The nucleotide sequence of the Friend murine leukaemia virus variant FIS-2 LTR has high identity with the closely related Friend murine leukaemia virus (F-MuLV) LTR, except for the deletion of one direct repeat, a few point mutations and the generation of a glucocorticoid response element (GRE) in the U3 region. The GRE can mediate gene induction by glucocorticoids, mineral corticoids, progesterone and androgens, and it has been shown that incorporation of a GRE(s) within the LTR can increase the transcriptional activity of retroviral enhancers. We have previously reported an increased early virus replication in male mice compared with female mice when infected with a virus containing the FIS-2 LTR and have proposed that the GRE might contribute to this sex difference. In the present study, we introduced a single point mutation in the GRE and performed comparative studies in NIH 3T3 cells and in young adult male and female NMRI mice. We found that significantly more virus was produced from NIH 3T3 cells infected with wt FIS-2 than from cells infected with the FIS-2 GRE mutant and that this difference was further augmented by glucocorticoids. The glucocorticoid antagonist RU486 inhibited virus production in a dose-dependent manner. The wt FIS-2 disseminated significantly faster than the FIS-2 GRE mutant in both male and female mice. There was no significant difference in the dissemination rate between male and female mice infected with the FIS-2 GRE mutant. Hence, the GRE in the FIS-2 LTR is one determinant of the significant sex difference in susceptibility to FIS-2 infection.
INTRODUCTIONFriend immunosuppressive virus (FIS-2) is a low-oncogenic, immunosuppressive variant of Friend murine leukaemia virus, F-MuLV (Faxvaag et al., 1993;Dai et al., 1994Dai et al., , 1998. We have previously reported that young adult NMRI male mice are more susceptible to FIS-2 infection than female mice (Bruland et al., 2001). The level of virus in serum, bone marrow and spleen was initially higher in male mice. Male mice were also more susceptible to FIS-2-induced immunosuppression, and together these results indicated a more efficient virus replication and dissemination in male mice. Studies with recombinant viruses between FIS-2 and the prototype F-MuLV clone 57 revealed that the FIS-2 LTR was one major factor contributing to the observed sex difference in early virus replication.The nucleotide sequence of the FIS-2 LTR shows high identity with that of the F-MuLV LTR, except for the deletion of one of the two direct repeats (DRs) in F-MuLV LTR and a few point mutations (Dai et al., 1994). The deletion of the second copy of the DR in the FIS-2 LTR has been associated with the generation of a binding site for the glucocorticoid receptor (GR), 59-AGAACAGATGG-39, at the 39 end of the remaining DR (Dai et al., 1994). This glucocorticoid response element (GRE) contains a conserved hexanucleotide, 59-AGAACA-39, defined as the GRE core sequence (DeFranco & Yamamoto, 1986;Miksicek et al., 1986). The core motif also has affinity for the p...