Genetic instability occurs sooner than expected: promotion, progression and clonality during hepatocarcinogenesis in the rat

Genomic fluidity is a necessary event preceding the acquisition of tumorigenicity during spontaneous neoplastic transformation of WB-F344 rat liver epithelial cells

The genomic evolution of a cohort of WB-F344 rat liver epithelial cell lineages undergoing spontaneous neoplastic transformation was followed to define the mechanistic relationship between genomic instability and progression to the neoplastic phenotype. Eighteen independent populations of WB-F344 cells (initiated from a single diploid-founding population) were subjected to 12 cycles of selective growth at confluent cell density, and cellular DNA contents were measured after each selection cycle. Flow cytometry demonstrated significant gains in the amount of G1 DNA after selection cycles 3, 6, and 7 in 44% (8 of 18), 89% (16 of 18), and 39% (7 of 18) of the cell populations, respectively. All populations subsequently lost DNA and returned to a diploid or pseudo-diploid DNA content within 1 to 2 selection cycles after the appearance of an increased DNA content. Additionally, appearance and subsequent disappearance of aneuploid or tetraploid subpopulations was observed in 11% (2 of 18) and 83% (15 of 18) of the experimental lineages, respectively. Although perturbations of G1 DNA content were apparent as early as selection cycle 3, at least 8 cycles of selective growth were required for the acquisition of tumorigenicity. While the independent lineages demonstrated significant fluctuations in G1 DNA content between selection cycles 3 and 8, the majority (11 of 13) of the populations contained a diploid or pseudo-diploid DNA content at the time tumorigenicity was expressed. Genomic instability preceded the acquisition of tumorigenic potential in rat liver epithelial cells subjected to selective growth conditions of maintenance at confluence, and may be required for its expression.

Flow cytometric analysis of neoplastic nodules and hepatocellular carcinomas induced by ciprofibrate in the rat

Alterations in DNA ploidy accompany hepatocellular carcinoma (HCC). However, changes in DNA content are also seen in regenerating liver and with increasing age. Thus, to investigate the role of DNA ploidy changes in development of HCC, flow cytometric DNA content determinations were done in a rat model system of peroxisome proliferator-induced HCC. Paraffin blocks of liver isolated from 18 Fisher 344 male rats fed ciprofibrate for 20 weeks (4), 40 weeks (4) or 20 months (10) were examined. Livers from age-matched control rats were also examined. From the 20 month ciprofibrate group, nine neoplastic nodules (NNs), 27 HCCs and four non-tumorous surrounding tissue controls (NTCs) were examined. Significant DNA tetraploid populations were seen in both the NNs and NTCs. A significant increase in the percentage of DNA diploid cells was observed in the NN samples. No significant difference in the percentage S-phase cells was seen. Emergence of cell populations with new DNA ploidy classes (8c or DNA aneuploid) as compared with NTCs was only seen in HCCs (7 of 27), and five of these seven were DNA aneuploid, as distinct from DNA tetraploid, populations. A total of 16 of 24 HCC samples that were adequate for cell cycle analysis had average percent S-phase greater than the mean of the NTCs plus three standard deviations. Although a direct role cannot be inferred, these results support the hypothesis that increases in the fraction of diploid cells is an important early event in the development of rat HCC and that further alterations in DNA ploidy and increased proliferative fraction accompany the development of HCC.