Voltage-insensitive Ca2+ channels and Ca2+/calmodulin-dependent protein kinases propagate signals from endothelin-1 receptors to the c-fos promoter

The Effect of Chronic Fluorosis on Calcium Ions and CaMKIIα, and c-fos Expression in the Rat Hippocampus

This study investigated neurotoxicity of chronic fluorosis in the rat hippocampus. Newly weaning, male, Sprague-Dawley (SD) rats were administered 15, 30, and 60 mg/L sodium fluoride (NaF) solution (fluorine ion concentration 8.25, 16.50, and 33.00 mg/L, respectively), and tap water, for 18 months. The neurotoxicological mechanism was examined with a focus on intracellular calcium overload. Results showed that as the fluoride concentration increased, calcium ion concentration [Ca²⁺], the expression of calcium/calmodulin-dependent protein kinase II α (CaMKIIα), and the expression of catus proto-oncogene protein c-fos (c-fos) all tend to increase. Compared to the control group, Ca²⁺, CaMKIIα, and c-fos significantly increased (P < 0.05) in the moderate-fluoride and the high-fluoride groups. These results indicate that Ca²⁺/CaMKIIα/c-fos channel signal may be the molecular mechanism of central nervous system damage caused by chronic fluoride intoxication. Moreover, elevated Ca²⁺ concentration in the hippocampus may be the initiating factor of neuronal apoptosis induced by fluoride.

CaMKII is essential for the cellular clock and coupling between morning and evening behavioral rhythms

Daily behavioral rhythms in mammals are governed by the central circadian clock located in the suprachiasmatic nucleus (SCN), and calcium is thought to play a role in the oscillation of the SCN. Kon et al. now find that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity promotes dimerization of CLOCK and BMAL1 and is essential for the cellular oscillation and synchronization among oscillators in the SCN. Kinase-dead CaMKIIα weakened the behavioral rhythmicity and elicited decoupling between the morning and evening activity rhythms.

Daily behavioral rhythms in mammals are governed by the central circadian clock, located in the suprachiasmatic nucleus (SCN). The behavioral rhythms persist even in constant darkness, with a stable activity time due to coupling between two oscillators that determine the morning and evening activities. Accumulating evidence supports a prerequisite role for Ca²⁺ in the robust oscillation of the SCN, yet the underlying molecular mechanism remains elusive. Here, we show that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) activity is essential for not only the cellular oscillation but also synchronization among oscillators in the SCN. A kinase-dead mutation in mouse CaMKIIα weakened the behavioral rhythmicity and elicited decoupling between the morning and evening activity rhythms, sometimes causing arrhythmicity. In the mutant SCN, the right and left nuclei showed uncoupled oscillations. Cellular and biochemical analyses revealed that Ca²⁺–calmodulin–CaMKII signaling contributes to activation of E-box-dependent gene expression through promoting dimerization of circadian locomotor output cycles kaput (CLOCK) and brain and muscle Arnt-like protein 1 (BMAL1). These results demonstrate a dual role of CaMKII as a component of cell-autonomous clockwork and as a synchronizer integrating circadian behavioral activities.

The role of transforming growth factor β1 in the regulation of blood pressure

Although human association studies suggest a link between polymorphisms in the gene encoding transforming growth factor (TGF) β1 and differing blood pressure levels, a causative mechanism for this correlation remains elusive. Recently we have generated a series of mice with graded expression of TGFβ1, ranging from approximately 10% to 300% compared to normal. We have found that blood pressure and plasma volume are negatively regulated by TGFβ1. Of note, the 10% hypomorph exhibits primary aldosteronism and markedly impaired urinary excretion of water and electrolytes. We here review previous literature highlighting the importance of TGFβ signaling as a natriuretic system, which we postulate is a causative mechanism explaining how polymorphisms in TGFβ1 could influence blood pressure levels.

Alpha-CaMKII plays a critical role in determining the aggressive behavior of human osteosarcoma

Osteosarcoma is among the most frequently occurring primary bone tumors, primarily affecting adolescents and young adults. Despite improvements in osteosarcoma treatment, more specific molecular targets are needed as potential therapeutic options. One target of interest is α-Ca(2+)/calmodulin-dependent protein kinase II (α-CaMKII), a ubiquitous mediator of Ca(2+)-linked signaling, which has been shown to regulate tumor cell proliferation and differentiation. Here, we investigate the role of α-CaMKII in the growth and tumorigenicity of human osteosarcoma. We show that α-CaMKII is highly expressed in primary osteosarcoma tissue derived from 114 patients, and is expressed in varying levels in different human osteosarcoma (OS) cell lines [MG-63, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)/HOS, and 143B). To examine whether α-CaMKII regulates osteosarcoma tumorigenic properties, we genetically inhibited α-CaMKII in two osteosarcoma cell lines using two different α-CaMKII shRNAs delivered by lentiviral vectors and overexpressed α-CaMKII by retrovirus. The genetic deletion of α-CaMKII by short hairpin RNA (shRNA) in MG-63 and 143B cells resulted in decreased proliferation (50% and 41%), migration (22% and 25%), and invasion (95% and 90%), respectively. The overexpression of α-CaMKII in HOS cells resulted in increased proliferation (240%), migration (640%), and invasion (10,000%). Furthermore, α-CaMKII deletion in MG-63 cells significantly reduced tumor burden in vivo (65%), whereas α-CaMKII overexpression resulted in tumor formation in a previously nontumor forming osteosarcoma cell line (HOS). Our results suggest that α-CaMKII plays a critical role in determining the aggressive phenotype of osteosarcoma, and its inhibition could be an attractive therapeutic target to combat this devastating adolescent disease.

AMP-activated protein kinase inhibits transforming growth factor-beta-induced Smad3-dependent transcription and myofibroblast transdifferentiation

In wound healing, myofibroblast transdifferentiation (MFT) is a metaplastic change in phenotype producing profibrotic effector cells that secrete and remodel the extracellular matrix. Unlike pathways that induce MFT, the molecular mechanisms that negatively regulate MFT are poorly understood. Here, we report that AMP-activated protein kinase (AMPK) blocks MFT in response to transforming growth factor-beta (TGFbeta). Pharmacological activation of AMPK inhibited TGFbeta-induced secretion of extracellular matrix proteins collagen types I and IV and fibronectin. AMPK activation also prevented induction of the myofibroblast phenotype markers alpha-smooth muscle actin and the ED-A fibronectin splice variant. AMPK activators did not prevent MFT in cells transduced with an adenovirus expressing dominant negative, kinase-dead AMPKalpha2. Moreover, AMPK activators did not inhibit MFT induction in AMPK(alpha1,2)(-/-) fibroblasts, demonstrating a requirement for AMPK(alpha) expression. Adenoviral transduction of constitutively active AMPK(alpha2) was sufficient to prevent TGFbeta-induced collagen I, alpha-smooth muscle actin, and ED-A fibronectin. AMPK did not reduce TGFbeta-stimulated Smad3 COOH-terminal phosphorylation and nuclear translocation, which are necessary for MFT. However, AMPK activation inhibited TGFbeta-induced transcription driven by Smad3-binding cis-elements. Consistent with a role for AMPK in transcriptional regulation, nuclear translocation of AMPKalpha2 correlated with the appearance of active AMPKalpha in the nucleus. Collectively, these results demonstrate that AMPK inhibits TGFbeta-induced transcription downstream of Smad3 COOH-terminal phosphorylation and nuclear translocation. Furthermore, activation of AMPK is sufficient to negatively regulate MFT in vitro.

