Significance to Female Reproductive Physiology1

[CANCER RESEARCH (SUPPL.) 42, 3342s-3344s, August 1982] 0008-5472/82/0042-OOOOS02.00 Biochemistry of Aromatase: Significance to Female Reproductive Physiology1

Kenneth J. Ryan

Laboratory of Human Reproduction and Reproductive Biology, Department of Obstetrics and Gynecology. Harvard Medical School, Boston, Massachusetts 02115

Abstract adrenal, fetal liver, skin, fat, breast, and bone marrow and, at least in the brain, ovary, and placenta, the enzyme is present The formation of estrogens in mammals via aromatase in in many different vertebrate species (3, 5). In the ovary, aro volves the relatively unique capacity to form an aromatic ring matase is the final step in a multienzyme system that synthe de novo in contrast to most other aromatic substances (essen sizes estrogen from acetate and cholesterol while, in most tial amino acids) which are obtained only in the diet. The other tissues, aromatase acts on preformed plasma precursor reaction is the only example of a cytochrome P-450 system androgens secreted by the adrenal or gonad (16). For many which resides in both the mitochondrial and microsomal frac species, the source of the androgen substrate for placental tions of the cell. It occurs widely throughout the body in diverse aromatase is the fetal adrenal (14). tissues and functions via both de novo synthesis and transfor Aromatase is unique in that it resides in both the microsomal mation of prehormones (androstenedione and testosterone). It and mitochondrial fractions of cells and as far as can be is found widely in animal species in both the brain and gonads determined is the same type of enzyme in both cellular subfrac- even in phylogenetically primitive species. tions (4). While the existence of more than one form of aro Placental aromatase appears to be associated with the evo matase has been suggested for differently substituted andro lution of viviparity and an extended gestational period in utero. gen substrates, a clear resolution of this has not yet been Follicular aromatase which is dependent upon follicle-stimulat achieved (11). Similarly, the detailed mechanisms for the en ing hormone stimulation appears to be essential for oogÃ©nesis, zymatic transformation of androgens to estrogens involving ovulation, and normal luteal functions while central nervous molecular oxygen, removal of an angular methyl group, and system aromatase serves to determine sexual behavior and the introduction of extra double bonds are not precisely estab neurohormonal link to the hypothalamus and pituitary for ovar lished (5, 13). ian cyclicity. The estrogen synthesized by the aromatase may be secreted While estrogens are the key to pituitary, breast, and endo- to function in a typical hormonal fashion and/or may be used metrial growth and development, this hormone is one of the locally by the tissue in which the estrogen is produced (7). The few examples of an endogenous steroid that has been impli primary estrogenic products of aromatase are estrone derived cated as a carcinogen or a stimulant for carcinogenesis. from androstenedione, 17/?-estradiol from testosterone, estriol from 16a-hydroxytestosterone, and 16a-hydroxyestrone from 16a-hydroxyandrostenedione. Various substituted Ci9 steroids can also be aromatized as long as their conformation and ring substituents do not hinder the reaction (5, 13). Aromatase and Estrogen Biosynthesis

The enzymatic pathway for biosynthesis of the aromatic Estrogen Metabolism estrogens remained a mystery for many years after they were first discovered. Aromatic compounds are not usually synthe The primary biologically active estrogen, 17/S-estradiol, can sized in higher vertebrate species, and substances like the be produced by aromatase from testosterone or converted aromatic amino acid phenylalanine must be obtained via the from estrone (derived from androstenedione aromatization) via diet. Phenylalanine is in turn used as a building block for the a 17ÃŸ-o\oxidoreductase. The estrogen molecule can be mod hormones epinephrine and thyroxin. The preformed cyclic sub ified by substitutions on at least 12 of its 18-carbon skeleton. stances quinic acid and hexahydrobenzoic acid obtained from For the most part, metabolism of estradiol reduces or abolishes the diet can be aromatized by gut microorganisms or liver its biological activities, but in some instances it is believed that enzymes, and for a long time it was believed that the aromatic an oxygen function at carbon atoms 2, 4, or 16 may confer ring of estrogens was similarly derived (5). This was found not special properties of biological importance (17). to be the case when aromatase was described in some detail in human placental microsomes with a requirement for reduced Significance of Aromatase in Reproductive Function pyridine nucleotide and oxygen (12). Unlike many other mixed- function cytochrome P-450 oxidases, this reaction is not read In the evolution from unicellular to multicellular organisms, ily inhibited by carbon monoxide, probably due to the high cells have become limited to highly specialized functions. This affinity of the enzyme for its major substrate, androstenedione specialization results in the enzymatic production of sub (8, 9). stances (hormones) that coordinate and regulate differentiated Aromatase has also been described in brain, ovary, testis, cells in the shared economy of a multicellular organism. It also allows coordination of behavior between 2 separate organisms ' Presented at the Conference "Aromatase: New Perspectives for Breast when interaction is required for reproduction and/or survival. Cancer," December 6 to 9, 1981, Key Biscayne, Fla. Aromatase in the specialized cells of the ovary, hypothala-

