G-protein- and capacitatively regulated Ca2+ entry pathways are activated by muscarinic receptor stimulation in a human submandibular ductal cell line

Zn2+ stimulates salivary secretions via metabotropic zinc receptor ZnR/GPR39 in human salivary gland cells

Zn²⁺ is a divalent cation that is essential for many biological activities, as it influences many ion channels and enzymatic activities. Zn²⁺ can evoke G-protein-coupled receptor signaling via activation of the metabotropic zinc receptor ZnR/GPR39. In spite of evidence suggesting the presence of ZnR/GPR39 in salivary gland cells, there has been no evidence of ZnR/GPR39-mediated modulation of salivary gland function. Here we characterized the role of ZnR/GPR39 in human submandibular gland cells. A 0.25% ZnCl₂ solution evoked secretion of unstimulated and stimulated whole saliva in humans. We found that ZnR/GPR39 is expressed in human submandibular glands and HSG cells. Zn²⁺ increased cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in a concentration-dependent manner. Muscarinic antagonist had no effect on Zn²⁺-induced [Ca²⁺]_i increase, which was completely blocked by the phospholipase C-β inhibitor. As with muscarinic agonist, Zn²⁺ also induced the translocation of aquaporin-5 (AQP-5) to the plasma membrane, which was drastically decreased in ZnR/GPR39-knockdown cells. These data suggest that the metabotropic Zn²⁺ receptor ZnR/GPR39 can modulate salivary secretion in human submandibular gland cells independent of muscarinic or histamine receptor signaling.

Calcium antagonists cause dry mouth by inhibiting resting saliva secretion

Ca2+ antagonists cause dry mouth by inhibiting saliva secretion. The present study was undertaken to elucidate the mechanism by which Ca2+ antagonists cause dry mouth. Since the intracellular Ca2+ concentration ([Ca2+]i) is closely related to saliva secretion, [Ca2+]i was measured with a video-imaging analysis system by using human submandibular gland (HSG) cells as the material. The Ca2+ antagonist, nifedipine, inhibited the elevation in [Ca2+]i induced by 1-10 microM carbachol (CCh), but had no inhibitory effect on that induced by 30 and 100 microM CCh. The other kinds of Ca2+ antagonists, verapamil (10 microM), diltiazem (10 microM), and the inorganic Ca2+ channel blocker, CdCl2 (50 microM), also inhibited the [Ca2+]i elevation induced by 10 microM CCh. The Ca2+ channel activator, Bay K 8644 (5 microM), significantly enhanced the CCh (10 microM)-induced [Ca2+]i elevation. Endothelin-1 and norepinephrine also increased the CCh (10 microM)-induced [Ca2+]i elevation. SKF-96365 reversed the enhancement of the CCh (10 microM)-induced [Ca2+]i elevation caused by AlF4- and phenylephrine. The phospholipase Cbeta (PLCbeta) inhibitor, U-73122 (5 microM), significantly inhibited the [Ca2+]i elevation induced by 100 microM CCh compared with that induced by 10 microM CCh, while the PLCbeta activator, m-3M3FBS (20 microM), significantly increased the [Ca2+]i elevation induced by 100 microM CCh compared with that induced by 10 microM CCh. We therefore conclude that non-selective cation and voltage-dependent Ca2+ channels are involved in resting salivation and that Ca2+ antagonists depress H2O secretion by blocking the Ca2+ channels and thereby cause dry mouth.

