Andrew Lawrence Glasebrook

Identification of an Estrogen Response Element Activated by Metabolites of 17β-Estradiol and Raloxifene

17β-Estradiol modulates gene transcription through the estrogen receptor and the estrogen response element in DNA. The human transforming growth factor-β3 gene was shown to be activated by the estrogen receptor in the presence of estrogen metabolites or estrogen antagonists. Activation was mediated by a polypurine sequence, termed the raloxifene response element, and did not require the DNA binding domain of the estrogen receptor. Interaction of the estrogen receptor with the raloxifene response element appears to require a cellular adapter protein. The observation that individual estrogens modulate multiple DNA response elements may explain the tissue-selective estrogen agonist or antagonist activity of compounds such as raloxifene.

Environmental estrogens : Effects on cholesterol lowering and bone in the ovariectomized rat

Representative non-steroidal estrogens, from common environmental sources such as plants, pesticides, surfactants, plastics, and animal health products, demonstrated an ability to lower serum cholesterol and prevent bone loss. Specifically, select environmental estrogens (coumestrol, genistein, methoxychlor, bisphenol A, and zeranol) effectively lowered total serum cholesterol in an estrogen-dependent animal model, the ovariectomized rat. Of these entities, coumestrol, methoxychlor, and zeranol prevented ovariectomy-induced bone loss. In an in vitro environment, these compounds competed with 17beta-estradiol for estrogen receptor binding and stimulated cell proliferation in a human breast cancer cell line (MCF-7). In addition to their well-documented effects on reproductive tissue, various environmental estrogens can dramatically affect non-reproductive parameters such as cholesterol lowering and bone metabolism.

Raloxifene is a Tissue‐Selective Agonist/Antagonist That Functions through the Estrogen Receptor

Raloxifene (LY 139481), previously called keoxifene, is a benzothiophene derivative that binds to the estrogen receptor (ER) with high affinity (kd = 0.54-0.11 nM).’ This compound has tissue-specific activity, acting as an antiestrogen in breast and uterus, but functioning as an estrogen agonist in bone and on lipid metabolism. The molecular mechanisms of this tissue specificity are still under investigation, but preliminary evidence indicates that raloxifene-ER complexes associate with unique response elements, thus indicating that multiple transcriptional pathways may mediate these e f f e ~ t s . ~ ’ ~

Enhanced CD4+ T cell proliferation and Th2 cytokine production in DR6-deficient mice.

We have found that DR6, a member of the TNF receptor family, is highly expressed in resting T cells and downregulated in activated T cells. DR6-targeted mutant mice were generated and showed normal development. However, DR6(-/-) CD4(+) T cells hyperproliferated in response to TCR-mediated stimulation and protein antigen challenge. Activated DR6(-/-) CD4(+) T cells exhibited upregulated CD25 expression and enhanced proliferation in response to exogenous IL-2 stimulation. In addition, increased CD28 and reduced CTLA-4 expression were observed in these cells. Enhanced Th2 cytokine production by activated DR6(-/-) CD4(+) T cells was associated with the increased transcription factor NF-ATc in nuclei. DR6, therefore, functions as a regulatory receptor for mediating CD4(+) T cell activation and maintaining proper immune responses.

An estrogen receptor basis for raloxifene action in boneProceedings of Xth International Congress on Hormonal Steroids, Quebec, Canada, 17–21 June 1998.