Cadmium activates CaMK-II and initiates CaMK-II-dependent apoptosis in mesangial cells

Cadmium is a toxic metal that initiates both mitogenic responses and cell death. We show that Cd(2+) increases phosphorylation and activity of Ca(2+)/calmodulin-dependent protein kinase II (CaMK-II) in mesangial cells, in a concentration-dependent manner. Activation is biphasic with peaks at 1-5 min and 4-6 h. Cadmium also activates Erk, but this appears to be independent of CaMK-II. At 10-20 microM, Cd(2+) initiates apoptosis in 25-55% of mesangial cells by 6h. Inhibition of CaMK-II, but not of Erk, suppresses Cd(2+)-induced apoptosis. We conclude that activation of CaMK-II by Cd(2+) contributes to apoptotic cell death, independent of Erk activation.

Coping with excess salt: adaptive functions of extrarenal osmoregulatory organs in vertebrates

In all organisms, changing environmental conditions require appropriate regulatory measures to physiologically adjust to the altered situation. Uptake of excess salt in non-mammalian vertebrates having limited or no access to freshwater is balanced by extrarenal salt excretion through specialized structures called 'salt glands'. Nasal salt glands of marine birds are usually fully developed in very early stages of their lives since individuals of these species are exposed to salt soon after hatching. In individuals of other bird species, salt uptake may occur infrequently. In these animals, glands are usually quiescent and glandular cells are kept in a fairly undifferentiated state. This is the situation in 'naive' ducklings, Anas platyrhynchos, which have never been exposed to excess salt. When these animals become initially osmotically stressed, the nasal glands start to secrete a moderately hypertonic sodium chloride solution but secretory performance is meager. Within 48 h after the initial stimulus, however, the number of cells per gland is elevated by a factor of 2-3, the secretory cells differentiate and acquire full secretory capacity. During this differentiation process, extensive surface specializations are formed. The number of mitochondria is increased and metabolic enzymes and transporters are upregulated. These adaptive growth and differentiation processes result in a much higher efficiency of salt excretion in acclimated ducklings compared with naive animals. Receptors and signal transduction pathways in salt gland cells controling the adaptive processes seem to be the same as those controling salt secretion, namely muscarinic acetylcholine receptors and receptors for vasoactive intestinal peptide. This review focusses on signal transduction pathways activated by muscarinic receptors which seem to fine-tune salt secretion in salt-adapted ducklings and may control adaptive growth and differentiation processes in the nasal gland of naive animals.

Role of Ca2+/calmodulin-dependent kinases in skeletal muscle plasticity

In skeletal muscle, the increase in intracellular Ca(2+) resulting from motor activation plays a key role in both contractile activity-dependent and fiber type-specific gene expression. These motor activation-dependent signals are linked to the amplitude and duration of the Ca(2+) transients that are decoded downstream by Ca(2+)-dependent transcriptional pathways. Evidence is mounting that the Ca(2+)/calmodulin-dependent kinases (CaMKs) such as CaMKII play an important role in regulating oxidative enzyme expression, mitochondrial biogenesis, and expression of fiber type-specific myofibrillar proteins. CaMKIV has been shown to promote mitochondrial biogenesis and a mild fast-to-slow fiber type transition but has recently been shown to not be required for activity-dependent changes in muscle phenotype. CaMKII is known to decode frequency-dependent information and is activated during hypertrophic growth and endurance adaptations and also is upregulated during muscle atrophy. CaMKII has also been shown to remain active in a Ca(2+)-independent manner after acute and prolonged exercise, and, therefore, is implicated as a mechanism for muscle memory. This mechanism can sense altered functional demands and trigger activation of an adaptational response that is dose dependently related to the activation level. This class of enzymes may therefore be the ideal decoders of information encoded by the intensity, frequency, and duty cycle of muscle activation and thus translate level of muscle activation into phenotypic adaptations through regulation of important muscle genes.

Calmodulin and Calmodulin-dependent Kinase IIα Regulate Osteoblast Differentiation by Controlling c-fos Expression

Ca(2+)/calmodulin-dependent protein kinase IIalpha (alpha-CaMKII) was once thought to be exclusively expressed in neuronal tissue, but it is becoming increasingly evident that CaMKII is also expressed in various extraneural cells. CaMKII plays a critical role in regulating various signaling pathways leading to modulation of several aspects of cellular functions, including proliferation, differentiation, cytoskeletal structure, and gene expression. The purpose of this study was to examine the expression of CaMKII in osteoblast-like cells (MC4) and to elucidate its role in osteoblast differentiation. We demonstrated that CaMKII, specifically the alpha isoform, is expressed in osteoblasts both in vitro and in vivo. Inhibition of CaMKII by the calmodulin antagonist trifluoperazine or the CaMKII antagonist KN93 reduces alkaline phosphatase activity and mineralization, as well as causes 85 and 56% decreases in alkaline phosphatase and osteocalcin gene expression, respectively. CaM and CaMKII antagonists, using the newborn mouse calvaria in vivo model, cause a 50% decrease in osteoblast number (N.Ob-BS) and a 32% decrease in mineralization (BV/TV). Pharmacologic and genetic inhibition of alpha-CaMKII by using trifluoperazine, KN93, and alpha-CaMKII small interfering RNA decreases the phosphorylation of ERK and of cAMP-response element-binding protein, leading to a significant decrease in the transactivation of serum response element and cAMP-response element. Inhibition of alpha-CaMKII decreases the expression of c-fos, AP-1 transactivation, and AP-1 DNA binding activity. Our findings demonstrated that alpha-CaMKII is expressed in osteoblasts and is involved in c-fos expression via regulation of serum response element and cAMP-response element. Inhibition of alpha-CaMKII results in a decrease in c-fos expression and AP-1 activation, leading to inhibition of osteoblast differentiation.

Ca2+/calmodulin kinase-dependent activation of hypoxia inducible factor 1 transcriptional activity in cells subjected to intermittent hypoxia

Intermittent hypoxia (IH) occurs in many pathological conditions. However, very little is known about the molecular mechanisms associated with IH. Hypoxia-inducible factor 1 (HIF-1) mediates transcriptional responses to continuous hypoxia. In the present study, we investigated whether IH activates HIF-1 and, if so, which signaling pathways are involved. PC12 cells were exposed to either to 20% O2 (non-hypoxic control) or to 60 cycles consisting of 30 s at 1.5% O2, followed by 4 min at 20% O2 (IH). Western blot analysis revealed significant increases in HIF-1alpha protein in nuclear extracts of cells subjected to IH. Expression of a HIF-1-dependent reporter gene was increased 3-fold in cells subjected to IH. Although IH induced the activation of ERK1, ERK2, JNK, PKC-alpha, and PKC-gamma, inhibitors of these kinases and of phosphatidylinositol 3-kinase did not block HIF-1-mediated reporter gene expression induced by IH, indicating that signaling via these kinases was not required. In contrast, addition of the intracellular Ca2+ chelator BAPTA-AM or the Ca2+/calmodulin-dependent (CaM) kinase inhibitor KN93 blocked reporter gene activation in response to IH. CaM kinase activity was increased 5-fold in cells subjected to IH. KN 93 prevented IH-induced transactivation mediated by HIF-1alpha, and its coactivator p300, which was phosphorylated by CaM kinase II in vitro. Expression of the HIF-1-regulated gene encoding tyrosine hydroxylase was induced by IH and this effect was blocked by KN93. These observations suggest that IH induces HIF-1 transcriptional activity via a novel signaling pathway involving CaM kinase.

