Estrogen Receptor alpha Monoclonal Antibody
Excerpts from
Sir George Beatson proposed a connection between breast cancer and
ovary more than a century ago. It took several decades to the discovery of hormone
estrogen and a few more decades when Elwood Jensen announced the discovery of
estrogen receptor. His work led to our understanding of how hormones control target gene transcription via their receptors. Several laboratories made major contributions toward our understanding of hormone action. To date, 49 nuclear hormone
receptors have been identified that form the nuclear receptor family and carry out a
myriad of metabolic functions. Most notably, they are targets for therapy. Jensen
and colleagues made ER antibodies that were used to develop ER assay kits in
breast cancer specimens. ER contents in breast cancer patients proved to be useful
in deciding mode of treatment. Soon after the discovery of ER, the antiestrogen,tamoxifen, which was originally developed as a female contraceptive, was repurposed for breast cancer management, and later it was used as a prophylactic in those women who were in high risk for breast cancer.
Elwood Jensen's discovery of the estrogen receptor (ER) made a paradigm shift
toward our understanding of steroid hormone action. It launched the field of
nuclear receptors, which has profoundly impacted the discipline of molecular
medicine. A perfect example of "Bench to Bedside" translational research, his
work has saved thousands of lives of breast cancer patients. Moreover, his work
has led to the understanding of how ligand-dependent transcription factors mediate
the cell-type-specific gene expression in amplifying hormonal actions.
The Elusive Mechanism of Estrogen Action
Adolf Butenandt and Edward Doisy independently purified estrogen in 1929. Over
the following decades, observations accumulated that tiny amount of the hormone
could cause profound target tissue growth (e.g., uterus). However, the mechanism of
its action remained elusive. In the 1950s, being the era of enzymology, research
community assumed that enzymes mediate the hormone-dependent tissue growth.
The popular belief was that the mechanism of estrogen action entailed trans-
hydrogenation in which the 17-hydroxyl group of estradiol is oxidized by one coenzyme and the resulting estrone reduced by another, thus using NADH to produce
NADPH l. However, there was one caveat with this thought process: such a mechanism would not explain the uterotropic actions of diethylstilbestrol (DES), a synthetic estrogen that lacks any aliphatic hydroxyl group and thus could not undergo that reversible oxidation/reduction
2 Fellowship in Zurich and the Matterhorn Experience
While doing a steroid chemistry fellowship in Zurich with the Nobel Laureate,
Professor Leopold Ruzicka, Elwood was fascinated by the natural beauty of the sur-
rounding areas. He was particularly attracted toward the towering Matterhorn.
Although never climbed a mountain, physically, he was in good condition from his
collegiate sports activities (Boxing/Judorrennis) and decided to scale the Matterhorn.
He teamed up with a lab colleague with mountaineering experience and a guide to
climb the Matterhorn (Fig. l) from an alternate route (Swiss side), rather than from
the seemingly simple but more hazardous Italian side. The latter approach was used
by Edward Whymper to scale the Matterhorn peak but at the cost of many unsuc-
cessful attempts and a few human lives. Matterhorn was the last European mountain
to be climbed. The successful Matterhorn experience by a novice like Elwood
Jensen instilled a lifelong passion of applying "alternative strategy" approaches in
his research pursuits .
\3 Faculty Position at the University of Chicago:
From Chemist to Endocrinologist
When Charles Huggins, who won the Nobel Prize for his work on prostate cancer,
recruited Elwood at the University of Chicago, a vexing question in the endocri-
nology field was "how does tiny amount of estrogen induce massive uterine
growth" Ill? Elwood Jensen, himself a chemist, and his postdoctoral fellow,
Herbert Jacobson, who earned his PhD degree with the famous chemist Morris
Kharasch at the University of Chicago, embarked upon solving an agelong endocrinology problem: what is the mechanism of estrogen action? Elwood invoked
the "alternative approach" and decided to understand the fate of the hormone
itself rather than what hormone does to the tissue—the prevailing approach in the
field at the time. Because estrogens are active at such low doses (in nanomolar
range), they planned to label the hormone with tritium and follow the radioactivity in various rat tissues. However, their experimental strategy entailed using the
hormone radioactively labeled to prohibitively high specific activity, normally
not permitted by the university regulatory authorities. But as luck would have it,
the "Fermi Lab"—an epicenter of the Second World War "Atomic Bomb
located in the nearby Argonne National Laboratory and was made
accessible to the Jensen team They built an apparatus, tritiumator (Fig. 2), to
measure the uptake of tritium by a catalytic reduction of a double bond in the
precursor. They reasoned that one could radiolabel the sixth and seventh position
of the hormone with carrier-free tritium gas. Thus, using the Fermi Lab facilities
to handle carrier-free tritium (60 Ci/mmole), they succeeded in labeling highspecific activity estradiol. When they injected the tritiated estradiol to immature
rats or to castrated rats, to their surprise, they found that the hormone remained
biochemically unchanged and the uterus showed the usual massive growth.
