H2O2 is required for UVB-induced EGF receptor and downstream signaling pathway activation

Ultraviolet radiation (UVR)-induced receptor phosphorylation is increasingly recognized as a widely occurring phenomenon. However, the mechanisms, mediators, and sequence of events involved in this process remain ill-defined. We have recently shown that exposure of human keratinocytes to physiologic doses of ultraviolet B radiation (UVB) activates epidermal growth factor receptor (EGFR)/extracellular-regulated kinase 1 and 2 (ERK1/2), and p38 signaling pathways via reactive oxygen species. Here we demonstrate that UVB exposure increased intra- and extracellular H2O2 production rapidly in a time-dependent manner. An EGFR-specific monoclonal antibody abrogated EGFR autophosphorylation and markedly decreased the phosphorylation of ERK1/2 whereas p38 activation was unaffected. Overexpression of catalase strongly inhibited UVB-induced EGFR/ERK1/2 pathway activation. These findings establish the sequence of events after UVB irradiation: (i) H2O2 generation, (ii) EGFR phosphorylation, and (iii) ERK activation. Our results identify UVB-induced H2O2 as a second messenger that is required for EGFR and dependent downstream signaling pathways activation.     

Vitamin E analog modulates UVB-induced signaling pathway activation and enhances cell survival

We have recently shown that exposure of human keratinocytes to physiologic doses of ultraviolet B (UVB) activates epidermal growth factor receptor (EGFR)/extracellular-regulated kinases 1 and 2 (ERK1/2) and p38 signaling pathways via reactive oxygen species, an effect that can be modulated by antioxidants. Trolox, a water-soluble vitamin E analog, is among the antioxidants that are currently being investigated for their preventive and protective potential against harmful effects of UV radiation to the skin. We found that Trolox inhibits both basal and UVB-induced intracellular H(2)O(2) generation in primary keratinocytes in a concentration-dependent manner. Trolox did not significantly affect UVB-induced phosphorylation of EGFR. Stronger inhibition was observed for ERK1/2 activation at lower, and for p38 activation at higher, concentrations of Trolox added to cells before exposure to UVB. Similarly different effects were found with regard to length of pretreatment with Trolox before UVB exposure-increasing inhibition for ERK1/2 activation at shorter, and for p38 activation at longer, pretreatment intervals. UVB-induced c-jun-N-terminal kinase activation was potently suppressed by Trolox. Also, increasing the pretreatment time of Trolox decreased the rate of cell death following UVB. In conclusion, UVB-induced signaling pathway activation is differentially modulated by Trolox. Further investigation into the time-dependent biologic activation of Trolox and its metabolic products, and modulation of signal transduction with cell outcome should facilitate development of rational strategies for pharmacologic applications.     

ATP-mediated Activation of the NADPH Oxidase DUOX1 Mediates Airway Epithelial Responses to Bacterial Stimuli

Activation of the NADPH oxidase homolog dual oxidase 1 (DUOX1) within the airway epithelium represents a key mechanism of innate airway host defense, through enhanced production of H₂O₂, which mediates cellular signaling pathways that regulate the production of various inflammatory mediators. Production of the CXC chemokine interleukin (IL)-8/CXCL8 forms a common epithelial response to many diverse stimuli, including bacterial and viral triggers, environmental oxidants, and other biological mediators, suggesting the potential involvement of a common signaling pathway that may involve DUOX1-dependent H₂O₂ production. Following previous reports showing that DUOX1 is activated by extracellular ATP and purinergic receptor stimulation, this study demonstrates that airway epithelial IL-8 production in response to several bacterial stimuli involves ATP release and DUOX1 activation. ATP-mediated DUOX1 activation resulted in the activation of ERK1/2 and NF-κB pathways, which was associated with epidermal growth factor receptor (EGFR) ligand shedding by ADAM17 (a disintegrin and metalloproteinase-17). Although ATP-mediated ADAM17 activation and IL-8 release were not prevented by extracellular H₂O₂ scavenging by catalase, these responses were attenuated by intracellular scavengers of H₂O₂ or related oxidants, suggesting an intracellular redox signaling mechanism. Both ADAM17 activation and IL-8 release were suppressed by inhibitors of EGFR/ERK1/2 signaling, which can regulate ADAM17 activity by serine/threonine phosphorylation. Collectively, our results indicate that ATP-mediated DUOX1 activation represents a common response mechanism to several environmental stimuli, involving H₂O₂-dependent EGFR/ERK activation, ADAM17 activation, and EGFR ligand shedding, leading to amplified epithelial EGFR activation and IL-8 production.     

