MNAR plays an important role in ERa activation of Src/MAPK and PI3K/Akt signaling pathways

Estrogens play a critical role in the regulation of cellular proliferation, differentiation, and apoptosis. Evidence indicates that this regulation is mediated by a complex interface of direct control of gene expression (so-called "genomic action") and by regulation of cell-signaling/phosphorylation cascades (referred to as the "non-genomic", or "extranuclear" action). However, the mechanisms of the non-genomic action of estrogens are not well defined. We have recently described the identification of a novel scaffold protein termed MNAR (modulator of non-genomic action of estrogen receptor), that couples conventional steroid receptors with extranuclear signal transduction pathways, thus potentially providing additional and tissue- or cell-specific level of steroid hormone regulation of cell functions. We have demonstrated that the MNAR is required for ER alpha (ERa) interaction with p60(src) (Src), which leads to activation of Src/MAPK pathway. Our new data also suggest that activation of cSrc in response to E2 leads to MNAR phosphorylation, interaction with p85, and activation of the PI3 and Akt kinases. These data therefore suggest that MNAR acts as an important scaffold that integrates ERa action in regulation of important signaling pathways. ERa non-genomic action has been suggested to play a key role in estrogen-induced cardio-, neuro-, and osteo-protection. Therefore, evaluation of the molecular crosstalk between MNAR and ERa may lead to development of functionally selective ER modulators that can separate between beneficial, prodifferentiative effects in bone, the cardiovascular system and the CNS and the "detrimental", proliferative effects in reproductive tissues and organs.     

Direct binding and activation of protein kinase C isoforms by steroid hormones

The non-genomic action of steroid hormones regulates a wide variety of cellular responses including regulation of ion transport, cell proliferation, migration, death and differentiation. In order to achieve such plethora of effects steroid hormones utilize nearly all known signal transduction pathways. One of the key signalling molecules regulating the non-genomic action of steroid hormones is protein kinase C (PKC). It is thought that rapid action of steroids hormones results from the activation of plasma membrane receptors; however, their molecular identity remains elusive. In recent years, an increasing number of studies have pointed at the selective binding and activation of specific PKC isoforms by steroid hormones. This has led to the hypothesis that PKC could act as a receptor as well as a transducer of the non-genomic effects of these hormones. In this review we summarize the current knowledge of the direct binding and activation of PKC by steroid hormones.     

Dexamethasone induces rapid tyrosine-phosphorylation of ZAP-70 in Jurkat cells

Steroid hormones are known to mediate rapid non-genomic effects occurring within minutes, besides the classical genomic actions mediated by the nuclear translocation of the cytoplasmic glucocorticoid receptor (GR). The glucocorticoid hormone (GC) has significant role in the regulation of T-cell activation; however, the cross-talk between the GC and T-cell receptor (TcR) signal transducing pathways are still to be elucidated. We examined the rapid effects of GC exposure on in vitro cultured human T-cells. Our results showed that Dexamethasone (DX), a GC analogue, when applied at high dose (10 microM), induced rapid (within 5 min) tyrosine-phosphorylation events in Jurkat cells. Short DX pre-treatment strongly inhibited the tyrosine-phosphorylation stimulated by CD3 cross-linking. Furthermore, we also investigated the phosphorylation status of ZAP-70, an important member of tyrosine kinase mediated signalling pathway of TcR-elicited T-cell activation. Here, we demonstrate that high dose DX induced a rapid ZAP-70 tyrosine-phosphorylation in Jurkat T-cells. DX-induced ZAP-70 phosphorylation could be inhibited by RU486 (GR antagonist), suggesting that this process was GR mediated. DX-induced ZAP-70 phosphorylation did not occur in the absence of active p56-lck as examined in the p56-lck kinase-deficient Jurkat cell line JCaM1.6. Our results show that DX, at a high dose, can rapidly influence the initial tyrosine-phosphorylation events of the CD3 signalling pathway in Jurkat cells, thereby modifying TcR-derived signals. Lck and ZAP-70 represent an important molecular link between the TcR and GC signalling pathways.     

Non-genomic oestrogen receptor signal in B lymphocytes: An approach towards therapeutic interventions for infection,autoimmunity and cancer

The non-genomic membrane bound oestrogen receptor (mER) regulates intracellular signals through receptor-ligand interactions. The mER, along with G-protein coupled oestrogen receptor GPR 30 (GPER), induces diverse cell signalling pathways in murine lymphocytes. The mER isoform ER-alpha46 has recently been demonstrated in human B and T lymphocytes as an analogue receptor for chemokine CCL18, the signalling events of which are not clearly understood. Ligand-induced mER and GPER signalling events are shared with BCR, CD19 mediated intracellular signalling through phospholipase C, PIP2/IP3/PI3 mediated activation of Akt, MAP kinase, and mTOR. Oestrogen has the ability to induce CD40-mediated activation of B cells. The complete signalling pathways of mER, GPR30 and their interaction with other signals are targeted areas for novel drug development in B cells during infection, autoimmunity and cancer. Therefore, an in depth investigation is critical for determining shared signal outputs during B cell activation. Here, we focus on the mode of action of membrane bound ER in B cells as therapeutic checkpoints.     

