Negative feedback regulation of MAPK signaling is an important driver of chronic lymphocytic leukemia progression

Despite available targeted treatments for the disease, drug-resistant chronic lymphocytic leukemia (CLL) poses a clinical challenge. The objective of this study is to examine whether the dual-specific phosphatases DUSP1 and DUSP6 are required to negatively regulate mitogen-activated protein kinases (MAPKs) and thus counterbalance excessive MAPK activity. We show that high expression of DUSP6 in CLL correlates with poor clinical prognosis. Importantly, genetic deletion of the inhibitory phosphatase DUSP1 or DUSP6 and blocking DUSP1/6 function using a small-molecule inhibitor reduces CLL cell survival in vitro and in vivo. Using global phospho-proteome approaches, we observe acute activation of MAPK signaling by DUSP1/6 inhibition. This promotes accumulation of mitochondrial reactive oxygen species and, thereby, DNA damage and apoptotic cell death in CLL cells. Finally, we observe that DUSP1/6 inhibition is particularly effective against treatment-resistant CLL and therefore suggest transient DUSP1/6 inhibition as a promising treatment concept to eliminate drug-resistant CLL cells

Fatty acid metabolism in aggressive B-cell lymphoma is inhibited by tetraspanin CD37

The importance of fatty acid (FA) metabolism in cancer is well-established, yet the mechanisms underlying metabolic reprogramming remain elusive. Here, we identify tetraspanin CD37, a prognostic marker for aggressive B-cell lymphoma, as essential membrane-localized inhibitor of FA metabolism. Deletion of CD37 on lymphoma cells results in increased FA oxidation shown by functional assays and metabolomics. Furthermore, CD37-negative lymphomas selectively deplete palmitate from serum in mouse studies. Mechanistically, CD37 inhibits the FA transporter FATP1 through molecular interaction. Consequently, deletion of CD37 induces uptake and processing of exogenous palmitate into energy and essential building blocks for proliferation, and inhibition of FATP1 reverses this phenotype. Large lipid deposits and intracellular lipid droplets are observed in CD37-negative lymphoma tissues of patients. Moreover, inhibition of carnitine palmitoyl transferase 1 A significantly compromises viability and proliferation of CD37-deficient lymphomas. Collectively, our results identify CD37 as a direct gatekeeper of the FA metabolic switch in aggressive B-cell lymphoma

NFATc1 affects mouse splenic B cell function by controlling the calcineurin–NFAT signaling network

Mouse B cells lacking NFATc1 exhibit defective proliferation, survival, isotype class switching, cytokine production, and T cell help

