Generation Of DAG Research Articles

The Evolution of Lipidomics during Oil Accumulation of Plukenetia volubilis Seeds.

Sacha inchi (Plukenetia volubilis) is a valuable oilseed crop with a high content of polyunsaturated fatty acids (PUFAs). However, there is a lack of in-depth understanding of the lipidomics in Sacha inchi seeds (SIDs). Saturated fatty acids occupied more than half of the proportion (59.31%) in early development, while PUFAs accounted for 78.92% at maturation. The main triacylglycerols were TAG(18:3/18:3/18:3), TAG(18:2/18:2/18:3), and TAG(16:0/18:2/18:2). The corresponding species (18:3/18:3, 18:2/18:2, and 16:0/18:2) were also the main ingredients in diacylglycerol and phosphatidic acid, indicating high PUFA composition in the sn-1 and sn-2 positions of TAG. Only LPC(18:3), LPC(18:2), and LPC(16:0) were identified in SIDs, implying that those PUFAs on the sn-2 positions of the PC(18:3/-), PC(18:2/-), and PC(16:0/-) categories were released into the acyl-CoA pool for the Kennedy pathway. Conversely, the PC(18:1/-) and PC(18:0/-) categories might be responsible for the generation of PC-derived DAG and TAG. The lipidomics data will contribute to understanding the TAG assembly in developing SIDs, especially for PUFAs.

How the calcium influx and homeostasis might be disrupted in the Purkinje cell degeneration and cerebellar dysfunction?

In cerebellar circuitry the Purkinje cells (PCs) are the most prominent neuronal element and are characterized by thick and highly branched dendritic tree. The signaling molecule PKCγ is critically linked to Purkinje cell function and has a strong negative impact on the dendritic growth and development of PCs. Recent evidence links enhanced PKCγ to long‐term alteration in calcium equilibrium in PCs. Observations also show that blocking the voltage gated calcium channels could partially or completely protect Purkinje cell dendrites from stunted growth thought to be associated with enhanced PKCγ activity. Additional evidence from moon walker mutant mice also link the increased calcium influx through TRPC3 channels to dampened dendritic morphology. In this study we investigate that how the calcium influx and homoeostasis is regulated in PCs? Here, we propose an integrated model of calcium equilibrium in PCs, by converging the mGluR1 induced membrane generation of DAG and calcium influx through TRPC3 channels into an integrated signaling cascade. The model proposed here link the DAG generation after membrane depolarization to PKCγ activation as well as the activation of TRPC3 through direct binding of DAG. This proposed signaling cascade suggests that DAG signaling in Purkinje cells is tightly regulated through a double‐negative feed‐back loop. The results show that DAG‐ PKCγ‐DGKγ axis forms the first negative feedback loop whereas, the DAG‐TRPC3‐PKCγ‐DGKγ axis forms the second negative feedback loop where, both PKCγ and DGKγ molecules in second negative feedback loop are contributing due to Ca+2 influx through TRPC3 channel, which in turn, stimulates the cytosol‐to‐membrane translocation of these molecules in PCs. Our results suggest that for wild‐type case the double negative feedback on DAG and subsequently on TRPC3 channel is exquisite and specific enough to maintain the calcium homeostasis in PCs, thus ensuring their viability. However, for PKCγ with enhanced activity the second double negative feedback loop is not able to tightly regulate the gating characteristics of TRPC3 channels thus disrupting the calcium homeostasis and leading to degeneration and dysfunction of PCs.Support or Funding InformationPrivate FundingThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Soluble IP4 limits NK cell effector functions by controlling PI3K signaling (INM2P.434)

Abstract NK cells have important functions in cancer immunosurveillance, BM allograft rejection, fighting infections and reproduction. NK cell-based therapies are promising blood cancer treatments. Overcoming their limited efficacy requires a better understanding of the molecular mechanisms dampening their effector functions. NK cells recognize and kill pathogen-infected or tumor cells through invariant NK cell receptors (NKR). Two second-messenger pathways downstream of NKRs are required for NK cell maturation and effector responses: PIP3-generation by PI3K, and generation of DAG and IP3 by PLCγ. IP3 plays a key signaling role by mobilizing Calcium, but can also be converted into soluble IP4. We and others previously showed that IP4 can act as a soluble analog of the PI3K lipid-product PIP3 and control PIP3-mediated protein membrane recruitment and activation in thymocytes and granulocytes. Here, we show data which suggest a novel IP4 function in promoting NK cell terminal differentiation and acquisition of a mature NKR repertoire. However, in mature NK cells, IP4 limits NKR induced IFNγ secretion, granule exocytosis and target-cell clearance, in part by inhibiting the PIP3 effector Akt. This identifies IP4 as an important novel regulator of NK cell development and function, and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP4 is a broadly important signaling paradigm.

