Negative regulation of phosphatidylinositol 3-kinase and Akt signalling pathway by PKC

NADPH Oxidase 1 Mediates Acute Blood Pressure Response to Angiotensin II by Contributing to Calcium Influx in Vascular Smooth Muscle Cells

BACKGROUND

Reactive oxygen species (ROS) and calcium ions (Ca²⁺) are among the major effectors of Ang II (angiotensin II) in vascular smooth muscle cells. ROS are related to Ca²⁺ signaling or contraction induced by Ang II, but little is known about their detailed functions. Here, NOX (NADPH oxidase), a major ROS source responsive to Ang II, was investigated regarding its contribution to Ca²⁺ signaling.

METHODS

Vascular smooth muscle cells were primary cultured from rat aorta. Ca²⁺ and ROS were monitored mainly using fura-2 and HyPer family probes' respectively. Signals activating NOX were examined with relevant pharmacological inhibitors and genetic manipulation techniques.

RESULTS

Ang II-induced ROS generation was found to be biphasic: the first phase of ROS production, which was mainly mediated by NOX1, was small and transient, preceding a rise in Ca²⁺, and the second phase of ROS generation, mediated by NOX1 and NOX4, was slow but sizeable, continuing over tens of minutes. NOX1-derived superoxide in the first phase is required for Ca²⁺ influx through nonselective cation channels. AT1R (Ang II type 1 receptor)-G_βγ-PI3K_γ (phosphoinositide 3-kinase γ) signaling pathway was responsible for the rapid activation of NOX1 in the first phase, while in the second phase, NOX1 was further activated by a separate AT1R-Gα_q/11-PLC (phospholipase C)-PKC_β (protein kinase C β) signaling axis. Consistent with these observations, aortas from NOX1-knockout mice exhibited reduced contractility in response to Ang II, and thus the acute pressor response to Ang II was also attenuated in NOX1-knockout mice.

CONCLUSIONS

NOX1 mediates Ca²⁺ signal generation and thereby contributes to vascular contraction and blood pressure elevation by Ang II.

Insulin and Exendin-4 Reduced Mutated Huntingtin Accumulation in Neuronal Cells

Patients with diabetes mellitus (DM) are more prone to develop cognitive decline and neurodegenerative diseases. A pathological association between an autosomal dominant neurological disorder caused by brain accumulation in mutated huntingtin (mHTT), known as Huntington disease (HD), and DM, has been reported. By using a diabetic mouse model, we previously suggested a central role of the metabolic pathways of HTT, further suggesting the relevance of this protein in the pathology of DM. Furthermore, it has also been reported that intranasal insulin (Ins) administration improved cognitive function in patients with neurodegenerative disorders such as Alzheimer disease, and that exendin-4 (Ex-4) enhanced lifespan and ameliorated glucose homeostasis in a mouse model of HD. Although antioxidant properties have been proposed, the underlying molecular mechanisms are still missing. Therefore, the aim of the present study was to investigate the intracellular pathways leading to neuroprotective effect of Ins and Ex-4 hypoglycemic drugs by using an in vitro model of HD, developed by differentiated dopaminergic neurons treated with the pro-oxidant neurotoxic compound 6-hydroxydopamine (6-ohda). Our results showed that 6-ohda increased mHTT expression and reduced HTT phosphorylation at Ser421, a post-translational modification, which protects against mHTT accumulation. Pre-treatment with Ins or Ex-4 reverted the harmful effect induced by 6-ohda by activating AKT1 and SGK1 kinases, and by reducing the phosphatase PP2B. AKT1 and SGK1 are crucial nodes on the Ins activation pathway and powerful antioxidants, while PP2B dephosphorylates HTT contributing to mHTT neurotoxic effect. In conclusion, present results highlight that Ins and Ex-4 may counteract the neurotoxic effect induced by mHTT, opening novel pharmacological therapeutic strategies against neurodegenerative disorders, with the main focus on HD, still considered an orphan illness.

TNF-α stimulates endothelial palmitic acid transcytosis and promotes insulin resistance

Persistent elevation of plasma TNF-α is a marker of low grade systemic inflammation. Palmitic acid (PA) is the most abundant type of saturated fatty acid in human body. PA is bound with albumin in plasma and could not pass through endothelial barrier freely. Albumin-bound PA has to be transported across monolayer endothelial cells through intracellular transcytosis, but not intercellular diffusion. In the present study, we discovered that TNF-α might stimulate PA transcytosis across cardiac microvascular endothelial cells, which further impaired the insulin-stimulated glucose uptake by cardiomyocytes and promoted insulin resistance. In this process, TNF-α-stimulated endothelial autophagy and NF-κB signaling crosstalk with each other and orchestrate the whole event, ultimately result in increased expression of fatty acid transporter protein 4 (FATP4) in endothelial cells and mediate the increased PA transcytosis across microvascular endothelial cells. Hopefully the present study discovered a novel missing link between low grade systemic inflammation and insulin resistance.

Increased PKC activity and altered GSK3β/NMDAR function drive behavior cycling in HINT1-deficient mice: bipolarity or opposing forces

Mice with histidine triad nucleotide-binding protein 1 (HINT1) deletion exhibit manic-like symptoms that evolve into depressive-like behavior in response to stressful paradigms. Molecular and electrophysiological studies have indicated that HINT1^−/− mice exhibit increased PKC, PKA, and GSK3β activities, as well as glutamate N-methyl-D-aspartate receptor (NMDAR)/α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic receptor (AMPAR) and NR2B/NR2A subunit ratios. Pharmacological interventions stabilized their behavior but through different mechanisms. GSK3β inhibitors and valproate directly attenuated the expression of the manic-like symptoms, whereas PKC inhibition, lamotrigine, or risperidone promoted NMDAR-mediated depressive-like behaviors that counterbalanced the preexisting manic-like symptoms. Naïve HINT1^−/− mice exposed to stressful paradigms rapidly manifested depressive-like behaviors in subsequent stressful situations, a capacity that persisted for a couple of weeks thereafter. During the depressive-like phase, citalopram, amitriptyline and MK801 precipitated manic-like behaviors in stressed HINT1^−/− mice. Notably, the antagonism of NMDARs prevented HINT1^−/− mice from alternating behaviors in response to stress. A comparison with “manic” Black Swiss mice indicated that in HINT1^−/− mice, PKC supports manic-like symptoms and reduces the expression of depressive-like behaviors via activation of GSK3β and regulation of NR2B-enriched NMDARs. HINT1^−/− mice represent a suitable model for studying human BPD and may facilitate the identification of novel targets and drugs to treat this mental disorder.

Targeting protein kinase C in mantle cell lymphoma

Although targeting the Bruton tyrosine kinase (BTK) with ibrutinib has changed lymphoma treatment, patients with mantle cell lymphoma (MCL) remain incurable. In this study, we characterized a broad range of MCL cell lines and primary MCL cells with respect to the response to the BTK inhibitor, ibrutinib, and compared it with the response to the protein kinase C (PKC) inhibitor, sotrastaurin. At clinically relevant concentrations, each drug induced potent cell death only in the REC-1 cell line, which was accompanied by robust inhibition of AKT and ERK1/ERK2 (ERK1/2, also termed MAPK3/MAPK1) phosphorylation. In sensitive REC-1 cells, the drug-mediated impaired phosphorylation was obvious on the levels of B-cell receptor-induced and basal phosphorylation. Similar results were obtained in primary MCL cells with ibrutinib and in a subset with sotrastaurin. The various drug-resistant MCL cell lines showed very distinct responses in terms of basal AKT and ERK1/2 phosphorylation. Interestingly, targeting PKC and BTK at the same time led to ibrutinib-mediated rescue of a weak sotrastaurin-induced apoptosis in MINO cells. Additional targeting of AKT sensitized MINO cells to inhibitor-mediated cytotoxicity. In summary, MCL cells are heterogeneous in their response to BTK or PKC inhibition, indicating the need for even more individualized targeted treatment approaches in subsets of MCL patients.

Distinct and complementary roles for α and β isoenzymes of PKC in mediating vasoconstrictor responses to acutely elevated glucose

BACKGROUND AND PURPOSE

We investigated the hypothesis that elevated glucose increases contractile responses in vascular smooth muscle and that this enhanced constriction occurs due to the glucose-induced PKC-dependent inhibition of voltage-gated potassium channels.

EXPERIMENTAL APPROACH

Patch-clamp electrophysiology in rat isolated mesenteric arterial myocytes was performed to investigate the glucose-induced inhibition of voltage-gated potassium (Kv ) current. To determine the effects of glucose in whole vessel, wire myography was performed in rat mesenteric, porcine coronary and human internal mammary arteries.

KEY RESULTS

Glucose-induced inhibition of Kv was PKC-dependent and could be pharmacologically dissected using PKC isoenzyme-specific inhibitors to reveal a PKCβ-dependent component of Kv inhibition dominating between 0 and 10 mM glucose with an additional PKCα-dependent component becoming evident at concentrations greater than 10 mM. These findings were supported using wire myography in all artery types used, where contractile responses to vessel depolarization and vasoconstrictors were enhanced by increasing bathing glucose concentration, again with evidence for distinct and complementary PKCα/PKCβ-mediated components.

