Expression of Mammary Tumor Virus Proteins in Preneoplastic Outgrowth Lines and Mammary Tumors of BALB/cV Mice

A subline of BALB/c mice, designated BALB/cV, has been segregated which exhibits an intermediate mammary tumor in cidence and which harbors a unique milk-transmitted virus. Six stable hyperplastic alveolar nodule outgrowth lines were estab lished from chemical carcinogen-treated and hormonally stimu lated mice. Tumor incidences exhibited by the individual preneoplastic lines ranged from 22 to 95%; no differences in tumorproducing capability were observed when lines were trans planted in virus-negative (BALB/c) or virus-positive (BALB/cV) animals. Viral antigen expression was monitored using antisera prepared against C3H mouse mammary tumor virus (MMTV) proteins. The preneoplastic lines exhibited more virus antigenpositive cells in a peroxidase-antiperoxidase immunocytochemical assay than did primary tumors which arose from the hyperplastic alveolar nodule transplants. Analysis of BALB/cV preneo plastic and tumor tissue by metabolic labeling and by protein electroblctting methodologies revealed that viral precursor and structural proteins were expressed in both types of tissue; the polypeptides were similar in molecular weight to those encoded by exogenous MMTVs. These studies demonstrate that the coding capacity of the BALB/cV isolate of MMTV is similar to that of known MMTV isolates and that each of the BALB/cV structural polypeptides shares group-specific antigenic determi nants with the analogous protein encoded by MMTV from the C3H mouse. The hyperplastic outgrowth lines established in the BALB/cV subline provide an additional system for the study of mammary tumorigenesis.