential defect, and protected the cells from apoptosis. These results show that proteasomes can modulate apoptosis of lymphocytes by affecting the half-life of Bcl-2 family members, Bik being one of them.

Phosphorylation of the pro-apoptotic protein BIK: mapping of phosphorylation sites and effect on apoptosis

BIK is a pro-apoptotic BCL-2 family member and is the founding member of a subfamily of pro-apoptotic proteins known as "BH3-alone" proteins. Ectopic expression of BIK induces apoptosis in variety of mammalian cells. BIK complexes with various anti-apoptotic BCL-2 family proteins such as adenovirus E1B-19K and BCL-2 via the BH3 domain. However, the heterodimerization activity of BIK alone is insufficient for its apoptotic activity. Previous studies have shown that phosphorylation regulates the functional activity of both anti-apoptotic and pro-apoptotic members of the BCL-2 family. Here, we have examined phosphorylation of BIK and its effect on the apoptotic activity of BIK. We show that BIK exists as a phosphoprotein and is phosphorylated at residues 33 (threonine) and 35 (serine). Mutation of the phosphorylation sites, in which the Thr and Ser residues were changed to alanine residues, reduced the apoptotic activity of BIK without significantly affecting its ability to heterodimerize with BCL-2. Our results suggest that phosphorylation of BIK is required for eliciting efficient apoptotic activity. Partial purification of the protein kinase from HeLa cell cytoplasmic extracts suggest that BIK may be phosphorylated by a casein kinase II-related enzyme.

Structural analysis of the human pro-apoptotic gene Bik: chromosomal localization, genomic organization and localization of promoter sequences

The human Bik gene codes for a strong pro-apoptotic protein BIK. We have used fluorescent in-situ hybridization to establish the chromosomal localization of the Bik gene to 22q13.3. Genomic clones of the Bik gene were identified from a cosmid library of chromosome 22. Detailed analysis of the Bik gene revealed that it spans a region of about 19kb and comprises of five exons. Sequence analysis indicated that the 5' flanking region of Bik lacks canonical TATA and CAAT boxes but directs transcriptional initiation from a single site. A 1.9kb region containing the promoter elements of the Bik gene was identified and was found to direct expression of the reporter cat gene in transient transfection studies. By mutational analysis, the minimal Bik promoter was localized to a region between -211 to +153. Northern blot analysis showed a ubiquitous expression profile of the Bik mRNA with elevated levels of expression in heart and skeletal muscle.

Functional identification of the apoptosis effector BH3 domain in cellular protein BNIP1

BCL-2 family proteins play a central role in apoptosis regulation in mammals and in C. elegans. Mammalian cellular and viral anti-apoptosis proteins such as BCL-2 and E1B-19K interact with several cellular proteins. Some of these interacting proteins promote apoptosis and belong to the BCL-2 family. Certain BCL-2 family proapoptotic proteins such as BAX and BAK share extensive sequence homology with BCL-2. In contrast, certain pro-apoptotic proteins such as BIK and BID share a single death effector domain, BH3, with other BCL-2 family proteins. By mutational analysis, we show that one of the cellular proteins, BNIP1 (previously Nip-1), that interacts with BCL-2 family anti-apoptosis proteins is a 'BH3 alone' pro-apoptotic protein. Transient transfection of BNIP1 induces a moderate level of apoptosis. Deletions of the N-terminal 32 amino acid region and the C-terminal trans-membrane domain did not significantly affect pro-apoptotic activity. In contrast, deletions encompassing a region containing a motif similar to the BH3-domain abrogated the apoptotic activity. Substitution of BNIP1 BH3 domain for the corresponding sequence in BAX efficiently restored the apoptotic activity of BAX, establishing the functional identity of the BH3 domain of BNIP1. The N-terminal deletions of BNIP1 (that retain the BH3 domain) enhanced the level of interaction with BCL-XL. Mutants containing the BH3 deletions were still able to heterodimerize with BCL-XL while mutants lacking both the N-terminal region and the BH3 domain were unable to heterodimerize, suggesting that BNIP1 may bind to BCL-XL via two different binding motifs.

