Specific Deactivation of the Mouse Mammary Tumor Virus Long Terminal Repeat Promoter upon Continuous Hormone Treatment

We have studied the transcriptional behavior of the mouse mammary tumor virus long repeat (MMTV-LTR) promoter during a prolonged exposure to glucocorticoids. When integrated into XC-derived cells, MMTV-LTR expression reached its maximum during the first day of dexamethasone treatment, but longer exposure to the hormone resulted in the deactivation of the promoter. In contrast, glucocorticoid-responsive resident genes or MMTV-based transiently transfected plasmids maintained or even increased their mRNA levels during the same period of hormone treatment. An integrated chimeric construct containing the hormone-responsive elements from MMTV-LTR but in different sequence context became also deactivated after a prolonged hormone treatment but with a deactivation kinetics significantly slower than constructs containing the entire, chromatin-positioning MMTV-LTR sequence. The decrease on MMTV-LTR-driven transcription was concomitant with a parallel closure of the MMTV-LTR chromatin and with a decrease in glucocorticoid receptor (GR) concentration in the cell. We concluded that the chromatin-organized MMTV-LTR promoter is particularly sensitive to any decrease on GR levels. We propose that chromatin structure may contribute decisively to the differential expression of MMTV-LTR by two mechanisms: limiting MMTV-LTR accessibility to activating transcription factors and accelerating its shutting down upon a decrease on GR levels.