The importance of drug transporters in human pluripotent stem cells and in early tissue differentiation

The Role of ATP-Binding Cassette Proteins in Stem Cell Pluripotency

Pluripotent stem cells (PSCs) are highly proliferative cells that can self-renew indefinitely in vitro. Upon receiving appropriate signals, PSCs undergo differentiation and can generate every cell type in the body. These unique properties of PSCs require specific gene expression patterns that define stem cell identity and dynamic regulation of intracellular metabolism to support cell growth and cell fate transitions. PSCs are prone to DNA damage due to elevated replicative and transcriptional stress. Therefore, mechanisms to prevent deleterious mutations in PSCs that compromise stem cell function or increase the risk of tumor formation from becoming amplified and propagated to progenitor cells are essential for embryonic development and for using PSCs including induced PSCs (iPSCs) as a cell source for regenerative medicine. In this review, we discuss the role of the ATP-binding cassette (ABC) superfamily in maintaining PSC homeostasis, and propose how their activities can influence cellular signaling and stem cell fate decisions. Finally, we highlight recent discoveries that not all ABC family members perform only canonical metabolite and peptide transport functions in PSCs; rather, they can participate in diverse cellular processes from genome surveillance to gene transcription and mRNA translation, which are likely to maintain the pristine state of PSCs.

Trophoblast Differentiation Affects Crucial Nutritive Functions of Placental Membrane Transporters

Cytotrophoblasts are progenitor cells that proliferate and fuse to form the multinucleated syncytiotrophoblast layer, implicated in placental endocrine and transport functions. While membrane transporters play a critical role in the distribution of nutrients, hormones, and xenobiotics at the maternal-fetal interface, their selectivity to the syncytiotrophoblast layer is poorly characterized. We aimed to evaluate the regulation of placental transporters in response to trophoblast differentiation in vitro. Experiments were carried out in isolated primary human trophoblast cells before and after syncytialization. Gene expression of six molecular markers and thirty membrane transporters was investigated by qPCR analysis. Subsequently, functional expression was evaluated for proteins involved in the transplacental transfer of essential nutrients i.e., cholesterol (ABCA1, ABCG1), glucose (SLC2A1), leucine (SLC3A2, SLC7A5), and iron (transferrin receptor, TfR1). We identified that human chorionic gonadotropin, placental lactogen, endoglin, and cadherin-11 serve as optimal gene markers for the syncytialization process. We showed that trophoblast differentiation was associated with differential gene expression (mostly up-regulation) of several nutrient and drug transporters. Further, we revealed enhanced protein expression and activity of ABCG1, SLC3A2, SLC7A5, and TfR1 in syncytialized cells, with ABCA1 and GLUT1 displaying no change. Taken together, these results indicate that the syncytiotrophoblast has a dominant role in transporting essential nutrients cholesterol, leucine, and iron. Nonetheless, we present evidence that the cytotrophoblast cells may also be linked to transport functions that could be critical for the cell fusion processes. Our findings collectively yield new insights into the cellular functions associated with or altered by the trophoblast fusion. Importantly, defective syncytialization could lead to nutrient transfer imbalance, ultimately compromising fetal development and programming.

Posttranscriptional Regulation of the Human ABCG2 Multidrug Transporter Protein by Artificial Mirtrons

ABCG2 is a membrane transporter protein that has been associated with multidrug resistance phenotype and tumor development. Additionally, it is expressed in various stem cells, providing cellular protection against endobiotics and xenobiotics. In this study, we designed artificial mirtrons to regulate ABCG2 expression posttranscriptionally. Applying EGFP as a host gene, we could achieve efficient silencing not only in luciferase reporter systems but also at the ABCG2 protein level. Moreover, we observed important new sequential-functional features of the designed mirtrons. Mismatch at the first position of the mirtron-derived small RNA resulted in better silencing than full complementarity, while the investigated middle and 3′ mismatches did not enhance silencing. These latter small RNAs operated most probably via non-seed specific translational inhibition in luciferase assays. Additionally, we found that a mismatch in the first position has not, but a second mismatch in the third position has abolished target mRNA decay. Besides, one nucleotide mismatch in the seed region did not impair efficient silencing at the protein level, providing the possibility to silence targets carrying single nucleotide polymorphisms or mutations. Taken together, we believe that apart from establishing an efficient ABCG2 silencing system, our designing pipeline and results on sequential-functional features are beneficial for developing artificial mirtrons for other targets.

