Wortmannin, an inhibitor of phospholipase D activation, selectively blocks major histocompatibility complex class II-restricted antigen presentation

24R,25-Dihydroxyvitamin D3 regulates breast cancer cells in vitro and in vivo

BACKGROUND

Epidemiological studies indicate high serum 25(OH)D₃ is associated with increased survival in breast cancer patients. Pre-clinical studies attributed this to anti-tumorigenic properties of its metabolite 1α,25(OH)₂D₃. However, 1α,25(OH)₂D₃ is highly calcemic and thus has a narrow therapeutic window. Here we propose another metabolite, 24R,25(OH)₂D₃, as an alternative non-calcemic vitamin D₃ supplement.

METHODS

NOD-SCID-IL2γR null female mice with MCF7 breast cancer xenografts in the mammary fat pad were treated with 24R,25(OH)₂D₃ and changes in tumor burden and metastases were assessed. ERα66+ MCF7 and T47D cells, and ERα66- HCC38 cells were treated with 24R,25(OH)₂D₃in vitro to assess effects on proliferation and apoptosis. Effects on migration and metastatic markers were assessed in MCF7.

RESULTS

24R,25(OH)₂D₃ reduced MCF7 tumor growth and metastasis in vivo. In vitro results indicate that this was not due to an anti-proliferative effect; 24R,25(OH)₂D₃ stimulated DNA synthesis in MCF7 and T47D. In contrast, markers of invasion and metastasis were decreased. 24R,25(OH)₂D₃ caused dose-dependent increases in apoptosis in MCF7 and T47D, but not HCC38 cells. Inhibitors to palmitoylation, caveolae integrity, phospholipase-D, and estrogen receptors (ER) demonstrate that 24R,25(OH)₂D₃ acts on MCF7 cells through caveolae-associated, phospholipase D-dependent mechanisms via cross-talk with ERs.

CONCLUSION

These results indicate that 24R,25(OH)₂D₃ shows promise in treatment of breast cancer by stimulating tumor apoptosis and reducing metastasis.

GENERAL SIGNIFICANCE

24R,25(OH)₂D₃ regulates breast cancer cell survival through ER-associated mechanisms similar to 24R,25(OH)₂D₃ effects on chondrocytes. Thus, 24R,25(OH)₂D₃ may modulate cell survival in other estrogen-responsive cell types, and its therapeutic potential should be investigated in ER-associated pathologies.

Sphingosine 1-phosphate promotes antigen processing and presentation to CD4+ T cells in Mycobacterium tuberculosis-infected monocytes

Sphingosine 1-phosphate (S1P) has recently been described to induce antimycobacterial activity. The present study analyses the role played by S1P in antigen presentation of monocytes and in the next activation of Mycobacterium tuberculosis (MTB)-specific CD4+ T cell response. Results reported herein show that S1P stimulation of MTB-infected monocytes (i) inhibits intracellular mycobacterial growth, (ii) enhances phagolysosome maturation and the transit of mycobacteria in MHC class II compartments, (iii) increases the frequency of MTB-specific CD4+CD69+ T cells, expressing the inflammatory homing receptor CCR5, derived from tuberculosis patients and PPD+, BCG naïve, healthy subjects, and (iv) induces IFN-gamma production in CD4+CD69+CCR5+ T cells derived from PPD+ healthy individuals, only. Altogether, these results show that S1P promotes antigen processing and presentation in monocytes, increases the frequency of MTB-specific CD4+ T cells and can regulate IFN-gamma production by antigen specific CD4+ T cells in the course of active disease.

Understanding phospholipase D (PLD) using leukocytes: PLD involvement in cell adhesion and chemotaxis

Phospholipase D (PLD) is an enzyme that catalyzes the conversion of membrane phosphatidylcholine to choline and phosphatidic acid (PA; a second messenger). PLD is expressed in nearly all types of leukocytes and has been associated with phagocytosis, degranulation, microbial killing, and leukocyte maturation. With the application of recently developed molecular tools (i.e., expression vectors, silencing RNA, and specific antibodies), the demonstration of a key role for PLD in those and related cellular actions has contributed to a better awareness of its importance. A case in point is the recent findings that RNA interference-mediated depletion of PLD results in impaired leukocyte adhesion and chemotaxis toward a gradient of chemokines, implying that PLD is necessary for leukocyte movement. We forecast that based on results such as those, leukocytes may prove to be useful tools to unravel still-unresolved mechanistic issues in the complex biology of PLD. Three such issues are considered here: first, whether the cellular actions of PLD are mediated entirely by PA (the product of its enzymatic reaction) or whether PLD by itself interacts with other protein signaling molecules; second, the current difficulty of defining a "PA consensus site" in the various intracellular protein targets of PA; and third, the resolution of specific PLD location (upstream or downstream) in a particular effector signaling cascade. There are reasons to expect that leukocytes and their leukemic cell line counterparts will continue yielding invaluable information to cell biologists to resolve standing molecular and functional issues concerning PLD.