Ca(2+)/calmodulin-dependent protein kinase II inhibition by heparin in mesangial cells

Heparin exerts an antiproliferative effect in smooth muscle cells, and the Ca(2+)/calmodulin-dependent protein kinase (CaMK) signaling pathway is heparin sensitive. Here, we report that transfection with a truncated 326-amino acid fragment of CaMK-IIalpha increases basal activity of CaMK-II in mesangial cells. Ionomycin increased CaMK-II activity in both transfected and untransfected cells, with a concomitant increase in activated Ca(2+)/calmodulin. Heparin (1 microg/ml), but not chondroitin or dermatan sulfate, significantly attenuated both serum- or ionomycin-induced CaMK-II activity, and attendant c-fos mRNA expression, but did not affect upstream Ca(2+)/calmodulin. Autophosphorylation of Thr286 generates an autonomously active CaMK-II. Both serum and ionomycin increased phosphorylation at this site and increased CaMK-II activity in antiphosphothreonine immunoprecipitates. Heparin (1 microg/ml) did not inhibit phosphorylation of Thr286 (although much higher concentrations did). Replacement of Thr286 with Asp produces a constitutively active mutant that was insensitive to ionomycin but was inhibited by heparin maximally at 1 microg/ml. These results suggest that heparin at physiological concentrations acts at or downstream of CaMK-II to suppress its activity independent of an effect on autophosphorylation.

Force-independent expression of c-fos mRNA by endothelin-1 in rat intact small mesenteric arteries

AIM

Wall stress-independent signalling pathways were studied for endothelin-1 (ET-1)-induced c-fos expression in rat intact mesenteric small arteries.

METHODS

Arteries were kept unmounted in Krebs buffer, equilibrated for 1 h and stimulated with vasoactive substances for 15-60 min. The c-fos mRNA expression was determined by real-time polymerase chain reaction.

RESULTS

Stimulation with fetal bovine serum (FBS), phorbol 12-myristate 13-acetate (PMA) and ET-1 caused about a doubling of c-fos mRNA. The ET-1-induced c-fos expression was steady (15-60 min) and was inhibited by the inhibitor of the ET(A) receptor, BQ-123. Platelet-derived growth factor-B, angiotensin II and U46619 did not cause increased c-fos mRNA levels. The broad specificity inhibitor staurosporine inhibited the response to ET-1, but inhibitors of Rho-A kinase and phosphatidylinositol 3-kinase had no effect. However, inhibitors to tyrosine kinases, the MAP kinases [extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun amino-terminal kinase, p38], and to conventional protein kinase C showed no inhibition. Consistent with these findings, ET-1 did not cause activation of ERK1/2, a finding also seen in vessels held under pressure. In contrast, ET-1-induced c-fos expression was inhibited by the calcium chelator BAPTA, suggesting a role for intracellular calcium. This possibility was supported by the finding that raising the extracellular K(+) concentration caused increased expression of c-fos in a concentration-dependent manner.

CONCLUSION

The results suggest that in the absence of wall stress, ET-1 is able to induce increased expression of c-fos independent of traditional growth pathways, such as MAP kinase. The mechanism appears to be calcium-dependent.

Role of oxidative stress in intermittent hypoxia-induced immediate early gene activation in rat PC12 cells

Intermittent hypoxia (IH) occurs in many pathophysiological conditions. The molecular mechanisms associated with IH, however, have received little attention. Previous studies have reported that the c-fos gene via formation of activator protein-1 (AP-1) transcription factor contributes to adaptive responses to continuous hypoxia. In the present study, using a cell culture model we examined whether IH activates c-fos and AP-1 and if so, by what mechanisms. Experiments were performed on rat phaeochromocytoma cells exposed to 21% O(2) (normoxia) or 60 and 120 cycles of IH, each cycle consisting 15 s of hypoxia followed by 4 min of normoxia. IH resulted in a significant elevation of c-fos mRNA as well as transcriptional activation. IH was more potent and induced a longer lasting activation of c-fos than comparable cumulative duration of continuous hypoxia. IH increased AP-1 activity and tyrosine hydroxylase (TH) mRNA, an AP-1-regulated downstream gene, and these effects were prevented by antisense c-fos. Superoxide dismutase mimetic, a potent scavenger of superoxide anions, prevented IH-induced c-fos, AP-1 and TH activations. IH increased superoxide anion levels in mitochondria as evidenced by decreased aconitase enzyme activity and increased levels of hydrogen peroxide, a stable dismutated product of superoxide anions. Complex I of the mitochondrial electron transport chain was markedly inhibited in IH exposed cells. Pharmacological inhibitors of complex I mimicked the effects of IH during normoxia and occluded the effects of IH on c-fos activation, suggesting the involvement of the mitochondrial electron transport chain in the generation of superoxide anions during IH. These results suggest IH-induced c-fos-mediated transcriptional activation involves oxidative stress.

Ca2+/calmodulin-dependent protein kinase II stimulates c-fos transcription and DNA synthesis by a Src-based mechanism in glomerular mesangial cells

Mesangial cell growth factors elevate intracellular free [Ca2+]i, but mechanisms linking [Ca2+]i to gene expression and DNA synthesis are unclear. This study investigated the hypothesis that Ca2+/calmodulin-dependent protein kinase II (CaMK II), which is activated by elevated [Ca2+]i, increases c-fos transcription and DNA synthesis via a Src-based mechanism. In cultured rat mesangial cells, dominant negative Src (SrcK-) blocked activation of the c-fos gene promoter by CaMK II 290, a constitutively active form of CaMK IIalpha. Activation of the c-fos promoter by CaMK II 290 was also blocked by COOH-terminal Src kinase, which phosphorylates and inactivates c-Src. A pharmacologic CaMK inhibitor, KN-93, did not block activation of the c-fos promoter by ectopically expressed v-Src. Stimulation of c-Src by endothelin-1 required CaMK II activity, further supporting the notion that CaMK II acts upstream of Src in a signaling cassette. Activation of the c-fos promoter by CaMKII290 and Src required the c-fos serum response element. Dominant negative SrcK- also blocked induction of DNA synthesis in mesangial cells by CaMK II 290. Collectively, these results suggest that in mesangial cells Src protein tyrosine kinases act downstream of CaMKII in a signaling pathway in which [Ca2+]i induces the c-fos promoter and increases DNA synthesis.

Transcriptional regulation of the mdm2 oncogene by p53 requires TRRAP acetyltransferase complexes

The p53 tumor suppressor regulates the cellular response to genetic damage through its function as a sequence-specific transcription factor. Among the most well-characterized transcriptional targets of p53 is the mdm2 oncogene. Activation of mdm2 is critical in the p53 pathway because the mdm2 protein marks p53 for proteosome-mediated degradation, thereby providing a negative-feedback loop. Here we show that the ATM-related TRRAP protein functionally cooperates with p53 to activate mdm2 transcription. TRRAP is a component of several multiprotein acetyltransferase complexes implicated in both transcriptional regulation and DNA repair. In support of a role for these complexes in mdm2 expression, we show that transactivation of the mdm2 gene is augmented by pharmacological inhibition of cellular deacetylases. In vitro analysis demonstrates that p53 directly binds to a TRRAP domain previously shown to be an activator docking site. Furthermore, transfection of cells with antisense TRRAP blocks p53-dependent transcription of mdm2. Finally, using chromatin immunoprecipitation, we demonstrate direct p53-dependent recruitment of TRRAP to the mdm2 promoter, followed by increased histone acetylation. These findings suggest a model in which p53 directly recruits a TRRAP/acetyltransferase complex to the mdm2 gene to activate transcription. In addition, this study defines a novel biochemical mechanism utilized by the p53 tumor suppressor to regulate gene expression.

