Introduction: The Y-box binding protein YB-1 is a member of the cold shock domain protein superfamily and represents one of the most evolutionary conserved nucleic-acid binding proteins. Yb-1 is involved in a wide variety of cellular functions, such as regulation of transcription and translation, but also in DNA-repair and stress response to extracellular signals. Furthermore, recent reports from our group could show that overexpressed YB-1 plays a role in drug resistance of breast cancer cells and might act as an oncogene. The goal of this study was to investigate a potential pathogenetic role of YB-1 in MM.

Material and Methods: For the detection of YB-1 expression in vivo, bone marrow biopsies of MM patients were analyzed by immunohistochemistry. The expression of YB-1 protein in a number of MM cell lines was analyzed by Western Blotting. The regulation of YB-1 expression through major signaling pathways, e.g. IL-6R/STAT3, Ras/MAPK and PI3K/AKT, was analyzed using specific inhibitors of these pathways (Sant7, PD98059 and Ly294002). To determine the role of YB-1 for survival and proliferation, siRNA technology was exploited to transiently knockdown the expression of YB-1 in the MM cell lines INA-6 and MM.1s. In addition, these YB-1 knockdown cells were exposed to doxorubicin and melphalan to evaluate the influence of YB-1 on drug resistance.

Results: Immunohistochemical analyses of bone marrow biopsies revealed that YB-1 is strongly expressed only in MM cells of samples that show a highly proliferative phenotype. This subgroup of MM patients was characterized by an aggressive clinical course. In contrast, MM cells of samples with a slow proliferative status did not show YB-1 expression. Interestingly, tumor cells of patients that responded to chemotherapy did not express YB-1. Furthermore, neither normal bone marrow plasma cells nor premalignant plasma cells of MGUS patients showed YB-1 expression. YB-1 was detected in all of the evaluated MM cell lines. YB-1 expression was not regulated by the IL-6R/STAT3, Ras/MAPK and PI3K/AKT pathways. Knockdown of YB-1 in INA-6 and MM1.s cells by siRNA resulted in a slow proliferation phenotype with a higher apoptotic cell fraction. In addition, we observed that YB-1 knockdown remarkably sensitized cells towards drug-induced apoptosis.

Conclusions: YB-1 expression in MM appears to be correlated with a highly proliferative phenotype and with disease progression. YB-1 contributes to the malignant growth and drug resistance and might be therefore an attractive therapeutical target to circumvent aquired drug resistance in MM.

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