Differential regulation of growth plate chondrocytes by 1alpha,25-(OH)2D3 and 24R,25-(OH)2D3 involves cell-maturation-specific membrane-receptor-activated phospholipid metabolism

This review discusses the regulation of growth plate chondrocytes by vitamin D(3). Over the past ten years, our understanding of how two vitamin D metabolites, 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3), exert their effects on endochondral ossification has undergone considerable advances through the use of cell biology and signal transduction methodologies. These studies have shown that each metabolite affects a primary target cell within the endochondral developmental lineage. 1alpha,25-(OH)(2)D(3) affects primarily growth zone cells, and 24R,25-(OH)(2)D(3) affects primarily resting zone cells. In addition, 24R,25-(OH)(2)D(3) initiates a differentiation cascade that results in down-regulation of responsiveness to 24R,25-(OH)(2)D(3) and up-regulation of responsiveness to 1alpha,25-(OH)(2)D(3). 1alpha,25-(OH)(2)D(3) regulates growth zone chondrocytes both through the nuclear vitamin D receptor, and through a membrane-associated receptor that mediates its effects via a protein kinase C (PKC) signal transduction pathway. PKCalpha is increased via a phosphatidylinositol-specific phospholipase C (PLC)-dependent mechanism, as well as through the stimulation of phospholipase A(2) (PLA(2)) activity. Arachidonic acid and its downstream metabolite prostaglandin E(2) (PGE(2)) also modulate cell response to 1alpha,25-(OH)(2)D(3). In contrast, 24R,25-(OH)(2)D(3) exerts its effects on resting zone cells through a separate, membrane-associated receptor that also involves PKC pathways. PKCalpha is increased via a phospholipase D (PLD)-mediated mechanism, as well as through inhibition of the PLA(2) pathway. The target-cell-specific effects of each metabolite are also seen in the regulation of matrix vesicles by vitamin D(3). However, the PKC isoform involved is PKCzeta, and its activity is inhibited, providing a mechanism for differential autocrine regulation of the cell and events in the matrix by these two vitamin D(3) metabolites.

1alpha,25-dihydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) modulate growth plate chondrocyte physiology via protein kinase C-dependent phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase

Membrane-mediated increases in protein kinase C (PKC) activity and PKC-dependent physiological responses of growth plate chondrocytes to vitamin D metabolites depend on the state of endochondral maturation; 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)] regulates growth zone (GC) cells, whereas 24R,25-(OH)(2)D(3) regulates resting zone (RC) cells. Different mechanisms, including protein kinase A signaling, mediate the effects of 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) on PKC, suggesting that different mechanisms may also regulate any MAPK involvement in the physiological responses. This study used confluent cultures of rat costochondral chondrocytes as a model. 1alpha,25-(OH)(2)D(3) stimulated MAPK specific activity in GC in a time- and dose-dependent manner, evident within 9 min. 24R,25-(OH)(2)D(3) stimulated MAPK in RC; increases were dose dependent, occurred after 9 min, and were greatest at 90 min. In both cells the effect was due to ERK1/2 activation (p42 > p44 in GC; p42 = p44 in RC). MAPK activation was dependent on PKC, but not protein kinase A. The effect of 1alpha,25-(OH)(2)D(3) required phospholipase C, and the effect of 24R,25-(OH)(2)D(3) required phospholipase D. Inhibition of cyclooxygenase activity reduced the effect of 1alpha,25-(OH)(2)D(3) on MAPK in GC and enhanced the effect of 24R,25-(OH)(2)D(3) in RC. Based on MAPK inhibition with PD98059, ERK1/2 MAPK mediated the effect of 24R,25-(OH)(2)D(3) on [(3)H]thymidine incorporation and [(35)S]sulfate incorporation by RC, but only partially mediated the effect of 1alpha,25-(OH)(2)D(3) on GC. ERK1/2 was not involved in the regulation of alkaline phosphatase specific activity by either metabolite. This paper supports the hypothesis that 1alpha,25-(OH)(2)D(3) regulates the physiology of GC via rapid membrane-mediated signaling pathways, and some, but not all, of the response to 1alpha,25-(OH)(2)D(3) is via the ERK family of MAPKs. In contrast, 24R,25-(OH)(2)D(3) exerts its effects on RC via PKC-dependent MAPK. Whereas 1alpha,25-(OH)(2)D(3) increases MAPK activity via phospholipase C and increased prostaglandin production, 24R,25-(OH)(2)D(3) increases MAPK via phospholipase D and decreased prostaglandin production. The cell specificity, metabolite stereospecificity, and the dependence on PKC argue for the participation of membrane receptors for 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) in the regulation of ERK1/2 in the growth plate.

1alpha,25(OH)2D3 regulates chondrocyte matrix vesicle protein kinase C (PKC) directly via G-protein-dependent mechanisms and indirectly via incorporation of PKC during matrix vesicle biogenesis

Matrix vesicles are extracellular organelles involved in mineral formation that are regulated by 1alpha,25(OH)(2)D(3). Prior studies have shown that protein kinase C (PKC) activity is involved in mediating the effects of 1alpha,25(OH)(2)D(3) in both matrix vesicles and plasma membranes. Here, we examined the regulation of matrix vesicle PKC by 1alpha,25(OH)(2)D(3) during biogenesis and after deposition in the matrix. When growth zone costochondral chondrocytes were treated for 9 min with 1alpha,25(OH)(2)D(3), PKCzeta in matrix vesicles was inhibited, while PKCalpha in plasma membranes was increased. In contrast, after treatment for 12 or 24 h, PKCzeta in matrix vesicles was increased, while PKCalpha in plasma membranes was unchanged. The effect of 1alpha,25(OH)(2)D(3) was stereospecific and metabolite-specific. Monensin blocked the increase in matrix vesicle PKC after 24 h, suggesting the secosteroid-regulated packaging of PKC. In addition, the 1alpha,25(OH)(2)D(3) membrane vitamin D receptor (1,25-mVDR) was involved, since a specific antibody blocked the 1alpha,25(OH)(2)D(3)-dependent changes in PKC after both long and short treatment times. In contrast, antibodies to annexin II had no effect, and there was no evidence for the presence of the nuclear VDR on Western blots. To investigate the signaling pathways involved in regulating matrix vesicle PKC activity after biosynthesis, matrix vesicles were isolated and then treated for 9 min with 1alpha,25(OH)(2)D(3) in the presence and absence of specific inhibitors. Inhibition of phosphatidylinositol-phospholipase C, phospholipase D, or G(i)/G(s) had no effect. However, inhibition of G(q) blocked the effect of 1alpha,25(OH)(2)D(3). The rapid effect of 1alpha,25(OH)(2)D(3) also involved the 1,25-mVDR. Moreover, arachidonic acid was found to stimulate PKC when added directly to isolated matrix vesicles. These results indicate that matrix vesicle PKC is regulated by 1alpha,25(OH)(2)D(3) at three levels: 1) during matrix vesicle biogenesis; 2) through direct action on the membrane; and 3) through production of other factors such as arachidonic acid.