Soluble IP4 limits NK cell effector functions by controlling PI3K signaling (1013.1)

NK cells are important for cancer immunosurveillance, bone marrow allograft rejection, fighting infections and reproduction. Overcoming their limited efficacy in cancer immunotherapies requires a better understanding of the mechanisms limiting NK cell effector functions. NK cells recognize and kill pathogen‐infected or tumor cells through NK cell receptors (NKR). Two second‐messenger downstream pathways are required for NK cell maturation and function: PIP3‐generation by PI3K, and generation of DAG and IP3 by PLCγ. IP3 has a key signaling role by mobilizing Calcium, but can also be converted into soluble inositol(1,3,4,5)tetrakisphosphate (IP4). IP4 can act as a soluble analog of the PI3K lipid‐product phosphatidylinositol(3,4,5)trisphosphate (PIP3) and control PIP3‐mediated signaling protein membrane recruitment and activation in thymocytes and granulocytes. Here, we show published and unpublished data which suggest that IP4 limits NKR induced IFNγ secretion, granule exocytosis and target‐cell clearance, in part by inhibiting the PIP3 effector‐kinase Akt in NK cells. This identifies IP4 as an important novel regulator of NK cell function, and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP4 is a broadly important signaling paradigm.Grant Funding Source: Supported by NIH grants AI070845, GM088647, AI089805 & Leukemia & Lymphoma Society Award 1440‐11

Soluble IP4 limits NK cell effector functions by controlling PI3K signaling (P1198)

Abstract NK cells have important functions in cancer immunosurveillance, bone marrow allograft rejection, fighting infections and reproduction. NK cell-based therapies are promising blood cancer treatments. Overcoming their currently limited efficacy requires a better understanding of the molecular mechanisms controlling NK cell development and dampening their effector functions. NK cells recognize pathogen-infected or tumor cells through invariant NK cell receptors (NKR), and then kill such stressed cells. Two second-messenger pathways downstream of NKRs are required for NK cell maturation and effector responses: PIP3-generation by PI3K, and generation of DAG and IP3 by PLCγ. IP3 plays a key signaling role by mobilizing Calcium, but can also be converted into soluble inositol(1,3,4,5)tetrakisphosphate (IP4). We and others previously showed that IP4 can act as a soluble analog of the PI3K lipid-product phosphatidylinositol(3,4,5) trisphosphate (PIP3) and control PIP3-mediated signaling protein membrane recruitment and activation in thymocytes and granulocytes. Here, we show data which suggest a novel IP4 function in promoting NK cell terminal differentiation and acquisition of a mature NKR repertoire. However, in mature NK cells, IP4 limits NKR induced IFNγ secretion, granule exocytosis and target-cell clearance, in part by inhibiting the PIP3 effector-kinase Akt. This identifies IP4 as an important novel regulator of NK cell development and function, and expands our understanding of the therapeutically important mechanisms dampening NK cell responses. Our results further suggest that PI3K regulation by soluble IP4 is a broadly important signaling paradigm.

Protein Kinase Cθ C2 Domain Is a Phosphotyrosine Binding Module That Plays a Key Role in Its Activation

Protein kinase Cθ (PKCθ) is a novel PKC that plays a key role in T lymphocyte activation. To understand how PKCθ is regulated in T cells, we investigated the properties of its N-terminal C2 domain that functions as an autoinhibitory domain. Our measurements show that a Tyr(P)-containing peptide derived from CDCP1 binds the C2 domain of PKCθ with high affinity and activates the enzyme activity of the intact protein. The Tyr(P) peptide also binds the C2 domain of PKCδ tightly, but no enzyme activation was observed with PKCδ. Mutations of PKCθ-C2 residues involved in Tyr(P) binding abrogated the enzyme activation and association of PKCθ with Tyr-phosphorylated full-length CDCP1 and severely inhibited the T cell receptor/CD28-mediated activation of a PKCθ-dependent reporter gene in T cells. Collectively, these studies establish the C2 domain of PKCθ as a Tyr(P)-binding domain and suggest that the domain may play a major role in PKCθ activation via its Tyr(P) binding.