Mechanism of hydrogen peroxide-induced keratinocyte migration in a scratch-wound model

Recent studies have shown that low concentrations of H₂O₂ are produced endogenously by nonphagocytes after wounding. We observed that H₂O₂ at such concentrations can stimulate proliferation as well as migration of keratinocytes in a scratch-wound assay. Both wounding and H₂O₂ can induce phosphorylation of ERK1/2 via EGFR, but the activation of ERK1/2 by H₂O₂ is more sustained and can last more than 8 h. Sustained ERK1/2 activation is required for the increased proliferation and migration induced by H₂O₂. The p38 MAPK was also found to be phosphorylated upon treatment with H₂O₂ but it was not required for H₂O₂-induced migration or proliferation. Furthermore, it was observed that there is a cross talk between the ERK1/2 and the p38 pathways whereby inhibition of either pathway can lead to activation of the other. As a result, the motogenic effects of H₂O₂ were further enhanced when p38 was inhibited. Our data are consistent with the view that H₂O₂ may play an important signaling role in wound healing.     

MAPKs and Mst1/Caspase-3 pathways contribute to H2B phosphorylation during UVB-induced apoptosis

Apoptosis is a highly coordinated or programmed cell suicide mechanism in eukaryotes. Histone modification is associated with nuclear events in apoptotic cells. Specifically H2B phosphorylation at serine 14 (Ser14) catalyzed by Mst1 kinase has been linked to chromatin condensation during apoptosis. We report that activation of MAPKs (ERK1/2, JNK1/2 and p38) together with Mst1 and caspase-3 is required for phosphorylation of H2B (Ser14) during ultraviolet B light (UVB)-induced apoptosis. UVB can trigger activation of MAPKs and induce H2B phosphorylation at Ser14 but not acetylation in a time-dependent manner. Inhibition of ERK1/2, JNK1/2 or p38 activity blocked H2B phosphorylation (Ser14). Furthermore, caspase-3 was activated by UVB to regulate Mst1 activity, which phosphorylates H2B at Ser14, leading to chromatin condensation. Full inhibition of caspase-3 activity reduced Mst1 activation and partially inhibited H2B phosphorylation (Ser14), but ERK1/2, JNK1/2 and p38 activities were not affected. Taken together, these data revealed that H2B phosphorylation is regulated by both MAPKs and caspase-3/Mst1 pathways during UVB-induced apoptosis.     

CaMKII knockdown attenuates H2O2-induced phosphorylation of ERK1/2, PKB/Akt, and IGF-1R in vascular smooth muscle cells

We have shown earlier a requirement for Ca²⁺ and calmodulin (CaM) in the H₂O₂-induced activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and protein kinase B (PKB), key mediators of growth-promoting, proliferative, and hypertrophic responses in vascular smooth muscle cells (VSMC). Because the effect of CaM is mediated through CaM-dependent protein kinase II (CaMKII), we have investigated here the potential role of CaMKII in H₂O₂-induced ERK1/2 and PKB phosphorylation by using pharmacological inhibitors of CaM and CaMKII, a CaMKII inhibitor peptide, and siRNA knockdown strategies for CaMKIIα. Calmidazolium and W-7, antagonists of CaM, as well as KN-93, a specific inhibitor of CaMKII, attenuated H₂O₂-induced responses of ERK1/2 and PKB phosphorylation in a dose-dependent fashion. Similar to H₂O₂, calmidazolium and KN-93 also exhibited an inhibitory effect on glucose/glucose oxidase-induced phosphorylation of ERK1/2 and PKB in these cells. Transfection of VSMC with CaMKII autoinhibitory peptide corresponding to the autoinhibitory domain (aa 281–309) of CaMKII and with siRNA of CaMKIIα attenuated the H₂O₂-induced phosphorylation of ERK1/2 and PKB. In addition, calmidazolium and KN-93 blocked H₂O₂-induced Pyk2 and insulin-like growth factor-1 receptor (IGF-1R) phosphorylation. Moreover, treatment of VSMC with CaMKIIα siRNA abolished the H₂O₂-induced IGF-1R phosphorylation. H₂O₂ treatment also induced Thr²⁸⁶ phosphorylation of CaMKII, which was inhibited by both calmidazolium and KN-93. These results demonstrate that CaMKII plays a critical upstream role in mediating the effects of H₂O₂ on ERK1/2, PKB, and IGF-1R phosphorylation.     

