Activation of c-myc expression by c-Abl is independent of both the DNA binding function of c-Abl and the c-myc EP site

c-Abl tyrosine kinase activates p21 transcription via interaction with p53

c-Abl non-receptor tyrosine kinase has been implicated in many cellular processes including cell differentiation, stress response and regulating gene transcription. The mechanism by which c-Abl is involved in the regulation of gene transcription remains to be elucidated. In this study, we investigated the functions of c-Abl in the activation of p21 promoter. Our results showed that overexpression of c-Abl tyrosine kinase activated p21 promoter and endogenous p21 transcription in U2OS cells. We found that p53 is involved in the activation of p21 promoter by c-Abl, and integrative structure of p53 is required for regulating p21 transcription. In addition, the chromatin immunoprecipitation study demonstrated that c-Abl and p53 can be recruited to the region containing p53 binding site of p21 promoter, and c-Abl increases the DNA binding activity of p53 to the p21 promoter. Furthermore, not only the activation of p21 promoter but also the recruitment to p21 promoter by c-Abl is dependent on the interaction between c-Abl and p53 protein.

c-Abl tyrosine kinase regulates c-fos gene expression via phosphorylating RNA polymerase II

c-Abl tyrosine kinase, predominantly distributed in nucleus, has been implicated in many important cellular processes including the regulation of gene transcription. In this study, we showed that c-Abl promoted the transcription of c-fos gene, both exogenously and endogenously. The nuclear localization and tyrosine kinase activity of c-Abl were required for the activation of c-fos gene. c-Abl was associated with RNA polymerase II (RNAP II) in vivo and augmented the tyrosine phosphorylation of the largest subunit of RNAP II. In addition, c-Abl and RNAP II could be recruited to the region of c-fos promoter. The combined results suggest that c-Abl plays an important role in the transcriptional regulation of c-fos gene and the tyrosine phosphorylation of the largest subunit of RNAP II by c-Abl is involved in the regulating process.

Hoechst 33258 binds to G-quadruplex in the promoter region of human c-myc

In vitro binding of Hoechst 33258 to the promoter region of human c-myc, d(GG GGAGGG TGG GGA GGG TGG GGA AGG TGG GG) which forms G-quadruplex, both in vitro and in vivo in the presence of metal ions, was investigated by equilibrium absorption, fluorescence, and kinetic surface plasmon resonance methods. Hypochromic effect in UV absorption spectra and blue shift in fluorescence emission maxima of Hoechst in the presence of quadruplex revealed that Hoechst binds to the quadruplex. Analysis of UV and fluorescence titration data revealed that Hoechst binds to quadruplex with binding affinity of the order of 10(6). Anisotropy measurements and higher lifetime obtained from time-resolved decay experiments revealed that quadruplex-bound Hoechst is rotationally restricted in a less polar environment than the bulk buffer medium. From surface plasmon resonance studies, we obtained kinetic association (k(a)) and dissociation (k(d)) of 1.23+/-0.04 x 10(5)M(-1)s(-1) and 0.686+/-0.009 s(-1), respectively. As Hoechst is known to bind A-T-rich region of duplex DNA, here we propose the likelihood of Hoechst interacting with the AAGGT loop of the quadruplex.

The importance of drug scheduling and recovery phases in determining drug activity. Improving etoposide efficacy in BCR-ABL-positive CML cells

K562 leukaemic cells are known to be less sensitive to etoposide than other cell lines, despite having similar topo II mRNA levels and cleavable complex formation. We have investigated the effect of etoposide schedule on cell cycle distribution, apoptosis and p21(waf1) and cdk1(p34) status in two bcr-abl-positive chronic myeloid leukaemia (CML) cell lines (K562 and KU812) and two small cell lung cancer (SCLC) cell lines (H69 and GLC4). During a continuous 5-day exposure, the SCLC cell lines showed a time and concentration-dependent loss of cell viability, with an initial block in the G2/M phase of the cell cycle followed by apoptosis. In contrast, the two CML cell lines showed no significant apoptosis or loss of viability after a similar block in G2/M. However, when K562 or KU812 cells were placed in drug-free medium following a 3-day drug exposure there was marked, concentration-dependent apoptosis (% apoptosis after release at 1 microM etoposide in K562, 10% at 24 h, 30% at 48 h). Our data also show that p21(waf1) does not increase after etoposide treatment in either H69 or GLC4 (both with mutated-p53). Although K562 and KU812 cells are null-p53, the arrest in G2/M during drug exposure was associated with increased p21(waf1) and a decrease in cdk1 (both P<0.001 compared with controls). Upon release of these cells from drug-medium, p21(waf1) gradually returned to control levels, which was associated with an easing of the block at G2/M and an induction of apoptosis. This study highlights the importance of cell cycle regulatory proteins in drug sensitivity and resistance, and suggests that in cells such as K562 and KU812, a pulsed schedule may be more active than a single prolonged exposure.

