Ethanol potentiates the mitogenic effects of sphingosine 1-phosphate by a zinc- and calcium-dependent mechanism in fibroblasts

Modulation of oxidative stress and apoptosis by alteration of bioactive lipids in the pancreas, and effect of zinc chelation in a rat model of Alzheimer's disease

INTRODUCTION

Increasing epidemiological evidence highlights the association between systemic insulin resistance and Alzheimer's disease (AD). It is known that peripheral insulin resistance in the early stages of AD precedes and is a precursor to amyloid-β (Aβ) deposition. Although it is known that improving the CNS insulin sensitivity of AD patients is an important therapeutic goal and that the majority of insulin in the brain comes from the periphery, there has been little attention to the changes that occur in the pancreatic tissue of AD patients. Therefore, it is crucial to elucidate the mechanisms affecting insulin resistance in pancreatic tissue in AD. It is known that zinc (Zn2+) chelation is effective in reducing peripheral insulin resistance, cell apoptosis, cell death, and oxidative stress.

OBJECTIVE

It was aimed to determine the changes in bioactive lipids, amylin (AIPP), oxidative stress and apoptosis in pancreatic cells in the early stages of Alzheimer's disease. The main aim is to reveal the therapeutic effect of the Cyclo-Z agent on these changes seen in the pancreas due to AD disease.

METHODS

AD and ADC rats were intracerebroventricular (i.c.v.) Aβ1-42 oligomers. Cyclo-Z gavage was applied to ADC and SHC rats for 21 days. First of all, the effects of AIPP, bioactive ceramides, apoptosis and oxidative stress on the pancreatic tissue of AD group rats were evaluated. Then, the effect of Cyclo-Z treatment on these was examined. ELISA kit was used in biochemical analyses.

RESULTS

AIPP and ceramide (CER) levels and CER/ sphingosine-1 phosphate (S1P) ratio were increased in the pancreatic tissue of AD rats. It also increased the level of CER kinase (CERK), which is known to increase the concentration of CER 1-phosphate (C1P), which is known to be toxic to cells in the presence of excessive CER concentration. Due to the increase in CER level, it was observed that apoptosis and oxidative stress increased in the pancreatic cells of AD group rats.

CONCLUSION

Cyclo-Z, which has Zn2+ chelating properties, reduced AD model rats' AIPP level and oxidative stress and could prevent pancreatic apoptosis. Similar therapeutic effects were not observed in the pancreatic tissue of Cyclo-Z administered to the SH group. For this reason, it is thought that Cyclo-Z agent may have a therapeutic effect on the peripheral hyperinsulinemia observed in the early stages of AD disease and the resulting low amount of insulin transported to the brain, by protecting pancreatic cells from apoptosis and oxidative stress by regulating their bioactive metabolites.

Fingolimod suppresses neuronal autophagy through the mTOR/p70S6K pathway and alleviates ischemic brain damage in mice

The bioactive, signaling lipid, sphingosine-1-phosphate (S1P), and its analog, fingolimod (FTY720), have previously shown neuroprotective effects against ischemic brain injury. However, the underlying mechanisms have not yet been fully clarified. The roles of autophagy in ischemic stroke are being increasingly recognized. In the present study, we sought to determine whether the S1P pathway is involved in neuronal autophagy and investigate its possible mechanisms following stroke. Interestingly, we found that FTY720 significantly attenuates infarct volumes and reduces neuronal apoptosis on days 1 and 3 post stroke, accompanied by amelioration of functional deficits. Additionally, FTY720 was found to decrease the induction of autophagosome proteins, microtubule-associated protein 1 light chain 3(LC3-II) and Beclin1, following ischemic stroke in a dose-dependent manner. Meanwhile, protein levels of the mammalian target of rapamycin (mTOR) and the 70-kDa ribosomal protein, S6 kinase1 (p70S6K), were also up-regulated in FTY720-treated animals, and the nonspecific SphK inhibitor, N,N-dimethylsphingosine (DMS), was found to cause a reverse effect. Our results indicate that modulation of the S1P signaling pathway by FTY720 could effectively decrease neuronal autophagy through the mTOR/p70S6K pathway and attenuate ischemic brain injury in mice.

