Alteration of expression of Ca2+ signaling proteins and adaptation of Ca2+ signaling in SERCA2+/- mouse parotid acini

PURPOSE

The sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), encoded by ATP2A2, is an essential component for G-protein coupled receptor (GPCR)-dependent Ca2+ signaling. However, whether the changes in Ca2+ signaling and Ca2+ signaling proteins in parotid acinar cells are affected by a partial loss of SERCA2 are not known.

MATERIALS AND METHODS

In SERCA2+/- mouse parotid gland acinar cells, Ca2+ signaling, expression levels of Ca2+ signaling proteins, and amylase secretion were investigated.

RESULTS

SERCA2+/- mice showed decreased SERCA2 expression and an upregulation of the plasma membrane Ca2+ ATPase. A partial loss of SERCA2 changed the expression level of 1, 4, 5-tris-inositolphosphate receptors (IP3Rs), but the localization and activities of IP3Rs were not altered. In SERCA2+/- mice, muscarinic stimulation resulted in greater amylase release, and the expression of synaptotagmin was increased compared to wild type mice.

CONCLUSION

These results suggest that a partial loss of SERCA2 affects the expression and activity of Ca2+ signaling proteins in the parotid gland acini, however, overall Ca2+ signaling is unchanged.

Homer proteins in Ca2+ signaling by excitable and non-excitable cells

Homers are scaffolding proteins that bind Ca(2+) signaling proteins in cellular microdomains. The Homers participate in targeting and localization of Ca(2+) signaling proteins in signaling complexes. However, recent work showed that the Homers are not passive scaffolding proteins, but rather they regulate the activity of several proteins within the Ca(2+) signaling complex in an isoform-specific manner. Homer2 increases the GAP activity of RGS proteins and PLCbeta that accelerate the GTPase activity of Galpha subunits. Homer1 gates the activity of TRPC channels, controls the rates of their translocation and retrieval from the plasma membrane and mediates the conformational coupling between TRPC channels and IP(3)Rs. Homer1 stimulates the activity of the cardiac and neuronal L-type Ca(2+) channels Ca(v)1.2 and Ca(v)1.3. Homer1 also mediates the communication between the cardiac and smooth muscle ryanodine receptor RyR2 and Ca(v)1.2 to regulate E-C coupling. In many cases the Homers function as a buffer to reduce the intensity of Ca(2+) signaling and create a negative bias that can be reversed by the immediate early gene form of Homer1. Hence, the Homers should be viewed as the buffers of Ca(2+) signaling that ensure a high spatial and temporal fidelity of the Ca(2+) signaling and activation of downstream effects.

Initiation site of Ca(2+) entry evoked by endoplasmic reticulum Ca(2+) depletion in mouse parotid and pancreatic acinar cells

PURPOSE

In non-excitable cells, which include parotid and pancreatic acinar cells, Ca(2+) entry is triggered via a mechanism known as capacitative Ca(2+) entry, or store-operated Ca(2+) entry. This process is initiated by the perception of the filling state of endoplasmic reticulum (ER) and the depletion of internal Ca(2+) stores, which acts as an important factor triggering Ca(2+) entry. However, both the mechanism of store-mediated Ca(2+) entry and the molecular identity of store-operated Ca(2+) channel (SOCC) remain uncertain.

MATERIALS AND METHODS

In the present study we investigated the Ca(2+) entry initiation site evoked by depletion of ER to identify the localization of SOCC in mouse parotid and pancreatic acinar cells with microfluorometeric imaging system.

RESULTS

Treatment with thapsigargin (Tg), an inhibitor of sarco/endoplasmic reticulum Ca(2+)-ATPase, in an extracellular Ca(2+) free state, and subsequent exposure to a high external calcium state evoked Ca(2+) entry, while treatment with lanthanum, a non-specific blocker of plasma Ca(2+) channel, completely blocked Tg-induced Ca(2+) entry. Microfluorometric imaging showed that Tg-induced Ca(2+) entry started at a basal membrane, not a apical membrane.

