Steroid receptors are hormone-activated
transcription factors. Sex steroids play a
central role in the growth and maintenance of a
number of tissues and the tumors originating from
these tissues. Steroids of the glucocorticoid class
control many aspects of metabolism and homeostasis.
They have potent anti-inflammatory activities and
are widely used in medicine. Steroid hormones
achieve these wide ranging actions by regulating
gene expression. The hormones bind to specific
receptors, which, via a series of events, become
active transcription factors. The receptors for
progesterone, a sex steroid, and cortisol, a
glucocorticoid, are related. Indeed, the two
receptors bind to the same target sequences in DNA.
Thus, a fundamental issue is how the two receptors
mediate the regulation of distinct sets of genes in
tissues that contain both receptors.
How do two transcription factors that recognized the
same DNA site regulate different genes? One of
the major areas of work in the laboratory
investigates this conundrum. One set of studies has
developed a novel cre-lox based enhancer trap system
to identify genes differentially induced by
glucocorticoids and progestins. In parallel,
microarray analyses have identified a set of
differentially regulated genes. Current studies are
investigating the mechanisms that underlie the
differential regulation by the two receptors. Other
studies have shown that a steroid-regulated
promoter, the mouse mammary tumor virus promoter,
can be regulated very differently by glucocorticoids
and progestins depending on the site of integration
of the promoter in the genome. Thus, integration of
the mouse mammary tumor virus promoter into one site
results in a transcription unit that can be
regulated by both hormone receptors. However, when
integrated into another site in the same cell, the
same promoter retains its glucocorticoid
responsiveness while being nearly refractory to
progestins. Glucocorticoids can elicit remodeling of
mouse mammary tumor virus chromatin at both loci
whereas progestins only promote remodeling and
chromatin modification at the responsive locus. This
implies that the surrounding chromatin can impose
differential regulation on the transcription control
elements of this promoter in such a way as to
distinguish two closely related receptors.
Chromatin and gene regulation. We are
currently employing a variety of methodologies to
investigate the mechanisms that underlie the role of
chromatin in regulating the steroid response.
Current studies are analyzing transcription factor
loading and histone acetylation at target genes
using chromatin immunoprecipitation assays. These
studies have found, contrary to prevailing
assumptions, that histone acetylation is not
required for hormone-receptor mediated gene
induction. More recently we have begun to employ a
newly developed in vitro transcription system using
chromatin-assembled templates to elucidate
mechanisms of regulation of gene expression by
estrogen and progesterone receptors. A focus in
these in vitro studies is the role of
chromatin remodeling, coactivators, and histone
acetylation in the activation of gene expression by
steroid receptors in the reconstituted system.
Glucocorticoid-induced cell death. Additional
research endeavors in the laboratory seek to
identify glucocorticoid-induced genes that mediate
the hormonal induction of apoptosis in T-cells. A
candidate gene has been identified and a
multifaceted approach is being applied to test
whether the manipulation of expression of the
candidate gene has the functional consequences
expected of a gene that serves as a central mediator
of hormone-induced cell death.
Selected Publications
Wan Y, Coxe KK,
Thackray VG, Housley PR, and Nordeen SK (2001)
Separable features of the ligand-binding domain
determine the differential subcellular localization
and ligand-binding specificity of glucocorticoid
receptor and progesterone receptor. Molecular
Endocrinology 15:17-31.
Nordeen SK,
Housley PR, Wan Y, and Day RN (2001) The application
of green fluorescent protein to the study of the
dynamic protein-protein interactions and subcellular
trafficking of steroid receptors. In Methods and
Protocols in Steroid Receptor Biology, ed. B.A.
Lieberman, Humana Press, pp.179-199.
Lambert, J.R. and
Nordeen, S.K. Analysis of steroid hormone-induced
acetylation by chromatin (ChIP) assay. In Methods
and Protocols in Steroid Receptor Biology, Humana
Press, 2001 pp. 273-282.
Wan, Y., and
Nordeen, S.K. (2002) Identification of genes
differentially regulated by glucocorticoids and
progestins using a Cre/loxP-mediated
retroviral promoter trapping strategy. (2002) J.
Molecular Endocrinology 28: 177-192.
Wan Y., and
Nordeen S.K. (2002) Overlapping but distinct gene
regulation profiles by glucocorticoids and
progestins in human breast cancer cells.
Molecular Endocrinology 16: 1204-1214.
Thackray VG,
Nordeen SK (2002) High yield purification of
functional, full-length steroid receptor coactivator
1 expressed in insect cells. BioTechniques
32: 260-263.
Lambert, JR and
Nordeen SK (2003) CBP Recruitment and Histone
Acetylation in Differential Gene Induction by
Glucocorticoids and Progestins. Molecular
Endocrinology (in press)
Thackray VG, Toft
DO, Nordeen SK (2003) Novel Activation Step Required
for Transcriptional Competence of Progesterone
Receptor on Chromatin Templates (submitted)
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