Estrogen: Actions and Functions

Types of Estrogen

• Lippincott Illustrated Reviews: Pharmacology, p. 832

Mechanism of Action

Transcriptional regulation by intracellular steroid hormone receptors (ER = estrogen receptor; ERE = estrogen response element)

1. Free estradiol dissociates from sex hormone-binding globulin (SHBG) or albumin in plasma.
2. It diffuses across the cell membrane and binds with high affinity to nuclear estrogen receptors (ERα or ERβ), which are inactive monomers bound to heat shock protein 90 (HSP90).
3. Hormone binding causes a conformational change - HSPs dissociate, the receptor dimerizes (homodimers ERα/ERα, ERβ/ERβ, or ERα/ERβ heterodimers), and the dimer binds to estrogen response elements (EREs) on DNA.
4. Together with coactivator proteins, the complex activates RNA polymerase II, initiating transcription of target genes and synthesis of specific proteins.

• Goodman & Gilman's Pharmacological Basis of Therapeutics, p. [block13]
• Lippincott Illustrated Reviews: Pharmacology, p. 832

Estrogen Receptor Subtypes

• Goodman & Gilman's Pharmacological Basis of Therapeutics

Physiological Actions of Estrogen

1. Female Reproductive Tract

• Maturation and maintenance of the uterus, fallopian tubes, cervix, and vagina.
• In the uterus: causes cell proliferation, cell growth, and increased contractility; stimulates growth of the endometrium, glands, stroma, and spiral arteries during the follicular (proliferative) phase.
• In the cervix: makes mucus copious, watery, and elastic ("ferning" pattern on glass slide), allowing sperm penetration.
• In the fallopian tubes: stimulates ciliary activity and contractility to aid sperm transport.
• In the vagina: stimulates epithelial cell proliferation.

2. Secondary Sexual Characteristics at Puberty

• Responsible for development of female secondary sex characteristics: growth of the uterus, vagina, labia, pubic/axillary hair distribution, female fat distribution (hips/thighs/buttocks), and female body contour.

3. Breast Development

• Required for development of the breasts; stimulates ductal and stromal growth, fat deposition.
• Works in coordination with progesterone and prolactin for full lactation capability.

4. Hypothalamic-Pituitary Feedback

• Negative feedback during most of the follicular phase: suppresses GnRH (hypothalamus) and FSH/LH (anterior pituitary).
• Positive feedback at midcycle: when estradiol levels rise above ~200 pg/mL, it up-regulates GnRH receptors on the anterior pituitary and causes the LH/FSH surge, triggering ovulation.
• Up-regulates progesterone receptors in target tissues, preparing them to respond to progesterone (the follicular phase "primes" the luteal phase).

5. Bone

• Reduces bone resorption by decreasing the bone-resorbing action of PTH.
• Estrogen receptors are present on bone cells; estrogen has direct effects on bone remodeling.
• Deficiency (as at menopause) leads to accelerated bone loss and osteoporosis.
• Men with aromatase deficiency or absent ER show osteopenia and failure of epiphyseal closure, confirming estrogen's role in bone in both sexes.
• Katzung's Basic and Clinical Pharmacology, 16th Edition

6. Cardiovascular Effects

• Increases HDL cholesterol, decreases LDL cholesterol (favorable lipid profile).
• Direct effects on vascular endothelium and smooth muscle.
• Note: The Women's Health Initiative demonstrated that hormone replacement therapy (HRT) significantly increases risks of heart disease and breast cancer; HRT is now reserved for short-term relief of vasomotor symptoms.

7. Skin and Connective Tissue

• Maintains skin thickness and collagen content; estrogen deficiency after menopause accelerates collagen loss.

8. CNS Effects

• Higher estrogen levels transiently increase CNS sensitivity to serotonin.
• Sexual dimorphisms in brain function partially reflect acute and reversible actions of sex steroid concentrations.
• Involved in thermoregulation - estrogen withdrawal leads to vasomotor instability ("hot flashes").

9. Metabolic Effects

• Increases hepatic production of sex hormone-binding globulin (SHBG), coagulation factors, and vitamin D-binding protein (DBP).
• Increases 1,25(OH)₂D levels in vivo (indirect effect via changes in PTH, calcium, and phosphate).

Summary Table: Actions of Estrogen on Target Tissues

• Costanzo Physiology, 7th Edition, Table 10.2

Therapeutic Uses

1. Contraception - combined with a progestogen (oral, transdermal, vaginal).
2. Menopausal hormone therapy - vasomotor symptoms, urogenital atrophy (short-term use preferred due to risks).
3. Osteoporosis prevention - established efficacy, though now largely superseded by bisphosphonates and SERMs due to adverse effects.

• Lippincott Illustrated Reviews: Pharmacology, p. 832-833
• Katzung's Basic and Clinical Pharmacology, 16th Edition