DUAL PHENOTYPIC-EXPRESSION OF HEPATOCYTES AND BILE DUCTULAR MARKERS IN DEVELOPING AND PRENEOPLASTIC RAT-LIVER

This study supports the existence of a pluripotent liver stem cell pop ulation which has the potential to differentiate into hepatocytes and bile ductular cells. We compared the expression of hepatocyte-specific and bile ductular-specific markers in fetal and preneoplastic rat liv er. L-pyruvate kinase (L-PK) and alpha glutathione S-transferase (GST) were used as adult hepatocyte-specific markers, while cytokeratin 19 (CK19) was used as a bile ductular-specific marker. pi GST and M(2)-py ruvate kinase (M(2)-PK), which are fetal hepatocyte-specific and expre ssed at high levels in the oval and duct-like cells, were also used. W e characterized fetal liver derived from 13-21 days of gestation (E13- E21). pi GST was detected in the E18 hepatoblasts, which form the intr ahepatic bile ducts, while CK19 was detected at E19. Some of these cel ls express alpha GST and L-PK from E19 to E21. Oval, duct-like and bil e ductular cells in rats treated with a choline-deficient diet contain ing 0.07% ethionine(CDE diet) for up to 8 weeks were characterized by double immunocytochemistry. L-PK and alpha GST are absent from bile du ctular cells in the normal adult liver and up to 3 weeks of CDE treatm ent. After 4-5 weeks on CDE treatment, the majority of bile ductular c ells express L-PK, while at 6 weeks some co-express L-PK and alpha GST . There are two populations of oval cells, a major population expressi ng only the fetal hepatocyte markers, while a minor population express es the fetal hepatocyte, adult hepatocyte and bile ductular markers. T here are at least three different duct-like cell populations which co- express different markers and have characteristics of fetal hepatocyte s at sequential stages of differentiation. One population co-expresses pi GST and M(2)-PK and is similar to fetal hepatocytes derived from E 13-E14 fetuses. The second expresses the two fetal markers and L-PK, a nd this reflects characteristics of E15 hepatocytes. The third express es pi GST, M(2)-PK, L-PK and alpha GST which is characteristic of E16- E19 hepatocytes. Upon withdrawal of the CDE diet, autoradiography usin g tritiated thymidine shows that oval and duct-like cells differentiat e into hepatocytes. This study demonstrates that oval and duct-like ce lls express both hepatocytic and bile ductular markers, and have the c apacity to differentiate into hepatocytes, characteristics similar to hepatoblasts in the developing rat liver.