Levels and membrane localization of the c-K-ras p21 protein in lungs of mice of different genetic strains and effects of 2,3,7,8- tetrachlorodibenzo-p-dioxin (TCDD) and Aroclor 1254

Abstract

Mutational activation of the K-ras oncogene often occurs in human and mouse lung adenocarcinomas. Since K-ras p21 functions in trans-membrane signaling, we have investigated whether the amount of this protein in lung cell membranes is a variable that could influence lung tumorigenesis, either due to genetic differences or in response to tumor promoters. The six mouse strains assessed showed little difference in the total lung K-ras p21 after immunoprecipitation and immunoblotting. However, amount of ras p21 in the membrane fraction showed significant differences, with C57BL/6 and BALB/c having 3-5-fold more than NIH Swiss, AKR and DBA mice. Interestingly, a congenic AKR strain having the Ahr(b-1) Ah receptor allele from C57BL/6 mice (designated AKR.B6Ah) had high lung membrane K-ras p21 similar to that of C57BL/6. To test for possible changes related to lung tumor promotion, mice were treated with a promotional dose of TCDD (5 nmol/kg). After 48 h C57BL/6 lungs showed an increase in p21 in both total and membrane fractions. BALB/c, DBA and Swiss mice showed an increase only in membranes. There was no change in the AKR and AKR.B6Ah. Aroclor 1254 (250 mg/kg) caused an increase in membrane/cytosol ratio in Swiss mice. Thus the membrane:cytosol K-ras p21 ratio may be influenced by the Ahr phenotype, and TCDD and PCBs can induce p21 or increase its membrane level in certain strains, but these properties are not fully dependent on Ahr receptor type. In confirmation of the relevance of these findings for the tumor target cell type, the immortalized alveolar type 2 E10 cell line presented K-ras p21 in membrane, and this was increased 4-fold by treatment with 10 nM TCDD.