CONCLUSION

These results suggest that Ca2+ entry by depletion of the ER initiates at the basal pole in polarized exocrine cells and may help to characterize the nature of SOCC.

TRPC channels: interacting proteins

TRP channels, in particular the TRPC and TRPV subfamilies, have emerged as important constituents of the receptor-activated Ca2+ influx mechanism triggered by hormones, growth factors, and neurotransmitters through activation ofphospholipase C (PLC). Several TRPC channels are also activated by passive depletion of endoplasmic reticulum (ER) Ca2+. Although in several studies the native TRP channels faithfully reproduce the respective recombinant channels, more often the properties of Ca2+ entry and/or the store-operated current are strikingly different from that of the TRP channels expressed in the same cells. The present review aims to discuss this disparity in the context of interaction of TRPC channels with auxiliary proteins that may alter the permeation and regulation of TRPC channels.

Calcium signaling complexes in microdomains of polarized secretory cells

The highly polarized nature of epithelial cells in exocrine glands necessitates targeting, assembly into complexes and confinement of the molecules comprising the Ca(2+) signaling apparatus, to cellular microdomains. Such high degree of polarized localization has been shown for all Ca(2+) signaling molecules tested, including G protein coupled receptors and their associated proteins, Ca(2+) pumps, Ca(2+) influx channels at the plasma membrane and Ca(2+) release channels in the endoplasmic reticulum. Although the physiological significance of polarized Ca(2+) signaling is clear, little is known about the mechanism of targeting, assembly and retention of Ca(2+) signaling complexes in cellular microdomains. The present review attempts to summarize the evidence in favor of polarized expression of Ca(2+) signaling proteins at the apical pole of secretory cells with emphasis on the role of scaffolding proteins in the assembly and function of the Ca(2+) signaling complexes. The consequence of polarized enrichment of Ca(2+) signaling complexes at the apical pole is generation of an apical to basal pole gradient of cell responsiveness that, at low physiological agonist concentrations, limits Ca(2+) spikes to the apical pole, and when a Ca(2+) wave occurs, it always propagates from the apical to the basal pole. Our understanding of Ca(2+) signaling in microdomains is likely to increase rapidly with the application of techniques to controllably and selectively disrupt components of the complexes and apply high resolution recording techniques, such as TIRF microscopy to this problem.

Critical role of phospholipase Cgamma1 in the generation of H2O2-evoked [Ca2+]i oscillations in cultured rat cortical astrocytes

Reactive oxygen species, such as the superoxide anion, H2O2, and the hydroxyl radical, have been considered as cytotoxic by-products of cellular metabolism. However, recent studies have provided evidence that H2O2 serves as a signaling molecule modulating various physiological functions. Here we investigated the effect of H2O2 on the regulation of intracellular Ca2+ signaling in rat cortical astrocytes. H2O2 triggered the generation of oscillations of intracellular Ca2+ concentration ([Ca2+]i) in a concentration-dependent manner over the range 10-100 microM. The H2O2-induced [Ca2+]i oscillations persisted in the absence of extracellular Ca2+ and were prevented by depletion of intracellular Ca2+ stores with thapsigargin. The H2O2-induced [Ca2+]i oscillations were not inhibited by pretreatment with ryanodine but were prevented by 2-aminoethoxydiphenyl borate and caffeine, known antagonists of inositol 1,4,5-trisphosphate receptors. H2O2 activated phospholipase C (PLC) gamma1 in a dose-dependent manner, and U73122, an inhibitor of PLC, completely abolished the H2O2-induced [Ca2+]i oscillations. In addition, RNA interference against PLCgamma1 and the expression of the inositol 1,4,5-trisphosphate-sequestering "sponge" prevented the generation of [Ca2+]i oscillations. H2O2-induced [Ca2+]i oscillations and PLC1 phosphorylation were inhibited by pretreatment with dithiothreitol, a sulfhydryl-reducing agent. Finally, epidermal growth factor induced H2O2 production, PLCgamma1 activation, and [Ca2+]i increases, which were attenuated by N-acetylcysteine and diphenyleneiodonium and by the overexpression of peroxiredoxin type II. Therefore, we conclude that low concentrations of exogenously applied H2O2 generate [Ca2+]i oscillations by activating PLCgamma1 through sulfhydryl oxidation-dependent mechanisms. Furthermore, we show that this mechanism underlies the modulatory effect of endogenously produced H2O2 on epidermal growth factor-induced Ca2+ signaling in rat cortical astrocytes.

