key: cord-0029509-3wwy89sj
authors: Liu, Shuang; Wang, Hongyan; Wang, Xiang; Lu, Lin; Gao, Ning; Rowe, Peter S. N.; Hu, Baocheng; Wang, Ya
title: MEPE/OF45 protects cells from DNA damage induced killing via stabilizing CHK1
date: 2009-10-06
journal: Nucleic Acids Res
DOI: 10.1093/nar/gkp768
sha: a1f1e3ee7363c51088fb0d73f6a44a94036ecbb7
doc_id: 29509
cord_uid: 3wwy89sj

Matrix extracellular phosphoglycoprotein/osteoblast factor 45 (MEPE/OF45) was cloned in 2000 with functions related to bone metabolism. We identified MEPE/OF45 for the first time as a new co-factor of CHK1 in mammalian cells to protect cells from DNA damage induced killing. We demonstrate here that MEPE/OF45 directly interacts with CHK1. Knocking down MEPE/OF45 decreases CHK1 levels and sensitizes the cells to DNA damage inducers such as ionizing radiation (IR) or camptothicin (CPT)-induced killing. Over-expressing wild-type MEPE/OF45, but not the mutant MEPE/OF45 (depleted the key domain to interact with CHK1) increases CHK1 levels in the cells and increases the resistance of the cells to IR or CPT. MEPE/OF45, interacting with CHK1, increases CHK1 half-life and decreases CHK1 degradation through the ubiquitine-mediated pathway. In addition, the interaction of MEPE/OF45 with CHK1 decreases CHK1 levels in the ubiquitin E3 ligases (Cul1 and Cul4A) complex, which suggests that MEPE/OF45 competes with the ubiquitin E3 ligases binding to CHK1 and thus decreases CHK1 from ubiquitin-mediated proteolysis. These findings reveal an important role of MEPE/OF45 in protecting cells from DNA damage induced killing through stabilizing CHK1, which would provide MEPE/OF45 as a new target for sensitizing tumor cells to radiotherapy or chemotherapy.

CHK1 is one of the essential checkpoint proteins involved in cellular response to multiple DNA damage inducers (1) (2) (3) (4) . It is believed that up-regulated CHK1 protects cells from ionizing radiation (IR) or camptothecin (CPT)-induced killing (4) (5) (6) (7) (8) , which is related to the role of CHK1 in promoting homologous recombination repair (9, 10) . Despite the importance of CHK1 in DNA damage response, the regulation of CHK1 in mammalian cells is not well understood, partly because of its essential nature (3, 11) .

Matrix extracellular phosphoglycoprotein/osteoblast factor 45 (MEPE/OF45) was originally cloned from a human oncogenetic hypophosphatemia tumor (12) in 2000, and then was identified in rat (13) and mouse (14) . Since MEPE/OF45 was identified, its function related to bone metabolism has been widely investigated. We show here for the first time that MEPE/OF45, as a co-factor of CHK1, protects cells from DNA damage induced killing. This work was initiated by looking for co-factor(s) of CHK1 from one pair of transformed rat embryo fibroblasts: A1-5 and B4 cells.