CONCLUSIONS AND IMPLICATIONS

Our results provide compelling evidence that glucose-induced PKCα/PKCβ-mediated inhibition of Kv current in vascular smooth muscle causes an enhanced constrictor response. Inhibition of Kv current causes a significant depolarization of vascular myocytes leading to marked vasoconstriction. The PKC dependence of this enhanced constrictor response may present a potential therapeutic target for improving microvascular perfusion following percutaneous coronary intervention after myocardial infarction in hyperglycaemic patients.

Combining Normobaric Oxygen with Ethanol or Hypothermia Prevents Brain Damage from Thromboembolic Stroke via PKC-Akt-NOX Modulation

In a thromboembolic stroke model after reperfusion by recombinant tissue plasminogen activator (rt-PA), we aimed to determine whether therapeutic hypothermia (TH) and ethanol (EtOH) in combination with low concentration (60 %) of normobaric oxygen (NBO) enhanced neuroprotection, as compared to using each of these agents alone. We further aimed to elucidate a potential role of the NADPH oxidase (NOX), phosphorylated protein kinase B (Akt), and protein kinase C-δ (PKC-δ) pathway in oxidative stress and neuroprotection. In Sprague-Dawley rats, a focal middle cerebral artery (MCA) occlusion was induced by an autologous embolus in the following experimental groups: rt-PA treatment alone, rt-PA + NBO treatment, rt-PA + TH at 33 °C, rt-PA + EtOH, rt-PA + NBO + EtOH, rt-PA + NBO + TH, rt-PA + NOX inhibitor, rt-PA + EtOH + NOX inhibitor, or rt-PA + EtOH + Akt inhibitor. Control groups included sham-operated without stroke or stroke without treatment. Infarct volume and neurological deficit were assessed at 24 h after rt-PA-induced reperfusion with or without treatments. ROS levels, NOX activity, and the protein expression of NOX subunits p22^phox, p47^phox, p67^phox, gp91^phox, as well as PKC-δ and phosphorylated Akt were measured at 3 and 24 h after rt-PA-induced reperfusion. Following rt-PA in thromboembolic stroke rats, NBO combined with TH or EtOH more effectively decreased infarct volume and neurological deficit, as well as reactive oxygen species (ROS) production than with any of the used monotherapies. NOX activity and subunit expressions were downregulated and temporally associated with reduced PKC-δ and increased p-Akt expression. The present study demonstrated that combining NBO with either TH or EtOH conferred similar neuroprotection via modulation of NOX activation. The results suggest a role of Akt in NOX activation and implicate an upstream PKC-δ pathway in the Akt regulation of NOX. It is possible to substitute EtOH for TH, thus circumventing the difficulties in clinical application of TH through the comparatively easier usage of EtOH as a potential stroke management.

Reduced Apoptosis by Ethanol and Its Association with PKC-δ and Akt Signaling in Ischemic Stroke

Along with thrombolytic therapy, which has a number of limitations, stroke outcome may be improved with neuroprotective therapies that disrupt ischemic cell death. Recent research has shown a neuroprotective role of ethanol administration during ischemic stroke, such as its ability to reduce infarct volume and neurologic deficit. In order to investigate this further, we assessed the hypothesis that ethanol's neuroprotective effect is through reduction of apoptosis and the modulation of the important apoptotic PKC-δ and Akt signaling pathway. Ethanol (1.5 g/kg) was given by intraperitoneal injections to 54 Sprague-Dawley rats after 2 hours of middle cerebral artery (MCA) occlusion, followed by 3 or 24 hours of reperfusion. We measured apoptotic cell death, PKC-δ, and Akt mRNA and protein expressions in each of ischemic groups with or without ethanol treatment using ELISA, real-time PCR and Western blot analysis. Our results showed that cell death was significantly increased in rats following 2 hour MCA occlusion and 24 hour reperfusion. Subsequently, cell death was significantly reduced by an administration of ethanol. We further found that ethanol administration, prior to either 3 or 24 hours of reperfusion, significantly decreased the expression of PKC-δ while simultaneously increasing the expression Akt at both mRNA and protein levels at the two points. In conclusion, our study suggests that ethanol administration following ischemic stroke modulates the gene and protein profile in such a way that it increased expression of anti-apoptotic Akt and decreased the pro-apoptotic PKC-δ. This ultimately results in a decrease in neuronal apoptosis, thus conferring neuroprotection.

The phosphoinositide 3-kinase α selective inhibitor BYL719 enhances the effect of the protein kinase C inhibitor AEB071 in GNAQ/GNA11-mutant uveal melanoma cells

G-protein mutations are one of the most common mutations occurring in uveal melanoma activating the protein kinase C (PKC)/mitogen-activated protein kinase and phosphoinositide 3-kinase (PI3K)/AKT pathways. In this study, we described the effect of dual pathway inhibition in uveal melanoma harboring GNAQ and GNA11 mutations via PKC inhibition with AEB071 (sotrastaurin) and PI3K/AKT inhibition with BYL719, a selective PI3Kα inhibitor. Growth inhibition was observed in GNAQ/GNA11-mutant cells with AEB071 versus no activity in wild-type cells. In the GNAQ-mutant cells, AEB071 decreased phosphorylation of myristoylated alanine-rich C-kinase substrate, a substrate of PKC, along with ERK1/2 and ribosomal S6, but persistent AKT activation was present. BYL719 had minimal antiproliferative activity in all uveal melanoma cell lines, and inhibited phosphorylation of AKT in most cell lines. In the GNA11-mutant cell line, similar effects were observed with ERK1/2 inhibition, mostly inhibited by BYL719. With the combination treatment, both GNAQ- and GNA11-mutant cell lines showed synergistic inhibition of cell proliferation and apoptotic cell death. In vivo studies correlated with in vitro findings showing reduced xenograft tumor growth with the combination therapy in a GNAQ-mutant model. These findings suggest a new therapy treatment option for G-protein-mutant uveal melanoma with a focus on specific targeting of multiple downstream pathways as part of combination therapy.

Protein interaction network of Arabidopsis thaliana female gametophyte development identifies novel proteins and relations

Although the female gametophyte in angiosperms consists of just seven cells, it has a complex biological network. In this study, female gametophyte microarray data from Arabidopsis thaliana were integrated into the Arabidopsis interactome database to generate a putative interaction map of the female gametophyte development including proteome map based on biological processes and molecular functions of proteins. Biological and functional groups as well as topological characteristics of the network were investigated by analyzing phytohormones, plant defense, cell death, transporters, regulatory factors, and hydrolases. This approach led to the prediction of critical members and bottlenecks of the network. Seventy-four and 24 upregulated genes as well as 171 and 3 downregulated genes were identified in subtracted networks based on biological processes and molecular function respectively, including novel genes such as the pathogenesis-related protein 4, ER type Ca(2+) ATPase 3, dihydroflavonol reductase, and ATP disulfate isomerase. Biologically important relationships between genes, critical nodes, and new essential proteins such as AT1G26830, AT5G20850, CYP74A, AT1G42396, PR4 and MEA were found in the interactome's network. The positions of novel genes, both upregulated and downregulated, and their relationships with biological pathways, in particular phytohormones, were highlighted in this study.

Inhibition of anti-apoptotic signals by Wortmannin induces apoptosis in the remote myocardium after LAD ligation: evidence for a protein kinase C-δ-dependent pathway

It has been shown that, in the remote myocardium after infarction (MI), protein kinase C (PKC) inhibition reduces apoptosis both by blocking proapoptotic pathways and by activating antiapoptotic signals including the Akt pathway. However, it was open if vice versa, blockade of antiapoptotic pathways may influence proapoptotic signals. To clarify this, the present study tested the effects of the PI3-kinase blocker Wortmannin on proapoptotic signals and on apoptosis execution in the remote myocardium after infarction. Rats were subjected to MI by LAD ligation in situ. Some were pre-treated with Wortmannin alone or in combination with the PKC inhibitor Chelerythrine. After 24 h, pro- and anti-apoptotic signals (caspase-3, PKC isoforms, p38-MAPK, p42/44-MAPK, Akt, Bad), and marker of apoptosis execution (TUNEL) were quantified in the myocardium remote from the infarction. Wortmannin treatment increased apoptosis in the remote myocardium both at baseline and after MI, together with an activation of the PKC-δ/p38-MAPK-pathway. PKC-ε and p42/44-MAPK were unaffected. Combined treatment with Wortmannin and Chelerythrine fully reversed the pro-apoptotic effects of Wortmannin both at baseline and after MI. The PKC-δ-p38-MAPK-pathway as a strong signal for apoptosis in the non-infarcted myocardium can be influenced by targeting the anti-apoptotic PI3-kinase pathway. This gives evidence of a bi-directional crosstalk of pro- and anti-apoptotic signals after infarction.

