Regulatory mechanisms and roles of phospholipase D (PLD) in cellular responses

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We have found the alternatively spliced isoform of human phospholipase D1 (hPLD1). Moreover, the entire coding regions of rat PLD1 and PLD2 were isolated and sequenced. A shorter alternatively spliced form of rPLD1 (rPLD1b) was also identified. Chromosomal locations of these genes were determined in the rat and mouse by FISH.The PLD1 and PLD2 genes were localized to rat Chromosome 2q23.3-34 proximal end and Chromosome 10q23.3-24 proximal end, respectively. During cell differentiation or apoptotic processes, the expression of PLD1 and PLD2 mRNAs were drastically changed in rat phochromocytoma PC12 cells, human leukemic HL-60 cells and rat basophilic leukemia RBL-2H3 cells. In the presence of ethanol or butanol which is utilized for transphosphatidylation reaction of PLD to produce phosphatidylalcohol at the expense of phosphatidic acid, the expression of transcription factors, c-jun and c-fos in response to growth signal was inhibited, suggesting that PLD is closely involved in growth signal transduction. The regulatory mechanism of PLD was investigated in differentiated HL-60 cells and PC12 cells. Inhibitors of phosphatidylinositol 3-kinase (PI-3K), wortmannin and LY294002 revealed that PI-3K is located between receptor and PLD in differentiated HL-60 cells. Moreover, a kind of tyrosine kinase may work at the downstream of PI-3K.In PC12 cells, possible involvement of a calcium-sensitive tyrosine kinase in carbachol-induced PLD activation was suggested. Hydrogen peroxide (H_2O_2) greatly stimulated PLD activity in PC12 cells. An unidentified tyrosine kinase is also appeared to function to activate PLD in H_2O_2-stimulated PC12 cells. We are currently working on to identify the tyrosine kinase involved in these processes and also planning to use antisense oligonucleotides and cells transfected with PLD genes to further investigate the roles of PLD in cellular responses.