Although A1-5 and B4 cells have similar genetic backgrounds (15), as we reported previously, A1-5 cells are much more resistant to IR or CPT induced killing than B4 cells (5, 6) . The resistance of A1-5 cells to DNA damage induced killing is due to the high levels of CHK1 (5, 6) . By looking for the co-factor(s) of CHK1 in A1-5 cells, we identified the new role of MEPE/OF45 in protecting cells from DNA damage induced killing. This new role of MEPE/OF45 depends on the interaction of MEPE/ OF45 with CHK1, which maintains CHK1 levels by reducing CHK1 degradation. digested with RsaI and ligated with adapters according to the manufacturer's protocol. In the first round, the cDNA obtained from A1-5 cells was the tester and the cDNA obtained from B4 cells was the driver. In the second round, the cDNA obtained from A1-5 cells was the driver and the cDNA obtained from B4 cells was the tester. Two hybridizations were then performed. A secondary PCR amplification was performed with nested primers to further reduce any background PCR products and enrich them for differentially expressed sequences. In total 158 cDNA fragments were differentially expressed between A1-5 and B4 cells by comparing the two populations of mRNA. Eighty-one cDNA fragments that were different sections of the same gene were excluded by analyzing the cDNA fragments in EST and BLAST databases. The remaining 77 cDNA fragments were individually labeled with a 32 P-dCTP by using a random priming method and were hybridized with the total RNAs from A1-5 or B4 cells. Ten of the 77 cDNA fragments were confirmed and differentially expressed between A1-5 and B4 cells by the hybridization. The full length of one of the 10 cDNA fragments, which showed less homology to any cDNA fragment in EST and BLAST databases in early 2000, was cloned by rapid amplification of cDNA ends (RACE) with the marathon DNA amplification kit (Clontech Laboratories, Inc.).

Total RNA was isolated from A1-5 and B4 cells by using Trizol purification kit (Invitrogen Corp.), according to the manufacturer's instructions. Two ml of RNA and 1 ml of oligo dT primer were mixed with 10 ml of RNase free water. The mixture was first incubated at 70 C for 10 min, then was cooled down to room temperature. Reverse transcription analyses were performed using the RT-PCR kit (TaKaRa, Inc.), according to the manufacturer's instructions. The primer sets used in this study were: Mepe/Of45-forward, 5 0 -CGG GAT CCC AGG CTG TGT CTG-3 0 ; Mepe/Of45-reverse, 5 0 -CGC AAG CTT GCG AGA GCA GTG GTG ACT AG-3 0 ; GAPDH-forward, 5 0 -ACC ACA GTC CAT GCC ATC ACT G-3 0 ; GAPDH-reverse, 5 0 -GTC CAC CAC CCT GTT GCT GTA G-3 0 . The PCR (30 cycles) condition was used as follows: denaturation at 95 C for 5 min, then denaturation at 94 C for 50 s, annealing at 55 C for 50 s and elongation at 72 C for 70 s, running 30 cycles. In Mepe/Of45 mRNA expression experiments, GAPDH was used as a housekeeping gene.

The transformed rat embryo fibroblast cells, A1-5 and B4, obtained from Dr A. Levine's laboratory (15) , were grown in DMEM supplemented with 10% calf serum. The antibody against rat MEPE/OF45 was made by our laboratory. Briefly, fragments of rat Mepe/Of45 cDNA (100-498 bp) were generated by PCR with primers 5 0 -CA TATGAAGCTGAATGAAGAT-3 0 and 5 0 -GGATCCTT ACATTTGGGGGAGATG-3 0 and cloned in pET-28a(+) plasmid (Novagen, Inc.). The protein was expressed in BL21 (DE3) cells and purified by using Ni-NTA spin columns (Qiagen, Inc.), according to the manufacturer's instructions. The purified protein was analyzed by SDS-PAGE and was then injected into mice. The antisera of MEPE/OF45 were collected and verified by Western Blot. The antibody against C-terminal of human MEPE/OF45 was as described (12, 17) . Purified HA-tagged JNK2 protein (sc-4062) and antibodies against CHK1 (sc-8408 and sc-7898), HA (sc-805, sc-7392), PCNA (sc-56), Cul4 (sc-10782), a-Tubulin (sc-8035), and ubquitin (Ub) (sc-9133) were purchased from Santa Cruz Biotechnology, Inc. The antibody against Cul1 was purchased from Zymed Laboratories. The antibody against GFP (AVAMM 26001) was purchased from AVIVA System Biology, Inc. The antibody against H2A (07-146) was purchased from Upstate Biotechnology, Inc. The antibody against DsRed (632496) was purchased from Clontech Laboratories, Inc.

