Multiple Myeloma (MM) is a malignancy of clonal plasma cells residing mostly in the bone marrow. The bone microenvironment constitutes a prominent gradient of SDF-1 (Stromal-derived factor-1), a chemokine required for MM cell homing to bone. With disease progression, MM cells establish a positive feedback loop of osteoclast activation resulting in lytic bone disease. MM cells are found dispersed in interstitial marrow as well as in focal aggregates that potentially develop into sites of osteolytic lesions. BTK (Bruton's tyrosine kinase), a TEC (Tyrosine kinase of hepatocellular carcinoma) family kinase, is a cytoplasmic protein mainly involved in B cell homing, adhesion and maturation, while its expression is lost during their differentiation into antibody-producing plasma cells. During RANKL (Receptor activator of nuclear factor kappa-B) -mediated osteoclast maturation, BTK forms a critical part of NFATc1 (Nuclear factor of activated T cells c1) complex, the master transcription factor for osteoclast differentiation. Considering its expression both in MM plasma cells and differentiating osteoclasts, we investigated whether BTK contributes to metastasis, tumor growth and associated bone disease. BTK was highly expressed in patient MM cells, stroma- and IL-6 (Interleukin-6)-dependent MM lines, while its expression in majority of MM lines was very low. Cell-surface CXCR4 [(C-X-C motif) receptor 4] expression highly correlated to BTK expression in MM cells. SDF-1 stimulation induced activation of BTK and chemotaxis in MM cells, an effect abrogated by the pharmacological BTK inhibitor LFM-A13. BTK inhibition reduced MM cell adhesion to stroma and clonogenicity. In vivo, LFM-A13 reduced homing of MM cells to the implanted bone, while BTK silencing exclusively in MM cells reduced metastasis to distant bone but promoted tumor growth in primary site. In a novel hematopoietic bone marrow coculture model, the irreversible BTK inhibitor ibrutinib promoted patient MM cell growth. Systemic BTK inhibition did not reduce MM tumor burden and effectively attenuated MM-induced bone disease. BTK and CXCR4 expression in patients' MM cells from random marrow sites were higher than in those from focal lesions. Gene expression analysis of tumor-derived MM cells with BTK silencing revealed downregulation of metastasis-related and upregulation of tumor growth factors. Together, our findings indicate that BTK is required for MM cell migration, adhesion, clonogenicity, metastases and osteoclast-mediated bone disease. Loss of BTK in MM cells promotes tumor growth particularly in association with bone marrow microenvironmental cells such as osteoclasts.
|Commitee:||Epstein, Joshnua, Griffin, Robert J., Kelly, Thomas J., O'Brien, Charles A.|
|School:||University of Arkansas for Medical Sciences|
|Department:||Interdisciplinary Biomedical Sciences|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 75/09(E), Dissertation Abstracts International|
|Subjects:||Molecular biology, Oncology|
|Keywords:||Bone, Bruton’s tyrosine kinase, CXCR4, Cancer, Metastasis, Multiple-myeloma|
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