The direct action of estrone on vascular tissue involves genomic and non-genomic actions

A two step model mechanism of steroid action has been recently postulated. In this study, we test the hypothesis that, the biochemical action of estrone (E(1)) on vascular tissue could be performed via genomic and non-genomic actions. Rat aortic rings or vascular smooth muscle cell cultures (VSMC) were used to test the effect of the hormone on nitric oxide (NO) production, protein kinases activities and cell proliferation. Our data showed that estrone increased NO synthesis between 30 s and 20 min treatment, and this stimulatory effect was dependent on MAPK cascade activation, since it was prevented in the presence of a MAPK inhibitor (PD98059). Using a phosphorylation assay, we also showed that E(1) significantly increased MAPK activity. The effect of the hormone on PKC activity was measured in concentrations and time course studies. Direct treatment of rat aortic homogenates with E(1) significantly enhanced PKC activity (1-10 fold increase, p<0.01) at all concentrations (1; 10; 50 nM) and time tested (1-10 min). We demonstrated that 24 h of E(1) treatment markedly increased VSMC proliferation (53% above control), and this effect was suppressed by a PKC inhibitor. The rapid and the long term effects of the hormone were completely suppressed in the presence of an estradiol receptor antagonist (ICI 182780). In summary, we provided evidence that, the steroid exerts both non-genomic and genomic actions, the former associated with MAPK kinase dependent on NO production, and the latter related with induction of VSMC proliferation involving PKC pathway activation.     

Mitogen activated protein kinase (MAPK) mediates non-genomic pathway of estrogen on T cell cytokine production following trauma-hemorrhage

Although studies have shown 17beta-estradiol (E2) administration following trauma-hemorrhage (T-H) attenuates alterations in T cell cytokine production, it remains unknown whether such effects of E2 are mediated via genomic or non-genomic pathways. In this study, we determined the non-genomic effects of E2 on splenic T cell cytokine production and the role of MAPK following T-H. Male Sprague-Dawley rats underwent T-H (mean BP 40 mmHg for 90 min, then resuscitation). E2, E2 conjugated with BSA (E2-BSA, 1 mg/kg E2) with or without an estrogen receptor antagonist (ICI 182 780), or vehicle was administered during resuscitation. Two hours thereafter, T cell production of IL-2 and IFN-gamma and activation of MAPK (p38, ERK-1/2 and JNK) were determined. The effect of selective MAPK inhibitors on cytokine production was also examined in vitro. IL-2 and IFN-gamma production capacity and MAPK activation decreased in T cells following T-H. However, E2 administration normalized these parameters. Although E2-BSA administration also attenuated suppression in cytokine production, the values were lower compared to sham. In contrast, E2-BSA prevented T-H-induced suppression in MAPK activation to the same extent as E2. Co-administration of ICI 182 780 abolished E2-BSA effects. These findings suggest E2 effects on T cell cytokine production following T-H are mediated at least in part via non-genomic pathway and these non-genomic effects are likely mediated via MAPK pathways.     

Sensor types in signal transduction pathways in plant cells responding to abiotic stressors: do they depend on stress intensity?

Despite the fast growing knowledge about the components of the signalling pathways involved in the activation of drought‐, cold‐, osmotic stress‐ and salt‐responsive genes, relatively little is known about the sensor types responsible for the induction of the pathways. It is thought that different signals have their own cognate receptors which independently, or in cooperation, initiate a downstream signalling cascade ( Xiong and Zhu 2002 ). On the other hand, the stress level‐dependent activation of different receptors has been proposed in plants responding to osmotic stress ( Munnik and Meijer 2001 ). The question arises as to whether the activation of different signalling systems will depend on the nature of the sensors or on the stress‐induced primary event, which may differ in cells subjected to moderate or severe stress. In this article, a discussion is given of the available literature data concerning this controversial question. It is proposed that, in plants responding to mild stress, a disturbance of water balance is the primary stress‐induced event affecting the cell wall–plasma membrane interactions, resulting in the activation of receptor‐like kinases, including wall‐associated kinases, cytoskeleton‐related mechanosensors, stretch‐dependent ion (calcium) channels and redox‐mediated systems. The mild stress‐sensing systems, assisted by an increased supply of abscisic acid, seem to be involved in the activation of the pathways that enable the adjustment of plant growth and metabolism to the stressful conditions, i.e. allowing acclimation. Severe or suddenly acting stressors are sensed by membrane destabilization (membrane depolarization, alterations in ion transport systems), which results in the triggering of phospholipid signalling. This may lead to the increased production of reactive oxygen species, the accumulation of H₂O₂, lipid peroxidation and increased synthesis of hormones such as jasmonates and ethylene. These are characteristic features of the alarm situation, which may result in irreversible injury and cell death, or in cell recovery, depending on stress impact.