Secretion of salivary statherin is compromised in uncontrolled diabetic patients.

BACKGROUND: Statherin is an important salivary protein for maintaining oral health. The purpose of the current study was to determine if differences in statherin levels exist between diabetic and healthy subjects. METHODS: A total of 48 diabetic and healthy controls were randomly selected from a community-based database. Diabetic subjects (n=24) had fasting glucose levels >180 mg/dL, while controls (n=24) had levels <110 mg/dL. Parotid saliva (PS) and sublingual/submandibular saliva (SS) were collected and salivary flow rates determined. Salivary statherin levels were determined by densitometry of Western blots. Blood hemoglobin A1c (HbA1c) and total protein in saliva were also obtained. RESULTS: SS, but not PS, salivary flow rate and total protein in diabetics were significantly less than in healthy controls (p=0.021 & p<0.001 respectively). Correlation analysis revealed the existence of a negative correlation between PS statherin levels and HbA1c (p=0.012) and fasting glucose (p=0.021) levels, while no such correlation was found for SS statherin levels. When statherin levels were normalized to total salivary protein, the proportion of PS statherin, but not SS statherin, in diabetics was significantly less than controls (p=0.032). In contrast, the amount of statherin secretion in SS, but not PS, was significantly decreased in diabetics compared to controls (p=0.016). CONCLUSIONS AND GENERAL SIGNIFICANCE: The results show that synthesis and secretion of statherin is reduced in diabetics and this reduction is salivary gland specific. As compromised salivary statherin secretion leads to increased oral health risk, this study indicates that routine oral health assessment of these patients is warranted.

Silk fibroin scaffolds promote formation of the ex vivo niche for salivary gland epithelial cell growth, matrix formation, and retention of differentiated function.

Salivary gland hypofunction often results from a number of causes, including the use of various medications, radiation for head and neck tumors, autoimmune diseases, diabetes, and aging. Since treatments for this condition are lacking and adult salivary glands have little regenerative capacity, there is a need for cell-based therapies to restore salivary gland function. Development of these treatment strategies requires the establishment of a system that is capable of replicating the salivary gland cell "niche" to support the proliferation and differentiation of salivary gland progenitor cells. In this study, a culture system using three-dimensional silk fibroin scaffolds (SFS) and primary salivary gland epithelial cells (pSGECs) from rat submandibular (SM) gland and parotid gland (PG) was established and characterized. pSGECs grown on SFS, but not tissue culture plastic (TCP), formed aggregates of cells with morphological features resembling secretory acini. High levels of amylase were released into the media by both cell types after extended periods in culture on SFS. Remarkably, cultures of PG-derived cells on SFS, but not SM cells, responded to isoproterenol, a ß-adrenergic receptor agonist, with increased enzyme release. This behavior mimics that of the salivary glands in vivo. Decellularized extracellular matrix (ECM) formed by pSGECs in culture on SFS contained type IV collagen, a major component of the basement membrane. These results demonstrate that pSGECs grown on SFS, but not TCP, retain important functional and structural features of differentiated salivary glands and produce an ECM that mimics the native salivary gland cell niche. These results demonstrate that SFS has potential as a scaffold for creating the salivary gland cell niche in vitro and may provide an approach for inducing multipotent stem cells to provide therapeutically meaningful numbers of salivary gland progenitor cells for regenerating these tissues in patients.

A melatonin-based fluorescence method for the measurement of mitochondrial complex III function in intact cells.

