Expression of p21(Cip1/Waf1/Sdi1) and p27(Kip1) cyclin-dependent kinase inhibitors during human hematopoiesis.
Expression of p21 and p27 cyclin-dependent kinase inhibitors is associated with induced differentiation and cell-cycle arrest in some hematopoietic cell lines. However, it is not clear how these inhibitors are expressed during normal hematopoiesis. We examined various human hematopoietic colonies derived from cord blood CD34(+) cells, bone marrow, and peripheral blood cells using a quantitative reverse transcription-polymerase chain reaction assay, immunochemistry, and/or Western blot analysis. p21 mRNA was expressed increasingly over time in all of the colonies examined (granulocytes, macrophages, megakaryocytes, and erythroblasts), whereas p27 mRNA levels remained low, except for erythroid bursts. Erythroid bursts expressed both p21 and p27 mRNAs with differentiation but expressed neither protein, whereas both proteins were expressed in megakaryocytes and peripheral blood monocytes. In bone marrow, p21 was immunostained almost exclusively in a subset of megakaryocytes and p27 protein was present in megakaryocytes, plasma cells, and endothelial cells. In megakaryocytes, reciprocal expression of p27 to Ki-67 was evident and an inverse relationship between p21 and Ki-67 positivities was also present, albeit less obvious. These observations suggest that a complex lineage-specific regulation is involved in p21 and p27 expression and that these inhibitors are involved in cell-cycle exit in megakaryocytes.