Protease-activated Receptor-2 Increases Exocytosis via Multiple Signal Transduction Pathways in Pancreatic Duct Epithelial Cells

Protease-activated receptor-2 (PAR-2) is activated when trypsin cleaves its NH(2) terminus to expose a tethered ligand. We previously demonstrated that PAR-2 activates ion channels in pancreatic duct epithelial cells (PDEC). Using real-time optical fluorescent probes, cyan fluorescence protein-Epac1-yellow fluorescence protein for cAMP, PH(PLC-delta1)-enhanced green fluorescent protein for phosphatidylinositol 4,5-bisphosphate, and protein kinase Cgamma (PKCgamma)-C1-yellow fluorescence protein for diacylglycerol, we now define the signaling pathways mediating PAR-2 effect in dog PDEC. Although PAR-2 activation does not stimulate a cAMP increase, it induces phospholipase C to hydrolyze phosphatidylinositol 4,5-bisphosphate into inositol 1,4,5-trisphosphate and diacylglycerol. Intracellular Ca(2+) mobilization from inositol 1,4,5-trisphosphate-sensitive Ca(2+) stores and a subsequent Ca(2+) influx through store-operated Ca(2+) channels cause a biphasic increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), measured with Indo-1 dye. Single-cell amperometry demonstrated that this increase in [Ca(2+)](i) in turn causes a biphasic increase in exocytosis. A protein kinase assay revealed that trypsin also activates PKC isozymes to stimulate additional exocytosis. Paralleling the increased exocytosis, mucin secretion from PDEC was also induced by trypsin or the PAR-2 activating peptide. Consistent with the serosal localization of PAR-2, 1 microm luminal trypsin did not induce exocytosis in polarized PDEC monolayers; on the other hand, 10 microm trypsin at 37 degrees C damaged the epithelial barrier sufficiently so that it could reach and activate the serosal PAR-2 to stimulate exocytosis. Thus, in PDEC, PAR-2 activation increases [Ca(2+)](i) and activates PKC to stimulate exocytosis and mucin secretion. These functions may mediate the reported protective role of PAR-2 in different models of pancreatitis.

Positive regulation of Itk PH domain function by soluble IP4 is required for thymocyte positive selection but dispensable for negative selection

Membrane recruitment of Itk and PLCγ1 require interactions between their Pleckstrin homology (PH) domains and PIP3, the phospholipid product of PI‐3 kinase. Dual anchoring via SH2 and PH domains are required for optimal Itk‐mediated activation of PLCγ1 and the subsequent generation of IP3 and DAG. We have identified a novel mechanism in which conversion of IP3 into IP4 by ItpkB creates a physiologically important soluble ligand for the Itk PH domain. IP4 specifically promotes PH domain‐dependent recruitment of Itk but not PLCγ1 to PIP3 and is required for optimal Itk activation. PH‐domain mediated oligomerization of Itk leads us to propose a model in which IP4 binding to one Itk PH domain can allosterically increase the affinity of associated Itk PH domains for membrane PIP3. IP4‐mediated positive feedback of Itk activation is required for positive selection during T cell development. Dose dependent reduction of single positive thymocytes suggests that ItpkB limits the efficiency of T cell maturation. Interestingly, our data indicates that thymocyte negative selection is normal in the absence of ItpkB and suggests that ItpkB participates in distinguishing TCR‐dependent death or survival signals.

