Muscarinic Receptors and BK Channels Are Affected by Lipid Raft Disruption of Salivary Gland Cells.

Activity-dependent fluid secretion is the most important physiological function of salivary glands and is regulated via muscarinic receptor signaling. Lipid rafts are important for G-protein coupled receptor (GPCR) signaling and ion channels in plasma membranes. However, it is not well understood whether lipid raft disruption affects all membrane events or only specific functions in muscarinic receptor-mediated water secretion in salivary gland cells. We investigated the effects of lipid raft disruption on the major membrane events of muscarinic transcellular water movement in human salivary gland (HSG) cells. We found that incubation with methyl-ß-cyclodextrin (MßCD), which depletes lipid rafts, inhibited muscarinic receptor-mediated Ca2+ signaling in HSG cells and isolated mouse submandibular acinar cells. However, MßCD did not inhibit a Ca2+ increase induced by thapsigargin, which activates store-operated Ca2+ entry (SOCE). Interestingly, MßCD increased the activity of the large-conductance Ca2+-activated K+ channel (BK channel). Finally, we found that MßCD did not directly affect the translocation of aquaporin-5 (AQP5) into the plasma membrane. Our results suggest that lipid rafts maintain muscarinic Ca2+ signaling at the receptor level without directly affecting the activation of SOCE induced by intracellular Ca2+ pool depletion or the translocation of AQP5 into the plasma membrane.

Human salivary gland cells express bradykinin receptors that modulate the expression of proinflammatory cytokines.

Bradykinin is an important peptide modulator that affects the function of neurons and immune cells. However, there is no evidence of the bradykinin receptors and their functions in human salivary glands. Here we have identified and characterized bradykinin receptors on human submandibular gland cells. Both bradykinin B1 and B2 receptors are expressed on human submandibular gland cells, A253 cells, and HSG cells. Bradykinin increased the intracellular Ca2+ concentration ([Ca2+ ]i ) in a concentration-dependent manner. Interestingly, a specific agonist of the B1 receptor did not have any effect on [Ca2+ ]i in HSG cells, whereas specific agonists of the B2 receptor had a Ca2+ mobilizing effect. Furthermore, application of the B1 receptor antagonist, R715, did not alter the bradykinin-mediated increase in cytosolic Ca2+ , whereas the B2 receptor antagonist, HOE140, showed a strong inhibitory effect, which implies that bradykinin B2 receptors are functional in modulating the concentration of cytosolic Ca2+ . Bradykinin did not affect a carbachol-induced rise of [Ca2+ ]i and did not modulate translocation of aquaporin-5. However, bradykinin did promote the expression of proinflammatory cytokines, including tumor necrosis factor-α (TNF-α), implying the role of bradykinin in salivary gland inflammation. These data suggest that bradykinin receptors are involved in Ca2+ signaling in human submandibular gland cells and serve a unique role, which is separate from that of other salivary gland G protein-coupled receptors.

Polychlorinated biphenyl 19 blocks the most common form of store-operated Ca2+ entry through Orai.

PCB19, a 2,2',6-trichlorinated biphenyl, is one of many non-dioxin-like polychlorinated biphenyls (NDL-PCBs), which are ubiquitous pollutants. NDL-PCBs affect cytosolic Ca2+ signaling by promoting Ca2+ release from ryanodine receptor-sensitive Ca2+ pools and inhibiting store-operated Ca2+ entry (SOCE) from the extracellular space. However, NDL-PCB-mediated SOCE inhibition has only been demonstrated in PC12 cells, in which SOCE is thought to be mainly mediated by TRPC family channels. Here, we investigated the effect of PCB19 on SOCE using human embryonic kidney 293 (HEK293) cells, human leukemia T cell line Jurkat-T cells and human promyelocytoma HL-60 cells which are the cell lines that are previously demonstrated to mediate the most common form of SOCE solely by the intrinsic Orai channels. PCB19 reduced thapsigargin-induced Ca2+ influx after Ca2+ pool depletion in HEK293 cells. SOCEs in HEK293, Jurkat T, HL-60 and PC12 cells showed distinct sensitivities to SOCE inhibitors such as Gd3+ and ML-9; however, PCB19 also showed a common effect of inhibiting SOCEs in all cell lines. PCB19-mediated SOCE inhibition was confirmed by demonstrating the ability of PCB19 to inhibit the SOCE current but not the TRPM7 current. These results imply that PCB19 inhibits not only TRPC-mediated SOCE as in PC12 cells but also Orai-mediated SOCE as in many other cells including HEK293, Jurkat T and HL-60 cells.

