Insulin second messengers

The molecular pathways for insulin's signal transduction from its cell surface receptor to the cell's interior metabolic machinery remain in many ways uncharted. Lately two molecules have been proposed as second messengers transducing the insulin signal into the target cell. One is a phospholigosaccharide/inositolphosphoglycan and the other is diacylglycerol, both deriving from the same plasma membrane glycolipid, which is hydrolysed in response to insulin treatment. The phospho-oligosaccharide appears to mediate many metabolic effects of insulin through control of the phosphorylation state of key regulatory metabolic enzymes. Diacylglycerol may mediate insulin's stimulation of glucose transport over the plasma membrane. The glycolipid precursor of these putative second messengers, as well as the receptor for insulin, appear to be localized in caveolae microdomains of the plasma membrane, and glucose transporters accumulate in caveolae in response to insulin treatment, suggesting a focal role for caveolae in insulin signalling.     

Intracellular second messengers mediate stress inducible hormesis and Programmed Cell Death: A review

Alterations in the levels of numerous second messengers are ubiquitous responses to all stresses that lead to apoptotic or hormetic responses. The sheer number and vast diversity of different second messenger systems activated in response to stresses belies a complexity that is often overlooked. This negligence is in large part due to the excessive focus on classical stress responsive second messenger mediators of stress especially Reactive Oxygen Species (ROS) but also others like calcium and ceramide. Here we review the many different intracellular second messengers that are involved in stress responses. We further integrate this information to emphasize that initial stress mediated responses consist of increased levels of a multitude of intracellular second messengers that serve to elicit the appropriate cell survival and/or cell death responses. We suggest that a greater focus on second messenger systems may shed more light on the processes that serve in the initiation of stress mediated PCD.     

Glucose metabolism, second messengers and insulin secretion

Insulin secretion from beta cells of the islets of Langerhans in the endocrine pancreas is regulated by glucose, glucose metabolites, metabolic intermediates such as ATP, acetyl CoA and reduced pyridine nucleotides, and classical second messengers. Receptor responses transduced by guanine nucleotide binding proteins modulate metabolic activity, the generation of second messengers, and cell depolarization during stimulus-response coupling in the beta cell. This review will consider insulin secretion as regulated by glucose metabolic pathways and second messengers.     

G protein-linked effector and second messenger systems involved in melatonin signal transduction

The isolation and characterization of multiple melatonin receptors in a wide range of tissues and cells signifies the functional diversity of melatonin. In different cellular environments, melatonin can regulate distinct second messengers or even positively or negatively regulate the same signal transduction pathway. The capacity by which melatonin receptors modulate the activities of various effector molecules is determined by the complement of signaling components present in any particular cell type. The specific interactions between many signaling molecules have been discerned in an increasing number of cellular systems and this information is being used to explain the physiological actions of melatonin. This review will attempt to summarize recent research by many groups that has revealed numerous subtleties of the melatonin-coupled signaling pathways.     

The role of lipid second messengers in aldosterone synthesis and secretion

Second messengers are small rapidly diffusing molecules or ions that relay signals between receptors and effector proteins to produce a physiological effect. Lipid messengers constitute one of the four major classes of second messengers. The hydrolysis of two main classes of lipids, glycerophospholipids and sphingolipids, generate parallel profiles of lipid second messengers: phosphatidic acid (PA), diacylglycerol (DAG), and lysophosphatidic acid versus ceramide, ceramide-1-phosphate, sphingosine, and sphingosine-1-phosphate, respectively. In this review, we examine the mechanisms by which these lipid second messengers modulate aldosterone production at multiple levels. Aldosterone is a mineralocorticoid hormone responsible for maintaining fluid volume, electrolyte balance, and blood pressure homeostasis. Primary aldosteronism is a frequent endocrine cause of secondary hypertension. A thorough understanding of the signaling events regulating aldosterone biosynthesis may lead to the identification of novel therapeutic targets. The cumulative evidence in this literature emphasizes the critical roles of PA, DAG, and sphingolipid metabolites in aldosterone synthesis and secretion. However, it also highlights the gaps in our knowledge, such as the preference for phospholipase D-generated PA or DAG, as well as the need for further investigation to elucidate the precise mechanisms by which these lipid second messengers regulate optimal aldosterone production.     

The influence of second messengers and related substances on the sensitivity of early embryos to cytostatic neurochemicals

"Prenervous" neurotransmitters interact with second messengers in the regulation of early embryogenesis of sea urchins and the clawed frog Xenopus laevis. Propranolol and atropine inhibit cleavage divisions of X. laevis only after their intracellular administration. The level of the membrane potential (including a zero level) of the embryos studied is not essential for the cleavage divisions. Possible mechanisms of action of the "prenervous" neurotransmitters at the intracellular level and their coupling with second messengers are discussed.     

