Christian-Jacques Larsen

Human ARF binds E2F1 and inhibits its transcriptional activity

The INK4a/ARF locus which is frequently inactivated in human tumours encodes two different tumour suppressive proteins, p16INK4a and ARF. p16INK4a is a major component of the RB pathway. ARF is part of an ARF-mdm2-p53 network that exerts a negative control on hyperproliferative signals emanating from oncogenic stimuli. Among these is the transcription factor E2F1, a final effector of the RB pathway, that induces ARF expression. Recent data suggest that ARF function is not restricted to the p53 pathway. However, ARF target(s) implicated in this p53-independent function remains to be identified. We show that ARF is able to inhibit the proliferation of human cell lines independently of their p53 status. In this context, we demonstrate that ARF interacts physically with E2F1 and inhibits its transcriptional activity. Moreover, we show that mdm2 is required for the modulation of E2F1 activity by ARF. Beside the well-known p53 and mdm2 partners, these results identify E2F1 as a new ARF target. Thus, ARF can be viewed as a dual-acting tumour suppressor protein in both the p53 and RB pathways, further emphasizing its role in tumour surveillance.

Human ARF protein interacts with Topoisomerase I and stimulates its activity

The ARF gene (p19ARF in mouse and p14ARF in man) has become a central actor of the cell cycle regulation process as it participates to the ARF-MDM2-p53 pathway and the Rb-E2F-1 pathway. By use of immunoprecipitation and Western blotting (IP/WB), we now show that ARF physically associates with Topoisomerase I (Topo I). ARF-Topo I immune complexes were detected in SF9 insect cells infected with recombinant baculoviruses encoding the two genes as well as in 293 cells that express endogeneously these proteins. Preparations of a GST–ARF recombinant protein stimulated the DNA relaxation activity of Topo I but, in contrast, had no effect on the decatenation activity of Topo II. The Topo I stimulation was also detected in cell extracts of SF9 cells expressing both proteins. A confocal microscopy study indicated that part of ARF and Topo I colocalized in the granular component structure of the nucleolus. As a whole, our data indicate that Topo I is a new partner of ARF and suggest that ARF is involved in cell reactions that require Topo I.

Loss of heterozygosity of the L-myc oncogene in human breast tumors

SummaryRecent studies suggest that loss of heterozygosity may play an important role in various human neoplasia. Cytogenetic abnormalities detected in primary breast tumors led us to examine breast tumor DNAs for deletions. In the present study, we demonstrate, using restriction fragment length polymorphism (RFLP) analysis at the L-myc proto-oncogene (chromosome 1p32), a frequent loss of heterozygosity in primary breast tumor DNAs (55 out of 152 informative tumor DNAs). Most of these deletions appear to be limited to chromosome 1p. No correlation was observed between this genetic alteration and several parameters of each patients history or characteristics of the tumor. However, a significantly (P = 0.011) shorter survival period after relapse was observed for patients with loss of heterozygosity at L-myc in primary tumor DNAs compared with patients with tumor DNAs lacking this alteration.

The human protein p19ARF is not detected in hemopoietic human cell lines that abundantly express the alternative beta transcript of the p16INK4a/MTS1 gene.

The p16/MTS1/CDKN2 gene on human chromosome band 9p21 encodes two unrelated proteins: p16INK4a, a specific inhibitor of the cyclin D-dependent kinases CKD4 and CDK6, and the structurally unrelated p19ARF protein that arrests cell growth in G1/S and also in G2/M. By use of polyclonal antibodies, the human p19ARF (hp19ARF) protein has been identified in the nucleus of various cells including normal cultured fibroblasts. The level of this protein did not fluctuate throughout the cell cycle and was more elevated in fibroblasts with limited or arrested growth, suggesting that p19ARF accumulated in presenescent or senescent cells. Interestingly, hp19ARF was not detected in several hemopoietic tumor cell lines (mainly of B-type lymphoid origin) that expressed abundant amounts of the p16β transcript. This finding indicates that in certain tumors, the expression of hp19ARF RNA and protein may be uncoupled. Furthermore, it suggests that disruption of a translational mechanism may be involved in the inactivation of hp19ARF.

Human Arf tumor suppressor specifically interacts with chromatin containing the promoter of rRNA genes

The tumor suppressor Arf (Alternative Reading Frame) protein (p14ARF in human and p19ARF in mouse) is mainly located in the nucleolus consistent with its subcellular localization, the protein has been shown to specifically interact with 5.8S rRNA and with B23/Nucleophosmin and to regulate ribosome biogenesis. Here, we show that the p14ARF protein interacts with chromatin and is recovered by chromatin immunoprecipitation (ChIP) in a fraction that contains a DNA sequence of the rRNA gene promoter. In addition, topoisomerase I (Topo I) that has been shown to interact with p14ARF coprecipitates with p14ARF containing chromatin. These data, in view of the function for Topo I in rRNA transcription, are consistent with a role for the p14ARF-Topo I complex in rRNA transcription and/or maturation.

