Dae-Sik Lim

ATM phosphorylates p95/nbs1 in an S-phase checkpoint pathway.

The rare diseases ataxia-telangiectasia (AT), caused by mutations in the ATM gene, and Nijmegen breakage syndrome (NBS), with mutations in the p95/nbs1 gene, share a variety of phenotypic abnormalities such as chromosomal instability, radiation sensitivity and defects in cell-cycle checkpoints in response to ionizing radiation. The ATM gene encodes a protein kinase that is activated by ionizing radiation or radiomimetic drugs, whereas p95/nbs1 is part of a protein complex that is involved in responses to DNA double-strand breaks. Here, because of the similarities between AT and NBS, we evaluated the functional interactions between ATM and p95/nbs1. Activation of the ATM kinase by ionizing radiation and induction of ATM-dependent responses in NBS cells indicated that p95/nbs1 may not be required for signalling to ATM after ionizing radiation. However, p95/nbs1 was phosphorylated on serine 343 in an ATM-dependent manner in vitro and in vivo after ionizing radiation. A p95/nbs1 construct mutated at the ATM phosphorylation site abrogated an S-phase checkpoint induced by ionizing radiation in normal cells and failed to compensate for this functional deficiency in NBS cells. These observations link ATM and p95/nbs1 in a common signalling pathway and provide an explanation for phenotypic similarities in these two diseases.

Substrate Specificities and Identification of Putative Substrates of ATM Kinase Family Members

Ataxia telangiectasia mutated (ATM) phosphorylates p53 protein in response to ionizing radiation, but the complex phenotype of AT cells suggests that it must have other cellular substrates as well. To identify substrates for ATM and the related kinases ATR and DNA-PK, we optimized in vitro kinase assays and developed a rapid peptide screening method to determine general phosphorylation consensus sequences. ATM and ATR require Mn2+, but not DNA ends or Ku proteins, for optimal in vitro activity while DNA-PKCs requires Mg2+, DNA ends, and Ku proteins. From p53 peptide mutagenesis analysis, we found that the sequence S/TQ is a minimal essential requirement for all three kinases. In addition, hydrophobic amino acids and negatively charged amino acids immediately NH2-terminal to serine or threonine are positive determinants and positively charged amino acids in the region are negative determinants for substrate phosphorylation. We determined a general phosphorylation consensus sequence for ATM and identified putative in vitro targets by using glutathioneS-transferase peptides as substrates. Putative ATM in vitro targets include p95/nibrin, Mre11, Brca1, Rad17, PTS, WRN, and ATM (S440) itself. Brca2, phosphatidylinositol 3-kinase, and DNA-5B peptides were phosphorylated specifically by ATR, and DNA Ligase IV is a specific in vitro substrate of DNA-PK.

A mutation in mouse rad51 results in an early embryonic lethal that is suppressed by a mutation in p53.

RecA in Escherichia coli and its homolog, ScRad51 in Saccharomyces cerevisiae, are known to be essential for recombinational repair. The homolog of RecA and ScRad51 in mice, MmRad51, was mutated to determine its function. Mutant embryos arrested early during development. A decrease in cell proliferation, followed by programmed cell death and chromosome loss, was observed. Radiation sensitivity was demonstrated in trophectoderm-derived cells. Interestingly, embryonic development progressed further in a p53 null background; however, fibroblasts derived from double-mutant embryos failed to proliferate in tissue culture.

Two molecularly distinct G(2)/M checkpoints are induced by ionizing irradiation.

ABSTRACT Cell cycle checkpoints are among the multiple mechanisms that eukaryotic cells possess to maintain genomic integrity and minimize tumorigenesis. Ionizing irradiation (IR) induces measurable arrests in the G1, S, and G2 phases of the mammalian cell cycle, and the ATM (ataxia telangiectasia mutated) protein plays a role in initiating checkpoint pathways in all three of these cell cycle phases. However, cells lacking ATM function exhibit both a defective G2 checkpoint and a prolonged G2 arrest after IR, suggesting the existence of different types of G2 arrest. Two molecularly distinct G2/M checkpoints were identified, and the critical importance of the choice of G2/M checkpoint assay was demonstrated. The first of these G2/M checkpoints occurs early after IR, is very transient, is ATM dependent and dose independent (between 1 and 10 Gy), and represents the failure of cells which had been in G2 at the time of irradiation to progress into mitosis. Cell cycle assays that can distinguish mitotic cells from G2 cells must be used to assess this arrest. In contrast, G2/M accumulation, typically assessed by propidium iodide staining, begins to be measurable only several hours after IR, is ATM independent, is dose dependent, and represents the accumulation of cells that had been in earlier phases of the cell cycle at the time of exposure to radiation. G2/M accumulation after IR is not affected by the early G2/M checkpoint and is enhanced in cells lacking the IR-induced S-phase checkpoint, such as those lacking Nbs1 or Brca1 function, because of a prolonged G2 arrest of cells that had been in S phase at the time of irradiation. Finally, neither the S-phase checkpoint nor the G2 checkpoints appear to affect survival following irradiation. Thus, two different G2 arrest mechanisms are present in mammalian cells, and the type of cell cycle checkpoint assay to be used in experimental investigation must be thoughtfully selected.