Mitochondrial complex III (MC-3) plays a pivotal role in electron transfer and oxidative phosphorylation. Impaired MC-3 functions may contribute to a variety of diseases by interrupting normal bioenergetics and increasing reactive oxygen production and oxidative stress. Currently, MC-3 function is assessed by measuring the cytochrome c reductase activity spectrophotometrically in isolated mitochondria or MC-3. The cytoplasmic microenvironment critical for mitochondrial complex functions may be depleted during these isolation processes. The development of a reliable method to measure MC-3 activities in intact cells or tissues is highly desirable. This report describes a novel fluorescence-based method to assess MC-3 functions, i.e., Qi site electron transfer, in the intact cells. Human mesangial and teratocarcinoma NT2 cells were used to demonstrate that melatonin-induced oxidation of 2',7'-dichlorodihydrofluorescein (H2 DCF) was inhibited by antimycin A, the MC-3 Qi site-specific inhibitor, but not by myxothiazol, the MC-3 Qo site-specific inhibitor, nor rotenone, the mitochondrial complex I inhibitor. These results indicate that melatonin-induced oxidation of H2 DCF is reflecting MC-3 Qi site electron transfer activities. Modifying structures of the side groups at the R3 and R5 positions of the indole ring of melatonin diminished its efficacy for inducing H2 DCF oxidation, suggesting a specific interaction of melatonin with the MC-3 Qi site. These results suggest that the fluorogenic property of melatonin-induced H2 DCF oxidation provides a MC-3 Qi site electron transfer-specific measurement in intact cells. Interestingly, using this method, the Qi site electron transfer activity in transformed or immortalized cells was found to be significantly higher than the nontransformed cells.

Geldanamycin derivative ameliorates high fat diet-induced renal failure in diabetes.

Diabetic nephropathy is a serious complication of longstanding diabetes and its pathogenesis remains unclear. Oxidative stress may play a critical role in the pathogenesis and progression of diabetic nephropathy. Our previous studies have demonstrated that polyunsaturated fatty acids (PUFA) induce peroxynitrite generation in primary human kidney mesangial cells and heat shock protein 90ß1 (hsp90ß1) is indispensable for the PUFA action. Here we investigated the effects of high fat diet (HFD) on kidney function and structure of db/db mice, a widely used rodent model of type 2 diabetes. Our results indicated that HFD dramatically increased the 24 h-urine output and worsened albuminuria in db/db mice. Discontinuation of HFD reversed the exacerbated albuminuria but not the increased urine output. Prolonged HFD feeding resulted in early death of db/db mice, which was associated with oliguria and anuria. Treatment with the geldanamycin derivative, 17-(dimethylaminoehtylamino)-17-demethoxygeldanamycin (17-DMAG), an hsp90 inhibitor, preserved kidney function, and ameliorated glomerular and tubular damage by HFD. 17-DMAG also significantly extended survival of the animals and protected them from the high mortality associated with renal failure. The benefit effect of 17-DMAG on renal function and structure was associated with a decreased level of kidney nitrotyrosine and a diminished kidney mitochondrial Ca(2+) efflux in HFD-fed db/db mice. These results suggest that hsp90ß1 is a potential target for the treatment of nephropathy and renal failure in diabetes.

Role of ß-adrenergic receptors in regulation of hepatic fat accumulation during aging.

Excessive fat accumulation in liver (hepatic steatosis) predisposes to hepatic functional and structural impairment and overall metabolic risk. Previous studies noted an association between hepatic steatosis and age in humans and rodents. However, the mechanisms leading to age-associated hepatic fat accumulation remain unknown. Earlier work from our group showed that ß-adrenergic receptor (ß-AR) levels and ß-AR-stimulated adenylyl cyclase activity increase in rat liver during aging. Here we investigated whether age-associated increases in ß-AR signaling play a role in augmenting hepatic lipid accumulation. We demonstrate an increase in hepatic lipid content during senescence and a significant correlation between hepatic fat content and stimulation of adenylyl cyclase activity by the ß-AR agonist isoproterenol in rat liver. Isoproterenol administration to young and old rodents in vivo increased hepatic lipid accumulation. Furthermore, in vitro overexpression of ß1- and ß2-AR subtypes in hepatocytes from young rodents increased cellular lipid content, whereas inhibition of ß-ARs by receptor subtype-specific inhibitors reduced lipid levels in hepatocytes from senescent animals. Isoproterenol-induced hepatic lipid accumulation in vivo was prevented by the ß-AR nonselective blocker propranolol, suggesting a novel therapeutic effect of this class of drugs in hepatic steatosis. Acipimox, which inhibits adipose tissue lipolysis, did not alter isoproterenol-mediated hepatic fat accumulation; thus ß-AR responsive hepatic lipid accumulation does not appear to be related primarily to altered lipolysis. These findings suggest that augmented hepatic ß-AR signaling during aging may increase lipid accumulation in liver and advocate a possible role for ß-adrenergic blockers in preventing or retarding the development of hepatic steatosis.

