Hepatocytes from induced pluripotent stem cells: a giant leap forward for hepatology

Concise Review: Differentiation of Human Adult Stem Cells Into Hepatocyte-like Cells In vitro

Adult stem cells (ASCs) are undifferentiated cells found throughout the body that divide to replenish dying cells and regenerate damaged tissues, which are the powerful sources for cell therapy and tissue engineering. Bone marrow-derived mesenchymal stem cells (BMSCs), adipose tissue-derived mesenchymal stem cells (ADSCs), and peripheral blood monocytes (PBMCs) are the common ASCs, and many studies indicated that ASCs isolated from various adult tissues could be induced to hepatocyte-like cells in vitro. However, the isolation, culture protocols, characterization of ASCs and hepatocyte-like cells are different. This review aims to describe the isolation and culture procedures for ASCs, to summarize the molecular characterization of ASCs, to characterize function of hepatocyte-like cells, and to discuss the future role of ASCs in cell therapy and tissue engineering.

Direct transdifferentiation of spermatogonial stem cells to morphological, phenotypic and functional hepatocyte-like cells via the ERK1/2 and Smad2/3 signaling pathways and the inactivation of cyclin A, cyclin B and cyclin E

Background

Severe shortage of liver donors and hepatocytes highlights urgent requirement of extra-liver and stem cell source of hepatocytes for treating liver-related diseases. Here we hypothesized that spermatogonial stem cells (SSCs) can directly transdifferentiate to hepatic stem-like cells capable of differentiating into mature hepatocyte-like cells in vitro without an intervening pluripotent state.

Results

SSCs first changed into hepatic stem-like cells since they resembled hepatic oval cells in morphology and expressed Ck8, Ck18, Ck7, Ck19, OV6, and albumin. Importantly, they co-expressed CK8 and CK19 but not ES cell markers. Hepatic stem-like cells derived from SSCs could differentiate into small hepatocytes based upon their morphological features and expression of numerous hepatic cell markers but lacking of bile epithelial cell hallmarks. Small hepatocytes were further coaxed to differentiate into mature hepatocyte-like cells, as identified by their morphological traits and strong expression of Ck8, Ck18, Cyp7a1, Hnf3b, Alb, Tat, Ttr, albumin, and CYP1A2 but not Ck7 or CK19. Notably, these differentiated cells acquired functional attributes of hepatocyte-like cells because they secreted albumin, synthesized urea, and uptake and released indocyanine green. Moreover, phosphorylation of ERK1/2 and Smad2/3 rather than Akt was activated in hepatic stem cells and mature hepatocytes. Additionally, cyclin A, cyclin B and cyclin E transcripts and proteins but not cyclin D1 or CDK1 and CDK2 transcripts or proteins were reduced in mature hepatocyte-like cells or hepatic stem-like cells derived from SSCs compared to SSCs.

Conclusions

SSCs can transdifferentiate to hepatic stem-like cells capable of differentiating into cells with morphological, phenotypic and functional characteristics of mature hepatocytes via the activation of ERK1/2 and Smad2/3 signaling pathways and the inactivation of cyclin A, cyclin B and cyclin E. This study thus provides an invaluable source of mature hepatocytes for treating liver-related diseases and drug toxicity screening and offers novel insights into mechanisms of liver development and cell reprogramming.

Induced pluripotent stem cells: from Nobel Prizes to clinical applications

Advances in basic hepatology have been constrained for many years by the inability to culture primary hepatocytes in vitro, until just over five years ago when the scientific playing field was changed beyond recognition with the demonstration that human skin fibroblasts could be reprogrammed to resemble embryonic cells. The reprogrammed cells, known as induced pluripotent stem cells (iPSCs), were then shown to have the capacity to re-differentiate into almost any human cell type, including hepatocytes. The unlimited number and isogenic nature of the cells that can be generated from tiny fragments of tissue have massive implications for the study of human liver diseases in vitro. Of more immediate clinical importance were recent data demonstrating precision gene therapy on patient specific iPSCs, which opens up the real and exciting possibility of autologous hepatocyte transplantation as a substitute for allogeneic whole liver transplantation, which has been an effective approach to end-stage liver disease, but one that has now been outstripped by demand. In this review, we describe the historical development, current technology and potential clinical applications of induced pluripotency, concluding with a perspective on possible future directions in this dynamic field.

