The p16/Cdkn2a/Ink4a gene is frequently deleted in nitrosourea-induced rat glial tumors

The present study investigates nitrosourea-induced rat (Rattus norvegicus) glioma cell lines for the functional status of the p16/Cdkn2a/Ink4a gene, which encodes the p16 cdk4 inhibitor and the alternative reading frame protein, p19ARF. We detected homozygous deletions of the p16/Cdkn2a/Ink4a gene locus in 4 of 5 glioma cell lines (C6, F98, RG2, and RGL.3), but not in the 9L gliosarcoma cell line or in a rat primary fibroblast cell line. RT-PCR demonstrated expression of the p16 and p19ARF mRNAs only in 9L cells and in rat fibroblasts. Comparative genomic in situ hybridization showed that the copy number of rat chromosome RNO5 was not altered in any of the glioma cell lines investigated, indicating that the deletions result from a discrete loss in the region of the p16/Cdkn2a/Ink4a locus. This is the first report of p16/Cdkn2a/Ink4a deletions present in nitrosourea-induced rat glioma cell lines. Since this genetic alteration is also commonly observed in human malignant glial tumors, our results validate the use of chemically induced rat glioma cell lines as an experimental model in the development of gene therapy strategies.

Suppression of cell growth by wild-type p53 in tumor-derived rat esophageal epithelial cells

Our laboratory previously identified a frameshift mutation in codon 176 of the p53 gene in the cell line RE-149DHD derived from transformation of rat esophageal epithelial cells by benzo(a)pyrene dihydrodiol. This mutation correlated with tumorigenic potential of the cells. To investigate the effect of rat wt-p53 expression on cell growth we constructed a recombinant expression vector, pcDNA3-RP53, in which wt-p53 cDNA is under transcriptional control of a cytomegalovirus promoter, and introduced the vector into RE-149DHD cells. A stable clonal cell line was isolated from each of four colonies transfected by pcDNA3-RP53 vector. Exogenous p53 DNA fragments were successfully amplified from three of the four cell lines with the sense primer complementary to T7 promoter sequences downstream of the CMV promoter. Restriction digestion analysis of these amplified PCR fragments showed fragments of the expected size for exogenous p53 DNA, indicating successful introduction of the pcDNA3-RP53 vector into RE-149DHD cells. However, expression of exogenous p53 mRNA was not detected by RT-PCR in these cell lines. To further confirm absence of exogenous p53 expression, a different set of primers, spanning codon 176 of the p53 gene, was used to amplify a 183-bp fragment of p53 mRNA from all transfected cell lines, the parental RE-149DHD cell line and normal esophageal tissue. Subsequent SSCP analysis of the RT-PCR products from all transfected cell lines and the parental cell line RE-149DHD showed the same mobility shift, indicating expression of endogenous mutant p53 alone, and no exogenous expression of p53 mRNA in all four stable clonal cell transfected with the pcDNA3-RP53 vector. This data suggests that wt-p53 may play a negative role in growth regulation of tumor-derived rat esophageal epithelial cells.