Liver regeneration: A consequence of complex, well‐orchestrated signals
Authors: Thomas D. Shupe, Bryon E. Petersen
Journal: Hepatology (2015)

This commentary will address the article published in the Journal of Clinical Investigation in December 2014 by Kordes et al.1 The article describes a method for isolating a subpopulation of hepatic stellate cells (HSCs) based on retinoid storage. The researchers isolated these cells from GFP+ rats and transplanted them into wild-type rats. Recipient rats were then subjected to one of two different models for liver injury (retrorsine/partial hepatectomy [PHx] and 2-acetylaminofluorene/PHx). Liver regeneration in each of these models is mediated primarily by liver progenitor cells, and in each case, GFP+ cells were found to contribute to restoration of the mesenchymal cell, progenitor cell, hepatocyte, and cholangiocyte populations. The data presented by Kordes et al. clearly establish the multipotency of hepatic retinoid storing cells within the liver. Ever since hepatic "oval" cells were first characterized by Dr. Farber in the 1950s, the field of liver progenitor cell research has attempted to define this cell population based on the expression of a discrete panel of markers, much as was done with hematopoietic stem cells.2 Liver regeneration is a very complex process that depends on a well-orchestrated series of signals that govern the repopulation and reorganization of damaged liver tissue. For the most part, hepatocytes are able to effectively restore any parenchymal volume lost to injury. However, in the case of chronic liver injury and in certain liver diseases, mature hepatocytes are unable to affect liver repair. In these scenarios, facultative liver stem cells are recruited to augment the regenerative response. Over the past four decades, a variety of strategies have been developed for the isolation of these liver progenitor cells. These strategies have included density gradient centrifugation, cell sorting based on membrane antigen expression (e.g., EpCAM and Thy-1), and cell panning to deplete unwanted cells. However, no consensus has yet been reached on a definitive strategy for the isolation of liver progenitor cells. Two key factors have complicated the universal acceptance of a single isolation technique. The first of these is that the progenitor cell population represents a spectrum of differentiation states, presenting an enormous challenge in the development of a unified isolation strategy. Second, it has become apparent that different types of injuries activate different progenitor cell populations. This creates a division among groups that favor various experimental injury models. An article by Kuwahara et al. describes four distinct microanatomical compartments from which asymmetrically dividing cells emerge in response to specific types of liver injury.3 The first location lies at the junction of the hepatocyte bile canaliculi and the proximal biliary ductules at the canals of Hering. The second location is within the intralobular bile ducts. The third is peripheral to the bile ducts and is represented by a cell that does not express classical stem cell markers. Cells matching this description have been identified after treatment with agents that induce portal zone necrosis, particularly allyl alcohol.4 The final HSPC compartment identified by Kuwahara et al.'s study lies within the hepatic parenchyma, harboring a cell that resembles a small, transitional hepatocyte. Grisham et al.5 have published several articles on the retrorsine/PHx model, which seems to induce proliferation of this particular cell population. In the current study, Kordes et al. present a series of experiments showing that an isolated population of retinoid storing HSCs are able to give rise to a variety of differentiated progeny within the liver. The experiments are eloquently designed and the data lend themselves to an unambiguous conclusion. Retinoid storing HSCs are able to contribute to liver regeneration and give rise to several lineages of functional cells. Of course, these data are open to the same skepticism that similar cell transplant studies have been subjected to (i.e., potential confounding of data by events such as cell fusion, transdifferentiation, and mesenchymal-to-epithelial transition). It should also be noted that the transplanted cell population has not been meticulously characterized. Given the intimate spatial relationship between previously described hepatic progenitor cell populations and stellate cells, it is critical to exclude the possibility of coselecting these two cell types. It would also be interesting to understand the growth kinetics of the transplanted population, and this effort would be simplified by quantitating the percentage of HSC engraftment. This point is particularly relevant in light of studies published by other groups that indicate that the mode of injury can affect cell transplant efficiency.6 Last, it is important to understand that there are limitations to any cell transplant model. Depending on where the transplanted cells engraft into the recipient liver, alterations in cell-cell and cell-matrix associations can result in a subtly different microenvironment that can affect cell phenotype and lineage potential. It is generally accepted that the body has multiple mechanisms of liver repair. A study published by Wang et al.7 demonstrates that, in a mouse model for tyrosinemia, cell fusion was the predominant mechanism for repair. Data published by Michalopoulos et al.8 indicate that, under certain physiological conditions, mature hepatocytes have the ability to transdifferentiate into cholangiocytes. Additionally, Schaub et al. recently published an article that challenged the long-held tenant that oval/stem cells contribute directly to liver regeneration. In addition, they make the accurate observation that a vast majority of our understanding of hepatic oval/stem cells has been generated in the rat model, but this has not been confirmed in other animal species or humans.9 Nevertheless, it cannot be disputed that a reservoir of cells other than hepatocytes exists that is capable of regenerating liver parenchyma postinjury. It is likely that there are more than one cell type with this capability, and for any specific injury, the easiest repair pathway is utilized. This notion is in good alignment with the principal of Occam's Razor, which states that among a series of potential explanations, the simple answer tends to be the most likely. Several studies on the isolation of liver progenitor cells have targeted different cell populations. The simplest explanation for this fact would be that there are multiple populations of cells that are able to affect repair of the liver. The liver is the only solid organ in the body that has the ability to regenerate itself in response to massive injury. The speed and efficiency of this regeneration is quite remarkable. Throughout the decades since the liver progenitor cell was first described, many reports have been published that identify specific populations with such potential. The article by Kordes et al. clearly describes another multipotent cell population with the potential to give rise to many cell types within the liver. Moving forward, it is becoming increasingly important to define the molecular regulation of these cells and determine their therapeutic potential for cell therapies, tissue engineering, and regenerative medicine. Thomas D. Shupe, Ph.D.1Bryon E. Petersen, Ph.D.2 1Wake Forest Institute for Regenerative Medicine Wake Forest School of Medicine Winston-Salem, NC 2Department of Pediatrics Universty of Florida Gainesville, FL