MicroRNAs 221 and 222 Inhibit Normal Erythropoiesis and Erythroleukemic Cell Growth Via Kit Receptor Downmodulation.

MicroRNAs (miRs) are small non-coding RNAs that regulate gene expression primarily through translational repression. In unilineage erythropoietic (E) culture of cord blood (CB) CD34+ progenitor cells, the level of miR 221 and 222 is gradually and sharply downmodulated. Hypothetically, this decline could promote erythropoiesis by unblocking expression of key functional proteins. Our studies indicate that miR 221 and 222 target the Kit receptor: specifically, (a) the luciferase targeting assay showed that miR 221 and 222 directly interact with the 3′UTR of Kit mRNA; (b) in E culture the miR 221 and 222 level is inversely related to Kit protein expression, whereas the abundance of Kit mRNA is relatively stable. Functional studies show that treatment of CD34+ cells with miR 221 and 222, via oligonucleotide transfection or lentiviral vector infection, causes impaired proliferation and accelerated differentiation of E cells, coupled with downmodulation of Kit protein: this phenomenon, observed in E culture releasing endogenous Kit ligand (KL), is magnified in E culture supplemented with KL. Furthermore, transplantation experiments into NOD-SCID mice reveal that miR 221 or 222 treatment of CD34+ cells impairs their engraftment capacity and stem cell activity. Finally, miR 221 and 222 gene transfer impairs proliferation of the TF1 erythroleukemic cell line, expressing the Kit receptor. Altogether, our studies indicate that in human erythropoiesis the decline of miR 221 and 222 unblocks Kit protein production at translational level, thus leading to expansion of early E cells. Furthermore, overexpression of miR 221 and 222 inhibits proliferation of Kit+ erythroleukemic cells, suggesting a potential role of these microRNAs in cancer therapy.