MECHANISMS OF THYROID HORMONE ACTION 
P. Reed Larsen, M.D., Investigator 
Dr. Larsen's laboratory is investigating the mecha- 
nism of thyroid hormone action. He has focused on 
two steps in this process: 1) the deiodination of the 
prohormone thyroxine (T^ to produce the active 
hormone 3, 5, 3 -triiodothyronine (T^ and 2) the 
mechanism by v^^hich alters the expression of the 
thyroid hormone-dependent protein, rat growth 
hormone (rGH). 
The deiodination of T^ to produce the active hor- 
mone Tj is a complex process. Several goals must 
be achieved. Circulating T^ for use by muscles, 
heart, liver, and kidney must be produced. A local 
source of T^ for the brain must be provided, since 
circulating T^, which does not enter this tissue effi- 
ciently, is required for normal central nervous sys- 
tem development. Finally, both the circulating ac- 
tive hormone, T^, as well as the prohormone, T^, 
must be monitored. Dr. Larsen's previous work elu- 
cidated the relative roles of the two 5'-iodothyro- 
nine deiodinases that subserve these goals. One en- 
zyme, the type I deiodinase, is present at highest 
concentration in the liver and kidney and provides 
most of the circulating T^. Type II deiodinase pro- 
duces —80% of the specifically bound intranuclear 
Tj for the brain and —50% of that found in nuclei 
of pituitary and brown adipose tissue. In the pitu- 
itary, type II deiodinase permits the thyrotroph to 
sense the circulating T^ concentration; in the 
brown adipose tissue, the T^ generated by adrener- 
gic stimulation of the type II deiodinase enzyme is 
required for a normal thermogenic response to 
cold stress. Type II deiodinase can provide a signifi- 
cant fraction of the body's circulating T^ under spe- 
cial circumstances (e.g., in the neonatal or hypothy- 
roid rat), since its activity is increased and that of 
type I deiodinase is reduced. 
A major goal of Dr. Larsen's studies has been to 
identify these membrane-bound deiodinase pro- 
teins, learn how they are regulated in different tis- 
sues, and analyze their structural-functional rela- 
tionships. Because of the 20- to 30-fold increases in 
type II deiodinase activity induced by sympathetic 
stimulation in brown fat, dispersed brown ad- 
ipocytes were employed to evaluate its regulation. 
In earlier studies, catecholamines were found to 
stimulate type II deiodinase activity, and the effect 
of the dual agonist norepinephrine (NE) was en- 
hanced by (3-adrenergic blockade in cells from eu- 
thyroid rats. This emphasized the important role of 
ttj-agonists in the control of this enzyme. NE stimu- 
lation of type II deiodinase was greater in hypothy- 
roid rats but was inhibited by P-blockade. The re- 
cent studies showed that both (B-adrenergic agents 
(through cAMP) and ttj-agonists (probably via in- 
creasing intracellular calcium) were required for 
deiodinase stimulation. In both euthyroid and hy- 
pothyroid cells, the response to these agents was 
synergistic. For example, in hypothyroid cells the 
increment with combined a^/p-stimulation was four 
times that which could be explained on the basis of 
the additive effects of the two pathways. Maximal 
stimulation required a concomitant exposure of 
—1.5 h to both a J- and (J-agonists. It was blocked 
by actinomycin D and thus required the synthesis 
of a critical protein, either the enzyme or a species 
that can activate it. The cAMP response to P-adren- 
ergic agents was reduced in hypothyroid cells, al- 
though this became apparent only over a 2 h incu- 
bation. In euthyroid cells the magnitude of the 
deiodinase response to Bt^ cAMP or forskolin alone 
was biphasic, with lower stimulation occurring at 
higher cAMP concentrations. This explained the ob- 
servation that P-blockade enhanced the response of 
such cells to NE. Thus the attenuation of the cAMP 
response in hypothyroid cells was coupled with a 
greater stimulation in the presence of a^-agonists. 
These results are the first demonstration of a com- 
pensatory mechanism by which hypothyroidism en- 
hances a cAMP-dependent process. The importance 
of facultative thermogenesis to survival in the cold 
points to a teleological rationale for this phenome- 
non, but the actual mechanism for the increase in 
type II deiodinase under the a/p-adrenergic influ- 
ence remains to be determined. It will require the 
identification of the type II deiodinase or its mRNA. 
Although Dr. Larsen's group has obtained several 
hundred-fold purification of this enzyme using 
combinations of hydrophobic interaction chroma- 
tography and ion-exchange techniques, several 
bands are still present on silver stains. Therefore 
they decided to pursue an alternate strategy to iso- 
late these deiodinases. Dr. Maria Berry has em- 
ployed the Xenopus oocyte expression system. The 
oocyte expresses type I deiodinase activity after in- 
jection of poly(A)^ mRNA isolated from hyperthy- 
roid rat liver. Size fractionation shows the mRNA to 
be —1.9-3 kb in length. Preliminary results are en- 
couraging, in that deiodinase activity was found 
after injection of mRNA generated from a pool of 
20,000 \-phage cDNA clones prepared from hyper- 
Continued 
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