Production of inositol-phospholipid-derived second messengers in a rat natural killer cell line exposed to susceptible tumor targets

It is currently believed that natural killer (NK) cells kill bound target cells by exocytosis of cytotoxic granules via a calcium-dependent process. After confirming that NK-mediated killing was indeed dependent upon extracellular calcium, we investigated the production of inositol-phospholipid-derived second messengers in a rat NK cell line, RNK, upon exposure to susceptible target cells. These messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG), are associated with calcium-dependent secretory processes in a number of cell types. When RNK cells were exposed to susceptible YAC-1 tumor targets significant amounts of both IP3 and DAG were produced. The levels of the membrane phospholipid parent molecules of these second messengers declined in similarly stimulated RNK cells over a comparable time period. Using three different target cell lines, it was found that the levels of DAG that RNK produced in response to the different targets followed the same rank order as their susceptibility to RNK-mediated lysis. These data suggest that IP3 and DAG are produced in NK cells in response to tumor target cells, and these second messengers may have a functional role in NK-mediated killing.     

Lipid second messengers and cell signaling in vascular wall

Agonists of cellular receptors, such as receptor tyrosine kinases, G protein-coupled receptors, cytokine receptors, etc., activate phospholipases (C(gamma), C(beta), A(2), D), sphingomyelinase, and phosphatidylinositol-3-kinase. This produces active lipid metabolites, some of which are second messengers: inositol trisphosphate, diacylglycerides, ceramide, and phosphatidylinositol 3,4,5-trisphosphate. These universal mechanisms are involved in signal transduction to maintain blood vessel functions: regulation of vasodilation and vasoconstriction, mechanical stress resistance, and anticoagulant properties of the vessel lumen surface. Different signaling pathways realized through lipid second messengers interact to one another and modulate intracellular events. In early stages of atherogenesis, namely, accumulation of low density lipoproteins in the vascular wall, cascades of pro-atherogenic signal transduction are triggered through lipid second messengers. This leads to atherosclerosis, the general immuno-inflammatory disease of the vascular system.     

Cross-Talk between Signaling Pathways Can Generate Robust Oscillations in Calcium and cAMP

Background

To control and manipulate cellular signaling, we need to understand cellular strategies for information transfer, integration, and decision-making. A key feature of signal transduction is the generation of only a few intracellular messengers by many extracellular stimuli.

Methodology/Principal Findings

Here we model molecular cross-talk between two classic second messengers, cyclic AMP (cAMP) and calcium, and show that the dynamical complexity of the response of both messengers increases substantially through their interaction. In our model of a non-excitable cell, both cAMP and calcium concentrations can oscillate. If mutually inhibitory, cross-talk between the two second messengers can increase the range of agonist concentrations for which oscillations occur. If mutually activating, cross-talk decreases the oscillation range, but can generate ‘bursting’ oscillations of calcium and may enable better filtering of noise.

Conclusion

We postulate that this increased dynamical complexity allows the cell to encode more information, particularly if both second messengers encode signals. In their native environments, it is unlikely that cells are exposed to one stimulus at a time, and cross-talk may help generate sufficiently complex responses to allow the cell to discriminate between different combinations and concentrations of extracellular agonists.     

Role of phosphoinositide-derived second messengers in mediating anti-IgM-induced growth arrest of WEHI-231 B lymphoma cells

Anti-IgM irreversibly inhibits the growth of WEHI-231 B lymphoma cells and induces phosphoinositide hydrolysis--producing diacylglycerol, which activates protein kinase C, inositol 1,4,5-trisphosphate, which induces the release of calcium from intracellular storage sites into the cytoplasm, and other inositol polyphosphates. The roles of two of the possible second messengers, cytoplasmic free calcium and diacylglycerol, in mediating the action of anti-IgM on WEHI-231 cells were assessed by elevating [Ca2+]i with ionomycin and by activating protein kinase C with phorbol 12,13-dibutyrate (PdBu). The combination of 250 nM ionomycin and 4 to 7 nM PdBu was found to cause growth arrest and cell volume decrease responses in WEHI-231 cells which were similar to those caused by anti-IgM, although clearly slower. Both anti-IgM and the combination of mimicking reagents induced growth arrest of WEHI-231 cells in the G1 phase of the cell cycle. In both cases, this growth arrest was mitigated by addition of bacterial LPS. Moreover, 250 nM ionomycin plus 4 to 7 nM PdBu did not inhibit the growth of two other murine B lymphoma cell lines, each of which did exhibit increased phosphoinositide hydrolysis but not growth arrest in response to anti-Ig. Taken together, these results suggest that ionomycin and PdBu, at the concentrations used, did not inhibit WEHI-231 growth by general toxicity, but rather by mimicking the effects of the natural second messengers generated from Ag receptor cross-linking. Thus, the phosphoinositide-derived second messengers Ca2+i and diacylglycerol are capable of playing important roles in mediating the action of anti-IgM on WEHI-231 B lymphoma cells. However, the response of WEHI-231 cells to anti-IgM could not be fully reproduced with ionomycin and phorbol diester. These results suggest that another second messenger induced by anti-IgM may also play an important role in mediating the growth arrest of these cells.     

