ABCG2 is a plasma membrane multidrug transporter with an established role in the cancer drug-resistance phenotype. This protein is expressed in a variety of tissues, including several types of stem cell. Although ABCG2 is not essential for life, knock-out mice were found to be hypersensitive to xenobiotics and had reduced levels of the side population of hematopoietic stem cells. Previously we have shown that ABCG2 is present in human embryonic stem cell (hESC) lines, with a heterogeneous expression pattern. In this study we examined this heterogeneity, and investigated whether it is related to stress responses in hESCs. We did not find any difference between expression of pluripotency markers in ABCG2-positive and negative hESCs; however, ABCG2-expressing cells had a higher growth rate after cell separation. We found that some harmful conditions (physical stress, drugs, and UV light exposure) are tolerated much better in the presence of ABCG2 protein. This property can be explained by the transporter function which eliminates potential toxic metabolites accumulated during stress conditions. In contrast, mild oxidative stress in hESCs caused rapid internalization of ABCG2, indicating that some environmental factors may induce removal of this transporter from the plasma membrane. On the basis of these results we suggest that a dynamic balance of ABCG2 expression at the population level has the advantage of enabling prompt response to changes in the cellular environment. Such actively maintained heterogeneity might be of evolutionary benefit in protecting special cell types, including pluripotent stem cells.

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Background: ATP-binding cassette (ABC) transporters have key roles in various physiological functionsas well as providing chemical defense and stress tolerance in human tissues. In this study, we have examined the expression pattern of all ABC proteins in pluripotent human embryonic stem cells (hESCs) and in their differentiated progenies. We paid special attention to the cellular expression and localization of multidrug transporter ABC proteins.Methods: Stem cell differentiation was carried out without chemical induction or cell sorting, and specialized cell types were separated mechanically. Cellular features regarding pluripotency and tissue identity, as well as ABC transporter expression were studied by flow cytomtery, immuno-microscopy, and qPCR-based low-density arrays.Results: Pluripotent hESCs and differentiated cell types (cardiomyocytes, neuronal cells, and mesenchymal stem cells) were distinguished by morphology, immunostaining markers, and selected mRNA expression patterns. We found that the mRNA expression levels of the 48 human ABC proteins also clearly distinguished the pluripotent and the respective differentiated cell types. When multidrug and lipid transporter ABC protein expression was examined by using well characterized specific antibodies by flow cytometry and confocal microscopy, the protein expression data corresponded well to the mRNA expression results. Moreover, the cellular localization of these important human ABC transporter proteins could be established in the pluripotent and differentiated hESC derived samples.Conclusions: These studies provide valuable information regarding ABC protein expression in human stem cells and their differentiated offspring. The results may also help to obtain further information concerning the specialized cellular functions of selected ABC transporters. V C 2014 International Clinical Cytometry Society