New roles for introns: sites of combinatorial regulation of Ca2+- and cyclic AMP-dependent gene transcription

Because some proteins can facilitate cell division or the expression of many genes simultaneously, it comes as no surprise that the expression of very important gene products is a tightly controlled process. Although gene expression is often thought of in terms of complexes of transcription factors binding to promoter elements, some studies indicate that intronic DNA sequences may also regulate gene expression. Finkbeiner examines recent work by Schlegel and colleagues demonstrating that sequences within the first intron of the c-fos gene help to regulate Fos expression under different conditions.

Activation of calcium/calmodulin-dependent protein kinase IV in long term potentiation in the rat hippocampal CA1 region

The importance of well characterized calcium/calmodulin-dependent protein kinase (CaMK) II in hippocampal long term potentiation (LTP) is widely well established; however, several CaMKs other than CaMKII are not yet clearly characterized and understood. Here we report the activation of CaMKIV, which is phosphorylated by CaMK kinase and localized predominantly in neuronal nuclei, and its functional role as a cyclic AMP-responsive element-binding protein (CREB) kinase in high frequency stimulation (HFS)-induced LTP in the rat hippocampal CA1 region. CaMKIV was transiently activated in neuronal nuclei after HFS, and the activation returned to the basal level within 30 min. Phosphorylation of CREB, which is a CaMKIV substrate, and expression of c-Fos protein, which is regulated by CREB, increased during LTP. This increase was inhibited mainly by CaMK inhibitors and also by an inhibitor for mitogen-activated protein kinase cascade, although to a lesser extent. Our results suggest that CaMKIV functions as a CREB kinase and controls CREB-regulated gene expression during HFS-induced LTP in the rat hippocampal CA1 region.

Ca(2+)/CaM-dependent kinases: from activation to function

Calmodulin (CaM) is an essential protein that serves as a ubiquitous intracellular receptor for Ca(2+). The Ca(2+)/CaM complex initiates a plethora of signaling cascades that culminate in alteration of cellular functions. Among the many Ca(2+)/CaM-binding proteins to be discovered, the multifunctional protein kinases CaMKI, II, and IV play pivotal roles. Our review focuses on this class of CaM kinases to illustrate the structural and biochemical basis for Ca(2+)/CaM interaction with and regulation of its target enzymes. Gene transcription has been chosen as the functional endpoint to illustrate the recent advances in Ca(2+)/CaM-mediated signal transduction mechanisms.

Recombinant perciform GnRH-R activates different signaling pathways in fish and mammalian heterologous cell lines

Perciforms have three forms of gonadotropin-releasing hormone (GnRH) in their brain. All three GnRHs are potent secretogogues for luteinizing hormone (LH) from the pituitary. The pivotal role of GnRH-R-GnRH interactions in reproductive homeostasis is well established; however, there is a paucity of information on how a GnRH-R responds to the three endogenous GnRH forms in a perciform species. In this study, a recombinant pituitary GnRH-R from striped bass (stb) was expressed in a mammalian cell line (COS-7) and a fish cell line (CHSE-214). Activation of the signaling pathways was monitored by reporter gene (luciferase) based assays, which were specific for cAMP-PKA or Ca 2+/calmodulin kinase (activated via c-fos promoter) signaling pathways. The stbGnRH-R expressed in two different cell lines triggered different downstream signaling in response to the treatments with chicken (c) GnRH II. Interestingly, when endogenous GnRHs were used in combinations, the luciferase activity was significantly attenuated in transfected CHSE-214 cells.

Characterization of a pituitary GnRH-receptor from a perciform fish, Morone saxatilis: functional expression in a fish cell line

Gonadotropin-releasing hormones (GnRHs) bind to the specific receptor present on the gonadotrophs to activate the synthesis and release of gonadotropins (follicle stimulating hormone or FSH and luteinizing hormone or LH), which in turn control gonadal maturation, gametogenesis and gamete release. Perciform species have three endogenous GnRHs. The main objective of this study was to characterize the gonadotropin-releasing hormone receptor (GnRH-R) present in the pituitary of a perciform species, striped bass (Morone saxatilis) and demonstrate how it interacts with its potential ligand. In this study, a cDNA for GnRH-R from the pituitaries of striped bass was cloned. The cloned cDNA has an open reading frame (ORF) that codes for a 419 amino acids peptide. Like other G-protein coupled receptors including the non-mammalian GnRH-Rs, the peptide has seven putative transmembrane domains and a C-terminal tail. Comparative analysis of the amino acid sequence of striped bass (stb) GnRH-R shows 38-87% similarity with the known GnRH-Rs. A Northern blot analysis revealed a single GnRH-R transcript in the pituitary; however, its expression in various extrapituitary tissues was demonstrated by a reverse-transcription-PCR (RT-PCR). Functionally, upon induction by endogenous forms of GnRHs (seabream, chicken II and salmon GnRHs) and a mammalian GnRH-agonist, the recombinant stbGnRH-R mediated a reporter gene (luciferase) activity in a fish cell line (CHSE-214). A real-time relative quantitation method established that significantly higher (P<0.05) levels of stbGnRH-R mRNA were present in the pituitaries of striped bass with advanced stages of ovarian development, compared to the pituitaries of fish with less developed ovaries.

Intracellular pathways linking hypoxia to activation of c-fos and AP-1

Organisms respond to hypoxia through detection of blood oxygen levels by sensors at peripheral chemoreceptors and by receptors in certain key cells of the body. The pathways over which peripheral chemoreceptor signals are transmitted to respiratory muscles are well established. However, the intracellular pathways that transmit hypoxic stimulus to gene activation are just being identified. Using anti-sense c-fos strategy, we have shown that c-fos is essential for the activation of activator protein-1 transcription factor complex (AP-1) and subsequent stimulation of downstream genes such as tyrosine hydroxylase (TH; Mishra et al. 1998). The purpose of the present study was to identify intracellular pathways that link hypoxia to activation of c-fos. The results of the present study show that hypoxia causes Ca2+ influx through L-type voltage gated Ca2+ channels and that hypoxia-induced c-fos gene expression is Ca2+/calmodulin dependent. We also demonstrate that hypoxia activates the extracellular-regulated kinase (ERK) and p38, but not JNK. Further, phosphorylation of ERK is essential for c-fos activation via SRE cis-element. Further characterization of nuclear signalling pathways provides evidence for the involvement of Src, a non receptor protein tyrosine kinase, and Ras, a small G protein, in the hypoxia-induced c-fos gene expression. These results suggest a possible role for non-receptor protein tyrosine kinases in propagating signals from G-protein coupled receptors to the activation of immediate early genes such as c-fos during hypoxia.

Effect of endothelin-1 (1-31) on human mesangial cell proliferation

It was previously found that human chymase cleaves big endothelins (ETs) at the Tyr31-Gly32 bond and produces 31-amino acid ETs (1-31). In the present study, human plasma concentrations of ET-1 (1-31) and ET-1 were examined and the effect of synthetic ET-1 (1-31) on the proliferation of cultured human mesangial cells (HMCs) was investigated. The proliferative effect of ET-1 (1-31) was evaluated from the [3H]-thymidine uptake. The activity of extracellular signal-regulated kinase (ERK) and DNA binding activity of activator protein-1 were determined by using an in-gel kinase assay and gel mobility shift assay, respectively. Immunoreactive ET-1 (1-31) was detectable in plasma, but the level was slightly lower than that of ET-1. ET-1 (1-31) increased [3H]-thymidine incorporation in HMCs to a degree similar to that induced by ET-1. ET-1 (1-31) also activated ERK1/2. Inhibition of protein kinase C and ERK kinase caused a reduction of ET-1 (1-31)-induced ERK1/2 activation. The ERK1/2 activation was followed by an increase in transcription factor activator protein-1 DNA binding activity. These findings suggest that ET-1 (1-31) is a bioactive peptide in humans and ET-1 (1-31) itself stimulates HMC proliferation.