Impaired mitochondrial complex III and melatonin responsive reactive oxygen species generation in kidney mitochondria of db/db mice.

We have previously demonstrated that melatonin, at pharmacological concentrations, causes rapid reactive oxygen species (ROS) generation at the antimycin-A sensitive site of mitochondrial complex III (MC-3). In the current work, we used this melatonin response to investigate the role of mitochondrial dysfunction in the development of diabetic nephropathy. We find that the development of diabetic nephropathy, as indicated by hyperfiltration and histopathological lesions in the kidney of db/db mice, is associated with diminished melatonin-induced ROS generation and MC-3 activity, indicating impaired MC-3 at the antimycin-A site. The MC-3 protein level in the renal mitochondria was equivalent in db/db and the nondiabetic db/m mice, whereas mitochondrial complex I (MC-1) protein was dramatically upregulated in the db/db mice. This differential regulation in mitochondrial complexes may alter the equilibrium of the electron transport in renal mitochondria and contribute to ROS overproduction. The study provides one mechanism of enhanced oxidative stress that may be involved in the pathogenesis of diabetic nephropathy in db/db mice.

The role of mitochondrial complex III in melatonin-induced ROS production in cultured mesangial cells.

Melatonin is a potent scavenger of reactive oxygen (ROS) and reactive nitrogen species (RNS). At pharmacological concentrations, however, melatonin is documented to cause ROS/RNS production, especially in cultured cancerous cells. Currently, the mechanism responsible for melatonin-induced ROS generation remains elusive. In this study, we provided evidence that melatonin, at micromolar concentrations, induced rapid ROS generation by a mitochondrial-dependent mechanism in primary human mesangial (HM) cells. The melatonin-induced ROS production occurred independent of changes in Ca(2+) concentrations in the cytosol and/or in mitochondria. In mitochondria isolated from HM cells and mice kidney tissues, melatonin caused ROS production; this melatonin response was completely blocked by the complex III inhibitor antimycin A. In contrast, both the mitochondrial complex I inhibitor, rotenone, and another complex III inhibitor, myxothiazol, which interacts with complex III at a distinct site, had no significant inhibitory effect on melatonin-induced ROS generation. These results demonstrate that melatonin induced rapid ROS generation via the antimycin A-sensitive site of mitochondrial complex III.

Loss of manganese superoxide dismutase leads to abnormal growth and signal transduction in mouse embryonic fibroblasts.

Manganese superoxide dismutase (MnSOD) in the mitochondria plays an important role in cellular defense against oxidative damage. Homozygous MnSOD knockout (Sod2(-/-)) mice are neonatal lethal, indicating the essential role of MnSOD in early development. To investigate the potential cellular abnormalities underlying the aborted development of Sod2(-/-) mice, we examined the growth of isolated mouse embryonic fibroblasts (MEFs) from Sod2(-/-) mice. We found that the proliferation of Sod2(-/-) MEFs was significantly decreased compared with wild-type MEFs despite the absence of morphological differences. The Sod2(-/-) MEFs produced less cellular ATP, had lower O(2) consumption, generated more superoxide, and expressed less Prdx3 protein. Furthermore, the loss of MnSOD dramatically altered several markers involved in cell proliferation and growth, including decreased growth stimulatory function of mTOR signaling and enhanced growth inhibitory function of GSK-3ß signaling. Interestingly, the G-protein-coupled receptor-mediated intracellular Ca(2+) signal transduction was also severely suppressed in Sod2(-/-) MEFs. Finally, the ratio of microtubule-associated protein light chain 3 (LC3)-II/LC3-I, an index of autophagic activity, was increased in Sod2(-/-) MEFs, consistent with a reduction in mTOR signal transduction. These data demonstrate that MnSOD deficiency results in alterations in several key signaling pathways, which may contribute to the lethal phenotype of Sod2(-/-) mice.

Linoleic acid-induced mitochondrial Ca(2+) efflux causes peroxynitrite generation and protein nitrotyrosylation.

