| Abstract: | The importance of mutational activation of the Ha-ras protooncogene in polycyclic aromatic hydrocarbon-induced mouse skin tumors was investigated in a complete carcinogenesis model using repetitive applications of 7,12-dimethylbenza]anthracene (DMBA), or in an initiation-promotion model using a single application of dibenzc,h]acridine (DBc,h]ACR) or benzoa]pyrene (Ba]BP) followed by chronic treatment with phorbol 12-myristate 13-acetate. DNA isolated from carcinomas induced by DMBA or DBc,h]ACR, but not by Ba]P, efficiently transformed NIH 3T3 cells, and a high percentage of the transformed foci had an amplified Ha-ras gene. Restriction enzyme Southern blot analysis and DNA sequencing revealed that the amplified Ha-ras genes of the transformants had an A----T transversion in the second position of the 61st codon. The same mutation was also detected in primary tumor DNA in a high percentage of the DMBA- or DBc,h]ACR-induced carcinomas. Identification of the mutation in NIH 3T3 cells transformed with DNA from DBc,h]ACR-induced benign skin papillomas suggests that it is an early event in skin carcinogenesis. Thus, mutation of the 61st codon of the Ha-ras-1 gene appears to be a critical step in the formation of mouse skin tumors induced in both of the two models tested. Our analyses also delineate two other classes of hydrocarbon-induced carcinomas--namely, tumors whose DNAs efficiently transform 3T3 cells but do not contain mutated ras genes and tumors whose DNAs do not transform 3T3 cells. |