Although controversy remains regarding direct effects of estrogen on bone, in vivo data clearly show that estrogens suppress bone turnover, resulting in decreased bone resorption and formation activity. Selective estrogen receptor modulators (SERMs), such as raloxifene, produce effects on bone which are very similar to those of estrogen. In vitro, both raloxifene and estrogen inhibit mammalian osteoclast differentiation and bone resorption activity, but only in the presence of IL-6. Data from a number of ovariectomized rat model manipulations (i.e. hypophysectomy, low calcium diet and drug combinations) demonstrate a strong parallel between the antiosteopenic effects of raloxifene and estrogen. A characteristic action of estrogens on the skeleton is inhibition of longitudinal bone growth, an effect which is not observed with other resorption inhibitors, including calcitonin and bisphosphonates. Consistent with an estrogen-like mechanism on bone, raloxifene inhibits longitudinal bone growth in growing rats. In addition to the overall similarity of the bone activity profile in animals, estrogen and raloxifene also produce similar effects on various signaling pathways relative to the antiosteopenic effect of these two agents. For example, IL-6, a cytokine involved in high turnover bone resorption following estrogen deficiency in rats, is suppressed by both raloxifene and estrogen. Raloxifene and estrogen also produce a similar activation of TGF-beta3 (a cytokine associated with inhibition of osteoclast differentiation and activity) in ovariectomized rats. Like 17beta-estradiol, raloxifene binds with high affinity to both estrogen receptor-alpha (ER alpha) and estrogen receptor-beta (ER beta). Crystal structure analyses have shown that 17beta-estradiol and raloxifene bind to ER alpha with small, but important, differences in three dimensional structure. These subtle differences in the conformation of the ligand:receptor complex are likely the basis for the key pharmacological differences between estrogens and the various SERMs (i.e. raloxifene vs tamoxifen). Raloxifene also produces estrogen-like effects on serum cholesterol metabolism and the vasculature. Thus, while raloxifene exhibits a complete estrogen antagonist in mammary tissue and the uterus, it produces beneficial effects on the cardiovascular system and prevents bone loss via an estrogen receptor mediated mechanism.

Raloxifene: A selective estrogen receptor modulator

Raloxifene is a benzothiophene derivative that was first examined ten years ago and has variously been referred to as keoxifene, LY139431, or LY156758. Raloxifene was classified generally as an anti-estrogen, because like tamoxifen it antagonizes the stiimulatory effects of estrogen on uterine weight. However, raloxifene inhibited the proliferation of the human breast cancer cell line, MCF-7, with IC50=0.2 nM compared to IC50=200 nM for tamoxifen; and histological analysis of uteri from ovariectomized rats showed that raloxifene does not significantly affect epithelial cell height or induce eosinophilia as tamoxifen does. At other sites, raloxifene has estrogen and tamoxifen-like effects on bone and serum cholesterol levels in ovariectomized rats. At 5 weeks, ovariectomy induced a 9% and 18% decrease in the bone mineral density of the lumbar vertebrae L1-4 and proximal tibiae, respectively. Raloxifene prevented this loss of bone with EC50=0.3 mg/kg/day for both the axial and appendicular skeleton. The bone effects of raloxifene could not be distinguished from estrogen effects, at the respective maximally efficacious doses. Raloxifene also lowered serum cholesterol levels to below Sham with ED50=0.2 mg/kg/day, in a manner similar to estrogen and tamoxifen. These data show that raloxifene has tissue specific effects that are distinct from either estrogen or tamoxifen. With breast tissue and uteri, raloxifene functions as a complete estrogen antagonist; but in bone and serum lipids, it behaves as a potent estrogen agonist. Because of this unique pharmmacological profile, we have classified raloxifene as a “selective estrogen receptor modulator (SERM).” We also note that the data taken together suggests that raloxifene may have tissue selective properties that may be advantageous to clinicians treating post-menopausal osteoporosis.

A pharmacological review of raloxifene

SummaryIn view of its highly tissue-selective pharmacological properties (i.e., relatively pure antagonist in reproductive tissue with minimal agonist effects to nearly full agonist properties in bone and on cholesterol metabolism), terms used to define compounds with slightly related pharmacology (i.e., antiestrogen, partial estrogen agonist) do not adequately describe raloxifenes activity. Thus, raloxifene is distinct from agents such as tamoxifen (which does stimulate the uterus), or frank estrogen (which do not sufficiently antagonize estrogens agonistic effects in reproductive tissue). In this regard, raloxifene and its pyrrolidine analogue, LY117018, (81) are the first representatives of a novel class of pharmacological agents, which we have termed “selective estrogen receptor modulator” (SERM). While we now have considerable evidence to distinguish estrogen recepto-mediated effects on bone from those on reproductive tissue, the precise mechanism for this tissue-specific mechanism remains an active area of investigation. Clearly, many important issues remain to be explored.