Signal transduction and cellular radiation responses

Exposure of cells to ionizing radiation results in complex cellular responses resulting in cell death and altered proliferation states. The underlying cytotoxic, cytoprotective and cellular stress responses to radiation are mediated by existing signaling pathways, activation of which may be amplified by intrinsic cellular radical production systems. These signaling responses include the activation of plasma membrane receptors, the stimulation of cytoplasmic protein kinases, transcriptional activation, and altered cell cycle regulation. From the data presented, there is increasing evidence for the functional links between cellular signal transduction responses and DNA damage recognition and repair, cell survival, or cell death through apoptosis or reproductive mechanisms.

c-Myb protein binds to the EP element of the HBV enhancer and regulates transcription in synergy with NF-M

The hepatitis B virus (HBV) enhancer contains multiple active elements, one of which is the EP element, a 15 bp site important for its regulation by acting on other functional elements like the E site. The EP element, in the HBV enhancer context, contains two putative binding sites for c-myb family gene products. Electrophoretic mobility shift assays showed that the minimal c-Myb DNA-binding domain binds to the EP sequence. DNase I footprinting experiments revealed that only one consensus binding site was effectively protected. We found that c-Myb down-regulates transcription driving by the HBV enhancer in CAT assays performed in a haematopoietic (K562) and in a hepatic (HepG2) cell line. Interestingly, co-expression of both c-Myb and NF-M, a C/EBPbeta homologue which recognises the E element of the HBV enhancer, showed a synergistic transactivation of the HBV enhancer while, separately, each of them had an inhibitory effect on transcription in HepG2 and K562 cell lines, two cell types potentially infected by the hepatitis B virus.

The DNA binding domain of the human c-Abl tyrosine kinase preferentially binds to DNA sequences containing an AAC motif and to distorted DNA structures

The c-Abl tyrosine kinase protein is implicated in the signaling pathway as well as in transcription, DNA repair, apoptosis, and several other vital biological processes essential for cell proliferation or differentiation. The interaction of c-Abl with DNA is important for some of these functions, but the exact nature of this interaction is still a matter of controversy. The present study addresses the DNA-binding properties of the human c-Abl protein. Using CASTing experiments, the consensus binding site 5'-AA/CAACAAA/C was determined. The central highly conserved AAC triplet appears to constitute the crucial core element in the binding sequences of the c-Abl protein. The c-Abl DNA-binding domain recognizes specific sequences and interacts with deformed DNA structures such as four-way junctions and bubble DNA containing a large single-stranded loop, as determined by electromobility shift assay, melting temperature studies, and binding to specific oligonucleotides covalently linked to beads. Additional competition experiments suggest that the interaction mainly involves contacts within the minor groove of the double helix. The DNA-binding properties of c-Abl are reminiscent of those of high-mobility group (HMG)-like proteins such as LEF-1 and SRY. However, the circular permutation and ring closure assays and DNA unwinding experiments reveal that, unlike HMGs, c-Abl does not bend its target sequence. In addition, it is shown that the protein potentiates the DNA relaxation activity of topoisomerase I. These findings indicate that the interaction of c-Abl with DNA is both sequence-selective and structure-dependent.

Determination of the human c-Abl consensus DNA binding site

c-Abl tyrosine kinase, an essential protein of the cell cycle signalling pathways, is implicated in the regulation of RNA polymerase II activity, apoptosis and DNA repair. Its DNA binding activity is important for its biological functions. However, the molecular basis of c-Abl interaction with DNA remains largely unclear. We delimited the human c-Abl DNA binding domain and identified its preferred binding site, 5'-A(A/C)AACAA(A/C). The central AAC motif is highly conserved and constitutes the major core element in the binding sites. EMSAs and footprinting experiments were performed to explore how the c-Abl fusion protein recognizes specific sequences in DNA.