Modulation of mitochondrial Ca(2+) homeostasis by Bcl-2

We have investigated the role of mitochondrial Ca(2+) (Ca(m)) homeostasis in cell survival. Disruption of Ca(m) homeostasis via depletion of the mitochondrial Ca(2+) store was the earliest event that occurred during staurosporine-induced apoptosis in neuroblastoma cells (SH-SY5Y). The decrease of Ca(m) preceded activation of the caspase cascade and DNA fragmentation. Overexpression of the anti-apoptosis protein Bcl-2 led to increased Ca(m) load, increased mitochondrial membrane potential (DeltaPsi(m)), and inhibition of staurosporine-induced apoptosis. On the other hand, ectopic expression of the pro-apoptotic protein Bik led to decreased Ca(m) load and decreased DeltaPsi(m). Inhibition of calcium uptake into mitochondria by ruthenium red induced a dose-dependent apoptosis as determined by nuclear staining and DNA ladder assay. Similarly, reducing the Ca(m) load by lowering the extracellular calcium concentration also led to apoptosis. We suggest that the anti-apoptotic effect of Bcl-2 is related to its ability to maintain a threshold level of Ca(m) and DeltaPsi(m) while the pro-apoptotic protein Bik has the opposite effect. Furthermore, both ER and mitochondrial Ca(2+) stores are important, and the depletion of either one will result in apoptosis. Thus, our results, for the first time, provide evidence that the maintenance of Ca(m) homeostasis is essential for cell survival.

Binding of CtIP to the BRCT repeats of BRCA1 involved in the transcription regulation of p21 is disrupted upon DNA damage

Mutations in BRCA1 are responsible for nearly all of the hereditary ovarian and breast cancers, and about half of those in breast cancer-only kindreds. The ability of BRCA1 to transactivate the p21 promoter can be inactivated by mutation of the conserved BRCA1 C-terminal (BRCT) repeats. To explore the mechanisms of this BRCA1 function, the BRCT repeats were used as bait in a yeast two-hybrid screen. A known protein, CtIP, a co-repressor with CtBP, was found. CtIP interacts specifically with the BRCT repeats of BRCA1, both in vitro and in vivo, and tumor-derived mutations in this region abolished these interactions. The association of BRCA1 with CtIP was also abrogated in cells treated with DNA-damaging agents including UV, gamma-irradiation, and adriamycin, a response correlated with BRCA1 phosphorylation. The transactivation of the p21 promoter by BRCA1 was diminished by expression of exogenous CtIP and CtBP. These results suggest that the binding of the BRCT repeats of BRCA1 to CtIP/CtBP is critical in mediating transcriptional regulation of p21 in response to DNA damage.

BNIP3alpha: a human homolog of mitochondrial proapoptotic protein BNIP3

Apoptosis is regulated by interaction of viral and cellular BCL-2 family antiapoptotic proteins with various pro-apoptotic proteins, several of which are also members of the BCL-2 family. Cellular protein BNIP3 is a BCL-2 family proapoptotic protein that interacts with viral antiapoptosis proteins such as adenoviruses E1B-19K and EBV-BHRF1 and cellular antiapoptosis proteins such as BCL-2 and BCL-xL. Database searches indicate that the human genome encodes an open reading frame for a protein, BNIP3alpha, that shares substantial homology with BNIP3. The BNIP3alpha open reading frame encodes a protein of 219 amino acids that contains a conserved BH3 domain and a COOH-terminal trans-membrane domain, characteristic of several BCL-2 family proapoptotic proteins. BNIP3alpha interacts with viral antiapoptosis protein E1B-19K and cellular antiapoptosis proteins BCL-2 and BCL-xL. Overexpression of BNIP3alpha in transfected cells results in apoptosis and suppresses the antiapoptosis activity of E1B-19K and BCL-xL. Like BNIP3, BNIP3alpha seems to be predominantly localized in mitochondria. These results suggest that BNIP3alpha is a structural and functional homologue of BNIP3. BNIP3 and BNIP3alpha seem to be the first examples of homologues among the various human proapoptotic proteins. Northern blot analysis reveals that BNIP3alpha is expressed ubiquitously in most human tissues. In contrast, BNIP3 is expressed well in several human tissues and less abundantly in certain tissues such as placenta and lung. These results suggest that although BNIP3 and BNIP3alpha may promote apoptosis simultaneously in most human tissues, BNIP3alpha may play a more universal role.