The vector encoding antisense of rat Mepe/Of45 was generated by using PCR with the primers 5 0 -CG GGA TCC ATG GTC ACC ACT GCT CTC-3 0 and 5 0 -CGC AAG CTT CTA CAG GCT GTG TCT GTT GG-3 0 , and was cloned into the plasmid pREP10 (Invitrogen Corp.). The vector pLEGFP-C1 with NLS (Clontech Laboratories, Inc.) encoding the full length cDNA of rat Chk1 and/or the vector pDsRed1-N1 without NLS (Clontech Laboratories, Inc.) encoding the full length cDNA of rat Mepe/Of45 was generated. pREP10-HA was generated by inserting the HA-tag into plasmid pREP10. Wild-type MEPE/OF45 or MEPE/OF45 deleted mutants were cloned in pREP10-HA. GST-fulllength MEPE/OF45 or GST-truncated MEPE/OF45s were generated by the standard methods. All the tags including GST, red fluorescence protein and HA were designed at the C-terminal of MEPE/OF45. A1-5 or B4 cells were transfected with the empty plasmid, the plasmid encoding the antisense of Mepe/Of45, wild-type Mepe/ Of45 or deletion mutant Mepe/Of45 by Lipofectamine 2000 (Invitrogen Corp.), according to the manufacturer's instructions. Stable cell lines were selected from hygromycin resistant colonies and the transfected gene was verified by PCR or by Western blot with an HA antibody. In immunostaining experiments, the cells were collected at 48 h after transfected or co-transfected with the vector(s) (GFP-Chk1 in pLEGFP-C1 and red fluorescence-Mepe/Of45 in pDsRed1-N1).

Rat Mepe/Of45 siRNAs (5 0 -GGG ATC TAC TAC TCC AGA AGA-3 0 , 5 0 -GAT AAT GAT GTC CCT CCT TTC-3 0 ) were designed to specifically target the sequence of 423-444 or 572-593 from the start codon region of the rat Mepe/Of45. The siRNA of CHK1 is as previously described (8) . Human MEPE/OF45 siRNA (5 0 -AAC TAA GCA AAG CTG TGT GGA-3 0 ) was designed to specifically target the sequence of 74-95 from the start codon region of the human MEPE/OF45. These siRNAs were synthesized by Dharmacon, Inc. Scrambled duplex RNAs (Dharmacon, Inc.) were used as the control transfection. The RNAs were delivered to the cells by OLIGOFECTAMINE TM (Invitrogen Corp.), according to the manufacturer's instructions. The cells were analyzed at 36-48 h post-transfection.

Rat Mepe/Of45 and rat Chk1 were expressed in both pGBKT7 and pGADT7 (Clontech Laboratories, Inc.) as fusion proteins. The interaction was monitored by the ability of the transformants to grow on synthetic medium lacking Leu, Trp and His according to the manufacturer's instructions, which occurred only when MEPE/OF45 interacted with CHK1.

Cells transfected with vector pLEGFP-C1, containing nuclei localization signal (NLS) encoding Chk1 and/or vector pDsRed1-N1 encoding Mepe/Of45 were grown on glass coverslips in 35 mm dishes. Cells were fixed with 4% paraformaldehyde. The fluorescent image was observed by using a confocal laser microscope (Radiance 2100, Bio-Rad Laboratories). IP was performed as described previously (18) . The samples were washed with the buffer (0.5% NP-40, 1 mM Na 3 VO 4 , 5 mM NaF, 0.2 mM PMSF in PBS buffer) and boiled in 30 ml of protein loading buffer. The samples were loaded to 10% Tris-PAGE gel followed by the standard Western Blot.

Surviving fractions were obtained from clonogenic assay that was performed as previously described (5, 6) .