The importance of transporters and cell polarization for the evaluation of human stem cell-derived hepatic cells

One of the most promising applications of human pluripotent stem cells is their utilization for human-based pharmacological models. Despite the fact that membrane transporters expressed in the liver play pivotal role in various hepatic functions, thus far only little attention was devoted to the membrane transporter composition of the stem cell-derived liver models. In the present work, we have differentiated HUES9, a human embryonic stem cell line, toward the hepatic lineage, and monitored the expression levels of numerous differentiation marker and liver transporter genes with special focus on ABC transporters. In addition, the effect of bile acid treatment and polarizing culturing conditions on hepatic maturation has been assessed. We found that most transporter genes crucial for hepatic functions are markedly induced during hepatic differentiation; however, as regards the transporter composition the end-stage cells still exhibited dual, hepatocyte and cholangiocyte character. Although the bile acid treatment and sandwich culturing only slightly influenced the gene expressions, the stimulated cell polarization resulted in formation of bile canaliculi and proper localization of transporters. Our results point to the importance of membrane transporters in human stem cell-derived hepatic models and demonstrate the relevance of cell polarization in generation of applicable cellular models with correctly localized transporters. On the basis of our observations we suggest that conventional criteria for the evaluation of the quality of stem cell-derived hepatocyte-like cells ought to be augmented with additional elements, such as polarized and functional expression of hepatic transporters.

Metformin Preconditioning of Human Induced Pluripotent Stem Cell-Derived Neural Stem Cells Promotes Their Engraftment and Improves Post-Stroke Regeneration and Recovery

While transplantation of human induced pluripotent stem cell-derived neural stem cells (hiPSC-NSCs) shows therapeutic potential in animal stroke models, major concerns for translating hiPSC therapy to the clinic are efficacy and safety. Therefore, there is a demand to develop an optimal strategy to enhance the engraftment and regenerative capacity of transplanted hiPSC-NSCs to produce fully differentiated neural cells to replace lost brain tissues. Metformin, an FDA-approved drug, is an optimal neuroregenerative agent that not only promotes NSC proliferation but also drives NSCs toward differentiation. In this regard, we hypothesize that preconditioning of hiPSC-NSCs with metformin before transplantation into the stroke-damaged brain will improve engraftment and regenerative capabilities of hiPSC-NSCs, ultimately enhancing functional recovery. In this study, we show that pretreatment of hiPSC-NSCs with metformin enhances the proliferation and differentiation of hiPSC-NSCs in culture. Furthermore, metformin-preconditioned hiPSC-NSCs show increased engraftment 1 week post-transplantation in a rat endothelin-1 focal ischemic stroke model. In addition, metformin-preconditioned cell grafts exhibit increased survival compared to naive cell grafts at 7 weeks post-transplantation. Analysis of the grafts demonstrates that metformin preconditioning enhances the differentiation of hiPSC-NSCs at the expense of their proliferation. As an outcome, rats receiving metformin-preconditioned cells display accelerated gross motor recovery and reduced infarct volume. These studies represent a vital step forward in the optimization of hiPSC-NSC-based transplantation to promote post-stroke recovery.

Generation of multidrug resistant human tissues by overexpression of the ABCG2 multidrug transporter in embryonic stem cells

The ABCG2 multidrug transporter provides resistance against various endo- and xenobiotics, and protects the stem cells against toxins and stress conditions. We have shown earlier that a GFP-tagged version of ABCG2 is fully functional and may be used to follow the expression, localization and function of this transporter in living cells. In the present work we have overexpressed GFP-ABCG2, driven by a constitutive (CAG) promoter, in HUES9 human embryonic stem cells. Stem cell clones were generated to express the wild-type and a substrate-mutant (R482G) GFP-ABCG2 variant, by using the Sleeping Beauty transposon system. We found that the stable overexpression of these transgenes did not change the pluripotency and growth properties of the stem cells, nor their differentiation capacity to hepatocytes or cardiomyocytes. ABCG2 overexpression provided increased toxin resistance in the stem cells, and protected the derived cardiomyocytes against doxorubicin toxicity. These studies document the potential of a stable ABCG2 expression for engineering toxin-resistant human pluripotent stem cells and selected stem cell derived tissues.

Placental ABC Transporters: Biological Impact and Pharmaceutical Significance

The human placenta fulfills a variety of essential functions during prenatal life. Several ABC transporters are expressed in the human placenta, where they play a role in the transport of endogenous compounds and may protect the fetus from exogenous compounds such as therapeutic agents, drugs of abuse, and other xenobiotics. To date, considerable progress has been made toward understanding ABC transporters in the placenta. Recent studies on the expression and functional activities are discussed. This review discusses the placental expression and functional roles of several members of ABC transporter subfamilies B, C, and G including MDR1/P-glycoprotein, the MRPs, and BCRP, respectively. Since placental ABC transporters modulate fetal exposure to various compounds, an understanding of their functional and regulatory mechanisms will lead to more optimal medication use when necessary in pregnancy.