Activation of phospholipase D in porcine tracheal smooth muscle: role of phosphatidylinositol 3-kinase and RhoA activation

Muscarinic receptor agonists transiently activate phospholipase D in tracheal smooth muscle. Muscarinic activation of phospholipase D in this tissue is dependent on activation of protein kinase C and an unidentified pathway that is not protein kinase C dependent. Cholinergic agents have also been shown to activate phospholipase D by pathways linked to the small G protein, RhoA. This study explores the relationship between muscarinic activation of phophatidylinositol 3-kinase and activation of RhoA, and examines whether phospholipase D activation is dependent on either pathway in tracheal smooth muscle. Wortmannin or 2-(4-morphonyl)-8-phenyl-4H-1-benzopyran-4-one (LY-294002), putative specific inhibitors of phophatidylinositol 3-kinase, significantly inhibit acetylcholine-induced formation of phosphatidylethanol and also block acetylcholine-induced translocation of RhoA to the membrane. In previous experiments calphostin C, a protein kinase C inhibitor, partially inhibited both acetylcholine-induced and phorbol-12-myristate-13-acetate (PMA)-induced phosphatidylethanol formation. In the present study calphostin C did not block acetylcholine-induced RhoA translocation to the membrane. However, the Rho kinase inhibitor, N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide (Y-27632), significantly inhibited acetylcholine-induced phosphatidylethanol formation, but had no effect on activation of phospholipase D by PMA. Acetylcholine treatment also stimulated the phosphorylation of the 110-kDa subunit of phosphatidylinositol 3-kinase. Phosphorylation of phosphatidylinositol 3-kinase 110-kDa subunit could be blocked by wortmannin in a concentration-dependent manner, and acetylcholine-induced phosphatidylinositol 3-kinase activity was significantly inhibited by wortmannin. LY-294002 also inhibited acetylcholine-induced phosphorylation of 110-kDa subunit and activation of phosphatidylinositol 3-kinase. These results suggest that acetylcholine stimulation translocates RhoA to the membrane by a phosphatidylinositol 3-kinase-dependent mechanism and acetylcholine-induced phospholipase D stimulation is at least partly mediated via phosphatidylinositol 3-kinase, however, protein kinase C appears to activate phospholipase D independent of phosphatidylinositol 3-kinase or RhoA activation in porcine tracheal smooth muscle.

The effect of 24R,25-(OH)(2)D(3) on protein kinase C activity in chondrocytes is mediated by phospholipase D whereas the effect of 1alpha,25-(OH)(2)D(3) is mediated by phospholipase C

1alpha,25-(OH)(2)D(3) regulates protein kinase C (PKC) activity in growth zone chondrocytes by stimulating increased phosphatidylinositol-specific phospholipase C (PI-PLC) activity and subsequent production of diacylglycerol (DAG). In contrast, 24R,25-(OH)(2)D(3) regulates PKC activity in resting zone (RC) cells, but PLC does not appear to be involved, suggesting that phospholipase D (PLD) may play a role in DAG production. In the present study, we examined the role of PLD in the physiological response of RC cells to 24R,25-(OH)(2)D(3) and determined the role of phospholipases D, C, and A(2) as well as G-proteins in mediating the effects of vitamin D(3) metabolites on PKC activity in RC and GC cells. Inhibition of PLD with wortmannin or EDS caused a dose-dependent inhibition of basal [3H]-thymidine incorporation by RC cells and further increased the inhibitory effect of 24R,25-(OH)(2)D(3). Wortmannin also inhibited basal alkaline phosphatase activity and [35]-sulfate incorporation and decreased the stimulatory effect of 24R,25-(OH)(2)D(3). This inhibitory effect of wortmannin was not seen in cultures treated with the PI-3-kinase inhibitor LY294002, verifying that wortmannin affected PLD. Wortmannin also inhibited basal PKC activity and partially blocked the stimulatory effect of 24R,25-(OH)(2)D(3) on this enzyme activity. Neither inhibition of PI-PLC with U73122, nor PC-PLC with D609, modulated PKC activity. Wortmannin had no effect on basal PLD in GC cells, nor on 1alpha,25-(OH)(2)D(3)-dependent PKC. Inhibition of PI-PLC blocked the 1alpha,25-(OH)(2)D(3)-dependent increase in PKC activity but inhibition of PC-PLC had no effect. Activation of PLA(2) with melittin inhibited basal and 24R,25-(OH)(2)D(3)-stimulated PKC in RC cells and stimulated basal and 1alpha,25-(OH)(2)D(3)-stimulated PKC in GC cells, but wortmannin had no effect on the melittin-induced changes in either cell type. Pertussis toxin modestly increased the effect of 24R,25-(OH)(2)D(3) on PKC, whereas GDPbetaS had no effect, suggesting that PLD2 is the isoform responsible. This indicates that 1alpha,25-(OH)(2)D(3) regulates PKC in GC cells via PI-PLC and PLA(2), but not PC-PLC or PLD, whereas 24R,25-(OH)(2)D(3) regulates PKC in RC cells via PLD2.