In brief, 2 Â 10 5 cells were plated, per 60 mm dish with 3 ml of medium. Thirty hours later, cells were either treated with 10 nM taxotere (to increase mitotic ratio) for 20 h, and were irradiated with 10 Gy or treated with 10 nM taxotere and 2 mM hydroxyurea (HU) for 20 h. Cells were harvested with trypsin and a Premature chromatin condensation (PCC) assay was performed as described (19, 20) . Normal mitosis characteristics include wellformed oblong chromatids present in pairs and at least 20 such chromosome pairs were found in a cluster. Approximately 100 mitotic cells were counted per sample.

Cells were transiently transfected with a plasmid encoding myc-ub [obtained from Dr. Chen's and Dr. Yu's laboratory (21) ]. Thirty-six hours later the cells were in the presence of 4 mM of LLnV for an additional 4 h. The cells were collected and whole cell lyses were prepared. Cell lyses (1 mg) were mixed with 2 mg of CHK1 monoclonal antibody or Ub antibody and 20 ml of 50% (v/v) protein A-Sepharose slurry (RepliGen) in 1 ml of buffer (0.5% NP40, 1 mM Na 3 VO 4 , 5 mM NaF and 0.2 mM PMSF in PBS) and were rotated at 4 C overnight. The immune complexes were washed three times with the same buffer, boiled with protein loading buffer and run on SDS-PAGE gels. The gels were not cut or cut at the site marked 60 kDa. The cut gel with >60 kDa samples were immunoblotted with a-Ub antibody and the gel with <60 kDa was immunoblotted with a-CHK1 polyclonal antibody. The non-cut gel was immunoblotted with CHK1 antibody.

To look for the co-factors of CHK1, we chose the PCRselect cDNA subtraction (PSDS) approach to screen the different expressed genes between A1-5 cells and B4 cells. By using this approach, we identified one fragment that was expressed higher in A1-5 cells than in B4 cells (Supplementary Figure S1A) . RT-PCR results confirmed the different expression between these two cell lines (Supplementary Figure S1B ) and the full length of this gene cloned from our laboratory in late 2000 (Supplementary Figure S1C ) was found to be identical to the sequence of rat Mepe/Of45 published by another group (13) . The rat OF45 is the homologue of human MEPE that was related to bone metabolism. However, there was no report on the connection of MEPE/OF45 to DNA damage induced killing, and we were interested in knowing whether the highly expressed MEPE/OF45 was related to the resistance of A1-5 cells. For this purpose, we established stable cell lines that either knocked down MEPE/OF45 in A1-5 cells or up-expressed MEPE/OF45 in B4 cells and tested the sensitivities of the cell lines to IR or CPT. The results showed that when MEPE/OF45 was knocked down, the cells (Aa in Figure 1A and A-a-MEPE/OF45 in Figure 1B ) became more sensitive to IR or CPT than the parental cell line (A in Figure 1A and A1-5 in Figure 1B ) and the cells transfected with the vector alone (Ap in Figure 1A and A-pREP10 in Figure 1B ). The Mepe/Of45 siRNAs data (Supplementary Figure S2 ) exclude the off-target effects. The cells became sensitive to DNA damage inducers when either CHK1 alone was knocked down (5,6) or MEPE/ OF45 alone was knocked down ( Figure 1B) . To exclude the possibility that the results reflected only in the particular cell lines, we examined the effects of knocking down MEPE/OF45 on human cell sensitivity to DNA damage inducers. The results showed that similar to the REF cells, knocking down MEPE/OF45 also sensitized HeLa cells to DNA damage inducers (Supplementary Figure S3) . Interestingly, the cells could not increase their sensitivity to DNA damage further when the two proteins were knocked down at the same time (Supplementary Figure  S3) , suggesting that MEPE/OF45 and CHK1 are in the same pathway for protecting cells from DNA damage induced killing. When the cells expressed with exogenous wild-type MEPE/OF45 (Bw in Figure 1A and B-w-MEPE/OF45 in Figure 1B) , they became more resistant to IR or CPT than the parental cell line (B in Figure 1A , and B4 in Figure 1B ) and the cells transfected with the vector alone (Bp in Figure 1A and B-pREP10 in Figure  1B ). These results demonstrate for the first time that MEPE/OF45, the protein involved in bone metabolism, does protect mammalian cells from DNA damageinduced killing. These data indicate that MEPE/OF45 is a multi-function protein. The relationship between the two functions of MEPE/OF45 needs to be elucidated in the future. Although CHK1 affecting cell sensitivity to DNA damage is involved in p53 stature (22) , A1-5 and B4 cells expressed similar levels of wild type and mutant p53 (15) (our unpublished data), we could exclude the effects of p53 on the different sensitivities of these cells to DNA damage.