Moreover, when they examined various rat tissues for the uptake of radioactive
estradiol (Fig. 3), they found uptake and retention was 100-fold higher in uterus
and vagina than in nonreproductive tissue such as blood II, 21. Some skeptics
raised the concern that estrogen might have undergone oxidoreduction of its
17-beta hydroxyl group such that the hydrogen atom lost during oxidation is not
the same one that replaces it during reduction. Jensen and colleagues addressed
this by injecting a mixture of 6,7 tritium-labeled estrogen plus 17-tritium-labeled
estrogen in their rat model and clearly demonstrated that there was no loss of
tritium from position 17 of estrogen during the hormone-induced uterotropic
growth
4 Birth of the Nuclear Receptor Family
When Elwood announced his groundbreaking findings at the International
Congress in Vienna, five people came to listen to him—three of whom were
speakers. Whereas, in a concurrent plenary session, hundreds went to hear the
now debunked enzymatic theories of estrogen action! The "factor" that Jensen
initially termed "estrophilin" is now known as "estrogen receptor" (ER). This
momentous discovery shifted attention away from the involvement of enzymes in
the mechanism of hormone action. Subsequently, Elwood's contemporary, Jack
Gorski at University of Illinois at Urbana/Champaign used state-of-the-art sedi-
mentation gradient procedures to isolate and characterize a macromolecular com-
ponent, which possessed the attributes of a specific receptor for estrogens 131.
The sedimentation density gradient would go on to play a critical role in the
Jensen laboratory in research related to ER. These findings stimulated the search
for other hormone receptors. The pioneering work by John Baxter, Pierre
Chambon, Ron Evans, Jan-Ake Gustafsson, Bert O'Malley, and Keith Yamamoto
led to the discoveries of the glucocorticoid receptor, progesterone receptor, reti-
noic acid receptor, and orphan receptors. In a remarkably short span of time, the
49 nuclear receptors described to date have become a Receptor
Family" [4]. At the 2004 Lasker Award ceremony, Nobel Laureate, Joseph
Goldstein, paid tribute to the discovery and called Elwood Jensen the patriarch
and estrogen receptor the matriarch of the family [5]. The Lasker Foundation
recognized these discoveries with Lasker Awards to Drs. Jensen, Chambon, and
Evans• man in the field believe that it is deservin of reco nition b the Nobel
5 Estrogen Receptor and RNA Synthesis
After distinguishing two forms of the receptor [cytoplasmic and nuclear], Elwood
Jensen, as well as Jack Gorski [3, 6], showed that the hormone-receptor complex
becomes tightly bound in the nucleus and enhances RNA synthesis (transcrip-
tion) in nuclei specifically isolated from hormone-dependent tissues [71. Shortly
thereafter, Bert O'Malley's group used estrogen-stimulated chicken oviduct sys-
tem and published landmark papers not only describing the receptor for proges-
terone but also showing that it also stimulated transcription of specific mRNAs
[8, 9]. This phenomenon of hormone-induced receptor activation has since
proved to be a key step in the actions of various classes of steroid hormones, and
it identified a definitive biochemical role for the steroid.
6 Estrogen Receptor Domain Structure and Ligand-
Dependent Receptor Dimerization5 Estrogen Receptor and RNA Synthesis
After distinguishing two forms of the receptor [cytoplasmic and nuclear], Elwood
Jensen, as well as Jack Gorski [3, 6], showed that the hormone-receptor complex
becomes tightly bound in the nucleus and enhances RNA synthesis (transcrip-
tion) in nuclei specifically isolated from hormone-dependent tissues [71. Shortly
thereafter, Bert O'Malley's group used estrogen-stimulated chicken oviduct sys-
tem and published landmark papers not only describing the receptor for proges-
terone but also showing that it also stimulated transcription of specific mRNAs
[8, 9]. This phenomenon of hormone-induced receptor activation has since
proved to be a key step in the actions of various classes of steroid hormones, and
it identified a definitive biochemical role for the steroid.
6 Estrogen Receptor Domain Structure and Ligand-
Dependent Receptor Dimerization
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