Transduction pathways mediated by second messengers including cAMP in the sugar receptor cell of the blow fly: study by the whole cell clamp method

The whole cell clamp method was directly applied to the sensory receptor neurons isolated from the adult labellar hair of the blow fly Phormia regina to locate the signal transduction pathways mediated by second messengers. First, the cAMP-mediated transduction pathway was examined to specify its location in the sugar receptor cell. Sugar receptor cell was identified by recording inward current flow under the voltage clamp applying sucrose solution to the surface of the taste neurons. When cyclic nucleotides, such as cGMP and cAMP, were introduced into the sugar receptor cell, inward current was observed (cGMP, 70pA; cAMP, 300pA at 350microM). Inhibitors and activators for the second messengers (GDPbetaS and forskolin) and non-cyclic nucleotides were also examined. Second, non-nucleotide second messengers (IP3 and Ca2+) were examined. The sugar receptor cell was activated when it was injected with IP3 or Ca2+. All the obtained results suggest that the cAMP-mediated signal transduction pathway plays a major role in the sugar receptor cell. The possibility of other transduction pathways mediated by IP3 or Ca2+ was not excluded.     

Effects of second messengers on serine/threonine protein phosphatases in insulin-secreting cells

Reversible protein phosphorylation is an important and versatile mechanism by which cells transduce external signals into biological responses. Cellular levels of protein phosphorylation are determined by the balanced actions of both protein kinases and protein phosphatases (PPases). Compared with protein kinases, however, serine/threonine PPases have received less attention. In the present study, the effects of certain insulin secretagogues and intracellular second messengers, known to stimulate or inhibit insulin secretion, on the activities of cation-independent serine/threonine PPases were investigated in insulin-secreting RINm5F insulinoma cells. Raising cellular cAMP through adenylyl cyclase activation and phosphodiesterase inhibition in intact cells, evoked inhibitory effects on PPase activities. The addition of a nitric oxide donor, cyclic nucleotides, or proinflammatory prostaglandins to RINm5F cell homogenates at widely different concentrations did not affect type-1 or -2A PPase activities. Phosphatidyl serine seemingly activated PPase-1, while inactivating PPase-2A. A protein kinase C-activating phorbol ester produced the opposite results when added to RINm5F cell homogenates. These studies suggest that several known intracellular second messengers are without effect on beta-cell PPase activities. However, phosphatidyl serine and protein kinase C activation, whose activity is transiently increased by glucose, may promote insulin release through PPase inactivation, likely contributing to the increase in phosphorylation state that occurs after stimulation of insulin release. Thus, inhibition of protein dephosphorylation may be a novel regulatory mechanism, assisting in activation of the stimulus-secretion coupling in insulin-producing cells.     

Lipids at membrane contact sites: cell signaling and ion transport

Communication between organelles is essential to coordinate cellular functions and the cell's response to physiological and pathological stimuli. Organellar communication occurs at membrane contact sites (MCSs), where the endoplasmic reticulum (ER) membrane is tethered to cellular organelle membranes by specific tether proteins and where lipid transfer proteins and cell signaling proteins are located. MCSs have many cellular functions and are the sites of lipid and ion transfer between organelles and generation of second messengers. This review discusses several aspects of MCSs in the context of lipid transfer, formation of lipid domains, generation of Ca²⁺ and cAMP second messengers, and regulation of ion transporters by lipids.     

Mercury-arc photolysis: a method for examining second messenger regulation of endothelial cell monolayer integrity.

Cell-cell apposition in bovine pulmonary endothelial cell monolayers was modulated by inducing transient increases in intracellular adenosine 3':5'-cyclic monophosphate (cAMP) and 1,4,5-inositol triphosphate (IP3). This was accomplished by mercury-arc flash photolysis of o-nitrobenzyl derivatives of the second messengers (caged compounds). Second messenger release by the mercury-arc lamp was determined by radioimmunoassay of cAMP to have a t1/2 of approximately 8 min. Each second messenger induced the phosphorylation of a distinct subset of cytoskeletal proteins; however, both IP3 and cAMP increased vimentin phosphorylation. Actin isoform patterns were not altered by the second messengers. Intracellular pulses of IP3 in pulmonary endothelial cells caused disruption of endothelial monolayer integrity as determined by phase-contrast microscopy and by visualization of actin stress fibers with rhodamine-phalloidin. Intracellular pulses of cAMP increased cell-cell contact, cell surface area, and apposition. IP3 appeared to have its greatest effect on the actin peripheral band. In silicone rubber contractility assays this agent caused contraction of pulmonary microvascular endothelial cells as visualized by an increase in wrinkles beneath the cells. On the other hand, cAMP appeared to effect both the peripheral band and centralized actin domains. Caged cAMP caused relaxation of endothelial cells as visualized by a disappearance of wrinkles beneath the cells.     