Docosahexaenoic acid inhibits UVB-induced activation of NF-κB and expression of COX-2 and NOX-4 in HR-1 hairless mouse skin by blocking MSK1 signaling

Exposure to ultraviolet-B (UVB) radiation induces inflammation and photocarcinogenesis in mammalian skin. Docosahexaenoic acid (DHA), a representative ω-3 polyunsaturated fatty acid, has been reported to possess anti-inflammatory and chemopreventive properties. In the present study, we investigated the molecular mechanisms underlying the inhibitory effects of DHA on UVB-induced inflammation in mouse skin. Our study revealed that topical application of DHA prior to UVB irradiation attenuated the expression of cyclooxygenase-2 (COX-2) and NAD(P)H:oxidase-4 (NOX-4) in hairless mouse skin. DHA pretreatment also attenuated UVB-induced DNA binding of nuclear factor-kappaB (NF-κB) through the inhibition of phosphorylation of IκB kinase-α/β, phosphorylation and degradation of IκBα and nuclear translocation of p50 and p65. In addition, UVB-induced phosphorylation of p65 at the serine 276 residue was significantly inhibited by topical application of DHA. Irradiation with UVB induced phosphorylation of mitogen and stress-activated kinase-1 (MSK1), extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinase, and all these events were attenuated by pretreatment with DHA. Blocking ERK and p38 MAP kinase signaling by U0126 and SB203580, respectively, diminished MSK1 phosphorylation in UVB-irradiated mouse skin. Pretreatment with H-89, a pharmacological inhibitor of MSK1, abrogated UVB-induced activation of NF-κB and the expression of COX-2 and NOX-4 in mouse skin. In conclusion, topically applied DHA inhibits the UVB-induced activation of NF-κB and the expression of COX-2 and NOX-4 by blocking the phosphorylation of MSK1, a kinase downstream of ERK and p38 MAP kinase, in hairless mouse skin.     

Tissue kallikrein induces SH-SY5Y cell proliferation via epidermal growth factor receptor and extracellular signal-regulated kinase1/2 pathway

Tissue kallikrein (TK) is well known to take most of its biological functions through bradykinin receptors. In the present study, we found a novel signaling pathway mediated by TK through epidermal growth factor receptor (EGFR) in human SH-SY5Y cells. We discovered that TK facilitated the activation of EGFR, extracellular signal-regulated kinase (ERK) 1/2 and p38 cascade. Interestingly, not p38 but ERK1/2 phosphorylation was severely compromised in cells depleted of EGFR. Nevertheless, impairment of signaling of ERK1/2 seemed not to be restricted to EGFR phosphorylation. We also observed that TK stimulation could induce SH-SY5Y cell proliferation, which was reduced by EGFR down-regulation or ERK1/2 inhibitor. Overall, our findings provided convincing evidence that TK could mediate cell proliferation via EGFR and ERK1/2 pathway in vitro.     

The relationship between p53/p21/Rb and MAPK signaling pathways in human endometrium-derived stem cells under oxidative stress

Human endometrium-derived mesenchymal stem cells (hMESC) under the sublethal oxidative stress induced by H₂O₂ activate both the p53/p21/Rb and p38/MAPKAPK-2 pathways that are responsible for the induction of hMESC premature senescence (Borodkina et al., 2014). However, the interrelations between the p53/p21/Rb and MAPK signaling pathways, including ERK1/2, p38, and JNK, remain yet unexplored. Here, we used the specific inhibitors—pifithrin-α (PFT), U0126, SB203580, and SP600125 to “switch off” one of the proteins in these cascades and to evaluate the functional status alterations of the rest of the proteins. Each MAPK suppression significantly increased the p53 phosphorylation level, as well as p21 protein expression followed by Rb hypophosphorylation. On the other hand, PFT-induced p53 inhibition enhanced mostly the ERK1/2 activation rather than p38 and JNK. These results suggest the existence of a reciprocal negative regulation between p53- and MAPK-dependent signaling pathways. By analyzing the possible interactions among the members of the MAPK family, we showed that p38 and JNK can function as ERK antagonists: JNK is able to activate ERK, while p38 may block ERK activation. Together, these results demonstrate the existence of complex links between different signaling cascades in stressed hMESC, implicating ERK, p38, and JNK in regulation of premature senescence via the p53/p21/Rb pathway.     

