Insulin-induced Ca(2+) entry in hepatocytes is important for PI 3-kinase activation, but not for insulin receptor and IRS-1 tyrosine phosphorylation

Insulin Signaling in Bupivacaine-induced Cardiac Toxicity: Sensitization during Recovery and Potentiation by Lipid Emulsion

BACKGROUND

The impact of local anesthetics on the regulation of glucose homeostasis by protein kinase B (Akt) and 5'-adenosine monophosphate-activated protein kinase (AMPK) is unclear but important because of the implications for both local anesthetic toxicity and its reversal by IV lipid emulsion (ILE).

METHODS

Sprague-Dawley rats received 10 mg/kg bupivacaine over 20 s followed by nothing or 10 ml/kg ILE (or ILE without bupivacaine). At key time points, heart and kidney were excised. Glycogen content and phosphorylation levels of Akt, p70 s6 kinase, s6, insulin receptor substrate-1, glycogen synthase kinase-3β, AMPK, acetyl-CoA carboxylase, and tuberous sclerosis 2 were quantified. Three animals received Wortmannin to irreversibly inhibit phosphoinositide-3-kinase (Pi3k) signaling. Isolated heart studies were conducted with bupivacaine and LY294002-a reversible Pi3K inhibitor.

RESULTS

Bupivacaine cardiotoxicity rapidly dephosphorylated Akt at S473 to 63 ± 5% of baseline and phosphorylated AMPK to 151 ± 19%. AMPK activation inhibited targets downstream of mammalian target of rapamycin complex 1 via tuberous sclerosis 2. Feedback dephosphorylation of IRS1 to 31 ± 8% of baseline sensitized Akt signaling in hearts resulting in hyperphosphorylation of Akt at T308 and glycogen synthase kinase-3β to 390 ± 64% and 293 ± 50% of baseline, respectively. Glycogen accumulated to 142 ± 7% of baseline. Irreversible inhibition of Pi3k upstream of Akt exacerbated bupivacaine cardiotoxicity, whereas pretreating with a reversible inhibitor delayed the onset of toxicity. ILE rapidly phosphorylated Akt at S473 and T308 to 150 ± 23% and 167 ± 10% of baseline, respectively, but did not interfere with AMPK or targets of mammalian target of rapamycin complex 1.

CONCLUSION

Glucose handling by Akt and AMPK is integral to recovery from bupivacaine cardiotoxicity and modulation of these pathways by ILE contributes to lipid resuscitation.

Effects of tyroserleutide on gene expression of calmodulin and PI3K in hepatocellular carcinoma

Tyroserleutide (YSL) is a tripeptide compound that has exhibited inhibitory effects on hepatocellular carcinoma in our previous research. The mechanism of this antitumor activity involves the second messenger, Ca(2+). Ca(2+) influences cell function through the Ca(2+)/calmodulin (CaM) pathway, and abnormality of the Ca(2+)/CaM system correlates closely with the occurrence of tumors. In addition, CaM associates with phosphatidylinositol 3 kinase (PI3K), thereby enhancing the activity of PI3K, which promotes cell proliferation. In order to investigate its anti-tumor mechanism, we studied the effects of YSL on CaM protein expression and mRNA level, PI3K activity, PI3K regulatory subunit p85 protein expression and mRNA level, and the mRNA level of PI3K catalytic subunits p110alpha and p110gamma in human hepatocellular carcinoma BEL-7402 xenograft tumors in nude mice. Our results showed that YSL decreased the mRNA level and protein expression of CaM, inhibited the activity of PI3K, and reduced the mRNA level and protein expression of the PI3K regulatory subunit p85 and mRNA level of PI3K catalytic subunits p110alpha and p110gamma. Accordingly, it is suggestive that the anti-tumor effects of YSL may be mediated by down regulation of CaM and PI3K subunits p85 and p110, influencing the signal transduction pathway in the tumor cells and perhaps overcoming the dysfunctional PI3K activity in tumors.

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

Activation of Akt by mechanical stretching in human epidermal keratinocytes

Mechanical stretching represents an important part of the signaling in skin. We have previously demonstrated that mechanical stretching induced proliferative phenotypes in human keratinocytes, as shown in increased 5-bromo-2'-deoxyuridine (BrdU) incorporation, ERK1/2 activation, and keratin K6 induction. Here we have further investigated the antiapoptotic signals in human keratinocytes provoked by mechanical stretching in vitro. Keratinocytes were plated on flexible silicone supports to transmit mechanical stretching to keratinocytes, involving continuous stretching by +20%. Stretching of these cells for 15-30 min caused the phosphorylation and activation of Akt. Inhibition of mitogen and extracellular signal-regulated kinase (MEK1/2) with U0126 and phosphoinositide 3-OH kinase (PI 3-K) with Wortmannin attenuated Akt activation. The epidermal growth factor (EGF) receptor kinase inhibitor, AG1478, and calcium channel inhibitor, gadolinium (Gd3+), also inhibited Akt activation. In summary, our results demonstrate the activation of the Akt signaling pathway by mechanical stretching, generating not only proliferative but also antiapoptotic signals in human keratinocytes.