It is well known that excessive non-esterified fatty acids in diabetes contribute to the pathogenesis of renal complications although the mechanism remains elusive. Enhanced oxidative stress has been hypothesized as a unified factor contributing to diabetic complications and increased protein nitrotyrosylation has been reported in the kidneys of diabetic patients. In the current manuscript we described that linoleic acid (LA) caused mitochondrial Ca(2+) efflux and peroxynitrite production, along with increased nitrotyrosine levels of cellular proteins in primary human mesangial cells. The peroxynitrite production by LA was found to depend on mitochondrial Ca(2+) efflux. Downregulation of hsp90beta1, which has been previously shown to be essential for polyunsaturated fatty acid-induced mitochondrial Ca(2+) efflux, significantly diminished LA-responsive mitochondrial Ca(2+) efflux and the coupled peroxynitrite generation, implicating a critical role of hsp90beta1 in the LA responses. Our results further demonstrated that mitochondrial complexes I and III were directly involved in the LA-induced peroxynitrite generation. Using the well established type 2 diabetic animal model db/db mice, we observed a dramatically enhanced LA responsive mitochondrial Ca(2+) efflux and protein nitrotyrosylation in the kidney. Our study thus demonstrates a cause-effect relationship between LA and peroxynitrite or protein nitrotyrosylation and provides a novel mechanism for lipid-induced nephropathy in diabetes.

Distinct pathways of ERK activation by the muscarinic agonists pilocarpine and carbachol in a human salivary cell line.

Cholinergic-muscarinic receptor agonists are used to alleviate mouth dryness, although the cellular signals mediating the actions of these agents on salivary glands have not been identified. We examined the activation of ERK1/2 by two muscarinic agonists, pilocarpine and carbachol, in a human salivary cell line (HSY). Immunoblot analysis revealed that both agonists induced transient activation of ERK1/2. Whereas pilocarpine induced phosphorylation of the epidermal growth factor (EGF) receptor, carbachol did not. Moreover, ERK activation by pilocarpine, but not carbachol, was abolished by the EGF receptor inhibitor AG-1478. Downregulation of PKC by prolonged treatment of cells with the phorbol ester PMA diminished carbachol-induced ERK phosphorylation but had no effect on pilocarpine responsiveness. Depletion of intracellular Ca2+ ([Ca2+]i by EGTA did not affect ERK activation by either agent. In contrast to carbachol, pilocarpine did not elicit [Ca2+]i mobilization in HSY cells. Treatment of cells with the muscarinic receptor subtype 3 (M3) antagonist N-(3-chloropropyl)-4-piperidnyl diphenylacetate decreased ERK responsiveness to both agents, whereas the subtype 1 (M1) antagonist pirenzepine reduced only the carbachol response. Stimulation of ERKs by pilocarpine was also decreased by M3, but not M1, receptor small interfering RNA. The Src inhibitor PP2 blocked pilocarpine-induced ERK activation and EGF receptor phosphorylation, without affecting ERK activation by carbachol. Our results demonstrate that the actions of pilocarpine and carbachol in salivary cells are mediated through two distinct signaling mechanisms-pilocarpine acting via M3 receptors and Src-dependent transactivation of EGF receptors, and carbachol via M1/M3 receptors and PKC-converging on the ERK pathway.

Heat shock protein 90beta1 is essential for polyunsaturated fatty acid-induced mitochondrial Ca2+ efflux.

Nonesterified fatty acids may influence mitochondrial function by alterations in gene expression, metabolism, and/or mitochondrial Ca(2+) ([Ca(2+)](m)) homeostasis. We have previously reported that polyunsaturated fatty acids induce Ca(2+) efflux from mitochondria, an action that may deplete [Ca(2+)](m) and thus contribute to nonesterified fatty acid-responsive mitochondrial dysfunction. Here we show that the chaperone protein heat shock protein 90 beta1 (hsp90beta1) is required for polyunsaturated fatty acid-induced mitochondrial Ca(2+) efflux (PIMCE). Retinoic acid induced differentiation of human teratocarcinoma NT2 cells in association with attenuation of PIMCE. Proteomic analysis of mitochondrial proteins revealed that hsp90beta1, among other proteins, was reduced in retinoic acid-differentiated cells. Blockade of PIMCE in NT2 cells by 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin, a known inhibitor of the chaperone activity of hsp90, and hsp90beta1 RNA interference demonstrated that hsp90beta1 is essential for PIMCE. We also show localization of hsp90beta1 in mitochondria by Western blot and immunofluorescence. Distinctive effects of inhibitors binding to the N or C terminus of hsp90 on PIMCE in isolated mitochondria suggested that the C terminus of hsp90beta1 plays a critical role in PIMCE.