Changes in functioning of rat submandibular salivary gland under streptozotocin-induced diabetes are associated with alterations of Ca2+ signaling and Ca2+ transporting pumps

Xerostomia and pathological thirst are troublesome complications of diabetes mellitus associated with impaired functioning of salivary glands; however, their cellular mechanisms are not yet determined. Isolated acinar cells were loaded with Ca2+ indicators fura-2/AM for measuring cytosolic Ca2+ concentration ([Ca2+]i) or mag-fura-2/AM-inside the endoplasmic reticulum (ER). We found a dramatic decrease in pilocarpine-stimulated saliva flow, protein content and amylase activity in rats after 6 weeks of diabetes vs. healthy animals. This was accompanied with rise in resting [Ca2+]i and increased potency of acetylcholine (ACh) and carbachol (CCh) but not norepinephrine (NE) to induce [Ca2+]i transients in acinar cells from diabetic animals. However, [Ca2+]i transients mediated by Ca2+ release from ER stores (induced by application of either ACh, CCh, NE, or ionomycin in Ca2+-free extracellular medium) were decreased under diabetes. Application of inositol-1,4,5-trisphosphate led to smaller Ca2+ release from ER under the diabetes. Both plasmalemma and ER Ca2+-ATPases activity was reduced and the latter showed the increased affinity to ATP under the diabetes. We conclude that the diabetes caused impairment of salivary cells functions that, on the cellular level, associates with Ca2+ overload, increased Ca2+-mobilizing ability of muscarinic but not adrenergic receptors, decreased Ca2+-ATPases activity and ER Ca2+ content.

Using HSV-thymidine kinase for safety in an allogeneic salivary graft cell line

Extreme salivary hypofunction is a result of tissue damage caused by irradiation therapy for cancer in the head and neck region. Unfortunately, there is no currently satisfactory treatment for this condition that affects up to 40,000 people in the United States every year. As a novel approach to managing this problem, we are attempting to develop an orally implantable, fluid-secreting device (an artificial salivary gland). We are using the well-studied HSG salivary cell line as a potential allogeneic graft cell for this device. One drawback of using a cell line is the potential for malignant transformation. If such an untoward response occurred, the device could be removed. However, in the event that any HSG cells escaped, we wished to provide additional patient protection. Accordingly, we have engineered HSG cells with a hybrid adeno-retroviral vector, AdLTR.CMV-tk, to express the herpes simplex virus thymidine kinase (HSV-tk) suicide gene as a novel safety factor. Cells were grown on plastic plates or on poly-L-lactic acid disks and then transduced with different multiplicities of infection (MOIs) of the hybrid vector. Thereafter, various concentrations of ganciclovir (GCV) were added, and cell viability was tested. Transduced HSG cells expressed HSV-tk and were sensitive to GCV treatment. Maximal effects were seen at a MOI of 10 with 50 microM of GCV, achieving 95% cell killing on the poly-L-lactic acid substrate. These results suggest that engineering the expression of a suicide gene in an allogeneic graft cell may provide additional safety for use in an artificial salivary gland device.

EGF inhibits muscarinic receptor-mediated calcium signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on intracellular calcium ([Ca(2+)](i)) responses to the muscarinic agonist carbachol were studied in a human salivary cell line (HSY). Carbachol (10(-4) M)-stimulated [Ca(2+)](i) mobilization was inhibited by 40% after 48-h treatment with 5 x 10(-10) M EGF. EGF also reduced carbachol-induced [Ca(2+)](i) in Ca(2+)-free medium and Ca(2+) influx following repletion of extracellular Ca(2+). Under Ca(2+)-free conditions, thapsigargin, an inhibitor of Ca(2+) uptake to internal stores, induced similar [Ca(2+)](i) signals in control and EGF-treated cells, indicating that internal Ca(2+) stores were unaffected by EGF; however, in cells exposed to thapsigargin, Ca(2+) influx following Ca(2+) repletion was reduced by EGF. Muscarinic receptor density, assessed by binding of the muscarinic receptor antagonist L-[benzilic-4,4'-(3)HCN]quinuclidinyl benzilate ([(3)H]QNB), was decreased by 20% after EGF treatment. Inhibition of the carbachol response by EGF was not altered by phorbol ester-induced downregulation of protein kinase C (PKC) but was enhanced upon PKC activation by a diacylglycerol analog. Phosphorylation of mitogen-activated protein kinase (MAP kinase) and inhibition of the carbachol response by EGF were both blocked by the MAP kinase pathway inhibitor PD-98059. The results suggest that EGF decreases carbachol-induced Ca(2+) release from internal stores and also exerts a direct inhibitory action on Ca(2+) influx. A decline in muscarinic receptor density may contribute to EGF inhibition of carbachol responsiveness. The inhibitory effect of EGF is mediated by the MAP kinase pathway and is potentiated by a distinct modulatory cascade involving activation of PKC. EGF may play a physiological role in regulating muscarinic receptor-stimulated salivary secretion.