Regulation of apoptosis by a Caenorhabditis elegans BNIP3 homolog

We have identified a C. elegans protein, ceBNIP3, homologous to the human BCL-2/EIB-19K interacting BCL-2 family pro-apoptotic protein BNIP3. In transiently transfected mammalian cells, ceBNIP3 complexes with CED-9, the worm homolog of BCL-2. CeBNIP3 also efficiently heterodimerizes with the cell death protease proCED-3 by direct binding via the prodomain. Transfection of ceBNIP3 and CED-3 results in enhanced proteolytic processing of the CED-3 zymogen and in cooperative induction of apoptosis. Coexpression of CED-9 suppresses the cooperative cell death induced by ceBNIP3 and CED-3. In cells coexpressing CED-9, ceBNIP3 and CED-3, all three proteins exist as a ternary complex suggesting that CED-9 may suppress cooperative apoptosis induced by CED-3 and ceBNIP3 by simultaneous complex formation with CED-3 and ceBNIP3. Our results suggest that ceBNIP3 may be a novel component of the C. elegans apoptosis paradigm and may initiate apoptosis by recruiting CED-3 to mitochondria and other cytoplasmic membranes.

Structural determinants present in the C-terminal binding protein binding site of adenovirus early region 1A proteins

The C-terminal binding protein (CtBP) has previously been shown to bind to a highly conserved six-amino acid motif very close to the C terminus of adenovirus early region 1A (Ad E1A) proteins. We have developed an enzyme-linked immunosorbent assay that has facilitated the screening of synthetic peptides identical or similar to the binding site on Ad E1A for their ability to bind CtBP and thus inhibit its interaction with Ad12 E1A. It has been shown that amino acids both C-terminal and N-terminal to the original proposed binding site contribute to the interaction of peptides with CtBP. Single amino acid substitutions across the binding site appreciably alter the Kd of the peptide for CtBP, indicative of a marked reduction in the affinity of the peptide for CtBP. The solution structures of synthetic peptides equivalent to the C termini of both Ad5 and Ad12 E1A and two substituted forms of these have been determined by proton NMR spectroscopy. Both the Ad12 and Ad5 peptides dissolved in trifluoroethanol/water mixtures were found to adopt regular secondary structural conformations seen as a series of beta-turns. An Ad12 peptide bearing a substitution that resulted in only very weak binding to CtBP (Ad12 L258G) was found to be random coil in solution. However, a second mutant (Ad12 V256K), which bound to CtBP rather more strongly (although not as well as the wild type), adopted a conformation similar to that of the wild type. We conclude that secondary structure (beta-turns) and an appropriate series of amino acid side chains are necessary for recognition by CtBP.

A potent cell death activity associated with transient high level expression of BCL-2

The BCL-2 proto-oncogene contains unusually long untranslated 5' and 3' sequences. Deletion of the sequences flanking the BCL-2 open reading frame dramatically increases the level of protein expression. Transient high level BCL-2 protein expression mediated by plasmid transfection or by infection with recombinant adenovirus results in potent apoptosis of several cell lines. Detailed mutational (deletion and add-back) analysis reveals that both 5'- and 3'-flanking sequences contribute to the negative modulation of protein expression from the BCL-2 open reading frame. It appears that these sequences exert the negative regulatory effect in an orientation-dependent manner. Analysis of BCL-2 RNA levels indicate that elevated levels of mRNA may be the primary cause of elevated levels of protein expression. Apoptosis induced by adenovirus vectors expressing elevated levels of BCL-2 can be readily inhibited by the caspase inhibitor z-VAD-fmk, suggesting that high levels of BCL-2 expression induce apoptosis via the caspase cascade. Mutational analysis of BCL-2 indicates that its pro-apoptotic activity is separable from its anti-apoptosis activity. Our results raise the possibility that oncogenic conversion of BCL-2 may require somatic mutations in the pro-apoptotic activity, in addition to other activating mutations that result in enhanced expression. Consistent with this hypothesis, a somatic mutation of BCL-2 observed in multiple human tumors results in reduced apoptosis activity.