In vivo footprinting analysis of the hepatic control region of the human apolipoprotein E/C-I/C-IV/C-II gene locus

Expression of both the apolipoprotein (apo)E and apoC-I genes in the liver is specified by a 319-nucleotide hepatic control region (HCR-1) that is located 15 kilobase pairs downstream of the apoE gene and 5 kilobase pairs downstream of the apoC-I gene. In vivo footprint analysis of HCR-1 in intact nuclei revealed several liver-specific protein-binding sites that were not detectable by in vitro methods. In addition to three previously identified in vitro footprints, four in vivo footprints were identified in a region of HCR-1 that is required for directing gene expression to hepatocytes. Prominent liver-specific DNase I-hypersensitive sites were associated with these footprints. Liver-specific nuclear protein binding to these sites was confirmed by oligonucleotide gel-retention assays. The in vivo analysis also identified a cluster of nuclear protein-binding sites in the Alu family repeat segment adjacent to the domain required for liver expression. Micrococcal nuclease digestion indicated the presence of a nucleosome in the central domain of HCR-1 in liver chromatin that was in phase with the nucleosome location in tissues that did not express the transgene. These results suggest that HCR-1 functions in a highly structured chromatin environment requiring a complex interaction of liver-enriched transcription factors.

Interactions of the transcription factors MIBP1 and RFX1 with the EP element of the hepatitis B virus enhancer

We previously demonstrated that MIBP1 and RFX1 polypeptides associate in vivo to form a complex that binds to the MIF-1 element in the c-myc gene and the major histocompatibility complex class II X-box recognition sequence. We now show that the EP element, a key regulatory sequence within hepatitis B virus enhancer I, also associates with MIBP1 and RFX1. Using polyclonal antisera directed against either oligonucleotide-purified MIBP1 or a peptide derived from the major histocompatibility complex class II promoter-binding protein RFX1, we showed that MIBP1 and RFX1 are both present in the DNA-protein complexes at the EP site. In addition, while the EP element can act cooperatively with several adjacent elements to transactivate hepatitis B virus expression, we demonstrated that the EP site alone can repress transcription of simian virus 40 promoter in a position- and orientation-independent manner, suggesting a silencer function in hepatocarcinoma cells.

Repression of the c-myb gene by WT1 protein in T and B cell lines

The c-myb gene is primarily expressed in immature hematopoietic cells, and it is overexpressed in many leukemias. We have investigated the role of negative regulatory sites in the c-myb promoter in the Molt-4 T cell line and in the DHL-9 B cell line. A potential binding site for either the EGR-1 or WT1 protein was identified by in vivo footprinting in the 5'-flanking region of c-myb in a region of negative regulatory activity in T cells. We showed by electrophoretic mobility shift assay and electrophoretic mobility shift assay Western that WT1, EGR-1, and Sp1 bound to this site. A mutation of this site which prevented protein binding increased the activity of the c-myb promoter by 2.5-fold. In the DHL-9 B cell line, this site was nonfunctional; however, we found a potential EGF-1/WT1 site located more 3' in a region of negative regulatory activity. We showed that WT1, EGR-1, and Sp1 bound to this site, and that mutation of this site increased the activity of the c-myb promoter by 3.2-fold. Cotransfection of a WT1 expression vector repressed the activity of the c-myb promoter in both cell lines, and this repression was relieved when the EGR-1/WT1 sites were removed. Cotransfection of either an EGR-1 or Sp1 expression vector had no significant effect on the activity of the c-myb promoter. We conclude that WT1 is a negative regulator of c-myb expression in both T and B cell lines.

The transcription factor, Nm23H2, binds to and activates the translocated c-myc allele in Burkitt's lymphoma

We have identified an in vivo footprint over the PuF site on the translocated c-myc allele in Burkitt's lymphoma cells. The PuF site on the silent normal c-myc allele was unoccupied. We demonstrated by electrophoretic mobility shift assay, electrophoretic mobility shift assay with antibody, UV cross-linking followed by SDS-gel electrophoresis, and Western analysis that Nm23H2 in B cell nuclear extracts bound to the c-myc PuF site. Transfection experiments with c-myc promoter constructs in both DHL-9 and Raji cells revealed that the PuF site functioned as a positive regulatory element in B cells with a drop in activity with mutation of this site. Access to this site is blocked in the normal silent c-myc allele; these data suggest that the Nm23H2 protein is involved in deregulation of the translocated c-myc allele in Burkitt's lymphoma cells.