Carl W. Anderson

DNA-dependent protein kinase catalytic subunit: a relative of phosphatidylinositol 3-kinase and the ataxia telangiectasia gene product

DNA-dependent protein kinase (DNA-PK), which is involved in DNA double-stranded break repair and V(D)J recombination, comprises a DNA-targeting component called Ku and an approximately 460 kDa catalytic subunit, DNA-PKcs. Here, we describe the cloning of the DNA-PKcs cDNA and show that DNA-PKcs falls into the phosphatidylinositol (PI) 3-kinase family. Biochemical assays, however, indicate that DNA-PK phosphorylates proteins but has no detectable activity toward lipids. Strikingly, DNA-PKcs is most similar to PI kinase family members involved in cell cycle control, DNA repair, and DNA damage responses. These include the FKBP12-rapamycin-binding proteins Tor1p, Tor2p, and FRAP, S. pombe rad3, and the product of the ataxia telangiectasia gene, mutations in which lead to genomic instability and predisposition to cancer. The relationship of these proteins to DNA-PKcs provides important clues to their mechanisms of action.

Phosphorylation of human p53 by p38 kinase coordinates N-terminal phosphorylation and apoptosis in response to UV radiation

Components of the ras signaling pathway contribute to activation of cellular p53. In MCF‐7 cells, p38 kinase activated p53 more effectively than other members of the ras pathway. p53 and p38 kinase exist in the same physical complex, and co‐expression of p38 stabilized p53 protein. In vitro, p38 kinase phosphorylated p53 at Ser33 and Ser46, a newly identified site. Mutation of these sites decreased p53‐mediated and UV‐induced apoptosis, and the reduction correlated with total abrogation of UV‐induced phosphorylation on Ser37 and a significant decrease in Ser15 phosphorylation in mutant p53 containing alanine at Ser33 and Ser46. Inhibition of p38 activation after UV irradiation decreased phosphorylation of Ser33, Ser37 and Ser15, and also markedly reduced UV‐induced apoptosis in a p53‐dependent manner. These results suggest that p38 kinase plays a prominent role in an integrated regulation of N‐terminal phosphorylation that regulates p53‐mediated apoptosis after UV radiation.

Human DNA-activated protein kinase phosphorylates serines 15 and 37 in the amino-terminal transactivation domain of human p53.

Human DNA-PK is a nuclear, serine/threonine protein kinase that, when activated by DNA, phosphorylates several DNA-binding substrates, including the tumor suppressor protein p53. To identify which p53 residues are phosphorylated, we examined DNA-PKs ability to phosphorylate synthetic peptides corresponding to human p53 sequences. Serines 15 and 37 in the amino-terminal transactivation domain of human p53, and serines 7 and 18 of mouse p53, were phosphorylated by DNA-PK in the context of synthetic peptides. Other serines in these p53 peptides, and serines in other p53 peptides, including peptides containing the serine 315 p34cdc2 site and the serine 392 casein kinase II site, were not recognized by DNA-PK or were phosphorylated less efficiently. Phosphorylation of the conserved serine 15 in human p53 peptides depended on the presence of an adjacent glutamine, and phosphorylation was inhibited by the presence of a nearby lysine. Phosphorylation of recombinant wild-type mouse p53 was inhibited at high DNA concentrations, suggesting that DNA-PK may phosphorylate p53 only when both are bound to DNA at nearby sites. Our study suggests that DNA-PK may have a role in regulating cell growth and indicates how phosphorylation of serine 15 in DNA-bound p53 could alter p53 function.