Expression of Ca2+-dependent synaptotagmin isoforms in mouse and rat parotid acinar cells

Synaptotagmin is a Ca2+ sensing protein, which triggers a fusion of synaptic vesicles in neuronal transmission. Little is known regarding the expression of Ca2+-dependent synaptotagmin isoforms and their contribution to the release of secretory vesicles in mouse and rat parotid acinar cells. We investigated a type of Ca2+-dependent synaptotagmin and Ca2+ signaling in both rat and mouse parotid acinar cells using RT-PCR, microfluorometry, and amylase assay. Mouse parotid acinar cells exhibited much more sensitive amylase release in response to muscarinic stimulation than did rat parotid acinar cells. However, transient [Ca2+]i increases and Ca2+ influx in response to muscarinic stimulation in both cells were identical, suggesting that the expression or activity of the Ca2+ sensing proteins is different. Seven Ca2+-dependent synaptotagmins, from 1 to 7, were expressed in the mouse parotid acinar cells. However, in the rat parotid acinar cells, only synaptotagmins 1, 3, 4 and 7 were expressed. These results indicate that the expression of Ca2+-dependent synaptotagmins may contribute to the release of secretory vesicles in parotid acinar cells.

Aberrant localization of intracellular organelles, Ca2+ signaling, and exocytosis in Mist1 null mice

Ca2+ signaling and exocytosis are highly polarized functions of pancreatic acinar cells. The role of cellular architecture in these activities and the capacity of animals to tolerate aberrant acinar cell function are not known. A key regulator of acinar cell polarity is Mist1, a basic helix-loop-helix transcription factor. Ca2+ signaling and amylase release were examined in pancreatic acini of wild type and Mist1 null mice to gain insight into the importance of cellular architecture for Ca2+ signaling and regulated exocytosis. Mist1-/- acinar cells exhibited dramatically altered Ca2+ signaling with up-regulation of the cholecystokinin receptor but minimal effect upon expression of the M3 receptor. However, stimulation of inositol 1,4,5-trisphosphate production by cholecystokinin and carbachol was inefficient in Mist1-/- cells. Although agonist stimulation of Mist1-/- cells evoked a Ca2+ signal, often the Ca2+ increase was not in the form of typical Ca2+ oscillations but rather in the form of a peak/plateau-type response. Mist1-/- cells also displayed distorted apical-to-basal Ca2+ waves. The aberrant Ca2+ signaling was associated with mislocalization and reduced Ca2+ uptake by the mitochondria of stimulated Mist1-/- cells. Deletion of Mist1 also led to mislocalization of the Golgi apparatus and markedly reduced digestive enzyme content. The combination of aberrant Ca2+ signaling and reduced digestive enzyme content resulted in poor secretion of digestive enzymes. Yet, food consumption and growth of Mist1-/- mice were normal for at least 32 weeks. These findings reveal that Mist1 is critical to normal organelle localization in exocrine cells and highlight the critical importance of maintaining cellular architecture and polarized localization of cellular organelles in generating a propagating apical-to-basal Ca2+ wave. The studies also reveal the spare capacity of the exocrine pancreas that allows normal growth and development in the face of compromised exocrine pancreatic function.

TRPC3 channels confer cellular memory of recent neuromuscular activity

Skeletal muscle adapts to different patterns of motor nerve activity by alterations in gene expression that match specialized properties of contraction, metabolism, and muscle mass to changing work demands (muscle plasticity). Calcineurin, a calcium/calmodulin-dependent, serine-threonine protein phosphatase, has been shown to control programs of gene expression in skeletal muscles, as in other cell types, through the transcription factor nuclear factor of activated T cells (NFAT). This study provides evidence that the function of NFAT as a transcriptional activator is regulated by neuromuscular stimulation in muscles of intact animals and that calcium influx from the transient receptor potential (TRPC3) channel is an important determinant of NFAT activity. Expression of TRPC3 channels in skeletal myocytes is up-regulated by neuromuscular activity in a calcineurin-dependent manner. These data suggest a mechanism for cellular memory in skeletal muscles whereby repeated bouts of contractile activity drive progressively greater remodeling events.