Regulation of B1 cells and systemic autoimmunity in mice by Siglec-G

Siglec-G ist ein Inhibitor des BZR-vermittelten Signals auf B1a-Zellen. Diese Population der B-Zellen ist in Siglec-G-defizienten Mäusen stark vergrößert. Der exakte Mechanismus dieser Expansion war aber bis jetzt unbekannt. Im Rahmen dieser Arbeit wurde gezeigt, dass Siglec-G-defiziente B1a-Zellen eine niedrigere Apoptoserate haben und länger überleben. Die verringerte Apoptoserate könnte durch die höhere Expression von NFATc1 in Siglec-G-defizienten B1a-Zellen erklärt werden. Weiterhin zeigen Siglec-G-defiziente B1a-Zellen ein verändertes Ig-Repertoire im Vergleich zu Wildtyp-B1a-Zellen, was auf eine veränderte Selektion von B1a-Zellen in Siglec-G-defizienten Mäusen deutet. Siglec-G-defiziente Mäuse zeigen eine vergrößerte Population von B1a-Zellen und erhöhte Serum-IgM-Spiegel. Deshalb wurde die Suszeptibilität von Siglec-G-defizienten Mäusen gegenüber einer S.pneumoniae Infektion untersucht. Siglec-G-defiziente Mäuse schienen die Bakterien effektiver zu bekämpfen, wenn sie mit einem S.pneumoniae Stamm infiziert wurden, der eine niedrige Pathogenizität besaß. Sie unterlagen einer letalen Infektion aber schneller als Wildtyp-Mäuse. Diese Ergebnisse sind bisher als vorläufig zu betrachten. CD22 ist ebenfalls ein negativer Regulator des BZR-vermittelten Signals. CD22 scheint aber nur auf B2-Zellen eine dominante Rolle zu spielen. Obwohl CD22 auch auf B1a-Zellen exprimiert wird, zeigen diese Zellen keinen Phänotyp in CD22-defizienten Mäusen. Andererseits wird Siglec-G auch auf B2-Zellen exprimiert, diese Population scheint in Siglec-G-defizienten Mäusen aber normal zu sein. Weder Siglecg-/- noch Cd22-/- Mäuse auf einem reinen BALB/c bzw. C57BL/6 Hintergrund entwickeln Autoimmunität. Mit Hilfe von Siglec-G x CD22 doppelt-defizienten Mäusen konnte in dieser Arbeit gezeigt werden, dass Siglec-G und CD22 teilweise redundante Funktionen in B1- und B2-Zellen erfüllen. Siglec-G x CD22 doppelt-defiziente Mäuse zeigen ein verstärktes Calciumsignal in beiden B-Zell-Populationen- den B1- und den B2-Zellen, erhöhte Serum IgM Spiegel und eine vergrößerte Population an B1-Zellen. Diese Expansion übertrifft sogar die von Siglecg-/- Mäusen. Weiterhin zeigen Siglec-G x CD22 doppelt-defiziente Mäuse eine verringerte Immunantwort auf TD- und TI-II- Antigene. Ältere Siglec-G x CD22 doppelt-defiziente Mäuse entwickeln anti-DNA und anti-RNA Autoantikörper, die zu einer leichten Form von Glomerulonephritis führen. Diese Ergebnisse zeigen, dass Siglec-G und CD22 wichtig für die Aufrechterhaltung von Toleranz sind.Siglec-G is an inhibitor of BCR-mediated signaling in B1a cells. This population of B cells is highly increased in Siglec-G-deficient mice, but the exact mechanism of this expansion was not known to this date. In this study, it was demonstrated that Siglec-G-deficient B1a cells show a lower level of spontaneous apoptosis and a prolonged life span. The lower apoptosis could result from higher expression levels of the transcription factor NFATc1 in Siglec-G-deficient B1a-cells. Further, Sigleg-G-deficient B1a-cells display a changed Ig Repertoire compared to wild type B1a cells, this suggests that the selection into the B1a cell population is altered in Siglec-G deficient mice. Siglec-G-deficient mice show an enlarged population of B1a cells and increased levels of Serum IgM. Therefore, the susceptibility of Siglec-G-deficient mice to the infection with S.pneumoniae was analysed. Siglec-G-deficient mice seemed to clear the bacteria more efficiently than wildtype mice when infected with a S.pneumoniae strain with low pathogenicity, but showed a higher susceptibility during lethal challenge. However, these experiments need to be repeated. CD22 is also a negative regulator of BCR mediated signaling, but seems to play a dominant role only on B2 cells. Even though CD22 is also expressed on B1a cells, those cells show no phenotype in CD22 deficient mice. On the other hand, Siglec-G is also expressed on B2 cells, but this population is normal in Siglec-G deficient mice. Neither Siglecg-/- nor Cd22-/- mice on a pure BALB/c or C57BL/6 background, respectively, develop autoimmunity. Using Siglec-G x CD22 double-deficient mice it was demonstrated in this study, that Siglec-G and CD22 have partly redundant functions. Siglec-G x CD22 double deficient mice show elevated calcium responses in both B1 and B2 cells, increased serum IgM levels and an enlarged population of B1 cells. This enlargement is even higher than in Siglecg-/- mice. Furthermore Siglec-G x CD22 double-deficient mice show a diminished immune response to both TD and TI-II Antigens. Aged Siglec-G x CD22 double-deficient mice developed anti-DNA and anti-RNA antibodies, which resulted in a moderate form of glomerulonephritis. These results show that Siglec-G and CD22 are crucial in maintaining B cell tolerance

Context-Specific BAFF-R Signaling by the NF-κB and PI3K Pathways

BAFF is a soluble factor required for B cell maturation and survival. BAFF-R signals via the noncanonical NF-κB pathway regulated by the TRAF3/NIK/IKK1 axis. We show that deletion of Ikk1 during early B cell development causes a partial impairment in B cell maturation and BAFF-dependent survival, but inactivation of Ikk1 in mature B cells does not affect survival. We further show that BAFF-R employs CD19 to promote survival via phosphatidylinositol 3-kinase (PI3K), and that coinactivation of Cd19 and Ikk1 causes a profound block in B cell maturation at the transitional stage. Consistent with a role for PI3K in BAFF-R function, inactivation of PTEN mediates a partial rescue of B cell maturation and function in Baff−/− animals. Elevated PI3K signaling also circumvents BAFF-dependent survival in a spontaneous B cell lymphoma model. These findings indicate that the combined activities of PI3K and IKK1 drive peripheral B cell differentiation and survival in a context-dependent manner