The growth and morphological behavior of salivary epithelial cells on matrix protein-coated biodegradable substrata

The purpose of this study was to examine the growth and morphology of a salivary epithelial cell line (HSG) in vitro on several biodegradable substrata as an important step toward developing an artificial salivary gland. The substrates examined were poly-L-lactic acid (PLLA), polyglycolic acid (PGA), and two co-polymers, 85% and 50% PLGA, respectively. The substrates were formed into 20- to 25-mm disks, and the cells were seeded directly onto the polymers or onto polymers coated with specific extracellular matrix proteins. The two copolymer substrates became friable over time in aqueous media and proved not useful for these experiments. The purified matrix proteins examined included fibronectin (FN), laminin (LN), collagen I, collagen IV, and gelatin. In the absence of preadsorbed proteins, HSG cells did not attach to the polymer disks. The cells, in general, behaved similarly on both PLLA and PGA, although optimal results were obtained consistently in PLLA. On FN-coated PLLA disks, HSG cells were able to form a uniform monolayer, which was dependent on time and FN concentration. Coating of disks with LN, collagen I, and gelatin also promoted monolayer growth. This study defines the conditions necessary for establishing a monolayer organization of salivary epithelial cells with rapid proliferation on a biodegradable substrate useful for tissue engineering.

Ca2+-dependent inactivation of a store-operated Ca2+ current in human submandibular gland cells. Role of a staurosporine-sensitive protein kinase and the intracellular Ca2+ pump

Stimulation of human submandibular gland cells with carbachol, inositol trisphosphate (IP3), thapsigargin, or tert-butylhydroxyquinone induced an inward current that was sensitive to external Ca2+ concentration ([Ca2+]e) and was also carried by external Na+ or Ba2+ (in a Ca2+-free medium) with amplitudes in the order Ca2+ > Ba2+ > Na+. All cation currents were blocked by La3+ and Gd3+ but not by Zn2+. The IP3-stimulated current with 10 microM 3-deoxy-3-fluoro-D-myo-inositol 1,4,5-triphosphate and 10 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid in the pipette solution, showed 50% inactivation in <5 min and >5 min with 10 and 1 mM [Ca2+]e, respectively. The Na+ current was not inactivated, whereas the Ba2+ current inactivated at a slower rate. The protein kinase inhibitor, staurosporine, delayed the inactivation and increased the amplitude of the current, whereas the protein Ser/Thr phosphatase inhibitor, calyculin A, reduced the current. Thapsigargin- and tert-butylhydroxyquinone-stimulated Ca2+ currents inactivated faster. Importantly, these agents accelerated the inactivation of the IP3-stimulated current. The data demonstrate that internal Ca2+ store depletion-activated Ca2+ current (ISOC) in this salivary cell line is regulated by a Ca2+-dependent feedback mechanism involving a staurosporine-sensitive protein kinase and the intracellular Ca2+ pump. We suggest that the Ca2+ pump modulates ISOC by regulating [Ca2+]i in the region of Ca2+ influx.

Gamma-irradiation-induced cell cycle arrest and cell death in a human submandibular gland cell line: effect of E2F1 expression

