Phospholipase C-gamma mediates the hydrolysis of phosphatidylinositol, but not of phosphatidylinositol 4,5-bisphoshate, in carbamylcholine-stimulated islets of langerhans

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

The diverse roles of protein kinase C in pancreatic beta-cell function

Members of the serine/threonine PKC (protein kinase C) family perform diverse functions in multiple cell types. All members of the family are activated in signalling cascades triggered by occupation of cell surface receptors, but the cPKC (conventional PKC) and nPKC (novel PKC) isoforms are also responsive to fatty acid metabolites. PKC isoforms are involved in various aspects of pancreatic beta-cell function, including cell proliferation, differentiation and death, as well as regulation of secretion in response to glucose and muscarinic receptor agonists. Recently, the nPKC isoform, PKCepsilon, has also been implicated in the loss of insulin secretory responsiveness that underpins the development of Type 2 diabetes.

Sustained diacylglycerol accumulation resulting from prolonged G protein-coupled receptor agonist-induced phosphoinositide breakdown in hepatocytes

Studies in various cells have led to the idea that agonist-stimulated diacylglycerol (DAG) generation results from an early, transient phospholipase C (PLC)-catalyzed phosphoinositide breakdown, while a more sustained elevation of DAG originates from phosphatidylcholine (PC). We have examined this issue further, using cultured rat hepatocytes, and report here that various G protein-coupled receptor (GPCR) agonists, including vasopressin (VP), angiotensin II (Ang.II), prostaglandin F2alpha, and norepinephrine (NE), may give rise to a prolonged phosphoinositide hydrolysis. Preincubation of hepatocytes with 1-butanol to prevent conversion of phosphatidic acid (PA) did not affect the agonist-induced DAG accumulation, suggesting that phospholipase D-mediated breakdown of PC was not involved. In contrast, the GPCR agonists induced phosphoinositide turnover, assessed by accumulation of inositol phosphates, that was sustained for up to 18 h, even under conditions where PLC was partially desensitized. Pretreatment of hepatocytes with wortmannin, to inhibit synthesis of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PIP2), prevented agonist-induced inositol phosphate and DAG accumulation. Upon VP stimulation the level of PIP) declined, but only transiently, while increases in inositol 1,4,5-trisphosphate (InsP3) and DAG mass were sustained, suggesting that efficient resynthesis of PIP2 allowed sustained PLC activity. This was confirmed when cells were pretreated with wortmannin to prevent resynthesis of PIP2. Furthermore, metabolism of InsP3 was rapid, compared to that of DAG, with a more than 20-fold difference in half-life. Thus, rapid metabolism of InsP3 and efficient resynthesis of PIP2 may account for the larger amount of DAG generated and the more sustained time course, compared to InsP3. The results suggest that DAG accumulation that is sustained for many hours in response to VP, Ang.II, NE, and prostaglandin F2alpha in hepatocytes is mainly due to phosphoinositide breakdown.

Oscillations of phospholipase C activity triggered by depolarization and Ca2+ influx in insulin-secreting cells

Phospholipase C (PLC) is a ubiquitous enzyme involved in the regulation of a variety of cellular processes. Its dependence on Ca2+ is well recognized, but it is not known how PLC activity is affected by physiological variations of the cytoplasmic Ca2+ concentration ([Ca2+](i)). Here, we applied evanescent wave microscopy to monitor PLC activity in parallel with [Ca2+](i) in individual insulin-secreting INS-1 cells using the phosphatidylinositol 4,5-bisphosphate- and inositol 1,4,5-trisphosphate-binding pleckstrin homology domain from PLCdelta(1) fused to green fluorescent protein (PH(PLCdelta1)-GFP) and the Ca2+ indicator fura red. In resting cells, PH(PLCdelta1)-GFP was located predominantly at the plasma membrane. Activation of PLC by muscarinic or purinergic receptor stimulation resulted in PH(PLCdelta1)-GFP translocation from the plasma membrane to the cytoplasm, detected as a decrease in evanescent wave-excited PH(PLCdelta1)-GFP fluorescence. Using this translocation as a measure of PLC activity, we found that depolarization by raising extracellular [K+] triggered activation of the enzyme. This effect could be attributed both to a rise of [Ca2+](i) and to depolarization per se, because some translocation persisted during depolarization in a Ca2+-deficient medium containing the Ca2+ chelator EGTA. Moreover, oscillations of [Ca2+](i) resulting from depolarization with Ca2+ influx evoked concentration-dependent periodic activation of PLC. We conclude that PLC activity is under tight dynamic control of [Ca2+](i). In insulin-secreting beta-cells, this mechanism provides a link between Ca2+ influx and release from intracellular stores that may be important in the regulation of insulin secretion.

