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Despite the fact that many novel therapies have been introduced in the treatment of MM, drug resistance develops and all patients eventually relapse. Therefore, it is critical to identify factors, which mediate drug resistance in MM. C/EBPb belongs to the C/EBP family of transcription factors which share substantial sequence similarity in the C-terminal basic leucine zipper region and play an important role in the regulation of cell proliferation and differentiation. Increased levels of C/EBPb have been found in different types of tumors, and mice deficient in C/EBPb show impaired generation of B lymphocytes, suggesting that C/EBPb plays a critical role in B lymphopoiesis. The promoter of IL-6, the most important growth and survival factor for MM cells, is activated by C/EBPb, and thus C/EBPb was originally named NF-IL6 (nuclear factor involved in IL-6 expression) (Akira et al. 1990). In addition, it has been shown that MIP-1α, which is also a major growth and survival factor for MM, increased expression of C/EBPb (Matsumoto et al. 1998). These data suggest that C/EBPb might play an important role in the pathogenesis of MM. In proliferation assays we analyzed 27 malignant hematological cell lines for their sensitivity to IMiD CC-4047, including multiple myeloma, M. Hodgkin, Burkitt’s lymphoma, pre-B-ALL and T-cell leukemia cell lines, Our analyses revealed that 13 cell lines were sensitive to CC-4047 with an inhibition of proliferation of at least 60%. To examine the mechanisms that mediate resistance or sensitivity to CC-4047, we investigated the transcriptional profile of CC-4047 treated MM.1S MM cells by oligonucleotide microarray analysis. This analysis revealed that treatment with CC-4047 resulted in a significant regulation of several genes, including FHF-2, C/EBPb, c-myb, GFI-1 and HUMSPARC. MM.1S cells, which were sensitive to CC-4047, showed a strong down-regulation of C/EBPb mRNA in response to CC-4047. In western blotting, C/EBPb protein including all isoforms (LIP, LAP) completely disappeared with treatment by CC-4047 in MM.1S cells. In contrast, treatment with Thal had only minor effects on C/EBPb expression. Interestingly, pretreatment of MM1.S cells with IL-1β increased C/EBPb expression, but this induction could also be completely abrogated by CC-4047 treatment. Further experiments showed, that all cell lines sensitive to CC-4047 consistently demonstrated a down-regulation of all C/EBPb isoforms. Our studies suggest that C/EBPb might be an important transcription factor controlling drug resistance towards Thal and its derivatives in MM cells. Further studies are warranted to understand the molecular basis underlying the resistance towards IMiDs in order to define new strategies for breaking drug resistance in MM cells.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]