Aurore Keutgens

The Repressing Function of the Oncoprotein BCL-3 Requires CtBP, while Its Polyubiquitination and Degradation Involve the E3 Ligase TBLR1

ABSTRACT The nuclear and oncogenic BCL-3 protein activates or represses gene transcription when bound to NF-κB proteins p50 and p52, yet the molecules that specifically interact with BCL-3 and drive BCL-3-mediated effects on gene expression remain largely uncharacterized. Moreover, GSK3-mediated phosphorylation of BCL-3 triggers its degradation through the proteasome, but the proteins involved in this degradative pathway are poorly characterized. Biochemical purification of interacting partners of BCL-3 led to the identification of CtBP as a molecule required for the ability of BCL-3 to repress gene transcription. CtBP is also required for the oncogenic potential of BCL-3 and for its ability to inhibit UV-mediated cell apoptosis in keratinocytes. We also defined the E3 ligase TBLR1 as a protein involved in BCL-3 degradation through a GSK3-independent pathway. Thus, our data demonstrate that the LSD1/CtBP complex is required for the repressing abilities of an oncogenic IκB protein, and they establish a functional link between the E3 ligase TBLR1 and NF-κB.

Matrix Metalloproteinase-9 gene induction by a truncated oncogenic NF-κB2 protein involves the recruitment of MLL1 and MLL2 H3K4 histone methyltransferase complexes.

Constitutive nuclear factor (NF)-κB activation in haematological malignancies is caused in several cases by loss of function mutations within the coding sequence of NF-κB inhibitory molecules such as IκBα or p100. Hut-78, a truncated form of p100, constitutively generates p52 and contributes to the development of T-cell lymphomas but the molecular mechanism underlying this oncogenic potential remains unclear. We show here that MMP9 gene expression is induced through the alternative NF-κB-activating pathway in fibroblasts and also on Hut-78 or p52 overexpression in fibroblasts as well as in lymphoma cells. p52 is critical for Hut-78-mediated MMP9 gene induction as a Hut-78 mutant as well as other truncated NF-κB2 proteins that are not processed into p52 failed to induce the expression of this metalloproteinase. Conversely, MMP9 gene expression is impaired in p52-depleted HUT-78 cells. Interestingly, MLL1 and MLL2 H3K4 methyltransferase complexes are tethered by p52 on the MMP9 but not on the IκBα promoter, and the H3K4 trimethyltransferase activity recruited on the MMP9 promoter is impaired in p52-depleted HUT-78 cells. Moreover, MLL1 and MLL2 are associated with Hut-78 in a native chromatin-enriched extract. Thus, we identified a molecular mechanism by which the recruitment of a H3K4 histone methyltransferase complex on the promoter of a NF-κB-dependent gene induces its expression and potentially the invasive potential of lymphoma cells harbouring constitutive activity of the alternative NF-κB-activating pathway.

BCL-3 degradation involves its polyubiquitination through a FBW7-independent pathway and its binding to the proteasome subunit PSMB1

The oncogenic protein BCL-3 activates or represses gene transcription through binding with the NF-κB proteins p50 and p52 and is degraded through a phospho- and GSK3-dependent pathway. However, the mechanisms underlying its degradation remain poorly understood. Yeast two-hybrid analysis led to the identification of the proteasome subunit PSMB1 as a BCL-3-associated protein. The binding of BCL-3 to PSMB1 is required for its degradation through the proteasome. Indeed, PSMB1-depleted cells are defective in degrading polyubiquitinated BCL-3. The N-terminal part of BCL-3 includes lysines 13 and 26 required for the Lys48-linked polyubiquitination of BCL-3. Moreover, the E3 ligase FBW7, known to polyubiquitinate a variety of substrates phosphorylated by GSK3, is dispensable for BCL-3 degradation. Thus, our data defined a unique motif of BCL-3 that is needed for its recruitment to the proteasome and identified PSMB1 as a key protein required for the proteasome-mediated degradation of a nuclear and oncogenic IκB protein.

Lymphocytoses B monoclonales : de la revue de la littérature à la pratique de laboratoire.

Monoclonal B-cell lymphocytosis (MBL) is defined as an asymptomatic condition characterized by the presence of less than 5,000 monoclonal B-cells per microliter and the absence of clinical signs or symptoms of a B-cell lymphoproliferative disorder. Most MBL cases involve B cells presenting an identical phenotype to CLL (CLL-like MBL) with a Catovsky-Matutes score of 3 to 5 and share the same chromosomal abnormalities than CLL. Depending on the absolute B cell count, one may distinguish low-count CLL-like MBL (<500 B cells/μL) which have no evidence of progression, no reduction in overall survival, no increase in infection risk and do not require any specific follow-up. Patients with clinical CLL-like MBL (>500 B cells/μL) have a 1% to 2% per year risk of progression to CLL requiring therapy, a higher risk of infectious complications and mortality implicating an annual follow-up by hematologist. MBL may also express other less common phenotypes and are named atypical MBL in case of CD5 antigen expression (Catovsky-Matutes score: 1-2) and non-CLL-like MBL for CD5 negative cases (Catovsky-Matutes score: 0-2). Their poorer prognosis implicates imaging studies, bone marrow biopsy and cytogenetic analysis in addition to physical examination in order to rule out non-hodgkinien lymphoma, and require a more frequent follow-up. This review focuses on key concepts in the classification, diagnosis, monitoring and biology of MBL in laboratory practice.

Atypical plasma cells with coexpression of myeloid markers and bundles of Auer rod-like inclusions.

Sir, Although variable morphological and immunophenotypic features of plasma cells are described [1], plasma cells with bundles of Auer rod-like inclusions and myeloid markers coexpression have never been reported. A 76-year-old patient complaining of a long-lasting bone pain in the lumbar region was admitted in the hospital. Physical examination revealed a patient in poor condition. Laboratory tests showed a mild anemia (hemoglobin 9.8 g/dL) with normal leukocyte and platelet counts, elevated LDH, and renal failure. Serum protein electrophoresis revealed a severe hypogammaglobulinemia (0.36 g/dL) and quantitative evaluation of immunoglobulins showed normal IgA but decreased IgG and IgM. The urinary protein level was increased at 1073 mg/L with predominant monoclonal kappa light chains. A scintigraphy showed multiple nonspecific hyperfixation sites. X-ray skeleton examination and magnetic resonance imaging had not been performed when bone marrow cytology was carried out. The bone marrow (BM) aspirate smear revealed a hypercellular marrow with 15% infiltration by two atypical cell populations. The first population corresponded to plasma cells containing intracytoplasmic azurophilic granules, termed Buhot plasma cells. The second population contained bundles of Auer rod-like inclusions and a fragmenting cytoplasm. We also detected extracellular Auer rod-like depositions (Figure 1). Even if Auer rods are indicative of the myeloid lineage of blasts [2], Auer rod-like inclusions have seldom been reported in B-cell neoplasms, including chronic lymphocytic leukemia and