Ha-rasVa112 but not p53Ser247 leads to a significant neoplastic transformation rate of the putative rat liver stem cells (oval cell)

In order to test the controversially discussed hypothesis thatoval cells are part of a liver stem cell compartment and cangive rise to cholangiocellular as well as hepatocellular carcinomasin the course of liver carcinogenesis, we trans-fected an ovalcell line established in our laboratory with an oncogenicallyactivated genomic Ha-ras clone (pUC EJ 6.6), carrying a valineat position 12 instead of the wild-type glycine, or a rat p53cDNA mutated by site-directed mutagenes is at codon 247, whichcorresponds to codon 249 in the human p53. This codon is ofparticular interest since it represents a mutation hotspot observedin hepatocellular carcinoma especially in regions with highaflatoxin B₁ exposure. Independent Ha-ras^Val112Wain and p53Ser247recombinant clones were subcutaneously injected into syngeneicnewborn rats and the resulting tumours were analysed histopathologically.Each of two p53^Ser247 clones gave negligible tumour yields (onetumour out of 13 injected animals), whereas each of two Ha-ras^Val12clones gave marked tumour yields (four tumours out of 13 andseven out of 12 treated animals, respectively). In addition,the p53^Ser247-induced tumours appeared only after 11 monthsand were small, whereas the Ha-ras-induced tumours appearedalready after 6–8 weeks and grew rapidly. Histopathologicalanalysis of the tumours revealed only undifferentiated carcinomas.Interestingly, one tumour that arose upon injection of Ha-ras^Val12-transfectedcells stained positive for albumin, showing at least a partialhepatocytic differentiation.