Expression of tumor-associated aldehyde dehydrogenase during rat hepatocarcinogenesis using the resistant hepatocyte model

The resistant hepatocyte model was used to study expressionof tumor-associated aldehyde dehydrogenase (ALDH) activity duringthe course of rat hepatocarcinogenesis. The hepatic ALDH phenotypewas determined at intervals over 280 days by histochemical analysis,total ALDH activity assays and gel electrophoresis, using propionaldehydeand NAD (P/NAD) to characterize normal liver ALDH activity orbenzaldehyde and NADP (B/NADP) to determine tumor-associatedALDH activity. By total activity assays and gel electrophoresis,no significant changes in ALDH activity occurred until day 70.However, histochemical analysis clearly demonstrated changesin ALDH activity early in neoplastic development. Intense focalhepatocyte staining with P/NAD and/or B/NADP was first detectableat day 28. The number of P/NAD-positive foci increased untilday 35 then declined until day 70. The number of B/NADP-positivefoci also increased until day 35, but then remained relativelyconstant for the remainder of the experiment. GGT activity ofserial sections indicated that early ALDH-positive lesions representa small subpopulation (9%) of all GGT-positive foci. However,by day 168 a significant portion (80%) of persistent GGT-positiveneoplastic nodules were also B%NADP-positive histochemically.In addition, virtually all hepatocellular carcinomas (96%) generatedby this protocol possessed significantly elevated levels oftumor-associated ALDH by histochemical analysis, total ALDHactivity and gel electrophoresis. These results indicate thatearly appearing ALDH-positive lesions may define one early subpopulationof all initiated cells that have a high probability of progressingto the ultimate neoplasm.