The petroleum ether extract of Nigella sativa exerts lipid-lowering and insulin-sensitizing actions in the rat

We studied the effect of a 4-week intragastric gavage with a petroleum ether extract of Nigella sativa seeds on blood glucose, insulin and lipids in the normal rat. Petroleum ether extract caused a 25% reduction in food intake that translated into a transient weight loss. No sign of toxicity of the plant could be seen in vivo or in vitro. Fasting plasma glucose remained stable throughout Nigella sativa treatment. At the end of the 4-week treatment, Nigella sativa-treated rats had lower fasting plasma levels of insulin and triglycerides, and higher HDL-cholesterol as compared to pair-fed controls. Response to insulin was evaluated in hepatocytes isolated from animals of all groups by Western blot analysis of phosphorylated MAPK p44/42erk and PKB. In vivo Nigella sativa treatment resulted in greater dose-dependent activation of MAPK and PKB in response to insulin. These results suggest that the petroleum ether extract of Nigella sativa has a slight anorexic effect, and that it contains the hypolipidemic activity previously obtained with the plant. More significantly, our data demonstrate that in vivo treatment with the petroleum ether extract exerts an insulin-sensitizing action by enhancing the activity of the two major intracellular signal transduction pathways of the hormone's receptor.

SAM68: a downstream target of angiotensin II signaling in vascular smooth muscle cells in genetic hypertension

We investigated whether phosphatidylinositol 3-kinase (PI3K) and 68-kDa Src associated during mitosis (SAM68) are involved in angiotensin II (ANG II) growth signaling in vascular smooth muscle cells (VSMCs) from spontaneously hypertensive rats (SHR). PI3K activity was assessed by measuring the phosphorylation of the regulatory subunit p85alpha and kinase activity of the catalytic 110-kDa subunit of PI3K. The PI3K-SAM68 interaction was assessed by coimmunoprecipitation, and SAM68 activity was evaluated by poly(U) binding. SAM68 expression was manipulated by SAM68 antisense oligonucleotide transfection. VSMC growth was evaluated by measuring [3H]leucine and [3H]thymidine incorporation as indexes of protein and DNA synthesis, respectively. ANG II increased the phosphorylation of p85alpha and kinase activity of the 110-kDa PI3K subunit in VSMCs from SHR and transiently increased p85alpha-SAM68 association. In Wistar-Kyoto (WKY) rat cells, ANG II increased SAM68 phosphorylation without influencing poly(U) binding. In SHR, ANG II did not influence SAM68 phosphorylation but increased SAM68 binding to poly(U). ANG II stimulated phosphoinositol phosphate synthesis by PI3K in SAM68 immunoprecipitates in both groups, with significantly enhanced effects in SHR. Inhibition of PI3K, using the selective inhibitor LY-294002, and downregulation of SAM68, by antisense oligonucleotides, significantly decreased ANG II-stimulated incorporation of [3H]leucine and [3H]thymidine in VSMCs, showing the functional significance of PI3K and SAM68. Our data demonstrate that PI3K and SAM68 are involved in ANG II signaling and that SAM68 is differentially regulated in VSMCs from SHR. These processes may contribute to the enhanced ANG II signaling and altered VSMC growth in SHR.

Synergistic activation of mitogen-activated protein kinase by insulin and adenosine triphosphate in liver cells: permissive role of Ca2+

We have previously demonstrated that insulin and G(q)-coupled receptor agonists individually activate mitogen-activated protein kinase (MAPK) in liver cells and both effects involve an influx of extracellular Ca(2+). Yet, these agonists have opposing physiological actions on hepatocyte glucose metabolism. We thus investigated the interaction between insulin and the P2Y(2) purinergic agonist adenosine triphosphate (ATP) on MAPK in HTC cells, a model hepatocyte cell line, and determined the involvement of cytosolic Ca(2+). Insulin and ATP each induced a dose-dependent phosphorylation of p44/42 MAPK that was partially inhibited by EGTA. However, pretreatment with insulin markedly increased the MAPK phosphorylation response to ATP. This potentiation was canceled by chelation of extracellular Ca(2+) with EGTA. We used patch clamp electrophysiology and fluorescence microscopy to understand the role of intracellular Ca(2+) in this effect. Insulin and ATP, respectively, induced monophasic and multiphasic changes in membrane potential and intracellular Ca(2+) as expected. Pretreatment with 10 nmol/L insulin significantly decreased the initial rapid depolarization (inward nonselective cation current [NSCC]), as well as the compounded Ca(2+) response induced by 100 micro mol/L ATP. However, in Ca(2+)-free conditions, insulin did not modify the Ca(2+) mobilized from internal pools after stimulation with ATP. Upon Ca(2+) readmission, internal store depletion by ATP or thapsigargin doubled the rate of capacitative Ca(2+) influx, whereas insulin increased this influx 1.32-fold. On the other hand, insulin pretreatment counteracted the increased rate of Ca(2+) influx induced by ATP but not by thapsigargin. In summary, insulin counteracts the membrane potential and Ca(2+) responses to ATP in HTC cells. However, insulin and ATP effects on MAPK activation are synergistic and Ca(2+) influx plays a permissive role. Therefore, the opposing metabolic actions of insulin and G(q)-coupled receptor agonists involve an interaction in signaling pathways that resides downstream of Ca(2+) influx.