Baicalein protects against 6-OHDA-induced neurotoxicity through activation of Keap1/Nrf2/HO-1 and involving PKCα and PI3K/AKT signaling pathways

Baicalein, one of the major flavonoids found in Scutellaria baicalensis Georgi, displays neuroprotective effects on experimental models of Parkinson's disease (PD) in vitro and in vivo. Although the antioxidative and/or anti-inflammatory activity of baicalein likely contributes to these neuroprotective effects, other modes of action remain largely uncharacterized. In the present study, baicalein pretreatment significantly prevented cells from 6-hydroxydopamine (6-OHDA)-induced damage by attenuating cellular apoptosis. However, post-treatment with baicalein did not show any restorative effect against 6-OHDA-induced cellular damage. We found that baicalein increased transcriptional factor NF-E2-related factor 2 (Nrf2)/hemo oxygenase 1(HO-1) protein expression and decreased Kelch-like ECH-associated protein 1 (Keap1) in a time- and concentration-dependent manner in PC12 cells. In addition, baicalein induced Nrf2 nuclear translocation and enhanced antioxidant response element (ARE) transcriptional activity, which conferred cytoprotection against 6-OHDA-induced oxidative injury. Moreover, we demonstrated that cytoprotective effects of baicalein could be attenuated by Nrf2 siRNA transfection and the HO-1 inhibitor zinc protoporphyrin (Znpp) as well as the proteasome inhibitor MG132. Furthermore, PKCα and AKT protein phosphorylation were up-regulated by baicalein pretreatment, and selective inhibitors targeted to PKC, PI3K, and AKT could block the cytoprotective effects of baicalein. Taken together, our results indicate that baicalein prevented PC12 cells from 6-OHDA-induced oxidative damage via the activation of Keap1/Nrf2/HO-1, and it also involves the PKCα and PI3K/AKT signaling pathway. Ultimately, the neuroprotective effects of baicalein may endue baicalein as a promising candidate for the prevention of PD.

Luteolin ameliorates cardiac failure in type I diabetic cardiomyopathy

OBJECTIVE

The study aimed to determine whether luteolin can confer cardioprotective effects against diabetic cardiomyopathy in relation to specific and quantitative markers of oxidative stress.

METHODS

We examined diabetic cardiomyopathy by left ventricular hemodynamic analysis. Myocardial oxidative stress was assessed by measuring the activity of superoxide dismutase (SOD) as well as the level of malondialdehyde (MDA). Hypolipidaemic effects of luteolin were also investigated in STZ-induced diabetic rats. Myocardial Akt/PKB phosphorylation, heme oxygenase-1 (HO-1) and connective tissue growth factor (CTGF) protein levels were measured by Western blot in all rats at the end of the study.

RESULTS

This study showed a significant increase in serum triacylglycerol (TG), total cholesterol (TC), lower density lipoprotein (LDL), MDA content, creatine kinase (CK), lactate dehydrogenase (LDH), and myocardial CTGF and a significant decrease in high density lipoprotein (HDL), SOD and Akt phosphorylation level in the diabetic group compared to the control group. Luteolin treatment induced a significant decrease in serum TG, TC, LDL, MDA, CK, LDH, and myocardial CTGF and a significant increase in HDL, SOD and Akt phosphorylation levels in comparison with the diabetic group.

CONCLUSION

These results show that luteolin protects against the progression of diabetes mellitus-induced cardiac dysfunction by attenuation of myocardial oxidative stress probably through its antioxidant properties.

Signaling regulation of fetoplacental angiogenesis

During normal pregnancy, dramatically increased placental blood flow is critical for fetal growth and survival as well as neonatal birth weights and survivability. This increased blood flow results from angiogenesis, vasodilatation, and vascular remodeling. Locally produced growth factors including fibroblast growth factor 2 (FGF2) and vascular endothelial growth factor A (VEGFA) are key regulators of placental endothelial functions including cell proliferation, migration, and vasodilatation. However, the precise signaling mechanisms underlying such regulation in fetoplacental endothelium are less well defined, specifically with regard to the interactions amongst protein kinases (PKs), protein phosphatase, and nitric oxide (NO). Recently, we and other researchers have obtained solid evidence showing that different signaling mechanisms participate in FGF2- and VEGFA-regulated fetoplacental endothelial cell proliferation and migration as well as NO production. This review will briefly summarize currently available data on signaling mediating fetoplacental angiogenesis with a specific emphasis on PKs, ERK1/2, AKT1, and p38 MAPK and protein phosphatases, PPP2 and PPP3.

AP-1 elements and TCL1 protein regulate expression of the gene encoding protein tyrosine phosphatase PTPROt in leukemia

We previously demonstrated that the gene encoding PTPROt, the truncated form of protein tyrosine phosphatase receptor type O expressed predominantly in hematopoietic cells, is a candidate tumor suppressor and is down-regulated in chronic lymphocytic leukemia (CLL). Here, we show that PTPROt expression is significantly reduced in CD19(+) spleen B cells from Eμ-T cell leukemia 1 (TCL1) transgenic mice relative to the wild-type mice. Strikingly, as much as a 60% decrease in PTPROt expression occurs at 7 weeks independently of promoter methylation. To elucidate the potential mechanism for this early suppression of PTPROt in these mice, we explored the role of activating protein-1 (AP-1) in its expression. We first demonstrate that AP-1 activation by 12-O-tetradecanoylphorbol-13-acetate induces PTPROt expression with concurrent recruitment of c-fos and c-jun to its promoter. The PTPROt promoter is also responsive to over- and underexpression of AP-1, confirming the role of AP-1 in PTPROt expression. Next, we demonstrate that TCL1 can repress the PTPROt promoter by altering c-fos expression and c-jun activation state. Finally, using primary CLL cells we have shown an inverse relationship between TCL1 and PTPROt expression. These findings further substantiate the role of TCL1 in PTPROt suppression and its importance in the pathogenesis of CLL.

Beta-HPV 5 and 8 E6 promote p300 degradation by blocking AKT/p300 association

The E6 oncoprotein from high-risk genus alpha human papillomaviruses (α-HPVs), such as HPV 16, has been well characterized with respect to the host-cell proteins it interacts with and corresponding signaling pathways that are disrupted due to these interactions. Less is known regarding the interacting partners of E6 from the genus beta papillomaviruses (β-HPVs); however, it is generally thought that β-HPV E6 proteins do not interact with many of the proteins known to bind to α-HPV E6. Here we identify p300 as a protein that interacts directly with E6 from both α- and β-HPV types. Importantly, this association appears much stronger with β-HPV types 5 and 8-E6 than with α-HPV type 16-E6 or β-HPV type 38-E6. We demonstrate that the enhanced association between 5/8-E6 and p300 leads to p300 degradation in a proteasomal-dependent but E6AP-independent manner. Rather, 5/8-E6 inhibit the association of AKT with p300, an event necessary to ensure p300 stability within the cell. Finally, we demonstrate that the decreased p300 protein levels concomitantly affect downstream signaling events, such as the expression of differentiation markers K1, K10 and Involucrin. Together, these results demonstrate a unique way in which β-HPV E6 proteins are able to affect host-cell signaling in a manner distinct from that of the α-HPVs.

Dual function of protein kinase C (PKC) in 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced manganese superoxide dismutase (MnSOD) expression: activation of CREB and FOXO3a by PKC-alpha phosphorylation and by PKC-mediated inactivation of Akt, respectively

12-O-tetradecanoylphorbol-13-acetate (TPA) has been shown to induce transcriptional activation of human manganese superoxide dismutase (MnSOD) mRNA in human lung carcinoma cells, A549, mediated by a protein kinase C (PKC)-dependent activation of cAMP-responsive element-binding protein (CREB)-1/ATF-1-like factors. In this study, we showed that MnSOD protein expression was elevated in response to TPA or TNF-α, but not to hydrogen peroxide treatment. TPA-induced generation of reactive oxygen species (ROS) was blocked by pretreatment of the PKC inhibitor BIM and NADPH oxidase inhibitor DPI. Small interfering RNA (siRNA) experiments indicated that knocking down the NADPH oxidase components e.g. Rac1, p22(phox), p67(phox), and NOXO1 in A549 cells impaired TPA-induced MnSOD expression. To identify the PKC isozyme involved, we used a sod2 gene response reporter plasmid, pSODLUC-3340-I2E-C, capable of sensing the effect of TNF-α and TPA, to monitor the effects of PKC isozyme-specific inhibitors and siRNA-induced knockdown of specific PKC isozyme. Our data indicate that TPA-induced MnSOD expression was independent of p53 and most likely mediated by PKC-α-, and -ε-dependent signaling pathways. Furthermore, siRNA-induced knock-down of CREB and Forkhead box class O (FOXO) 3a led to a reduction in TPA-induced MnSOD gene expression. Together, our results revealed that TPA up-regulates, in part, two PKC-dependent transcriptional pathways to induce MnSOD expression. One pathway involves PKC-α catalyzed phosphorylation of CREB and the other involves a PKC-mediated the PP2A catalyzed dephosphorylation of Akt at Ser(473) which in turn leads to FOXO3a Ser(253) dephosphorylation and its activation.

Protein kinase C: an attractive target for cancer therapy

Apoptosis plays an important role during all stages of carcinogenesis and the development of chemoresistance in tumor cells may be due to their selective defects in the intracellular signaling proteins, central to apoptotic pathways. Consequently, many studies have focused on rendering the chemotherapy more effective in order to prevent chemoresistance and pre-clinical and clinical data has suggested that protein kinase C (PKC) may represent an attractive target for cancer therapy. Therefore, a complete understanding of how PKC regulates apoptosis and chemoresistance may lead to obtaining a PKC-based therapy that is able to reduce drug dosages and to prevent the development of chemoresistance.

