Abstract 1391: Multidrug Resistant Protein 1 But Not Breast Cancer Resistant Protein 1 Confers the Phenotype of Cardiac Side Population Cells
Background: Side population (SP) cells are highly enriched for progenitor cell capacity. They are characterized by their ability to efflux Hoechst dye via ATP-binding cassette (ABC) transporters such as breast cancer resistant protein 1 (Bcrp1) and multidrug resistant protein 1 (Mdr1). First identified in bone marrow (BMSP), SP cells with tissue specific progenitor capacity were found in various solid organs. Recently, we have isolated cardiac SP cells (CSP) from adult myocardium. These CSP cells exhibit high proliferative capacity and the ability to differentiate into functional cardiomyocytes in vitro. While Bcrp1 confers the SP phenotype in BMSP cells and is implicated in their proliferation, little is known about the role of Bcrp1 and Mdr1 in CSP cells.
Methods and Results: To determine the role of Bcrp1 and Mdr1 in mediating CSP cell phenotype, we isolated CSP and BMSP cells from wildtype (WT) and mice lacking Bcrp1 (Bcrp1-ko) or Mdr1a/1b (Mdr1-ko) using fluorescence assisted cell sorting (FACS). WT-mice exhibited BMSP and CSP. RT-PCR analysis of sorted SP cells confirmed the expression of both Bcrp1 and Mdr1a/1b in WT-BMSP and WT-CSP cells. Bcrp1-ko mice lacked a BMSP population, though exhibited a clear CSP population. In contrast, Mdr1-ko mice exhibited BMSP cells, but their hearts lacked detectable CSP cells. The proliferation capacity was also determined in CSP isolated from Brcp1-ko and WT mice. Interestingly, CSP cells lacking Bcrp1 demonstrated significantly impaired proliferative capacity as compared to WT CSP cells (fold increase in cell number at day 9: Bcrp1-ko 3.4±0.5; WT 38.3±1.7).
Conclusion: The ABC transporters Mdr1 and Bcrp1 specifically mediate CSP cell dye efflux and proliferation, respectively. Stimulation of these individual transporters may therefore be used to enhance cardiac progenitor cell survival and proliferation.