Characterization of a phosphatidic acid phosphatase from rat brain cell membranes

The lymphocyte function-associated antigen-1 receptor costimulates plasma membrane Ras via phospholipase D2

Ras activation as a consequence of antigen receptor (T-cell receptor; TCR) engagement on T lymphocytes is required for T-cell development, selection and function. Lymphocyte function-associated antigen-1 (LFA-1) mediates lymphocyte adhesion, stabilization of the immune synapse and bidirectional signalling. Using a fluorescent biosensor we found that TCR activation with or without costimulation of CD28 led to activation of Ras only on the Golgi apparatus, whereas costimulation with LFA-1 induced Ras activation on both the Golgi and the plasma membrane. Ras activation on both compartments required RasGRP1, an exchange factor regulated by calcium and diacylglycerol (DAG), but phospholipase C (PLC) activity was required only for activation on the Golgi. Engagement of LFA-1 increased DAG levels at the plasma membrane by stimulating phospholipase D (PLD). PLD2 and phosphatidic acid phosphatase (PAP) were required for Ras activation on the plasma membrane. Thus, LFA-1 acts through PLD2 to reshape the pattern of Ras activation downstream of the TCR.

The inhibitory effect of local anesthetics on bradykinin-induced phospholipase D activation in rat pheochromocytoma PC12 cells

Bradykinin induces activation of phospholipase D (PLD) via B(2) receptors in neuronal cells. To demonstrate molecular mechanism(s) of local anesthetics, we examined whether and how local anesthetics affect bradykinin-induced PLD activation in PC12 cells. Using [(3)H]Palmitic acid-labeled PC12 cells stimulated with bradykinin, formation of [(3)H]phosphatidylbutanol was measured as a variable of PLD activity. Bradykinin-stimulated PLD activity seemed to peak at 2 min. Procaine, lidocaine, ropivacaine, bupivacaine, and tetracaine suppressed the bradykinin-induced PLD activation. We chose tetracaine, the most potent drug among the local anesthetics tested, to examine how local anesthetics affect phospholipase C, protein tyrosine kinase, and extracellular signal-regulated kinase, which are the molecules upstream of PLD. Tetracaine at clinically relevant concentrations (1 approximately 10 x 10(-4) M) inhibited the bradykinin-induced PLD activation in a dose- and time-dependent manner, but neither tetrodotoxin nor nifedipine affected the PLD activation. Tetracaine (5 x 10(-4) M) slightly potentiated brady-kinin-induced phospholipase C activation. Bradykinin-stimulated protein tyrosine-phosphorylation and extracellular signal-regulated kinase activation were not affected by tetracaine. Tetracaine significantly decreased PLD activity of membrane fraction in PC12 cells. These results indicate that local anesthetics depress bradykinin-induced lipid signaling pathway(s) and may provide some clues to understanding the molecular mechanisms of these drugs for anesthesia or analgesia.

IMPLICATIONS

Local anesthetics depressed the bradykinin-induced activation of phospholipase D (PLD) in PC12 cells. The effects of tetracaine, the most potent among the anesthetics tested, on the bradykinin-induced intracellular signaling molecules were examined. The bradykinin-induced PLD activation could be one of the potential intracellular signaling molecular sites of local anesthetic action.

Identification of the phosphatidic acid phosphatase type 2a isozyme as an androgen-regulated gene in the human prostatic adenocarcinoma cell line LNCaP

Differential display was used to identify novel androgen-regulated genes in the human prostatic adenocarcinoma cell line LNCaP. A 322-base pair cDNA fragment that was consistently induced by the synthetic androgen R1881 revealed 100% homology with the human phosphatidic acid phosphatase type 2a isozyme very recently reported by Kai et al. (PAP-2a; Kai., M., Wada, I., Imai, S.-i., Sakane, F., and Kanoh, H. (1997) J. Biol. Chem. 272, 24572-24578). The fragment was used to clone the corresponding cDNA from a human prostate library. The deduced amino acid sequence confirmed the identity with human PAP-2a. The inducibility of PAP-2a mRNA by androgens was confirmed by Northern blot hybridization. The effect was time- and dose-dependent with a maximal stimulation (4-fold) after 24 h of treatment with 10(-9) M R1881. The steroid specificity of PAP-2a mRNA regulation was found to be in agreement with the aberrant ligand specificity of the mutated androgen receptor in LNCaP cells, supporting the involvement of the androgen receptor in the induction process. Furthermore, low basal levels of PAP-2a mRNA and absence of androgen inducibility were observed in the poorly differentiated and androgen receptor-negative cell lines PC-3 and DU-145. Induction of PAP-2a mRNA was not affected by the protein synthesis inhibitor cycloheximide and was accompanied by a marked increase in PAP-2 activity as measured by the conversion of phosphatidic acid into diacylglycerol in membrane fractions of LNCaP. Comparison of the expression of PAP-2a mRNA in 50 different human tissues revealed ubiquitous expression. The highest levels, however, were observed in the prostate. Since PAP-2 plays a pivotal role in the control of signal transduction by lipid mediators such as phosphatidate, lysophosphatidate, and ceramide-1-phosphate, the ability of androgens to stimulate the expression and activity of this enzyme in prostatic cells may provide an important opportunity for cross-talk between signaling pathways involving lipid mediators and androgens.