A new hypothesis for how sex steroid hormones regulate bone mass Joseph Lorenzo J Clin Invest. 2003;111(11):1641-1643. https://doi.org/10.1172/JCI18812. Commentary It has been proposed — but remains controversial — that estrogen’s effects on various tissues may be mediated by different cell signaling pathways. Researchers have identified a synthetic ligand that activates only a subset of these pathways, suggesting that bypass of the traditional estrogen pathway can prevent bone loss without associated side effects on reproductive organs. Find the latest version: https://jci.me/18812/pdf COMMENTARIES A new hypothesis for how sex steroid 3′,5′-cyclic adenosine monophosphate (cAMP) levels within 15 seconds. hormones regulate bone mass Pietras and Szego (7) found that the outer surface of endometrial cells con- Joseph Lorenzo tained estrogen-binding sites. In recent years it has become apparent The University of Connecticut Health Center, Farmington, Connecticut, USA that rapid effects of steroid hormones are mediated by interactions with It has been proposed — but remains controversial — that estrogen’s components of various signal trans- effects on various tissues may be mediated by different cell signaling duction pathways, including adenylyl pathways. Researchers have identified (see related article beginning on cyclase, MAPKs, and PI3K. page 1651) a synthetic ligand that activates only a subset of these path- Interestingly, the receptors that bind ways, suggesting that bypass of the traditional estrogen pathway can pre- sex steroid ligands and initiate these vent bone loss without associated side effects on reproductive organs. responses can be identical to or differ- ent from known steroid receptors J. Clin. Invest. 111:1641–1643 (2003). doi:10.1172/JCI200318812. (Figure 1). Estrogen can bind the G protein–coupled receptor homolog Sex steroid hormones exert a variety of androgen receptor (AR). It is now GPR30 and activate extracellular sig- important actions. In addition to their appreciated that the relative balance of nal–regulated kinases (ERKs) 1 and 2 role as regulators of reproductive func- receptors, coactivator, and corepressor in SK-BR-3 breast cancer cells (8). tions, they have potent effects on the proteins is a critical determinant of the ERK-1 and ERK-2 are members of the nervous and cardiovascular system and ability of this classical pathway to initi- MAPK family. Significantly, SK-BR-3 are major determinants of the develop- ate responses. Since the relative con- cells are devoid of both ERα and ERβ. ment and the structural integrity of the centrations of these molecules is cell Activation of ERKs by estrogen in skeleton. Estrogens and androgens reg- specific, sex steroid hormones can have SK-BR-3 cells was not prevented by an ulate such diverse cellular actions as vastly different functions in different ER antagonist, which blocks binding proliferation, morphogenesis, differ- tissues of the same organism. Varia- of estrogen to ER (8). entiation, and apoptosis, or pro- tions in the recruitment of coregulato- Estrogen is also capable of inducing grammed cell death. However, our ry molecules also appear to be the rapid effects via membrane-bound understanding of the pathways by mechanism by which selective estrogen ERs. Typically, the majority of either which sex steroid hormones influence receptor modulators (SERMs) produce ERα or ERβ is found in the nucleus. cellular functions is incomplete. their tissue-specific effects (3). However, small amounts (2%) can The classical mechanism of steroid A second mechanism of action for associate with the cell membrane. hormone action involves interactions the classic pathway involves protein- Accumulation of sex steroid hormone with intracellular receptors (Figure 1), protein interactions (Figure 1). Typi- receptors in the cell membrane is which are either cytoplasmic or nuclear cally, in this pathway, receptor-ligand patchy, suggesting the presence of (1, 2). Subsequently, the receptor-ligand complexes interact with transcrip- structures that are critical for their complex binds to specific cis-active ele- tion factors to alter the ability of the localization. Both ERα and ERβ as ments of the promoter regions in the transcription factor to influence gene well as AR can accumulate in caveolae, DNA of responsive genes. A key role of transcription. An example of this which are 50- to 100-nm flask-shaped sex steroid hormones in the regulation mechanism is the capacity of ligated- subcompartments in the cell mem- of gene transcription is the recruitment ER complexes to influence the func- brane that contains the 22-kDa trans- of a complex of coactivators and core- tion of activator protein-1 and spe- membrane phosphoprotein caveolin. pressors to the receptor-ligand-DNA