IL-6-Induced C-MYC Down-Regulates the Expression of CD33 in CD33(+) Myeloma Cell Lines

Since human myeloma cells specifically lose the expression of PAX-5 gene, the master gene of B cell lineage, we frequently detect the ectopic expressions of non-B cell lineage markers such as CD56 and CD33. As for the expression of CD33, we confirmed that CD33 expression was found in primary myeloma cells from about 12% of 120 cases with overt myeloma and also in 2 myeloma cell lines (ILKM8 and Liu01 (a subclone of U266)) in the protein and mRNA levels. In primary myeloma cells as well as CD33(–) myeloma cell lines, DMSO treatment could induce the expression of CD33 in the in vitro culture. In these CD33(+) myeloma cell lines and DMSO-induced myeloma cells, we found that expressions of C/EBPα and PU.1 were markedly increased by gene expression profiling. On the other hand, IL-6 down-regulated the expression of CD33 in CD33(+) myeloma cell lines accompanying with down-regulation of C/EBPα and PU.1 expressions. Also, we found that IL-6 up-regulated the expression of C-MYC and the increased C-MYC bound to the promoter region of C/EBPα gene followed by the down-regulation of C/EBPα expression. It was confirmed that introduction of sh-RNA for C-MYC to two CD33(+) myeloma cell lines blocked the IL-6-induced down-regulation of CD33 and C/EBPα expression. Therefore, these results indicate that IL-6 can reverse the ectopic expression of non-B cell lineage markers by up-regulating C-MYC followed by its down-regulation of C/EBPα expression and also suggest that constitutive activation of STAT3 by IL-6 may keep PAX-5(–) myeloma cells being uncommitted cells to any lineage; myeloma cells could be one of stem cell-like cancer cells.