【摘要】 mre11在dna损伤反应的信号转导通路中起着重要的作用。本研究查明hmre11与依托泊甙所致人单核细胞白血病细胞株u937 dna双链断裂的关系。在依托泊甙（vp16）处理u937细胞后,运用脉冲凝胶电泳(pfge)检测dna双链断裂, 采用rtpcr技术检测mre11的转录水平,借助免疫荧光技术检测hmre11蛋白的分布改变,并通过流式细胞术检测其细胞周期。结果表明：vp16诱导u937细胞dna双链断裂的发生率与其剂量高度相关，从2 μg/ml时的 (13.0±2.3)% 增至20 μg/ml时的(32.0±4.3)%（p<0.01）。但vp16诱导u937细胞前后不同时间内hmre11 mrna 水平未见变化（p>0.05）。hmre11蛋白丰富而均匀分布于核仁外的细胞核中，但经vp16处理后则形成独立的核灶（nuclear focus），并且存在这种核灶的细胞数和细胞中的核灶数量随vp16的剂量增加而增多。经100 μg/ml vp16处理（2小时）后8小时形成hmre11蛋白灶的u937细胞数达到（61.54±3.6）％［对照组为（0.47±1.17）％，p﹤0.01］，而且47.55±2.35%的u937细胞［对照组（21.95±2.91）%，p﹤0.05］停滞于s期，然后逐渐下降。结论：hmre11 蛋白核灶形成可能参与vp16所致人单核白血病细胞株u937的dna损伤修复过程。
【关键词】
j exp hematol 2007; 15(1)hmre11 plays an important role in u937 cellular response to dna doublestrand breaks following etoposide
in both mitotic and meiotic processes, cellular surveillance of the integrity of genetic information transmission from parental cells to their subsequent generations is carried out by a network of proteins primarily involved in cellcycle regulation, dna replication, dna repair, and chromosome segregation［1-3］. dna doublestrand breaks (dsbs) occur frequently during dna replication and are induced by ionizing radiation and certain chemical substances［4］. in this context, the mammalian mre11 represents an essential multifunctional protein that promotes the repair of dna dsbs and plays a role in the signal transduction of dna damage response［5，6］.
it has been shown that etoposide（vp16）, a topoisomerase ii inhibitor, greatly reduces the ability of topoisomerase ii to create proteinbridged dsbs, and hmre11 protein can play an important role during this process［7］. in this research we speculated the relationship between hmre11 focus formation and dna dsbs caused by etoposide in human promonocytic cells u937.
materials and methods
cell culture
human promonocytic cells u937 were obtained from a preservation center for typical culture in wuhan university（wuhan）and were maintained as suspension cultures in rpmi 1640 (hyclone) containing 10% (v/v) fetal bovine serum (gibco) , 100 u/ml penicilline and 100 μg/ml streptomycin in an humidified atmosphere with 5% co2 at 37℃ (ph 7.1). then the u937 cells growing to a logarithmic growth phase were passaged and cultured at different concentration of vp16 and for different time in the following experiments.
reagents
trizol kit was purchased from invivogen company (usa). the reverse transcription kit, taq dna polymerase and dntps were all purchased from mbi company (usa). the rabbit antihuman hmre11 polyclonal antibody（pc388t）was purchased from oncogene company (usa). the fitc labelled goat antirabbit igg was from pierce company (usa).
pulsedfield gel electrophoresis
u937 cells were washed in pbs and resuspended. an equal volume of 2% lowmeltingpoint agarose (gibco) at 55°c was added to the cell suspension, and the melting gel was cast in plug molds (90 μl per plug). the plugs were transferred to 0.5 mol/l edta (ph 9.0), 10 mmol/l triscl (ph 7.5), 20 mg/ml sarkosyl, and 2 mg/ml proteinase k; and incubated at 50°c for 24 hours. after washing in tris/edta buffer［1 mmol/l edta (ph 8.0), 10 mmol/l triscl (ph 7.0)］,the resultant dna plugs were then incubated for 1 hours in tris/edta buffer containing 0.2 mg/ml rnase a. the agarose plugs containing purified dna were kept in tris/edta buffer at 4°c and used for electrophoresis. pulsedfield gel electrophoresis（pfge） was carried out in 1% agarose gels (0.5×tbe) on hexafield horizontal gel electrophoresis apparatus (invitrogen) at 14°c for 22 hours at 6 v/cm, which includes a hexagonal array of electrodes having a reorientation angle of 120°. after electrophoresis, gels were stained with ethidium bromide and then photographed on a uvtransilluminator. for liquid scintillation counting the gel lanes were cut into 5 mm segment. each segments was dissolved in scintillation liquid and counted in a canberrapackard minaxi tricarb 4 000 counter. the counts were normalized as the ratio of (count of band)/(total dna count).