Regulation of STIM1, store-operated Ca2+ influx, and nitric oxide generation by retinoic acid in rat mesangial cells.

It has been shown that store-operated Ca(2+) influx (SOC) plays critical roles in the activation of endothelial nitric oxide (NO) synthase (eNOS) and generation of NO in endothelial cells. Recent studies indicate stromal interaction molecule 1 (STIM1) is the molecule responsible for SOC activation following Ca(2+) depletion in the ER. Retinoic acids (RA) have beneficial effects in the treatment of renal diseases. The mechanism of the RA action is still largely unknown. In the current study, we used primary cultured rat mesangial cells to examine the effect of RA on SOC and STIM1. In these cells, BK caused concentration-dependent [Ca(2+)](i) mobilization. Treatment of the cells with RA, while it had no effect on the initial peak, reduced the plateau phase of BK-mediated [Ca(2+)](i) response, indicating the inhibition of SOC by RA. The level of STIM1 protein but not mRNA in RA-treated cells was significantly reduced. RA treatment did not affect TGF-beta-mediated gradual Ca(2+) influx which occurred by superoxide anion-mediated mechanism, indicating RA treatment specifically inhibited SOC in mesangial cells. RT-PCR and Western blot analysis demonstrated that eNOS was expressed in rat mesangial cells grown in media containing 11 and 30 but not 5.5 mM glucose. Downregulation of STIM1 protein and BK-induced SOC by RA treatment or STIM1 dsRNA were associated with abolished NO production. The 26S proteasome inhibitor lactacystin blocked the RA-mediated downregulation of BK-induced SOC, suggesting that ubiquitin-proteasome pathway may be involved in RA-mediated STIM1 protein downregulation in rat mesangial cells. Our data suggest that glucose-induced eNOS expression and NO production in mesangial cells may contribute to hyperfiltration in diabetes and RA may exert beneficial effects by downregulation of STIM1 and SOC in mesangial cells.

Inhibition of CD95-mediated apoptosis through beta 1 integrin in the HSG epithelial cell line.

The HSG cell line serves as a model for salivary gland epithelial progenitor cell differentiation. In order for a progenitor cell to differentiate, the cell must maintain viability within its niche. Studies were designed to elucidate the mechanism for integrin-mediated HSG cell survival. HSG cells, grown on Matrigel, were resistant to CD95-mediated apoptosis. Western blot analysis showed that Matrigel induced the expression of bcl-2, bcl-xL, p63, and DeltaNp63. This induction occurred by as early as 2 hrs and remained for 24 hrs. CD95-mediated apoptosis resistance was dependent, however, upon the expression of the bcl-2 family. Furthermore, Matrigel induced bcl-2 family expression was dependent on the transactivation of the EGF receptor pathway since PD98059 and AG1478 inhibited Matrigel induced bcl-2 family expression and caused HSG cells to be sensitive to CD95-mediated apoptosis. Activation of the EGF receptor pathway, by itself, however, was not sufficient to inhibit apoptosis. Blocking antibody showed that bcl-2 family expression was mediated through beta1 integrin. These studies show that salivary progenitor epithelial cell survival is integrin dependent and involves the transactivation of the EGF receptor pathway.

Polyunsaturated fatty acids mobilize intracellular Ca2+ in NT2 human teratocarcinoma cells by causing release of Ca2+ from mitochondria.

