Principles of Biochemistry Volume 3 - A. Lehninger 1985

Selected Aspects of Human Biochemistry
Hormones
The mechanism of action of estrogens on target cells is gradually becoming clearer

The primary estrogen in the female body is ß-estradiol, which is produced in the Ovaries from—paradoxically enough—the main male sex hormone, testosterone (Figs. 25-26). Truly, Eve was created from more than just Adam's rib! Specific intracellular receptors for ß-estradiol are found in primary target Tissues, namely the Uterus and Mammary Glands. The estrogen receptor, known as estrophilin I, has a Molecular Weight of ~200,000. Upon binding an estrogen molecule, estrophilin I undergoes molecular modification, transforming into estrophilin II, which can be regarded as a secondary messenger in estrogen action. Estrophilin II enters the Cell Nucleus, where it interacts with Chromatin, triggering the activation of specific genes and the synthesis of specialized Proteins characteristic of stimulated Accessory Organs of the Reproductive System (Figs. 25-26). For instance, administering estradiol to chicks causes a significant increase in The rate of synthesis of unique egg proteins in the oviduct, specifically Ovalbumin and ovovitellin. In this way, estradiol prepares the oviduct for egg formation.

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Fig. 25-26. Schematic illustration of estrogen action on target Cells in the chicken oviduct. As a lipid-soluble compound, estrogen passes through The Cell membrane and binds to the estrogen receptor—a protein with a sedimentation coefficient of 4S. Subsequently, the estrogen-receptor complex converts into the active 5S form and, acting as a secondary messenger, enters The Nucleus. There, by interacting with specific chromatin regions, it induces the METABOLISM/31.html">Transcription of target genes to produce corresponding mRNAs. These mRNAs exit the nucleus and serve as templates for Protein Synthesis on Ribosomes. As a result, a series of proteins characteristic of stimulated oviducts is synthesized, such as ovalbumin.

The number of estrogen receptors in the female mammary gland decreases during the development and growth of breast Cancer. Measuring the level of estrogen receptors in a small breast tissue sample is used as a diagnostic test to determine the stage of the disease and guide Treatment decisions. The progression of early-stage breast cancer can sometimes be halted by altering the balance of androgens and estrogens in the body.