Transforming growth factor β1 mediates cell-cycle arrest of primitive hematopoietic cells independent of p21 Cip1/Waf1 or p27 Kip1

The regulation of stem cell proliferation is a poorly understood process balancing rapid, massive blood cell production in times of stress with maintenance of a multipotent stem cell pool over decades of life. Transforming growth factor β1 (TGF-β1) has pleiotropic effects on hematopoietic cells, including the inhibition of primitive cell proliferation. It was recently demonstrated that the cyclin-dependent kinase inhibitors, p21 Cip1/Waf1 (p21) and p27 Kip1 (p27), can inhibit the proliferation of hematopoietic stem cells and progenitor cells, respectively. The relation of TGF-β1 stimulation to p21 and p27 was examined using a fine-mapping approach to gene expression in individual cells. Abundant TGF-β1 expression and p21 expression were documented in quiescent, cytokine-resistant hematopoietic stem cells and in terminally differentiated mature blood cells, but not in proliferating progenitor cell populations. TGF-β1 receptor (TβR II) was expressed ubiquitously without apparent modulation. Cell- cycle–synchronized 32D cells exposed to TGF-β1 demonstrated a marked antiproliferative effect of TGF-β1, yet neither the level of p21 mRNA nor the protein level of either p21 or p27 was altered. To corroborate these observations in primary cells, bone marrow mononuclear cells derived from mice engineered to be deficient in p21 or p27 were assessed. Progenitor and primitive cell function was inhibited by TGF-β1 equivalently in −/− and +/+ littermate controls. These data indicate that TGF-β1 exerts its inhibition on cell cycling independent of p21 and p27 in hematopoietic cells. TGF-β1 and p21 or p27 participate in independent pathways of stem cell regulation, suggesting that targeting each may provide complementary strategies for enhancing stem or progenitor cell expansion and gene transduction.