Somatic Stem Cell Populations and Studies on the Functional Role and Regulation of ABCG2




Tunison, Mary Katherine

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ATP binding cassette transporters use ATP to transport substances such as sterols, peptides, drugs and other toxins across membranes. Abcg2 is a member of the G family of transporters that was identified in breast cancer cells due to its ability to efflux chemotherapeutic drugs. Abcg2 can also efflux Hoechst 33342, resulting in a side population phenotype when stained cells are sorted by FACS. Recent studies have suggested that Abcg2 may be a marker for stem and progenitor cells. This paper presents experiments that were undertaken to further evaluate the functional role and regulation of Abcg2. Preliminary data obtained from a microarray performed on main and side population cells indicated that side population cells were enriched for genes that are important in cytoprotection and cell cycle control. To confirm this observation, cell cycle analysis was performed on C2C12 myoblasts transfected with an Abcg2 overexpressing plasmid. Those cells that overexpressed Abcg2 and consequently effluxed Hoechst 33342 dye were arrested in G0/G1 phase of the cell cycle, consistent with the microarray results. To determine whether Abcg2 played a cytoprotective role I attempted to measure the viability of main population versus side population cells when exposed to the oxidative compound menadione. These experiments were inconclusive, most likely due to limitations of the cell line used. To assess the regulation of Abcg2 I examined evolutionary conservation between the upstream regions of mouse and human Abcg2. Conserved regions were identified up to 11kb upstream of the mouse gene. In these regions, putative binding sites were discovered for transcription factors that are involved in stem and progenitor cell regulation. Present studies support that Abcg2 is a marker for stem and progenitor cells. Future studies will uncover additional functional roles of the transporter.

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