Chk2-deficient mice exhibit radioresistance and defective p53-mediated transcription

The mammalian Chk2 kinase is thought to mediate ATM‐dependent signaling in response to DNA damage. The physiological role of mammalian Chk2 has now been investigated by the generation of Chk2‐deficient mice. Although Chk2−/− mice appeared normal, they were resistant to ionizing radiation (IR) as a result of the preservation of splenic lymphocytes. Thymocytes and neurons of the developing brain were also resistant to IR‐induced apoptosis. The IR‐induced G1/S cell cycle checkpoint, but not the G2/M or S phase checkpoints, was impaired in embryonic fibroblasts derived from Chk2−/− mice. IR‐induced stabilization of p53 in Chk2−/− cells was 50–70% of that in wild‐type cells. Caffeine further reduced p53 accumulation, suggesting the existence of an ATM/ATR‐dependent but Chk2‐independent pathway for p53 stabilization. In spite of p53 protein stabilization and phosphorylation of Ser23, p53‐dependent transcriptional induction of target genes, such as p21 and Noxa, was not observed in Chk2−/− cells. Our results show that Chk2 plays a critical role in p53 function in response to IR by regulating its transcriptional activity as well as its stability.

Amplification of PPM1D in human tumors abrogates p53 tumor-suppressor activity

Expression of oncogenic Ras in primary human cells activates p53, thereby protecting cells from transformation. We show that in Ras-expressing IMR-90 cells, p53 is phosphorylated at Ser33 and Ser46 by the p38 mitogen-activated protein kinase (MAPK). Activity of p38 MAPK is regulated by the p53-inducible phosphatase PPM1D, creating a potential feedback loop. Expression of oncogenic Ras suppresses PPM1D mRNA induction, leaving p53 phosphorylated at Ser33 and Ser46 and in an active state. Retrovirus-mediated overexpression of PPM1D reduced p53 phosphorylation at these sites, abrogated Ras-induced apoptosis and partially rescued cells from cell-cycle arrest. Inactivation of p38 MAPK (the product of Mapk14) in vivo by gene targeting or by PPM1D overexpression expedited tumor formation after injection of mouse embryo fibroblasts (MEFs) expressing E1A+Ras into nude mice. The gene encoding PPM1D (PPM1D, at 17q22/q23) is amplified in human breast-tumor cell lines and in approximately 11% of primary breast tumors, most of which harbor wildtype p53. These findings suggest that inactivation of the p38 MAPK through PPM1D overexpression resulting from PPM1D amplification contributes to the development of human cancers by suppressing p53 activation.

Inactivation of the Wip1 phosphatase inhibits mammary tumorigenesis through p38 MAPK-mediated activation of the p16(Ink4a)-p19(Arf) pathway.

Modulation of tumor suppressor activities may provide new opportunities for cancer therapy. Here we show that disruption of the gene Ppm1d encoding Wip1 phosphatase activated the p53 and p16 (also called Ink4a)–p19 (also called ARF) pathways through p38 MAPK signaling and suppressed in vitro transformation of mouse embryo fibroblasts (MEFs) by oncogenes. Disruption of the gene Cdkn2a (encoding p16 and p19), but not of Trp53 (encoding p53), reconstituted cell transformation in Ppm1d-null MEFs. In vivo, deletion of Ppm1d in mice bearing mouse mammary tumor virus (MMTV) promoter–driven oncogenes Erbb2 (also called c-neu) or Hras1 impaired mammary carcinogenesis, whereas reduced expression of p16 and p19 by methylation-induced silencing or inactivation of p38 MAPK correlated with tumor appearance. We conclude that inactivation or depletion of the Wip1 phosphatase with resultant p38 MAPK activation suppresses tumor appearance by modulating the Cdkn2a tumor-suppressor locus.

Human cells contain a DNA-activated protein kinase that phosphorylates simian virus 40 T antigen, mouse p53, and the human Ku autoantigen.

