The cystine/glutamate antiporter xCT is a key regulator of EphA2 S897 phosphorylation under glucose-limited conditions

EphA2, which belongs to the Eph family of receptor tyrosine kinases, is overexpressed in a variety of human cancers. Serine 897 (S897) phosphorylation of EphA2 is known to promote cancer cell migration and proliferation in a ligand-independent manner. In this study, we show that glucose deprivation induces S897 phosphorylation of EphA2 in glioblastoma cells. The phosphorylation requires the activity of the cystine/glutamate antiporter xCT and reactive oxygen species (ROS)-dependent ERK and RSK activation. Furthermore, depletion of EphA2 in glioblastoma cells leads to decreased cell viability under glucose starvation. Our results suggest a role of EphA2 in glioblastoma cell viability under glucose-limited conditions

Role of ferritinophagy in cystine deprivation-induced cell death in glioblastoma cells

Ferroptosis is a form of cell death caused by iron-dependent lipid peroxidation. Cancer cells increase cystine uptake for the synthesis of glutathione (GSH), which is used by glutathione peroxidase 4 to reduce lipid peroxides. Here, we report that cystine deprivation in glioblastoma cells, but not inhibition of GSH synthesis by l-buthionine sulfoximine (BSO), induces ferroptosis. We found that cystine deprivation decreased the protein levels of ferritin heavy chain FTH1, whereas it was increased by BSO treatment. The lysosome inhibitor bafilomycin A1 or deletion of nuclear receptor coactivator 4 (NCOA4) inhibited cystine deprivation-induced decrease in FTH1 protein levels and cell death. In addition, cystine deprivation induced microtubule-associated protein light chain 3 (LC3)-II protein accumulation, suggesting that cystine deprivation induces ferritinophagy. BSO causes cell death when glioblastoma cells are treated with iron inducers, ferrous ammonium sulfate or hemin. On the other hand, cystine deprivation-induced degradation of FTH1 and cell death required glutamine. This study suggests that ferritinophagy, in addition to GSH depletion, plays an important role in cystine deprivation-induced ferroptosis in glioblastoma cells

Semaphorin 4D/Plexin-B1–mediated R-Ras GAP activity inhibits cell migration by regulating β1 integrin activity

Plexins are cell surface receptors for semaphorins and regulate cell migration in many cell types. We recently reported that the semaphorin 4D (Sema4D) receptor Plexin-B1 functions as a GTPase-activating protein (GAP) for R-Ras, a member of Ras family GTPases implicated in regulation of integrin activity and cell migration (Oinuma, I., Y. Ishikawa, H. Katoh, and M. Negishi. 2004. Science. 305:862–865). We characterized the role of R-Ras downstream of Sema4D/Plexin-B1 in cell migration. Activation of Plexin-B1 by Sema4D suppressed the ECM-dependent R-Ras activation, R-Ras–mediated phosphatydylinositol 3-kinase activation, and β1 integrin activation through its R-Ras GAP domain, leading to inhibition of cell migration. In addition, inactivation of R-Ras by overexpression of the R-Ras–specific GAP or knockdown of R-Ras by RNA interference was sufficient for suppressing β1 integrin activation and cell migration in response to the ECM stimulation. Thus, we conclude that R-Ras activity is critical for ECM-mediated β1 integrin activation and cell migration and that inactivation of R-Ras by Sema4D/Plexin-B1–mediated R-Ras GAP activity controls cell migration by modulating the activity of β1 integrins

Tyrosine phosphorylation of SGEF regulates RhoG activity and cell migration

SGEF and Ephexin4 are members of the Ephexin subfamily of RhoGEFs that specifically activate the small GTPase RhoG. It is reported that Ephexin1 and Ephexin5, two well-characterized Ephexin subfamily RhoGEFs, are tyrosine-phosphorylated by Src, and that their phosphorylation affect their activities and functions. In this study, we show that SGEF, but not Ephexin4, is tyrosine-phosphorylated by Src. Tyrosine phosphorylation of SGEF suppresses its interaction with RhoG, the elevation of RhoG activity, and SGEF-mediated promotion of cell migration. We identified tyrosine 530 (Y530), which is located within the Dbl homology domain, as a major phosphorylation site of SGEF by Src, and Y530F mutation blocked the inhibitory effect of Src on SGEF. Taken together, these results suggest that the activity of SGEF is negatively regulated by tyrosine phosphorylation of the DH domain

