The involvement of specific phospholipase C isozymes in catecholamine release from digitonin permeabilized bovine adrenal medullary chromaffin cells

Regulation of catecholamine release in human adrenal chromaffin cells by β-adrenoceptors

The adrenal gland plays a fundamental role in the response to a variety of stress situations. After a stress condition, adrenal medullary chromaffin cells release, by exocytosis, high quantities of catecholamine (epinephrine, EP; norepinephrine, NE), especially EP. Once in the blood stream, catecholamines reach different target organs, and induce their biological actions through the activation of different adrenoceptors. Adrenal gland cells may also be activated by catecholamines, through hormonal, paracrine and/or autocrine system. The presence of functional adrenoceptors on human adrenal medulla and their involvement on catecholamines secretion was not previously evaluated. In the present study we investigated the role of β(1)-, β(2)- and β(3)-adrenoceptors on catecholamine release from human adrenal chromaffin cells in culture. We observed that the β-adrenoceptor agonist (isoproterenol) and β(2)-adrenoceptor agonist (salbutamol) stimulated catecholamine (NE and EP) release from human adrenal chromaffin cells. Furthermore, the β(2)-adrenoceptor antagonist (ICI 118,551; 100 nM) and β(3)-adrenoceptor antagonist (SR 59230A; 100 nM) inhibited the catecholamine release stimulated by isoproterenol and nicotine in chromaffin cells. The β(1)-adrenoceptor antagonist (atenolol; 100 nM) did not change the isoproterenol- neither the nicotine-evoked catecholamine release from human adrenal chromaffin cells. Moreover, our results show that the protein kinase A (PKA), protein kinase C (PKC), mitogen-activated protein kinase (MAPK) and phospholipase C (PLC) are intracellular mechanisms involved in the catecholamine release evoked by salbutamol. In conclusion, our data suggest that the activation of β(2)- and β(3)-adrenoceptors modulate the basal and evoked catecholamine release, NE and EP, via an autocrine positive feedback loop in human adrenal chromaffin cells.

Elimination of plasma membrane phosphatidylinositol (4,5)-bisphosphate is required for exocytosis from mast cells

The inositol lipid phosphatidylinositol (4,5)-bisphosphate [PtdIns(4,5)P2] is involved in a myriad of cellular processes, including the regulation of exocytosis and endocytosis. In this paper, we address the role of PtdIns(4,5)P2 in compound exocytosis from rat peritoneal mast cells. This process involves granule-plasma membrane fusion as well as homotypic granule membrane fusion and occurs without any immediate compensatory endocytosis. Using a novel quantitative immunofluorescence technique, we report that plasma membrane PtdIns(4,5)P2 becomes transiently depleted upon activation of exocytosis, and is not detected on the membranes of fusing granules. Depletion is caused by phospholipase C activity, and is mandatory for exocytosis. Although phospholipase C is required for Ca2+ release from internal stores, the majority of the requirement for PtdIns(4,5)P2 hydrolysis occurs downstream of Ca2+ signalling - as shown in permeabilised cells, where the inositol (1,4,5)-trisphosphate-Ca2+ pathway is bypassed. Neither generation of the PtdIns(4,5)P2 metabolite, diacylglycerol (DAG) or simple removal and/or sequestration of PtdIns(4,5)P2 are sufficient for exocytosis to occur. However, treatment of permeabilised cells with DAG induces a small potentiation of exocytosis, indicating that it may be required. We propose that a cycle of PtdIns(4,5)P2 synthesis and breakdown is crucial for exocytosis to occur in mast cells, and may have a more general role in all professional secretory cells.

Phospholipase C isozymes are differentially distributed in the rat adrenal medulla

Immunohistochemistry has been used to examine the distribution of selected phospholipase C (PLC) isozymes within the adrenal medulla of the rat. PLCbeta isozymes were expressed at moderate levels in the chromaffin cells but more strongly in association with ganglion cell clusters. PLCbeta2 and PLCbeta3 staining of clusters did not overlap suggesting selective PLC isozyme expression in two distinct ganglionic types. The distribution of PLCbeta4 immunoreactivity was very similar to PLCbeta3 with the strongest staining observed in the same cell clusters. Antibodies to PLCbeta1 labelled multiple bands on Western blots and were not therefore used for immunohistochemistry. The chromaffin cells were also immunoreactive for PLCgamma1, although the strongest staining with this antibody was seen in cells surrounding large sinus vessels. PLCdelta1 and PLCdelta2 had quite distinct distributions, with the former selectively localized to an endothelial cell population surrounding the chromaffin cells. This observation was supported by experiments on isolated bovine adrenal medullary cells where PLCdelta1 expression was lost when the cell preparation was enriched for chromaffin cells. Antibodies to PLCdelta2 labelled a network of nerve fibres throughout the medulla and clusters of ganglion cells located primarily at the medullary-cortical boundary. PLCdelta2 immunoreactivity was also present in nerve fibres within the adrenal capsule where it appeared to be co-localized with PLCbeta4 staining.

Phospholipases C and A2 control lysosome-mediated IL-1 beta secretion: Implications for inflammatory processes

Blocking the activity of IL-1 beta has entered the clinical arena of treating autoimmune diseases. However, a successful outcome of this approach requires a clear definition of the mechanisms controlling IL-1 beta release. These are still unclear as IL-1 beta, lacking a secretory signal peptide, follows a nonclassical pathway of secretion. Here, we analyze the molecular mechanism(s) undergoing IL-1 beta processing and release in human monocytes and provide a unifying model for the regulated secretion of the cytokine. Our data show that in a first step, pro-caspase-1 and endotoxin-induced pro-IL-1 beta are targeted in part to specialized secretory lysosomes, where they colocalize with other lysosomal proteins. Externalization of mature IL-1 beta and caspase-1 together with lysosomal proteins is then facilitated by extracellular ATP. ATP triggers the efflux of K(+) from the cell, followed by Ca(2+) influx and activation of three phospholipases: phosphatidylcholine-specific phospholipase C and calcium-independent and -dependent phospholipase A(2). Whereas calcium-independent phospholipase A(2) is involved in processing, phosphatidylcholine-specific phospholipase C and calcium-dependent phospholipase A(2) are required for secretion. Dissection of the events that follow ATP triggering allowed to demonstrate that K(+) efflux is responsible for phosphatidylcholine-specific phospholipase C induction, which in turn allows the rise in intracellular free calcium concentration required for activation of phospholipase A(2). This activation is ultimately responsible for lysosome exocytosis and IL-1 beta secretion.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.