17 beta-estradiol-BSA conjugates and 17 beta-estradiol regulate growth plate chondrocytes by common membrane associated mechanisms involving PKC dependent and independent signal transduction

Nuclear receptors for 17 beta-estradiol (E(2)) are present in growth plate chondrocytes from both male and female rats and regulation of chondrocytes through these receptors has been studied for many years; however, recent studies indicate that an alternative pathway involving a membrane receptor may also be involved in the cell response. E(2) was found to directly affect the fluidity of chondrocyte membranes derived from female, but not male, rats. In addition, E(2) activates protein kinase C (PKC) in a nongenomic manner in female cells, and chelerythrine, a specific inhibitor of PKC, inhibits E(2)-dependent alkaline phosphatase activity and proteoglycan sulfation in these cells, indicating PKC is involved in the signal transduction mechanism. The aims of the present study were: (1) to examine the effect of a cell membrane-impermeable 17 beta-estradiol-bovine serum albumin conjugate (E(2)-BSA) on chondrocyte proliferation, differentiation, and matrix synthesis; (2) to determine the pathway that mediates the membrane effect of E(2)-BSA on PKC; and (3) to compare the action of E(2)-BSA to that of E(2). Confluent, fourth passage resting zone (RC) and growth zone (GC) chondrocytes from female rat costochondral cartilage were treated with 10(-9) to 10(-7) M E(2) or E(2)-BSA and changes in alkaline phosphatase specific activity, proteoglycan sulfation, and [(3)H]-thymidine incorporation measured. To examine the pathway of PKC activation, chondrocyte cultures were treated with E(2)-BSA in the presence or absence of GDP beta S (inhibitor of G-proteins), GTP gamma S (activator of G-proteins), U73122 or D609 (inhibitors of phospholipase C [PLC]), wortmannin (inhibitor of phospholipase D [PLD]) or LY294002 (inhibitor of phosphatidylinositol 3-kinase). E(2)-BSA mimicked the effects of E(2) on alkaline phosphatase specific activity and proteoglycan sulfation, causing dose-dependent increases in both RC and GC cell cultures. Both forms of estradiol inhibited [(3)H]-thymidine incorporation, and the effect was dose-dependent. E(2)-BSA caused time-dependent increases in PKC in RC and GC cells; effects were observed within three minutes in RC cells and within one minute in GC cells. Response to E(2) was more robust in RC cells, whereas in GC cells, E(2) and E(2)-BSA caused a comparable increase in PKC. GDP beta S inhibited the activation of PKC in E(2)-BSA-stimulated RC and GC cells. GTP gamma S increased PKC in E(2)-BSA-stimulated GC cells, but had no effect in E(2)-BSA-stimulated RC cells. The phosphatidylinositol-specific PLC inhibitor U73122 blocked E(2)-BSA-stimulated PKC activity in both RC and GC cells, whereas the phosphatidylcholine-specific PLC inhibitor D609 had no effect. Neither the PLD inhibitor wortmannin nor the phosphatidylinositol 3-kinase inhibitor LY294022 had any effect on E(2)-BSA-stimulated PKC activity in either RC or GC cells. The classical estrogen receptor antagonist ICI 182780 was unable to block the stimulatory effect of E(2)-BSA on PKC. Moreover, the classical receptor agonist diethylstilbestrol (DES) had no effect on PKC, nor did it alter the stimulatory effect of E(2)-BSA. The specificity of the membrane response to E(2) was also demonstrated by showing that the membrane receptor for 1 alpha,25-(OH)(2)D(3) was not involved. These data indicate that the rapid nongenomic effect of E(2)-BSA on PKC activity in RC and GC cells is dependent on G-protein-coupled PLC and support the hypothesis that many of the effects of E(2) involve membrane-associated mechanisms independent of classical estrogen receptors. (c) 2001 Wiley-Liss, Inc.

Regulation of phospholipase D (PLD) in growth plate chondrocytes by 24R,25-(OH)2D3 is dependent on cell maturation state (resting zone cells) and is specific to the PLD2 isoform

Many of the effects of 1alpha,25-(OH)2D3 and 24R,25-(OH)2D3 on costochondral chondrocytes are mediated by the protein kinase C (PKC) signal transduction pathway. 1alpha,25-(OH)2D3 activates PKC in costochondral growth zone chondrocytes through a specific membrane receptor (1alpha,25-mVDR), involving rapid increases in diacylglycerol via a phospholipase C (PLC)-dependent mechanism. 24R,25-(OH)2D3 activates PKC in resting zone chondrocytes. Although diacylglycerol is increased by 24R,25-(OH)2D3, PLC is not involved, suggesting a phospholipase D (PLD)-dependent mechanism. Here, we show that resting zone and growth zone cells express mRNAs for PLD1a, PLD1b, and PLD2. Both cell types have PLD activity, but levels are higher in resting zone cells. 24R,25-(OH)2D3, but not 24S,25-(OH)2D3 or 1alpha,25-(OH)2D3, stimulates PLD activity in resting zone cells within 3 min via nongenomic mechanisms. Neither 1alpha,25-(OH)2D3 nor 24R,25-(OH)2D3 affected PLD in growth zone cells. Basal and 24R,25-(OH)2D3-stimulated PLD were inhibited by the PLD inhibitors wortmannin and EDS. Inhibition of phosphatidylinositol 3-kinase (PI 3-kinase), PKC, phosphatidylinositol-specific PLC (PI-PLC), and phosphatidylcholine-specific PLC (PC-PLC) had no effect on PLD activity. Thus, 24R,25-(OH)2D3 stimulates PLD, and PI 3-kinase, PI-PLC and PKC are not involved, whereas PLD is required for stimulation of PKC by 24R,25-(OH)2D3. Pertussis toxin, GDPbetaS, and GTPgammaS had no effect on 24R,25-(OH)2D3-dependent PLD when added to cell cultures, indicating that G-proteins are not involved. These data show that PKC activation in resting zone cells is mediated by PLD and suggest that a functional 24R,25-(OH)2D3-mVDR is required. The results also support the conclusion that the 24R,25-(OH)2D3-responsive PLD is PLD2, since this PLD isoform is G-protein-independent.

