Protein kinase C controls Fc gamma receptor-mediated endocytosis in human neutrophils

Targeting the mercapturic acid pathway for the treatment of melanoma

The treatment of metastatic melanoma is greatly hampered by the simultaneous dysregulation of several major signaling pathways that suppress apoptosis and promote its growth and invasion. The global resistance of melanomas to therapeutics is also supported by a highly active mercapturic acid pathway (MAP), which is responsible for the metabolism and excretion of numerous chemotherapy agents. The relative importance of the MAP in melanoma survival was not recognized until demonstrated that B16 melanoma undergoes dramatic apoptosis and regression upon the depletion or inhibition of the MAP transporter protein RLIP. RLIP is a multi-functional protein that couples ATP hydrolysis with the movement of substances. As the rate-limiting step of the MAP, the primary function of RLIP in the plasma membrane is to catalyze the ATP-dependent efflux of unmetabolized drugs and toxins, including glutathione (GSH) conjugates of electrophilic toxins (GS-Es), which are the precursors of mercapturic acids. Clathrin-dependent endocytosis (CDE) is an essential mechanism for internalizing ligand-receptor complexes that promote tumor cell proliferation through autocrine stimulation (Wnt5a, PDGF, βFGF, TNFα) or paracrine stimulation by hormones produced by fibroblasts (IGF1, HGF) or inflammatory cells (IL8). Aberrant functioning of these pathways appears critical for melanoma cell invasion, metastasis, and evasion of apoptosis. This review focuses on the selective depletion or inhibition of RLIP as a highly effective targeted therapy for melanoma that could cause the simultaneous disruption of the MAP and critical peptide hormone signaling that relies on CDE.

Sphingolipids in neutrophil function and inflammatory responses: Mechanisms and implications for intestinal immunity and inflammation in ulcerative colitis

Bioactive sphingolipids are regulators of immune cell function and play critical roles in inflammatory conditions including ulcerative colitis. As one of the major forms of inflammatory bowel disease, ulcerative colitis pathophysiology is characterized by an aberrant intestinal inflammatory response that persists causing chronic inflammation and tissue injury. Innate immune cells play an integral role in normal intestinal homeostasis but their dysregulation is thought to contribute to the pathogenesis of ulcerative colitis. In particular, neutrophils are key effector cells and are first line defenders against invading pathogens. While the activity of neutrophils in the intestinal mucosa is required for homeostasis, regulatory mechanisms are equally important to prevent unnecessary activation. In ulcerative colitis, unregulated neutrophil inflammatory mechanisms promote tissue injury and loss of homeostasis. Aberrant neutrophil function represents an early checkpoint in the detrimental cycle of chronic intestinal inflammation; thus, dissecting the mechanisms by which these cells are regulated both before and during disease is essential for understanding the pathogenesis of ulcerative colitis. We present an analysis of the role of sphingolipids in the regulation of neutrophil function and the implication of this relationship in ulcerative colitis.

Acylcarnitines--old actors auditioning for new roles in metabolic physiology

Perturbations in metabolic pathways can cause substantial increases in plasma and tissue concentrations of long-chain acylcarnitines (LCACs). For example, the levels of LCACs and other acylcarnitines rise in the blood and muscle during exercise, as changes in tissue pools of acyl-coenzyme A reflect accelerated fuel flux that is incompletely coupled to mitochondrial energy demand and capacity of the tricarboxylic acid cycle. This natural ebb and flow of acylcarnitine generation and accumulation contrasts with that of inherited fatty acid oxidation disorders (FAODs), cardiac ischaemia or type 2 diabetes mellitus. These conditions are characterized by very high (FAODs, ischaemia) or modestly increased (type 2 diabetes mellitus) tissue and blood levels of LCACs. Although specific plasma concentrations of LCACs and chain-lengths are widely used as diagnostic markers of FAODs, research into the potential effects of excessive LCAC accumulation or the roles of acylcarnitines as physiological modulators of cell metabolism is lacking. Nevertheless, a growing body of evidence has highlighted possible effects of LCACs on disparate aspects of pathophysiology, such as cardiac ischaemia outcomes, insulin sensitivity and inflammation. This Review, therefore, aims to provide a theoretical framework for the potential consequences of tissue build-up of LCACs among individuals with metabolic disorders.