In a variety of disorders, overaccumulation of lipid in nonadipose tissues, including the heart, skeletal muscle, kidney, and liver, is associated with deterioration of normal organ function, and is accompanied by excessive plasma and cellular levels of free fatty acids (FA). Increased concentrations of FA may lead to defects in mitochondrial function found in diverse diseases. One of the most important regulators of mitochondrial function is mitochondrial Ca(2+) ([Ca(2+)](m)), which fluctuates in coordination with intracellular Ca(2+) ([Ca(2+)](i)). Polyunsaturated FA (PUFA) have been shown to cause [Ca(2+)](i) mobilization albeit by unknown mechanisms. We have found that PUFA but not monounsaturated or saturated FA cause [Ca(2+)](i) mobilization in NT2 human teratocarcinoma cells. Unlike the [Ca(2+)](i) response to the muscarinic G protein-coupled receptor agonist carbachol, PUFA-mediated [Ca(2+)](i) mobilization in NT2 cells is independent of phospholipase C and inositol-1,4,5-trisphospate (IP(3)) receptor activation, as well as IP(3)-sensitive internal Ca(2+) stores. Furthermore, PUFA-mediated [Ca(2+)](i) mobilization is inhibited by the mitochondria uncoupler carboxyl cyanide m-chlorophenylhydrozone. Direct measurements of [Ca(2+)](m) with X-rhod-1 and (45)Ca(2+) indicate that PUFA induce Ca(2+) efflux from mitochondria. Further studies show that ruthenium red, an inhibitor of the mitochondrial Ca(2+) uniporter, blocks PUFA-induced Ca(2+) efflux from mitochondria, whereas inhibitors of the mitochondrial permeability transition pore cyclosporin A and bongkrekic acid have no effect. Thus PUFA-gated Ca(2+) release from mitochondria, possibly via the Ca(2+) uniporter, appears to be the underlying mechanism for PUFA-induced [Ca(2+)](i) mobilization in NT2 cells.

Differential regulation of intracellular calcium oscillations by mitochondria and gap junctions.

Fluctuations of intracellular Ca2+ ([Ca2+]i) regulate a variety of cellular functions. The classical Ca2+ transport pathways in the endoplasmic reticulum (ER) and plasma membrane are essential to [Ca2+]i oscillations. Although mitochondria have recently been shown to absorb and release Ca2+ during G protein-coupled receptor (GPCR) activation, the role of mitochondria in [Ca2+]i oscillations remains to be elucidated. Using fluo-3-loaded human teratocarcinoma NT2 cells, we investigated the regulation of [Ca2+]i oscillations by mitochondria. Both the muscarinic GPCR agonist carbachol and the ER Ca2+-adenosine triphosphate inhibitor thapsigargin (Tg) induced [Ca2+]i oscillations in NT2 cells. The [Ca2+]i oscillations induced by carbachol were unsynchronized among individual NT2 cells; in contrast, Tg-induced oscillations were synchronized. Inhibition of mitochondrial functions with either mitochondrial blockers or depletion of mitochondrial DNA eliminated carbachol--but not Tg-induced [Ca2+]i oscillations. Furthermore, carbachol-induced [Ca2+]i oscillations were partially restored to mitochondrial DNA-depleted NT2 cells by introduction of exogenous mitochondria. Treatment of NT2 cells with gap junction blockers prevented Tg-induced but not carbachol-induced [Ca2+]i oscillations. These data suggest that the distinct patterns of [Ca2+]i oscillations induced by GPCR and Tg are differentially modulated by mitochondria and gap junctions.

beta-Adrenergic-responsive activation of extracellular signal-regulated protein kinases in salivary cells: role of epidermal growth factor receptor and cAMP.