Regulation of gene expression by the intracellular second messengers IP3 and diacylglycerol

In Dictyostelium, extracellular cAMP interacts specifically with cell-surface receptors to promote the accumulation of a variety of intracellular second messengers, such as 3'-5' cyclic adenosine monophosphate (cAMP) and 1,4,5 inositol trisphosphate (IP3). We and others have shown that activation of the cell-surface cAMP receptor can also modulate the expression of the Dictyostelium genome during development. In at least one instance, synthesis of intracellular cAMP is required for appropriate gene regulation. However, the induction of most cAMP-dependent gene expression can occur in the absence of receptor-mediated activation of adenylate cyclase and a consequent accumulation of intracellular cAMP. These results suggest that other intracellular second messengers produced in response to receptor activation may potentially act as signal transducers to modulate gene expression during development. In vertebrate cells, IP3 and diacylglycerol (DAG) are intracellular activators of specific protein kinases; they are produced in equimolar amounts by cleavage of phosphoinositol bisphosphate after a receptor-mediated activation of a membrane-bound phosphodiesterase. IP3 and, thus, by inference, diacyl-glycerol are synthesized in Dictyostelium as a response to cAMP interacting with its cell-surface receptor. Using defined conditions to inhibit the accumulation of extracellular cAMP, we have examined the effects of these compounds on the expression of genes that require cAMP for their maximal expression. Our results suggest that intracellular IP3 and DAG may in part mediate the action of extracellular cAMP on the expression of the Dictyostelium genome.     

Autophagy paradox and ceramide

Sphingolipid molecules act as bioactive lipid messengers and exert their actions on the regulation of various cellular signaling pathways. Sphingolipids play essential roles in numerous cellular functions, including controlling cell inflammation, proliferation, death, migration, senescence, tumor metastasis and/or autophagy. Dysregulated sphingolipid metabolism has been also implicated in many human cancers. Macroautophagy (referred to here as autophagy) “self-eating” is characterized by nonselective sequestering of cytosolic materials by an isolation membrane, which can be either protective or lethal for cells. Ceramide (Cer), a central molecule of sphingolipid metabolism, has been extensively implicated in the control of autophagy. The increasing evidence suggests that Cer is highly involved in mediating two opposing autophagic pathways, which regulate either cell survival or death, which is referred here as autophagy paradox. However, the underlying mechanism that regulates the autophagy paradox remains unclear. Therefore, this review focuses on recent studies with regard to the regulation of autophagy by Cer and elucidates the roles and mechanisms of action of Cer in controlling autophagy paradox. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.     

Cell-specific manipulation of second messengers; a toolbox for integrative physiology in Drosophila

Every living cell must detect, and respond appropriately to, external signals. The functions of intracellular second messengers, such as guanosine 3'5'-cyclic monophosphate (cGMP), adenosine 3'5'-cyclic monophosphate (cAMP), and intracellular calcium, are thus intensively studied. However, artifact-free manipulation of these messengers is problematic, and simple pharmacology may not allow selective intervention in distinct cell types in a real, complex tissue. We have devised a method by which second messenger levels can be manipulated in cells of choice using the GAL4/UAS system. By placing different receptors (rat atrial natriuretic peptide [ANP] receptor and Drosophila serotonin receptors [5HT(Dro7) and 5HT(Dro1A)]) under UAS control, they can be targeted to arbitrary defined populations of cells in any tissue of the fly, and second messenger levels can be manipulated simply by adding the natural ligand. The potential of the system is illustrated in the Drosophila renal (Malpighian) tubule, where each receptor was shown to stimulate fluid secretion, to act through its cognate second messenger, and to be blocked by appropriate pharmacological antagonists. The results uncovered a new role for cGMP signaling in tubule and also demonstrate the utility of the tubule as a possible in vivo test bed for novel receptors, ligands, or agonists/antagonists.     

刺激剂(elicitor)在植物细胞培养次生代谢物生产中的应用

刺激剂（ｅｌｉｃｉｔｏｒ）在植物细胞培养中被用来作为提高次生代谢物产量的手段。文中概括介绍了微生物、寡聚糖、蛋白质、第二信使及其他物质作为刺激剂在植物细胞培养中的应用及其研究成果。