3342s CANCER RESEARCH VOL. 42

Downloaded from cancerres.aacrjournals.org on September 23, 2021. © 1982 American Association for Cancer Research. Aromatase and Female Reproduction mus, and placenta clearly serves such a role in reproduction brain aromatase in so many animal species, including phylo for mammalian and other vertebrates. While aromatase is pres genetically primitive ones, has led us to speculate that it played ent in many diverse tissues, such as skin, fat, bone marrow, an important role in the evolution of reproductive cycles and liver, adrenal, and testes, its biological role in such sites is not behavior (3). as well understood. Placental Aromatase. This aromatase is present in many Throughout evolution, the role of aromatase via its estrogen mammalian species with a gestational period longer than 60 products appears to be centered on reproductive functions: days. It is suggested that the estrogen produced by the pla oogÃ©nesis, egg processing, sexual behavior, neuroendocrine centa provides a backup and then a replacement of ovarian synchronization, mammary development, and accommodation estrogen to ensure continued hormone during the longer ges to viviparity. Even the so-called "metabolic" functions of estro tation period of some viviparous species including the human gens in mammals appear to be left over from phylogenetically (14,15). more primitive reproductive mechanisms in the evolution from Breast Development. This development evolved with mam ovaparity to viviparity. The estrogenic hormone has been malian viviparity, and estrogen was again recruited as the basic hormone for this "reproductive target tissue." In the breast as adapted to many diverse functions as reproduction evolved. Ovarian Aromatase. This aromatase produces the bulk of in most other estrogenic target organs, the estrogen provides systemic estrogens for general reproductive functions. These the initial growth and development but also prepares the tissue estrogens also act locally. Follicles that produce estrogens for subsequent effects by other hormones such as progester have sequestered pituitary follicle-stimulating hormone which one and prolactin. Aromatase is present in breast tissue and in turn stimulates the aromatase reaction. Such follicles can breast fat and may play some role in local tissue function (10). undergo normal development and ovulation and contain eggs that readily resume meiosis when released. In the absence of an active local aromatase (i.e., no follicle-stimulating hormone), Aromatase and Cancer the follicles and oocytes become atretic and regress without Estrogens have a phenolic ring structure in common with the ovulating. If aromatase is present, the estrogen and follicle- carcinogenic hydrocarbons, and this has been invoked to stimulating hormone can further develop the follicular cells for explain the association of estrogen effects with cancer in target normal luteal function after ovulation takes place (6, 7). tissues. Although excess exogenous estrogens or estrogens The systemic estrogen produced by ovarian aromatase mod produced by the ovary unopposed by progesterone have been ulates central nervous system and pituitary functions for the common features in experimental or clinical examples of en ovarian cycle and in spontaneously ovulating mammals triggers dometrial and breast cancer, estrogen produced by extraglan- the release of the ovulatory surge of luteinizing hormone. dular aromatase in the absence of active ovarian function has Ovarian estrogen is the major component of negative and also been considered a possible factor in carcinogenesis (16). positive feedback for pituitary release of gonadotrophic hor Estradiol and estrone have been shown to bind covalently to mones (6, 7). The most primitive vetebrate reproductive "tract" consists DNA during incubations with liver microsomal preparations, a property shared by diethylstilbestrol and carcinogenic aromatic of a genital pore that merely discharges eggs outside the body, hydrocarbons (1, 2). Whether estrogen is directly carcinogenic but in evolution the oviduct became a conduit and a way station or renders the tissue more susceptible to other carcinogenic for processing eggs (15). Such specialized functions as oviduct influences is still uncertain. A key issue is whether control of contractility and protein secretion are under estrogenic control. cancer can be accomplished by aromatase inhibition of estro In the mammalian oviduct and uterus, the specialized epithelial gen production or whether it would be more profitable to try to cells of the tube, the endometrial lining, the muscle layers, and influence the effects of estrogen at its site of action. rich blood supply are all developed by estrogenic stimulation. In birds, reptiles, and amphibians, estrogen stimulates the production of phosphoproteins and lipoproteins by the liver for References incorporation into the yolk of the growing oocyte (18). While 1. Blackburn, G. M., Orgee, L., and Williams, G. M. Oxidative bonding of this function is missing in most mammals, estrogens still stim natural oestrogens to DNA by chemical and metabolic means. J. Chem. Soc. Chem. Commun., 386-387, 1977. ulate the production by liver of lipoproteins and other products 2. Blackburn, G. M., Thompson, M. H., and King, H. W. S. Binding of diethyl- which have more generalized metabolic functions. It is likely stilboestrol to deoxyribonucleic acid by rat liver microsomal fractions in vitro that these "metabolic" effects of estrogen are a carry-over and in mouse foetal cells in culture. Biochem. J., 158: 643-646, 1976. 3. Gallarli G. V., Retro, Z., and Ryan, K. J. Conversion of androgen to estrogen from a reproductive mechanism. and other steroids in the vertebrate brain. Am. Zool., 78. 511-523, 1978. Brain Aromatase. This aromatase is present in every verte 4. Canick, J. A., and Ryan, K. J. Properties of the aromatase system associated brate species tested except the most primitive cyclostomes with the mitochondrial fraction of human placenta. Steroids, 32: 499-509. 1978. and is represented in homologous areas of the brain throughout 5. Engel, L. L. The biosynthesis of estrogens. In: R. O. Greep and E. B. phylogeny (3). In some species (teleosts), aromatase is more Astwood (eds.). Handbook of Physiology, Vol 2, pp. 467-483. Washington, active in the brain than in the gonad. There is a functional D. C.: American Physiological Society, 1973. 6. McNatty, K. P., Makris, A., De Grazia, C., Osathanondh, R., and Ryan, K. J. sexual dimorphism of the hypothalamus in several mammalian 23. Steroidogenesis in granulosa cells and corpus luteum: the production of species which specifies male (tonic) or female (cyclic) repro progesterone, androgens and oestrogens by human granulosa cells in vitro and in vivo. J. Steroid Biochem., 11: 775-779, 1979. ductive hormonal patterns and sexual behavior. The male pat 7. McNatty, K. P., Smith D. M., Makris, A., Osathanondh, R., and Ryan, K. J. tern is developed in the rodent in the first 10 days of life when The microenvironment of the human antral follicle: interrelationships among testicular androgen is converted to estrogen by hypothalamic the steroid levels in antral fluid, the population of granulosa cells, and the status of the oocyte in vivo and in vitro. J. Clin. Endocrinol. Metab., 49: aromatase. In addition, brain aromatase controls both male and 851-860, 1979. female adult sexual behavior in some species. The presence of 8. Meigs, R. A., and Ryan, K. J. Cytochrome P-450 and steroid biosynthesis in

