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Key Points• Inhibition of Bruton's tyrosine kinase is as effective in vitro against AML as chronic lymphocytic leukemia.• Ibrutinib shows activity in AML because Bruton's tyrosine kinase is constitutively active.Bruton's tyrosine kinase (BTK) is a cytoplasmic protein found in all hematopoietic cell lineages except for T cells. BTK mediates signaling downstream of a number of receptors. Pharmacologic targeting of BTK using ibrutinib (previously PCI-32765) has recently shown encouraging clinical activity in a range of lymphoid malignancies. This study reports for the first time that ibrutinib inhibits blast proliferation from human acute myeloid leukemia (AML) and that treatment with ibrutinib significantly augmented cytotoxic activities of standard AML chemotherapy cytarabine or daunorubicin. Here we describe that BTK is constitutively phosphorylated in the majority of AML samples tested, with BTK phosphorylation correlating highly with the cell's cytotoxic sensitivity toward ibrutinib. BTK-targeted RNAi knockdown reduced colony-forming capacity of primary AML blasts and proliferation of AML cell lines. We showed that ibrutinib binds at nanomolar range to BTK. Furthermore, we showed ibrutinib's antiproliferative effects in AML are mediated via an inhibitory effect on downstream nuclear factor-kB survival pathways. Moreover, ibrutinib inhibited AML cell adhesion to bone marrow stroma. Furthermore, these effects of ibrutinib in AML were seen at comparable concentrations efficacious in chronic lymphocytic leukemia. These results provide a biological rationale for clinical evaluation of

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Dexamethasone is a key front-line chemotherapeutic for B-cell malignant multiple myeloma (MM). Dexamethasone modulates MM cell survival signaling but fails to induce marked cytotoxicity when used as a monotherapy. We demonstrate here the mechanism behind this insufficient responsiveness of MM cells toward dexamethasone, revealing in MM a dramatic anti-apoptotic role for microRNA (miRNA)-125b in the insensitivity toward dexamethasone-induced apoptosis. MM cells responding to dexamethasone exhibited enhanced expression of oncogenic miR-125b. Dexamethasone also induced expression of miR-34a, which acts to suppress SIRT1 deacetylase, and thus allows maintained acetylation and inactivation of p53. p53 mRNA is also suppressed by miR-125b targeting. Reporter assays showed that both these dexamethasone-induced miRNAs act downstream of their target genes to prevent p53 tumor suppressor actions and, ultimately, resist cytotoxic responses in MM. Use of antisense miR-125b transcripts enhanced expression of pro-apoptotic p53, repressed expression of anti-apoptotic SIRT1 and, importantly, significantly enhanced dexamethasone-induced cell death responses in MM. Pharmacological manipulations showed that the key regulation enabling complete dexamethasone sensitivity in MM cells lies with miR-125b. In summary, dexamethasone-induced miR-125b induces cell death resistance mechanisms in MM cells via the p53/miR-34a/SIRT1 signaling network and provides these cells with an enhanced level of resistance to cytotoxic chemotherapeutics. Clearly, such anti-apoptotic mechanisms will need to be overcome to more effectively treat nascent, refractory and relapsed MM patients. These mechanisms provide insight into the role of miRNA regulation of apoptosis and their promotion of MM cell proliferative mechanisms.

This study was designed to identify metalloproteinase determinants of macrophage migration and led to the specific hypothesis that matrix metalloproteinase 10 (MMP10/stromelysin-2) facilitates macrophage migration. We first profiled expression of all MMPs in LPS-stimulated primary murine bone marrow-derived macrophages and Raw264.7 cells and found that MMP10 was stimulated early (3 h) and down-regulated later (24 h). Based on this pattern of expression, we speculated that MMP10 plays a role in macrophage responses, such as migration. Indeed, using time lapse microscopy, we found that RNAi silencing of MMP10 in primary macrophages resulted in markedly reduced migration, which was reversed with exogenous active MMP10 protein. Mmp10 −/− bone marrow-derived macrophages displayed significantly reduced migration over a two-dimensional fibronectin matrix. Invasion of primary wild-type macrophages into Matrigel supplemented with fibronectin was also markedly impaired in Mmp10 −/− cells. MMP10 expression in macrophages thus emerges as an important moderator of cell migration and invasion. These findings support the hypothesis that MMP10 promotes macrophage movement and may have implications in understanding the control of macrophages in several pathologies, including the abnormal wound healing response associated with pro-inflammatory conditions.