12-O-tetradecanoylphorbol-1, 3-acetate induces the negative regulation of protein kinase B by protein kinase Calpha during gastric cancer cell apoptosis

The PKB signaling pathway is essential for cell survival and the inhibition of apoptosis, but its functional mechanisms have not been fully explored. Previously, we reported that TPA effectively inhibited PKB activity and caused PKB degradation, which was correlated with the repression of PKB phosphorylation at Ser473. In this study, we focus on how PKB is regulated by TPA in gastric cancer cells. One of the TPA targets, PKCα, was found to mediate the inhibition of PKB phosphorylation and degredation caused by TPA. Furthermore, TPA induced the import of PKCα into the nucleus, where PKCα exerted an inhibitory effect on PKB expression and phosphorylation. As a result, cancer cell proliferation was arrested. Our study characterizes a novel function of PKCα in mediating the negative regulation of PKB by TPA, and suggests a potential application in the clinical treatment of gastric cancer.

Functional links between diacylglycerol and phosphatidylinositol signaling systems in human leukocyte-derived cell lines

In leukocytes, diacylglycerol (DAG) regulates the small GTPase Ras through the agency of Ras guanyl releasing proteins (RasGRPs). Ras is thought to regulate phosphatidylinositol 3-kinase (PI3K). Therefore, DAG signaling is hypothesized to impact PI3K activity. The DAG analogue "pico" was used to activate RasGRPs in leukocyte-derived cell lines. PI3K signaling was monitored using antibodies that recognize the activated form of the PI3K-regulated protein kinase Akt. Diverse responses were documented. Some cell lines exhibit a DAG analogue-stimulated increase in phospho-Akt but this response proceeded even when Ras activation was blocked. In some Epstein-Barr virus-associated malignant cell lines and transformed B cells, high basal phospho-Akt was decreased by DAG analogue treatment. The pan-PKC inhibitor Bisindolymaleimide I blocked this effect. Basal phospho-Akt was also decreased by treatment with Go6976, an inhibitor of conventional protein kinase C and protein kinase D. While the proposed RasGRP-Ras-PI3K-Akt signaling axis may operate in some situations, our results indicate that alternative links between DAG targets such as protein kinases and the PI3K signaling system are more prominent.

PKC-dependent superoxide production by the renal medullary thick ascending limb from diabetic rats

Type 1 diabetes (T1D) is a state of oxidative stress accompanied by PKC activation in many tissues. The primary site of O2*- production by the normal rat kidney is the medullary thick ascending limb (mTAL). We hypothesized that T1D increases O2*- production by the mTAL through a PKC-dependent mechanism involving increased expression and translocation of one or more PKC isoforms. mTAL suspensions were prepared from rats with streptozotocin-induced T1D (STZ mTALs) and from normal or sham rats (normal/sham mTALs). O2*- production by STZ mTALs was fivefold higher than normal/sham mTALs (P < 0.05). PMA (30 min) mimicked the effect of T1D on O2*- production. Exposure to calphostin C or chelerythrine (PKC inhibitors), Gö6976 (PKCalpha/beta inhibitor), or rottlerin (PKCdelta inhibitor) decreased O2*- production to <20% of untreated baseline in both normal/sham and STZ mTALs. PKCbeta inhibitors had no effect. PKC activity was increased in STZ mTALs (P < 0.05 vs. normal/sham mTALs) and was unaltered by antioxidant exposure (tempol). PKCalpha protein levels were increased by 70% in STZ mTALs, with a approximately 30% increase in the fraction associated with the membrane (both P < 0.05 vs. sham). PKCbeta protein levels were elevated by 29% in STZ mTALs (P < 0.05 vs. sham) with no change in the membrane-bound fraction. Neither PKCdelta protein levels nor its membrane-bound fraction differed between groups. Thus STZ mTALs display PKC activation, upregulation of PKCalpha and PKCbeta protein levels, increased PKCalpha translocation to the membrane, and accelerated O2*- production that is eradicated by inhibition of PKCalpha or PKCdelta (but not PKCbeta). We conclude that increased PKCalpha expression and activity are primarily responsible for PKC-dependent O2*- production by the mTAL during T1D.

Gastrointestinal growth factors and hormones have divergent effects on Akt activation

Akt is a central regulator of apoptosis, cell growth and survival. Growth factors and some G-protein-coupled receptors (GPCR) regulate Akt. Whereas growth-factor activation of Akt has been extensively studied, the regulation of Akt by GPCR's, especially gastrointestinal hormones/neurotransmitters, remains unclear. To address this area, in this study the effects of GI growth factors and hormones/neurotransmitters were investigated in rat pancreatic acinar cells which are high responsive to these agents. Pancreatic acini expressed Akt and 5 of 7 known pancreatic growth-factors stimulate Akt phosphorylation (T308, S473) and translocation. These effects are mediated by p85 phosphorylation and activation of PI3K. GI hormones increasing intracellular cAMP had similar effects. However, GI-hormones/neurotransmitters [CCK, bombesin, carbachol] activating phospholipase C (PLC) inhibited basal and growth-factor-stimulated Akt activation. Detailed studies with CCK, which has both physiological and pathophysiological effects on pancreatic acinar cells at different concentrations, demonstrated CCK has a biphasic effect: at low concentrations (pM) stimulating Akt by a Src-dependent mechanism and at higher concentrations (nM) inhibited basal and stimulated Akt translocation, phosphorylation and activation, by de-phosphorylating p85 resulting in decreasing PI3K activity. This effect required activation of both limbs of the PLC-pathway and a protein tyrosine phosphatase, but was not mediated by p44/42 MAPK, Src or activation of a serine phosphatase. Akt inhibition by CCK was also found in vivo and in Panc-1 cancer cells where it inhibited serum-mediated rescue from apoptosis. These results demonstrate that GI growth factors as well as gastrointestinal hormones/neurotransmitters with different cellular basis of action can all regulate Akt phosphorylation in pancreatic acinar cells. This regulation is complex with phospholipase C agents such as CCK, because both stimulatory and inhibitory effects can be seen, which are mediated by different mechanisms.

Stimulation of adenosine A2b receptors blocks apoptosis in the non-infarcted myocardium even when administered after the onset of infarction

OBJECTIVE

Chronic adenosine A2b receptor stimulation has been shown to prevent ventricular remodelling after myocardial infarction (MI). We hypothesized that this effect is due to the inhibition of cardiac myocyte apoptosis in the myocardium remote from the infarction.

METHODS

Rats were subjected to MI by LAD ligation in situ. Some animals were pre-treated with the stable adenosine analogue 2-chloro-adenosine (CADO). After 24 h, pro- and anti-apoptotic signals (protein kinase C isoforms, p38, g proteins, Bcl-2/Bax ratio, Akt, Bad), and marker of apoptosis execution (caspase-3, TUNEL) were quantified in the remote myocardium.

RESULTS

CADO prevented the occurrence of apoptosis in the remote myocardium of an infarcted heart. This effect occured not only when CADO was started before the onset of ischemia but also when it started 3 h after the infarction. The anti-apoptotic effect of CADO was blocked by simultaneous administration of the selective adenosine A2b receptor antagonist MRS1754 (1 mg/kg). The anti-apoptotic effect of CADO seems to be mediated by g(alphaq) and by the activation of survival kinases (Bad) and by inhibition of the pro-apoptotic PKC-delta/p38-MAPK-pathway.

CONCLUSION

Chronic adenosine A2b receptor stimulation blocks cardiac myocyte apoptosis in the remote myocardium even when started after the onset of infarction. This may explain the anti-remodelling-effect of the A2b receptor stimulation after infarction.

PKC-beta1 mediates glucose-induced Akt activation and TGF-beta1 upregulation in mesangial cells

Accumulation of glomerular matrix is a hallmark of diabetic nephropathy. The serine/threonine kinase Akt mediates glucose-induced upregulation of collagen I in mesangial cells through transactivation of the EGF receptor (EGFR). In addition, in renal tubular cells, glucose-induced secretion of TGF-beta requires phosphoinositide-3-OH kinase, suggesting a possible role for Akt in the modulation of TGF-beta expression, but the mechanisms of Akt activation and its involvement in TGF-beta regulation are unknown. Here, in primary mesangial cells, high glucose induced AktS473 phosphorylation, which correlates with its activation, in a protein kinase C beta (PKC-beta)-dependent manner. Glucose led to PKC-beta1 membrane translocation and association with Akt, and PKC-beta1 immunoprecipitated from glucose-treated cells phosphorylated recombinant Akt on S473. PKC is known to mediate glucose-induced TGF-beta1 upregulation through the transcription factor AP-1; here, inhibitors of phosphoinositide-3-OH kinase, PKC-beta and Akt, and dominant-negative Akt all prevented glucose-induced activation of AP-1 and upregulation of TGF-beta1. Finally, pharmacologic and dominant negative inhibition of EGFR blocked glucose-induced activation of PKC-beta1, phosphorylation of AktS473, activation of AP-1, and upregulation of TGF-beta1. In vivo, the PKC-beta inhibitor ruboxistaurin prevented Akt activation in the renal cortex of diabetic rats. In conclusion, PKC-beta1 is an Akt S473 kinase in glucose-treated mesangial cells, and TGF-beta1 transcriptional upregulation requires EGFR/PKC-beta1/Akt signaling. New therapeutic approaches for diabetic nephropathy may result from targeting components of this pathway, particularly the initial EGFR transactivation.

