Nucleophosmin interacts with and inhibits the catalytic function of eukaryotic initiation factor 2 kinase PKR.

In normal cells the protein kinase PKR effects apoptosis in response to various extra and intracellular cues and can also function to suppress the neoplastic phenotype. Because most neoplastic cells are resistant to certain apoptotic cues, we reasoned that an early molecular event in carcinogenesis or leukemogenesis might be the inactivation of PKR by expression or activation of intracellular PKR inhibitors. Seeking novel PKR-modulating proteins we report here that nucleophosmin (NPM), a protein frequently overexpressed in a variety of human malignancies, binds to PKR, and inhibits its activation. Co-immunoprecipitation and in vitro binding experiments showed that NPM associated with PKR. Kinase assays demonstrated that recombinant NPM inhibited PKR activation in a dose-dependent manner. In addition, purified recombinant NPM was phosphorylated by activated PKR. Most importantly, overexpression of NPM suppressed PKR activity, enhanced protein synthesis, and inhibited apoptosis. Lymphoblasts from patients with Fanconi anemia (FA) expressed low levels of NPM, which correlated with high ground-state activation of PKR and cellular hypersensitivity to apoptotic cues, but enforced expression of NPM in these mutant cells reduced aberrant apoptotic responses. Inhibition of PKR by NPM may be one mechanism by which neoplastic clones evolve in sporadic malignancies and in neoplastic cells arising in the context of the cancer predisposition syndrome, Fanconi anemia.

Acquired FANCA dysfunction and cytogenetic instability in adult acute myelogenous leukemia.

Myelodysplastic and leukemic stem cell clones that evolve in children and adults with Fanconi anemia universally bear complex cytogenetic abnormalities. The abnormalities are generally recurring deletions or chromosomal loss and involve precisely the same chromosomes with the same frequency as has been described in marrow cells from patients with secondary acute leukemia induced by alkylating agents. Reasoning that acquired Fanconi anemia protein dysfunction might contribute to cytogenetic instability in secondary acute myelogenous leukemia (AML) cells, we analyzed leukemic cells bearing characteristic complex cytogenetic defects obtained from a 68-year-old man whose lymphoblasts showed no evidence of Fanconi anemia. Unlike the lymphoblasts, this myeloid leukemia cell line (UoC-M1) was hypersensitive to mitomycin-C (MMC) and diepoxybutane (DEB) and exhibited a marked decrease in nuclear FANCA, FANCG, and FANCD2-L. Retroviral transduction of FANCA significantly reduced MMC sensitivity but FANCF, FANCG, and FANCC did not. Overexpression of FANCA restored levels of both FANCA and FANCG, whereas overexpression of FANCG or FANCC did not restore FANCA levels. The molecular mass of cytoplasmic FANCA, FANCG, FANCC, and nuclear FANCD2 were normal. All exons of FANCA and FANCG were sequenced, and no mutations were found. We conclude that perturbations of as yet unidentified factors that govern the binding activity or intracellular localization of FANCA may promote cytogenetic instability and clonal progression in patients with AML who do not have Fanconi anemia.

The anti-apoptotic function of Hsp70 in the interferon-inducible double-stranded RNA-dependent protein kinase-mediated death signaling pathway requires the Fanconi anemia protein, FANCC.

Proteins encoded by five of the six known Fanconi anemia (FA) genes form a heteromeric complex that facilitates repair of DNA damage induced by cross-linking agents. A certain number of these proteins, notably FANCC, also function independently to modulate apoptotic signaling, at least in part, by suppressing ground state activation of the pro-apoptotic interferon-inducible double-stranded RNA-dependent protein kinase (PKR). Because certain FANCC mutations interdict its anti-apoptotic function without interfering with the capacity of FANCC to participate functionally in the FA multimeric complex, we suspected that FANCC enhances cell survival independent of its participation in the complex. By investigating this function in both mammalian cells and in yeast, an organism with no FA orthologs, we show that FANCC inhibited the kinase activity of PKR both in vivo and in vitro, and this effect depended upon a physical interaction between FANCC and Hsp70 but not on interactions of FANCC with other Fanconi proteins. Hsp70, FANCC, and PKR form a ternary complex in lymphoblasts and in yeast expressing PKR. We conclude that Hsp70 requires the cooperation of FANCC to suppress PKR activity and support survival of hematopoietic cells and that FANCC does not require the multimeric FA complex to exert this function.

Expression of apoptosis-related genes in chronic cyclosporine nephrotoxicity in mice.

To define the mechanism of cyclosporine (CsA)-induced apoptosis, we investigated the expression of apoptosis-related genes in experimental chronic CsA nephrotoxicity. Mice on a low-salt (0.01%) diet were given vehicle (VH, olive oil, 1 mg/kg/day), or CsA (30 mg/kg/day), and sacrificed at 1 and 4 weeks. Apoptosis was detected with deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) stain, and the expressions of apoptosis-related genes were evaluated by reverse transcription-polymerase chain reaction, immunoblot or immunohistochemistry. The activity of caspase 1 and 3 was also evaluated. The CsA group showed increases in apoptotic cells compared with the VH group (54 +/- 41 vs. 3 +/- 3, p < 0.05), and the number of apoptotic cells correlated well with interstitial fibrosis scores (r = 0.83, p < 0.01). The CsA group showed a significant increase in Fas-ligand mRNA (0.20 vs. 0.02 amol/microgram total RNA, p < 0.05) and Fas protein expression (146% vs. 95%, p < 0.05), compared with the VH group. The CsA group showed significant increases in ICE mRNA (0.21 vs. 0.03 amol/microgram total RNA at 4 weeks, p < 0.05) and CPP32 mRNA (0.18 vs. 0.03 amol/microgram total RNA at 4 weeks, p < 0.05), compared with the VH group. The enzymatic activity of ICE (16.6 vs. 7.9 rho mol/microgram/h, p < 0.05) and CPP32 protease (15.6 vs. 2.7 rho mol/microgram/h, p < 0.05) proteases were increased in the CsA group, compared with the VH group. The ratio between bax and bcl-2 protein increased significantly in the CsA group (5.3-fold), compared with the VH group. Levels of p53 protein also increased in the CsA group. Immunohistochemical detection of Fas, Fas-ligand, ICE and CPP32 revealed strong immunoreactivity in renal tubular cells in areas of structural injury. These findings suggest that local activation of the apoptosis-related genes is associated with CsA-induced apoptotic cell death.