Cloning and functional expression of a thyrotropin receptor from the gonads of a vertebrate (bony fish): potential thyroid-independent role for thyrotropin in reproduction

The thyroid stimulating hormone receptor (TSHR) mediates the pituitary control of the development, growth and function of the thyroid. The expression of the gene encoding this receptor is known only in the thyroid, lymphocytes, fibroblasts, retro-orbital tissue and fat cells. We have cloned a TSHR from the gonads of a non-mammalian vertebrate, a bony fish (striped bass, Morone saxatilis) in the course of our search for gonadotropin receptors (follicle stimulating hormone receptor, FSHR and luteinizing hormone receptor, LHR). RT-PCR analysis demonstrated that the striped bass TSHR (stbTSHR) transcripts were abundant in both the thyroid and gonads and detectable in skeletal muscle, heart and brain tissues. The stbTSHR cDNA encoded a 779-amino acid glycoprotein hormone receptor with much higher homology (57-59%) to the mammalian TSH receptors than the mammalian LH receptors (47-49%) and FSH receptors (47%), and salmon and catfish gonadotropin receptors (42-45%). There was a TSHR-specific insertion in the extracellular domain as seen in mammalian receptors. Moreover, PCR analysis of genomic DNA indicated the absence of the LHR-specific intron in the striped bass TSHR gene. Recombinant stbTSHR expressed in COS1 cells activated reporter genes (luciferase) driven by either a cAMP response element or the c-fos promoter in response to bovine TSH, stbLH or hCG, but not human FSH. In situ hybridization studies revealed the presence of stbTSHR transcripts in the gametes but not in the follicular cells. This pattern of expression is unique and suggests a direct, albeit unknown, role for TSH in gamete physiology.

Elevated endothelin-1 in tubular epithelium is associated with renal allograft rejection

Vascular endothelin-1 (ET-1) levels are elevated in patients with renal allograft rejection, and the mitogenic and pressor actions of ET-1 might contribute to transplant vasculopathy, posttransplantation hypertension, and ischemia-reperfusion injury. In contrast, relatively little is known about tubular expression of ET-1 in acute or chronic rejection of renal allografts. We sought to determine whether tubular ET-1 levels were altered in patients with acute or chronic renal allograft rejection. Immunohistochemical analysis of tubular ET-1 was performed in renal biopsy specimens from 18 patients with acute rejection, 7 patients with chronic rejection, and 5 normal kidneys excised for localized neoplasm. The diagnosis of acute or chronic rejection in each patient was verified and graded using the Banff schema. Renal tubular epithelium from patients with allograft rejection had markedly elevated staining for ET-1 compared with normal kidneys. Tubular ET-1 levels were elevated in 18 of 18 patients with acute rejection and 5 of 7 patients with chronic rejection. Tubular ET-1 staining was graded from 0 to +3 as follows: normal kidneys, 1.2 +/- 0.2; acute rejection, 2.3 +/- 0.4 (P < 0.01); and chronic rejection, 2.2 +/- 0.5 (P < 0.01). ET-1 staining was prominent in both proximal and distal tubules, and we observed abundant ET-1 secretion from proximal tubular epithelium in culture. Moreover, ET-1 activated the c-fos immediate early gene promoter in proximal tubular cells transfected with a c-fos luciferase reporter. We conclude that elevated tubular ET-1 levels are associated with acute and chronic rejection of renal allografts. Our results also suggest distinct pathophysiological roles for the tubular and vascular ET-1 systems in renal allograft rejection.

Ca2+/calmodulin-dependent kinase II and calcineurin play critical roles in endothelin-1-induced cardiomyocyte hypertrophy

Endothelin-1 (ET-1) induces cardiac hypertrophy. Because Ca(2+) is a major second messenger of ET-1, the role of Ca(2+) in ET-1-induced hypertrophic responses in cultured cardiac myocytes of neonatal rats was examined. ET-1 activated the promoter of the beta-type myosin heavy chain gene (beta-MHC) (-354 to +34 base pairs) by about 4-fold. This activation was inhibited by chelation of Ca(2+) and the blocking of protein kinase C activity. Similarly, the beta-MHC promoter was activated by Ca(2+) ionophores and a protein kinase C activator. beta-MHC promoter activation induced by ET-1 was suppressed by pretreatment with the calmodulin inhibitor, W7, the Ca(2+)/calmodulin-dependent kinase II (CaMKII) inhibitor, KN62, and the calcineurin inhibitor, cyclosporin A. beta-MHC promoter activation by ET-1 was also attenuated by overexpression of dominant-negative mutants of CaMKII and calcineurin. ET-1 increased the activity of CaMKII and calcineurin in cardiac myocytes. Pretreatment with KN62 and cyclosporin A strongly suppressed ET-1-induced increases in [(3)H]phenylalanine uptake and in cell size. These results suggest that Ca(2+) plays a critical role in ET-1-induced cardiomyocyte hypertrophy by activating CaMKII- and calcineurin-dependent pathways.

Increase of brain-derived neurotrophic factor gene expression in NG108-15 cells by the nuclear isoforms of Ca2+/ calmodulin-dependent protein kinase II

We have reported that the delta3 isoform of Ca2+/ calmodulin-dependent protein kinase II (CaM kinase II) is abundant in the nucleus in cerebellar granule cells. To examine the possibility that the nuclear isoforms of CaM kinase II are involved in the expression of brain-derived neurotrophic factor (BDNF), we transiently overexpressed the delta3 isoform in NG108-15 cells. The quantitative RT-PCR analysis revealed that rat cerebellum and NG108-15 cells expressed the exon IV-containing mRNA of BDNF (exon IV-BDNF mRNA) more than the exon III-BDNF mRNA. Treatment of NG108-15 cells with Bay K 8644 increased both exon III- and exon IV-BDNF mRNAs, and overexpression of the 83 isoform potentiated the expression of the exon IV-BDNF mRNA. The potentiation was not observed in the cells that were overexpressed with either the 61 isoform, a nonnuclear isoform, or the inactive mutant of the delta3 isoform. We constructed the luciferase reporter gene following the promoter upstream of exon IV and confirmed that overexpression of the delta3 isoform increased luciferase gene expression. Double-immunostaining of NG108-15 cells with the antibodies to CaM kinase II and BDNF clearly showed that BDNF was highly expressed in the cells that were overexpressed with the delta3 isoform or the alphaB isoform, another nuclear isoform of CaM kinase II. These results suggest that the nuclear isoforms of CaM kinase II are involved in the expression of BDNF.

