REVERSION BY DELETION OF TRANSFORMING ONCOGENE FOLLOWING INTERFERON-BETA AND RETINOIC ACID TREATMENT

We have previously shown that prolonged interferon-beta (IFN-beta) tre atment of RS485 cells (NIH3T3 cells transformed with multiple copies o f an LTR-cHa-ras oncogene) resulted in the phenotypic reversion of 1%- 5% of the culture, depending on the conditions used, This reversion pe rsisted after IFN-beta was discontinued, although the revertants retai ned the LTR-cHa-ras and continued la, express ras mRNA and p21. Clones were prepared of such persistent revertant cell lines (PRs), Expressi on off lysyl oxidase (LOX), which appears to act as a suppressor of ra s transformation, was downregulated in RS485 and upregulated in the PR s, When retinoic acid (RA) was combined with IFN-beta treatment of the RS485 cultures, a different mechanism of reversion predominated, Foll owing 60 days of treatment with 20 IU/ml of IFN-beta and 10 mu M RA, a ll of the multiple (3-5) copies of the transforming LTR-c-Ha-ras origi nally 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-beta alone, LOX mRNA expression was upregulat ed in all of the revertants that resulted from the treatment with IFN plus RA, The level of LOX mRNA expression acts, therefore, as an indic ator of transformation in this system.