Protein phosphatase 3 differentially modulates vascular endothelial growth factor- and fibroblast growth factor 2-stimulated cell proliferation and signaling in ovine fetoplacental artery endothelial cells

A critical process for vascular endothelial growth factor (VEGF)- and fibroblast growth factor 2 (FGF2)-regulated cellular function is reversible protein phosphorylation, which is tightly controlled by a balance of protein kinases and phosphatases. We have reported that in ovine fetoplacental artery endothelial (OFPAE) cells, VEGF and FGF2 stimulate cell proliferation in part via activation of mitogen-activated protein kinase kinase 1/2 (MAP2K1/2)/mitogen-activated protein kinase 3/1 (MAPK3/1) and phosphoinositide 3-kinase (PI3K)/v-akt murine thymoma viral oncogene homolog 1 (AKT1) pathways. In the present study, we examined if protein phosphatase 3 (PPP3) mediated VEGF- and FGF2-stimulated OFPAE cell proliferation via modulating activation of MAPK3/1 and AKT1. Small interfering RNA (siRNA) targeting human PPP3 catalytic subunit alpha (PPP3CA) was used to suppress PPP3CA protein expression in OFPAE cells. Compared with the scrambled siRNA, PPP3CA siRNA decreased PPP3CA protein levels by approximately 97% without altering protein levels of protein phosphatase 2 catalytic subunit alpha, total MAPK3/1, total AKT1, or glyceraldehyde-3-phosphate dehydrogenase. Knockdown of PPP3CA protein expression enhanced VEGF-stimulated, but not FGF2-stimulated, cell proliferation. Knockdown of PPP3CA protein expression did not significantly affect VEGF-induced MAPK3/1 and AKT1 phosphorylation but attenuated FGF2-induced MAPK3/1 and AKT1 phosphorylation. Thus, to our knowledge, the present study is the first to demonstrate successful knockdown of PPP3CA protein expression in any cell model using a single pair of double-strained siRNA. Moreover, specific knockdown of PPP3CA protein expression enhances VEGF-stimulated, but not FGF2-stimulated, OFPAE cell proliferation and attenuates FGF2-induced, but not VEGF-induced, MAPK3/1 and AKT1 activation. Thus, PPP3CA differentially modulates the VEGF- and FGF2-stimulated cell proliferation and signaling cascades in OFPAE cells. These data also suggest that signaling molecules other than MAPK3/1 and AKT1 play an important role in VEGF- and FGF2-stimulated cell proliferation after knockdown of PPP3CA in OFPAE cells.

Regulation of H(2)O(2)-induced necrosis by PKC and AMP-activated kinase signaling in primary cultured hepatocytes

Recent studies have suggested that, in certain cases, necrosis, like apoptosis, may be programmed, involving the activation and inhibition of many signaling pathways. In this study, we examined whether necrosis induced by H(2)O(2) is regulated by signaling pathways in primary hepatocytes. A detailed time course revealed that H(2)O(2) treated to hepatocytes is consumed within minutes, but hepatocytes undergo necrosis several hours later. Thus, H(2)O(2) treatment induces a "lag phase" where signaling changes occur, including PKC activation, Akt (PKB) downregulation, activation of JNK, and downregulation of AMP-activated kinase (AMPK). Investigation of various inhibitors demonstrated that PKC inhibitors were effective in reducing necrosis caused by H(2)O(2) (~80%). PKC inhibitor treatment decreased PKC activity but, surprisingly, also upregulated Akt and AMPK, suggesting that various PKC isoforms negatively regulate Akt and AMPK. Akt did not appear to play a significant role in H(2)O(2)-induced necrosis, since PKC inhibitor treatment protected hepatocytes from H(2)O(2) even when Akt was inhibited. On the other hand, compound C, a selective AMPK inhibitor, abrogated the protective effect of PKC inhibitors against necrosis induced by H(2)O(2). Furthermore, AMPK activators protected against H(2)O(2)-induced necrosis, suggesting that much of the protective effect of PKC inhibition was mediated through the upregulation of AMPK. Work with PKC inhibitors suggested that atypical PKC downregulates AMPK in response to H(2)O(2). Knockdown of PKC-alpha using antisense oligonucleotides also slightly protected (~22%) against H(2)O(2). Taken together, our data demonstrate that the modulation of signaling pathways involving PKC and AMPK can alter H(2)O(2)-induced necrosis, suggesting that a signaling "program" is important in mediating H(2)O(2)-induced necrosis in primary hepatocytes.

Diacylglycerol kinase zeta inhibits myocardial atrophy and restores cardiac dysfunction in streptozotocin-induced diabetes mellitus

Background

Activation of the diacylglycerol (DAG)-protein kinase C (PKC) pathway has been implicated in the pathogenesis of a number of diabetic complications. Diacylglycerol kinase (DGK) converts DAG to phosphatidic acid and acts as an endogenous regulator of PKC activity. Akt/PKB is associated with a downstream insulin signaling, and PKCβ attenuates insulin-stimulated Akt phosphorylation.

Methods and Results

We examined transgenic mice with cardiac-specific overexpression of DGKζ (DGKζ-TG) compared to wild type (WT) mice in streptozotocin-induced (STZ, 150 mg/kg) diabetic and nondiabetic conditions. After 8 weeks, decreases in heart weight and heart weight/body weight ratio in diabetic WT mice were inhibited in DGKζ-TG mice. Echocardiography at 8 weeks after STZ-injection demonstrated that decreases in left ventricular end-diastolic diameter and fractional shortening observed in WT mice were attenuated in DGKζ-TG mice. Thinning of the interventricular septum and the posterior wall in diabetic WT hearts were blocked in DGKζ-TG mice. Reduction of transverse diameter of cardiomyocytes isolated from the left ventricle in diabetic WT mice was attenuated in DGKζ-TG mice. Cardiac fibrosis was much less in diabetic DGKζ-TG than in diabetic WT mice. Western blots showed translocation of PKCβ and δ isoforms to membrane fraction and decreased Akt/PKB phosphorylation in diabetic WT mouse hearts. However in diabetic DGKζ-TG mice, neither translocation of PKC nor changes Akt/PKB phosphorylation was observed.

Conclusion

DGKζ modulates intracellular signaling and improves the course of diabetic cardiomyopathy. These data may suggest that DGKζ is a new therapeutic target to prevent or reverse diabetic cardiomyopathy.

Protein kinase B (c-akt) regulates hematopoietic lineage choice decisions during myelopoiesis

Hematopoiesis is a highly regulated process resulting in the formation of all blood lineages. Aberrant regulation of phosphatidylinositol-3-kinase (PI3K) signaling has been observed in hematopoietic malignancies, suggesting that regulated PI3K signaling is critical for regulation of blood cell production. An ex vivo differentiation system was used to investigate the role of PI3K and its downstream effector, protein kinase B (PKB/c-akt) in myelopoiesis. PI3K activity was essential for hematopoietic progenitor survival. High PKB activity was found to promote neutrophil and monocyte development, while, conversely, reduction of PKB activity was required to induce optimal eosinophil differentiation. In addition, transplantation of β2-microglobulin (−/−) NOD/SCID mice with CD34+ cells ectopically expressing constitutively active PKB resulted in enhanced neutrophil and monocyte development, whereas ectopic expression of dominant-negative PKB induced eosinophil development in vivo. Inhibitory phosphorylation of C/EBPα on Thr222/226 was abrogated upon PKB activation in hematopoietic progenitors. Ectopic expression of a nonphosphorylatable C/EBPα mutant inhibited eosinophil differentiation ex vivo, whereas neutrophil development was induced, demonstrating the importance of PKB-mediated C/EBPα phosphorylation in regulation of granulopoiesis. These results identify an important novel role for PKB in regulation of cell fate choices during hematopoietic lineage commitment.

Protein kinase C-betaII represses hepatocyte growth factor-induced invasion by preventing the association of adapter protein Gab1 and phosphatidylinositol 3-kinase in melanoma cells

The hepatocyte growth factor (HGF) signaling pathway was examined in human normal melanocytes and three malignant melanoma cell lines. HGF-induced activation of c-Met, its receptor-tyrosine kinase, was observed in both melanocytes and melanoma cells, whereas phosphatidylinositol 3-kinase (PI3K), a downstream target of c-Met, was not activated in the melanocytes but enhanced in the melanoma cell lines. The electrophoretic mobility of Gab1, the scaffolding adapter protein that couples activated c-Met and PI3K, was slower in the melanocytes than that in the melanoma cells, and the mobility shifted to that of the melanoma cells after treatment with alkaline phosphatase, indicating that Gab1 is highly phosphorylated on serine and threonine in the melanocytes. Introduction of protein kinase C (PKC)-betaII into the melanoma cells, which is expressed in melanocytes but absent in melanoma cells, resulted in serine and threonine phosphorylation of Gab1 and also prevented tyrosine phosphorylation of Gab1 and its association with PI3K. Furthermore, the introduction of PKC-betaII suppressed HGF-induced activation of PI3K, and attenuated the in vitro invasion activity of the melanoma cells. These results indicate that the HGF signaling process from Gab1 to PI3K is negatively regulated by PKC-betaII, and its loss is critical for melanoma cells to gain invasive potential.

Protein kinase Cepsilon makes the life and death decision

Cancer is caused by dysregulation in cellular signaling systems that control cell proliferation, differentiation and cell death. Protein kinase C (PKC), a family of serine/threonine kinases, plays an important role in the growth factor signal transduction pathway. PKCepsilon, however, is the only PKCepsilon isozyme that has been considered as an oncogene. It can contribute to malignancy by enhancing cell proliferation or by inhibiting cell death. This review focuses on how PKCepsilon collaborates with other signaling pathways, such as Ras/Raf/ERK and Akt, to regulate cell survival and cell death. We have also discussed how PKCepsilon mediates its antiapoptotic signal by altering the level or function of pro- and antiapoptotic Bcl-2 family members.