To test whether there was a functional link between MEPE/OF45 and CHK1, we examined the physical interaction between MEPE/OF45 and CHK1 by using yeast two-hybrid assay. The results showed that MEPE/OF45 did interact with CHK1 ( Figure 2A ). Although MEPE/ OF45 could be secreted to extracellular matrix (12, 13) , the endogenous MEPE/OF45 in the cell is mainly located in the nuclei, which is similar to CHK1 ( Figure  2B , left panel). By using IP assay, we showed that the endogenous MEPE/OF45 and CHK1 interacted with each other (Figure 2B, right panel lanes 3 and 4) , which is not affected by DNA damage (Supplementary Figure  S4) . To confirm the interaction of MEPE/OF45 with CHK1 in vivo, we developed a simple method by using two plasmids encoding either GFP-CHK1 fusion protein with a nucleus localization signal (NLS) or red fluorescence-MEPE/OF45 fusion protein without NLS. Although the cDNA of Chk1 contains a NLS, partial exogenous CHK1 would locate in cytoplasm without the additional NLS in the plasmid (data not shown). The results showed that the exogenous MEPE/OF45 alone located mainly in the cytoplasm ( Figure 2C , #1) and the exogenous CHK1 alone located mainly in the nuclei ( Figure 2C, #2) . However, when the cells were co-transfected with these two genes, MEPE/OF45 was Cell lines (A1-5, A-pREP10, A-a-MEPE/OF45, B4, B-pREP10 and Bw-MEPE/OF45) were either exposed to different doses of X-rays or treated with different doses of CPT for 8 h. Surviving fractions were obtained from clonogenic assay as previously described (5, 6) . The data represented the average of three independent experiments and were expressed as a percent of cell colonies without treatment. merged with CHK1 to the nuclei ( Figure 2C, #3-5) , indicating that the exogenous CHK1 interacted with the exogenous MEPE/OF45 and pulled MEPE/OF45 into the nucleus. These results provide additional evidence that MEPE/OF45 interacts with CHK1.

To identify the main domain of MEPE/OF45 interaction with CHK1, we combined the approach described above with GST-pull down assay to examine the interaction of different deleted MEPE/OF45 fragments with CHK1. We discovered that the mutant MEPE/OF45 protein that deleted 400-418 amino acids (aa) lost its major interaction between MEPE/OF45 and CHK1 (the B4 cells transiently co-transtected with pLEGFP-CHK1 and pDsRed-deleted MEPE/OF45 #9, D9 in Supplementary Figure S5C and D). Although the mutant MEPE/OF45 still showed a weak interaction with CHK1 by IP assay (D9 in Supplementary Figure S5D) , it lost its ability to merge with CHK1 when B4 cells were co-expressed with the two proteins ( Figure 3A ). We then established stable cell lines by expressing the wild-type MEPE/OF45 or the mutant MEPE/OF45 in B4 cells. Interestingly, the HAtagged mutant MEPE/OF45 (deleted 400-418 aa) mainly locates in the cytoplasm although the HA-tagged wildtype MEPE/OF45 is still located in nuclei ( Figure 3B , Top-panel). The IP results further confirm that the 400-418 aa of rat MEPE/OF45 is the main domain for MEPE/OF45 interacting with CHK1 ( Figure 3B , Bottom-panel). More importantly, when B4 cells were transfected with the wild-type MEPE/OF45 (Bw in Figure  3B , B-w-MEPE/OF45 in Figure 3C ) but not the mutant MEPE/OF45 (Bm in Figure 3B , B-m-MEPE/OF45 in Figure 3C ), they became resistant to IR or CPT. These results indicate that MEPE/OF45 protecting cells from DNA damage-induced killing depends on its interaction with CHK1. To further study the functional relationship between MEPE/OF45 and CHK1, we examined the effects of MEPE/OF45 on premature chromatin condensation (PCC) because CHK1 plays an essential role in preventing DNA damage-induced PCC (19, 20) . The results showed that the cells with higher expression of wild-type (A1-5, A-p, B-w), but not mutant MEPE/OF45 (B-m) had less DNA damage-induced PCC than the cells with lower expression of MEPE/OF45 (A-a, B4 in Figure 3D ). This indicates that the interaction of MEPE/OF45 with CHK1 is important for MEPE/OF45 to protect cells from DNA damage-induced PCC, further supporting that MEPE/ OF45 affecting cellular response to DNA damage is through the CHK1 pathway.