Wortmannin, a PI3-kinase inhibitor: promoting effect on insulin secretion from pancreatic beta cells through a cAMP-dependent pathway

To determine the role of phosphatidylinositol 3-kinase (PI3-kinase) in the regulation of insulin secretion, we examined the effect of wortmannin, a PI3-kinase inhibitor, on insulin secretion using the isolated perfused rat pancreas and freshly isolated islets. In the perfused pancreas, 10(-8) M wortmannin significantly enhanced the insulin secretion induced by the combination of 8.3 mM glucose and 10(-5) M forskolin. In isolated islets, cyclic AMP (cAMP) content was significantly increased by wortmannin in the presence of 3.3 mM, 8.3 mM, and 16.7 mM glucose with or without forskolin. In the presence of 16.7 mM glucose with or without forskolin, wortmannin promoted insulin secretion significantly. On the other hand, in the presence of 8.3 mM glucose with forskolin, wortmannin augmented insulin secretion significantly; although wortmannin tended to promote insulin secretion in the presence of glucose alone, it was not significant. To determine if wortmannin increases cAMP content by promoting cAMP production or by inhibiting cAMP reduction, we examined the effects of wortmannin on 10(-4) M 3-isobutyl-1-methylxantine (IBMX)-induced insulin secretion and cAMP content. In contrast to the effect on forskolin-induced secretion, wortmannin had no effect on IBMX-induced insulin secretion or cAMP content. Moreover, wortmannin had no effect on nonhydrolyzable cAMP analog-induced insulin secretion in the perfusion study. These data indicate that wortmannin induces insulin secretion by inhibiting phosphodiesterase to increase cAMP content, and suggest that PI3-kinase inhibits insulin secretion by activating phosphodiesterase to reduce cAMP content.

Endothelin-1 (ET-1)-potentiated insulin secretion: involvement of protein kinase C and the ET(A) receptor subtype

Endothelin-1 (ET-1), a potent vasoconstrictor peptide of endothelial origin, is capable of influencing hormone secretion from endocrine tissues, eg, pancreatic islet cells. We have shown a direct stimulatory effect of ET-1 on insulin secretion from isolated mouse islets of Langerhans. However, it is unknown as to whether the peptide acts through specific receptors on the islet cells and which mechanisms are involved in this insulinotropic action. We have therefore used the specific ET(A) receptor antagonist BQ123, the ET(B) receptor agonist BQ3020, and classic alpha- and beta-adrenergic and cholinergic antagonists. ET-1 (100 nmol/L) stimulated insulin secretion from islets incubated at 8.3, 11.1, 16.7, and 25 mmol/L glucose (P < .05). At 3.3 mmol/L glucose, no alteration in insulin secretion was found. The cholinergic receptor antagonist atropine (5 micromol/L) or the adrenergic receptor antagonists propranolol (5 micromol/L) or phentolamine (5 micromol/L) did not affect ET-1 (100 nmol/L)-stimulated insulin secretion. BQ123 (10 pmol/L to 10 nmol/L) and BQ3020 (1 nmol/L to 1 micromol/L) had no effect on glucose (16.7 mmol/L)-stimulated insulin secretion, but BQ123 counteracted the stimulatory effect of ET-1 (100 nmol/L) at concentrations of 1 nmol/L to 10 micromol/L (P < .01). We also studied the relative role of protein kinase C (PKC) and a Wortmannin-sensitive pathway for ET-1-induced insulin secretion using 12-O-tetradecanoyl phorbol-13-acetate (TPA), Calphostin C, and Wortmannin, respectively. At 5.6 mmol/L glucose, ET-1 (100 nmol/L) had no effect per se, whereas in the presence of 1 micromol/L TPA, which acutely stimulates PKC, the peptide did potentiate insulin secretion (P < .05). Furthermore, the insulinotropic effect of ET-1 at 16.7 mmol/L glucose was counteracted by the PKC inhibitor Calphostin C (P < .05) and by downregulation of PKC by 24 hours of exposure of islets to TPA (0.5 micromol/L, P < .05). Wortmannin (1 micromol/L) did not alter ET-1-potentiated insulin secretion. In conclusion, our results suggest that ET-1 acts through specific ET-1 receptors, most likely the ETA subtype. Furthermore, PKC plays an essential role in the insulinotropic action of ET-1 in mouse islets.

Intracellular traffic to compartments for MHC class II peptide loading: signals for endosomal and polarized sorting

In this review we focus on the traffic of MHC class II and endocytosed antigens to intracellular compartments where antigenic peptides are loaded. We also discuss briefly the nature of the peptide loading compartment and the sorting signals known to direct antigen receptors and MHC class II and associated molecules to this location. MHC class II molecules are expressed on a variety of polarized epithelial and endothelial cells, and polarized cells are thus potentially important for antigen presentation. Here we review some cell biological aspects of polarized sorting of MHC class II and the associated invariant chain and the signals that are involved in the sorting process to the basolateral domain. The molecules involved in sorting and loading of peptide may modulate antigen presentation, and in particular we discuss how invariant chain may change the cellular phenotype and the kinetics of the endosomal pathway.