Caspase-3-dependent Cleavage of Bcl-2 Promotes Release of Cytochrome c

Caspases are cysteine proteases that mediate apoptosis by proteolysis of specific substrates. Although many caspase substrates have been identified, for most substrates the physiologic caspase(s) required for cleavage is unknown. The Bcl-2 protein, which inhibits apoptosis, is cleaved at Asp-34 by caspases during apoptosis and by recombinant caspase-3 in vitro. In the present study, we show that endogenous caspase-3 is a physiologic caspase for Bcl-2. Apoptotic extracts from 293 cells cleave Bcl-2 but not Bax, even though Bax is cleaved to an 18-kDa fragment in SK-NSH cells treated with ionizing radiation. In contrast to Bcl-2, cleavage of Bax was only partially blocked by caspase inhibitors. Inhibitor profiles indicate that Bax may be cleaved by more than one type of noncaspase protease. Immunodepletion of caspase-3 from 293 extracts abolished cleavage of Bcl-2 and caspase-7, whereas immunodepletion of caspase-7 had no effect on Bcl-2 cleavage. Furthermore, MCF-7 cells, which lack caspase-3 expression, do not cleave Bcl-2 following staurosporine-induced cell death. However, transient transfection of caspase-3 into MCF-7 cells restores Bcl-2 cleavage after staurosporine treatment. These results demonstrate that in these models of apoptosis, specific cleavage of Bcl-2 requires activation of caspase-3. When the pro-apoptotic caspase cleavage fragment of Bcl-2 is transfected into baby hamster kidney cells, it localizes to mitochondria and causes the release of cytochrome c into the cytosol. Therefore, caspase-3-dependent cleavage of Bcl-2 appears to promote further caspase activation as part of a positive feedback loop for executing the cell.

Ku86-Deficient Mice Exhibit Severe Combined Immunodeficiency and Defective Processing of V(D)J Recombination Intermediates

Ku is a heterodimeric DNA end binding complex composed of 70 and 86 kDa subunits. Here, we show that Ku86 is essential for normal V(D)J recombination in vivo, as Ku86-deficient mice are severely defective for formation of coding joints. Unlike severe combined immunodeficient (scid) mice, Ku86-deficient mice are also defective for signal joint formation. Both hairpin coding ends and blunt full-length signal ends accumulate. Contrary to expectation, Ku86 is evidently not required for protection of either type of V(D)J recombination intermediate. Instead, V(D)J recombination appears to be arrested after the cleavage step in Ku86-deficient mice. We suggest that Ku86 may be required to remodel or disassemble DNA-protein complexes containing broken ends, making them available for further processing and joining.

The tumour suppressor RASSF1A regulates mitosis by inhibiting the APC–Cdc20 complex

The tumour suppressor gene RASSF1A is frequently silenced in lung cancer and other sporadic tumours as a result of hypermethylation of a CpG island in its promoter. However, the precise mechanism by which RASSF1A functions in cell cycle regulation and tumour suppression has remained unknown. Here we show that RASSF1A regulates the stability of mitotic cyclins and the timing of mitotic progression. RASSF1A localizes to microtubules during interphase and to centrosomes and the spindle during mitosis. The overexpression of RASSF1A induced stabilization of mitotic cyclins and mitotic arrest at prometaphase. RASSF1A interacts with Cdc20, an activator of the anaphase-promoting complex (APC), resulting in the inhibition of APC activity. Although RASSF1A does not contribute to either the Mad2-dependent spindle assembly checkpoint or the function of Emi1 (ref. 1), depletion of RASSF1A by RNA interference accelerated the mitotic cyclin degradation and mitotic progression as a result of premature APC activation. It also caused a cell division defect characterized by centrosome abnormalities and multipolar spindles. These findings implicate RASSF1A in the regulation of both APC–Cdc20 activity and mitotic progression.

