Green fluorescent protein (GFP)-labeled marrow cells transplanted into lethally irradiated mice can be detected in the lungs of transplanted mice and have been shown to express lung-specific proteins while lacking the expression of hematopoietic markers. We have studied marrow cells induced to transit the cell cycle by exposure to interleukin-3 (IL-3), IL-6, IL-11, and Steel factor at different times of culture corresponding to different phases of cell cycle. We have found that marrow cells at the G1 /S interface of the cell cycle have a three-fold increase in cells that assume a nonhematopoietic or pulmonary epithelial cell phenotype and that this increase is no longer seen in late S/G 2. These cells have been characterized as GFP+ CD45 - and GFP+ cytokeratin+. Thus, marrow cells with the capacity to convert into cells with a lung phenotype after transplantation show a reversible increase with cytokine-induced cell cycle transit. Previous studies have shown that the phenotype of bone marrow stem cells fluctuates reversibly as these cells traverse the cell cycle, leading to a continuum model of stem cell regulation. The present study indicates that marrow stem cell production of nonhematopoietic cells also fluctuates on a continuum.
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