Stem cells from bone marrow, skeletal muscle and possibly other tissues can be identified by the 'side-population' (SP) phenotype. Although it has been assumed that expression of ABC transporters is responsible for this phenotype, the specific molecules involved have not been defined. Here we show that expression of the Bcrp1 (also known as Abcg2 murine/ABCG2 human) gene is a conserved feature of stem cells from a wide variety of sources. Bcrp1 mRNA was expressed at high levels in primitive murine hematopoietic stem cells, and was sharply downregulated with differentiation. Enforced expression of the ABCG2 cDNA directly conferred the SP phenotype to bone-marrow cells and caused a reduction in maturing progeny both in vitro and in transplantation-based assays. These results show that expression of the Bcrp1/ABCG2 gene is an important determinant of the SP phenotype, and that it might serve as a marker for stem cells from various sources.

IL-10 regulates inflammation by reducing cytokine and chemokine production from activated macrophages. We performed microarray experiments to identify possible effector molecules of IL-10 and to investigate the global effect of IL-10 on the transcriptional response induced in LPS-activated macrophages. To exclude background effects of endogenous IL-10, macrophages from IL-10-deficient mice were used. IL-10 up-regulated expression of a small number of genes (26 and 37 after 45 min and 3 h, respectively), including newly identified and previously documented targets such as suppressor of cytokine signaling-3 and IL-1 receptor antagonist. However, the activation program triggered by LPS was profoundly affected by IL-10. IL-10 repressed 62 and further increased 15 of 259 LPS-induced genes. For all genes examined, the effects of IL-10 were determined to be STAT3-dependent. These results suggest that IL-10 regulates STAT3-dependent pathways that selectively target a broad component of LPS-induced genes at the mRNA level.

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Retinoblastoma-1 (RB1), and the RB1-related proteins p107 and p130, are key regulators of the cell cycle. Although RB1 is required for normal erythroid development in vitro, it is largely dispensable for erythropoiesis in vivo. The modest phenotype caused by RB1 deficiency in mice raises questions about redundancy within the RB1 family, and the role of RB1 in erythroid differentiation. Here we show that RB1 is the major pocket protein that regulates terminal erythroid differentiation. Erythroid cells lacking all pocket proteins exhibit the same cell cycle defects as those deficient for RB1 alone. RB1 has broad repressive effects on gene transcription in erythroid cells. As a group, RB1-repressed genes are generally well expressed but downregulated at the final stage of erythroid development. Repression correlates with E2F binding, implicating E2Fs in the recruitment of RB1 to repressed genes. Merging differential and time-dependent changes in expression, we define a group of approximately 800 RB1-repressed genes. Bioinformatics analysis shows that this list is enriched for terms related to the cell cycle, but also for terms related to terminal differentiation. Some of these have not been previously linked to RB1. These results expand the range of processes potentially regulated by RB1, and suggest that a principal role of RB1 in development is coordinating the events required for terminal differentiation.

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