Interactions of epigallo-catechin 3-gallate and ovalbumin, the major allergen of egg white

Polyphenols, the potent plant secondary metabolites, have beneficial effects on human health, but the mechanism(s) by which these effects are exerted is not well understood. Here, we present the detailed analysis of the interactions between the major green tea catechin, epigallo-catechin 3-gallate (EGCG), and the major dietary protein and allergen, ovalbumin (OVA). We show that EGCG binds to the pocket that partly overlaps with the previously identified IgE-binding region in OVA, and that this interaction induces structural changes in the allergen. Moreover, our ex vivo studies reveal that OVA binds IgE and stimulates degranulation of basophils, and that its uptake by monocytes proceeds at a slower rate in the presence of EGCG. This study provides further evidence in support of the proposed mechanism by which EGCG interactions with the food allergens contribute to its diverse biological activities and may impair antigen uptake by antigen-presenting cells.

Diacylglycerol-stimulated endocytosis of transferrin in trypanosomatids is dependent on tyrosine kinase activity

Small molecule regulation of cell function is an understudied area of trypanosomatid biology. In Trypanosoma brucei diacylglycerol (DAG) stimulates endocytosis of transferrin (Tf). However, it is not known whether other trypanosomatidae respond similarly to the lipid. Further, the biochemical pathways involved in DAG signaling to the endocytic system in T. brucei are unknown, as the parasite genome does not encode canonical DAG receptors (e.g. C1-domains). We established that DAG stimulates endocytosis of Tf in Leishmania major, and we evaluated possible effector enzymes in the pathway with multiple approaches. First, a heterologously expressed glycosylphosphatidylinositol phospholipase C (GPI-PLC) activated endocytosis of Tf 300% in L. major. Second, exogenous phorbol ester and DAGs promoted Tf endocytosis in L. major. In search of possible effectors of DAG signaling, we discovered a novel C1-like domain (i.e. C1_5) in trypanosomatids, and we identified protein Tyr kinases (PTKs) linked with C1_5 domains in T. brucei, T. cruzi, and L. major. Consequently, we hypothesized that trypanosome PTKs might be effector enzymes for DAG signaling. General uptake of Tf was reduced by inhibitors of either Ser/Thr or Tyr kinases. However, DAG-stimulated endocytosis of Tf was blocked only by an inhibitor of PTKs, in both T. brucei and L. major. We conclude that (i) DAG activates Tf endocytosis in L. major, and that (ii) PTKs are effectors of DAG-stimulated endocytosis of Tf in trypanosomatids. DAG-stimulated endocytosis of Tf may be a T. brucei adaptation to compete effectively with host cells for vertebrate Tf in blood, since DAG does not enhance endocytosis of Tf in human cells.

The transferrin receptor and the tetraspanin web molecules CD9, CD81, and CD9P-1 are differentially sorted into exosomes after TPA treatment of K562 cells

Here we show that treatment of K562 cells with the phorbol ester TPA induces the down-modulation of various surface antigens. Among them, the transferrin receptor (TfR), the tetraspanin CD81, and a CD81-associated protein, CD9P-1, were unique in that their expression levels were lower after 24 h incubation than after 3 h. We demonstrated that like the TfR, CD81 was internalized at early times, and was less synthesized at latter times. Despite the association of a fraction of the TfR with CD81, these two molecules were subjected to different fates. TPA increased targeting of CD81 and CD9P-1 into exosomes but strongly reduced the localization of the TfR in these vesicles. Using this model we have shown that a fraction of CD81 and CD9P-1 in exosomes comes from a surface pool and that these molecules remain associated in exosomes. However, CD9P-1 could be targeted to exosomes in the absence of CD81 and of another tetraspanin, CD9. The targeting of CD9 into exosomes did not require palmitoylation of the protein. J. Cell. Biochem. 102: 650-664, 2007. (c) 2007 Wiley-Liss, Inc.

