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Dev Cell
2015 Aug 10;343:364-72. doi: 10.1016/j.devcel.2015.06.002.
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RAD18 Is a Maternal Limiting Factor Silencing the UV-Dependent DNA Damage Checkpoint in Xenopus Embryos.
Kermi C
,
Prieto S
,
van der Laan S
,
Tsanov N
,
Recolin B
,
Uro-Coste E
,
Delisle MB
,
Maiorano D
.
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In early embryos, the DNA damage checkpoint is silent until the midblastula transition (MBT) because of maternal limiting factors of unknown identity. Here we identify the RAD18 ubiquitin ligase as one such factor in Xenopus. We show, in vitro and in vivo, that inactivation of RAD18 function leads to DNA damage-dependent checkpoint activation, monitored by CHK1 phosphorylation. Moreover, we show that the abundance of both RAD18 and PCNA monoubiquitylated (mUb) are developmentally regulated. Increased DNA abundance limits the availability of RAD18 close to the MBT, thereby reducing PCNA(mUb) and inducing checkpoint derepression. Furthermore, we show that this embryonic-like regulation can be reactivated in somatic mammalian cells by ectopic RAD18 expression, therefore conferring resistance to DNA damage. Finally, we find high RAD18 expression in cancer stem cells highly resistant to DNA damage. Together, these data propose RAD18 as a critical embryonic checkpoint-inhibiting factor and suggest that RAD18 deregulation may have unexpected oncogenic potential.
Figure 1. RAD18 Is Limiting near the MBT(A) Constitutive POLη chromatin binding and PCNAmUb at a low N/C ratio. Shown is a western blot of nucleosolic (top) or chromatin (bottom) fractions obtained from egg extracts containing sperm nuclei at a low (100 nuclei/μl) or high (1000 nuclei/μl) N/C ratio, UV-irradiated (+UV) or not (−UV), upon 50-min incubation at room temperature. Histone H3 served as the chromatin loading control.(B) Abundance of the indicated proteins (determined by western blot) remaining in the eggcytoplasm (left) or chromatin (right) after 90-min incubation with sperm chromatin at a low or high N/C ratio.(C) RAD18 interacts with DRF1 at a low N/C ratio in Xenopus egg extracts. Shown is a western blot of RAD18 immunoprecipitated (IP) from eggcytoplasm after nuclear assembly at a low or high N/C ratio. Short (light) and long (dark) exposures of DRF1 are shown. 10-fold more RAD18 immunoprecipitates at a low N/C ratio are also shown (right).(D and E) RAD18 and PCNAmUb are developmentally regulated. (D) Shown are western blots of total embryo protein extracts at the indicated stages of development (numbers) in the absence (DMSO) or presence of the proteasome inhibitor MG132 (30 μM; E). RAD18 quantification is expressed as relative optical density (ROD). Means ± SD are represented (∗∗p < 0.001). See also Figure S1.
Figure 2. RAD18 Depletion Induces CHK1 Phosphorylation at a Low N/C Ratio upon UV Damage(A and B) Western blot of cytoplasm (A) or chromatin fractions (B) obtained at a low N/C ratio upon immunodepletion with RAD18 antibodies.(C) Coomassie blue stain of the recombinant His6-RAD6-RAD18 complex expressed and purified from insect cells. kDa, molecular weight of standard protein markers.(D) CHK1 phosphorylation analyzed by western blot in either mock-depleted or RAD18-depleted egg extracts with UV-irradiated (+UV) or not (−UV) sperm nuclei at a low N/C ratio as well as with recombinant (Rec) His6-RAD6-RAD18. CHK1 served as the loading control.(E) Left: western blot of chromatin fractions analyzed in the absence (−) or presence (+) of UV irradiation with or without the recombinant His6-RAD6-RAD18 complex added 30 min after incubation at room temperature at a high N/C ratio. Reactions were incubated at room temperature for 60 min. Right: quantification of RPA2 accumulation shown at the left. The numbers indicate the lanes shown at the left. Means ± SD are shown (n = 3).(F) Left: overexpression of RAD18C28F delays embryonic cleavages. Shown are images of stage 6.5 embryos injected with either water (mock), XRAD18WT, or XRAD18C28F mRNA and UV-irradiated (+UV) or not (–UV). Right: quantification of embryos shown at the left reaching stage 6.5 (pre-MBT). Means ±SD are represented (n = 3).(G) RAD18 overexpression inhibits UV-dependent CHK1 phosphorylation in Xenopus embryos. Shown is a western blot of protein extracts from stage 7 embryos obtained upon injection of RAD18 mRNAs (from F). See also Figure S2.
Figure 3. Ectopic RAD18 Expression Induces Spontaneous TLS POLη Foci and Inhibits UV-Dependent CHK1 Phosphorylation in Mammalian Cells(A) Western blot of HEK293T cell extracts obtained upon transfection with RAD18 or empty vector (pCDNA3).(B) Expression of RAD18, and not RAD6, induces constitutive POLη foci. HEK293T cells co-transfected with the indicated vectors and eGFP-POLη were stained with DAPI to visualize DNA and observed for eGFP fluorescence. Scale bar, 10 μm.(C) Quantification of eGFP-POLη foci from the experiment described in (B). Means ± SD are shown (n = 3). V, vector.(D) Western blot of CHK1S345 phosphorylation in HEK293T cells expressing empty vector or XRAD18 upon UV irradiation (+UV) at the indicated times. Quantification of CHK1S345 phosphorylation is also shown (n = 2).(E and F) Checkpoint inhibition and constitutive eGFP-POLη foci upon expression of PCNAK164R-mUb fusion. (E) Western blot of total extracts made from HEK293T cells UV-irradiated (+UV) or not (−UV) and expressing the indicated vectors. (F) Cells co-transfected with the indicated vectors and eGFP-POLη were analyzed as described in (B). Scale bar, 10 μm. Quantification of eGFP-POLη foci is also shown (right). Means and SD are shown (∗∗∗p < 0.0001; n = 3; see also Figure S3).
Figure 4. High RAD18 Expression Is Associated with Resistance to DNA Damage(A) RAD18 expression inhibits UV-dependent RPA focus formation in mammalian cells. HEK293T cells transfected with the indicated expression vectors stained with DAPI to visualize DNA and RPA2 antibodies were viewed by fluorescence microscopy. Scale bar, 10 μm. Quantification of RPA2 foci from the experiment described in (A) is also shown. Means ± SD are shown (∗∗p < 0.01, n = 3).(B and C) (B) Survival curves of asynchronous NIH 3T3 cells stably expressing either empty vector or low levels of RAD18WT or RAD18C28F mutant challenged by the indicated does of UV-C or cisplatin (CisPt, C) normalized to non-irradiated cells (mock). Means ± SD are shown (∗∗p < 0.01, n = 3).(D) Expression of RAD18 mRNA in gliospheres (CD133+, Glioma) compared with HeLa cells by RT-PCR. Means ± SD are shown (n = 3).(E) Western blot of total cell extracts from glioblastoma biopsies (grade 4), differentiated counterparts (progenitors, CD133−), or HeLa cells.(F) Top: western blot of U87 glioblastoma cell extracts treated with control siRNA (siLuc) or a RAD18-specific siRNA (siRAD18) or co-transfected with RAD18 siRNA and a plasmid expressing RAD18WT (siRAD6+ RAD18). Bottom: survival curves of U87 glioblastoma cells treated as described for the top and challenged with the indicated doses of cisplatin compared with non-treated cells (mock). Means ± SD are shown (n = 3, see also Figure S4).
