Factors influencing the expression of endogenous retrovirus-related sequences in the liver of B6C3 mice.

The expression of RNA transcripts from three families of endogenous retrovirus-related sequences was investigated during liver cell proliferation in B6C3 mice. Treatment with a single dose of the liver mitogen and promoter of mouse hepatocarcinogenesis 1, 4-bis[2-(3,5-dichloropyridyloxy)]benzene (TCPOBOP), or with carbon tetrachloride (CCl4), induced liver cell proliferation at days 2 and 3 after treatment. Both of these treatments led to a marked increase in Moloney murine leukemia virus-related 6 kilobase RNAs, which were most abundant at day 1 after TCPOBOP treatment and at day 2 after CCl4. Intracisternal A particle-related 6 kilobase RNAs were markedly increased at days 1 and 2 after TCPOBOP and at days 1, 2, and 3 after CCl4. VL30-related transcripts were slightly decreased after TCPOBOP, but they were markedly increased at days 1 and 2 following CCl4. The livers of 15-day-old untreated mice contained about a 3-fold higher level of Moloney murine leukemia virus-related RNAs than adult liver. Intracisternal A particle-related 6-kilobase transcripts were present at 3-fold higher abundance in 7-day-old than in 15-day-old or adult liver. RNAs homologous to VL30 were detected at about the same levels in infant as well as adult livers. Inhibition of protein synthesis by the administration of cycloheximide to adult mice caused a marked increase in the amount of Moloney murine leukemia virus-, intracisternal A particle-, and VL30-related RNAs in the livers of the treated mice, suggesting the existence of labile proteins that normally regulate the abundance of these transcripts. We postulate that the amounts of these putative regulatory proteins vary during both normal development and carcinogenesis and also in response to specific agents that induce liver cell proliferation.