2n-fatty acids from phosphatidylcholine label sphingolipids—A novel role of phospholipase A2?

In order to find out whether there is a phospholipase A2 (PLA2)-mediated link between glycerophospholipids and sphingolipids, L929 cells were labeled with 1n-palmitoyl-2n-[1-14C]palmitoyl phosphatidylcholine for 16-18 h or 90 min. After labeling for 16-18 h, 14C-sphingomyelin (SM), 14C-ceramide and 14C-sphingosine were demonstrated on autoradiograms of thin layer chromatograms of untreated or mildly hydrolyzed lipid extracts in different chromatographic systems. Strong hydrolysis of labeled SM proved that both possible moieties of SM, sphingosine and acyl moiety, had been labeled. The identity of SM and its enzymatic degradation product, ceramide, was verified by mass spectrometry. The label in SM-derived ceramide was demonstrated on an autoradiogram after thin layer chromatography. The inhibitor of (dihydro)ceramide synthase fumonisin B1 suppressed the label in sphingolipids significantly during 16-18 h (ceramide and SM), as well as during 90-min labeling (SM). The presence of inhibitors of PLA2 (bromoenol lactone, aristolochic acid and quinacrine dihydrochloride) diminished the label in SM significantly during the 90-min labeling. These results demonstrate a close metabolic relationship between glycerophospholipids and sphingolipids and give evidence for a novel role of PLA2.

Mechanisms involved in calcium oxalate endocytosis by Madin-Darby canine kidney cells

Calcium oxalate (CaOx) crystals adhere to and are internalized by tubular renal cells and it seems that this interaction is related (positively or negatively) to the appearance of urinary calculi. The present study analyzes a series of mechanisms possibly involved in CaOx uptake by Madin-Darby canine kidney (MDCK) cells. CaOx crystals were added to MDCK cell cultures and endocytosis was evaluated by polarized light microscopy. This process was inhibited by an increase in intracellular calcium by means of ionomycin (100 nM; N = 6; 43.9% inhibition; P<0.001) or thapsigargin (1 microM; N = 6; 33. 3% inhibition; P<0.005) administration, and via blockade of cytoskeleton assembly by the addition of colchicine (10 microM; N = 8; 46.1% inhibition; P<0.001) or cytochalasin B (10 microM; N = 8; 34.2% inhibition; P<0.001). Furthermore, CaOx uptake was reduced when the activity of protein kinase C was inhibited by staurosporine (10 nM; N = 6; 44% inhibition; P<0.01), or that of cyclo-oxygenase by indomethacin (3 microM; N = 12; 17.2% inhibition; P<0.05); however, the uptake was unaffected by modulation of potassium channel activity with glibenclamide (3 microM; N = 6), tetraethylammonium (1 mM; N = 6) or cromakalim (1 microM; N = 6). Taken together, these data indicate that the process of CaOx internalization by renal tubular cells is similar to the endocytosis reported for other systems. These findings may be relevant to cellular phenomena involved in early stages of the formation of renal stones.