Role of calcium in E-selectin induced phenotype of T84 colon carcinoma cells

The adhesion of cancer cells to the endothelium during the metastatic process involves the interaction of specific cell-cell adhesion receptors on the cell surface. E-selectin on endothelial cells and sialyl Lewis X carbohydrate component on tumor cells are mainly implicated in the adhesion of colon carcinoma cells to the endothelium of target organ. In this paper we show that binding of E-selectin to T84 colon tumor cells causes approximately a twofold increase in intracellular calcium concentration. In particular, using two inhibitors of receptor operated calcium channels, CAI and SK&F 96365, we present evidences that the augmentation in cytoplasmic calcium originates from ionic influx from extracellular sources. Furthermore, we demonstrated that modulation of [Ca2+]i by engagement of E-selectin receptor starts signal transduction pathways that affect cell spreading, tyrosine phosphorylation signaling, and cancer cell motility.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Ethanol-induced apoptosis in hepatoma cells proceeds via intracellular Ca(2+) elevation, activation of TLCK-sensitive proteases, and cytochrome c release

Ethanol is known to induce apoptosis in hepatocytes. However, intracellular signaling events of ethanol-induced death are still only partially understood. We studied such processes in ethanol-induced apoptosis in HepG2 cells as a model system for human liver cells. We determined the incidence of apoptosis by DNA fragmentation and tested the effects of various known inhibitors. Ethanol induces apoptosis in HepG2 cells in a dose- and time-dependent manner as well as in rat primary hepatocytes. This effect was not mediated through the death receptor CD95 and the tumor necrosis factor receptors. It was efficiently inhibited by the caspase inhibitor N-benzyloxycarbonyl-Asp-Glu-Val-Asp-fluoromethylketone (zVAD-fmk), the Ca(2+) chelator EGTA, and the serine protease inhibitor N-p-tosyl-l-lysine chloromethyl ketone (TLCK). Upon ethanol treatment, the intracellular calcium ion concentration was increased and cytochrome c was released from the mitochondria, and caspases were activated. EGTA and TLCK could inhibit cytochrome c release from the mitochondria. Furthermore, overexpression of Bcl-x(L) saved cells from ethanol-induced apoptosis. These data suggest that ethanol-induced apoptosis in liver cells is initiated by the intracellular Ca(2+) elevation in the cytoplasm and activation of TLCK-sensitive serine proteases. Our data provide new insight into ethanol-induced apoptosis in liver cells and may lead to therapeutic strategies to prevent liver damage.

Adrenomedullin induces endothelium-dependent vasorelaxation via the phosphatidylinositol 3-kinase/Akt-dependent pathway in rat aorta

To study the mechanisms by which adrenomedullin (AM) induces endothelium-dependent vasorelaxation, we examined whether AM-induced endothelium-dependent vasodilation was mediated by the phosphatidylinositol 3-kinase (PI3K)/Akt-dependent pathway in rat aorta, because it was recently reported that PI3K/Akt was implicated in the activation of endothelial NO synthase. AM-induced vasorelaxation in thoracic aorta with intact endothelium was inhibited by pretreatment with PI3K inhibitors to the same level as that in endothelium-denuded aorta. AM elicited Akt phosphorylation in a time- and dose-dependent manner. AM-induced Akt phosphorylation was inhibited by pretreatment with a calmodulin-dependent protein kinase inhibitor as well as with PI3K inhibitors. When an adenovirus construct expressing a dominant-negative Akt mutant (Ad/dnAkt) was injected into abdominal aortas so that the mutant was expressed predominantly in the endothelium layer, AM-induced vasodilation was diminished to the same level as that in endothelium-denuded aortas. Finally, AM-induced cGMP production, which was used as an indicator for NO production, was suppressed by PI3K inhibition or by Ad/dnAkt infection into the endothelium. These results suggested that AM induced Akt activation in the endothelium via the Ca(2+)/calmodulin-dependent pathway and that this was implicated in the production of NO, which in turn induced endothelium-dependent vasodilation in rat aorta.

