Retroviral transduction of the human c-Ha-ras-1 oncogene into midgestation mouse embryos promotes rapid epithelial hyperplasia

Induction of cell death by stimulation of protein kinase C in human epithelial cells expressing a mutant ras oncogene: a potential therapeutic target

Ras oncogene activation is a key genetic event in several types of human cancer, making its signal pathways an ideal target for novel therapies. We previously showed that expression of mutant ras sensitizes human thyroid epithelial cells to induction of cell death by treatment with phorbol 12-myristate 13-acetate (PMA) and other phorbol esters. We have now investigated further the nature and mechanism of this cell death using both primary and cell line models. The cytotoxic effect of PMA could be blocked by bisindolylmaleimide (GF 109203X), a well-characterized inhibitor of c and n protein kinase C (PKC) isoforms, and by prior down-regulation of PKC, indicating that it is mediated by acute stimulation, rather than down-regulation. Western analysis identified two candidate isoforms--alpha and epsilon--both of which showed PMA-induced subcellular translocation, either or both of which may be necessary for PMA-induced cell death. Immunofluorescence showed that PMA induced a rapid nuclear translocation of p42 MAP kinase of similar magnitude in the presence or absence of mutant ras expression. Cell death exhibited the microscopic features (chromatin condensation, TdT labelling) and DNA fragmentation typical of apoptosis but after a surprising lag (4 days). Taken together with recent models of ras-modulated apoptosis, our data suggest that activation of the MAPK pathway by PMA tips the balance of pro- and anti-apoptotic signals generated by ras in favour of apoptosis. The high frequency of ras mutations in some cancers, such as cancer of the pancreas, which are refractory to conventional chemotherapy, together with the potential for stimulating PKC by cell-permeant pharmacological agents, makes this an attractive therapeutic approach.

Development of transforming function during transduction of proto-ras into Harvey sarcoma virus

Oncogenic retroviruses are generated by transduction of the coding region of a protooncogene and acquire genetic changes during subsequent replication. Critical genetic events which occurred during and after transduction of rat proto-ras-1Ha into Harvey sarcoma virus were identified by evaluating the transforming activity of plausible synthetic progenitor proviruses encompassing the complete proto-ras genomic region with or without various 5' deletions. All progenitor proviruses induced phenotypic transformation of mouse NIH 3T3 cells, although with a 5- to 10-fold lower frequency than Harvey sarcoma provirus. Although no tumor formation was observed in vivo after inoculation in the absence of helper murine retrovirus, both wild-type and progenitor viruses inoculated in the presence of helper virus induced tumors in newborn BALB/c mice. No critical alterations of the p21ras coding region and no deletion of 5' genomic elements were detected in a progenitor virus encompassing the complete proto-ras genomic region that had been isolated from tumors. However, one progenitor virus that included all proto-ras exons induced tumors with a decreased latency. This virus contained a mutation in codon 12 (glycine to valine), which had apparently been selected during tumorigenesis in vivo. During the genesis of Harvey sarcoma virus, critical steps conferring transforming function are therefore transduction of coding proto-ras exons and enhancement of their transforming function by specific amino acid changes in p21ras.

Identification of transforming genes of subgroup A and C strains of Herpesvirus saimiri

Herpesvirus saimiri is an oncogenic herpesvirus that induces rapidly progressing lymphomas in New World primates. Using retrovirus vectors for gene transfer, specific open reading frames of H. saimiri were tested for their ability to transform rodent cells in culture. One open reading frame, designated STP-C488 (for saimiri-transformation-associated protein of the subgroup C strain 488), phenotypically transformed Rat-1 cells, resulting in formation of foci, growth at reduced serum concentration, and growth to higher cell densities. Cells transformed by STP-C488 formed invasive tumors in nude mice. The STP-A11 reading frame of strain 11 (subgroup A) was much less potent in its transforming ability than STP-C488. These results demonstrate the oncogene nature of these two open reading frames and provide a means for studying their transforming functions independent of the rest of the H. saimiri genome.

Oncogene activation in human benign tumors of the skin (keratoacanthomas): is HRAS involved in differentiation as well as proliferation?

In vitro DNA amplification followed by oligonucleotide mismatch hybridization was used to study the frequency of HRAS mutations in the benign self-regressing skin tumors keratoacanthomas and in squamous cell carcinomas. We used freshly obtained keratoacanthomas as well as Formalin-fixed paraffin-embedded tissues from both types of tumors. DNA from 50 samples of each tumor type was analyzed for activating mutations involving codons 12 and 61. A relatively high percentage (30%) of HRAS mutations was found in the keratoacanthomas compared with 13% in the squamous cell carcinomas. The most frequent mutation identified is the A-T-to-T.A transversion in the second position of codon 61. The present findings demonstrate the involvement of the HRAS oncogene in human benign tumors. Moreover, they indicate that an activated HRAS oncogene is not sufficient to maintain a neoplastic phenotype and argue against a role of HRAS in the progression of skin tumorigenesis.

The ras and myc oncogenes cooperate in tumor induction in many tissues when introduced into midgestation mouse embryos by retroviral vectors

Midgestation embryos were infected with replication-defective retroviral vectors that either transduced the myc oncogene, the ras oncogene, or both oncogenes simultaneously. The myc virus induced tumors in diverse organs at a very low frequency and with a long latency period, while approximately 20% of the mice derived from embryos infected with the ras virus developed tumors in the skin with a latency of 4-8 weeks. In contrast, infection of embryos with the ras/myc double oncogene virus resulted in 27% of the animals developing rapidly growing and malignant tumors in a great variety of tissues after a median latency period of 2-3 weeks. All tumors were of monoclonal origin, as shown by Southern analysis using the provirus as a molecular marker. Our results are consistent with the hypothesis that the ras and myc oncogenes cooperate in transforming cells, but that additional alterations are necessary for realization of the fully malignant phenotype. Our observations also suggest that a much wider range of cell types become targets for malignant transformation when the embryos are exposed to the myc and the ras oncogenes simultaneously than when exposed to the same oncogenes separately. Infection of mouse embryos with vectors carrying different oncogenes or oncogene combinations may be an efficient and rapid method for evaluating the spectrum of cell types at risk for malignant conversion following mutation of a protooncogene to a transforming gene.