Marker chromosome stability associated with neoplastic transformation of human uroepithelial cells

Transformation of SV40-immortalized human uroepithelial cells by 3-methylcholanthrene increases IFN- and Large T Antigen-induced transcripts

BACKGROUND

Simian Virus 40 (SV40) immortalization followed by treatment of cells with 3-methylcholanthrene (3-MC) has been used to elicit tumors in athymic mice. 3-MC carcinogenesis has been thoroughly studied, however gene-level interactions between 3-MC and SV40 that could have produced the observed tumors have not been explored. The commercially-available human uroepithelial cell lines were either SV40-immortalized (HUC) or SV40-immortalized and then 3-MC-transformed (HUC-TC).

RESULTS

To characterize the SV40 - 3MC interaction, we compared human gene expression in these cell lines using a human cancer array and confirmed selected changes by RT-PCR. Many viral Large T Antigen (Tag) expression-related changes occurred in HUC-TC, and it is concluded that SV40 and 3-MC may act synergistically to transform cells. Changes noted in IFP 9-27, 2'-5' OAS, IF 56, MxA and MxAB were typical of those that occur in response to viral exposure and are part of the innate immune response. Because interferon is crucial to innate immune host defenses and many gene changes were interferon-related, we explored cellular growth responses to exogenous IFN-gamma and found that treatment impeded growth in tumor, but not immortalized HUC on days 4 - 7. Cellular metabolism however, was inhibited in both cell types. We conclude that IFN-gamma metabolic responses were functional in both cell lines, but IFN-gamma anti-proliferative responses functioned only in tumor cells.

CONCLUSIONS

Synergism of SV40 with 3-MC or other environmental carcinogens may be of concern as SV40 is now endemic in 2-5.9% of the U.S. population. In addition, SV40-immortalization is a generally-accepted method used in many research materials, but the possibility of off-target effects in studies carried out using these cells has not been considered. We hope that our work will stimulate further study of this important phenomenon.

Assessing the significance of chromosome-loss data: where are suppressor genes for bladder cancer?

Cytogenetic analysis reveals alterations of chromosome structure (losses, gains, and rearrangements of genetic material) in bladder cancer cells generated using an in vitro/in vivo transformation system. To predict possible locations of bladder cancer suppressor genes, we performed a robust Bayesian analysis of the chromosome-loss data. We postulated a simple stochastic model to describe chromosome loss during tumour progression. Posterior computations are enabled by a dynamic simulation algorithm. Ordered by decreasing posterior probability of putatively harbouring a suppressor gene, we observe significant losses on chromosomes 3, 18, 13, 10, 11, and y.

Jumping translocations originate clonal rearrangements in SV40-transformed human fibroblasts

A comparative study of chromosomal rearrangements occurring in 4 independent clones obtained from SV40-transformed cornea and skin human fibroblasts was performed. Rearrangements principally affect some constitutive heterochromatin and, to a lesser degree, telomeric regions. This results in multiple exchanges between a limited number of chromosome structures, i.e., in jumping translocations. Such rearrangements occur even at early passages and some of them give rise to clonal rearrangements that accumulate at late passages. This process is responsible for progressive modification of the karyotypes, principally characterized by deletion of a number of chromosome segments. Thus, clonal rearrangements are selected among many others not occurring at random. The selective pressure retaining clonal rearrangement seems to be similar for the 4 independent clones, since selection of the derivative chromosomes leads to the same imbalances, whatever the origin of the clone. This sequence of events recalls that of human solid tumors, since jumping rearrangements are generally observed in pre-malignant conditions or in low-grade malignancies, whereas clonal rearrangements leading to typical imbalances are detected in more advanced malignant tumors.

Chromosome losses in tumorigenic revertants of EJ/ras-expressing somatic cell hybrids

Tumorigenic transformation of SV40-immortalized human uroepithelial cells (SV-HUC) after transfection with EJ/ras was previously reported to be a rare event. To test the hypothesis that ras transformation requires loss of suppressor genes, somatic cell hybrids were generated between a rare tumorigenic transformant and an isogeneic nontumorigenic EJ/ras transfectant obtained in the same experiment. Both parental cell lines, as well as all hybrid progeny, expressed mutant p21 ras protein, but injections of three such independent hybrids into athymic nude mice at passage (P) 4 demonstrated that tumorigenicity was suppressed at 20 of 22 sites. Two tumors developed, after a relatively long 17-week latent period, as compared with a 4-week latent period for the tumorigenic parent. All three hybrids produced tumors at P8, but these showed different latent periods (3-14 weeks). Revertant hybrid tumors were high-grade carcinomas. Cell lines derived from these tumors expressed mutant p21 ras and retained at least 1 EJ/ras integration site. Karyotypic analysis of six independent hybrid tumor revertants showed that each had a unique clonal karyotype. Losses of two or more homologues of 1p, 3p, 4, 8, 10p, 11p, 13q, and 18 were identified in one or more tumorigenic revertants. Losses of all these chromosomes were previously associated with transformation of SV-HUC by EJ/ras, but were also associated with chemical transformation of SV-HUC in tumors that did not express mutant ras. Genetic losses involving most of these chromosomes have also been identified in clinical bladder cancers (i.e., 1p, 3p, 8, 11p, 13 and 18q). These data show that expression of EJ/ras does not negate or significantly alter requirements for multiple genetic losses in HUC tumorigenesis.