Protein kinase A-mediated CREB phosphorylation is an oxidant-induced survival pathway in alveolar type II cells

Oxidant stress plays a role in the pathogenesis of pulmonary diseases, including fibrotic lung disease and cancer. We previously found that hydrogen peroxide (H₂O₂) initiates an increase in Ca²⁺/cAMP-response element binding protein (CREB) phosphorylation in C10 alveolar type II cells that requires activation of extracellular regulated kinases 1/2 (ERK1/2). Here, we investigated the role of crosstalk between protein kinase A (PKA) and epidermal growth factor receptor (EGFR) in oxidant-induced signaling to ERK1/2 and CREB in C10 cells. Application of H₂O₂ increased nuclear accumulation of PKA, and inhibition of PKA with H89 reduced oxidant-mediated phosphorylation of both CREB and ERK1/2. Single cell measurements of cAMP and redox status, using a FRET-based biosensor and a redox-sensitive GFP, respectively, indicated that H₂O₂ increases production of cAMP that correlates with redox state. Inhibition of EGFR activity decreased both H₂O₂-induced CREB phosphorylation and translocation of PKA to the nucleus, suggesting that crosstalk between PKA and EGFR underlies the oxidant-induced CREB response. Furthermore, knockdown of CREB expression using siRNA led to a decrease in bcl-2 and an increase in oxidant-induced apoptosis. Together these data reveal a novel role for crosstalk between PKA, ERK1/2 and CREB that mediates cell survival during oxidant stress.     

Role of receptor and nonreceptor protein tyrosine kinases in H2O2-induced PKB and ERK1/2 signaling

Excessive generation of reactive oxygen species (ROS) has been implicated in the pathogenesis of many diseases, including atherosclerosis, hypertension, and vascular complications of diabetes. However, the precise mechanisms by which ROS contribute to the development of these diseases are not fully characterized. Hydrogen peroxide (H₂O₂), a ROS, has been shown to activate several signaling protein kinases, such as extracellular signal-regulated kinase (ERK)1/2 and protein kinase B (PKB) in different cell types, notably in vascular smooth muscle cells. Because these pathways regulate cellular mitogenesis, migration, proliferation, survival, and death responses, their aberrant activtion has been suggested to be a potential mechanism of ROS-induced pathologies. The upstream elements responsible for H₂O₂-induced ERK1/2 and PKB activation remain poorly characterized, but a potential role of receptor and nonreceptor protein tyrosine kinases (PTKs) as triggers that initiate such events has been postulated. Therefore, the aim of this review is to highlight the involvement of receptor and nonreceptor PTKs in modulating H₂O₂-induced ERK1/2 and PKB signaling.     

Regulation of EGF-induced ERK/MAPK activation and EGFR internalization by G protein-coupled receptor kinase 2

G protein-coupled receptor kinases （GRKs） mediate agonist-induced phosphorylation and desensitization of various G protein-coupled receptors （GPCRs）. We investigate the role of GRK2 on epidermal growth factor （EGF） receptor signaling, including EGF-induced extracellular signal-regulated kinase and mitogen-activated protein kinase （ERK/MAPK） activation and EGFR internalization. Immunoprecipitation and immunofluorescence experiments show that EGF stimulates GRK2 binding to EGFR complex and GRK2 translocating from cytoplasm to the plasma membrane in human embryonic kidney 293 cells. Western blotting assay shows that EGF-induced ERK/MAPK phosphorylation increases 1.9-fold, 1.1-fold and 1.5fold （P〈0.05） at time point 30, 60 and 120 min, respectively when the cells were transfected with GRK2,suggesting the regulatory role of GRK2 on EGF-induced ERK/MAPK activation. Flow cytometry experiments show that GRK2 overexpression has no effect on EGF-induced EGFR internalization, however, it increases agonist-induced G protein-coupled δ5 opioid receptor internalization by approximately 40% （P〈0.01）. Overall,these data suggest that GRK2 has a regulatory role in EGF-induced ERK/MAPK activation, and that the mechanisms underlying the modulatory role of GRK2 in EGFR and GPCR signaling pathways are somewhat different at least in receptor internalization.     