Abbreviations: MM -multiple myeloma; PI -proteasome inhibitor; NF-kB -nuclear factor-kappa B; BMSC -bone marrow stromal cells; BTK -Bruton's tyrosine kinase.Multiple Myeloma (MM) is a haematologic malignancy characterized by the accumulation of clonal plasma cells in the bone marrow. Over the last 10-15 y the introduction of the proteasome-inhibitor bortezomib has improved MM prognosis, however relapse due to bortezomib-resistance is inevitable and the disease, at present, remains incurable. To model bortezomib-resistant MM we generated bortezomib-resistant MM cell lines (n D 4 ) and utilised primary malignant plasma cells from patients relapsing after bortezomib treatment (n D 6 ). We identified enhanced Bruton's tyrosine kinase (BTK) activity in bortezomib-resistant MM cells and found that inhibition of BTK, either pharmacologically with ibrutinib (0.5 mM) or via lenti-viral miRNA-targeted BTK interference, re-sensitized previously bortezomib-resistant MM cells to further bortezomib therapy at a physiologically relevant concentration (5 nM). Further analysis of pro-survival signaling revealed a role for the NF-kB p65 subunit in MM bortezomib-resistance, thus a combination of BTK and NF-kB p65 inhibition, either pharmacologically or via further lenti-viral miRNA NF-kB p65 interference, also restored sensitivity to bortezomib, significantly reducing cell viability (37.5 § 6 .9 %, ANOVA P 0 .001). Accordingly, we propose the clinical evaluation of a bortezomib/ibrutinib combination therapy, including in patients resistant to single-agent bortezomib.

Pharmacological targeting of BTK using ibrutinib has recently shown encouraging clinical activity in a range of lymphoid malignancies. Recently we reported that ibrutinib inhibits human acute myeloid leukemia (AML) blast proliferation and leukemic cell adhesion to the surrounding bone marrow stroma cells. Here we report that in human AML ibrutinib, in addition, functions to inhibit SDF1/CXCR4-mediated AML migration at concentrations achievable in vivo. It has previously been shown that SDF1/CXCR4-induced migration is dependent on activation of downstream BTK in chronic lymphocytic leukaemia (CLL) and multiple myeloma. Here we show that SDF-1 induces BTK phosphorylation and downstream MAPK signalling in primary AML blast. Furthermore, we show that ibrutinib can inhibit SDF1-induced AKT and MAPK activation. These results reported here provide a molecular mechanistic rationale for clinically evaluating BTK inhibition in AML patients and suggests that in some AML patients the blasts count may initially rise in response to ibrutinib therapy, analgous to similar clinical observations in CLL.

The introduction of the proteasome inhibitor bortezomib in 2003 significantly improved treatment of the B-cell malignancy MM (multiple myeloma). Relapse following bortezomib therapy is inevitable, however, and MM remains an incurable disease. In the present mini-review, we explore the mechanisms by which bortezomib resistance occurs in MM, including inherent and acquired mutation, and inducible pro-survival signalling. We also outline the importance of MM cell interaction with the BMSC (bone marrow stromal cell) microenvironment as a pro-survival mechanism, and examine some potential druggable targets within this milieu, such as IGFs (insulin-like growth factors) and Btk (Bruton's tyrosine kinase). Although our understanding of bortezomib resistance is far from complete, there are a number of scientific developments that can help inform clinical decisions in relapsed MM.