Christian R. Geest

Small-molecule drugs that target the B-cell antigen receptor (BCR) signalosome show clinical efficacy in the treatment of B-cell non-Hodgkin lymphoma. These agents, including the Bruton tyrosine kinase (BTK) inhibitor PCI-32765, display an unexpected response in patients with chronic lymphocytic leukemia (CLL): a rapid and sustained reduction of lymphadenopathy accompanied by transient lymphocytosis, which is reversible upon temporary drug deprivation. We hypothesized that this clinical response reflects impaired integrin-mediated adhesion and/or migration. Here, we show that PCI-32765 strongly inhibits BCR-controlled signaling and integrin α(4)β(1)-mediated adhesion to fibronectin and VCAM-1 of lymphoma cell lines and primary CLL cells. Furthermore, PCI-32765 also inhibits CXCL12-, CXCL13-, and CCL19-induced signaling, adhesion, and migration of primary CLL cells. Our data indicate that inhibition of BTK by PCI-32765 overcomes BCR- and chemokine-controlled integrin-mediated retention and homing of malignant B cells in their growth- and survival-supporting lymph node and bone marrow microenvironment, which results in clinically evident CLL regression.

Abstract Relevance of mitogen-activated protein kinases ERK, JNK, and p38 in the regulation of myeloid differentiation. The MAPKs are a family of serine/threonine kinases that play an essential role in connecting cell-surface receptors to changes in transcriptional programs. MAPKs are part of a three-component kinase module consisting of a MAPK, an upstream MEK, and a MEKK that couples the signals from cell-surface receptors to trigger downstream pathways. Three major groups of MAPKs have been characterized in mammals, including ERKs, JNKs, and p38MAPKs. Over the last decade, extensive work has established that these proteins play critical roles in the regulation of a wide variety of cellular processes including cell growth, migration, proliferation, differentiation, and survival. It has been demonstrated that ERK, JNK, and p38MAPK activity can be regulated in response to a plethora of hematopoietic cytokines and growth factors that play critical roles in hematopoiesis. In this review, we summarize the current understanding of MAPK function in the regulation of hematopoiesis in general and myelopoiesis in particular. In addition, the consequences of aberrant MAPK activation in the pathogenesis of various myeloid malignancies will be discussed.

General and simple principles are identified that govern signal transduction. The effects of kinase and phosphatase inhibition on a MAP kinase pathway are first examined in silico. Quantitative measures for the control of signal amplitude, duration and integral strength are introduced. We then identify and prove new principles, such that total control on signal amplitude and on final signal strength must amount to zero, and total control on signal duration and on integral signal intensity must equal -1. Collectively, kinases control amplitudes more than duration, whereas phosphatases tend to control both. We illustrate and validate these principles experimentally: (a) a kinase inhibitor affects the amplitude of EGF-induced ERK phosphorylation much more than its duration and (b) a phosphatase inhibitor influences both signal duration and signal amplitude, in particular long after EGF administration. Implications for the cellular decision between growth and differentiation are discussed.

Resistance to ABT-199 induced by microenvironmental signals in chronic lymphocytic leukemia can be counteracted by CD20 antibodies or kinase

PURPOSE: Chronic lymphocytic leukemia (CLL) cells in lymph nodes (LN), from which relapses are postulated to originate, display an antiapoptotic profile in contrast to CLL cells from peripheral blood (PB). The BH3 mimetic ABT-737 antagonizes the antiapoptotic proteins Bcl-X(L) and Bcl-2 but not Mcl-1 or Bfl-1. Previously, it was shown that CD40-stimulated CLL cells were resistant to ABT-737. We aimed to define which antiapoptotic proteins determine resistance to ABT-737 in CLL and whether combination of known antileukemia drugs and ABT-737 was able to induce apoptosis of CD40-stimulated CLL cells. EXPERIMENTAL DESIGN: To mimic the LN microenvironment, PB lymphocytes of CLL patients were cultured on feeder cells expressing CD40L and treated with ABT-737 with or without various drugs. In addition, we carried out overexpression or knockdown of pro- and antiapoptotic proteins in immortalized primary B cells. RESULTS: Upon CD40 stimulation patient-specific variations in ABT-737 sensitivity correlated with differences in levels of Mcl-1 and its antagonist Noxa. Knockdown of Noxa, as well as Mcl-1 overexpression, corroborated the importance of the Noxa/Mcl-1 ratio in determining the response to ABT-737. Inhibition of NF-κB resulted in increased Noxa levels and enhanced sensitivity to ABT-737. Interestingly, increasing the Noxa/Mcl-1 ratio, by decreasing Mcl-1 (dasatinib and roscovitine) or increasing Noxa levels (fludarabine and bortezomib), resulted in synergy with ABT-737. CONCLUSIONS: Thus, the Noxa/Mcl-1 balance determines sensitivity to ABT-737 in CD40-stimulated CLL cells. These data provide a rationale to investigate the combination of drugs which enhance the Noxa/Mcl-1 balance with ABT-737 to eradicate CLL in chemoresistant niches.