G Protein betagamma subunits augment UVB-induced apoptosis by stimulating the release of soluble heparin-binding epidermal growth factor from human keratinocytes

UV radiation induces various cellular responses by regulating the activity of many UV-responsive enzymes, including MAPKs. The betagamma subunit of the heterotrimeric GTP-binding protein (Gbetagamma) was found to mediate UV-induced p38 activation via epidermal growth factor receptor (EGFR). However, it is not known how Gbetagamma mediates the UVB-induced activation of EGFR, and thus we undertook this study to elucidate the mechanism. Treatment of HaCaT-immortalized human keratinocytes with conditioned medium obtained from UVB-irradiated cells induced the phosphorylations of EGFR, p38, and ERK but not that of JNK. Blockade of heparin-binding EGF-like growth factor (HB-EGF) by neutralizing antibody or CRM197 toxin inhibited the UVB-induced activations of EGFR, p38, and ERK in normal human epidermal keratinocytes and in HaCaT cells. Treatment with HB-EGF also activated EGFR, p38, and ERK. UVB radiation stimulated the processing of pro-HB-EGF and increased the secretion of soluble HB-EGF in medium, which was quantified by immunoblotting and protein staining. In addition, treatment with CRM179 toxin blocked UV-induced apoptosis, but HB-EGF augmented this apoptosis. Moreover, UVB-induced apoptosis was reduced by inhibiting EGFR or p38. The overexpression of Gbeta(1)gamma(2) increased EGFR-activating activity and soluble HB-EGF content in conditioned medium, but the sequestration of Gbetagamma by the carboxyl terminus of G protein-coupled receptor kinase 2 (GRK2ct) produced the opposite effect. The activation of Src increased UVB-induced, Gbetagamma-mediated HB-EGF secretion, but the inhibition of Src blocked that. Overexpression of Gbetagamma increased UVB-induced apoptosis, and the overexpression of GRK2ct decreased this apoptosis. We conclude that Gbetagamma mediates UVB-induced human keratinocyte apoptosis by augmenting the ectodomain shedding of HB-EGF, which sequentially activates EGFR and p38.     

Sphingosine-1-phosphate promotes mouse melanocyte survival via ERK and Akt activation

In this study, we investigated the signalling pathways induced by ultraviolet B (UVB) and the effects of sphingosine-1-phosphate on UVB-induced apoptosis of mouse melanocytes, Mel-Ab, and observed the cytoprotective effects of sphingosine-1-phosphate on UVB-induced apoptosis. Since sphingosine-1-phosphate is a well-known mitogenic agent, we thought it possible that the mitogenic effect of sphingosine-1-phosphate might contribute to cell survival. However, we found that sphingosine-1-phosphate significantly inhibits DNA synthesis. We next examined the regulation of the three major subfamilies of mitogen-activated protein (MAP) kinases and of the Akt pathway by sphingosine-1-phosphate against UVB-induced apoptosis. UVB irradiation resulted in the remarkable and sustained activation of c-Jun N-terminal kinase (JNK), while p38 MAP kinase was only transiently activated. The basal level of extracellular signal-regulated protein kinase (ERK) phosphorylation decreased 30 min after UVB irradiation, whereas the basal level of Akt phosphorylation was unaffected by UVB. We also found that sphingosine-1-phosphate potently stimulates the phosphorylation of both ERK and Akt, which are involved in the cell survival-signalling cascade. Furthermore, the specific inhibition of the ERK and Akt pathways by PD98059 and LY294002, respectively, restored the cytoprotective effect induced by sphingosine-1-phosphate. On the other hand, the p38 inhibitor SB203580 additively enhanced the cytoprotective effect on sphingosine-1-phosphate. Based on these results, we conclude that the activation of p38 MAP kinase plays an important role in UVB-induced apoptosis, and that sphingosine-1-phosphate probably exert its cytoprotective effect in Mel-Ab cells through ERK and Akt activation.     