Characterization of the lipolytic pathways that mediate free fatty acid release during Fas/CD95-induced apoptosis

We have undertaken a study to characterize the lipolytic pathway responsible for the generation of free fatty acids (FFA) during Fas/CD95-induced apoptosis in Jurkat cells. It was initially shown that the cellular lipid fraction that suffered the major quantitative decrease during Fas-induced apoptosis was that of phosphatidylcholine (PC). In addition, the secretion of palmitic acid-derived FFA was largely prevented by D609, an inhibitor of PC-specific phospholipase C (PC-PLC) and also by the diacylglycerol lipase (DAGL) inhibitor RHC-80267, suggesting that the secretion of these FFA during Fas-induced apoptosis is mediated by the generation of DAG by a PC-PLC activity and, sequentially, by a 1-DAGL activity which generates the FFA from its sn-1 position. The endocannabinoid 2-arachidonoyl glycerol (2-AG) should be generated as a sub-product of this pathway, but it did not accumulate inside the cells nor was secreted into the supernatant. Interestingly, the complete inhibition of free AA secretion during Fas-induced apoptosis was only achieved by using the AA trifluoromethylketone, which not only inhibits all types of phospholipase-A(2) (PLA(2)) activities, but also the described lytic activities on 2-AG. Using a combination of RHC-80267 and the iPLA(2)-specific inhibitor bromoenol lactone, it was shown that the DAGL pathway also cooperates with iPLA(2) in the generation of free arachidonate.

Activation of Human TRPC6 Channels by Receptor Stimulation

The human TRPC6 channel was expressed in human embryonic kidney (HEK) cells, and activity was monitored using the giga-seal technique. Whole cell membrane currents with distinctive inward and outward rectification were activated by carbachol (CCh) in TRPC6-expressing cells, but not in lacZ-transfected controls. The effect of CCh was steeply dose-dependent with a K(0.5) of approximately 10 microm and a Hill coefficient of 3-4. A steep concentration-response relationship was also observed when TRPC6 activity was measured using a fluorescence-based imaging plate reader (FLIPR) assay for membrane depolarization. Ionomycin, thapsigargin, and dialysis of the cell with inositol 1,4,5-trisphosphate via the patch pipette had no effect on TRPC6 currents, but exogenous application of 1-oleoyl acetyl-sn-glycerol (OAG, 30-300 microm) produced a slow increase in channel activity. The PKC activator, phorbol 12-myristate 13-acetate (PMA, 0.5 microm) had no significant acute effect on TRPC6, or on the subsequent response to OAG. In contrast, the response to CCh was blocked >90% by PMA pretreatment. To further explore the role of DAG in receptor stimulation, TRPC6 currents were monitored following the sequential addition of CCh and OAG. Surprisingly, concentrations of CCh that produced little or no response in the absence of OAG, produced increases in TRPC6 currents in the presence of OAG that were larger than the sum of either agent alone. Likewise, the response to OAG was superadditive following prior stimulation of the cells with near threshold concentrations of CCh. Overall, these results suggest that generation of DAG alone may not fully account for activation of TRPC6, and that other receptor-mediated events act synergistically with DAG to stimulate channel activity. This synergy may explain, at least in part, the steep dose-response relationship observed for CCh-induced TRPC6 currents expressed in HEK cells.

Phosphatidylcholine-specific phospholipase C, but not phospholipase D, is involved in pemphigus IgG-induced signal transduction.

The precise mechanism of the acantholysis after pemphigus IgGs bind to desmoglein (Dsg) 3 and/or Dsg 1 on the cell surface is as yet unknown. We have previously reported that pemphigus IgG (P-IgG) causes a transient increase in intracellular calcium and inositol 1,4,5-trisphosphate concentration, and subsequent activation of protein kinase C (PKC) in DJM-1 cells, a squamous cell carcinoma line. In order to see whether phosphatidylcholine (PC)-specific phospholipase C (PLC) or phospholipase D (PLD) is involved in the P-IgG-induced signaling process, the production of 1,2-diacylglycerol (DAG) and phosphatidylbutanol (PBut), a potential marker for the determination of PLD activity in the presence of butanol, was determined in DJM-1 cells. A biphasic accumulation of DAG, which consisted of a first transient phase and a second sustained phase, was observed. The second phase of DAG accumulation was profoundly inhibited by pretreatment with D609, a selective inhibitor of PC-PLC, but not by propranolol, an inhibitor of phosphatidate phosphohydrolase. Pemphigus serum after preadsortion of antibodies to Dsg 3 and Dsg 1 with recombinant Dsg 3 and Dsg 1 did not show formation of DAG. PBut was not generated following the addition of P-IgG. In addition, the levels of [3H]phosphocholine, a direct metabolite of PC-PLC, were elevated after the addition of P-IgG. These results suggest that the PC-PLC pathway plays a major role in P-IgG-induced transmembrane signaling by causing prolonged generation of DAG, which may lead to long-term activation of PKC.