Nonrandom chromosome losses in tumorigenic revertants of hybrids between isogeneic immortal and neoplastic human uroepithelial cells

Somatic cell hybrid analysis was used to examine the role of recessive cancer genes in tumorigenic transformation in vitro of human uroepithelial cells (HUC). Hybrids between nontumorigenic pseudodiploid SV40-immortalized HUC (SV-HUC) and two aggressive grade III transitional cell carcinomas (TCC) produced in nude mice after in vitro exposure of SV-HUC to 3-methylcholanthrene (MC) were completely suppressed for tumorigenicity at early passage. Tumorigenic reversion occurred after five or more passages in culture and was always accompanied by chromosome losses. Overall, the tumorigenic revertants showed statistically significant losses of chromosomes 1, 4, 5, 9q, 12, 14q, and 17 (all P less than or equal to 0.05) as compared to losses in suppressed hybrids. In addition, hybrid reversion was accompanied by losses that left specific tumors with a single remaining homolog of certain chromosomes (i.e., 3, 5q, 11p, 17p, and 18q). These losses were also considered significant because of the likelihood that genes on these chromosomes were reduced to homozygosity. Many of the significant losses (i.e., 5q, 9q, 11p, and 17p) were of chromosomes that are frequently lost in clinical TCC. Thus, these results support the hypothesis that these chromosomes contain genes whose loss leads to HUC tumorigenesis.

Allelic 3p deletions in high-grade carcinomas after transformation in vitro of human uroepithelial cells

Restriction fragment length polymorphism (RFLP) analysis for allelic losses on the chromosome arm 3p were performed on independent carcinomas produced in athymic nude mice after transformation in vitro of a pseudodiploid clonal SV40-immortalized human uroepithelial cell line (SV-HUC). We analyzed ten primary carcinomas with heterogeneous phenotypes for deletions on 3p by using three informative probes, D3S30, D3S2, and D3F15S2, which map to the 3p11-p14, 3p21.1, and 3p21 regions, respectively. Five of the ten primary cancers showed reduction to homozygosity with at least one of the probes, and all five cancers were high-grade and poorly differentiated. We also analyzed six carcinomas that arose after progression of low-grade cancers, either spontaneously or after exposure to a human bladder carcinogen, to higher grades (progressed carcinomas). Four of the six exhibited 3p allelic loss. No preferential loss of a specific 3p allele was observed in any of the carcinomas. In addition, whereas most of the carcinomas showed allelic loss for all three of the probes, indicating a large-scale deletion, several of the carcinomas exhibited losses for only one or two of the probes, thus making it possible, along with the cytogenetic data, to define the least common region of deletion to 3p13----p14.2. These results support the hypothesis that nonrandom loss of a gene or genes on 3p leads to the development of cancer. Furthermore, these findings associate deletion of a putative 3p13----p14.2 tumor suppressor gene region with the development of high-grade uroepithelial carcinomas.

Karyotype evolution in a simian virus 40-transformed tumorigenic human cell line

Normal human foreskin fibroblasts (HSF4) were transfected using the pSV3-neo plasmid. A pool of 10 G418-resistant colonies, HSF4-T12, showed a progressive increase in the expression of a number of in vitro transformation markers with passage in culture and became immortalized. Although no tumors were formed when cells were injected subcutaneously into nude mice, this cell line produced progressive tumors when cells were injected into preimplanted Gelfoam sponges in the mice. When these tumors were cultured in vitro and subsequently injected subcutaneously, progressive tumors were produced with median latency periods as short as 4 weeks. Three phases of cytogenetic change could be distinguished. At early passages after transfection. HSF4-T12 exhibited many random chromosomal changes. At a time just after immortalization, both flow karyotype and G-banded analyses showed the appearance of balanced clonal rearrangements. These included t(2;4), t(2;14), t(3;?), 6p-, i(6p), 8p-, t(14;15), i(15), and t(18;?). These clonal rearrangements were stable with passage in culture, and less variability from cell to cell was noted. The only consistent chromosomal loss observed was -Y. Analysis of three independent tumors showed characteristic loss of chromosomal material rather than balanced chromosomal rearrangements. Frequent loss of 6q and chromosomes #13, 15, 20, and Y was noted.