L-type Ca(2+) channel activation regulates induction of c-fos transcription by hypoxia

In the present study we examined the intracellular pathways that link hypoxia to activation of c-fos gene expression. Experiments were performed on rat pheocromocytoma-12 (PC-12) cells. c-fos mRNA and promoter activities were analyzed by RT-PCR and reporter gene assays, respectively. BAPTA, a Ca(2+) chelator, inhibited c-fos mRNA and promoter activation by hypoxia. Nitrendipine, an L-type Ca(2+)-channel blocker, abolished, whereas BAY K 8644, an L-type channel agonist, enhanced c-fos activation by hypoxia. Ca(2+) currents were augmented reversibly by hypoxia, suggesting that Ca(2+) influx mediated by L-type Ca(2+) channels is essential for c-fos activation by hypoxia. We next determined downstream pathways activated by intracellular Ca(2+) concentration. Immunoblot analysis revealed Ca(2+)/calmodulin-dependent kinase II (CaMKII) protein in PC-12 cells and revealed that hypoxia increased the enzyme activity. KN-93, a CaMK inhibitor, blocked CaMKII activation and c-fos promoter stimulation by hypoxia. Ectopic expression of an active mutant of CaMKII (pCaMKII290) stimulated c-fos promoter activity under normoxia. Hypoxia increased phosphorylation of CREB at the serine residue 133 (Ser-133), and KN-93 attenuated this effect. Point mutations at the Ca(2+)/cAMP-responsive cis-element (Ca/CRE) attenuated, whereas point mutations in the serum-responsive cis-element (SRE) abolished transcriptional activation of c-fos by hypoxia. These results demonstrate that c-fos activation by hypoxia involves CaMK activation and CREB phosphorylation at Ser-133 and requires Ca/CRE and SRE. These observations demonstrate that Ca(2+)-dependent signaling pathways play a crucial role in induction of c-fos gene expression, which may underlie long-term adaptive responses to hypoxia.

Ca(2+) and p38 MAP kinase regulate mAChR-mediated c-Fos expression in avian exocrine cells

Muscarinic acetylcholine receptors (mAChRs) in exocrine tissue from the avian nasal salt gland are coupled to phospholipase C and generate inositol phosphate and Ca(2+) signals upon activation. An early effect of receptor activation in the secretory cells is a transient accumulation of c-Fos protein. In cooperation with constitutively expressed Jun, Fos presumably serves as a transcription factor altering gene expression during cell growth and differentiation processes in the gland associated with adaptation to osmotic stress in animals. Nothing is known, however, about the mAChR-dependent signaling pathways leading to Fos expression in these cells. By incubation of isolated nasal gland tissue in short-term culture with activators or inhibitors of signaling pathways and quantitative Western blot analysis of Fos abundance, we have now identified the sustained elevation of the intracellular Ca(2+) concentration and the activation of the p38 mitogen-activated protein (MAP) kinase as intermediate signaling elements for the regulation of c-Fos by muscarinic receptor activation. It is suggested that p38 MAP kinase, rather than exclusively mediating stress responses, is involved in the regulation of cellular growth and differentiation controlled by G protein-coupled receptors.

A novel calcium signaling pathway targets the c-fos intragenic transcriptional pausing site

In many cell types, increased intracellular calcium gives rise to a robust induction of c-fos gene expression. Here we show that in mouse Ltk(-) fibroblasts, calcium ionophore acts in synergy with either cAMP or PMA to strongly induce the endogenous c-fos gene. Run-on analysis shows that this corresponds to a substantial increase in active polymerases on downstream gene sequences, i.e. relief of an elongation block by calcium. Correspondingly a chimeric gene, in which the human metallothionein promoter is fused to the fos gene, is strongly induced by ionophore alone, unlike a c-fos promoter/beta-globin coding unit chimeric construct. Internal deletions in the hMT-fos reporter localize the intragenic calcium regulatory element to the 5' portion of intron 1, thereby confirming and extending previous in vitro mapping data. Ionophore induced cAMP response element-binding protein phosphorylation on Ser(133) without affecting the extracellular signal-regulated kinase cascade. Surprisingly, induction involved neither CaM-Ks nor calcineurin, while the calmodulin antagonist W7 activated c-fos transcription on its own. These data suggest that a novel calcium signaling pathway mediates intragenic regulation of c-fos expression via suppression of a transcriptional pause site.

Calcium-calmodulin mediates bradykinin-induced MAPK phosphorylation and c-fos induction in vascular cells

The vasoactive peptide bradykinin (BK) has been implicated in the pathophysiology of a number of vascular wall abnormalities, but the cellular mechanisms by which BK generates second messengers that alter vascular function are as yet undefined. Exposure of vascular smooth muscle cells (VSMC) to BK (10(-7) M) produced a rapid and transient rise in intracellular calcium, which preceded an increase in tyrosine phosphorylation of mitogen-activated protein kinase (MAPK). MAPK activation by BK was observed as early as 1 min, peaked at 5 min, and returned to baseline by 20 min. Treatment of cells with the intracellular calcium chelator EGTA-acetoxymethyl ester inhibited BK-stimulated MAPK activation, suggesting that intracellular calcium mobilization contributes to the activation of MAPK. The calmodulin inhibitor W-7 also markedly inhibited BK-induced MAPK phosphorylation in the cytoplasm as well as in the nucleus. Moreover, the BK-induced increase in c-fos mRNA levels was significantly inhibited by the calmodulin inhibitor, indicating that calmodulin is required for BK signaling leading to c-fos induction. These results implicate the calcium-calmodulin pathway in the mechanisms for regulating MAPK activity and the resultant c-fos expression induced by BK in VSMC.

Distinct calcium-dependent pathways of epidermal growth factor receptor transactivation and PYK2 tyrosine phosphorylation in PC12 cells

Recently, we have demonstrated that in PC12 cells activation of the Ras/extracellular signal-regulated kinase pathway in response to membrane depolarization or bradykinin is mediated by calcium-dependent transactivation of the epidermal growth factor receptor (EGFR). Here we address the question whether Ca(2+)-calmodulin-dependent protein kinase (CaM kinase) has a role in the EGFR transactivation signal. Using compounds that selectively interfere with either CaM kinase activity or calmodulin function, we show that KCl-mediated membrane depolarization-triggered, but not bradykinin-mediated signals involve CaM kinase function upstream of the EGFR. Although both depolarization-induced calcium influx and bradykinin stimulation of PC12 cells were found to induce c-fos transcription through EGFR activation, the former signal is CaM kinase-dependent and the latter was shown to be independent. As PYK2 is also activated upon elevation of intracellular calcium, we investigated the potential involvement of this cytoplasmic tyrosine kinase in EGFR transactivation. Interestingly, we observed that inhibition of CaM kinase activity in PC12 cells abrogated tyrosine phosphorylation of PYK2 upon KCl but not bradykinin treatment. Nevertheless, PYK2 activation in response to both stimuli appeared to be mediated by pathways parallel to EGFR transactivation. Our data demonstrate the existence of two distinct calcium-dependent mechanisms leading either to EGFR-mediated extracellular signal-regulated activation or to PYK2 tyrosine phosphorylation. Both pathways either in concert or independently might contribute to the definition of biological responses in neuronal cell types.