detection of hmre11 mrna expression in u937 cells by rtpcr
total rna was extracted from u937 cells by trizol reagent (mbi), cdna was synthesized, then pcr was performed. βactin was used as control. the sequences of primers were as follows: hmre11 sense primer 5′tcgaagagtccagcagtg3′, and antisense primer 5′ctcgcagtcgtacaagag3′; β actin sense primer 5′aaggccaaccgcgagaag atg 3′, and antisense primer 5′acaggactcca tgcccaggaa 3′. reaction conditions for pcr: predenature at 95℃ for 3 minutes cycling parameters were as follows: denature at 94℃ for 60 seconds, anneal at 52℃ for 60 seconds, extend at 72℃for 90 seconds, totally for 30 cycles; for the last cycle extend at 72℃ for 8 minutes. pcr products were analyzed by electrophoresis on 1.2% agarose gel.
detection of hmre11 protein expression and distribution in u937 cells by immunofluorescence as described as maser rs［7］
u937 cells were treated with vp16 and the slides were prepared with cytospin. following fixation, slides were washed three times for 5 minutes each in pbs and blocked in 10% bsapbs for 1 hour at room temperature. slides were incubated overnight at 4℃ with rabbit antihmrel 1(1∶100 dilution), followed by fitcconjugated goat antirabbit secondary antiserum (1∶200 dilution) for 30 minutes at room temperature, and then were observed via fluorescence microscopy.
flow cytometry
the changes of cell cycle progression in u937 cells were determined before and after treatment with etoposide. approximately 1×106 cells in pbs were fixed as a single cell suspension in 70% (v/v) cold ethanol overnight at 4℃. samples were subsequently washed with pbs, resuspended in 0.02 mg/ml rnase a followed by incubation for 30 minutes at 37℃,then in a 0.1%(w/v)triton x100 solution containing 0.1 mg/ml propidium iodide for 30 min at 4℃. analysis was carried out on flow cytometer (facscalibur，becton dickinson) with an argon laser tuned to 488 nm.
statistical analysis
two sample means were compared by student's t tests. sample means over two were compared by f tests, the sample rates were compared using the χ2 test. a p value less than 0.05 was considered significant.
results
dna fragments of u937 cells were induced in a vp16 dosedependent manner
u937 cells were treated with different doses of vp16 for 2 hours, and the dsbs were assayed by pfge after 8 hours. the percentage of dsbs induced by vp16 increased following treatment with the vp16, from 13.0±2.3% in 2μg/ml to 32.0±4.3% in 20 μg/ml（p<0.01） (figure 1) .
mrna level of hmre11 gene in u937 cells is unchanged following vp16 treatment
after u937 cells were treated with 100 μg/ml vp16 for 2 hours and harvested at the indicated time (figure 2a, figure 2b) , the mrna level of hmre11 in these cells at different times was evaluated by using rtpcr and significant differences were found （p>0.05）.
hmre11 protein formed nuclear foci in response to dna damage of vp16
immunofluorescence was used to explore whether vp16 induces alterations in the subcellular distribution of
figure 1a. analysis of dna dsbs of u937 cells induced by vp16 at different doses by pfge. lane 1: 0 μg/ml vp16(control). lane 2: 2 μg/ml vp16. lane 3: 5 vp16. lane 4: 10 μg/ml vp16. lane 5: 20 μg/ml vp16.
figure 1b. kinetics of dna doublestrand breaks of u937 cells induced by vp16 at different doses (n=5).
figure 2a. mrna expression of hmre11 gene in u937 cells analyzed by rtpcr treated with vp16 (100 μg/ml) at different times (n=5). m: marker. lane 1: 0 hour (control). lane 2: 8 hours. lane 3 :12 hours. lane 4: 24 hours.
figure 2b. optical density of mrna of hmre11 gene in u937 cells treated by vp16(100 μg/ml) at different time(n=5).
hmre11 protein in the human promonocytic cell line u937. it was found that hmre11 protein was abundantly and uniformly distributed in the nuclei of untreated u937 cells outside of nucleoli(figure 3a), hower, it formed discrete nuclear focus following vp16 treatment (figure 3b).
figure 3. hmre11 protein of u937 cells forms nuclear foci following treatment with vp16. u937 cells were spread onto glass slides and either left untreated (a) or treated (b) with 100 μg/ml vp16 for 2 hours and fixed at 8 hours after treatment. cells were then stained with antihmre11 antibody followed by fitcconjugated goat antirabbit antiserum as described in materials and methods.
nuclear focus of hmre11 protein forms in a vp16 dosedependent manner
the doses dependence of hmre11 protein nuclear focus formation in u937 cells following vp16 treatment was examined. these cells were treated with 2, 5,10 and 20 μg/ml vp16 for 8 hours respectivey. nuclei were then scored by the number of observed hmre11 nuclear foci and divided in three categories: less than 5 (negative), 5 to 20, or>20 per nucleus［7］. it was found that the mean of nuclear foci was increased by 5-20 times following the drug treatment （p<0.01）(figure 4). an average of 5 nuclear foci per positive nucleus were observed at the dose of 2 μg/ml, and this was increased to an average of over 14 nuclear foci per positive nucleus at the dose of 20 μg/ml. the percentage of nuclei containing hmre11 nuclear foci was also increased following treatment with vp16 (figure 5), from less than 10% after 2 μg/ml to over 50% after 20 μg/ml（p<0.01）. besides, the time course of nuclear focus formation for u937 cells following exposure to vp16 was also determined. u937 cells were treated with 100 μg/ml vp16 for 2 hours and then fixed respectively at 4, 8, 12 and 24 hours. independent slides were stained for hmre11. the percentage of nuclei with hmre11 nuclear focus was increased to a maximum of greater than 50% at 8 hours, with a subsequent decrease in the percentage of nuclear focus positive cells by 24 hours (figure 6).