Non-Dioxin-Like Polychlorinated Biphenyls Inhibit G-Protein Coupled Receptor-Mediated Ca2+ Signaling by Blocking Store-Operated Ca2+ Entry.

Polychlorinated biphenyls (PCBs) are ubiquitous pollutants which accumulate in the food chain. Recently, several molecular mechanisms by which non-dioxin-like (NDL) PCBs mediate neurodevelopmental and neurobehavioral toxicity have been elucidated. However, although the G-protein coupled receptor (GPCR) is a significant target for neurobehavioral disturbance, our understanding of the effects of PCBs on GPCR signaling remains unclear. In this study, we investigated the effects of NDL-PCBs on GPCR-mediated Ca2+ signaling in PC12 cells. We found that ortho-substituted 2,2',6-trichlorinated biphenyl (PCB19) caused a rapid decline in the Ca2+ signaling of bradykinin, a typical Gq- and phospholipase Cß-coupled GPCR, without any effect on its inositol 1,4,5-trisphosphate production. PCB19 reduced thapsigargin-induced sustained cytosolic Ca2+ levels, suggesting that PCB19 inhibits SOCE. The abilities of other NDL-PCBs to inhibit store-operated Ca2+ entry (SOCE) were also examined and found to be of similar potencies to that of PCB19. PCB19 also showed a manner equivalent to that of known SOCE inhibitors. PCB19-mediated SOCE inhibition was confirmed by demonstrating the ability of PCB19 to inhibit the SOCE current and thapsigargin-induced Mn2+ influx. These results imply that one of the molecular mechanism by which NDL-PCBs cause neurobehavioral disturbances involves NDL-PCB-mediated inhibition of SOCE, thereby interfering with GPCR-mediated Ca2+ signaling.

Prostaglandin modulates TLR3-induced cytokine expression in human astroglioma cells.

Cyclooxygenase (COX) products and pattern recognition receptors are important modulators of neuroinflammation; however, the role of prostaglandins and toll-like receptor (TLR) signaling and the functional crosstalk between COX modulators remains unclear, especially in astrocytes that closely modulate neuronal functions. Here, we studied the effect of prostaglandins on toll-like receptor 3 (TLR3)-induced cytokine expression in human astroglioma CRT-MG cells. Prostaglandin E2 (PGE2) was shown to increase cytosolic cAMP levels in an EP2 receptor dependent manner. Interestingly, the TLR3 agonist polyinosinic:polycytidylic acid (poly(I:C)) mediated phosphorylation of NF-κB and extracellular stress-related kinase 1/2 (ERK1/2), which significantly decreased following PGE2 treatment. In addition, PGE2 increased the phosphorylation and inactivation of glycogen synthesis kinase-3ß (GSK-3ß), whereas poly(I:C) decreased it. We observed that PGE2 decreased tumor necrosis factor-α (TNF-α) production evoked by poly(I:C), whereas PGE2 potentiated poly(I:C)-triggered interleukin-8 (IL-8) production. These results suggest that prostaglandin modulates the TLR3-mediated cytokine profile in astrocytes via EP2 receptors and regulates the NF-κB, ERK1/2 and GSK-3ß signaling pathways.

Desipramine inhibits histamine H1 receptor-induced Ca2+ signaling in rat hypothalamic cells.

The hypothalamus in the brain is the main center for appetite control and integrates signals from adipose tissue and the gastrointestinal tract. Antidepressants are known to modulate the activities of hypothalamic neurons and affect food intake, but the cellular and molecular mechanisms by which antidepressants modulate hypothalamic function remain unclear. Here we have investigated how hypothalamic neurons respond to treatment with antidepressants, including desipramine and sibutramine. In primary cultured rat hypothalamic cells, desipramine markedly suppressed the elevation of intracellular Ca(2+) evoked by histamine H1 receptor activation. Desipramine also inhibited the histamine-induced Ca(2+) increase and the expression of corticotrophin-releasing hormone in hypothalamic GT1-1 cells. The effect of desipramine was not affected by pretreatment with prazosin or propranolol, excluding catecholamine reuptake activity of desipramine as an underlying mechanism. Sibutramine which is also an antidepressant but decreases food intake, had little effect on the histamine-induced Ca(2+) increase or AMP-activated protein kinase activity. Our results reveal that desipramine and sibutramine have different effects on histamine H1 receptor signaling in hypothalamic cells and suggest that distinct regulation of hypothalamic histamine signaling might underlie the differential regulation of food intake between antidepressants.