Involvement of sphingolipids in ethanol neurotoxicity in the developing brain

Ethanol-induced neuronal death during a sensitive period of brain development is considered one of the significant causes of fetal alcohol spectrum disorders (FASD). In rodent models, ethanol triggers robust apoptotic neurodegeneration during a period of active synaptogenesis that occurs around the first two postnatal weeks, equivalent to the third trimester in human fetuses. The ethanol-induced apoptosis is mitochondria-dependent, involving Bax and caspase-3 activation. Such apoptotic pathways are often mediated by sphingolipids, a class of bioactive lipids ubiquitously present in eukaryotic cellular membranes. While the central role of lipids in ethanol liver toxicity is well recognized, the involvement of sphingolipids in ethanol neurotoxicity is less explored despite mounting evidence of their importance in neuronal apoptosis. Nevertheless, recent studies indicate that ethanol-induced neuronal apoptosis in animal models of FASD is mediated or regulated by cellular sphingolipids, including via the pro-apoptotic action of ceramide and through the neuroprotective action of GM1 ganglioside. Such sphingolipid involvement in ethanol neurotoxicity in the developing brain may provide unique targets for therapeutic applications against FASD. Here we summarize findings describing the involvement of sphingolipids in ethanol-induced apoptosis and discuss the possibility that the combined action of various sphingolipids in mitochondria may control neuronal cell fate.

Sphingosine-1-phosphate induced mTOR-activation is mediated by the E3-ubiquitin ligase PAM

The signaling pathways that are regulated by sphingosine-1-phosphate (S1P) and mammalian target of rapamycin (mTOR) modulate cell growth, mitogenesis and apoptosis in various cell types and are of major interest for the development of new cancer therapeutics. Previous reports show that S1P can cross-activate the mTOR pathway although the mechanisms that connect both pathways are still unknown. We found that S1P-treatment activates mTOR in several cancer cell lines and primary cells. The activation was independent of ERK, Akt and PI3-kinase, but instead was mediated by the E3 ubiquitin ligase Protein Associated with Myc (PAM). Increased intracellular PAM concentrations facilitated S1P- and insulin-induced mTOR activation as well as p70S6K and 4EBP1 phosphorylation while genetic deletion of PAM decreased S1P- and insulin-induced mTOR activation. PAM activated by facilitating the GDP/GTP-exchange of Rheb which is an activator of mTOR. In conclusion we show that PAM is a novel regulator of the mTOR pathway and that PAM may directly activate Rheb as a guanosine exchange factor (GEF).

Involvement of S6 kinase and p38 mitogen activated protein kinase pathways in strain-induced alignment and proliferation of bovine aortic smooth muscle cells

Bovine aortic smooth muscle cell (SMC) phenotype can be altered by physical forces. This has been demonstrated by cyclic strain-induced changes in proliferation and alignment. However, the intracellular coupling pathways remain ill defined. In the present study, we examined whether the p38 and S6 kinase pathway were involved in the mitogenic and morphological changes seen in SMCs exposed to cyclic strain. We seeded bovine aortic SMCs on silastic membranes that were deformed with 150-mmHg vacuum. Cyclic strain induced both alignment and proliferation of SMCs. SB202190, a specific inhibitor of p38, hindered SMC alignment, but not proliferation. Rapamycin, a specific inhibitor of the mTOR-S6 kinase pathway, attenuated strain-induced proliferation, but not alignment. Peak activation of p38 and S6 kinase was 351 +/- 76.9% at 5 min and 363 +/- 56.2% at 60 min compared with static control, respectively (P < 0.05). The results suggest that strain-induced SMC alignment is dependent on activation of p38, but not S6 kinase. Strain induced SMC proliferation is S6 kinase, but not p38 activation, dependent.

