Bigger may not be better when it comes to hepatocytes

Hepatocyte transplantation has gained attention as a potential therapeutic intervention for a number of acute and chronic diseases including fulminant hepatic failure, metabolic liver disease, and cirrhosis. Several decades of research with small animal models has shown that hepatocyte transplants can support liver function and life in animals in acute liver failure. Animal models of metabolic liver disease including tyrosinemia type 1, Crigler-Najjar (Gunn rat), analbuminemia, urea cycle disorders, primary familial intrahepatic cholestasis (PFIC), and Wilson’s disease are among the many disorders that have been at least partially corrected by hepatocyte transplantation. A number of centers around the world have conducted hepatocyte transplants in patients in liver failure or even more frequently in patients with metabolic defects in liver function. The results in the clinics are in general agreement with the preclinical data with animal models and support the hypothesis that continued efforts in this field will help bring this cellular therapy to more general use. One of the problems with hepatocyte transplants is the availability of cells for transplantation. The normal sources of hepatocytes are donor livers that cannot be used for whole organ transplantation. This requires that transplant quality cells be isolated from organs that were rejected for whole organ transplant. New sources for hepatocytes will be needed to expand the use of hepatocyte transplants to additional medical centers. Suggested cell sources include immortalized human liver lines, xenotransplants, and stem cell derived sources. A report in this issue by Shibata et al., (this issue) suggests that populations of small hepatocytes (SH) derived over a 12-day period from cultured rat hepatocytes may be useful for cell transplantation. Following transplantation of fresh hepatocytes or SH, liver repopulation was nearly identical. However, according to the author’s calculations at least 10-fold fewer SH were transplanted as compared to freshly isolated total hepatocytes, suggesting that on a cell by cell basis SH may engraft or proliferate better following transplantation than the general population of cells isolated from adult liver. Small hepatocytes, a term used to describe the morphology of the proliferating colonies of cells that are observed when rat hepatocytes are cultured in media supplemented with nicotinamide and growth factors such as EGF, were first described in 1992 by Mitaka et al. Characteristics of SH cells put them somewhere between fully mature hepatocytes and progenitor cells such as oval cells in that SH express some adult liver functions such as albumin, transferrin, and cytokeratins 8 and 18 but express little to no alpha-fetoprotein, CK14, OC2, or the placental type of glutathione S-transferase as would be expected of a progenitor cell. Similar experiments conducted by other investigators confirm these observations. Importantly, SH display a high proliferative rate in culture with up to 86% of the cells labeled with BrdU out to 8 days of culture. Proliferation and maturation of liver functions of SH in this culture system is accompanied by the proliferation of non-parenchymal cells from the liver including epithelial, sinusoidal endothelial, and stellate or Ito cells that initially surround the colonies and later invade under the colonies and support their continued maturation. Similar observations were made by Michalopoulos et al.