Activation of VEGFR-2 signaling in response to moderate dose of ultraviolet B promotes survival of normal human keratinocytes

Mounting evidence indicates that signaling via VEGF receptors (VEGFRs) extends beyond blood vessel formation. Recently, VEGFRs are also found to be constitutively expressed in keratinocytes and epidermal appendages. Here, we show that the expression of VEGFRs (including VEGFR-1, VEGFR-2, and NRP-1) was significantly enhanced by moderate dose of ultraviolet B (UVB) in normal human keratinocytes and epidermis. The elevated expression of VEGFRs by UVB was independent of autocrine stimulation by their natural ligand, VEGF, but mainly mediated through hypoxia and oxidative stress. Moderate dose UVB also promoted tyrosine phosphorylation of VEGFR-1 and VEGFR-2, this effect was again VEGF independent. Both α and δ isoforms of protein kinase C (PKC) were required for UVB-induced phosphorylation of VEGFR-1, but only the δ isoform was required for VEGFR-2 phosphorylation. The phosphorylation of VEGFRs or isoforms of PKC was completely inhibited by PP2, a specific inhibitor for Src family kinases (SFKs), indicating that SFKs are upstream of PKC and VEGFRs. Moderate dose UVB-induced VEGF exerted an anti-apoptotic effect for keratinocytes, whereas high dose UVB-induced VEGF played as an inflammatory factor. Of note, neutralization of VEGFR-2 but not VEGFR-1 exacerbated UVB-induced cell death and reduced survival of keratinocytes. Furthermore, VEGFR-2 neutralization inhibited the activation of ERK1/2 and Akt by UVB, suggesting that VEGFR-2 signaling was involved in the pro-survival mechanism via ERK1/2 and PI3-K/Akt pathway. Taken together, we demonstrate for the first time that VEGFR-2 signaling is activated and promotes survival of keratinocytes under moderate dose of UVB irradiation.     

Inhibition of p38 mitogen-activated protein kinase and phosphatidylinositol 3-kinase decreases UVB-induced activator protein-1 and cyclooxygenase-2 in a SKH-1 hairless mouse model

Activation of activator protein-1 (AP-1) and increased expression of cyclooxygenase-2 (COX-2) have been clearly shown to play a functional role in UVB-induced skin tumor promotion. In this study, we examined UVB-induced signal transduction pathways in SKH-1 mouse epidermis leading to increases in COX-2 expression and AP-1 activity. We observed rapid increases in p38 mitogen-activated protein kinase (MAPK) signaling through activation of p38 MAPK and its downstream target, MAPK activated protein kinase-2. UVB also increased phosphatidylinositol 3-kinase (PI3K) signaling as observed through increases in AKT and GSK-3beta phosphorylation. Activation of the p38 MAPK and PI3K pathways results in the phosphorylation of cyclic AMP-responsive element binding protein, which was also observed in UVB-irradiated SKH-1 mice. Topical treatment with SB202190 (a specific inhibitor of p38 MAPK) or LY294002 (a specific inhibitor of PI3K) significantly decreased UVB-induced AP-1 activation by 84% and 68%, respectively, as well as COX-2 expression. Our data show that in mouse epidermis, UVB activation of the p38 MAPK and PI3K pathways leads to AP-1 activation and COX-2 expression.     

Neuroprotective Effects of Ugonin K on Hydrogen Peroxide-Induced Cell Death in Human Neuroblastoma SH-SY5Y Cells

Oxidative stress plays an important role in the pathological processes of various neurodegenerative diseases. Ugonin K, a flavonoid isolated from the rhizomes of Helminthostachys zeylanica, possesses potent antioxidant property. In this study, we investigate the neuroprotective effects of ugonin K on hydrogen peroxide (H₂O₂)-induced apoptosis in SH-SY5Y cells. Incubation of SH-SY5Y cells with H₂O₂ for 24 h induced cell death measured with MTT assay. Hoechst 33258 staining confirmed that the reduced cell viability by H₂O₂ was due to apoptosis. In addition, H₂O₂ increased the expression of 17-kDa cleaved fragment of caspase-3 which could be reversed by pretreatment with ugonin K. Pretreatment with ugonin K attenuated H₂O₂-induced cell death in a dose-dependent manner. Neuroprotective effect of ugonin K was abolished by ERK and PI3K inhibitors. Pretreatment with JNK kinase and p38 MAPK inhibitors had no effect on ugonin K-mediated protection against H₂O₂-induced apoptosis. Western blotting with anti-phospho-ERK1/2 and anti-phospho-Akt (pS473) antibodies showed that ugonin K increased both ERK1/2 and Akt phosphorylation. These results suggest that ugonin K by activation of ERK1/2 and PI3K/Akt signal pathways protects SH-SY5Y cells from H₂O₂-induced apoptosis.