This study examined the effect of gamma-irradiation (5 and 10 Gy) on the human submandibular cell line (HSG). Radiation treatment (5 Gy and 10 Gy) induced a dose-dependent decrease in cell proliferation, with a G2/M arrest of the cell cycle, and an increase in cell death (cells with <2n DNA increased from 7% in control cells to 34% and 40% in 5 and 10 Gy irradiated cells, respectively). [Ca2+]i measurements demonstrated that the status of internal Ca2+ stores, and muscarinic receptor-mediated Ca2+ mobilization, in irradiated cells was comparable to that in non-irradiated cells. These data suggest that 1) irradiated HSG cells maintain normal physiology and 2) internal Ca2+ store depletion does not account for the decreased cell proliferation. To manipulate the radiation-induced cell cycle arrest, we examined the effect of the transcription factor E2F1, which has been shown to induce cell cycle progression in HSG cells (Lillibridge and O'Connell, 1997, J. Cell. Physiol., 1 72:343-350). The ability of irradiated HSG cells to express and appropriately route proteins was demonstrated by using adenovirus-mediated expression of beta-galactosidase, alpha1-antitrypsin, and aquaporin-1. Infection of HSG cells with an adenoviral vector encoding E2F1, either 12 h before or immediately following irradiation, but not post-irradiation, induced maintenance of cells in the S phase of the cell cycle, reduced the number of cells arrested at G2/M, and decreased the rate of appearance of cells with <2n DNA. While the mechanism of irradiation-induced cell death has not yet been confirmed, these data suggest that expression of the E2F1 gene product in HSG cells can be a useful strategy to manipulate cell cycle events and reduce the initial loss of cells due to radiation.

Regulation of KCa current by store-operated Ca2+ influx depends on internal Ca2+ release in HSG cells

This study examines the Ca2+ influx-dependent regulation of the Ca2+-activated K+ channel (KCa) in human submandibular gland (HSG) cells. Carbachol (CCh) induced sustained increases in the KCa current and cytosolic Ca2+ concentration ([Ca2+]i), which were prevented by loading cells with 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA). Removal of extracellular Ca2+ and addition of La3+ or Gd3+, but not Zn2+, inhibited the increases in KCa current and [Ca2+]i. Ca2+ influx during refill (i.e., addition of Ca2+ to cells treated with CCh and then atropine in Ca2+-free medium) failed to evoke increases in the KCa current but achieved internal Ca2+ store refill. When refill was prevented by thapsigargin, Ca2+ readdition induced rapid activation of KCa. These data provide further evidence that intracellular Ca2+ accumulation provides tight buffering of [Ca2+]i at the site of Ca2+ influx (H. Mogami, K. Nakano, A. V. Tepikin, and O. H. Petersen. Cell 88: 49-55, 1997). We suggest that the Ca2+ influx-dependent regulation of the sustained KCa current in CCh-stimulated HSG cells is mediated by the uptake of Ca2+ into the internal Ca2+ store and release via the inositol 1,4,5-trisphosphate-sensitive channel.

Differential sensitivity to nickel and SK&F96365 of second messenger-operated and receptor-operated calcium channels in rat submandibular ductal cells

The intracellular concentration of calcium ([Ca2+]i) of rat submandibular ductal cells was measured with the intracellular fluorescent dye Fura-2. Carbachol (100 microM) and ATP (1 mM) both increased the [Ca2+]i. The late response to ATP was blocked by 0.5 mM Ni2+. This concentration of Ni2+ also blocked the increase of the [Ca2+]i and the uptake of manganese and calcium in response to 2'- and 3'-O-(4-benzoylbenzoyl) adenosine 5'-triphosphate (BzATP, 100 microM), a specific agonist of P2X receptors from salivary glands. The increase of the [Ca2+]i in response to 2-methylthioadenosine 5'-triphosphate (2-MeSATP, 100 microM) a specific P2Y agonist in salivary glands or to a muscarinic agonist (carbachol) was not affected by 0.5 mM Ni2+. Only higher concentrations of Ni2+ (in the millimolar range) inhibited the uptake of extracellular calcium in response to carbachol. SK&F96365, a blocker of store-operated calcium channels, inhibited the uptake of extracellular calcium in response to carbachol without affecting the response to BzATP. It is concluded that at low concentrations (below 0.5 mM), Ni2+ inhibits the non-specific cation channel coupled to P2X receptors. The uptake of extracellular calcium by store-operated calcium channels is inhibited by higher concentrations of Ni2+ and by SK&F96365.

