Abstract

ABCG2 is a transporter protein that has the ability to efflux many drugs and fluorescent dyes. Primitive haematopoietic stem cells highly express ABCG2 and the expression level decreases as these cells differentiate indicating a possible role of this transporter in HSC. In the present study, we have analyzed the role of ABCG2 in early haematopoietic stem cells by constitutively expressing ABCG2 in human CB derived CD133+ cells. This constitutive expression of ABCG2 demonstrated an enhancement of primary CFCs in vitro, including the most primitive clonogenic cells the CFU-GEMM (n=12, p<0.002). ABCG2 enhances the replating capacity of primary colonies with a mean 3.0 fold increase in the number of 2nd colonies (n=9, p<0.01), indicating a substantial enhancement of the proliferative potential of clonogenic progenitors by constitutive ABCG2 expression. Overexpression of ABCG2 did not have any positive effect on cell expansion in liquid culture as well as the frequency of LTC-IC, however, the production of CFC per LTC-IC was found to be enhanced, again supporting the fact that ABCG2 might play an important role in the differentiation and proliferation of clonogenic progenitors. Using the NOD/SCID mouse model, we were able to demonstrate that enforced expression of ABCG2 in human primitive haematopoietic cells leads to inversion of lymphoid-myeloid ratio, suggesting that ABCG2 perhaps alters the cell fate decisions of multipotent cells with myeloid and lymphoid differentiation capacity. An enhanced production of differentiated myeloid cells was observed on ABCG2 overexpression. In order to analyze the effect of ABCG2 on early haematopoietic cells, NOD/SCID mice transplanted with CB cells either expressing ABCG2 or the empty viral vector, were analyzed for the presence of HSC. Although the number of human CD34+ CD38- cells did not show any difference, the number of CD34+ CD38+ progenitor cells was significantly increased (n=5, p<0.05), indicating that ABCG2 plays a role in the differentiation of clonogenic progenitors. CRU assays were performed to detect the effect of ABCG2 expression on the frequency of SRC and did not show any significant increase in the frequency of SRC. Taken together, these results indicate that ABCG2 is a potent positive regulator of human hematopoiesis at the level of early haematopoietic development.