Estrogen stimulates protein tyrosine phosphorylation and Src kinase activity in avian osteoclasts

Ventromedial Hypothalamus and the Generation of Aggression

Aggression is a costly behavior, sometimes with severe consequences including death. Yet aggression is prevalent across animal species ranging from insects to humans, demonstrating its essential role in the survival of individuals and groups. The question of how the brain decides when to generate this costly behavior has intrigued neuroscientists for over a century and has led to the identification of relevant neural substrates. Various lesion and electric stimulation experiments have revealed that the hypothalamus, an ancient structure situated deep in the brain, is essential for expressing aggressive behaviors. More recently, studies using precise circuit manipulation tools have identified a small subnucleus in the medial hypothalamus, the ventrolateral part of the ventromedial hypothalamus (VMHvl), as a key structure for driving both aggression and aggression-seeking behaviors. Here, we provide an updated summary of the evidence that supports a role of the VMHvl in aggressive behaviors. We will consider our recent findings detailing the physiological response properties of populations of VMHvl cells during aggressive behaviors and provide new understanding regarding the role of the VMHvl embedded within the larger whole-brain circuit for social sensation and action.

Estrogen inhibits RANKL-stimulated osteoclastic differentiation of human monocytes through estrogen and RANKL-regulated interaction of estrogen receptor-alpha with BCAR1 and Traf6

The effects of estrogen on osteoclast survival and differentiation were studied using CD14-selected mononuclear osteoclast precursors from peripheral blood. Estradiol at approximately 1 nM reduced RANKL-dependent osteoclast differentiation by 40-50%. Osteoclast differentiation was suppressed 14 days after addition of RANKL even when estradiol was withdrawn after 18 h. In CD14+ cells apoptosis was rare and was not augmented by RANKL or by 17-beta-estradiol. Estrogen receptor-alpha (ERalpha) expression was strongly down-regulated by RANKL, whether or not estradiol was present. Mature human osteoclasts thus cannot respond to estrogen via ERalpha. However, ERalpha was present in CD14+ osteoclast progenitors, and a scaffolding protein, BCAR1, which binds ERalpha in the presence of estrogen, was abundant. Immunoprecipitation showed rapid (approximately 5 min) estrogen-dependent formation of ERalpha-BCAR1 complexes, which were increased by RANKL co-treatment. The RANKL-signaling intermediate Traf6, which regulates NF-kappaB activity, precipitated with this complex. Reduction of NF-kappaB nuclear localization occurred within 30 min of RANKL stimulation, and estradiol inhibited the phosphorylation of IkappaB in response to RANKL. Inhibition by estradiol was abolished by siRNA knockdown of BCAR1. We conclude that estrogen directly, but only partially, curtails human osteoclast formation. This effect requires BCAR1 and involves a non-genomic interaction with ERalpha.

G protein-coupled receptor 30 is an estrogen receptor in the plasma membrane

Recently, GPR30 was reported to be a novel estrogen receptor; however, its intracellular localization has remained controversial. To investigate the intracellular localization of GPR30 in vivo, we produced four kinds of polyclonal antibodies for distinct epitopes on GPR30. Immunocytochemical observations using anti-GPR30 antibody and anti-FLAG antibody show that FLAG-GPR30 localizes to the plasma membrane 24 h after transfection. Treatment with estrogen (17beta-estradiol or E2) causes an elevation in the intracellular Ca2+ concentration ([Ca2+]i) within 10 s in HeLa cells expressing FLAG-GPR30. In addition, E2 induces the translocation of GPR30 from the plasma membrane to the cytoplasm by 1 h after stimulation. Immunohistochemical analysis shows that GPR30 exists on the cell surface of CA2 pyramidal neuronal cells. The images on transmission electron microscopy show that GPR30 is localized to a particular region associated with the plasma membranes of the pyramidal cells. These data indicate that GPR30, a transmembrane receptor for estrogen, is localized to the plasma membrane of CA2 pyramidal neuronal cells of the hippocampus in rat brain.