Involvement of Phosphatidate Phosphohydrolase in Arachidonic Acid Mobilization in Human Amnionic WISH Cells

Prostaglandins are known to play a central role in the initiation of labor in humans, and amnionic cells constitute a major source of these compounds. Prostaglandin synthesis and release by amnion cells in response to hormones and ligands takes place after a characteristic 4-5 h lag. However, we report herein that free arachidonic acid (AA), the metabolic precursor of prostaglandins, can be induced at much shorter times (1 h) in human amnionic WISH cells by phorbol 12-myristate 13-acetate (PMA) through activation of protein kinase Calpha (PKCalpha). WISH cells were found to possess both cytosolic group IV phospholipase A2 (cPLA2) and Group VI Ca2+-independent phospholipase A2 (iPLA2). Of these, the cPLA2 was found to be the likely mediator of AA mobilization in PMA-activated WISH cells. PMA also activates phospholipase D (PLD) in these cells and ethanol, a compound that inhibits PLD-mediated phosphatidic acid (PA) formation, blocked AA release. Moreover, prevention of PA dephosphorylation by the PA phosphohydrolase inhibitors propranolol and bromoenol lactone, resulted in inhibition of AA release by PMA-treated WISH cells. Collectively, these data suggest that activation of cPLA2 and attendant AA release by phorbol esters in WISH cells requires prior generation of DAG by phosphatidate phosphohydrolase.

Ras‐p21 Activates phospholipase D and A2, but not phospholipase C or PKC, in Xenopus laevis Oocytes

Xenopus laevis oocytes are a powerful tool for the characterization of signal transduction pathways leading to the induction of DNA synthesis. Since activation of PLA2, PLC, or PLD has been postulated as a mediator of ras function, we have used the oocyte system to study the putative functional relationship between ras-p21 and these phospholipases. A rapid generation of PA and DAG was observed after ras-p21 microinjection, suggesting the activation of both PLC and PLD enzymes. However, production of DAG was sensitive to inhibition of the PA-hydrolase by propranolol, indicating that PLD is the enzyme responsible for the generation of both PA and DAG. Microinjection of PLD or ras-p21 induced the late production of lysophosphatidylcholine on a p42MAPK-dependent manner, an indication of the activation of a PLA2. Inhibition of this enzyme by quinacrine does not inhibit PLD- or ras-induced GVBD, suggesting that PLA2 activation is not needed for ras or PLD function. Contrary to 3T3 fibroblasts, where ras-p21 is functionally dependent for its mitogenic activity on TPA- and staurosporine-sensitive PKC isoforms, in Xenopus oocytes, induction of GVBD by ras-p21 was independent of PKC, while PLC-induced GVBD was sensitive to PKC inhibition. Thus, our results demonstrate the activation of PLD and PLA2 by ras-p21 proteins, while no effect on PLC was observed.

Interaction of Anesthetics and Catecholamines on Conduction in the Canine His--Purkinje System

The findings in papillary muscles that epinephrine facilitates conduction at Purkinje fiber-muscle junctions and in the endocardium are consistent with older observations that activation of myocardial beta-adrenergic receptors speeds conduction and activation in the heart and thereby increases the synergy of contraction (46,47). The cellular mechanism underlying this action is probably increased cell-to-cell coupling between muscle fibers secondary to elevation of cyclic AMP (19,48). However, the findings that epinephrine alone or with halothane transiently slows conduction in the Purkinje layer while simultaneously improving conduction across Purkinje-muscle junctions and in the endocardium may represent proarrhythmic actions. These actions could facilitate arrhythmogenesis by transiently increasing regional differences of activation and repolarization times in the conduction system and myocardium and thereby increasing vulnerability to induction of reentry by premature impulses. Such a proarrhythmic effect could explain an older observation that low-dose norepinephrine infusions decrease the threshold for induction of fibrillation by two premature beats in pentobarbital-anesthetized animals (49). The cellular basis underlying the different responses of Purkinje fibers and the endocardial muscle layer to catecholamines, in which velocity decreased and increased, respectively, is not known. Our working hypothesis to explain this action in canine Purkinje fibers is a mechanism involving activation of WB4101-sensitive alpha 1-adrenoceptor, G-protein coupling to phospholipase C and the generation of DAG and IP3 leading to modulation of cell-to-cell coupling, which is potentiated in the presence of partial uncoupling by halothane. The different responses of Purkinje and myocardial fibers are speculated to result from differences in the relative density of this subtype of alpha 1-adrenoceptor, differences in the subcellular effector coupling mechanisms, or differences in the specific connexin proteins forming gap junctions between Purkinje and myocardial fibers (50).