HeLa cells contain a serine/threonine protein kinase (DNA-PK) that is strongly activated in vitro by low concentrations of double-stranded DNA (dsDNA). Activation was specific for dsDNA; both natural DNAs and synthetic oligonucleotides functioned as kinase activators. The fact that DNA-PK activity was rapidly inhibited by incubation with dsDNA and ATP suggests that DNA-PK activity also may be regulated by autophosphorylation. During gel filtration, DNA-PK activity behaved as a 350-kDa protein, and highly purified DNA-PK contained a dsDNA-binding, 350-kDa polypeptide that was phosphorylated in a dsDNA-dependent manner. We conclude that this 350-kDa polypeptide is likely to be DNA-PK. Previously we showed that the dsDNA-activated kinase phosphorylates two threonines at the N terminus of hsp90 alpha (S. P. Lees-Miller and C. W. Anderson, J. Biol. Chem. 264:17275-17280, 1989). Here we show that DNA-PK also phosphorylates the simian virus 40 large tumor antigen, the mouse tumor-suppressor protein p53, the human Ku autoantigen, and two unidentified HeLa DNA-associated polypeptides of 52 and 110 kDa. Identification of these and other newly identified DNA-binding substrates suggest that the dsDNA-activated kinase may regulate transcription, DNA replication, or cell growth.

p53 transcriptional activity is essential for p53‐dependent apoptosis following DNA damage

p53‐mediated transcription activity is essential for cell cycle arrest, but its importance for apoptosis remains controversial. To address this question, we employed homologous recombination and LoxP/Cre‐mediated deletion to produce mutant murine embryonic stem (ES) cells that express p53 with Gln and Ser in place of Leu25 and Trp26, respectively. p53Gln25Ser26 was stable but did not accumulate after DNA damage; the expression of p21/Waf1 and PERP was not induced, and p53‐dependent repression of MAP4 expression was abolished. Therefore, p53Gln25Ser26 is completely deficient in transcriptional activation and repression activities. After DNA damage by UV radiation, p53Gln25Ser26 was phosphorylated at Ser18 but was not acetylated at C‐terminal sites, and its DNA binding activity did not increase, further supporting a role for p53 acetylation in the activation of sequence‐specific DNA binding activity. Most importantly, p53Gln25Ser26 mouse thymocytes and ES cells, like p53−/− cells, did not undergo DNA damage‐induced apoptosis. We conclude that the transcriptional activities of p53 are required for p53‐dependent apoptosis.

DNA damage and the DNA-activated protein kinase

DNA-activated protein kinase (DNA-PK) is a nuclear serine/threonine protein kinase that is activated in vitro by DNA fragments. The cellular targets of DNA-PK are nuclear, DNA-binding, regulatory proteins including Sp1, Fos, Jun, Myc, the tumor suppressor protein p53, and RNA polymerase II. These characteristics suggest a role for DNA-PK in coordinating nuclear processes and as a modulator of checkpoint mechanisms activated by DNA damage.

ATM Mediates Phosphorylation at Multiple p53 Sites, Including Ser46, in Response to Ionizing Radiation

The p53 tumor suppressor protein preserves genome integrity by regulating growth arrest and apoptosis in response to DNA damage. In response to ionizing radiation (IR), ATM, the gene product mutated in ataxia telangiectasia, stabilizes and activates p53 through phosphorylation of Ser15 and (indirectly) Ser20. Here we show that phosphorylation of p53 on Ser46, a residue important for p53 apoptotic activity, as well as on Ser9, in response to IR also is dependent on the ATM protein kinase. IR-induced phosphorylation at Ser46 was inhibited by wortmannin, a phosphatidylinositol 3-kinase inhibitor, but not PD169316, a p38 MAPK inhibitor. p53 C-terminal acetylation at Lys320 and Lys382, which may stabilize p53 and activate sequence-specific DNA binding, required Ser15phosphorylation by ATM and was enhanced by phosphorylation at nearby residues including Ser6, Ser9, and Thr18. These observations, together with the proposed role of Ser46 phosphorylation in mediating apoptosis, suggest that ATM is involved in the initiation of p53-dependent apoptosis after IR in human lymphoblastoid cells.