The Hippo–Salvador pathway restrains hepatic oval cell proliferation, liver size, and liver tumorigenesis

Loss of Hippo signaling in Drosophila leads to tissue overgrowth as a result of increased cell proliferation and decreased cell death. YAP (a homolog of Drosophila Yorkie and target of the Hippo pathway) was recently implicated in control of organ size, epithelial tissue development, and tumorigenesis in mammals. However, the role of the mammalian Hippo pathway in such regulation has remained unclear. We now show that mice with liver-specific ablation of WW45 (a homolog of Drosophila Salvador and adaptor for the Hippo kinase) manifest increased liver size and expansion of hepatic progenitor cells (oval cells) and eventually develop hepatomas. Moreover, ablation of WW45 increased the abundance of YAP and induced its localization to the nucleus in oval cells, likely accounting for their increased proliferative capacity, but not in hepatocytes. Liver tumors that developed in mice heterozygous for WW45 deletion or with liver-specific WW45 ablation showed a mixed pathology combining characteristics of hepatocellular carcinoma and cholangiocarcinoma and seemed to originate from oval cells. Together, our results suggest that the mammalian Hippo–Salvador pathway restricts the proliferation of hepatic oval cells and thereby controls liver size and prevents the development of oval cell–derived tumors.

ER71 Acts Downstream of BMP, Notch, and Wnt Signaling in Blood and Vessel Progenitor Specification

FLK1-expressing (FLK1(+)) mesoderm generates blood and vessels. Here, we show that combined BMP, Notch, and Wnt signaling is necessary for efficient FLK1(+) mesoderm formation from embryonic stem cells (ESCs). Inhibition of BMP, Notch, and Wnt signaling pathways greatly decreased the generation of FLK1(+) mesoderm and expression of the Ets transcription factor Er71. Enforced expression of ER71 in ESCs resulted in a robust induction of FLK1(+) mesoderm; rescued the generation of FLK1(+) mesoderm when blocked by BMP, Notch, and Wnt inhibition; and enhanced hematopoietic and endothelial cell generation. Er71-deficient mice had greatly reduced FLK1 expression, died early in gestation, and displayed severe blood and vessel defects that are highly reminiscent of the Flk1 null mouse phenotype. Collectively, we provide compelling evidence that ER71 functions downstream of BMP, Notch, and Wnt signals and regulates FLK1(+) mesoderm, blood, and vessel development.

Mst1 inhibits autophagy by promoting the interaction between Beclin1 and Bcl-2

Yasuhiro Maejima1,4, Shiori Kyoi1, Peiyong Zhai1, Tong Liu2, Hong Li2, Andreas Ivessa1, Sebastiano Sciarretta1, Dominic P. Del Re1, Daniela K. Zablocki1, Chiao-Po Hsu3, Dae-Sik Lim5, Mitsuaki Isobe4, and Junichi Sadoshima1,6 1Cardiovascular Research Institute, Department of Cell Biology and Molecular Medicine, Rutgers, The State University of New Jersey, New Jersey Medical School, Newark, NJ 07103Here we show that Mst1, a proapoptotic kinase, impairs protein quality control mechanisms in the heart through inhibition of autophagy. Stress-induced activation of Mst1 in cardiomyocytes promoted accumulation of p62 and aggresome formation, accompanied by the disappearance of autophagosomes. Mst1 phosphorylated the Thr108 residue in the BH3 domain of Beclin1, which enhanced the interaction between Beclin1 and Bcl-2 and/or Bcl-xL, stabilized the Beclin1 homodimer, inhibited the phosphatidylinositide 3-kinase activity of the Atg14L-Beclin1-Vps34 complex and suppressed autophagy. Furthermore, Mst1-induced sequestration of Bcl-2 and Bcl-xL by Beclin1 allows Bax to become active, thereby stimulating apoptosis. Mst1 promoted cardiac dysfunction in mice subjected to myocardial infarction by inhibiting autophagy, associated with increased levels of Thr108-phosphorylated Beclin1. Moreover, dilated cardiomyopathy in humans was associated with increased levels of Thr108-phosphorylated Beclin1 and signs of autophagic suppression. These results suggest that Mst1 coordinately regulates autophagy and apoptosis by phosphorylating Beclin1 and consequently modulating a three-way interaction among Bcl-2 proteins, Beclin1 and Bax.