Interferon-gamma induces a decrease in the intracellular calcium pump in a human salivary gland cell line

Interferon-gamma (IFN-gamma) +/- tumor necrosis factor-alpha (TNF-alpha) induces antiproliferation and intracellular Ca2+ store depletion in a human submandibular ductal cell line (HSG), which can be reversed on cytokine removal [A. J. Wu, G. C. Chen, B. J. Baum, and I. S. Ambudkar. Am. J. Physiol. 270 (Cell Physiol. 39): C514-C521, 1996]. Here we have examined a possible mechanism for the IFN-gamma-induced intracellular Ca2+ store depletion. There was a time-dependent decrease in thapsigargin-dependent internal Ca2+ release after exposure of the cells to the cytokines. The intracellular Ca2+ pump [sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA)] protein in lysates and membranes of cells treated with IFN-gamma +/- TNF-alpha, but not with TNF-alpha alone, showed a similar time-dependent decrease (examined using a SERCA2 antibody). Removal of the cytokines, which resulted in recovery of cell growth and refill of internal Ca2+ stores, also increased the level of SERCA protein. The decrease in SERCA is not a result of decreased cell proliferation, since thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone, or serum-free growth conditions induced antiproliferative effects on HSG cells without any corresponding decrease in SERCA. We suggest that the IFN-gamma-induced decrease in the level of SERCA accounts for the depleted state of internal Ca2+ stores in cytokine-treated HSG cells. These data suggest a novel mechanism for the inhibition of HSG cell growth by IFN-gamma.

Interferon-gamma-induced JAK2 and STAT1 signalling in a human salivary gland cell line

We have used a human salivary gland cell line (HSG) as a possible in vitro model to evaluate the effects of IFN-gamma on human salivary gland epithelium (Wu et al., 1994, 1996, 1997). In the present study, we examined the JAK-STAT signal-transduction pathway in IFN-gamma-treated HSG cells. We demonstrate that JAK2 and Stat1 are phosphorylated at tyrosine residues in a time- and concentration-dependent manner following exposure to IFN-gamma. In addition, we show that activation of this signalling pathway is decreased by the addition of a blocking antibody to the IFN-gamma receptor. The same maneuver is also able to reduce by approximately 50-70% the surface expression of two IFN-gamma-induced immunoregulatory molecules: HLA-DR and ICAM-1. These results demonstrate that the JAK2 and Stat1 signalling pathway is active in salivary-derived epithelial cells and may contribute to their immunopathologic destruction.

Calcium influx pathways in rat pancreatic ducts

A number of agonists increase intracellular Ca2+ activity, [Ca2+]i, in pancreatic ducts, but the influx/efflux pathways and intracellular Ca2+ stores in this epithelium are unknown. The aim of the present study was to characterise the Ca2+ influx pathways, especially their pH sensitivity, in native pancreatic ducts stimulated by ATP and carbachol, CCH. Under control conditions both agonists led to similar changes in [Ca2+]i. However, these Ca2+ transients, consisting of peak and plateau phases, showed different sensitivities to various experimental manoeuvres. In extracellular Ca2+-free solutions, the ATP-induced [Ca2+]i peak decreased by 25%, but the CCH-induced peak was unaffected; both plateaus were inhibited by 90%. Flufenamate inhibited the ATP-induced peak by 35%, but not the CCH-evoked peak; the plateaus were inhibited by 75-80%. La3+ inhibited the ATP-induced plateau fully, but that induced by CCH by 55%. In resting ducts, an increase in extracellular pH, pHe, by means of HEPES and HCO3-/CO2 buffers, increased [Ca2+]i; a decrease in pHe had the opposite effect. In stimulated ducts the pH-evoked effects on Ca2+ influx were more pronounced and depended on the agonist used. At pHe 6.5 both ATP- and CCH-evoked plateaus were inhibited by about 50%. At pH 8.0 the ATP-stimulated plateau was inhibited by 27%, but that stimulated by CCH was increased by 72%. Taken together, we show that CCH stimulates Ca2+ release followed by Ca2+ influx that is moderately sensitive to flufenamate, La3+, depolarisation, it is inhibited by low pH, but stimulated by high pH. ATP stimulates Ca2+ release and probably an early Ca2+ influx, which is more markedly sensitive to flufenamate and La3+, and is both inhibited by low and high pH. Thus our study indicates that there are at least two separate Ca2+ influx pathways in pancreatic ducts cells.

