The stimulation of PLD1 by cholinergic agonists most likely plays a key role in the regulation of protein secretion from the lacrimal gland as cholinergic agonists are a major stimuli of this response. PLD activity has also been shown to play key roles in exocytosis in a variety of tissues and cell lines, including insulin from pancreatic β-cells, catecholamines from chromaffin cells, and von Willebrand factor from endothelial cells.
25 –27 In lacrimal gland disease, overactivation of the PLD1 pathway or its components could block cholinergic agonist stimulation of secretion leading to dry eye disease. Thus inhibition of PLD1, ROCK, MEK, or ERK could be developed as a treatment for dry eye. In many cell types, PKC plays an important role in stimulation of PLD activity.
4,28 Regulation of PLD by PKC is not completely understood but appears to be a result of a direct interaction between the two molecules. Indeed, the C terminus of PKCα has been shown to bind to and activate PLD1 in cells overexpressing PLD through a direct interaction and not by phosphorylation.
28 The antibody used in this study to detect activated PLD is directed against Thr147, a site shown to be phosphorylated by PKCα.
29 PKCβ isoforms can also stimulate PLD1.
30 The lacrimal gland contains at least four PKC isoforms.
31 In these cells, with specific inhibitors of PKCα, -δ, and -ε, we determined that cholinergic agonists activate PKCα, -δ, and -ε to stimulate protein secretion.
32 PKC also activates Pyk2, cSrc, and ERK to attenuate protein secretion in these cells.
22 In addition, cholinergic agonist-stimulated ERK activation was inhibited by nearly 80% by two PKC inhibitors.
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