Insulin stimulates hydrolysis of plasmanylinositol-glycan and phosphatidylinositol-glycan in rat adipocytes. Insulin-induced generation of inositol glycan, alkylacylglycerol, and diacylglycerol.

Insulin binding to its receptor has been known to induce hydrolysis of phosphatidylinositol-glycan and release inositol-glycan and diacylglycerol, two putative second messengers of insulin actions. We metabolically labeled and purified PIG in rat cultured adipocytes. The treatment of [3H]glycerol-labeled PIG with phosphatidylinositol-specific phospholipase C released [3H]glycerol-labeled DAG and [3H]glycerol-labeled 1-alkyl,2-acyl-glycerol, suggesting that PIG has not only PIG but also plasmanylinositol-glycan moiety. Insulin induced hydrolysis of PIG/PMIG and generation of IG, DAG, and AAG in a dose-dependent manner. This report shows the first demonstration of insulin-sensitive PMIG in rat adipocytes. These results provide evidence that insulin-induced generation of IG, DAG, and AAG might be early events in the insulin-signaling mechanism in rat adipocytes, and insulin-releasable AAG seems to mediate some actions of insulin.

Mutated insulin receptor Val996 reduces insulin-dependent generation of inositol glycan and diacylglycerol.

We evaluated whether insulin-receptor tyrosine kinase activity is required for activation of PDH, insulin-induced hydrolysis of PIG and generation of IG and 1,2-DAG. For the analysis, we used stable-transfected CHO cell lines expressing wild-type human insulin receptor (CHO-wt cells) or the mutant receptor (Val996) that lacks tyrosine kinase activity (CHO-mut cells) (1,2). Insulin stimulated PDH activity in three CHO cell lines in a dose-dependent manner. Half-maximal concentrations of insulin to activate PDH was 7 x 10(-11) M in the CHO-wt cells, 10(-9) M in the parental cells, and 8 x 10(-9) M in the CHO-mut cells. Insulin stimulated hydrolysis of PIG and generation of IG and DAG in three CHO cell lines in a dose-dependent manner. Half-maximal concentrations of insulin to induce generation of IG was 8 x 10(-11) M in the CHO-wt cells, 10(-9) M in the parental CHO cells, and 10(-8) M in the CHO-mut cells. ED50 for the stimulation of DAG generation was 7 x 10(-11) M in the CHO-wt cells, 10(-9) M in the parental cells, and 10(-8) M in the CHO-mut cells. It is concluded that insulin-dependent PDH activation, PIG hydrolysis, and IG and DAG generation are mediated by the wild-type but not by the mutated insulin receptor of Val996. This study suggests that tyrosine kinase activity of the insulin receptor might be a prerequisite for insulin-stimulated generation of IG and DAG.

Diacylglycerol rather than Ca 2+ mediates GnRH inhibition of FSH induced steroidogenesis in ovarian granulosa cells

Treatment of cultured granulosa cells with PLC or GnRH stimulated the rapid generation of DAG and phosphoinositide turnover. The PKC activators PLC (3 mU/ml) and TPA (10 −7M) or the decapeptide GnRH (10 −6M) elicited similar inhibitory responses on FSH or cAMP stimulated granulosa cell steroidogenesis. Mobilization of intracellular Ca 2+ with A23187 (10 −8M) was followed by a slight increase in the steroidogenic activity of cultured granulosa cells, whereas elevation of extracellular K + (50 mM) largely augmented the steroid biosynthetic activity of the granulosa cells. These results suggest that the inhibitory effect of GnRH on granulosa cell steroidogenesis is mediated by generation of DAG, rather than by increases in intracellular Ca 2+ concentrations.

The role of protein kinase C in alpha-thrombin-mediated endothelial cell activation.