Interferon-gamma induces persistent depletion of internal Ca2+ stores in a human salivary gland cell line

Interferon-gamma (IFN-gamma), in the presence of tumor necrosis factor-alpha (TNF-alpha), decreases proliferation of a human salivary gland ductal cell line, HSG (Wu, A., R. Kurrasch, J. Katz, P. Fox, B. Baum, and J. Atkinson. J. Cell. Physiol. 161:217-226, 1994). We examined the possible effects of these cytokines (1,000 U/ml IFN-gamma +/- 20 U/ml TNF-alpha for 7 days) on Ca2+ mobilization in HSG cells. In HSG cells, fetal bovine serum (10%) or carbachol (100 microM) stimulated rapid increases in cytosolic Ca2+ concentration ([Ca2+]i), apparently mobilized from different thapsigargin-sensitive intracellular Ca2+ stores. Serum induced a proliferative effect on HSG cells, which was suppressed (> 90%) by treatment with IFN-gamma +/- TNF-alpha, but not with TNF-alpha alone. Serum-, carbachol-, and thapsigargin-stimulated [Ca2+]i elevations were reduced by 90, 60, and > 65%, respectively, in cells treated with IFN-gamma +/- TNF-alpha and 30, 45, and 45%, respectively, in cells treated with TNF-alpha. Removal of the cytokines from the growth medium induced recovery of both cell proliferation and Ca2+ mobilization responses within 7 days. Treatment of HSG cells with thapsigargin (0.02-2 nM) induced a dose-dependent decrease in cell proliferation. Additionally, acute treatment (< 10 min) of cells with IFN-gamma did not affect [Ca2+]i or alter carbachol-, thapsigargin-, or serum-induced changes in [Ca2+]i. These data demonstrate that prolonged treatment of HSG cells with IFN-gamma +/- TNF-alpha leads to a persistent depletion of intracellular Ca2+ stores. We suggest that this may have a role in cell growth.

Ca2+ entry following store depletion in SH-SY5Y neuroblastoma cells

Ca2+ entry following Ca2+ store depletion was examined in the human neuroblastoma cell line, SH-SY5Y, by measuring the concentration of intracellular free Ca2+ ([Ca2+]i) with fura-2. Application of the muscarinic agonist oxotremorine-M (oxo-M) caused an increase in [Ca2+]i. This consisted of a peak, mediated by release of Ca2+ from internal stores followed by a sustained plateau, mediated by Ca2+ entry across the plasma membrane. The Ca2+ entry resulted from depletion of intracellular Ca2+ stores This pathway was further characterized in the presence of thapsigargin, an inhibitor of the Ca2+ ATPase involved in replenishing IP3-sensitive stores. Stores were first depleted with oxo-M and thapsigargin in the absence of extracellular Ca2+. After washout of oxo-M, subsequent exposure to Ca2+ evoked reproducible increases in [Ca2+]i. Application of oxo-M plus Ca2+ had little effect on the increases in [Ca2+]i, indicating that in SH-SY5Y cells, agonist-dependent pathways contribute little to Ca2+ entry following store depletion. Mn2+, Sr2+ and Ba2+ were permeable through this pathway. Mn2+ and Ba2+ also showed slight permeability in the absence of store depletion. Ca2+ entry following store depletion was blocked by La3+ (IC50 = 75 nM) and by SKF 96365. La3+ blocked Mn2+ entry through the pathway activated by store depletion but did not affect basal Mn2+ permeability. These results indicate that SH-SY5Y neuroblastoma cells have an agonist-independent Ca2+ entry pathway activated by store depletion.