The major function of B lymphocytes is to sense antigens and to produce protective antibodies after activation. This function requires the expression of a B-cell antigen receptor (BCR), and evolutionary conserved mechanisms seem to exist that ensure that B cells without a BCR do not develop nor survive in the periphery. Here, we show that the loss of BCR expression on Burkitt lymphoma cells leads to decreased mitochondrial function and impaired metabolic flexibility. Strikingly, this phenotype does not result from the absence of a classical Syk-dependent BCR signal but rather from compromised ER expansion. We show that the reexpression of immunoglobulins (Ig) in the absence of the BCR signaling subunits Igα and Igβ rescues the observed metabolic defects. We demonstrate that immunoglobulin expression is needed to maintain ER homeostasis not only in lymphoma cells but also in resting B cells. Our study provides evidence that the expression of BCR components, which is sensed in the ER and shapes mitochondrial function, represents a novel mechanism of metabolic control in B cells.

Cholangiocarcinoma (CCA) is a rare but highly aggressive tumor entity for which systemic therapies only showed limited efficacy so far. As OSI-027—a dual kinase inhibitor targeting both mTOR complexes, mTORC1 and mTORC2 - showed improved anti-cancer effects, we sought to evaluate its impact on the migratory and metastatic capacity of CCA cells in vitro. We found that treatment with OSI-027 leads to reduced cell mobility and migration as well as a reduced surviving fraction in colony-forming ability. While neither cell viability nor proliferation rate was affected, OSI-027 decreased the expression of MMP2 and MMP9. Moreover, survival as well as anti-apoptotic signaling was impaired upon the use of OSI-027 as determined by AKT and MAPK blotting. Dual targeting of mTORC1/2 might therefore be a viable option for anti-neoplastic therapy in CCA.

In most cell types, nuclear beta-catenin functions as prominent oncogenic driver and pairs with TCF7-family factors for transcriptional activation of MYC. Surprisingly, B-lymphoid malignancies not only lacked expression and activating lesions of beta-catenin but critically depended on GSK3B for effective beta-catenin degradation. Our interactome studies in B-lymphoid tumors revealed that beta-catenin formed repressive complexes with lymphoid-specific Ikaros factors at the expense of TCF7. Instead of MYC-activation, beta-catenin was essential to enable Ikaros-mediated recruitment of nucleosome remodeling and deacetylation (NuRD) complexes for transcriptional repression of MYC. To leverage this previously unrecognized vulnerability of B-cell-specific repressive beta-catenin-Ikaros-complexes in refractory B-cell malignancies, we examined GSK3B small molecule inhibitors to subvert beta-catenin degradation. Clinically approved GSK3B-inhibitors that achieved favorable safety profiles at micromolar concentrations in clinical trials for neurological disorders and solid tumors were effective at low nanomolar concentrations in B-cell malignancies, induced massive accumulation of beta-catenin, repression of MYC and acute cell death. Preclinical in vivo treatment experiments in patient-derived xenografts validated small molecule GSK3B-inhibitors for targeted engagement of lymphoid-specific beta-catenin-Ikaros complexes as a novel strategy to overcome conventional mechanisms of drug-resistance in refractory B-cell malignancies. HIGHLIGHTS Unlike other cell lineages, B-cells express nuclear beta-catenin protein at low baseline levels and depend on GSK3B for its degradation. In B-cells, beta-catenin forms unique complexes with lymphoid-specific Ikaros factors and is required for Ikaros-mediated tumor suppression and assembly of repressive NuRD complexes. CRISPR-based knockin mutation of a single Ikaros-binding motif in a lymphoid MYC superenhancer region reversed beta-catenin-dependent Myc repression and induction of cell death. The discovery of GSK3B-dependent degradation of beta-catenin as unique B-lymphoid vulnerability provides a rationale to repurpose clinically approved GSK3B-inhibitors for the treatment of refractory B-cell malignancies.

No abstract

Glycogen synthase kinase 3 (GSK3) is a ubiquitously expressed kinase involved in a myriad of biological processes. Although GSK3 mediated phosphorylation has been shown to induce the degradation of many pro-survival and pro-proliferation factors, cancer cells of different origin show reduced proliferation or survival after GSK3 inhibition. Our current understanding of the role GSK3 plays in normal mature B cells, B cell precursors and transformed B cells is incomplete and does not allow to assess whether GSK3 inhibitors can be used to treat B cell derived malignancies. Here we identify β-catenin as the major factor driving GSK3-inhibition induced changes in B cells. We show that β-catenin accumulation has opposing effects on cell metabolism and survival in mature B cells and B cell precursors. Moreover, we demonstrate that β-catenin destabilizes the commitment to the B cell lineage. In summary, our study identifies β-catenin induced signaling as a factor that can be exploited to limit the survival of malignant B cells.