Effects of treatment with miR-16 antagomir on erythropoietin (EPO)-induced erythrocytosis in mice. (A) Normal C57Bl/6J mice were injected with EPO only (600 U/kg; EPO-mice) on day 0 with or without daily intravenous injections of antagomir-16 (55 mg/kg; antagomir mice) on days 0 to 2 or an equivalent volume of vehicle (sham mice). The data presented in the plots derive from 2 independent experiments that involved at least 3 mice per time point in each experiment. (B) Antagomir injection resulted in almost complete suppression of endogenous miR-16 in bone marrow (BM), spleen (SP), and liver (LV) extracts on day 7 (measured by RTQ-PCR). **P < .001. (C) A blood sample was collected from all mice on day −2 (baseline) and on days 4 and 7 after EPO administration to estimate reticulocyte count. The > 3-fold increase of reticulocytes that followed EPO administration in EPO-mice was largely prevented by antagomir-16 treatment. Similarly, the proportion of Ter-119⁺ cells in the bone marrow of mice at day 7 after EPO injection was significantly higher in EPO-mice than in those receiving antagomir (Antago) or in sham mice (D). The number of BFU-E colonies obtained from bone marrow cells of antagomir-treated mice was significantly lower than that of mice treated with EPO only at both day 4 and day 7 (E), whereas the number of CFU–granulocyte macrophages (CFU-GM) grown from the spleen and the bone marrow at day 4 (not shown) or day 7 was unaffected (F).