Brutons tyrosine kinase (BTK) is a non-receptor kinase that plays a crucial role in oncogenic signaling that is critical for proliferation and survival of leukemic cells in many B cell malignancies. effects beyond its classic role in BCR signaling. These involve B cell-intrinsic signaling pathways central to cellular survival, proliferation or Proteasome-IN-1 retention in supportive lymphoid niches. Moreover, BTK functions in several myeloid cell populations representing important components of the tumor microenvironment. As a result, there is currently a considerable interest in BTK inhibition as an anti-cancer therapy, not only in B cell malignancies but also in solid tumors. Efficacy of BTK inhibition as a single agent therapy is usually strong, but resistance may develop, fueling the development of combination therapies that improve clinical responses. In this review, we discuss the role of BTK in B cell differentiation and B cell malignancies and spotlight the importance of BTK inhibition in cancer therapy. (X-linked immunodeficiency) mice, manifest only minor Rabbit Polyclonal to OR2AG1/2 defects in B cell development in the bone marrow, but instead the differentiation and survival of mature peripheral B cells is usually severely impaired [7C10]. Importantly, BTK has received large interest since small-molecule inhibitors of this kinase have shown excellent anti-tumor activity in clinical studies [11, 12]. In particular, the orally administered BTK inhibitor ibrutinib, which forms a covalent bond with a cysteine residue in the BTK active site, was also approved for first-line treatment of patients with chronic lymphocytic leukemia (CLL) and small lymphocytic leukemia (SLL) in 2016 [13]. Shortly after its discovery as the non-receptor tyrosine kinase defective in XLA [3, 4], BTK was placed in the signal transduction pathway downstream of the B cell receptor (BCR). This receptor is usually expressed around the B cell surface and has the unique capacity to specifically recognize antigens due to hypervariable regions present in the immunoglobulin heavy (IGH) and light (IGL) chains that together form the BCR [14]. BTK is also involved in many other signaling pathways in B cells, including chemokine receptor, Toll-like receptor (TLR) and Fc receptor signaling. Expression of BTK is not restricted to B cells, as also cells of the myeloid lineage express BTK. In these cells, BTK acts also downstream of TLRs and e.g. the FcR in mast cells [15, 16] and the FcyRI in macrophages [17, 18]. In addition, BTK is usually involved in various other pathways, including Receptor activator of nuclear factor-B (RANK) in osteoclasts [19], collagen and CD32 signaling in platelets [20] and the NLRP3 inflammasome in macrophages and neutrophils [21]. Since myeloid cells are important components of the tumor microenvironment and particularly tumor-associated macrophages contribute to cancer progression [22, 23], there is currently a considerable interest in BTK inhibition as an anti-cancer therapy not only in B cell leukemias but also in other hematological malignancies Proteasome-IN-1 and solid tumors [24C27]. In this review, we describe the importance of BTK in multiple signaling pathways. We discuss the crucial function of BTK in different stages of normal B cell development. In addition, we discuss its role in oncogenic signaling in B cell malignancies associated with genetic events that result in increased BTK activity. We describe clinical benefits of targeting BTK with small molecule inhibitors in B cell malignancies. Finally, we discuss the Proteasome-IN-1 effects of BTK inhibitors on tumor growth in solid malignancies in the context of the function of myeloid cells in the tumor environment. BTK structure BTK is one of the five members of the TEC family of non-receptor tyrosine kinases – along with tyrosine kinase expressed in hepatocellular carcinoma (TEC), interleukin-2-inducible T cell kinase (ITK), resting lymphocyte kinase (RLK) and bone marrow expressed kinase (BMX) – which are strongly conserved throughout evolution [28]. BTK, TEC and ITK are most.