Comparison of in vitro hepatogenic differentiation potential between various placenta-derived stem cells and other adult stem cells as an alternative source of functional hepatocytes

Mesenchymal stem cells (MSCs) are powerful sources for cell therapy in regenerative medicine. The capability to obtain effective stem cell-derived hepatocytes would improve cell therapy for liver diseases. Recently, various placenta-derived stem cells (PDSCs) depending on the localization of placenta have been suggested as alternative sources of stem cells are similar to bone marrow-derived MSC (BM-MSCs) and adipose-derived MSC (AD-MSCs). However, comparative studies for the potentials of the hepatogenic differentiation among various MSCs largely lacking. Therefore, we investigated to compare the potentials for hepatogenic differentiation of PDSCs with BM-MSCs, AD-MSCs, and UCB-MSCs. Several MSCs were isolated from human term placenta, adipose tissue, and umbilical cord blood and characterized isolated MSCs and BM-MSCs was performed by quantitative reverse transcription-PCR (RT-PCR) and special stains after mesodermal differentiation. The hepatogenic potential of PDSCs was compared with AD-MSCs, UCB-MSCs, and BM-MSCs using RT-PCR, PAS stain, ICG up-take assays, albumin expression, urea production, and cytokine assays. MSCs isolated from different tissues all presented similar characteristics of MSCs. However, the proliferative potential of PDSCs and the expression of hepatogenic markers in differentiated PDSCs were higher than other MSCs. Interestingly, the expression of hepatocyte growth factor (HGF) increased in PDSCs after hepatogenic differentiation. Interestingly, stem cell factor (SCF) expression in chorionic plate-derived MSCs, one of the PDSCs, was significantly higher than in the other PDSCs. Taken together, the results of the present study suggest that MSCs isolated from various adult tissues can be induced to undergo hepatogenic differentiation in vitro, and that PDSCs may have the greatest potential for hepatogenic differentiation and proliferation. Therefore, PDSCs could be used as a stem cell source for cell therapy in liver diseases.

Colon cancer stem cells

Colorectal cancer (CRC) is the second leading cause of death from cancer in the United States. Aggressive research in the last decade has led to a wealth of information about this disease; for example, we now know that more than 80% of sporadic colon tumors contain mutations in the Wnt and TGFβ signaling pathways. The latest avenue of research is revealing the existence of and role for the cancer stem cell (CSC) model, which promotes the idea that malignancies originate from a small fraction of cancer cells that show self-renewal and multi- or pluripotency. The model also endorses that CSCs are capable of initiating and sustaining tumor growth. The body of evidence in favor of the CSC model is rapidly growing and includes analyses from flow cytometry of numerous CSC biomarkers, abnormal signaling pathways, symmetric division, dietary augmentation, and analysis of the behavior of these cells in spheroid culture formation. Although the incidence of death from CRC remains high, fervent research, both basic and translational, is beginning to improve patient outcomes. This paper focuses on stem cell biology in the context of CRC to help understand the mechanisms leading to tumor development and therapy resistance, with possible therapeutic indications.

Induced Pluripotent Stem Cells-A New Foundation in Medicine

Generation of induced pluripotent stem (iPS) cells using defined factors has been considered a ground-breaking step towards establishing patient-specific pluripotent stem cells for various applications. The isolation of human embryonic stem (ES) cells set the standard that pluripotent stem cells are attainable as potentially immortal cells for regeneration of many types of tissues. Different approaches have been tested to obtain pluripotent stem cells by circumventing the need for embryos. iPS cells appear to be an ideal substitute for ES cells. Since the first demonstration of creating iPS cells in 2006, tremendous efforts have been made into improving iPS cell generation methods and understanding the reprogramming mechanism as well as the nature of iPS cells. To improve iPS cell generation, several approaches have been taken: (1) eliminate the viral vector integration after delivering the defined factors; (2) select different cell types that more effectively give rise to iPS cells; (3) use of chemicals to facilitate reprogramming; (4) use of protein factors to reprogram cells. The iPS cells are also being rigorously characterized in comparison to ES cells. All these efforts are made for the purpose of making iPS cells closer to clinical applications. This article will give an overview of the following areas: (1) mechanisms of iPS cell derivation; (2) characterization of iPS cells; (3) iPS cells for cell-based therapy; and (4) iPS cells for studying disease mechanism. Questions as to what aspects of iPS cells require further understanding before they may be put to clinical use are also discussed.

Induced pluripotent stem cells--alchemist's tale or clinical reality?

Following Shinya Yamanaka's first report describing the reprogramming of fibroblasts into stem cells over three years ago, some sceptics initially drew analogies between this new field of research and the quasi-mystical practice of 'alchemy'. Unlike the alchemist, however, stem cell researchers have rigorously tested and repeated experiments, proving their very own brand of cellular 'alchemy' to be a reality, with potentially massive implications for the study of human biology and clinical medicine. These investigations have resulted in an explosion of related publications and initiated the field of stem cell research known as 'induced pluripotency'. In this review, we give an account of the historical development, current technologies and potential clinical applications of induced pluripotency and conclude with a perspective on the possible future directions for this dynamic field.