CHK1 is a fast turnover protein (23) . MEPE/OF45 levels correlate with the CHK1 levels in cells ( Figure 1A and Supplementary Figure S2 ). We were interested in studying whether MEPE/OF45 played any role in regulating CHK1 levels. We did not find any apparent difference in Chk1 mRNA levels between A1-5 and B4 cells (data not shown), excluding the possibility that MEPE/OF45 affects the transcription of Chk1, suggesting that MEPE/OF45 protects CHK1 from degradation. To test this hypothesis, we examined the half-life of CHK1 in these cell lines. The results showed that CHK1 half-life was longer in the cell lines with high levels of wild-type MEPE/OF45 (A: A1-5; Ap: A1-5 transfected with empty plasmid; Bw: B4 cells expressed with wild-type MEPE/ OF45 in Figure 4A ) than that in the cell lines with low levels of wild-type MEPE/OF45 (B: B4 cells; Aa: A1-5 cells with knocked down MEPE/OF45 in Figure 4A ) or with high levels of mutant MEPE/OF45 that lost interaction with CHK1 (Bm: B4 cells expressed with the mutant MEPE/OF45 in Figure 4 ). In order to clearly compare the changes of CHK1 levels among different cell lines, we compared the initial levels of CHK1 at the 0 h point and plotted the kinetics levels at different times treated with CHX (Supplementary Figure S6) . These results show that the interaction of MEPE/OF45 with CHK1 does play an important role for stabilizing CHK1. Because CHK1 degradation occurs through the ubiquitin-proteasome pathway (24), we wanted to know whether MEPE/OF45 affected the ubiquitin-mediated proteolysis of CHK1. For this purpose, we examined ubiquitinated CHK1 in these cell lines. After transiently expressing Myc-ubiquitin in the cell lines ( Figure 4B ), we examined the levels of CHK1 polyubiquitination in these Myc-ubiquitin expressed cells. The results showed that the signals of CHK1 polyubiquitination were weaker in the cells with high levels of wild-type MEPE/OF45 (A, Ap and Bw) than that in the cells with low levels of MEPE/OF45 (B and Aa) or cells with high levels of mutant MEPE/OF45 (Bm) ( Figure 4B, Supplementary Figure S7 ). These results provide direct evidence that supports that MEPE/OF45 plays an important role in delaying the ubiquitinmediated degradation of CHK1, and that the interaction of MEPE/OF45 with CHK1 is an essential factor for such functions.