Stabilization of tumor necrosis factor alpha mRNA by chronic ethanol: role of A + U-rich elements and p38 mitogen-activated protein kinase signaling pathway

Increased expression of tumor necrosis factor alpha (TNFalpha) in response to chronic ethanol has been implicated in the pathogenesis of alcoholic liver disease. However, the molecular mechanisms by which ethanol increases the levels of TNFalpha are not well characterized. Utilizing Kupffer cells isolated from rats fed an ethanol containing diet and a murine macrophage cell line, RAW264.7, exposed to ethanol in culture, we have demonstrated that exposure to chronic ethanol results in an enhanced expression of lipopolysaccharide (LPS)-induced TNFalpha. While chronic ethanol had no effect on the rate of LPS-induced TNFalpha transcription as measured by nuclear run-on experiments, TNFalpha mRNA half-life was increased by chronic ethanol. Chronic ethanol also potentiated the activation of LPS-induced p38 mitogen-activated protein (MAP) kinase in Kupffer cells, as well as in RAW264.7 cells. Specific inhibition of p38 MAP kinase activation by SB203580 in Kupffer cells or by overexpression of dominant negative p38 MAP kinase in RAW264.7 cells blocked ethanol-mediated TNFalpha mRNA stabilization. Furthermore, using chimeric reporter constructs, we have shown that A + U-rich elements in the 3'-untranslated region of TNFalpha mRNA are not sufficient to impart ethanol-mediated stabilization on TNFalpha mRNA.

Activation of mitogen-activated protein kinase by muscarinic receptors in astroglial cells: role in DNA synthesis and effect of ethanol

Mitogen-activated protein kinase (MAPK) can be phosphorylated by mitogens binding to G-protein-coupled receptors and is considered a major pathway involved in cell proliferation. In this study, we report on the activation of MAPK by muscarinic acetylcholine receptors in astroglial cells, namely the 1321N1 human astrocytoma cell line, primary rat cortical astrocytes, and fetal human astrocytes. Carbachol caused a rapid and transient phorphorylation of MAPK (ERK1/2) in all cell types, with an increase in MAPK activity, without changing the levels of MAPK proteins. Human astrocytoma cells were used to characterize the effect of carbachol on MAPK. Experiments with M2- and M3-receptor subtype-selective antagonists, and with pertussis toxin, indicated that the M3 subtype is responsible for activating MAPK in glial cells. Pretreatment of cells with the protein kinase C (PKC) inhibitor bisindolylmaleimide I, or downregulation of PKC by 24-h treatment with the phorbol ester TPA inhibited carbachol-induced MAPK activation. Additional experiments with PKC alpha- or PKC epsilon-specific compounds indicated that the epsilon isozyme of PKC is primarily involved in MAPK activation by carbachol. Chelation of calcium also inhibited MAPK activation by carbachol. Two MEK (MAPK kinase) inhibitors inhibited carbachol-induced DNA synthesis but only at concentrations that exceeded those sufficient to block carbachol-induced MAPK activation. Ethanol (< or =200 mM) had no effect on MAPK when present alone and did not affect carbachol-induced MAPK activation under various experimental conditions, although it inhibits carbachol-induced DNA synthesis at low concentrations (10-100 mM). These results suggest that activation of MAPK by carbachol may be necessary but not sufficient for its mitogenic effect in astroglial cells, and that does not represent a target for ethanol-induced inhibition of DNA synthesis elicited by muscarinic receptors.

Promitogenic effects of ethanol, methanol, and ethanolamine in insulin-treated fibroblasts

The zinc-dependent potentiating effect of ethanol (EtOH) on insulin-stimulated DNA synthesis was studied with a focus on the possible site of EtOH action and the ability of other alcohols to elicit similar promitogenic effects. In serum-starved (27 hr) NIH 3T3 fibroblasts, 200-300 mM methanol (MeOH) and 0.1-1.5 mM ethanolamine (Etn), but not 3- to 9-carbon normal alcohols, enhanced the effect of insulin on DNA synthesis to varying extents. The promitogenic effects of EtOH and MeOH, but not that of Etn, required the presence of 15-25 microM zinc. The potentiating effects of Etn were enhanced by 5 mM choline (Cho) and inhibited by 1-3 mM hemicholinium-3 (HC-3), an inhibitor of Cho transporter and Cho kinase. In the presence of 15 microM zinc, 40 mM EtOH, which had no effect on its own, inhibited the potentiating effects of Cho and enhanced the inhibitory effects of HC-3 on synergistic stimulation of DNA synthesis by Etn and insulin. On the other hand, both Cho and HC-3 partially inhibited the promitogenic effect of 80 mM EtOH in the presence of 25 microM zinc. After a 10-min incubation, EtOH decreased the amount of cell-associated [(14)C]Cho in the absence but not in the presence of HC-3. After a 40-min incubation, Cho (5 mM) partially inhibited the cellular uptake as well as the metabolism of [(14)C]Etn. Whereas after the 40-min incubation 80 mM EtOH had no effects on Etn metabolism, in the absence of Cho it decreased the amount of cell-associated [(14)C]Etn. However, EtOH had no detectable effects on cell association of [(14)C]Etn after the 10-min incubation. The results suggest that in NIH 3T3 fibroblasts EtOH is a remarkably specific promitogen, and that it may act via a cell membrane site(s), also regulated by Cho (agonist) and HC-3 (antagonist), which can influence membrane binding and the promitogenic activity of Etn.

