Reversion by deletion of transforming oncogene following interferon-β and retinoic acid treatment

We have previously shown that prolonged interferon-β (IFN-β) treatment of RS485 cells (NIH3T3 cells transformed with multiple copies of an LTR-cHa- ras oncogene) resulted in the phenotypic reversion of 1%-5% of the culture, depending on the conditions used. This reversion persisted after IFN-β was discontinued, although the revertants retained the LTR-cHa-ras and continued to express ras mRNA and p21. Clones were prepared of such persistent revertant cell lines (PRs). Expression of lysyl oxidase (LOX), which appears to act as a suppressor of ras transformation, was downregulated in RS485 and upregulated in the PRs. When retinoic acid (RA) was combined with IFN-β treatment of the RS485 cultures, a different mechanism of reversion predominated. Following 60 days of treatment with 20 IU/ml of IFN-β and 10 μM RA, all of the multiple (3-5) copies of the transforming LTR-c-Ha-ras originally present in RS485 cells were deleted from the genome in 72% of 54 revertant cell lines isolated. As in the case of revertants observed after treatment with IFN-β alone, LOX mRNA expression was upregulated in all of the revertants that resulted from the treatment with IFN plus RA. The level of LOX mRNA expression acts, therefore, as an indicator of transformation in this system.