Phosphorylated inositol compounds in beta -cell stimulus-response coupling

Pancreatic beta-cell function is essential for the regulation of glucose homeostasis in humans, and its impairment leads to the development of type 2 diabetes. Inputs from glucose and cell surface receptors act together to initiate the beta-cell stimulus-response coupling that ultimately leads to the release of insulin. Phosphorylated inositol compounds have recently emerged as key players at all levels of the stimulus-secretion coupling process. In this current review, we seek to highlight recent advances in beta-cell phosphoinositide research by dividing our examination into two sections. The first involves the events that lead to insulin secretion. This includes both new roles for inositol polyphosphates, particularly inositol hexakisphosphate, and both conventional and 3-phosphorylated inositol lipids. In the second section, we deal with the more novel concept of the autocrine role of insulin. Here, released insulin initiates signal transduction cascades, principally through the activity of phosphatidylinositol 3-kinase. This new round of signal transduction has been established to activate key beta-cell genes, particularly the insulin gene itself. More controversially, this insulin feedback has also been suggested to either terminate or enhance insulin secretion events.

Identification of myo-inositol 1,2-cyclic monophosphate by electrospray tandem mass spectrometry, a major constituent of EGF-stimulated phosphoinositide turnover in MDA 468 cells

Epidermal growth factor (EGF) caused an increase in phosphoinositide (PI) turnover in MDA 468 cells. This EGF-stimulated effect was inhibited by the protein tyrosine kinase inhibitor lavendustin A (LA). MDA 468 cells generated an atypical PI turnover profile. Examination and quantitation of the PI metabolite profile showed that even control cells produced a metabolite which was acid-labile and which formed about 60% of the total PI metabolites. By using the technique of electrospray ionization tandem mass spectrometry, we were able to confirm the identity of this acid-labile metabolite through the specific fragmentation as compared with the standard. The precursor molecule fragmented into two distinct productions with molar masses identical to that of the standard myo-inositol 1,2-cyclic monophosphate (cInsP). Changes in the PI turnover profile could be accounted for by the alterations in myo-inositol 1,2-cyclic monophosphate generated in these cells. We thus conclude that, by some as-yet-unidentified mechanism, cyclic inositol monophosphate forms a major constituent of EGF-stimulated PI turnover in MDA 468 cells.

Evidence for selective coupling of alpha 1-adrenergic receptors to phospholipase C-beta 1 in rat neonatal cardiomyocytes

Activation of phospholipase C (PLC) in neonatal rat cardiomyocytes (NCM) generates primarily inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) in response to rises in intracellular Ca(2+), or inositol 1,4-bisphosphate (Ins(1,4)P(2)) in response to norepinephrine (NE) (Matkovich, S. J. and Woodcock, E. A. (2000) J. Biol. Chem. 275, 10845-10850). To examine the PLC subtype mediating the alpha(1)-adrenergic receptor response, PLC-beta(1) and PLC-beta(3) were overexpressed in NCM using adenoviral infection (Ad-PLC-beta(1) NCM and Ad-PLC-beta(3) NCM, respectively) and PLC responses assessed from [(3)H]inositol phosphate (InsP) generation in the presence of 10 mm LiCl. The [(3)H]InsP response to NE (100 microm) was enhanced in Ad-PLC-beta(1) NCM relative to cells infected with blank virus (Ad-MX NCM), but was reduced in Ad-PLC-beta(3) NCM. In contrast, the [(3)H]InsP response to ATP (100 microm) was not elevated in Ad-PLC-beta(1) NCM, and was enhanced rather than diminished in Ad-PLC-beta(3) NCM, showing that effects of the two PLC-beta isoforms were specific for particular receptor types. PLC-delta(1) overexpression selectively reduced NE-induced [(3)H]InsP responses, without affecting the ATP stimulation. The reduced NE response was associated with a selective loss of PLC-beta(1) expression in Ad-PLC-delta(1) NCM. alpha(1)-Adrenergic receptor activation caused phosphorylation of PLC-beta(1) but not PLC-beta(3), whereas stimulation by ATP induced phosphorylation of PLC-beta(3) but not PLC-beta(1.) Taken together, these studies provide evidence that NE-stimulated InsP generation in NCM is primarily mediated by PLC-beta(1), despite the presence of both PLC-beta(1) and PLC-beta(3) isoforms.