Protein kinase C-mediated down-regulation of cyclin D1 involves activation of the translational repressor 4E-BP1 via a phosphoinositide 3-kinase/Akt-independent, protein phosphatase 2A-dependent mechanism in intestinal epithelial cells

We reported previously that protein kinase Calpha (PKCalpha), a negative regulator of cell growth in the intestinal epithelium, inhibits cyclin D1 translation by inducing hypophosphorylation/activation of the translational repressor 4E-BP1. The current study explores the molecular mechanisms underlying PKC/PKCalpha-induced activation of 4E-BP1 in IEC-18 nontransformed rat ileal crypt cells. PKC signaling is shown to promote dephosphorylation of Thr(45) and Ser(64) on 4E-BP1, residues directly involved in its association with eIF4E. Consistent with the known role of the phosphoinositide 3-kinase (PI3K)/Akt/mTOR pathway in regulation of 4E-BP1, PKC signaling transiently inhibited PI3K activity and Akt phosphorylation in IEC-18 cells. However, PKC/PKCalpha-induced activation of 4E-BP1 was not prevented by constitutively active mutants of PI3K or Akt, indicating that blockade of PI3K/Akt signaling is not the primary effector of 4E-BP1 activation. This idea is supported by the fact that PKC activation did not alter S6 kinase activity in these cells. Further analysis indicated that PKC-mediated 4E-BP1 hypophosphorylation is dependent on the activity of protein phosphatase 2A (PP2A). PKC signaling induced an approximately 2-fold increase in PP2A activity, and phosphatase inhibition blocked the effects of PKC agonists on 4E-BP1 phosphorylation and cyclin D1 expression. H(2)O(2) and ceramide, two naturally occurring PKCalpha agonists that promote growth arrest in intestinal cells, activate 4E-BP1 in PKC/PKCalpha-dependent manner, supporting the physiological significance of the findings. Together, our studies indicate that activation of PP2A is an important mechanism underlying PKC/PKCalpha-induced inhibition of cap-dependent translation and growth suppression in intestinal epithelial cells.

Neurotensin phosphorylates GSK-3alpha/beta through the activation of PKC in human colon cancer cells

Neurotensin (NT), a gastrointestinal hormone, binds its receptor [neurotensin receptor (NTR)] to regulate the growth of normal and neoplastic intestinal cells; molecular mechanisms remain largely undefined. Glycogen synthase kinase-3 (GSK-3) regulates diverse cellular processes, including cell growth and apoptosis. Here, we show that NT induces the phosphorylation of GSK-3alpha/beta in the human colon cancer cell line HT29, HCT116, or SW480, which possesses high-affinity NTR. The effect of NT was blocked by inhibitors of protein kinase C (PKC), but not by inhibitors of MEK1 or phosphatidylinositol-3 kinase, suggesting a predominant role for PKC in GSK-3beta phosphorylation by NT. Pretreatment with Gö6976 (which inhibits PKCalpha and PKCbeta1) or downregulation of endogenous PKCalpha or PKCbeta1 blocked NT-mediated GSK-3beta (but not GSK-3alpha) phosphorylation. Moreover, a selective PKCbeta inhibitor, LY379196, reduced NT-mediated GSK-3beta (but not GSK-3alpha) phosphorylation, suggesting a role for PKCbeta1 in the NT-mediated phosphorylation of GSK-3beta and an undefined kinase in the NT-mediated phosphorylation of GSK-3alpha. Treatment with NT or the GSK-3 inhibitor SB216763 increased the expression of cyclin D1, a downstream effector protein of GSK-3 and a critical protein for the proliferation of various cells. Our results indicate that NT uses PKC-dependent pathways to modulate GSK-3, which may play a role in the NT regulation of intestinal cell growth.

Protein kinase Cdelta participates in insulin-induced activation of PKB via PDK1

PKCdelta has been shown to be activated by insulin and to interact with insulin receptor and IRS. PKB(Akt) plays an important role in glucose transport and glycogen synthesis. In this study, we investigated the possibility that PKCdelta may be involved in insulin-induced activation of PKB. Studies were conducted on primary cultures of rat skeletal muscle. PKB was activated by insulin stimulation within 5min and reached a peak by 15-30min. Insulin also increased the physical association between PKCdelta with PKB and with PDK1. The insulin-induced PKCdelta-PKB association was PI3K dependent. PKB-PKCdelta association was accounted for by the involvement of PDK1. Overexpression of dominant negative PKCdelta abrogated insulin-induced association of PKCdelta with both PKB and PDK1. Blockade of PKCdelta also decreased insulin-induced Thr308 PKB phosphorylation and PKB translocation. Moreover, PKCdelta inhibition reduced insulin-induced GSK3 phosphorylation. The results indicate that insulin-activated PKCdelta interacts with PDK1 to regulate PKB.

Protein kinase Cepsilon activates protein kinase B/Akt via DNA-PK to protect against tumor necrosis factor-alpha-induced cell death

We have previously shown that protein kinase Cepsilon (PKCepsilon) protects breast cancer cells from tumor necrosis factor-alpha (TNF)-induced cell death. In the present study, we have investigated if the antiapoptotic function of PKCepsilon is mediated via Akt and the mechanism by which PKCepsilon regulates Akt activity. TNF caused a transient increase in Akt phosphorylation at Ser473 in MCF-7 cells. Overexpression of PKCepsilon in MCF-7 cells increased TNF-induced Akt phosphorylation at Ser473 resulting in its activation. Knockdown of PKCepsilon by small interfering RNA (siRNA) decreased TNF-induced Akt phosphorylation/activation and increased cell death. Introduction of constitutively active Akt protected breast cancer MCF-7 cells from TNF-mediated cell death and partially restored cell survival in PKCepsilon-depleted cells. Depletion of Akt in MCF-7 cells abolished the antiapoptotic effect of PKCepsilon on TNF-mediated cell death. Akt was constitutively associated with PKCepsilon and DNA-dependent protein kinase (DNA-PK), and this association was increased by TNF treatment. Overexpression of PKCepsilon enhanced the interaction between Akt and DNA-PK. Knockdown of DNA-PK by siRNA inhibited TNF-induced Akt phosphorylation and the antiapoptotic effect of Akt and PKCepsilon. These results suggest that PKCepsilon activates Akt via DNA-PK to mediate its antiapoptotic function. Furthermore, we report for the first time that DNA-PK can regulate receptor-initiated apoptosis via Akt.

Involvement of Protein Kinase Cε in the Negative Regulation of Akt Activation Stimulated by Granulocyte Colony-Stimulating Factor

Stimulation of cells with G-CSF activates multiple signaling cascades, including the serine/threonine kinase Akt pathway. We show in this study that G-CSF-induced activation of Akt in myeloid 32D was specifically inhibited by treatment with PMA, a protein kinase C (PKC) activator. PMA treatment also rapidly attenuated sustained Akt activation mediated by a carboxy truncated G-CSF receptor, expressed in patients with acute myeloid leukemia evolving from severe congenital neutropenia. The inhibitory effect of PMA was abolished by pretreatment of cells with specific PKC inhibitor GF109203X, suggesting that the PKC pathway negatively regulates Akt activation. Ro31-8820, a PKCepsilon inhibitor, also abrogated PMA-mediated inhibition of Akt activation, whereas rottlerin and Go6976, inhibitors of PKCdelta and PKCalphabetaI, respectively, exhibited no significant effects. Furthermore, overexpression of the wild-type and a constitutively active, but not a kinase-dead, forms of PKCepsilon markedly attenuated Akt activation, and inhibited the proliferation and survival of cells in response to G-CSF. The expression of PKCepsilon was down-regulated with G-CSF-induced terminal granulocytic differentiation. Together, these results implicate PKCepsilon as a negative regulator of Akt activation stimulated by G-CSF and indicate that PKCepsilon plays a negative role in cell proliferation and survival in response to G-CSF.

Inhibitors of the PI3-kinase/Akt pathway induce mitotic catastrophe in non-small cell lung cancer cells

Non-small cell lung cancer cells (NSCLC) are more resistant to anticancer treatment as compared with other types of cancer cells. Recently (Hemström et al., Exp Cell Res 2005;305:200-13) we showed that apoptosis of U1810 NSCLC cells induced by the staurosporine analog PKC 412 correlated with inhibition of Akt and ERK1/2, suggesting the involvement of these kinases in cell survival. Here we investigated the contribution of the PI3-kinase/Akt and MEK/ERK pathways to survival of NSCLC cells. The two signaling pathways were studied by using different combinations of the PI3-kinase inhibitors LY-294002 and wortmannin, the Akt activator Ro 31-8220, the MEK inhibitor PD 98059 and PKC 412. PI3-kinase inhibitors induced apoptosis-like death in U1810 cells. H157 cells in general were relatively resistant to PI3 kinase/Akt inhibitors yet these compounds sensitized cells to the DNA-damaging drug VP-16, while Ro 31-8220 could not. PD 98059 only had a sensitizing effect on H157 cells when combined with PI3-kinase inhibition and VP-16. Morphological data indicated that LY-294002 and PKC 412 induced cell death at anaphase and metaphase, respectively, suggesting death by mitotic catastrophe. Analyzes of cells blocked in G2/M-phase by nocodazol revealed that LY-294002 increased, while PKC 412 decreased histone H3 phosphorylation, suggesting that LY-294002 allowed, while PKC 412 inhibited cells to leave M-phase. Flow cytometric analysis of cell cycle distribution demonstrated that LY-294002 allowed cells to leave G2/M phase, while PKC 412 inhibited cytokinesis, resulting in formation of multinucleated cells. These results indicate that sensitization of NSCLC cells by PI3-kinase inhibition involves interplay between cell cycle regulation, mitotic catastrophe and apoptosis.

