Phospholipase A(2) is involved in thapsigargin-induced sodium influx in human lymphocytes

Discovery and Structure-Based Optimization of Novel Atg4B Inhibitors for the Treatment of Castration-Resistant Prostate Cancer

Autophagy inhibition is an attractive target for cancer therapy. In this study, we discovered inhibitors of Atg4B essential for autophagosome formation and evaluated their potential as therapeutics for prostate cancer. Seventeen compounds were identified as candidates after in silico screening and a thermal shift assay. Among them, compound 17 showed the most potent Atg4B inhibitory activity, inhibited autophagy induced by anti-castration-resistant prostate cancer (CRPC) drugs, and significantly enhanced apoptosis. Although 17 has been known as a phospholipase A₂ (PLA₂) inhibitor, other PLA₂ inhibitors had no effect on Atg4B and autophagy. We then performed structural optimization based on molecular modeling and succeeded in developing 21f (by shortening the alkyl chain of 17), which was a potent competitive inhibitor for Atg4B (K_i = 3.1 μM) with declining PLA₂ inhibitory potency. Compound 21f enhanced the anticancer activity of anti-CRPC drugs via autophagy inhibition. These findings suggest that 21f can be used as an adjuvant drug for therapy with anti-CRPC drugs.

A store-operated nonselective cation channel in human lymphocytes

1. Agonist interaction with phospholipase C-linked receptors at the plasma membrane can elicit both Ca2+ and Na+ influxes in lymphocytes. While Ca2+ influx is mediated by Ca2+ release-activated Ca2+ (CRAC) channels, the pathway responsible for Na+ influx is largely unknown. 2. We show that thapsigargin, ionomycin, ADP-ribose and IP3 activated a nonselective cation channel in lymphocytes that had a slightly outwardly rectifying I-V relationship, and a single channel conductance of 23.1 pS. We termed this channel a Ca2+ release-activated nonselective cation (CRANC) channel. 3. On activation in cell-attached configuration, switching to an inside-out configuration abolished CRANC channel activity. 4. Transfection of Jurkat T cells with antisense oligonucleotides for LTRPC2 reduced capacitative Ca2+ entry. 5. These results suggest that CRANC channels are responsible for the Na+ influx as well as a portion of the Ca2+ influx in lymphocytes induced by store depletion, that sustained activation of CRANC channels requires some property of the environment of a cell depleted of its Ca2+ stores; and that LTRPC2 protein is a likely component of the CRANC channel.

Studies of phospholipid metabolism, proliferation, and secretion of stably transfected insulinoma cells that overexpress group VIA phospholipase A2

A cytosolic 84 kDa Group VIA phospholipase A2 (iPLA2beta) that does not require Ca2+ for catalysis was cloned from Chinese hamster ovary (CHO) cells, murine P388D1 cells, pancreatic islet beta-cells, and other sources. Proposed iPLA2beta functions include participation in phosphatidylcholine (PC) homeostasis by degrading excess PC generated in CHO cells that overexpress CTP:phosphocholine cytidylyltransferase (CT), which catalyzes the rate-limiting step in PC biosynthesis; participation in biosynthesis of arachidonate-containing PC species in P388D1 cells by generating lysophosphatidylcholine (LPC) acceptors for arachidonate incorporation; and participation in signaling events in insulin secretion from islet beta-cells. To further examine iPLA2beta functions in beta-cells, we prepared stably transfected INS-1 insulinoma cell lines that overexpress iPLA2beta activity eightfold compared to parental INS-1 cells or to INS-1 cells transfected with an empty retroviral vector that did not contain iPLA2beta cDNA. The iPLA2beta-overexpressing cells exhibit a twofold increase in CT activity compared to parental cells but little change in rates of [3H]choline incorporation into or disappearance from PC. Electrospray ionization (ESI) tandem mass spectrometric measurements indicate that iPLA2beta-overexpressing cells have 1.5-fold higher LPC levels than parental INS-1 cells but do not exhibit increased rates of [3H]arachidonate incorporation into phospholipids, and incorporation is unaffected by a bromoenol lactone (BEL) suicide substrate inhibitor of iPLA2beta. The rate of appearance of arachidonate-containing phosphatidylethanolamine species visualized by ESI mass spectrometry is also similar in iPLA2beta-overexpressing and parental INS-1 cells incubated with supplemental arachidonic acid, and this process is unaffected by BEL. Compared to parental INS-1 cells, iPLA2beta-overexpressing cells proliferate more rapidly and exhibit amplified insulin secretory responses to a protein kinase C-activating phorbol ester, glucose, and a cAMP analog. These findings suggest that iPLA2beta plays a signaling role in beta-cells that differs from housekeeping functions in PC biosynthesis and degradation in P388D1 and CHO cells.

