Role of proto-oncogene c-Ki-ras amplification and overexpression in the malignancy of Y-1 adrenocortical tumor cells

Abstract

1. The proto-oncogene c-Ki-ras has been reported by others to be amplified and overexpressed in malignant Y-1 mouse adrenal cells. However, the role of this amplification in the origin and/or maintenance of malignancy in these cells has not been established. This question is addressed here by comparing the Y-1 cell line with its normal revertants. 2. In Y-1 cells, amplified c-Ki-ras is found in either double minute (DM) or in homogeneously stained region (HSR) chromosomes. Elimination of HSR containing Y-1 marker chromosomes m1 and m2 was demonstrated in the normal revertants by cytogenetic analysis and in situ hybridization. Southern and Northern hybridization did not reveal amplification or expression of c-Ki-ras in the normal revertants. 3. In contrast with c-Ki-ras, the proto-oncogene c-fos was not amplified in either malignant or normal revertant adrenal cells. Only a 5.4 Kbp Eco R1 restriction fragment typical of the normal mouse genome was found. 4. Highly abundant poly A+ RNA homologous to viral sequences was detected in both Y-1 cells and the normal revertants using probes derived from the pFBJ-2 provirus. 5. Spontaneous retransformation of Y-1 normal revertants yielded anchorage-independent non-tumorigenic cells in which c-Ki-ras is neither amplified nor overexpressed. 6. We propose that c-Ki-ras overexpression by way of amplification is necessary for the malignancy displayed by the Y-1 cell line.