The Role of Ca2+ in Insulin-stimulated Glucose Transport in 3T3-L1 Cells

We have examined the requirement for Ca2+ in the signaling and trafficking pathways involved in insulin-stimulated glucose uptake in 3T3-L1 adipocytes. Chelation of intracellular Ca2+, using 1,2-bis (o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetra (acetoxy- methyl) ester (BAPTA-AM), resulted in >95% inhibition of insulin-stimulated glucose uptake. The calmodulin antagonist, W13, inhibited insulin-stimulated glucose uptake by 60%. Both BAPTA-AM and W13 inhibited Akt phosphorylation by 70-75%. However, analysis of insulin-dose response curves indicated that this inhibition was not sufficient to explain the effects of BAPTA-AM and W13 on glucose uptake. BAPTA-AM inhibited insulin-stimulated translocation of GLUT4 by 50%, as determined by plasma membrane lawn assay and subcellular fractionation. In contrast, the insulin-stimulated appearance of HA-tagged GLUT4 at the cell surface, as measured by surface binding, was blocked by BAPTA-AM. While the ionophores or ionomycin prevented the inhibition of Akt phosphorylation and GLUT4 translocation by BAPTA-AM, they did not overcome the inhibition of glucose transport. Moreover, glucose uptake of cells pretreated with insulin followed by rapid cooling to 4 degrees C, to promote cell surface expression of GLUT4 and prevent subsequent endocytosis, was inhibited specifically by BAPTA-AM. This indicates that inhibition of glucose uptake by BAPTA-AM is independent of both trafficking and signal transduction. These data indicate that Ca2+ is involved in at least two different steps of the insulin-dependent recruitment of GLUT4 to the plasma membrane. One involves the translocation step. The second involves the fusion of GLUT4 vesicles with the plasma membrane. These data are consistent with the hypothesis that Ca2+/calmodulin plays a fundamental role in eukaryotic vesicle docking and fusion. Finally, BAPTA-AM may inhibit the activity of the facilitative transporters by binding directly to the transporter itself.

Insulin stimulates membrane conductance in a liver cell line: evidence for insertion of ion channels through a phosphoinositide 3-kinase-dependent mechanism

Activation of insulin receptors stimulates a rapid increase in the ion permeability of liver cells. To evaluate whether this response involves insertion of ion channels, plasma membrane turnover was measured in a model liver cell line using the fluorescent membrane marker FM1-43. Under basal conditions, the rate of constitutive membrane turnover was approximately 2%min(-1), and balanced exocytosis and endocytosis maintained the total cell membrane area constant. Exposure to insulin stimulated a transient increase in membrane turnover of up to 10-fold above constitutive rates. The response was concentration-dependent (0.001-10 microm). Insulin also caused a parallel increase in membrane conductance as measured by whole-cell patch clamp recording due to opening of Cl(-)- and K(+)-selective ion channels. The insulin-stimulated membrane turnover did not appear to involve the constitutive recycling compartments, suggesting that a distinct pool of vesicles may be involved. The effects of insulin on membrane turnover and membrane conductance were inhibited by blockers of phosphoinositide 3-kinase LY294002 and wortmannin or by disrupting microtubule assembly with nocodazole. Taken together, these findings indicate that insulin stimulates recruitment of new membranes through phosphoinositide 3-kinase-dependent mechanisms. Thus, regulated insertion of a separate population of ion channel-containing vesicles may represent one mechanism for mediating the changes in membrane conductance that are essential for the cellular response to insulin.

Insulin increased cAMP phosphodiesterase activity antagonizing metabolic actions of glucagon in rat hepatocytes cultured with herbimycin A

The baseline activity of cyclic nucleotide phosphodiesterase 4 was markedly lowered by primary culture of rat hepatocytes with herbimycin A for 4 h [Eur. J. Biochem. 260 (1999) 398-408.]. We now report that insulin added to this preparation of hepatocytes, which had been completely freed of herbimycin, increased the thus lowered phosphodiesterase activity, consequently antagonizing glucagon-induced production of cAMP and activation of glycogen phosphorylase. The insulin receptor beta-subunits and alpha-tubulin were tyrosine-phosphorylated upon the addition of insulin. The phosphorylation of alpha-tubulin afforded conditions unfavorable for microtubule assembly that is responsible for phosphodiesterase inhibition. These effects of insulin observed in herbimycin-pretreated hepatocytes were not inhibited by wortmannin that actually abolished insulin-induced activation of phosphatidylinositol 3-kinase (PtdIns 3-kinase) under the same conditions. The physiological significance of the insulin action not mediated by PtdIns 3-kinase in herbimycin-pretreated hepatocytes is discussed.