German cockroach extract activates protease-activated receptor 2 in human airway epithelial cells

BACKGROUND

The German cockroach has been reported to act as an allergen that might be associated with a protease reaction in asthma. However, the molecular identities of the antigens in German cockroach extract (GCE) with protease activity and the protease-activated receptors (PARs) that are activated by GCE in human airway epithelial cells have not been characterized.

OBJECTIVE

We investigated the direct effect of GCE on Ca(2+) signaling in human airway epithelial cells and the type of PARs activated by GCE.

METHODS

The Ca(2+)-sensitive dye Fura2 was used to determine intracellular Ca(2+) concentration ([Ca(2+)](i)) by means of spectrofluorometry.

RESULTS

GCE induced a baseline type of [Ca(2+)](i) oscillations in a dose-dependent manner. The oscillations persisted for long periods of time in the absence of Ca(2+) entry across the plasma membrane, suggesting that the observed [Ca(2+)](i) increases were due to Ca(2+) release from intracellular stores. Accordingly, after depleting endoplasmic reticulum Ca(2+) with thapsigargin, an endoplasmic reticulum Ca(2+) ATPase inhibitor, the GCE-mediated [Ca(2+)](i) signals were abolished. Whereas desensitization of PAR-1, PAR-3, and PAR-4 had no effect on GCE-mediated Ca(2+) mobilization, no GCE-mediated [Ca(2+)](i) increase was observed after desensitization of PAR-2.

CONCLUSIONS

These results indicate that GCE has a direct effect on human airway epithelial cells, in particular generating [Ca(2+)](i) oscillations through Ca(2+) release from thapsigargin-sensitive Ca(2+) stores through activation of PAR-2.

Partial inhibition of SERCA is responsible for extracellular Ca2+ dependence of AlF-4-induced [Ca2+]i oscillations in rat pancreatic

AlF4-is known to generate oscillations in intracellular Ca2+ concentration ([Ca2+]i) by activating G proteins in many cell types. However, in rat pancreatic acinar cells, AlF4--evoked [Ca2+]i oscillations were reported to be dependent on extracellular Ca2+, which contrasts with the [Ca2+]i oscillations induced by cholecystokinin (CCK). Therefore, we investigated the mechanisms by which AlF4- generates extracellular Ca2+-dependent [Ca2+]i oscillations in rat pancreatic acinar cells. AlF4(-)-induced [Ca2+]i oscillations were stopped rapidly by the removal of extracellular Ca2+ and were abolished on the addition of 20 mM caffeine and 2 microM thapsigargin, indicating that Ca2+ influx plays a crucial role in maintenance of the oscillations and that an inositol 1,4,5-trisphosphate-sensitive Ca2+ store is also required. The amount of Ca2+ in the intracellular Ca2+ store was decreased as the AlF4--induced [Ca2+]i oscillations continued. Measurement of 45Ca2+ influx into isolated microsomes revealed that AlF4-directly inhibited sarco/endoplasmic reticulum Ca2+-ATPase (SERCA). The activity of plasma membrane Ca2+-ATPase during AlF4- stimulation was not significantly different from that during CCK stimulation. After partial inhibition of SERCA with 1 nM thapsigargin, 20 pM CCK-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+. This study shows that AlF4- induces [Ca2+]i oscillations, probably by inositol 1,4,5-trisphosphate production via G protein activation but that these oscillations are strongly dependent on extracellular Ca2+ as a result of the partial inhibition of SERCA.