Protein kinase C negatively regulates Akt activity and modifies UVC-induced apoptosis in mouse keratinocytes

Skin keratinocytes are subject to frequent chemical and physical injury and have developed elaborate cell survival mechanisms to compensate. Among these, the Akt/protein kinase B (PKB) pathway protects keratinocytes from the toxic effects of ultraviolet light (UV). In contrast, the protein kinase C (PKC) family is involved in several keratinocyte death pathways. During an examination of potential interactions among these two pathways, we found that the insulin-like growth factor (IGF-1) activates both the PKC and the Akt signaling pathways in cultured primary mouse keratinocytes as indicated by increased phospho-PKC and phospho-Ser-473-Akt. IGF-1 also selectively induced translocation of PKCdelta and PKCepsilon from soluble to particulate fractions in mouse keratinocytes. Furthermore, the PKC-specific inhibitor, GF109203X, increased IGF-1-induced phospho-Ser-473-Akt and Akt kinase activity and enhanced IGF-1 protection from UVC-induced apoptosis. Selective activation of PKC by 12-O-tetradecanoylphorbol-13-acetate (TPA) reduced phospho-Ser-473-Akt, suggesting that activation of PKC inhibits Akt activity. TPA also attenuated IGF-1 and epidermal growth factor-induced phospho-Ser-473-Akt, reduced Akt kinase activity, and blocked IGF-1 protection from UVC-induced apoptosis. The inhibition of Akt activity by TPA was reduced by inhibitors of protein phosphatase 2A, and TPA stimulated the association of phosphatase 2A with Akt. Individual PKC isoforms were overexpressed in cultured keratinocytes by transduction with adenoviral vectors or inhibited with PKC-selective inhibitors. These studies indicated that PKCdelta and PKCepsilon were selectively potent at causing dephosphorylation of Akt and modifying cell survival, whereas PKCalpha enhanced phosphorylation of Akt on Ser-473. Our results suggested that activation of PKCdelta and PKCepsilon provide a negative regulation for Akt phosphorylation and kinase activity in mouse keratinocytes and serve as modulators of cell survival pathways in response to external stimuli.

Signal transduction pathways involved in oxidative stress-induced intestinal epithelial cell apoptosis

Necrotizing enterocolitis (NEC) is a devastating inflammatory condition of the gut that occurs in premature infants. Ischemia-reperfusion gut injury with production of reactive oxygen species (ROS) is thought to contribute to NEC; the exact cellular mechanisms involved are largely unknown. The purpose of this study was to determine the intracellular signaling transduction pathways involved in oxidative stress-induced intestinal epithelial cell apoptosis. H2O2 treatment resulted in rat intestinal epithelial cell apoptosis in a dose- and time-dependent manner; the caspase inhibitor, zVAD-fmk, blocked this response. Western blotting was performed to determine phosphorylation of kinases and ELISA was used to assess DNA fragmentation, as a measure of apoptosis. A rapid increase in phosphorylation of extracellular signal-related kinase (ERK)1/2, c-Jun N-terminal kinase (JNK)1/2, and Akt was noted. Inhibition of ERK and JNK decreased H2O2-induced apoptosis. Additionally, inhibition of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3-K) attenuated and enhanced H2O2-mediated apoptosis and mitochondrial membrane potential decrease, respectively. Furthermore, activation of PKC reduced the Akt phosphorylation, whereas inhibition of PKC attenuated H2O2-mediated activation of caspase-3 and enhanced the H2O2-induced Akt phosphorylation. This study shows that activation of multiple signaling transduction pathways occurs during oxidative stress-induced intestinal epithelial cell injury. In contrast to ERK, JNK, and PKC, PI3-K/Akt may play an important role as a protective cellular signaling pathway during this process.

Human papillomavirus can escape immune recognition through Langerhans cell phosphoinositide 3-kinase activation

Human papillomavirus (HPV) infection of cervical epithelium is linked to the generation of cervical cancer. Although most women infected with HPV clear their lesions, the long latency period from infection to resolution indicates that HPV evolved immune escape mechanisms. Dendritic cells, which are targeted by vaccination procedures, incubated with HPV virus-like particles induce an HPV-specific immune response. Langerhans cells (LC), which are located at the sites of primary infection, do not induce a response implicating the targeting of LC as an immune escape mechanism used by HPV. LC incubated with HPV virus-like particles up-regulate the phosphoinositide 3-kinase (PI3-K) pathway and down-regulate MAPK pathways. With the inhibition of PI3-K and incubation with HPV virus-like particles, LC initiate a potent HPV-specific response. PI3-K activation in LC defines a novel escape mechanism used by HPV, and PI3-K inhibition may serve as an effective clinical target to enhance HPV immunity.

T Cell Leukemia-1 Modulates TCR Signal Strength and IFN-γ Levels through Phosphatidylinositol 3-Kinase and Protein Kinase C Pathway Activation

A signaling role for T cell leukemia-1 (TCL1) during T cell development or in premalignant T cell expansions and mature T cell tumors is unknown. In this study, TCL1 is shown to regulate the growth and survival of peripheral T cells but not precursor thymocytes. Proliferation is increased by TCL1-induced lowering of the TCR threshold for CD4(+) and CD8(+) T cell activation through both PI3K-Akt and protein kinase C-MAPK-ERK signaling pathways. This effect is submaximal as CD28 costimulation coupled to TCL1 expression additively accelerates dose-dependent T cell growth. In addition to its role in T cell proliferation, TCL1 also increases IFN-gamma levels from Th1-differentiated T cells, an effect that may provide a survival advantage during premalignant T cell expansions and in clonal T cell tumors. Combined, these data indicate a role for TCL1 control of growth and effector T cell functions, paralleling features provided by TCR-CD28 costimulation. These results also provide a more detailed mechanism for TCL1-augmented signaling and help explain the delayed occurrence of mature T cell expansions and leukemias despite tumorigenic TCL1 dysregulation that begins in early thymocytes.

A comprehensive pathway map of epidermal growth factor receptor signaling

The epidermal growth factor receptor (EGFR) signaling pathway is one of the most important pathways that regulate growth, survival, proliferation, and differentiation in mammalian cells. Reflecting this importance, it is one of the best-investigated signaling systems, both experimentally and computationally, and several computational models have been developed for dynamic analysis. A map of molecular interactions of the EGFR signaling system is a valuable resource for research in this area. In this paper, we present a comprehensive pathway map of EGFR signaling and other related pathways. The map reveals that the overall architecture of the pathway is a bow-tie (or hourglass) structure with several feedback loops. The map is created using CellDesigner software that enables us to graphically represent interactions using a well-defined and consistent graphical notation, and to store it in Systems Biology Markup Language (SBML).

Effects of FSH and 17beta-estradiol on the transactivation of estrogen-regulated promoters and cell proliferation in L cells

In the present study, we analyzed human follicle-stimulating hormone (FSH)-induced cell proliferation and transactivation of estrogen-sensitive reporter genes-in L cells stably expressing the human FSH receptor [L-(hFSHR(+)) cells]. In order to dissect the signaling pathways involved in this process, L-(hFSHR(+)) cells were transiently transfected with either the 3X-ERE-TAT-Luc or the ERE-VitA2-TK-CAT reporter genes and treated with FSH or PKA activators (cholera toxin, forskolin and 8-Br-cAMP) in the presence or absence of various kinase inhibitors. We found that FSH and all PKA activators, specifically induced transactivation of both reporter genes. Transactivation of estrogen-sensitive genes by FSH or PKA activators were blocked (approximately 90%) by H89 (PKA inhibitor) and LY294002 but not by Wortmannin (PI3-K inhibitors), 4-OH-tamoxifen, ICI182,780 or SB203580 (p38 MAPK inhibitor); PD98059 (ERK1/2 inhibitor) partially (approximately 30%) blocked the FSH-mediated effect. The combination of FSH and estradiol resulted in a synergistic effect on transactivation as well as on cell proliferation, and this enhancement was attenuated by antiestrogens. We additionally analyzed the participation of the coactivators SRC-1 and cAMP response element binding protein (CREB)-binding protein (CBP) in FSH-evoked estrogen receptor (ER)-dependent transactivation; we found that CBP but not SRC-1 potentiated FSH-induced transcriptional activation of both ER-sensitive reporters, being this effect stronger on the ERE-VitA2-TK-CAT than on the 3X-ERE-TAT-Luc reporter. Thus, in L-(hFSHR(+)) cells FSH induces transcriptional activation of estrogen-sensitive genes through an A-kinase-triggered signaling pathway, using also to a lesser extent the ERK1/2 and p38 pathways. PI3-K is not apparently involved in this FSH-mediated process since LY294002, but not Wortmannin, specifically binds ERs and completely blocks estrogen action. Presumably, CBP cooperates with the ER on genes that contain estrogen responsive elements through mechanisms involving the participation of other proteins and/or basal transcription factors (e.g. CREB), which in turn mediate the transcriptional response of estrogen-sensitive reporter genes to FSH stimulation.