Enhancement of the T cell response to a mycobacterial peptide by conjugation to synthetic branched polypeptide

A peptide-based approach towards improving the immunodiagnosis of, and vaccination against, tuberculosis faces the problems of MHC restriction of T cell recognition and the poor immunogenicity of peptides in the absence of adjuvant. We sought to compensate this by the use of synthetic branched polypeptides of the poly[Lys-(Xi-DL-Alam)] type, containing a glutamic acid residue (EAK), and further modified either by succinylation (SucEAK) or acetylation (AcEAK). These carriers were conjugated to two permissively recognized peptides of Mycobacterium tuberculosis. The 38p350 - 369-SucEAK conjugate enhanced IFN-gamma production more than 13-fold (from 22.6 to 294 pg / ml, p = 0.001) in peripheral blood mononuclear cells from healthy subjects, and 8.7-fold (p = 0. 012) in cells from tuberculosis patients. The effect was dependent on the carrier used and on covalent linkage of SucEAK to 38p350 - 369. An increased response occurred best in cells from subjects bearing at least one HLA-DR allele for which 38p350 - 369 had high binding affinity and required cellular processing of the conjugate as inhibitors (chloroquine and wortmannin) blocked the IFN-gamma response. SucEAK conjugation of peptide 16p91 - 110 did not significantly increase IFN-gamma production, indicating that the ability of conjugation to enhance the response was peptide structure dependent. These data indicate that the use of SucEAK polymer coupled with permissively recognized peptides could contribute to the development of an improved immunodiagnostic or vaccine reagent for tuberculosis.

CD38 Stimulation Lowers the Activation Threshold and Enhances the Alloreactivity of Cord Blood T Cells by Activating the Phosphatidylinositol 3-Kinase Pathway and Inducing CD73 Expression

We have recently described in cord blood T cells (CBTC) a novel pathway linking CD38 and CD73, two signal transducers with ecto-enzyme activity. The aim of this study was 2-fold: first, to characterize the mechanisms by which CD38 regulates CD73 expression; and second, to determine whether surface-induced CD73 modulates CBTC responses. A marked increase in CD73 expression was observed in CD38+ cells after incubation with the appropriate CD38 mAbs. The induction of CD73 was blocked by wortmannin, a specific inhibitor of phosphatidylinositol 3-kinase (PI3-K). CD38 stimulation induced tyrosine phosphorylation of the p85 regulatory subunit of PI3-K and its association with other tyrosine-phosphorylated proteins. Surface-induced CD73 was as efficient in delivering activatory signals as the CD73 constitutively expressed on adult T cells. Highly CBTC, totally unresponsive to mitogenic concentrations of plastic-immobilized CD3 mAb, proliferated vigorously when exposed to the combination of plastic-immobilized CD3 and CD73 mAbs. The reactivity to allogeneic irradiated PBMC was also significantly enhanced by CD38 stimulation and was dependent on CD73 expression. Thus, CD38 stimulation lowers the activation threshold of CBTC by the CD3/TCR complex and enhances their reactivity to allogeneic cells via activation of the PI3-kinase pathway and CD73 expression.

Oxidation of defined antigens allows protein unfolding and increases both proteolytic processing and exposes peptide epitopes which are recognized by specific T cells

The participation of oxidative mechanisms in major histocompatibility complex (MHC) class II-restricted antigen presentation was studied in vitro. In general, antigen processing is inhibited when peritoneal macrophages (MO) are incubated with scavengers of reactive oxygen intermediates (ROI): mannitol (an.OH scavenger), dimethylurea (DMTU, which reacts with H2O2 and HOCl) and NCO-700 (an epoxysuccinic acid derivative which inhibits oxidant production by activated phagocytes and can scavenge reactive oxygen species in both NaOCl and hypoxanthine (XOD) systems). However, neither rotenone and antimycins (inhibitors of O-2 production at the NADH dehydrogenase and ubiquinone-cytochrome b regions, respectively) nor aminoguanidine (an inducible nitric oxide synthase inhibitor) impaired antigen presentation, thus indirectly discarding the participation of mitochondrial oxidation and reactive nitrogen intermediates (RNI) in antigen processing. ROI scavengers do not inhibit the MHC class II-restricted presentation of antigens that need processing but have their disulphide bonds reduced. It can be shown that oxidation of protein antigens (either by chlorination or performic acid treatment) allow protein unfolding and enhance both processing and exposure of immunogenic epitopes to specific T cells.

Characterization of human PLD2 and the analysis of PLD isoform splice variants

Phospholipase D (PLD) cleaves phosphatidylcholine in response to a variety of cell stimuli to release phosphatidic acid, which is associated with a number of cellular responses including regulated secretion, mitogenesis, and cytoskeletal changes. Recent advances in this field include the reports of cDNA sequences for two mammalian PLD isoforms: human PLD1 and rodent PLD2. We report the characterization of cDNA encoding human PLD2. In these experiments, we uncovered alternate splice variants of both human isoforms and evaluated the relative abundance of these messages by reverse transcriptase polymerase chain reaction, thereby indicating the physiologically relevant forms. Further, Northern hybridization experiments defined the tissue distribution of the human PLD messages. Human PLD1 does not appear to be an abundant message in any tissue tested whereas levels of human PLD2 mRNA apparently were higher and more variable. The specific activity and regulation of recombinant human PLD2 are indistinguishable from that of recombinant mouse PLD2. Analysis of the amino acid sequences of both human isoforms revealed important putative Pleckstrin homology domains and identified additional members of the PLD gene family that help to delimit the catalytic domain. The presence of Pleckstrin homology domains in the PLDs resolves several contradictory observations regarding PLD regulation and the domain structure of the proteins.

Effect of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, on the first mitotic divisions of the fertilized sea urchin egg

We have reported earlier that the polyphosphoinositide messenger system may control mitosis in sea urchin eggs. Besides phospholipase C activation and its second messengers, phosphatidylinositol (PI) 3-kinase has been proposed to affect a wide variety of cellular processes in other cellular systems. Therefore, we have investigated whether PI 3-kinase could play a role in regulating the sea urchin early embryonic development. Our data presented here suggest that PI 3-kinase is present in sea urchin eggs. We found that wortmannin, an inhibitor of PI 3-kinase, led to arrest of the cell cycle. Chromosome condensation, nuclear envelope breakdown, microtubular aster polymerization, protein and DNA synthesis were not affected when fertilization was performed in the presence of the drug. However, maturation-promoting factor (MPF) activation was inhibited and centrosome duplication was perturbed preventing the formation of a bipolar mitotic spindle in wortmannin treated eggs. We discuss how PI 3-kinase might be involved in the cascade of events leading to the first mitotic divisions of the fertilized sea urchin egg.