A novel 43-kDa glycoprotein is detected in the nucleus of mammalian cells by autoantibodies from dogs with autoimmune disorders

We have characterized a new antibody specificity in a panel of sera from dogs developing systemic lupus erythematosus (SLE) or clinically related autoimmune disorders. This antibody stains in a speckled fashion the nucleus of cells of different mammalian origins. The target antigen is a basic (pI 9.2) nuclear polypeptide with an apparent molecular weight of 43 kDa (p43) which is detected in various mammalian cell nuclei. p43, as studied in HeLa cells, appears to be cell cycle-independent. It is released from nuclei by salts (0.5 M NaCl or 0.25 M ammonium sulfate). Upon subfractionation of nuclear components, p43 is found in the fraction containing HnRNPs and is recovered in immunoprecipitates obtained with 4F4 monoclonal antibody to HnRNP C proteins. Immunoelectron microscopy revealed that p43 is concentrated over the dense chromatin periphery and interchromatin granule clusters. Another important feature of p43 is its ability to specifically bind wheat germ agglutinin lectin but not concanavalin A nor Ulex europaeus I, supporting the notion that p43 is a glycoprotein bearing an N-acetyl-glucosamine moiety. Consistent with this result, a radio-active p43 band is specifically immunoprecipitated by canine anti-p43 autoantibodies from HeLa cells metabolically labeled with [14C]glucosamine. Finally, anti-p43 antibodies do not immunoprecipitate SnRNA, indicating that p43 has no apparent association with SnRNPs.

Structural organization of the hCTLA-1 gene encoding human granzyme B

Cytotoxic T lymphocytes (CTLs) and natural killer/lymphokine-activated cells produce granzymes, a family of serine esterase proteins located in cytoplasmic granules. These might be involved in different cytotoxic pathways. We report the structural organization of the human gene encoding granzyme B (hCTLA-1). A 4.75-kb genomic DNA fragment containing all the sequences of granzyme B-encoding cDNA clones has been sequenced. The gene is composed of five exons and four introns. A comparison with the genomic organization of murine CCP1/CTLA-1 showed very similar structure and a 76% nucleotide homology in the coding sequences. This suggests that both genes may have a common ancestor. No typical regulatory element was detected in the 1160 bp upstream from the ATG start codon. The detection of a second locus related to hCTLA-1 is also described.

Delineation of the domains required for physical and functional interaction of p14ARF with human topoisomerase I.

We recently reported an interaction between the p14ARF protein and human topoisomerase I (Topo I) resulting in the stimulation of the relaxation activity of Topo I. Our data showed that the complex between the two proteins was located within the nucleolus. In the present work, we have investigated the regions of p14ARF involved in this interaction by using targeted point mutagenesis and deletion mutants. A region encompassing exon 2-encoded sequence was required for physical binding of p14ARF to Topo I as well as for stimulatory activity of the enzyme. Exon 1β-encoded segment was not implicated in the interaction. Moreover, among p14ARF point mutants selected for their high conservation among different mammalian species, mutant p14ARF (RR87, 88AA) did not stimulate Topo I in spite of its association with the enzyme, suggesting its direct implication in the functional activity of ARF. In contrast, one mutant, p14ARF (R71A), was more efficient than wild-type protein to activate Topo I, suggesting that this residue is a key element to modulate Topo I activity. Finally, only ARF–Topo I complexes containing p14ARF exon 2 segment were found to be localized in the nucleolus, suggesting that this subnuclear location is linked to the biological function of the ARF–Topo I complex.

Genetic variability of proto-oncogenes for breast cancer risk and prognosis☆

This paper summarizes the results of a study on human breast cancers performed mainly at the Centre René Huguenin in collaboration with other American and French groups, and supported in part by a Grant from the Association pour la Recherche sur le Cancer (ARC) Villejuif. During this work, the following conclusions emerged: c-myc proto-oncogene amplification is a common alteration in ductal invasive tumors, more frequently found in recurrent and metastatic tumors, suggesting a role for c-myc in tumor progression. However, in the current state of our study, it does not appear to be linked to prognosis; parts of the short arm of chromosome 11 are deleted in 20% of tumors resulting in hemizygosity for several genes (c-ha-ras, beta globin, pTH, calcitonin, catalase). These deletions seem to be linked with aggressiveness of tumors; a restriction fragment length polymorphism (RFLP) study of c-ha-ras has shown a significant association of the frequency of rare ha-ras alleles in cancer patients compared to that of normal individuals. Although this result is currently a matter of controversy, further studies must be independently repeated to be conclusive; -- another RFLP was found in c-mos proto-oncogene, which is detected only in patients with breast cancers or other types of tumors. The molecular basis for this RFLP has been elucidated. The significance of this association is unknown.

Presence of three recurrent chromosomal rearrangements, t(2;3)(p12;q37), del(8)(q24), and t(14;18), in an acute lymphoblastic leukemia

A complex chromosomal abnormality associating three recurrent rearrangements, t(2;3)((p12;q37), del (8)(q24) and t(14;18)(q32;q21), was detected in a patient with acute lymphoblastic leukemia of the Burkitt type. Southern blot studies showed rearrangements of the MYC, BCL2, and JH genes, thus confirming the cytogenetic data. However, no rearrangement of the LAZ3/BCL6 gene, normally localized on band 3q27, could be detected. The simultaneous presence of three recurrent rearrangements specific for lymphoid malignancies addresses the question of their timing in the malignant process and the prognostic significance of the association of such anomalies.