MicroRNAs 155 and 451 as Possible Key Molecules for Human Erythroid Differentiation.

MicroRNAs (miRNAs) are 18–25-nucleotide noncoding RNAs which play important roles for cell death, proliferation, development and differentiation. MiRNA is an important molecule to regulate genes by suppressing the translation or inducing instability of miRNAs, and is consist of the network system to regulate gene functions in combination with transcription factors. Many recent works demonstrated that some of miRNAs are playing key roles for hematopoiesis and leukemogenesis. In this study, we analyzed the expression of miRNAs(miRNA-155, miRNA-221, miRNA-223, miRNA-451) during differentiation of purified normal human eryhroid progenitors in the liquid culture system. Cells increased almost 500-folds in a number, and differentiated to benzidine-positive mature erythroblasts after days 7 to 9 which were partly red blood cells on days 12 to 14. Since mature erythroid cells loose cellular nucleic acids at the final maturation stages, we measured changes in U6 RNA contents as the internal control for assays of miRNA. Each expression levels of miRNAs were normalized using U6 RNA contents. Analyses of miRNA expressions using quantitative real-time reversetranscriptase polymerase chain reaction have shown that the expression level of miRNA-155 decreased about 200-folds from day 3 to day 12 with almost 87.5% reduction between days 3 and 5. On the other hand, the expression levels of miRNA-451 increased about 270-folds by day 12 in parallel to an increase in benzidine-positive cell numbers. To extend our observation on the up-regulation of miRNA-451 in mature blood cells, we analyzed the miRNA-451 levels in each mature blood cells (red blood cells, granulocytes, lymphocytes and monocytes, platelets) purified from normal human peripheral blood by using a density centrifugation method. miRNA-451 was expressed in red blood cells about 104 folds more than in granulocytes, about 102 folds more than in platelets. Moderate down-regulations of miRNAs 221 and 223 were observed. In conclusion, our observations suggest that the down-regulation of miRNA-155 and the up-regulation of miRNA-451 are key events for normal erythroid differentiation, and that quantitative assays of the two miRNAs may be useful tools for specifying the differentiation stage of each erythroid cells.