cific protein-1 (4, 5). However, a After binding ligand, ERs or ARs in the binding site. This is true for estrogen common characteristic of both of cell membrane can interact with c-Src receptors (ERs) α and β as well as the these classic pathways is that changes and activate MAPK pathways (1, 9). in gene transcription typically occur In earlier investigations, Kousteni Address correspondence to: Joseph Lorenzo, after 30–60 minutes. and colleagues examined the role of The University of Connecticut Health Center, In contrast, it is now appreciated estrogen in preventing the bone loss Division of Endocrinology L3104, MC-1317, that more rapid nonclassical pathways that develops relatively rapidly in mice Farmington, Connecticut 06030-1317, USA. Phone: (860) 679-8199; Fax: (860) 679-1040; of sex steroid hormone action allow after sex steroid hormones are with- E-mail: [email protected]. both estrogens and androgens to drawn (10, 11). In this issue of the JCI, Conflict of interest: The author has declared influence cellular function. Typically, Kousteni et al. (12) now present data that no conflict of interest exists. these effects occur within seconds to that link the rapid activation of MAPK Nonstandard abbreviations used: estrogen minutes. In 1967, Szego and Davis (6) by nonclassical pathways to the ability receptor (ER); androgen receptor (AR); β selective estrogen receptor modulator (SERM); demonstrated that 17 -estradiol of sex steroid hormones to regulate extracellular signal–regulated kinase (ERK). given intravenously increased uterine apoptosis in bone cells. The Journal of Clinical Investigation | June 2003 | Volume 111 | Number 11 1641 Figure 1 Sex steroid hormones can affect cellular function by a variety of mechanisms. The illustration depicts the mechanisms by which estrogen influences cells. The classical path- ways (I and II) depend on direct interaction of estrogen with its receptor in the nucleus. Once activated, the estrogen-receptor com- plex can directly mediate gene transcription (I) or interact with transcription factors (II) to influence their activity. The nonclassical pathways (III and IV) work more rapidly and depend on the ability of estrogen to interact with either nonsteroid hormone receptors (III) or steroid hormone receptors in the membrane (IV). Both nonclassical pathways activate kinases that ultimately regulate tran- scription of specific genes. Adapted with per- mission from ref. 16. Kousteni et al. (9) previously demon- signaling mechanisms in mediating rent study (12), the authors postulate strated in cell culture that the ability of the ability of sex steroid hormones to that estren has a different mechanism estrogens or androgens to regulate block apoptosis in cell culture models. of action than do SERMs, since the lat- apoptosis was mediated by the ligand- In addition, key to their studies is the ter exclusively use the classical path- binding domain of sex steroid hormone use of the synthetic ligand 4-estren- ways to regulate gene transcription in receptors that were localized exclusively 3α,17β-diol, which the authors refer to a cell-specific manner. The authors to the cell membrane. This response as estren. This compound showed no believe, based on their in vitro data, was seen in HeLa cells transfected with classical sex steroid hormone activity that estren influences cellular function ERs or ARs and in osteoblast and osteo- in the examined cell culture systems. through its ability to influence cell cyte cell culture models. The ability of However, estren was a potent activator membrane–mediated nonclassical sex steroid hormones to regulate apop- of the rapid cell membrane–mediated responses to sex steroid hormones. tosis in these models was not specific Src-MAPK pathways in cell culture This is a provocative and controver- for a particular sex steroid hormone models. Furthermore, it blocked apop- sial hypothesis that requires addition- receptor, since estrogens or androgens tosis in osteoblast models and stimu- al investigation before it can be fully were equally effective in mediating lated apoptosis in osteoclast models. accepted. The skeleton is a complex responses in HeLa cells transfected with In contrast, raloxifene — a SERM cur- organ system that is regulated by the either ERs or ARs. The authors hypoth- rently approved to treat osteoporosis interactions of both bone-forming esized that a major effect of sex steroid — did not mimic the apoptosis-regu- osteoblasts (derived from mesenchy- hormones in preventing bone loss lating effects of estren in cell culture mal precursor cells) and bone-resorb- results from inhibition of apoptosis in models. In addition, studies published ing osteoclasts (derived from hemato- osteoblasts and stimulation of apopto- last