Cytogenetic analysis of three rat liver epithelial cell lines (WBneo, WBHa-ras, and WBrasIIa) and correlation of an early chromosomal alteration with insulin-like growth factor II expression.

Cytogenetic changes that occur during the progression of rat hepatocarcinogenesis were assessed with three rat liver epithelial cell lines derived from WB cells. Previously characterized WBneo, WBras, and WBrasIIa cells were grown in culture and analyzed for structural and numerical chromosomal integrity by banded karyotype analysis. The WBneo cells had a low level of aneuploidy with a consistent loss of the Y chromosome by passage 7. The ras-transfected cell line selected for growth in soft agar, WBras, had acquired a loss of chromosome 3 (12%) or 3p (34%), a trisomy of chromosome 1, as well as the chromosome Y loss. The cell line produced from tumors generated by injection of the WBras cells into a syngeneic F344 rat, WBrasIIa, contained additional chromosomal changes. The WBrasIIa line comprised cells retaining a trisomy of chromosome 1 (55%) and cells with two copies of chromosome 1, with a minimal duplication of 1q3.7 to 1q4.3 (45%). This tumor-derived cell line contained, in addition, a higher percentage of cells with a loss of all or part of chromosomes 3 and 6, indicating the possible presence of tumor suppressor genes in this region. The smallest region of duplication of chromosome 1 was bands 1q3.7-4.3. The insulin-like growth factor II (IGF-II) gene is located within the region of duplication on chromosome 1. Because IGF-II is both a rat liver mitogen and an inhibitor of apoptosis, its expression was examined in these three rat liver epithelial cell lines. Northern blot analysis demonstrated an increase in IGF-II mRNA expression in the WBras and WBrasIIa cell lines relative to the WBneo control cell line. Several IGF-II transcripts analogous to those detected in fetal rat liver were observed. An additional IGF-II transcript that migrates above the 28S ribosomal marker was also observed. These results were confirmed at the protein level by immunohistochemical and Western blot analysis. This increased expression of IGF-II may confer a selective growth advantage to rat liver epithelial cells with a duplication of 1q3.7-4.3. This growth advantage may be enhanced by the further sequential loss of putative tumor suppressor genes on chromosomes 3 and 6.