Expression of p15INK4B in normal hematopoiesis

The cyclin-dependent kinase inhibitor (CDKI) p15INK4B (p15) induces cell cycle arrest in G0/G1 phase. Several studies report deletion or transcriptional loss of the p15 gene in myeloid and lymphoid hematological malignancies, and a possible role as a tumor suppressor gene has been proposed for this CDKI. In this study we evaluated the expression of p15 by cytofluorometric, immunohistochemical, and reverse transcriptase-polymerase chain reaction (RT-PCR) methods in CD34+ progenitors (both during steady state and after chemotherapy and/or granulocyte-colony stimulating factor [G-CSF] administration) and in cells belonging to different hematopoietic differentiative lineages. We found that p15 is not expressed in normal G0/G1-arrested peripheral blood (PB)- or bone marrow (BM)-CD34+ cells. Moreover, p15 is expressed in G0/G1-blocked CD34+ cells mobilized by chemotherapy and G-CSF but not in CD34+ cells mobilized by G-CSF alone. To clarify the role of p15 in normal hematopoiesis, we used flow cytometry to investigate its expression in normal differentiating BM and PB cells. We found that p15 was expressed in cells belonging to the granulocyte-monocyte lineage and in B and T lymphocytes, whereas erythroid and megakaryocytic cells were p15 negative. These findings, which were confirmed both by immunohistochemical and RT-PCR analysis, definitely establish a linkage between p15 expression and granulocyte-monocyte differentiation.