CHARACTERIZATION OF CONDITIONS FOR THE ACTIVATION OF ENDOGENOUS GUINEA-PIG RETROVIRUS IN CULTURED-CELLS BY 5-BROMO-2'-DEOXYURIDINE

Human endogenous retroviral sequences recently have been shown to be a ssociated with breast cancer and some leukemias, These retroviral sequ ences have similarities to an endogenous retrovirus expressed in guine a pigs. The conditions for activation of this guinea pig retrovirus (G PRV) in cultured guinea pig embryo (GPE) cells using 5-bromo-2'-deoxyu ridine (BrdU) was investigated. These studies employed the reverse tra nscriptase activity (RT) assay and electron microscopy (EM), in conjun ction with Northern blot analysis that utilized a 2.6 kb GPRV-specific cDNA probe. Contrary to published studies, dexamethasone at concentra tions ranging from 10(-8) to 10(-5) M appeared to play a minimal role in enhancing the production of GPRV, Following a 6 hr incubation with BrdU, GPRV mRNA was present in cultured GPE cells. Extracellular virio n release was also observed by EM 12 hr later, although RT activity wa s not detected. All three methods detected viral expression at 48 hr a fter the addition of the drug, Additionally, after 6 hr exposure to Br dU, detectable RT and mRNA levels were maintained through 44 days afte r the removal of BrdU in a stationary culture condition and through 31 days in cultures that were subcultured weekly in media not containing BrdU, Low levels of extracellular viruses were detected in these cult ures by electron microscopy through 49 days. Therefore, after only a 6 hr exposure to BrdU was extracellular GPRV detected 12 hr after drug removal and virus production continued for up to 49 days. This study p rovides information about an animal endogenous retroviral system that may be used as a model for the study of human endogenous retroviruses.