Bumetanide, the specific inhibitor of Na+-K+-2Cl- cotransport, inhibits 1alpha,25-dihydroxyvitamin D3-induced osteoclastogenesis in a mouse co-culture system

The Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) is responsible for ion transport across the secretory and absorptive epithelia, the regulation of cell volume, and possibly the modulation of cell growth and development. It has been reported that a variety of cells, including osteoblasts, contain this cotransporter. In this study, the physiological role of NKCC1 in osteoclastogenesis was exploited in a co-culture system. Bumetanide, a specific inhibitor of NKCC1, reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells. In order to investigate the mechanism by which bumetanide inhibits osteoclastogenesis, the mRNA expressions of the receptor activator of nuclear factor (NF)-kappaB ligand (RANKL) and osteoprotegerin (OPG) were analysed by RT-PCR. Exposure of osteoblastic cells to a medium containing 1 micro M bumetanide reduced RANKL mRNA expression induced by 10 nM 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3, in a dose-dependent manner. In addition, RANKL expression was also analysed with enzyme-linked immunosorbant assay (ELISA) using anti-RANKL antibody. The expression of RANKL was decreased with the increase of bumetanide concentration. In contrast, the expression of OPG mRNA, a novel tumour necrosis factor (TNF) receptor family member was increased in the presence of bumetanide. These results imply that bumetanide inhibits osteoclast differentiation by reducing the RANKL/OPG ratio in osteoblastic cells. However, no significant difference in M-CSF mRNA expression was observed when bumetanide was added. Also, we found that the phosphorylation of c-Jun NH2-terminal kinase (JNK), which regulates the activity of various transcriptional factors, was reduced by bumetanide treatment. Conclusively, these findings suggest that NKCC1 in osteoblasts has a pivotal role in 1alpha,25(OH)2D3-induced osteoclastogenesis partly via the phosphorylation of JNK.

Staurosporine-inhibitable protein kinase activity associated with secretory granule membranes isolated from rat submandibular gland cells

Protein kinases, such as protein kinase C, have been shown to be associated with secretory granules and to regulate the event of exocytosis in various tissues including parotid salivary acinar cells. However, in submandibular acinar cells that play an important role in the secretion of proteins into the oral cavity, kinase activity on the granule membrane has not been explored. Therefore, in the present study, we isolated the secretory granules from rat submandibular acinar cells and investigated the localisation of protein kinases on the granule membrane. Initially, we isolated and purified secretory granules from rat submandibular acinar cells. Addition of [gamma-32P] ATP to granule-membrane lysate phosphorylated the granule-membrane-associated 26, 32, 55 and 58kDa proteins, suggesting the presence of endogenous kinase activity on the membrane. Moreover, the phosphorylation of 26 and 32kDa proteins was inhibited by staurosporine and K252a, both non-specific protein kinase C inhibitors. However, the phosphorylation of 26 and 32kDa proteins was not inhibited by other protein kinase C inhibitors, such as calphostin C, GF109203X and chelerythrine, indicating that protein kinase C was not responsible for the phosphorylation. In addition, H-89, ML-9, KN-62 and genistein did not appear to inhibit this phosphorylation, indicating that protein kinase A, myosin light chain kinase (MLCK), Ca2+/calmodulin-dependent protein kinase II (CAMKII) and tyrosine kinase were not involved in the phosphorylation of 26 and 32kDa proteins. Moreover, Ca2+ had no effect on the kinase activity. Therefore, our results suggest that an unidentified, staurosporine-inhibitable protein kinase activity is associated with the secretory granule membrane of rat submandibular acinar cells.

Homer 2 tunes G protein-coupled receptors stimulus intensity by regulating RGS proteins and PLCbeta GAP activities