Stimulatory effect of naturally occurring flavonols quercetin and kaempferol on alkaline phosphatase activity in MG-63 human osteoblasts through ERK and estrogen receptor pathway

Many plant-derived substances have estrogenic activities. Due to their ability to bind the estrogen receptor (ER), these compounds have the potential to counteract the deleterious effects of estrogen deficiency on bone. In this study, we investigated the in vitro effect of two widespread flavonols, quercetin and kaempferol, on alkaline phosphatase (ALP) activity in MG-63 cultured human osteoblasts. We found that both flavonols significantly increased ALP activity. This effect was markedly reduced by PD 98059, an inhibitor of the extracellular regulated kinase (ERK) pathway, and by ICI 182780, an antagonist of ERs. Western blot studies confirmed that ERK is rapidly activated in cells treated by both flavonols. Finally, ICI 182780 markedly inhibits the flavonol-induced ERK activation. The data presented in this study support the conclusion that, in MG-63 osteoblasts (i) the increase in ALP activity by flavonols involves a rapid stimulation of ERK activation but also involves the ER, and that (ii) the activation of ERK by flavonols occurs most likely downstream of the ERs activation. Taken together, these results suggest that flavonols derivatives as quercetin and kaempferol can stimulate osteoblastic activity. Such compounds may represent new pharmacological tools for the treatment of osteoporosis.

17beta-estradiol downregulates beta3-integrin expression in differentiating and mature human osteoclasts

The increased bone resorption observed after estrogen withdrawal is responsible for bone loss and may lead to osteoporosis. The mechanism by which estradiol inhibits bone resorption is known to involve decreased osteoclastogenesis, however, the effect on osteoclast adhesion remains unclear. We examined the in vitro effect of estradiol and raloxifene on human osteoclast differentiation and function. Human peripheral blood mononuclear cells were cultured with M-CSF/RANK-L for 18 days, and we evaluated bone resorption, the expression of the protein and mRNA of the integrins, c-jun and c-fos in the presence or absence of estradiol. In this human model, beta3-integrin expression increased at the mRNA and protein levels during osteoclast differentiation, whereas that of beta5-integrin did not. We found that estradiol and raloxifene directly inhibited bone resorption on bone slices by 50%, and decreased the expression of beta3-integrin mRNA (60%) and protein (20%) in a time-dependent manner. Moreover, the mRNAs of c-fos and c-jun were both diminished by estradiol and raloxifene, particularly in early osteoclasts, but also to a lesser extent in mature cells. These findings suggest that the direct inhibitory action of estradiol on bone resorption may affect human osteoclast differentiation through downregulation of c-fos and c-jun and adhesion through modulation of beta3-integrin.

Membrane receptors for steroid hormones: signal transduction and physiological significance

Membrane receptors for steroid hormones affect signaling pathways that modulate nuclear function, influence neuronal activity, ion flow, and the circulatory system. Indeed, 'new' steroid hormones have been identified by their interaction with membrane-initiated signaling systems. A brief summary of the FASEB Summer Research Conference devoted to these topics is presented in this mini-review. In addition, attendees of the meeting propose introduction of the following terminology: membrane-initiated steroid signaling (MISS) and nuclear-initiated steroid signaling (NISS) to replace more inaccurate terms in current use.

Src kinase mediates phosphatidylinositol 3-kinase/Akt-dependent rapid endothelial nitric-oxide synthase activation by estrogen

17beta-Estradiol activates endothelial nitric oxide synthase (eNOS), enhancing nitric oxide (NO) release from endothelial cells via the phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway. The upstream regulators of this pathway are unknown. We now demonstrate that 17beta-estradiol rapidly activates eNOS through Src kinase in human endothelial cells. The Src family kinase specific-inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) abrogates 17beta-estradiol- but not ionomycin-stimulated NO release. Consistent with these results, PP2 blocked 17beta-estradiol-induced Akt phosphorylation but did not inhibit NO release from cells transduced with a constitutively active Akt. PP2 abrogated 17beta-estradiol-induced activation of PI3-kinase, indicating that the PP2-inhibitable kinase is upstream of PI3-kinase and Akt. A 17beta-estradiol-induced estrogen receptor/c-Src association correlated with rapid c-Src phosphorylation. Moreover, transfection of kinase-dead c-Src inhibited 17beta-estradiol-induced Akt phosphorylation, whereas constitutively active c-Src increased basal Akt phosphorylation. Estrogen stimulation of murine embryonic fibroblasts with homozygous deletions of the c-src, fyn, and yes genes failed to induce Akt phosphorylation, whereas cells maintaining c-Src expression demonstrated estrogen-induced Akt activation. Estrogen rapidly activated c-Src inducing an estrogen receptor, c-Src, and P85 (regulatory subunit of PI3-kinase) complex formation. This complex formation results in the successive activation of PI3-kinase, Akt, and eNOS with consequent enhanced NO release, implicating c-Src as a critical upstream regulator of the estrogen-stimulated PI3-kinase/Akt/eNOS pathway.