Ethanol enhances the stimulatory effects of lysophosphatidic acid on DNA synthesis but not cell proliferation in human and mouse fibroblasts

Lysophosphatidic acid (LPA), a constituent of serum, is a positive regulator of cell growth, while ethanol (EtOH) has been shown to exert both inhibitory and stimulatory effects on mitogenesis. In this work, we examined possible interactions between the effects of EtOH and LPA on DNA synthesis, cell proliferation, activating phosphorylation of p44/p42 mitogen-activated protein kinases (MAPK), and p70 S6 kinase (p70 S6K) activity. In fibroblasts derived from human or mouse embryo or the skin of healthy human subjects, LPA (1-20 microM) and EtOH (40-80 mM) synergistically stimulated DNA synthesis in a zinc-dependent manner. Nevertheless, EtOH did not modify the stimulatory effect of LPA on the proliferation of human embryonal fibroblasts. In the presence of zinc, EtOH did not affect LPA-induced activating phosphorylation of p42/p44 MAPKs, although an inhibitor of MAPK kinase inhibited the combined effects of LPA and EtOH on DNA synthesis. In contrast, in the presence of zinc, EtOH enhanced the stimulatory effect of LPA on p70 S6K activity. The results indicate that in human fibroblasts, in the presence of zinc, EtOH enhances the stimulatory effects of LPA on DNA synthesis, but not on cell proliferation, by a mechanism probably involving activation of p70 S6K.

Ethanol has multiple effects on DNA synthesis in fibroblasts depending on the presence of secreted growth regulators and zinc as well as the level of protein kinase C activation

Earlier we showed that in serum-starved (27 h), washed mouse fibroblasts and other cell lines 40-80 mM concentrations of ethanol (EtOH) potentiate, in a zinc (Zn2+)-dependent manner, the combined stimulatory effects of calcium (Ca2+) and insulin (Ins) on DNA synthesis. We now report that the promitogenic EtOH effects require removal of the used medium at least 6 h prior to treatments with EtOH, Zn2+, and Ins. If serum-starved (27 h) cells were continuously incubated for another 18-h period without replacing the medium, a secreted cellular factor moderately enhanced the mitogenic effect of Ins and simultaneously blocked the potentiating effect of EtOH on DNA synthesis measured during the last hour of treatments. However, the presence of Ca2+ (2.8 mM) plus Zn2+ (25 microM) or 25-300 nM phorbol 12-myristate 13-acetate (PMA) during the serum starvation period partially restored the promitogenic effect of EtOH. The PMA effect was blocked by the protein kinase C (PKC) inhibitor GF 109203X added for the second (18 h) period. Even at 300 nM, PMA failed to fully downregulate PKC-alpha, the major PKC isoform, over a 28-h period, suggesting that an activated PKC enzyme was involved in the restoration of EtOH effect. When EtOH (40-80 mM) was added for the entire serum starvation period and the incubations were continued for 18 h without removing the medium, EtOH inhibited both the combined actions of Ins and cellular factor as well as the promoting effect of newly added EtOH on Ins-dependent DNA synthesis. Coaddition of Zn2+ and PMA with EtOH prevented these inhibitory effects of EtOH. The results indicate that in mouse fibroblasts EtOH can both enhance and inhibit Ins-dependent DNA synthesis depending on the timing of EtOH treatment as well as the presence of Zn2+, cellular factors, and activators of the PKC system.