The ubiquitination of CHK1 is mediated by E3 ligase complexes containing Cul1 or Cul4A (24) . To study whether the interaction of MEPE/OF45 with CHK1 affected the interaction of Cul1 or Cul4A with CHK1, we examined the CHK1 levels in either Cul1 or Cul4A immunoprecipitated complexes from extracts derived from the cell lines. The results showed that the interaction of CHK1 with either Cul1 or Cul4A was lower in the cells with high levels of wild-type MEPE/OF45 than that in the cells with low levels of MEPE/OF45 or with high levels of mutant MEPE/OF45 ( Figure 4C ). These results indicate that the level of CHK1 interaction with either Cul1 or Cul4A is affected by the interaction of MEPE/OF45 with CHK1 ( Figure 4C) . These results suggest a model that the interaction of MEPE/OF45 with CHK1 competes with the interaction of either Cul1 or Cul4A with CHK1, which in turn affects CHK1 ubiquitinmediated degradation ( Figure 4D ). Confirming this hypothesis requires future identification of the key domain of CHK1 interacting with either MEPE/OF45 or the E3 ligases.

In summation, our results indicate that MEPE/OF45 protecting mammalian cells from DNA damage-induced killing mainly depends on its interaction with CHK1. The specific molecular mechanism can be linked to the function of MEPE/OF45 to delay ubiquitin-mediated degradation of CHK1 by competing with CHK1 E3 ligases. Very interestingly, the role of MEPE/OF45 in protecting rat cells from DNA damage induced killing through interacting with CHK1 is similar to that shown in different human cell lines (Supplementary Figure S3 and our unpublished data). These results indicate that MEPE/ OF45 is a new co-factor of CHK1, which could be a new target for sensitizing cells for DNA damage inducers and will benefit cancer treatment.

Analysis of Rad3 and Chk1 protein kinase defines different checkpoint responses

Requirement for Atr in phosphorylation of Chk1 and cell cycle regulation in response to DNA replication blocks and UV-damaged DNA in Xenopus egg extracts

Chk1 is an essential kinase that is regulated by Atr and required for the G 2 /M DNA damage checkpoint

Disruption of the checkpoint kinase 1/cell division cycle 25A pathway abrogates ionizing radiation-induced S and G2 checkpoints

The radioresistance to killing of A1-5 cells derives from activation of the Chk1 pathway

CHK1-regulated S-phase checkpoint response reduces camptothecin cytotoxicity

Essential and dispensable roles of ATR in cell cycle arrest and genome maintenance

Involvement of the Fhit gene in the ionizing radiation-activated ATR/CHK1 pathway

Fhit and CHK1 have opposing effects on homologous recombination repair

The cell-cycle checkpoint kinase Chk1 is required for mammalian homologous recombination repair

Chk1 is haploinsufficient for multiple functions critical to tumor suppression

MEPE, a new gene expressed in bone marrow and tumors causing osteomalacia

Identification of osteoblast/osteocyte factor 45 (OF45), a bone-specific cDNA encoding an RGD-containing protein that is highly expressed in osteoblasts and osteocytes

MEPE, the gene encoding a tumor-secreted protein in oncogenic hypophosphatemic osteomalacia, is expressed in bone

Cellular localization and cell cycle regulation by a temperaturesensitive p53 protein

PERP, an apoptosis-associated target of p53, is a novel member of the PMP-22/gas3 family

MEPE has the properties of an osteoblastic phosphatonin and minhibin

Ku affects the ataxia and Rad 3-related/ Chk1-dependent S-phase checkpoint response after camptothecin treatment

ATR inhibition selectively sensitizes G1 checkpoint-deficient cells to lethal premature chromatin condensation

Repeated phosphopeptide motifs in human claspin are phosphorylated by Chk1 and mediate claspin function

Ubiquitin-binding protein RAP80 mediates BRCA1-Dependent DNA damage response

Chk1 suppresses a caspase-2 apoptotic response to DNA damage that bypasses p53, Bcl-2, and Caspase-3

Cell cycle-dependent and ATM-independent expression of human Chk1 kinase

Genotoxic stress targets human Chk1 for degradation by the ubiquitin-proteasome pathway

The authors thank Drs A. Levine, Junjie Chen and Xiaochun Yu for reagents. 

Supplementary Data are available at NAR Online.Conflict of interest statement. None declared.