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Carbon tetrachloride induction of rapid changes in liver nuclear protein factors capable of sequence‐specific binding to regulatory elements in the long terminal repeat of polytropic‐class endogenous murine leukemia virus—related proviruses

Treatment of mice with hepatic carcinogens, including CCl4, has been shown to rapidly enhance the transcription of endogenous murine leukemia virus—related proviral sequences in the liver. To understand the mechanism for this transcriptional stimulation, we used nuclear protein preparations from mouse livers to perform DNase I protection analyses and identified nuclear protein binding on approximately 20 individual sequences within the regulatory regions of the long terminal repeat (LTR) of a polytropic‐class endogenous provirus clone. From 3 to 144 h after treatment with CCl4, the livers of FVB/N mice were analyzed for specific nuclear protein binding to the LTR DNA. Three to nine hours after CCI4 treatment, decreased protection was seen at potential regulatory cis‐elements throughout the LTR, including specific sites within the putative negative regulatory element (located 5′ of the consensus enhancer sequences) and the 3′ terminal portion of the polytropic class‐specific enhancer‐like inserted sequence element and around the CCAA(C/T) box in the promoter region. In addition, by 3–6 h after treatment, a transient increase in protection activity for the transcription initiation site occurred. The loss of cis‐element protection expanded to other binding sites and became most marked by 48 h after treatment. As the regenerating liver recovered, the nuclear protein binding activities for these LTR sequences also recovered, but protection at the TATAA and transcription initiation sites remained deprotected at 144 h after treatment. Nuclear protein protection of other sites, particularly in the conserved LTR enhancer sequences, was minimally affected by CCl4 treatment. Three nuclear protein binding sites that showed rapid CCI4‐induced kinetic changes were homologous to the consensus sequence for the binding of the transcription factor families MEF‐2, HNF‐1, and C/EBP. The complex kinetic changes in factors that may contribute to the rapid and transient induction of endogenous retroviral gene expression in the liver after CCI4 exposure are discussed.