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the human placenta. Biochim. Biophys. Acta, Ã65:476-482, 1968. ative approach. Excerpta Med. Int. Congr. Ser., 183: 120-131. 1968. 9. Meigs, R. A., and Ryan, K. J. Enzymatic aromatization of steroids I. Effects 15. Ryan, K. J. New concepts in hormonal control of parturition. Biol. Roprod . of oxygen and carbon monoxide on the intermediate steps of estrogen 76. 88-94, 1977. biosynthesis. J. Biol. Chem., 246: 83-87, 1971. 16. Siiteri, P. K., and MacDonald, P. C. Role of extraglandular estrogen in 10. Nimrod, A., and Ryan, K. J. Aromatization of androgens by human abdominal human endocrinology. In: R. 0. Greep and E. B. Astwood (eds.). Handbook and breast fat tissue. J. Clin. Endocrino!. Metab., 40: 367-372, 1975. of Physiology, Vol. 2, pp. 615-629. Washington, D. C.: American Physio 11. Preumont, P., Ryan, K. J., Younglai, E. V., and Soloman, S. Specificity of logical Society, 1973. aromatization and 16-hydroxylation in the human ovary. 29: 1394-1403, 17. Slaunwhite. W. R., Jr., Kirdani, R. Y . and Sandberg, A. A. Metabolic aspects 1969. of estrogens in man. In: R. O. Greep and E. B. Astwood (eds.). Handbook of 12. Ryan. K. J. Biological aromatization of steroids. J. Biol. Chem., 234. Physiology Vol. 2, pp. 485-523. Washington, D. C.: American Physiological 268-272, 1959. Society, 1973. 13. Ryan, K. J. Biogenesis of estrogens, in G. Popjak (ed.), Biosythesis of 18. Wallace, R. A. Oocyte growth in nonmammalianvertebrates, in: R. E. Jones Lipids. pp. 381-394. New York, Macmillan Publishing Co., Inc., 1963. (ed.), The Vertebrate Ovary, pp. 469-502. New York: Plenum Publishing 14. Ryan, K. J. Theoretical basis for endocrine control of gestationâ€”acompar Corp., 1978.

3344s CANCER RESEARCH VOL. 42

Kenneth J. Ryan

Cancer Res 1982;42:3342s-3344s.

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