Homocysteine induces cell cycle G1 arrest in endothelial cells through the PI3K/Akt/FOXO signaling pathway

OBJECTIVE

High levels of homocysteine (Hcy) induce a sustained injury on arterial endothelial cells, which accelerates the development of thrombosis and atherosclerosis. Hcy specifically inhibits the growth of endothelial cells. The present study investigated the signaling pathways underlying this cell-cycle effect.

METHODS

Human umbilical venous endothelial cells were treated with Hcy, and/or LY294002, okadaic acid, peroxovanadate (PV), antisense Akt, phosphorylation of Akt and FKHRL1 proteins. p27(kip1) protein levels were measured with Western blotting, and Akt kinase activity and cell cycle were measured with immunoprecipitation and flow cytometry, respectively.

RESULTS

We demonstrate that Hcy induces dephosphorylation of Akt and FKHRL1 and upregulates the cyclin-dependent kinase inhibitors p27(kip1) in a time- and dose-dependent manner. Phosphatidylinositol-3 kinase (PI3K) activator PV and phosphatase 2A inhibitor okadaic acid could reverse it, which suggests it was dependent on PI3K activity. Moreover, Hcy induces cell cycle G1 phase arrest prevented by pretreatment with PV and okadaic acid. Transfection with specific antisense oligonucleotides to Akt further proves the observations.

CONCLUSIONS

The studies implied that a novel signaling pathway, PI3K/Akt/FOXO, might play an important role in mediating cell cycle G1 arrest in endothelial cells.

Functional proteomic analysis of a three-tier PKCepsilon-Akt-eNOS signaling module in cardiac protection

Cardiac protective signaling networks have been shown to involve PKCepsilon. However, the molecular mechanisms by which PKCepsilon interacts with other members of these networks to form task-specific modules remain unknown. Among 93 different PKCepsilon-associated proteins that have been identified, Akt and endothelial nitric oxide (NO) synthase (eNOS) are of importance because of their independent abilities to promote cell survival and prevent cell death. The simultaneous association of PKCepsilon, Akt, and eNOS has not been examined, and, in particular, the formation of a module containing these three proteins and the role of such a module in the regulation of NO production and cardiac protection are unknown. The present study was undertaken to determine whether these molecules form a signaling module and, thereby, play a collective role in cardiac signaling. Using recombinant proteins in vitro and PKCepsilon transgenic mouse hearts, we demonstrate the following: 1) PKCepsilon, Akt, and eNOS interact and form signaling modules in vitro and in the mouse heart. Activation of either PKCepsilon or Akt enhances the formation of PKCepsilon-Akt-eNOS signaling modules. 2) PKCepsilon directly phosphorylates and enhances activation of Akt in vitro, and PKCepsilon activation increases phosphorylation and activation of Akt in PKCepsilon transgenic mouse hearts. 3) PKCepsilon directly phosphorylates eNOS in vitro, and this phosphorylation enhances eNOS activity. Activation of PKCepsilon in vivo increased phosphorylation of eNOS at Ser(1177), indicating eNOS activation. This study characterizes, for the first time, the physical, as well as functional, coupling of PKCepsilon, Akt, and eNOS in the heart and implicates these PKCepsilon-Akt-eNOS signaling modules as critical signaling elements during PKCepsilon-induced cardiac protection.

The luteinizing hormone-releasing hormone inhibits the anti-apoptotic activity of insulin-like growth factor-1 in pituitary alphaT3 cells by protein kinase Calpha-mediated negative regulation of Akt

The luteinizing hormone-releasing hormone (LHRH) receptor is a G protein-coupled receptor involved in the synthesis and release of pituitary gonadotropins and in the proliferation and apoptosis of pituitary cells. Insulin-like growth factor-1 receptor (IGF-1R) is a tyrosine kinase receptor that has a mitogenic effect on pituitary cells. In this study, we used the alphaT3 gonadotrope cell line as a model to characterize the IGF-1R signaling pathways and to investigate whether this receptor interacts with the LHRH cascade. We found that IGF-1 activated the IGF-1R, insulin receptor substrate (IRS)-1, phosphatidylinositol 3-kinase, and Akt in a time-dependent manner in alphaT3 cells. The MAPK (ERK1/2, p38, and JNK) pathways were only weakly activated by IGF-1. In contrast, LHRH strongly stimulated the MAPK pathways but had no effect on Akt activation. Cotreatment with IGF-1 and LHRH had various effects on these signaling pathways. 1) It strongly increased IGF-1-induced tyrosine phosphorylation of IRS-1 and IRS-1-associated phosphatidylinositol 3-kinase through activation of the epidermal growth factor receptor. 2) It had an additive effect on ERK1/2 activation without modifying the phosphorylation of p38 and JNK1/2. 3) It strongly reduced IGF-1 activation of Akt. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays and cell cycle analysis revealed that, in addition to having an additive effect on ERK1/2 activation, cotreatment with IGF-1 and LHRH also had an additive effect on cell proliferation. The LHRH-induced inhibition of Akt stimulated by IGF-1 was completely blocked by Safingol, a protein kinase C (PKC) alpha-specific inhibitor, and by a dominant negative form of PKCalpha. Finally, we showed that the inhibitory effect of LHRH on IGF-1-induced PKCalpha-mediated Akt activation was associated with a marked reduction in Bad phosphorylation and a substantial decrease in the ability of IGF-1 to rescue alphaT3 cells from apoptosis induced by serum starvation. Our results demonstrate for the first time that several interactions take place between IGF-1 and LHRH receptors in gonadotrope cells.

Inhibition of HMC-1 mast cell proliferation by vitamin E: involvement of the protein kinase B pathway

The effects of four natural tocopherols on the proliferation and signaling pathways were examined in the human mastocytoma cell line (HMC-1). The four tocopherols inhibited HMC-1 cell proliferation with different potency (delta > alpha = gamma > beta). Growth inhibition correlated with the reduction of PKB (protein kinase B) phosphorylation by the different tocopherols. The reduction of PKB phosphorylation led to a decrease of its activity, as judged from a parallel reduction of GSKalpha/beta phosphorylation. The translocation of PKB to the membrane, as a response to receptor stimulation by NGFbeta, is also prevented by treatment with tocopherols. In the presence of PKC or PP2A inhibitors, the reduction of PKB phosphorylation by tocopherols was still observed, thus excluding the direct involvement of these enzymes. Other pathways, such as the Ras-stimulated ERK1/2 (extracellular signal responsive kinase) pathway, were not affected by tocopherol treatment. The tocopherols did not significantly change oxidative stress in HMC-1 cells, suggesting that the observed effects are not the result of a general reduction of oxidative stress. Thus, the tocopherols interfere with PKB phosphorylation and reduce proliferation of HMC-1 cells, possibly by modulating either phosphatidylinositol 3-kinase, a kinase phosphorylating PKB (PDK1/2), or a phosphatase that dephosphorylates it. Inhibition of proliferation and PKB signaling in HMC-1 cells by vitamin E suggests a role in preventing diseases with mast cell involvement, such as allergies, atherosclerosis, and tumorigenesis.

Protein kinase C modulates negatively the hepatocyte growth factor-induced migration, integrin expression and phosphatidylinositol 3-kinase activation

Previously, we reported that, in hepatocyte growth factor (HGF)-induced HepG2 cells, protein kinase C (PKC) decreased the duration of intensive Erk1/Erk2 MAP kinase activation. This study shows that the inhibition of PKC enhanced significantly the HGF-induced integrin expression. Beside the prolonged activation of Erk1/Erk2, the activity of phosphatidylinositol 3-kinase (PI 3K) was required for growth factor-induced integrin expression. PI 3-kinase was activated to a higher extent in response to HGF than to epidermal growth factor (EGF), though the activation was transient in both cases. In EGF-induced cells, PI 3K activation was terminated by the loss of phosphotyrosine docking sites for PI 3K. To the contrary, the decrease of PI 3K activation, which followed the HGF-induced increase was not accompanied by the loss of phosphotyrosine docking sites and was prevented by the inhibition of PKC. The negative modulator effects of PKC on integrin expression and PI 3-kinase activation correlated with its ability to limit the HGF-induced motogen response.

A proline-rich motif in the C terminus of Akt contributes to its localization in the immunological synapse

The serine/threonine kinases of the Akt/protein kinase B family are regulated in part by recruitment to the plasma membrane, which is accomplished by the binding of an N-terminal PH domain to the phosphatidylinositol 3-kinase products phosphoinositol 3,4,5-trisphosphate and phosphoinositol 3,4-bisphosphate. We have examined Akt localization in a murine T cell clone (D10) before and after stimulation by APC/Ag, and we found that whereas the pleckstrin homology domain is required for plasma membrane recruitment of Akt upon T cell activation, the C terminus of the kinase restricts its cellular localization to the immunologic synapse formed at the site of T cell/APC contact. A recently described proline-rich motif in this region appears to be important for proper localization of full-length Akt. Moreover, a form of Akt in which this motif was mutated acts as a potent dominant negative construct to block T cell activation. Therefore, multiple mechanisms are involved in the proper targeting of Akt during the early events of T cell activation.