Casein kinase II activity is required for transferrin receptor endocytosis

The effect of protein kinase inhibitors on transferrin receptor (TR) internalization was examined in HeLa, A431, 3T3-L1 cells, and primary chicken embryo fibroblasts. We show that TR endocytosis is not affected by tyrosine kinase or protein kinase C inhibitors, but is inhibited by one serine/threonine kinase inhibitor, H-89. Inhibition occurred within 15 min, was completely reversible after H-89 withdrawal, and was specific for endocytosis rather than pinocytosis since a TR mutant lacking an internalization signal was not affected. Interestingly, H-89 also inhibited the internalization of a TR chimera containing the major histocompatibility complex class II invariant chain cytoplasmic tail, indicating that the effect was not specific for the TR. Since H-89 inhibits a number of kinases, we employed a permeabilized cell endocytosis assay to further characterize the kinase. In permeabilized 3T3-L1 cells, addition of pseudosubstrate inhibitor peptides of casein kinase II (CKII) blocked TR internalization by more than 50%, whereas pseudosubstrates of cyclic AMP-dependent kinase A, protein kinase C, and casein kinase I had no effect. Furthermore, addition of purified CKII to the cell-free reactions containing CKII pseudosubstrates reversed the endocytosis block, suggesting that CKII or a CKII-like activity is required for constitutive endocytosis.

Phospholipases and phagocytosis: the role of phospholipid-derived second messengers in phagocytosis

Phagocytosis, the process by which leukocytes recognize and destroy invading pathogens, is essential for host defense. The binding of foreign organisms to phagocytic leukocytes initiates a complex signaling cascade which ultimately results in the entrapment and destruction of the pathogen. The signal transduction pathway mediating phagocytosis is the subject of intense investigation and is known to include protein tyrosine kinases, GTP-binding proteins, protein kinase C (PKC), actin polymerization and membrane movement. A rapidly expanding body of evidence suggests that phospholipases play an integral role in phagocytosis by generating essential second messengers. Here we review the data linking activation of phospholipase A2 (PLA2), phospholipase C (PLC) phospholipase D (PLD), and phosphoinositide 3-OH kinase (PI(3)K) to antibody (IgG)-mediated phagocytosis. Evidence is presented that (1) PLA2-derived arachidonic acid (AA) stimulates NADPH oxidase and membrane redistribution during phagocytosis, (2) the inositol-3,4,5-triphosphate (IP3) and diacylglycerol (DAG) products of PLC activate NADPH oxidase and PKC, and (3) sequential activation of PLD and phosphatidic acid phosphohydrolase may provide an alternative pathway for generation of DAG. Additionally, considerable evidence exists that wortmannin, a PI(3)K inhibitor, depresses phagocytosis. This finding is discussed in the context of the extensive effects PI(3)K products have on endocytosis and exocytosis and the potential role of membrane redistribution in phagocytosis. Finally, a model is presented which integrates data obtained from a variety of phagocytic systems and illustrates potential interactions that may exist between phospholipase-derived second messengers and signaling events required for phagocytosis.

Protein kinase C isoforms in normal and chronic myeloid leukemic neutrophils. Distinct signal for PKC alpha by immunodetection on PVDF membrane, decreased expression of PKC alpha and increased expression of PKC delta in leukemic neutrophils

Neutrophils from patients with Chronic Myeloid Leukemia (CML) exhibit defects in several functions. They also show altered phosphorylation-dephosphorylation patterns of several proteins on stimulation with phorbol myristate acetate--a direct activator of protein kinase C (PKC). Since PKC mediates several functions of the neutrophil, in this study we investigate the PKC isoform profile and subcellular distribution in normal and CML neutrophils in an attempt to elucidate their role in CML signalling. Our results show the presence of PKC alpha, betaI, betaII and delta in both the cell types. A distinct and clear signal was obtained for PKC alpha, the isoform reported to be absent or present in very low amounts in normal neutrophils. In addition, PKC alpha was present in significantly lower levels in CML neutrophils while the PKC delta isoform was found in significantly higher amounts in the CML cytosol as compared to that in normal cells. PKC alpha, betaI, betaII and delta isoforms could not be detected in the nucleus of unstimulated normal and CML neutrophils. The altered levels of PKC alpha and delta may be one of the causes for the defects in function exhibited by the leukemic cells.