figure 4. mean value of foci in positive nuclei(5-20 foci)(n=5).
figure 5. percentage of nuclear focuspositive nuclei(>5 foci)(n=5)
figure 6. percentage of nuclei containing over 5 nuclear foci after treatment with vp16(100 μg/ml) at different time(n=5).
number of hmre11 protein focuspositive nuclei following vp16 treatment corresponded approximately to the percentage of sphase u937 cells
the cell cycle distribution of u937 cells at 8 hours after being treated with 100 μg/ml vp16 for 2 hours was as follows： g0/g1 phase 32.76±1.33%, s phase 47.55±2.35%, g2/m phase 19.69±2.11%; but that without 100 μg/ml vp16 were g0/g1 phase 49.29±0.83%, s 21.95±2.91% and g2/m phase 28.76±3.72%(figure 7). these suggest that the most of u937 cells treated with vp16 were arrested at sphase of cell cycle（p﹤0.05）. it was also found at the same time that the rate of u937 cells with the nuclear focus formation of hmre11 protein at 8 hours after being treated with 100 μg/ml vp16 reached 61.54±3.6％, but that of the control u937 cells only accounted for 0.47±1.17％（p﹤0.01）. this also hinted that a positive correlation between the numbers of u937 cells with hmre11 protein nuclear foci and sphase u937 cells.
figure 7. cell cycle distribution of u937 cells at 8 hours after being treated without (a) or with (b) 100 μg/ml vp16 for 2 hours. a: g0/g1phase 49.29±0.83%, s 21.95±2.91%, g2/mphase 28.76±3.72%; b: g0/g1phase 32.76±1.33%, sphase 47.55±2.35% g2/mphase 19.69±2.11%.
discussion
the mre11 protein participates in both the nonhomologous end joining （nhej）and homologous recombination （hr）dna repair pathways, as a part of complex of proteins including the hrad50 and nbs1 proteins［8-10］. our results suggest that the effect of vp16 inducing dna dsbs was linearly dependent upon its dose, but the hmre11 mrna levels are not altered following vp16 treatment though hmre11 protein nuclear foci are specific for dna dsbs, that is the same as steadystate protein levels evaluated by immunoblotting showed by maser rs［7］. therefore, this nuclear focus formation might result from changes in the location rather than the abundance of hmre11 protein.
in this study, immunofluorescence used to describe the redistribution of hmre11 protein to discrete foci within the nucleus of u937 cells following dna damage induced by vp16, a potent inducer of dna dsbs as topoisomerase ii inhibitor［11］. our results suggest that the number of nuclear focus per nucleus was also increased following vp16 treatment, indicating that the number of dna dsbs initially formed determines the multiplicity of the hmre11 nuclear focus response. immunofluorescence analysis in situ offers a way to correlate various metabolic processes with the locations of specific proteins and has provided evidence that certain functions may be regulated by their association with particular sites or structures within the nucleus. immunofluorescence has provided evidence that diverse processes, including dna replication and dna repair［12］, may be compartmentalized within the nucleus. we also noticed that the percentage of nuclei with hmre11 nuclear focus was increased to maximum at 8 hours after treatment by vp16, with a subsequent decrease in the percentage of nuclear focuspositive cells by 24 hours. vock eh［13］ described that vp16 induced dsbs in absence of cell viability impairment at 8 hours, while at 24 hours the reversed effect was found: reduction of cell viability occurred in absence of dsbs. thus, the hmre11 protein might mainly play roles in detecting the signals of dna damage from initiation to peak point at 8 hours after vp16 treatment, then the number of nuclear focus decreases with the increase of dsbs repairs. meanwhile, vp16 may also induce cell death by interfering with gene transcription since topoisomerases have been shown to play an important role in its regulation［14］.
it was found that the number of hmre11 protein focuspositive nuclei following vp16 treatment corresponded approximately to the percentage of sphase u937 cells. the reasons are as follows for this：(1) vp16 preferentially induces the damage of sphase u937 cells high expressing topoisomerase ⅱ［14］. (2) the hmre11 protein complex could make u937 cells arrested in s phase by transferring the signaling of dna damage response so as to get good repair of damaged dna.
in conclusion, this research has demonstrated that the hmre11 protein can play an important role in the human promonocytic cell u937 response to topoisomerase ⅱ inhibitor vp16 induced dna damage by its nuclear focus formation or protein complex transferring dna damage signal. further assessment of the function of hmre11 in this process need to identify dna dsbs in situ and thereby to localize nuclear foci of hmre11 protein to sites of damage. elucidation of the biochemical activities and composition of the hmre11 protein complex will also give important new insight to dna repair of u937 cells following vp16 treatment.
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