The beta-adrenergic receptor agonist isoproterenol exerts growth-promoting effects on salivary glands. In this study, activation of ERKs, members of the mitogen-activated protein kinase family, by isoproterenol was examined in a human salivary gland cell line (HSY). Immunoblot analysis indicated that isoproterenol (10(-5) M) induced transient activation of ERK1/2 (4.4-fold relative to basal at 10 min) similar to that caused by EGF (6.7 fold). Isoproterenol, like EGF, also induced phosphorylation of the EGF receptor. However, inhibition of EGF receptor phosphorylation by the tyrphostin AG-1478 only partially attenuated isoproterenol-induced ERK phosphorylation, whereas EGF-responsive ERK activation was completely blocked. The G(i) inhibitor pertussis toxin also caused partial inhibition of isoproterenol-stimulated ERK activation. The cAMP analog 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP) and the cAMP-elevating agents IBMX and cholera toxin produced transient ERK1/2 activation, similar to the effect of isoproterenol, in HSY cells. The stimulatory effects of isoproterenol and cAMP on ERK phosphorylation were not reduced by the PKA inhibitor H-89, whereas the Src family inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidase (PP2) and transfection of a dominant-negative Src construct diminished isoproterenol-induced ERK activation. Isoproterenol induced marked overexpression of the cell growth-related adhesion molecule CD44, and this effect of isoproterenol was abolished by the ERK pathway inhibitor PD-98059. In summary, we show a dual mechanism of isoproterenol-induced ERK phosphorylation in HSY cells-one pathway mediated by EGF receptor transactivation and the other by an EGF receptor-independent pathway possibly mediated by cAMP. Our results also suggest that isoproterenol-induced growth of salivary tissue may involve ERK-mediated CD44 expression.

Involvement of calcineurin in transforming growth factor-beta-mediated regulation of extracellular matrix accumulation.

Calcineurin is a calcium-dependent, serine/threonine phosphatase that functions as a signaling intermediate. In this study, we investigated the role of calcineurin in transforming growth factor-beta (TGF-beta)-mediated cellular effects and examined the signaling pathway involved in activation of calcineurin. Calcineurin is activated by TGF-beta in a time- and dose-dependent manner. Consistent with increased phosphatase activity, the calcineurin substrate, NFATc1, is dephosphorylated and transported to the nucleus. Inhibition of calcineurin prior to the addition of TGF-beta revealed that calcineurin is required for TGF-beta-mediated accumulation of extracellular matrix (ECM) proteins but not cell hypertrophy. Conversely, overexpression of constitutively active calcineurin was sufficient to induce ECM protein expression. The mechanism of calcineurin activation by TGF-beta was found to be induction of a low, sustained increase of intracellular calcium. Chelation of extracellular calcium blocked both TGF-beta-mediated calcium influx and calcineurin activity. Finally, calcium entry was found to be dependent upon generation of reactive oxygen species (ROS) including superoxide anion and hydrogen peroxide. Accordingly, inhibition of ROS generation also blocked TGF-beta-mediated calcineurin phosphatase activity and decreased ECM accumulation. In conclusion, this study describes a new pathway for TGF-beta-mediated regulation of ECM via generation of ROS, calcium influx, and activation of calcineurin.

Epidermal growth factor upregulates beta-adrenergic receptor signaling in a human salivary cell line.

The effects of epidermal growth factor (EGF) on the beta-adrenergic receptor-coupled adenylyl cyclase system were studied in a human salivary cell line (HSY). The beta-adrenergic agonist isoproterenol (10(-5) M) stimulated adenylyl cyclase activity by approximately 2-fold, and the isoproterenol response was increased 1.8-fold after prolonged (48 h) exposure to EGF (5 x 10(-10) M). In contrast, enzyme activation via stimulatory prostaglandin receptors and by agents acting on nonreceptor components of the adenylyl cyclase system was not enhanced by EGF. beta-Adrenergic receptor density, assessed by binding of the beta-adrenergic receptor antagonist (-)-[(125)I]iodopindolol, was increased threefold after EGF treatment. Competition binding studies with unlabeled antagonists selective for beta(1)- and beta(2)-adrenergic receptor subtypes indicated that the increase in (-)-[(125)I]iodopindolol binding sites induced by EGF reflected an increased number of beta(2)-adrenergic receptors. Likewise, Northern blot analysis of RNA from EGF-treated cells revealed selective induction of beta(2)-adrenergic receptor mRNA, which was blocked by the RNA synthesis inhibitor actinomycin D. The increase in beta-adrenergic receptor density produced by EGF was unaltered after phorbol ester-induced downregulation of protein kinase C (PKC). Enhancement of isoproterenol-responsive adenylyl cyclase activity and phosphorylation of mitogen-activated protein kinase (MAPK) by EGF were both blocked by the MAPK pathway inhibitor PD-98059. The results suggest that in HSY cells EGF enhances beta-adrenergic responsiveness by upregulating beta(2)-adrenergic receptor expression at the transcriptional level. Moreover, the stimulatory effect of EGF on beta(2)-adrenergic receptor signaling appears to be mediated by the MAPK pathway and independent of PKC activation.