Evaluation of the major metabolites of raloxifene as modulators of tissue selectivity

Raloxifene (LY139481 HCl) is a selective estrogen receptor modulator (SERM) which blocks the effects of estrogen on some tissues, such as the breast and uterus, while mimicking estrogen in other tissues, such as bone. To study the origins of this unique pharmacology, we have prepared the major metabolites of raloxifene as chemical probes for examining the estrogen receptor function in vitro and in vivo. In human breast cancer cell (MCF-7) related assays, these glucuronide conjugates show little affinity for the estrogen receptor and are more than two orders of magnitude less potent at inhibiting cell proliferation than raloxifene. In non-traditional estrogen target tissue, such as bone, these metabolites are less effective than the parent at inhibiting cytokine-stimulated bone resorbing activity in rat osteoclasts or producing transforming growth factor beta-3 (TGF-beta3). In animal models, tissue distribution studies with radiolabelled metabolite indicate that conversion to raloxifene occurs readily in a variety of tissues including the liver, lung, spleen, kidney, bone and uterus. Differential conversion of metabolite in target organs, such as bone and the uterus, is not observed indicating that the origin of raloxifenes pharmacology does not result from tissue-selective deconjugation of metabolite to parent.

Estriol: A potent regulator of TNF and IL-6 expression in a murine model of endotoxemia

The increased incidence of autoimmune disease in premenopausal women suggests the involvement of sex steroids in the pathogenesis of these disease processes. The effects of estrogen on autoimmunity and inflammation may involve changes in the secretion of inflammatory mediators by mononuclear phagocytes. Estradiol, for example, has been reported to regulate TNF, IL-6, IL-1 and JE expression. In the present study the effects of the estrogen agonist, estriol, on cytokine expression have been investigated in mice administered a sublethal lipopolysaccharide, LPS, challenge. Pretreatment of mice with pharmacologic doses of estriol, 0.4–2 mg/kg, resulted in a significant increase in serum TNF levels in both control and autoimmune MRL/lpr mice, following LPS challenge. This increase in TNF over the placebo group was blocked by the estrogen antagonist tamoxifen. Estriol treated mice also exhibited a rapid elevation in serum IL-6 levels following LPS challenge with the peak increase occurring 1 hr post LPS. This contrasted with the placebo group in which maximal serum IL-6 levels were detected at 3 hrs post challenge. This shift in the kinetics of IL-6 increase by estriol was inhibited by tamoxifen. The estriol mediated effects on TNF and IL-6 serum levels were consistent with the changes in TNF and IL-6 mRNA observed ex vivo in elicited peritoneal macrophages. Macrophage cultures from estriol treated animals however, did not demonstrate significant differences from the placebo group for TNF or NO secretion following in vitro LPS challenge. These results suggest that the estrogen agonist estriol can have significant quantitative, TNF, and kinetic, IL-6, effects on inflammatory monokines produced in response to an endotoxin challenge.

Enhanced B Cell Expansion, Survival, and Humoral Responses by Targeting Death Receptor 6

Targeted disruption of death receptor (DR)6 results in enhanced CD4+ T cell expansion and T helper cell type 2 differentiation after stimulation. Similar to T cells, DR6 is expressed on resting B cells but is down-regulated upon activation. We examined DR6−/− B cell responses both in vitro and in vivo. In vitro, DR6−/− B cells undergo increased proliferation in response to anti–immunoglobulin M, anti-CD40, and lipopolysaccharide. This hyperproliferative response was due, at least in part, to both increased cell division and reduced cell apoptosis when compared with wild-type B cells. Consistent with these observations, increased nuclear levels and activity of nuclear factor κB transcription factor, c-Rel, and elevated Bcl-xl expression were observed in DR6−/− B cells upon stimulation. In addition, DR6−/− B cells exhibited higher surface levels of CD86 upon activation and were more effective as antigen-presenting cells in an allogeneic T cell proliferation response. DR6−/− mice exhibited enhanced germinal center formation and increased titers of immunoglobulins to T-dependent as well as T-independent type I and II antigens. This is the first demonstration of a regulatory role of DR6 in the activation and function of B cells.