The Polycomb group (PcG) genes (bmi1 and mel-18) known as negative control factors of the Hox gene is thought to regulate the differentiation and self-renewal of hematopoietic stem cells (HSCs). The loss of mel-18 results in the promotion of HSC self-renewal, and the increase of mel-18 expression inversely leads to the differentiation of HSCs. On the other hand, the loss of bmi1 does not lead to self-renewal activity of HSCs. In this study we examined the effect of expression of bmi1 and mel-18 on the role of function in murine HSCs. Lineage-negative, Sca1-positive, and cKit-positive primitive hematopoietic cells were purified and the expression of PcG protein was evaluated from the intra-nuclear distribution of PcG proteins. The Bmi1-positive hematopoietic cells barely contained Mel-18, and the Mel-18-positive cells barely contained Bmi1. the frequency of positive cells for both Bmi1 and Mel-18 was less than 0.5% of purified primitive hematopoietic cells. The expression levels of the PcG genes, bmi1 and mel-18, in HSCs were knocked down by siRNA and then gene expression was assessed by quantitative real-time PCR. The introduction of siRNA against bmi1 or mell-18 resulted in approximate 50 to 60% decrease of each gene expression without affecting another gene expression. Primary colony-forming activity of knocked down cells in response to stem cell factor, thrombopoietin and the ligand for flt3 was not affected by the induction of siRNA. However, secondary colony-forming activity from primary colony-forming cells in bmi1-knockdown cells was significantly decreased when compared with that of control cells. Conversely, the mel-18-knockdown cells significantly increased, suggesting that mel-18-knockdown cells are capable of proliferating activity. Finally, bone marrow reconstitutive activity was examined by using Ly5.1 and Ly5.2 system. While the bmi1-knockdown marrow cells decreased the reconstitutive activity, the mel-18-knockdown marrow cells showed the increase of engraftment activity after 6 months of transplantation. From these results, we consider that mel-18 and bmi1 have reciprocal functions in HSCs. Mammalian PcG protein complexes can be classified into two distinct types, Polycomb repressive complexes 1 and 2 (PRC1 and PRC2). The Mel-18 protein is a constituent of mammalian PRC1 together with M33, Bmi1 or rae28, and Scmh1. The Mel-18 protein is composed of 342 amino acids and the N-terminal region of the 102 amino acid, which includes the RING finger motif, shares 93% homology with Bmi1 protein. In addition, its secondary structure shows high homology with the Mel-18 and Bmi1 proteins. We hypothesized that the opposite function is expressed in HSCs because Mel-18 and Bmi1 share the same structure and compete when in the complex form. These results suggest that mel-18 and bmi1 have inverse function in HSCs and that the balance of Bmi1 and Mel-18 may regulate the fate of self-renewal and differentiation in HSCs.

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