Signaling in human osteoblasts by extracellular nucleotides. Their weak induction of the c-fos proto-oncogene via Ca2+ mobilization is strongly potentiated by a parathyroid hormone/cAMP-dependent protein kinase pathway independently of mitogen-activated protein kinase

Extracellular nucleotides acting through specific P2 receptors activate intracellular signaling cascades. Consistent with the expression of G protein-coupled P2Y receptors in skeletal tissue, the human osteosarcoma cell line SaOS-2 and primary osteoblasts express P2Y1 and P2Y2 receptors, respectively. Their activation by nucleotide agonists (ADP and ATP for P2Y1; ATP and UTP for P2Y2) elevates [Ca2+]i and moderately induces expression of the c-fos proto-oncogene. A synergistic effect on c-fos induction is observed by combining ATP and parathyroid hormone, a key bone cell regulator. Parathyroid hormone elevates intracellular cAMP levels and correspondingly activates a stably integrated reporter gene driven by the Ca2+/cAMP-responsive element of the human c-fos promoter. Nucleotides have little effect on either cAMP levels or this reporter, instead activating luciferase controlled by the full c-fos promoter. This induction is reproduced by a stably integrated serum response element reporter independently of mitogen-activated protein kinase activation and ternary complex factor phosphorylation. This novel example of synergy between the cAMP-dependent protein kinase/CaCRE signaling module and a non-mitogen-activated protein kinase/ternary complex factor pathway that targets the serum response element shows that extracellular ATP, via P2Y receptors, can potentiate strong responses to ubiquitous growth and differentiative factors.

The retinoblastoma susceptibility gene product/Sp1 signalling pathway is modulated by Ca2+/calmodulin kinases II and IV activity

To investigate the possible link between Ca2+ signalling and cell cycle control we analysed Ca2+/calmodulin kinases (CamK) interaction with the retinoblastoma susceptibility gene product/SP1 pathway. CamK II and IV activate c-fos transcription through a short promoter region (-99 to -53) containing the retinoblastoma control element (RCE) and a cAMP response element (CRE) related sequences. Deletion analysis revealed that the RCE is a major CamK responsive element and is sufficient to confer CamK and Ca2+ regulation to a minimal promoter. Direct interactions between SP1 and RCE were confirmed by gel shift experiments. Using transient transfection experiments, we show that CamK-dependent transcription is regulated by the retinoblastoma (Rb) susceptibility gene product and the p107 Rb related protein. However, the stimulatory effects of CamKs and Rb on c-fos are blocked by overexpression of both proteins. These effects appear to be directly mediated by SP1 as shown by the use of a Gal4/SP1 fusion proteins. In conclusion, CamK II and IV, two major Ca2+-dependent intracellular effectors, may represent a molecular link between this second messenger transduction pathway and effectors that control cell cycle progression through Rb/SP1 signalling pathway.

Independent regulation of cyclo-oxygenase 2 expression by p42/44 mitogen-activated protein kinases and Ca2+/calmodulin-dependent kinase

5-Hydroxytryptamine (5-HT, 'serotonin') is a potent inducer of the early response gene cyclo-oxygenase 2 (Cox-2; prostaglandin G/H synthase) in mesangial cells. Protein kinase C (PKC), Ca2+-dependent enzymes and mitogen-activated protein kinase (p42/44 MAPK) have previously been shown to be essential modules of the signalling pathway leading from the pertussis-insensitive 5-HT2A receptor to the induction of Cox-2 mRNA expression. In the present study, PKC activation was linked to the 5-HT-mediated phosphorylation and thus the activation of p42/44 MAPK: the inhibition of PKC by the specific inhibitor GF109203x prevented p42/44 MAPK activation. Ca2+/calmodulin-dependent (CaM) kinase II delta2 was detected in mesangial cells by Western blot analysis. The inhibition of CaM kinase by the inhibitors KN62 or KN93 led to a partial inhibition of 5-HT-induced Cox-2 mRNA expression and decreased basal, but not PMA-mediated, Cox-2 expression. The 5-HT-mediated activation of MAPK was not decreased by KN62 or KN93, excluding CaM kinase as a signalling module upstream of p42/44 MAPK. Taken together, these results indicate a modulatory involvement of CaM kinase in the regulation of 5-HT-mediated Cox-2 mRNA expression in addition to the main pathway that consists of the activation of PKC and p42/44 MAPK.

Identification and expression of delta-isoforms of the multifunctional Ca2+/calmodulin-dependent protein kinase in failing and nonfailing human myocardium

Despite its importance for the regulation of heart function, little is known about the isoform expression of the multifunctional Ca2+/calmodulin-dependent protein kinase (CaMKII) in human myocardium. In this study, we investigated the spectrum of CaMKII isoforms delta2, delta3, delta4, delta8, and delta9 in human striated muscle tissue. Isoform delta3 is characteristically expressed in cardiac muscle. In skeletal muscle, specific expression of a new isoform termed delta11 is demonstrated. Complete sequencing of human delta2 cDNA, representing all common features of the investigated CaMKII subclass, revealed its high homology to the corresponding rat cDNA. Comparative semiquantitative reverse transcription-polymerase chain reaction analyses from left ventricular tissues of normal hearts and from patients suffering from dilated cardiomyopathy showed a significant increase in transcript levels of isoform delta3 relative to the expression of glyceraldehyde-3-phosphate dehydrogenase in diseased hearts (101. 6+/-11.0% versus 64.9+/-9.9% in the nonfailing group; P<0.05, n=6). Transcript levels of the other investigated cardiac CaMKII isoforms remained unchanged. At the protein level, by using a subclass-specific antibody, we observed a similar increase of a delta-CaMKII-specific signal (7.2+/-1.0 versus 3.8+/-0.7 optical density units in the nonfailing group; P<0.05, n=4 through 6). The diseased state of the failing hearts was confirmed by a significant increase in transcript levels for atrial natriuretic peptide (292. 9+/-76.4% versus 40.1+/-3.2% in the nonfailing group; P<0.05, n=3 through 6). Our data characterize for the first time the delta-CaMKII isoform expression pattern in human hearts and demonstrate changes in this expression pattern in heart failure.

Endothelin-1 stimulates c-fos mRNA expression in C6 glioma cells via MAP kinase pathway

Exposure of C6 glioma cells to endothelin-1 (ET-1) caused dose-dependent (10(-11) M to 10(-7) M) increments in intracellular calcium concentration ([Ca2+]i) and c-fos mRNA expression (4.5-fold) that were abolished by the endothelinA receptor antagonist, BQ610, and by inhibition of phospholipase C with U73122. ET-1 stimulated c-fos mRNA expression was also inhibited by protein kinase C inhibition (chelerythrine) and by the MAP kinase kinase inhibitor PD98059, but not by inhibitors of tyrosine kinases, protein kinase A type I or II, calmodulin kinase II, or calcium channel blockade. C6 cells treated with ET-1 demonstrated a significant increase in MAP kinase activity as evidenced by Western blotting. These results indicate a mechanism of long-term signaling by ET-1 involving an ET(A) receptor-mediated, phospholipase C(beta)-linked pathway that is dependent on protein kinase C and MAP kinase activation.

Endothelin-stimulated ERK activation in airway smooth-muscle cells requires calcium influx and Raf activation

Endothelin (ET)-1 is a 21-amino-acid peptide that is a potent vasoconstrictor and mitogen. By binding to its G-protein coupled receptor, ET-1 stimulates the proliferation of airway smooth-muscle (ASM) cells, which may be involved in the pathogenesis of asthma. The ETB receptor stimulates activation of the extracellular regulated kinase 2 (ERK2), which is thought to be required for proliferation of ASM cells. Our findings reveal that ET rapidly activates Raf, and that dominant-negative Raf interferes with ET-induced ERK activation in ASM cells. Expression of the amino-terminal Ras-binding domain of Raf inhibited ET-induced ERK activation, suggesting that ET-stimulated Raf activation is a Ras-dependent process. Furthermore, ET-stimulated ERK and Raf activation in ASM cells require calcium influx; chelating extracellular calcium or preventing calcium influx through calcium channels inhibited ET-stimulated, but not phorbol ester-stimulated, ERK and Raf activation.

