Concentrations of retention solutes in uremia vary over a broad range, from nanograms per liter to grams per liter. Low concentrations are found especially for the middle molecules. A substantial number of molecules are protein bound and/or middle molecules, and many of these exert toxicity and are characterized by a high range of toxic over normal concentration (CU/CN ratio). Hence, uremic retention is a complex problem that concerns many more solutes than the current markers of urea and creatinine alone. This list provides a basis for systematic analytic approaches to map the relative importance of the enlisted families of toxins.

Administration of mesenchymal stem cells (MSCs) improves the recovery from acute kidney injury (AKI).The mechanism may involve paracrine factors promoting proliferation of surviving intrinsic epithelial cells, but these factors remain unknown. In the current study, we found that microvesicles derived from human bone marrow MSCs stimulated proliferation in vitro and conferred resistance of tubular epithelial cells to apoptosis. The biologic action of microvesicles required their CD44-and ␤1-integrin-dependent incorporation into tubular cells. In vivo, microvesicles accelerated the morphologic and functional recovery of glycerol-induced AKI in SCID mice by inducing proliferation of tubular cells. The effect of microvesicles on the recovery of AKI was similar to the effect of human MSCs. RNase abolished the aforementioned effects of microvesicles in vitro and in vivo, suggesting RNA-dependent biologic effects. Microarray analysis and quantitative real time PCR of microvesicle-RNA extracts indicate that microvesicles shuttle a specific subset of cellular mRNA, such as mRNAs associated with the mesenchymal phenotype and with control of transcription, proliferation, and immunoregulation. These results suggest that microvesicles derived from MSCs may activate a proliferative program in surviving tubular cells after injury via a horizontal transfer of mRNA.

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell–derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.Electronic supplementary materialThe online version of this article (doi:10.1007/s00204-013-1078-5) contains supplementary material, which is available to authorized users.

Membrane-derived microvesicles (MVs) are released from the cell surface and are implicated in cell-to-cell communication. We evaluated whether MVs derived from endothelial progenitor cells (EPCs) are able to trigger angiogenesis. We found that EPC-derived MVs were incor- IntroductionStem cells have been proposed as a new opportunity for tissue repair in several diseases. Experimental studies have suggested that transplantation of stem and progenitor cells may have a beneficial effect on functional and structural recovery in several organs, including heart, liver, and kidney. [1][2][3] The mechanisms underlining stem-cell therapy are still intensely debated. Some studies have suggested an engraftment of stem cells by transdifferentiation or fusion in targeted organs. [1][2][3] However, a growing number of evidences indicate that transient cell localization in the injured tissue may be sufficient to favor functional and regenerative events, suggesting the release of paracrine mediators. [1][2][3] Several mechanisms involved in cell-tocell communication have been identified, including secretion of growth factors, cytokines, surface receptors, and nucleotides. [4][5][6][7] It has been suggested that microvesicles (MVs) actively released from cells may play an important role in cell-to-cell communication. [8][9][10][11] MVs are derived from the endosomal membrane compartment after fusion with the plasma membrane and are shed from the cell surface of activated cells. 12,13 Several studies suggest that MVs may stimulate target cells directly or by transferring surface receptors. [8][9][10]13,14 It has been shown that MVs derived from activated platelets induce metastasis and angiogenesis in lung cancer. 14 Moreover, tumorderived MVs were shown to transfer surface determinants and mRNA of tumor cells to monocytes. 15 It has been also postulated that MVs may contribute in spreading certain infective agents such as HIV or prions. 16,17 Embryonic stem cells were recently shown to represent an abundant source of MVs, and it was suggested that MVs derived from these cells may represent one of the critical components supporting self-renewal and expansion of stem cells. 18,19 In addition, Ratajczak et al 18 demonstrated that embryonic stem cell-derived MVs are able to reprogram hematopoietic progenitors by a horizontal transfer of mRNA and protein delivery.These experimental evidences rise the question whether a stemcell regenerative therapy is feasible without transplantation of stem cells by using MVs as a carrier of genetic information or proteins able to reprogram tissue resident cells to repair injury.In the present study, we aimed to investigate whether MVs, derived from human circulating endothelial progenitor cells (EPCs), were able to trigger neoangiogenesis. Materials and methodsApproval of the study was obtained from the Center for Molecular Biotechnology Institutional Review Board. Adult peripheral blood was collected from healthy volunteers with informed consent obtained in accordance with the Declaration of Hels...

Recent studies suggest that tumor-derived microvesicles (MV) act as a vehicle for exchange of genetic information between tumor and stromal cells, engendering a favorable microenvironment for cancer development. Within the tumor mass, all cell types may contribute to MV shedding, but specific contributions to tumor progression have yet to be established. Here we report that a subset of tumor-initiating cells expressing the mesenchymal stem cell marker CD105 in human renal cell carcinoma releases MVs that trigger angiogenesis and promote the formation of a premetastatic niche. MVs derived only from CD105-positive cancer stem cells conferred an activated angiogenic phenotype to normal human endothelial cells, stimulating their growth and vessel formation after in vivo implantation in immunocompromised severe combined immunodeficient (SCID) mice. Furthermore, treating SCID mice with MVs shed from CD105-positive cells greatly enhanced lung metastases induced by i.v. injection of renal carcinoma cells. Molecular characterization of CD105-positive MVs defines a set of proangiogenic mRNAs and microRNAs implicated in tumor progression and metastases. Our results define a specific source of cancer stem cell-derived MVs that contribute to triggering the angiogenic switch and coordinating metastatic diffusion during tumor progression. Cancer Res; 71(15); 5346-56. Ó2011 AACR.