Homers are scaffolding proteins that bind G protein-coupled receptors (GPCRs), inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs), ryanodine receptors, and TRP channels. However, their role in Ca2+ signaling in vivo is not known. Characterization of Ca2+ signaling in pancreatic acinar cells from Homer2-/- and Homer3-/- mice showed that Homer 3 has no discernible role in Ca2+ signaling in these cells. In contrast, we found that Homer 2 tunes intensity of Ca2+ signaling by GPCRs to regulate the frequency of [Ca2+]i oscillations. Thus, deletion of Homer 2 increased stimulus intensity by increasing the potency for agonists acting on various GPCRs to activate PLCbeta and evoke Ca2+ release and oscillations. This was not due to aberrant localization of IP3Rs in cellular microdomains or IP3R channel activity. Rather, deletion of Homer 2 reduced the effectiveness of exogenous regulators of G proteins signaling proteins (RGS) to inhibit Ca2+ signaling in vivo. Moreover, Homer 2 preferentially bound to PLCbeta in pancreatic acini and brain extracts and stimulated GAP activity of RGS4 and of PLCbeta in an in vitro reconstitution system, with minimal effect on PLCbeta-mediated PIP2 hydrolysis. These findings describe a novel, unexpected function of Homer proteins, demonstrate that RGS proteins and PLCbeta GAP activities are regulated functions, and provide a molecular mechanism for tuning signal intensity generated by GPCRs and, thus, the characteristics of [Ca2+]i oscillations.

A new mode of Ca2+ signaling by G protein-coupled receptors: gating of IP3 receptor Ca2+ release channels by Gbetagamma

The most common form of Ca(2+) signaling by Gq-coupled receptors entails activation of PLCbeta2 by Galphaq to generate IP(3) and evoke Ca(2+) release from the ER. Another form of Ca(2+) signaling by G protein-coupled receptors involves activation of Gi to release Gbetagamma, which activates PLCbeta1. Whether Gbetagamma has additional roles in Ca(2+) signaling is unknown. Introduction of Gbetagamma into cells activated Ca(2+) release from the IP(3) Ca(2+) pool and Ca(2) oscillations. This can be due to activation of PLCbeta1 or direct activation of the IP(3)R by Gbetagamma. We report here that Gbetagamma potently activates the IP(3) receptor. Thus, Gbetagamma-triggered [Ca(2+)](i) oscillations are not affected by inhibition of PLCbeta. Coimmunoprecipitation and competition experiments with Gbetagamma scavengers suggest binding of Gbetagamma to IP(3) receptors. Furthermore, Gbetagamma inhibited IP(3) binding to IP(3) receptors. Notably, Gbetagamma activated single IP(3)R channels in native ER as effectively as IP(3). The physiological significance of this form of signaling is demonstrated by the reciprocal sensitivity of Ca(2+) signals evoked by Gi- and Gq-coupled receptors to Gbetagamma scavenging and PLCbeta inhibition. We propose that gating of IP(3)R by Gbetagamma is a new mode of Ca(2+) signaling with particular significance for Gi-coupled receptors.

Signalling specificity in GPCR-dependent Ca2+ signalling

Cells use signalling networks to translate with high fidelity extracellular signals into specific cellular functions. Signalling networks are often composed of multiple signalling pathways that act in concert to regulate a particular cellular function. In the centre of the networks are the receptors that receive and transduce the signals. A versatile family of receptors that detect a remarkable variety of signals are the G protein-coupled receptors (GPCRs). Virtually all cells express several GPCRs that use the same biochemical machinery to transduce their signals. Considering the specificity and fidelity of signal transduction, a central question in cell signalling is how signalling specificity is achieved, in particular among GPCRs that use the same biochemical machinery. Ca(2+) signalling is particularly suitable to address such questions, since [Ca(2+)](i) can be recorded with excellent spatial and temporal resolutions in living cells and tissues and now in living animals. Ca(2+) is a unique second messenger in that both biochemical and biophysical components form the Ca(2+) signalling complex to regulate its concentration. Both components act in concert to generate repetitive [Ca(2+)](i) oscillations that can be either localized or in the form of global, propagating Ca(2+) waves. Most of the key proteins that form Ca(2+) signalling complexes are known and their activities are reasonably well understood on the biochemical and biophysical levels. We review here the information gained from studying Ca(2+) signalling by GPCRs to gain further understanding of the mechanisms used to generate cellular signalling specificity.