Thrombin, the key regulatory protein of hemostasis, has been implicated in a variety of important endothelial cell processes closely linked to endothelial signal transduction mechanisms. An initial event, following receptor binding by catalytically active alpha-thrombin, appears to be the activation of a G-protein-coupled, PI-specific PLC, with resultant generation of IP3 and DAG, with increases in [Ca2+]i, and activation and translocation of PKC (Fig. 9). PKC activation results in down-regulation of PLC, as demonstrated by inhibition of agonist-induced increases in [Ca2+]i, whereas PLA2 activity is up-regulated, with a resultant increase in endothelial PGI2 synthesis. Recently, we have demonstrated that activity of membrane-bound, endothelial PLD, is also up-regulated by PKC activation. In addition to its modulatory role in endothelial cell phospholipase activities, PKC activation appears to play a critical role in thrombin-mediated endothelial barrier dysfunction, likely via specific cytoskeletal protein phosphorylation. A temporal relationship between alpha-thrombin-mediated signal transduction and specific cellular responses, such as PGI2 synthesis and barrier dysfunction, can be established (Fig. 2). Further investigations are ongoing to identify more clearly the precise biochemical intermediates involved in the endothelial cell response to thrombin, as well as the role of differential phosphorylation by various protein kinase systems in thrombin-mediated signal transduction in vascular endothelium.

HIV inhibits the early steps of lymphocyte activation, including initiation of inositol phospholipid metabolism.

Mechanisms accounting for HIV-associated suppression of lymphocyte proliferation were investigated. In previous work we demonstrated that purified and inactivated HIV-suppressed lymphoid cell proliferation. In this report we used an inactivated preparation of HIV obtained from infected CEM cells grown in serum free media and demonstrated that this HIV-associated suppression acted in the early steps of activation to inhibit the incorporation of radiolabeled phosphorus into phosphatidylinositol 4,5-bisphosphate and phosphatidic acid. Initially we showed that both purified CD4 and CD8 T lymphocyte subsets were affected and HIV-associated inhibition did not require the CD4 molecule. Impaired lymphocyte blastogenesis (decreased size and granularity and decreased expression of receptors to IL-2 and transferrin) in response to PHA indicated an effect of inactivated HIV on the early steps of activation. This was confirmed by time studies where 1) a 2 min HIV-pretreatment followed by washing before stimulation was sufficient to inhibit PHA induced proliferation of normal lymphocytes, and 2) addition of HIV to PHA prestimulated lymphocytes failed to inhibit proliferation, e.g., there was no effect on preactivated lymphocytes. HIV was mainly inhibitory of lymphocyte proliferation induced by PHA or mAb to the CD3 receptor. In contrast to the effect on the CD3/TiR, responses via the CD2 receptor were not suppressed, e.g., stimulation with the monoclonal antibodies T11(2) + T11(3). Inasmuch as responses by direct A23187 + PMA stimulation of intracellular pathways were also inhibited, it appears that the HIV-induced defect was not (or not only) membrane receptor mediated. The earliest (min) measurable event after stimulation was the initial increase in intracellular Ca2+ which was unaffected by HIV pretreatment. The next measurable event (min to h) of stimulation is a sustained increase in inositol phospholipid turnover. Pretreatment of mononuclear cells with inactivated HIV resulted in a decreased inositol phospholipid turnover as judged from decreased 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate. This led to decreased generation of DAG as reflected in the reduced radiolabeling of its metabolite PA. Reduced availability of DAG presumably interferes with pkC activation and leads to decreased expression of receptors for IL-2 and transferrin and impaired proliferation.

Growth Hormone Stimulates the Formation ofsn-1,2 Diacylglycerol in Rat Hepatocytes

The transmembrane signaling events of GH were investigated in the liver, a major target organ of GH action. Recombinant human GH when added to freshly isolated rat hepatocytes rapidly stimulated the production of sn-1,2-diacylglycerol (DAG). The generation of DAG was biphasic with the first transient peak observed at 2 min and the second peak at 15 min (1.2-fold and 1.4-fold over control, respectively). Levels of DAG continued to be elevated above those in control cells at 30 min. The response was dose-dependent with an EC50 of 0.15 nM. Both bovine GH and rat GH, which bind to the rat GH receptor but not to the PRL receptor, also stimulated DAG production. Similarly, human PRL, which binds to the PRL but not GH receptor, stimulated DAG formation to a comparable extent. These results suggest that production of DAG may be an early signaling event mediated by hormone stimulation of both the GH and PRL receptors.