A store-operated nonselective cation channel in lymphocytes is activated directly by Ca(2+) influx factor and diacylglycerol

Agonist-receptor interactions at the plasma membrane often lead to activation of store-operated channels (SOCs) in the plasma membrane, allowing for sustained Ca(2+) influx. While Ca(2+) influx is important for many biological processes, little is known about the types of SOCs, the nature of the depletion signal, or how the SOCs are activated. We recently showed that in addition to the Ca(2+) release-activated Ca(2+) (CRAC) channel, both Jurkat T cells and human peripheral blood mononuclear cells express novel store-operated nonselective cation channels that we termed Ca(2+) release-activated nonselective cation (CRANC) channels. Here we demonstrate that activation of both CRAC and CRANC channels is accelerated by a soluble Ca(2+) influx factor (CIF). In addition, CRANC channels in inside-out plasma membrane patches are directly activated upon exposure of their cytoplasmic side to highly purified CIF preparations. Furthermore, CRANC channels are also directly activated by diacylglycerol. These results strongly suggest that the Ca(2+) store-depletion signal is a diffusible molecule and that at least some SOCs may have dual activation mechanisms.

Studies of insulin secretory responses and of arachidonic acid incorporation into phospholipids of stably transfected insulinoma cells that overexpress group VIA phospholipase A2 (iPLA2beta ) indicate a signaling rather than a housekeeping role for iPLA2beta

A cytosolic 84-kDa group VIA phospholipase A(2) (iPLA(2)beta) that does not require Ca(2+) for catalysis has been cloned from several sources, including rat and human pancreatic islet beta-cells and murine P388D1 cells. Many potential iPLA(2)beta functions have been proposed, including a signaling role in beta-cell insulin secretion and a role in generating lysophosphatidylcholine acceptors for arachidonic acid incorporation into P388D1 cell phosphatidylcholine (PC). Proposals for iPLA(2)beta function rest in part on effects of inhibiting iPLA(2)beta activity with a bromoenol lactone (BEL) suicide substrate, but BEL also inhibits phosphatidate phosphohydrolase-1 and a group VIB phospholipase A(2). Manipulation of iPLA(2)beta expression by molecular biologic means is an alternative approach to study iPLA(2)beta functions, and we have used a retroviral construct containing iPLA(2)beta cDNA to prepare two INS-1 insulinoma cell clonal lines that stably overexpress iPLA(2)beta. Compared with parental INS-1 cells or cells transfected with empty vector, both iPLA(2)beta-overexpressing lines exhibit amplified insulin secretory responses to glucose and cAMP-elevating agents, and BEL substantially attenuates stimulated secretion. Electrospray ionization mass spectrometric analyses of arachidonic acid incorporation into INS-1 cell PC indicate that neither overexpression nor inhibition of iPLA(2)beta affects the rate or extent of this process in INS-1 cells. Immunocytofluorescence studies with antibodies directed against iPLA(2)beta indicate that cAMP-elevating agents increase perinuclear fluorescence in INS-1 cells, suggesting that iPLA(2)beta associates with nuclei. These studies are more consistent with a signaling than with a housekeeping role for iPLA(2)beta in insulin-secreting beta-cells.