Proteolytic activity of the mucosa-associated lymphoid tissue lymphoma translocation protein-1 (MALT1) paracaspase is required for survival of the activated B cell subtype of diffuse large B cell lymphoma (ABC-DLBCL). We have identified distinct derivatives of medicinal active phenothiazines, namely mepazine, thioridazine, and promazine, as small molecule inhibitors of the MALT1 protease. These phenothiazines selectively inhibit cleavage activity of recombinant and cellular MALT1 by a noncompetitive mechanism. Consequently, the compounds inhibit anti-apoptotic NF-κB signaling and elicit toxic effects selectively on MALT1-dependent ABC-DLBCL cells in vitro and in vivo. Our data provide a conceptual proof for a clinical application of distinct phenothiazines in the treatment of ABC-DLBCL.

The activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL) represents a very aggressive human lymphoma entity. Constitutive NF-κB activation caused by chronic active B-cell receptor (BCR) signaling is common feature of many ABC DLBCL cells; however, the pathways linking BCR signaling to the NF-κB prosurvival network are largely unknown. Here we report that constitutive activity of PI3K and the downstream kinase PDK1 are essential for the viability of two ABC DLBCL cell lines that carry mutations in the BCR proximal signaling adaptor CD79B. In these cells, PI3K inhibition reduces NF-κB activity and decreases the expression of NF-κB target genes. Furthermore, PI3K and PDK1 are required for maintaining MALT1 protease activity, which promotes survival of the affected ABC DLBCL cells. These results demonstrate a critical function of PI3K-PDK1 signaling upstream of MALT1 protease and NF-κB in distinct ABC DLBCL cells and provide a rationale for the pharmacologic use of PI3K inhibitors in DLBCL therapy.

Current challenges in capturing naive human pluripotent stem cells (hPSCs) suggest that the factors regulating human naive versus primed pluripotency remain incompletely defined. Here we demonstrate that the widely used Essential 8 minimal medium (E8) captures hPSCs at a naive-to-primed intermediate state of pluripotency expressing several naive-like developmental, bioenergetic, and epigenomic features despite providing primed-statesustaining growth factor conditions. Transcriptionally, E8 hPSCs are marked by activated lipid biosynthesis and suppressed MAPK/TGF-b gene expression, resulting in endogenous ERK inhibition. These features are dependent on lipidfree culture conditions and are lost upon lipid exposure, whereas short-term pharmacological ERK inhibition restores naive-to-primed intermediate traits even in the presence of lipids. Finally, we identify de novo lipogenesis as a common transcriptional signature of E8 hPSCs and the pre-implantation human epiblast in vivo. These findings implicate exogenous lipid availability in regulating human pluripotency and define E8 hPSCs as a stable, naive-to-primed intermediate (NPI) pluripotent state.

The discovery of constitutive nuclear factor-κB (NF-κB) activation in Hodgkin's lymphoma tumor cells almost two decades ago was one of the first reports that directly connected deregulated NF-κB signaling to human cancer. Subsequent studies demonstrated that enhanced NF-κB signaling is a common hallmark of many lymphoid malignancies, including Hodgkin lymphoma, mucosa-associated lymphoid tissue lymphoma, diffuse large B-cell lymphoma and multiple myeloma. By inducing an anti-apoptotic and pro-proliferative gene program, NF-κB is involved in lymphoma survival and growth. Identification of somatic mutations that led to activation of oncogenes and inactivation of tumor suppressor genes in the pathway revealed that specific pathogenic mechanisms are responsible for constitutive NF-κB activation in different lymphoma entities. Thus, the identification of distinct oncogenic events is reflecting the diverse cellular origins of the different lymphomas. Further, elucidation of the mechanisms that drive NF-κB in lymphoma is of high clinical relevance as it will allow the design of target-directed precision therapy. Indeed, a number of drugs that impair constitutive NF-κB activation in lymphoid malignancies are currently in preclinical or clinical development.

Second site: In the crystal structure of human MALT1casp‐Ig3 (mucosa‐associated lymphoid tissue lymphoma translocation protein 1) in complex with the tricyclic phenothiazine derivative thioridazine (violet in the picture), the inhibitor is bound in a hydrophobic pocket far from the active site. This explains the action of phenothiazine derivatives as noncompetitive, reversible inhibitors.