p15Ink4b Maintains Normal Levels of Granulocyte-Macrophage Progenitors In Vivo.

Maintaining a balance between the proliferation and differentiation of hematopoietic stem and progenitor cells is crucial for normal maintenance of the hematopoietic system. The INK4 family of cyclin-dependent kinase inhibitors traditionally function in regulating cell proliferation by blocking G1/S phase transition during cell cycle. Loss of expression of one INK4 family member, p15INK4B, has been associated with acute myeloid leukemia (AML) and myelodysplasia and mice that lack p15Ink4b are more prone to developing retrovirus-induced AML. These data suggest an important role for p15INK4B in myeloid differentiation. Here we examined the hematopoietic progenitor pool in p15Ink4b deficient (Ink4b−/−) mice to determine its potential role in regulating hematopoietic precursors. In myeloid progenitor colony assays, bone marrow (BM) from Ink4b−/− mice were found to contain a 1.4 fold greater number of progenitors committed to the formation of granulocytes and macrophages (CFU-GM). This in vitro data was supported by flow cytometric analysis which determined that Ink4b−/− BM contained a 3-fold greater proportion of granulocyte-macrophage progenitors (GMP) (11+/−1.2% vs 34+/−9%) concomitant with a 2-fold decrease in common myeloid progenitors (CMP) (56+/−2 % vs 34+/−7 %). GMP isolated from Ink4b−/− BM also demonstrated a 3-fold greater propensity to form CFU-GM. Despite these differences, the proportion of cycling GMP or CMP in Ink4b−/− and wt mice were identical as determined by propidium iodide and Hoecsht dye DNA stains and in vivo BrdU incorporation. However, in response to specific cytokines, Ink4b−/− CMP showed an increased potential for GMP differentiation and a decreased capacity to form megakaryocyte-erythroid progenitors (MEP). Our work demonstrates that p15Ink4b functions in maintaining normal levels of CFU-GM and may regulate CMP differentiation in response to specific factors.