Lateral mobility of Fc gamma RIIa is reduced by protein kinase C activation

The lateral mobility of membrane proteins can reflect the extent of various protein-protein interactions. Using the fluorescence recovery after photobleaching technique, we have studied the lateral mobility of human Fc gamma RIIa and some Fc gamma RIIa mutants expressed in either P388D1 cells, a mouse macrophage-like cell line, or in Chinese hamster ovary (CHO) cells [1]. After treatment with phorbol myristate acetate (PMA), only the Fc gamma RIIa molecules capable of mediating rapid endocytosis of immune complexes exhibited a reduced lateral diffusion coefficient with respect to untreated controls. Wild type Fc gamma RIIa expressed in CHO cells, and nonfunctional Fc gamma RIIa mutants expressed in P388D1 cells did not show any differences upon PMA treatment. This finding suggests that protein kinase C activation evokes additional protein-protein interactions with the cytoplasmic domain of functional Fc gamma RIIa, which reduced receptor lateral mobility. The identity of these putative interacting proteins and the nature of the interactions remain to be elucidated.

Exposure of human ovarian carcinoma to cisplatin transiently sensitizes the tumor cells for liposome-mediated gene transfer

Human ovarian carcinoma cells (line 2008) grown as subcutaneous solid tumor in the severe combined immunodeficient mouse can be transfected by directly injecting a plasmid DNA-liposome complex into the tumor (in situ lipofection). The level of reporter gene expression in the tumor cells was significantly elevated if the animal received a single i.p. injection of cisplatin 1 week before the lipofection. Sensitization of the tumor for lipofection peaked 1 week after cisplatin injection and declined thereafter. Cells exposed to low concentration of cisplatin in vitro for four to five doubling times also showed elevated sensitivity for lipofection in vitro. Cisplatin was the only anticancer drug tested that exhibited this activity. These results suggest a sequential combinational gene therapy protocol with cisplatin for the ovarian carcinoma.

Differential phosphorylation in normal and leukemic granulocytes in response to phorbol 12-myristate 13-acetate

Granulocytes from the peripheral blood of patients with chronic myeloid leukemia (CML) exhibit a number of functional defects. To explore the relationship of these aberrations to signal transduction, granulocytes from normal subjects and CML patients were labelled with 32Pi, stimulated with phorbol myristate acetate (PMA) and the phosphoproteins (Pps) in the unstimulated and stimulated cells analyzed by 2D-SDS-PAGE followed by autoradiography. Results show that there are six distinct reproducibly phosphorylated proteins referred to as Pp1-Pp6 identifiable in the basal patterns of the resting granulocytes. Amongst these, Pp1 and Pp5 are more intensely phosphorylated and Pp3 is very faint or absent in unstimulated CML cells, relative to the normal granulocytes. On stimulation of normal cells with PMA, Pp1, Pp3, Pp4 and Pp6 exhibit distinct patterns of phosphorylation-dephosphorylation. In the CML cells, however, Pp1 and Pp4 are unresponsive to PMA. We conclude that PKC-mediated functions involving Pp1, Pp3 and Pp4 are most probably defective in CML cells.

Differential phosphorylation--cause for defective internalization of aggregated IgG by chronic myeloid leukemic granulocytes?

Granulocytes from the peripheral blood of patients with chronic myeloid leukemia (CML) are known to exhibit a defect in internalization of aggregated IgG relative to normal cells. As this aberration may arise due to defective transmembrane signalling, this study was undertaken to analyze alterations, if any, in protein phosphorylation between the two cell types. Normal and CML granulocytes were labeled with 32P-sodium orthophosphate and then stimulated with aggregated IgG. The phosphoproteins in the unstimulated and stimulated cells were analyzed by 2D-SDS-PAGE followed by autoradiography. The results show that there are five distinctly identifiable, reproducibly phosphorylated proteins referred to as Pp1-Pp5. In the unstimulated normal cells, Pp1 is less phosphorylated than Pp3, while in CML cells, Pp1 is more intense than Pp3. On stimulation of normal cells, with aggregated IgG, intensity of Pp1 increases while that of Pp3 decreases. In CML cells this response is reversed. We conclude that one of the causes for the defective internalization of IgG by CML granulocytes may probably be the observed differences in the phosphorylation of the proteins under study.