Staurosporine mobilizes Ca(2+) from secretory granules by inhibiting protein kinase C in rat submandibular acinar cells

Staurosporine was previously shown to mobilize Ca(2+) from the thapsigargin-insensitive Ca(2+) store in rat submandibular acinar cells. However, the nature of the store is not yet known. Therefore, in the present study, the staurosporine-releasable intracellular Ca(2+) store was characterized. Staurosporine increased the cytosolic Ca(2+) concentration ([Ca(2+)](c)) after the inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) store was depleted. Ionomycin caused only small increases in [Ca(2+)](c) after the depletion of the IP(3)-sensitive Ca(2+) store, whereas ionomycin+monensin caused large increases. However, ionomycin+monensin did not increase [Ca(2+)](c) when added after [Ca(2+)](c) was increased by staurosporine, indicating that the acidic Ca(2+) store was the main source of Ca(2+). The acidic Ca(2+) store appeared to be associated with secretory granules, since ionomycin+monensin- and staurosporine-induced [Ca(2+)](c) increases were significantly reduced when the acinar cells were degranulated. The effect of staurosporine on [Ca(2+)](c) was mimicked by other protein kinase C inhibitors. Therefore, we conclude that staurosporine mobilizes Ca(2+) from secretory granules, probably through the inhibition of protein kinase C in rat submandibular acinar cells.

Transporter-mediated bile acid uptake causes Ca2+-dependent cell death in rat pancreatic acinar cells

BACKGROUND & AIMS

The mechanism by which cholelithiasis increases the risk of acute pancreatitis remains obscure. Because bile acids can enter the pancreas either by luminal diffusion or by interstitial leakage during gallstone impaction and pancreatitis is associated with impaired Ca(2+) signaling, we examined the effect of bile acids on pancreatic acinar cell signaling and the associated intracellular events.

METHODS

Rat pancreatic acinar cells were isolated by collagenase digestion and the effects of bile acids on [Ca(2+)](i) signaling, cell survival, inflammatory signals, and the molecular and functional expressions of bile uptake transporters were analyzed.

RESULTS

Bile acids specifically inhibited the sarco/endoplasmic reticulum Ca(2+) ATPase pump to chronically deplete part of the Ca(2+) stored in the endoplasmic reticulum. This in turn led to the activation of capacitative Ca(2+) entry and a chronic [Ca(2+)](i) load. The increase in [Ca(2+)](i) and Ca(2+) load activated the inflammation-associated signals of c-Jun amino-terminal kinases and NF-kappaB and led to cell death, which was inhibited by buffering [Ca(2+)](i) with 1,2-bis(2-aminophenoxy)ethane-N,N,N,N'-tetraacetic acid. A comprehensive molecular analysis of bile acid transporters revealed that pancreatic acinar cells express the bile uptake transporters Na(+)-taurocholate co-transporting polypeptide and organic anion transporting polypeptide in the luminal and basolateral membranes, respectively. Bile acid uptake into acinar cells was in part Na(+)-dependent and in part Na(+)-independent, suggesting that both transporters contribute to bile acid influx into acinar cells.

CONCLUSIONS

These results suggest that bile acids can be transported into pancreatic acinar cells through specific membrane transporters and induce cell death by impairing cellular Ca(2+) signaling.

Phase II study of the antiangiogenesis agent thalidomide in recurrent or metastatic squamous cell carcinoma of the head and neck

BACKGROUND

Thalidomide has been shown to have antiangiogenic effects in preclinical models as well as a significant antitumor effect in hematologic tumors such as multiple myeloma. The authors performed this Phase II study to determine the activity, toxicity profile, and antiangiogenic effect of thalidomide in patients with locoregionally recurrent or metastatic squamous cell carcinoma of the head and neck.

METHODS

Twenty-one patients with recurrent or metastatic squamous cell carcinoma of the head and neck were treated with single-agent thalidomide. All patients had received radiation therapy, and most had undergone surgery (95%) and/or chemotherapy (90%). Thalidomide was initiated at 200 mg;3>daily and increased to a target dose of 1000 mg daily. Patients continued treatment until disease progression, unacceptable toxicity, or death occurred.