Sex steroid hormones act as growth factors

We observed that sex steroid hormones, like growth factors, stimulate the Src/Ras/erk pathway of cell lines derived from human mammary or prostate cancers. In addition, hormone-dependent pathway activation can be induced in Cos cells, upon transfection of classic steroid receptors. Cross-talks between sex steroid receptors regulate their association with Src and consequent pathway activation. Oestradiol treatment of MCF-7 cells triggers simultaneous association of ER with Src and p85, the regulatory subunit of phosphatidylinositol-3-kinase (PI3-kinase) and activation of Src- and PI3-K-dependent pathways. Activation of the latter pathway triggers cyclin D1 transcription, that is unaffected by Mek-1 activation. This suggests that simultaneous activation of different signalling effectors is required to target different cell cycle components. Thus, a novel reciprocal cross-talk between the two pathways appears to be mediated by the ER. In all tested cells, activation of the signalling pathways has a proliferative role. Transcriptionally inactive ER expressed in NIH 3T3 cells responds to hormone causing Src/Ras/Erk pathway activation and DNA synthesis. This suggests that in these cells genomic activity is required for later events of cell growth.

Down-regulation of osteoclast differentiation by daidzein via caspase 3

Phytoestrogens are plant-derived compounds with estrogen-like activity. Phytoestrogen-rich diets may prevent postmenopausal osteoporosis and these molecules maintain bone mass in ovariectomized animals. We compared the effects of the isoflavone daidzein, which has no action on tyrosine kinases, and 17beta-estradiol on the development and activity of osteoclasts in vitro. Nonadherent porcine bone marrow cells were cultured on dentine slices or on culture slides in the presence of 10-8 M of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], with or without 10(-8) M of daidzein, 10(-8) M of 17beta-estradiol for 9-11 days. Multinucleated tartrate-resistant acid phosphatase-positive (TRAP+) cells that resorbed bone (osteoclasts) developed in the presence of 1,25(OH)2D3. The number of osteoclasts formed in response to 1,25(OH)2D3 was reduced by 58 +/- 8% by daidzein and 52 +/- 5% by estrogen (p < 0.01); these effects were reversed by 10-6 M of ICI 182,780. The area resorbed by mature osteoclasts was reduced by 39 +/- 5% by daidzein and 42 +/- 6% by estradiol (p < 0.01). Both compounds also inhibited the 1,25(OH)2D3-induced differentiation of osteoclast progenitors (mononucleated TRAP+ cells), 53 +/- 8% by daidzein and 50 +/- 7% by estradiol (p < 0.05). Moreover, daidzein and estradiol promoted caspase-8 and caspase-3 cleavage and DNA fragmentation of monocytic bone marrow cells. Caspase-3 cleavage was reversed by 10-8 M of ICI 182,780. Both compounds up-regulated the expression of nuclear estrogen receptors ER-alpha and ER-beta. Thus, daidzein, at the same concentration as 17beta-estradiol, inhibits osteoclast differentiation and activity. This may be caused by, at least in part, greater apoptosis of osteoclast progenitors mediated by ERs.

The Src signaling pathway regulates osteoclast lysosomal enzyme secretion and is rapidly modulated by estrogen

To investigate the role of the pp60src signaling pathway in osteoclast activity, we have used dominant negative pp60src, c-ras, and c-raf expression vectors to individually disrupt their functions in osteoclasts. Osteoclasts were transiently transfected and secretions of cathepsin B/K and tartrate-resistant acid phosphatase (TRAP) were monitored. Expression of these constructs increased secretion of lysosomal enzymes. In contrast, constitutively active pp60src expression caused decreased lysosomal enzyme secretion. Osteoclasts respond to 17-beta estradiol (17betaE2) treatment with decreased lysosomal enzyme secretion. Therefore, we investigated the effects of E2 on pp60src kinase activity and observed an E2 time- and dose-dependent decrease in cytoskeletal membrane-associated pp60src tyrosine kinase activity. We have shown that estrogen decreases lysosomal enzyme gene expression and secretion; so we have examined the effects of the expression constructs on estrogen regulation of enzyme secretion. Constitutively active pp60src blocked E2 effects on secretion whereas expression of dominant negative pp60src, c-Ras, or c-Raf enhanced E2 effects. These data support that the kinase domain of cytoskeletal-associated pp60src is likely to be involved in the regulation of lysosomal enzyme secretion.