Activation of glomerular mesangial cells by hepatocyte growth factor through tyrosine kinase and protein kinase C

Hepatocyte growth factor (HGF) induces mitogenesis, chemotaxis, and tubule formation in renal epithelial cells. This study examined the effects of wortmannin and protein kinase C (PKC) inhibitors on HGF-mediated changes in metabolic activity in glomerular mesangial cells and renal epithelial carcinoma A498 cells. The extracellular acidification rate of transformed mouse glomerular mesangial cells and A498 cells was measured as an index of metabolic activity with a microphysiometer. HGF increased the acidification rate of mesangial cells and A498 cells in a concentration-dependent fashion that was inhibited completely by the tyrosine kinase inhibitor tyrophostin-23 (100 microM). The PKC inhibitors RO-32-0432 and SKF-57048 also inhibited HGF-induced acidification. The IC50 values for SKF-57048 were 59 +/- 2 and 20 +/- 10 nM in mesangial cells and A498 cells, respectively (P < 0.05). 12-O-Tetradecanoylphorbol 13-acetate (TPA), a phorbol ester that activates PKC, increased acidification in mesangial and epithelial cells similar to HGF. Wortmannin, an inhibitor of phosphatidylinositol (PI) 3-kinase (IC50 value 1-10 nM), inhibited HGF-induced acidification with an IC50 of 93 +/- 31 and 9 +/- 1 nM in mesangial and A498 cells, respectively (P < 0.05). In contrast, there was no significant difference in the IC50 value of wortmannin for epidermal growth factor (EGF)-induced acidification between mesangial and A498 cells (23 +/- 9 vs 14 +/- 1 nM, respectively). Because the IC50 value for wortmannin in inhibiting HGF but not EGF-induced acidification was an order of magnitude higher in mesangial cells than in epithelial A498 cells, a wortmannin-sensitive PI 3-kinase pathway may not be involved in HGF-mediated acidification in mesangial cells.

The consequences of the intracellular retention of pathogen-derived T-cell-independent antigens on protein presentation to T cells

Intracellular pathogens can be considered as particulate antigens chemically composed of a complex mixture of T-cell-dependent antigens (TD) (peptides and proteins) and T-cell-independent antigens (TI) (glycolipids and complex polysaccharides). A large range of saccharides (from oligosaccharides to complex polysaccharides) derived from pathogenic microorganisms are being isolated and characterized. They are currently implicated in signaling systems and concomitant host-parasite relationships. However, there are not many structure-function relationships described for these pathogens. This is particularly true of polysaccharides. In this report we have reviewed the role of defined TI antigens in the processing and presentation of defined TD antigens to specific T cells by antigen-presenting cells (APC). We also considered the importance of some of the chemical characteristics shared by different carbohydrates implicated in the inhibition of antigen presentation. These findings are discussed in relation to the clear immunopathological consequences of long retention periods of complex carbohydrate molecules derived from intracellular parasites inside certain APC and the absence of antigen presentation impairment in physiological situations such as the removal of senescent or damaged red blood cells by splenic macrophages or intracellular accumulation of carbohydrates in colostrum and milk macrophages during lactation.

Inhibitory effects of thymus-independent type 2 antigens on MHC class II-restricted antigen presentation: comparative analysis of carbohydrate structures and the antigen presenting cell

The role of thymus-independent type 2 (TI-2) antigens (polysaccharides) on the MHC-II-restricted processing of protein antigens was studied in vitro. In general, antigen presentation is inhibited when both peritoneal and splenic macrophages (M phi) as well as Küpffer cells (KC) are preincubated with acidic polysaccharides or branched dextrans. However, the inhibitory effect of neutral polysaccharides was minimal when KC were used as antigen presenting cells (APC). Morphological evaluation of the uptake of fluoresceinated polysaccharides clearly correlates with this selective and differential interference. Polysaccharides do not block MHC-I-restricted antigen presentation. Some chemical characteristics shared by different saccharides seem to be specially related to their potential inhibitory abilities: (i) those where two anomeric carbon atoms of two interlinked sugars and (ii) those containing several sulfate groups per disaccharide repeating unit. No polysaccharide being inhibitory in M phi abrogated antigen processing in other APC: lipopolysaccharide-activated B cells, B lymphoma cells, or dendritic cells (DC). Using radiolabeled polysaccharides it was observed that DC and B cells incorporated less radioactivity as a function of time than M phi. Morphological evaluation of these different APC incubated for extended periods of time with inhibitory concentrations of polysaccharides revealed intense cytoplasmic vacuolization in M phi but not in B cells or DC. The large majority of M phi lysosomes containing polysaccharides fail to fuse with incoming endocytic vesicles and delivery of fluid-phase tracers was reduced, suggesting that indigestible carbohydrates reduced the fusion of these loaded lysosomes with endosomes containing recently internalized tracers. It is suggested that the main causes of this antigen presentation blockade are (i) the chemical characteristics of certain carbohydrates and whether the specific enzymatic machinery for their intracellular degradation exists; and (ii) the different phagocytic abilities of distinct APC populations, fluid-phase pinocytosis and receptor-mediated saccharide uptake, and existence of a differential antigen-processing pathway in M phi and DC or B cells, which could be based on a polysaccharide-inhibited step present in M phi but unaffected or irrelevant in both B cells and DC.

