The Cyclic Amp/Phosphodiesterase4 Axis Regulates Proximal B Cell Receptor Signaling and Offers a New Therapeutic Strategy for B

Aberrant activation of the B cell receptor (BCR) is an emerging hallmark of mature B cell tumors. The BCR initiates a signaling cascade that activates multiple kinases, including SYK, BTK and PI3K?, leading to engagement of the NF-?B, PI3K/AKT, RAS and MAPK pathways, thus promoting proliferation/survival of B cells. Considering this pro-growth outcome, it is not surprising that malignant B cells hijack this signaling module for their benefit. Consequently, the development of therapies to inhibit the BCR has become a high priority and preliminary results suggest that this is a sound approach. However, these novel therapeutic approaches rely primarily on the targeting of one kinase at a time and only rarely induce complete responses. Moreover, acquired mutations within the BCR pathway (BTK/PLC?2) and subsequent loss of clinical response have recently been reported in patients treated with the BTK inhibitor Ibrutinib. Moreover, little is known about the signaling molecules that induce the termination of BCR activity. Earlier we identified phosphodiesterase 4B (PDE4B) as a highly expressed gene in fatal diffuse large B cell lymphoma (DLBCL). Subsequently, we demonstrated that PDE4B inhibition leads to a cyclic-AMP (cAMP) mediated suppression of SYK, BTK and PI3K, which associates with growth inhibition in lymphoma. These observations suggested a cross-talk between cAMP/PDE4 and the BCR, an observation that when fully understood can be developed into clinical initiatives. Here, we will test the hypothesis that PDE4 inhibition, by elevating cAMP levels, suppresses the activity of the BCR and consequently that of BCR-dependent kinases SYK, BTK and PI3K?. Building on our preliminary data, we also plan to test the idea that the cAMP/PDE4 axis modulates the BCR via activation of of the lipid phosphatase SHIP1, thus terminating PI3K activity. By extension, we propose that combining PDE4 inhibitors with agents that specifically target these BCR-related kinases will impart clinical benefit. We suggest that this vertical targeting approach will be synergistic and may block compensatory mechanisms derived from single agent strategies, thus limiting the emergence of resistance. To test these concepts, we will use genetically modified DLBCL cell lines, a Pdeb4 knock-out mouse, a new composite mouse model generated in our group that combines homozygous loss of Pde4b with a lymphomagenic Myc transgenic allele, and clinical grade inhibitors of PDE4, BTK and PI3K?. Our specific aims are: 1. Define in vitro and in vivo the role of the cAMP/PDE4 axis in modulating BCR activity in normal mature B lymphocytes and B cell lymphomas; 2. Determine in vitro and vivo the effects of the combinatorial targeting of PDE4 and BCR-dependent kinases in human and murine B cell lymphoma. Upon completion, this study may identify cAMP as mediator of BCR signal termination, and PDE4 inhibition as a novel approach to target the aberrant BCR signaling pathway. If positive, our data will fulfill one of the tenets of contemporary rational therapeutic design, the targeting of a critical pathway, the BCR, with distinct drug classes, phosphodiesterase and kinase inhibitors. PUBLIC HEALTH RELEVANCE: To improve the cure rate of B cell lymphomas we need to better understand how these cancers develop and progress. We will test if inhibiting the enzyme phopshodiesterase 4 (PDE4) suppresses the oncogenic activity of the B cell receptor and thus imparts clinical benefit to this group of cancer patients.