Contribution of the ABC Transporters Bcrp1 and Mdr1a/1b to the Side Population Phenotype in Mammary Gland and Bone Marrow of Mice

Abstract

The ability of cells to export Hoechst 33342 can be used to identify a subpopulation of cells (side population [SP]) with characteristics of stem cells in many tissues. The ATP-binding cassette transporters Bcrp1 (Abcg2) and Mdr1a/1b (Abcb1a/1b) have been implicated as being responsible for this phenotype. To further explore the involvement of these transporters in the SP phenotype, we have generated Bcrp1/ Mdr1a/1b triple knockout mice and studied the effect of their absence on the SP in bone marrow and mammary gland.Whereas in bone marrow Bcrp1 was almost exclusively responsible for the SP, both transporters contributed to the SP phenotype in the mammary gland, where their combined absence resulted in a nearly complete loss of SP. Interestingly, bone marrow of Mdr1a/1b −/− mice frequently displayed an elevated SP, which was reversible by the Bcrp1 inhibitor Ko143, suggesting that Bcrp1 can compensate for the loss of Mdr1a/1b in bone marrow. Stem Cells 2005;23:1059-1065

Drug transporters: recent advances concerning BCRP and tyrosine kinase inhibitors

“…The notion that only BCRP can efflux Hoechst demonstrated that this transporter was dominantly associated with the SP phenotype, which was further confirmed by BCRP transfection in bone marrow cells causing significant expansion of cells bearing the SP phenotype (Zhou et al, 2001). Also in Bcrp1/Mdr1a/1b triple knockout mice, it was shown that Bcrp1, but not Mdr1a/1b, is responsible for the SP phenotype in the bone marrow, while both transporters are required for the SP phenotype in the mammary gland (Jonker et al, 2005). It has now been demonstrated that SP cells are present in several tumour samples, possess stem cell-like properties, overexpress BCRP and possess inherent drug resistance (Haraguchi et al, 2006).…”

Normal ovarian surface epithelial label-retaining cells exhibit stem/progenitor cell characteristics

“…Additionally, somatic and cancer stem cells from various tissues have been identified by their ability to efflux Hoechst 33342 dye through ATP-binding-cassette transporters, such as Abcg2/Bcrp1 (11,(20)(21)(22)(23)(24)(25)(26)(27), including our recent identification of these ''side population'' (SP) cells as potential tumor-initiating cells in ovarian cancer (28). It has been postulated that this chemical-effluxing capability contributes to the cytopreservation necessary for the longevity attributed to stem/progenitor cells (29).…”

ABC transporters, neural stem cells and neurogenesis – a different perspective

“…In particular, ABCB1 is expressed in human CD34+ stem cells, which can be identified by their ability to transport fluorescent dyes like Rh123 and Hoechst-33342 [128,129,[137][138][139][140][141][142][143]. Interestingly, the SP phenotype in rodent and human tissues often appears to be specifically determined by the expression of an ABC transporter: for example, both ABCB1 and ABCG2 transporters are highly expressed in the SP of stem cells from different tissues such as brain, bone marrow, pancreas, liver and others, all of which can be isolated based on the cells' ability to promote the efflux of the Hoechst-33342 fluorescent dye [9,[129][130][131][132][133][134][135][136][142][143][144][145][146][147][148][149][150]. Moreover, different research teams have demonstrated that ABCA3 and ABCG2 were expressed at higher levels in SP cells than in non-SP cells in human, rhesus monkey and mouse hematopoietic tissues; and microarray analysis indicated that several genes related to stem cells were substantially upregulated in the SP cells in comparison to non-SP cells [130,151,152].…”