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Rapid, complete and reversible transformation by v-sis precedes irreversible transformation
D Mercola1, P M Carpenter, A Grover-Bardwick
1Department of Pathology, University of California, San Diego, La Jolla 92093.
Abstract:
v-sis is the oncogene of simian sarcoma virus, but whether tumor growth is maintained by v-sis expression alone or requires additional changes is unknown. To distinguish these possibilities we studied a model of reversible transformation including tumorigenicity using NIH3T3 cells bearing a metallothionein promoter-v-sis construction. Cells subcultured from 10 out of 18 tumors from athymic mice, all less than 0.1 g and less than or equal to 21 days in age, reverted to a normal phenotype but exhibited transformation upon addition of zinc as judged by morphology, growth rate, saturation density and anchorage independence of growth. Thus, activation of v-sis alone is sufficient for initiation and early autocrine-based growth of tumors. However, the cells from the remaining and predominantly larger, 0.5 +/- 0.7 g, tumors did not revert and exhibited zinc-independent transformation as judged by the same criteria. Southern analysis and examination of the regulation of v-sis product expression in cells derived from these tumors showed no change in zinc-dependent and reversible regulation of v-sis sequences. These results suggest that subsequent tumor growth strongly favors acquisition of additional irreversible change(s) in the tumor cell genome at high frequency (44%). Thus an early event of a multistep process stimulated by v-sis-dependent transformation best accounts for the sum of results.
Insights
Simian sarcoma virus oncogene v-sis alone can initiate tumor growth. However, sustained tumor expansion favors additional, irreversible genetic changes for zinc-independent transformation.
Area of Science:
- Oncogenomics
- Cancer Biology
- Molecular Oncology
Background:
- The v-sis oncogene from simian sarcoma virus drives cell transformation.
- It remains unclear if v-sis expression alone sustains tumor growth or if additional genetic alterations are required.
Purpose of the Study:
- To investigate whether v-sis activation is sufficient for tumor initiation and sustained growth.
- To determine the role of additional genetic changes in tumor progression.
Main Methods:
- Utilized NIH3T3 cells with a metallothionein promoter-v-sis construct for reversible transformation and tumorigenicity studies.
- Analyzed tumor cells from athymic mice for reversibility of transformation upon zinc withdrawal.
- Performed Southern analysis to examine v-sis gene regulation and identify genomic alterations.
Main Results:
- Early-stage tumors ( < 0.1g, < 21 days) showed zinc-dependent, reversible transformation upon v-sis activation.
- Later-stage, larger tumors (0.5 +/- 0.7g) exhibited zinc-independent transformation, indicating irreversible changes.
- No alterations in v-sis regulation were observed in non-reverting cells, suggesting independent genetic events.
Conclusions:
- v-sis activation is sufficient for initiating tumor growth through an autocrine mechanism.
- Sustained tumor growth necessitates additional, irreversible genetic alterations acquired at a high frequency (44%).
- Tumorigenesis driven by v-sis is a multistep process where early transformation is followed by genomic instability.