Epidermal growth factor-induced depletion of the intracellular Ca2+ store fails to activate capacitative Ca2+ entry in a human salivary cell line.

Epidermal growth factor (EGF) is a multifunctional factor known to influence proliferation and function of a variety of cells. The actions of EGF are mediated by EGF receptor tyrosine kinase pathways, including stimulation of phospholipase Cgamma and mobilization of intracellular Ca(2+) ([Ca(2+)](i)). Generally, agonist-mediated Ca(2+) mobilization involves both Ca(2+) release from internal stores and Ca(2+) influx activated by store depletion (i.e. capacitative or store-operated Ca(2+) influx). However, the role of capacitative Ca(2+) entry in EGF-mediated Ca(2+) mobilization is still largely unknown. In this study, we compared [Ca(2+)](i) signals elicited by EGF with those induced by agents (the muscarinic receptor agonist carbachol and thapsigargin (Tg)) known to activate capacitative Ca(2+) entry. Unlike carbachol and Tg, EGF (5 nm) elicited a transient [Ca(2+)](i) signal without a plateau phase in the presence of extracellular Ca(2+) and also failed to accelerate Mn(2+) entry. Repletion of extracellular Ca(2+) to cells stimulated with EGF in the absence of Ca(2+) elicited an increase in [Ca(2+)](i), indicating that EGF indeed stimulates Ca(2+) influx. However, the influx was activated at lower EGF concentrations than those required to stimulate Ca(2+) release. Interestingly, the phospholipase C inhibitor completely inhibited Ca(2+) release induced by both EGF and carbachol and also reduced Ca(2+) influx responsive to carbachol but had no effect on Ca(2+) influx induced by EGF. EGF-induced Ca(2+) influx was potentiated by low concentrations (<5 ng/ml) of oligomycin, a mitochondrial inhibitor that blocks capacitative Ca(2+) influx in other systems. Transient expression of the hTRPC3 protein enhanced Ca(2+) influx responsive to carbachol but did not increase EGF-activated Ca(2+) influx. Both EGF and carbachol depleted internal Ca(2+) stores. Our results demonstrate that EGF-induced Ca(2+) release from internal stores does not activate capacitative Ca(2+) influx. Rather, EGF stimulates Ca(2+) influx via a mechanism distinct from capacitative Ca(2+) influx induced by carbachol and Tg.

Extracellular Adenosine Triphosphate Triggers Arrhythmias and Elemental Redistribution in Electrically Stimulated Rat Cardiac Myocytes.

An increase in extracellular adenosine triphosphate (ATP) is arrhythmogenic in rat cardiac myocytes and extracellular ATP levels are elevated during cardiac ischemia. To gain insight into the mechanism by which the arrhythmic contractions are generated, we investigated changes in subcellular elemental content by electron probe microanalysis (EPMA) in isolated adult rat cardiac myocytes stimulated by the ATP analog, 2-methylthio-ATP (2-M-S-ATP). We also measured the effects of 2-M-S-ATP stimulation on myocyte cell shortening. In electrically stimulated myocytes, 2-M-S-ATP stimulation generated arrhythmic contractions and also increased the amplitude of cell shortening. However, only the arrhythmic contractions were reversed by 2-M-S-ATP washout. EPMA of freeze-dried cryosections of rapidly frozen 2-M-S-ATP-stimulated myocytes showed increased cytosolic Na and Cl, decreased K, but no significant change in mitochondrial Ca upon 2-M-S-ATP stimulation. The arrhythmias were abolished upon 2-M-S-ATP washout, and the observed changes in cytosolic elemental content also reversed upon agonist washout, thus suggesting that the increased Na and Cl, and decreased K, are specifically associated with the ATP-dependent spontaneous contractile activity. The observed increase in intracellular Na upon 2-M-S-ATP stimulation may explain our observation of prolonged relaxation time of 2-M-S-ATP-stimulated contractions. This may be due to inhibition of Ca(2+) efflux via the Na(+) Ca(2+) exchanger.