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Alterations in chromatin structure associated with glucocorticoid-induced expression of endogenous mouse mammary
Abstract:
Alterations in the chromatin structure of endogenous mouse mammary tumor virus genes accompany glucocorticoid induction of viral RNA synthesis in the C57BL/6 T lymphoma cell line T1M1. These alterations are defined by the appearance of sites of DNase I hypersensitivity within proviral DNA in isolated nuclei, as well as by changes in the moderate nuclease sensitivity of entire proviral transcription units. Induced hypersensitive sites, termed type I, appear with a time course comparable to that required for induction of the rate of viral RNA synthesis and are maintained only in the continuous presence of hormone. Two such sites map to analogous positions in the 5' and 3' long terminal repeats of proviral DNA within, or very near, sequences that have been shown to comprise positions of specific binding of the glucocorticoid receptor in vitro and that are required for hormone-inducible transcription in vivo. A third type I site maps to another position of in vitro receptor binding near the 3' long terminal repeat. Some sites of DNase I hypersensitivity, termed type II, appear not to be markedly hormone dependent; two such sites are present in corresponding positions in each long terminal repeat. Comparison of the moderate DNase I sensitivity of mouse mammary tumor virus proviral DNA suggests that the three different endogenous units in T1M1 cells can be maintained in distinct chromatin conformations that are determined by factors related to the site of provirus insertion. It seems possible that altered chromatin conformations may reflect, or actually encode, important mechanistic features of these hormone-responsive genes.
Insights
Glucocorticoids alter chromatin structure to induce mouse mammary tumor virus (MMTV) RNA synthesis. These changes involve specific DNA hypersensitivity sites linked to hormone receptor binding and viral gene expression.
Area of Science:
- Molecular Biology
- Epigenetics
- Virology
Background:
- Endogenous mouse mammary tumor virus (MMTV) genes are present in T1M1 lymphoma cells.
- Glucocorticoids are known to regulate gene expression.
Purpose of the Study:
- To investigate the chromatin alterations accompanying glucocorticoid induction of MMTV RNA synthesis.
- To identify specific DNA regions involved in hormone-dependent gene regulation.
Main Methods:
- DNase I hypersensitivity assays were used to probe chromatin structure.
- Analysis of proviral DNA in isolated nuclei from T1M1 cells.
- Comparison of chromatin conformations of different endogenous MMTV units.
Main Results:
- Glucocorticoid induction of MMTV RNA synthesis correlates with new DNase I hypersensitive sites (type I).
- Type I hypersensitive sites are hormone-dependent and located near glucocorticoid receptor binding sites.
- Other hypersensitive sites (type II) are hormone-independent.
- Distinct chromatin conformations exist for different endogenous MMTV proviruses, influenced by insertion site.
Conclusions:
- Chromatin structural changes, particularly hormone-inducible hypersensitive sites, are crucial for glucocorticoid-mediated MMTV gene regulation.
- The location of proviral insertion can influence chromatin conformation and gene expression.
- Altered chromatin structures may encode key mechanisms for hormone-responsive genes.