Inactivation of kinase cascades in mesangial cells grown on collagen type I

Growth on collagen type I gels is known to suppress the mitogenic responsiveness of mesangial cells. Because these cells proliferate in some renal diseases and themselves synthesize collagen type I, we examined the influence of growth on collagen upon several kinase signaling cascades involved in mesangial cell proliferation. Quiescent mesangial cells grown on collagen type I and then stimulated with serum showed a markedly diminished induction of the protooncogene c-fos, compared with their counterparts on plastic or fibronectin. This effect was accompanied by decreased activation of mitogen-activated (Erk family) and Ca2+/calmodulin-dependent protein kinases. Cells on collagen showed lower basal protein kinase C (PKC) activity and diminished levels of PKC-alpha and -zeta isoforms. Global phosphorylation of tyrosine residues was diminished on collagen, and tyrosine phosphorylation of Erk and focal adhesion kinase in response to serum was not detected, in contrast to cells on plastic. We conclude that attachment of mesangial cells to collagen type I results in a broad suppression of protein phosphorylation that is reflected in diminished induction of the c-fos gene and probably underlies the conversion of cultured mesangial cells to a nonproliferative phenotype.

Vascular and glomerular expression of endothelin-1 in normal human kidney

To understand better the function of endothelin-1 (ET-1) in renal physiology, we examined vascular and glomerular expression of ET-1 in normal human kidney and in lupus nephritis. Immunohistochemical analysis revealed that renal endothelium of glomeruli, arteries, veins, and capillaries expressed ET-1. Endothelial cells were the principal source of glomerular ET-1; positive immunostaining was detected only rarely in mesangial cells and vascular smooth muscle cells from normal kidney. However, mesangial staining for ET-1 was elevated in patients with lupus nephritis, suggesting that under certain conditions mesangial cells elaborate ET-1. Indeed cultured human mesangial cells from normal subjects secreted ET-1 peptide. ET-1 secretion was augmented by the protein kinase C activator phorbol ester and by transforming growth factor-beta1 (TGF-beta1), a cytokine implicated in the development of glomerulosclerosis. Transient transfection of cultured mesangial cells with a preproET-1 reporter construct showed that the preproET-1 promoter is transcriptionally active in mesangial cells and is stimulated by TGF-beta1, phorbol ester, or ectopic expression of protein kinase beta1. Cultured human mesangial cells have both ETA and ETB receptors that contribute to ET-1-stimulated mitogenesis. Taken together, these results demonstrate that ET-1 is expressed at sites where paracrine or autocrine signaling by ET-1 might control renal vasoconstriction, glomerular filtration rate, and remodeling of the glomerulus in renal disease.

Heparin inhibits Ca2+/calmodulin-dependent kinase II activation and c-fos induction in mesangial cells

Like vascular smooth-muscle cells, rat mesangial cells (RMCs) display an anti-mitogenic response to heparin. In particular, heparin partially suppresses the ability of quiescent RMCs to enter the cell cycle and induce c-fos expression. When the mitogenic stimulus is serum, phorbol ester or platelet-derived growth factor, this response appears to result from the ability of heparin to suppress activation of the extracellular-signal-regulated kinase family of mitogen-activated protein kinases. However, we have also shown that heparin suppresses c-fos expression in response to ionophores such as ionomycin, an event independent of mitogen-activated protein kinase [Miralem, Wang, Whiteside and Templeton (1996) J. Biol. Chem. 271, 17100-17106]. Here we identify this second heparin-sensitive pathway as involving Ca2+/calmodulin-dependent kinase (CaMK) II. Ionomycin (100 nM) caused a transient rise in intracellular Ca2+ concentration ([Ca2+]i) in quiescent RMCs to 386+/-55 nM, with an increase in CaMK II activity that peaked 30 s later. The accumulation of c-fos mRNA that ensued 30 min later was prevented when the increase in [Ca2+]i was prevented with the intracellular Ca2+ chelator, 1,2-bis-(2-aminophenyoxy)ethane-N,N,N',N'-tetra-acetic acid. The broad-specificity CaMK inhibitor, KT 5926, inhibited ionomycin-dependent c-fos induction at a concentration at which it was without effect on induction by serum or phorbol ester. The CaMK II-specific inhibitor, KN-93, likewise inhibited c-fos induction by ionomycin, but not by serum or phorbol ester. ML-7, an inhibitor of the CaMK-related myosin light-chain kinase (MLCK), was without effect. Heparin (1 microg/ml) suppressed ionomycin-dependent c-fos induction. It was without effect on [Ca2+]i, but inhibited the development of autonomous CaMK II activity. However, when heparin was added to the CaMK II assay solution in vitro, it was without effect on autonomous activity. Furthermore, heparin did not prevent full activation of CaMK II by the Ca2+-calmodulin complex in vitro. Heparin did not affect myosin light-chain phosphorylation or RMC contraction, processes mediated by MLCK. We conclude that ionomycin induces c-fos in RMCs through the CaMK II pathway, and that heparin prevents CaMK II activation by an indirect process mediated by other cell components. Heparin does not affect activation of the closely related CaMK, MLCK.

The nuclear deltaB isoform of Ca2+/calmodulin-dependent protein kinase II regulates atrial natriuretic factor gene expression in ventricular myocytes

Cultured neonatal ventricular myocytes display features of myocardial hypertrophy including increased cell size, myofilament organization, and reexpression of the embryonic gene for atrial natriuretic factor (ANF). KN-93, an inhibitor of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase II), blocked the induction of these responses by the alpha1-adrenergic receptor agonist phenylephrine, whereas its inactive analog KN-92 did not. To directly determine whether CaM kinase II could regulate ANF gene expression, we transiently expressed each of three isoforms of CaM kinase II (alpha, deltaB, and deltaC) along with an ANF promoter/luciferase reporter gene. The deltaB isoform markedly increased luciferase gene expression, whereas comparable levels of the deltaC and alpha isoforms were ineffective. Expression of deltaB-CaM kinase II also potentiated phenylephrine-mediated ANF gene expression, and this effect was blocked by KN-93 but not by KN-92. The ability of deltaB-CaM kinase II to transactivate a truncated ANF promoter, containing a serum response element (SRE) required for phenylephrine-inducible gene expression, was lost when this SRE was mutated. The deltaB isoform of CaM kinase II has been shown to exhibit nuclear localization. Coexpression of the non-nuclear deltaC or alpha isoforms, which can form multimers with the deltaB isoform, prevented the nuclear localization of deltaB-CaM kinase II and also blocked its effects on ANF reporter gene and protein expression. In addition, a chimeric alpha-CaM kinase II which contains the nuclear localization signal of the deltaB isoform was able to induce ANF reporter gene expression, albeit to a lesser extent than deltaB-CaM kinase II. These data are the first to assign a function to the deltaB isoform of CaM kinase II and to link its nuclear localization to subsequent activation of cardiac gene expression.

Role of Rho GTPase in the endothelin-1-induced nuclear signaling

Binding of Endothelin-1 (ET-1) to its heterotrimeric G protein-coupled receptors stimulates various signaling cascades involving the activation of phospholipase C-beta, phospholipase D, protein kinase C (PKC), tyrosine kinases, Ca2+/calmodulin-dependent kinase (CaMKs), and Ras, a small molecular weight G-protein, but, the role of Rho GTPase remains unclear. In this project, we examined whether RhoA contributes to the ET-1-induced signaling cascade to c-fos SRE activation in Rat-2 fibroblast cells. Our results demonstrate that Rho activation is critical for the signal transduction of ET-1 to c-fos SRE.