The lysosomotropic amines, chloroquine and hydroxychloroquine: a potentially novel therapy for graft-versus-host disease

We have recently shown that the lysosomotropic amine, chloroquine, can inhibit the development of graft-versus-host disease (GVHD) secondary to minor histocompatibility (MiHC) differences in mice. In addition, we have shown that both chloroquine and hydroxychloroquine can inhibit T cell responses in vitro to minor and major histocompatibility (MHC) antigens. We review the rationale for the use of lysosomotropic amines, whose primary mechanism of action appears to be inhibition of MHC class II antigen presentation, as therapy for GVHD in humans. Used in low concentrations, these agents appear to have no direct effect on T cells either in vitro or in vivo although they may have a direct effect at higher concentrations. The lysosomotropic amines, at low concentrations, in combination with the T cell-specific agent, cyclosporin A, synergistically suppresses the T cell response to MiHC and MHC in mouse and in human. We present the initial data from the human clinical trials using hydroxychloroquine. We hypothesize that the lysosomotropic amines may have unique beneficial effects on immune reconstitution following bone marrow transplantation. The lysosomotropic amines, hydroxychloroquine and chloroquine, represent agents with unique mechanisms of action that may be used to control GVHD in humans.

Wortmannin inhibits the production of reactive oxygen and nitrogen intermediates and the killing of the Saccharomyces cerevisiae by isolated chicken macrophages

The direct effects of wortmannin (0 to 1280 nM) on several functions in cultured macrophages isolated from Sephadex-elicited Leghorn chicken peritonea were studied. Under concentrations not affecting cell viability, wortmannin, as low as 5 nM, inhibited lipopolysaccharide (LPS)-induced nitric oxide production (P < 0.01). However, wortmannin (as high as 1280 nM) exposure 5 hours post LPS induction had no effect on nitric oxide production in macrophages, indicating a blockade of LPS-induction of a signaling pathway related to nitric oxide formation. Phorbol myristate acetate (PMA)-induced superoxide production was only inhibited (P < 0.001) by concurrent exposure to 1280 nM wortmannin. Prior exposure to 160 nM and higher of wortmannin for 24 hours reduced the average number of yeast cells ingested by or attached to a single macrophage (P < 0.001) and the ability of the macrophage to kill the baker's yeast (P < 0.05), while wortmannin itself did not affect the yeast. These data provide direct evidence for macrophages being the target cell of wortmannin and further support the notion that impaired macrophage functions are responsible for the immunosuppressive effect of wortmannin previously observed in birds.

Inhibition of phosphatidylinositol 3-kinase blocks T cell antigen receptor/CD3-induced activation of the mitogen-activated kinase Erk2

The production of 3-phosphorylated inositol phospholipids is implicated in regulation of cell growth and transformation. To explore the role of these lipids in T cell antigen receptor (TCR)/CD3-induced signaling, we have examined the effects of a specific phosphatidylinositol 3-kinase (PtdIns3K) inhibitor, wortmannin, and overexpression of two PtdIns3K constructs on the activation of down-stream effectors in anti-CD3 treated T cells. We report that treatment of cells with wortmannin blocked anti-CD3-induced activation of the mitogen-activation kinase Erk2 while not affecting phorbol-ester-induced Erk2 activation. An inactive analog of wortmannin, WM12, did not affect TCR/CD3-induced Erk2 activation, and wortmannin had no effect on the activity of Erk2 when added directly to the in vitro assays. Expression of a disruptive PtdIns3K construct also reduced Erk2 activation, while a construct that stimulates PtdIns3K enhanced the activation of Erk2. Receptor-induced activation of other Ser/Thr kinases, such as c-Raf, B-Raf, Mek1, Mek2, Mekk, was not affected by wortmannin. Our results suggest that the production of 3-phosphorylated inositol phospholipids is involved in the activation of Erk2, but does not regulate the enzymes that are thought to be upstream of Erk2.

A wortmannin-sensitive signal transduction pathway is involved in the stimulation of insulin release by vasoactive intestinal polypeptide and pituitary adenylate cyclase-activating polypeptide

Vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27), and PACAP-38 stimulated insulin release with EC50 values of 0.15, 0.15, and 0.06 nM respectively, as expected for the VIP2/PACAP3 receptor subtype. Secretion was stimulated promptly and peaked at 6-10 min. At 30 min, the secretion rate was still 2-3-fold higher than the control rate. The peptides increased cyclic AMP and [Ca2+]i transiently so that at 30 min they had returned to control values. Therefore, an additional signal is required to explain the prolonged stimulation of release. The prolonged effects, but not the acute effects of VIP and PACAP on insulin release were inhibited by low concentrations of wortmannin, a phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor. While wortmannin inhibited PI 3-kinase activity in cell lysates, no activation by the peptides was seen. Therefore, the wortmannin-sensitive pathway is either dependent on basal PI 3-kinase activity, or another target for wortmanin is responsible for inhibition of the peptide-stimulated secretion. It is concluded that the acute stimulation of insulin release by VIP and PACAP is mediated by increased cyclic AMP and [Ca2+]i, whereas the sustained release is mediated by a novel wortmannin-sensitive pathway.

Wortmannin inhibits transcytosis of dimeric IgA by the polymeric immunoglobulin receptor

Phosphatidyl inositol 3-kinase (PI3K) plays an essential role in numerous signaling events, and increasingly has been implicated in regulation of certain membrane traffic events. The polymeric immunoglobulin receptor (pIgR) transcytoses dimeric IgA (dIgA) across epithelial cells and into external secretions, where the dIgA forms the first specific immunological defense against infection. We show here that wortmannin, a highly specific inhibitor of PI3K, inhibits transcytosis of dIgA by the pIgR. Instead, the dIgA is recycled back to the basolateral surface of the epithelial cell. PI3K therefore plays an essential role in regulating the transcytosis of dIgA, a key step in the mucosal immune response.

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.