HORMONAL REGULATION OF GENE EXPRESSION 
William W. Chin, M.D., Investigator 
Dr. Chin and his colleagues have focused on the 
molecular mechanisms involved in hormonal regu- 
lation of gene expression, emphasizing thyroid 
hormone regulation of thyrotropin [thyroid-stim- 
ulating hormone (TSH)] and sex steroid/gonadal 
peptide regulation of gonadotropin [lutropin (LH) 
and foUitropin (FSH)] gene expression. In addition, 
Dr. Chin's studies on the molecular biology of thy- 
roid hormone receptors (THRs) have underscored 
them as prototypes of trans-acting factors involved 
in the regulation of gene expression. 
L Thyroid Hormone Regulation of TSH Gene 
Expression. 
Dr. Chin has continued to characterize both cis- 
and trans-acting elements involved in thyroid hor- 
mone negative regulation of TSH subunit gene ex- 
pression. He has previously shown that the rat 
TSHP subunit gene contains two transcriptional 
start sites with their associated TATA boxes located 
43 bp apart. The transcript corresponding to the 
downstream start site appears to be most regulated 
by thyroid hormone. Dr. Chin has used deletion 
mutants and gene transfer into GH^ cells to identify 
a 57 bp fragment, encompassing the downstream 
start site and the first exon, that can confer thyroid 
hormone-responsive negative regulation of the 
TSHp gene. Furthermore, a protein-DNA-binding 
assay employing avidin-biotin complex formation 
has been used to locate two THR binding sites 
within this fragment at positions -13 to +4 and 
-fll to +27. These data indicate that two putative 
thyroid hormone-responsive elements (TREs) 
downstream of the rat TSHp gene promoters may 
cooperate to permit thyroid hormone action. Sim- 
ilar work performed with the rat a-subunit gene 
has shown that its putative TRE is located at posi- 
tions -74 to -53, in a region that is devoid of well- 
known proximal promoter elements, such as GC- 
rich or CAAT boxes. This region also binds THR as 
protein in nuclear extracts or as in vitro translation 
products. These data suggest that THRs may inter- 
act with DNA by direct binding, with interference of 
gene transcription on a steric basis. 
II. Sex Steroid Hormone/Gonadal Peptide Regula- 
tion of LH and FSH Gene Expression. 
Dr. Chin and his colleagues have shown that es- 
trogen can regulate a, LHp, and FSHp mRNAs in pi- 
tuitary glands of intact mature female and male 
rats. In addition, they have provided evidence that 
estrogen may have a direct effect on LH synthesis in 
the pituitary gland, in part by regulating the 
synthesis of LHP and a mRNAs, with effects on sub- 
unit gene transcription. The 5 '-flanking region of 
the rat LHp gene was analyzed for a putative estro- 
gen-response element. Such an element is present 
at -1,200 and confers positive regulation by estro- 
gen on heterologous promoters. Another important 
point involves the differential regulation of FSHp 
mRNAs by androgens. Androgens have little effect 
on FSHp mRNA in vivo but markedly stimulate it in 
vitro. Dr. Chin has also shown that a number of go- 
nadal peptides, including inhibin A and B, 
foUistatin, and/or FSH-releasing peptide (activin) 
regulate FSHp mRNA in a specific manner without 
effect on a or LHp mRNA. In summary, the subunit- 
specific positive and negative regulation of gonado- 
tropin gene expression by various gonadal factors 
and hormones may provide insight into the com- 
plex regulation of gonadotropins in the intact 
animal. 
III. Thyroid Hormone Receptors. 
The first reports of the isolation and characteriza- 
tion of cDNAs encoding putative THRs were pro- 
vided by Drs. R. Evans and B. Vennstrom in late 
1986. The former reported that a proto-oncogene, 
c-erbA, as expressed in human placenta, encodes a 
P form of THR, and the latter described a related 
cDNA from chick embryo that encodes an a form of 
THR. Soon thereafter it was shown that c-erbAa 
and c-erMp are encoded by separate genes located 
on different chromosomes in humans. Thus there 
was early evidence of heterogeneity of mammalian 
THRs. 
Studies on the molecular biology of THRs in the 
rat have been performed. In addition to identifying 
a cDNA encoding rat c-erbAa. (410 aa; a-1) that was 
originally described by Dr. C. Thompson, a cDNA 
derived from a rat pituitary GH^ cell line and en- 
coding an isoform of rat c-erbAa (492 aa; a-2) was 
isolated. This form is nearly identical to a-1 in its 
first 370 amino acids, which encompass the entire 
DNA binding and most of the thyroid hormone- 
binding domains of the THR. The carboxyl-termi- 
nal 40 amino acids of a-1 (a region completely con- 
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