Function and mode of action of nuclear receptors: Estrogen, progesterone, and vitamin D

Estrogens and progestins play important roles in regulating growth and differentiation of a wide range of cell types, in both reproductive and nonreproductive tissues. Not surprisingly, therefore, endocrine active substances that mimic the actions of these steroid hormones have been demonstrated to have untoward effects on the reproductive function of a variety of animals. Although direct links between exposure to endocrine active substances of this class and reproductive abnormalities in humans have been difficult to establish, the potential for harm by this class of chemicals warrants further investigation. This chapter summarizes our current understanding of the molecular pharmacology of the estrogen and progesterone receptors, ER and PR, beginning with a historical perspective of ER pharmacology and ending with a comparison of these receptors to the vitamin D receptor (VDR), a nonsteroid hormone nuclear receptor involved in regulating growth and development in nonreproductive tissues. Major topics discussed include receptor subtypes (or isoforms), receptor functional domains, ligand-binding characteristics, receptor structure, cofactor binding, effects of phosphorylation, and nonclassical modes of action. This discussion will demonstrate the need for developing novel screens for potential endocrine disruptors that incorporate our current understanding of nuclear receptor pharmacology.