RESULTS

All 21 patients eventually developed progressive disease. Median time to progression was 50 days (95% confidence interval, 28-70), with median overall survival time of 194 days (95% lower confidence boundary, 151), similar to the progression and survival times reported for this patient group with other agents. Thalidomide was generally well tolerated, with few patients experiencing Grades 3 to 4 toxicities. Serum vascular endothelial growth factor and basic fibroblast growth factor levels increased in six of seven patients, for whom paired serum samples were available and all of whom had progressive disease.

CONCLUSIONS

In this heavily pretreated population of patients with advanced squamous cell carcinoma of the head and neck, thalidomide does not appear to have single-agent antitumor activity. Further evaluation of the mechanism of action of thalidomide is indicated. Potentially, future evaluations of thalidomide may be performed in combination with other antiangiogenic or cytotoxic agents in patients with earlier stage disease or in patients with minimal residual disease.

Polarized expression of G protein-coupled receptors and an all-or-none discharge of Ca2+ pools at initiation sites of [Ca2+]i waves in polarized exocrine cells

In the present work we examined localization and behavior of G protein-coupled receptors (GPCR) in polarized exocrine cells to address the questions of how luminal to basal Ca(2+) waves can be generated in a receptor-specific manner and whether quantal Ca(2+) release reflects partial release from a continuous pool or an all-or-none release from a compartmentalized pool. Immunolocalization revealed that expression of GPCRs in polarized cells is not uniform, with high levels of GPCR expression at or near the tight junctions. Measurement of phospholipase Cbeta activity and receptor-dependent recruitment and trapping of the box domain of RGS4 in GPCRs complexes indicated autonomous functioning of G(q)-coupled receptors in acinar cells. These findings explain the generation of receptor-specific Ca(2+) waves and why the waves are always initiated at the apical pole. The initiation site of Ca(2+) wave at the apical pole and the pattern of wave propagation were independent of inositol 1,4,5-trisphosphate concentration. Furthermore, a second Ca(2+) wave with the same initiation site and pattern was launched by inhibition of sarco/endoplasmic reticulum Ca(2+)-ATPase pumps of cells continuously stimulated with sub-maximal agonist concentration. By contrast, rapid sequential application of sub-maximal and maximal agonist concentrations to the same cell triggered Ca(2+) waves with different initiation sites. These findings indicate that signaling specificity in pancreatic acinar cells is aided by polarized expression and autonomous functioning of GPCRs and that quantal Ca(2+) release is not due to a partial Ca(2+) release from a continuous pool, but rather, it is due to an all-or-none Ca(2+) release from a compartmentalized Ca(2+) pool.

Regulation of Ca2+-release-activated Ca2+ current (Icrac) by ryanodine receptors in inositol 1,4,5-trisphosphate-receptor-deficient DT40 cells

Persistence of capacitative Ca(2+) influx in inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-deficient DT40 cells (DT40(IP(3)R-/-)) raises the question of whether gating of Ca(2+)-release activated Ca(2+) current (I(crac)) by conformational coupling to Ca(2+)-release channels is a general mechanism of gating of these channels. In the present work we examined the properties and mechanism of activation of I(crac) Ca(2+) current in wild-type and DT40(IP(3)R-/-) cells. In both cell types passive depletion of internal Ca(2+) stores by infusion of EGTA activated a Ca(2+) current with similar characteristics and time course. The current was highly Ca(2+)-selective and showed strong inward rectification, all typical of I(crac). The activator of ryanodine receptor (RyR), cADP-ribose (cADPR), facilitated activation of I(crac), and the inhibitors of the RyRs, 8-N-cADPR, ryanodine and Ruthenium Red, all inhibited I(crac) activation in DT40(IP(3)R-/-) cells, even after complete depletion of intracellular Ca(2+) stores by ionomycin. Wild-type and DT40(IP(3)R-/-) cells express RyR isoforms 1 and 3. RyR levels were adapted in DT40(IP(3)R-/-) cells to a lower RyR3/RyR1 ratio than in wild-type cells. These results suggest that IP(3)Rs and RyRs can efficiently gate I(crac) in DT40 cells and explain the persistence of I(crac) gating by internal stores in the absence of IP(3)Rs.