Tyrosine kinase activity of EphA2 promotes its S897 phosphorylation and glioblastoma cell proliferation

EphA2, a member of the Eph family of receptor tyrosine kinases, has been reported to promote tumor malignancy through phosphorylation of serine 897 (S897). Here, we found that overexpression of wild-type EphA2 induced S897 phosphorylation through ERK activation without growth factors or cytokines and promoted glioblastoma cell proliferation. However, overexpression of a kinase-inactive mutant of EphA2 failed to induce ERK activation, S897 phosphorylation, and promotion of glioblastoma cell proliferation. These data suggest that when overexpressed, EphA2 induces ERK activation through its tyrosine kinase activity, leading to S897 phosphorylation and promotion of glioblastoma cell proliferation. Our findings provide a new insight into how EphA2 mediates glioblastoma progression

Different membrane targeting of prostaglandin EP3 receptor isoforms dependent on their carboxy-terminal tail structures

AbstractMouse prostaglandin EP3 receptor consists of three isoforms, EP3α, β and γ, with different carboxy-terminal tails. To assess the role of their carboxy-terminal tails in membrane targeting, we examined subcellular localization of myc-tagged EP3 isoforms expressed in MDCK cells. Two isoforms, EP3α and EP3β, were localized in the intracellular compartment but not in the plasma membrane, while the EP3γ isoform was found in the lateral plasma membrane and in part in the intracellular compartment. Mutant EP3 receptor lacking the carboxy-terminal tail was localized in the intracellular compartment but not in the plasma membrane. Thus, EP3 isoforms differ in subcellular targeting, and the carboxy-terminal tails play an important role in determination of the membrane targeting of EP3 receptor

Medium-chain fatty acids suppress lipotoxicity-induced hepatic fibrosis via the immunomodulating receptor GPR84

食事性肥満から肝炎発症に関わる制御因子の同定 --中鎖脂肪酸油による予防・GPR84標的NASH治療薬の可能性--. 京都大学プレスリリース. 2023-01-18.Medium-chain triglycerides (MCTs), which consist of medium-chain fatty acids (MCFAs), are unique forms of dietary fat with various health benefits. G protein–coupled 84 (GPR84) acts as a receptor for MCFAs (especially C10:0 and C12:0); however, GPR84 is still considered an orphan receptor, and the nutritional signaling of endogenous and dietary MCFAs via GPR84 remains unclear. Here, we showed that endogenous MCFA-mediated GPR84 signaling protected hepatic functions from diet-induced lipotoxicity. Under high-fat diet (HFD) conditions, GPR84-deficient mice exhibited nonalcoholic steatohepatitis (NASH) and the progression of hepatic fibrosis but not steatosis. With markedly increased hepatic MCFA levels under HFD, GPR84 suppressed lipotoxicity-induced macrophage overactivation. Thus, GPR84 is an immunomodulating receptor that suppresses excessive dietary fat intake–induced toxicity by sensing increases in MCFAs. Additionally, administering MCTs, MCFAs (C10:0 or C12:0, but not C8:0), or GPR84 agonists effectively improved NASH in mouse models. Therefore, exogenous GPR84 stimulation is a potential strategy for treating NASH

Ephexin4 and EphA2 mediate cell migration through a RhoG-dependent mechanism

Ephexin4 is a RhoG-specific guanine nucleotide exchange factor that interacts with the EphA2 receptor in breast cancer cells

Pink-Colored Grape Berry Is the Result of Short Insertion in Intron of Color Regulatory Gene

We report here that pink grape berries were obtained by a short insertion in the intron of the MybA1 gene, a gene that regulates grape berry color. Genetic variation was detected among the MybA1 genes from grapes cultivated worldwide. PCR analysis of the MybA1 gene demonstrated that the size of the MybA1 gene in the red allele differs among grapes. Oriental V. vinifera bearing pink berries has the longest MybA1 gene among grapes, whereas the shortest MybA1 gene was detected in occidental V. vinifera grapes. The nucleotide sequences of the MybA1 genes demonstrated that oriental V. vinifera has two additional gene fragments (44 bp and 111 bp) in the promoter region of the MybA1 gene in the red allele and another 33 bp fragment in the second intron of the MybA1 gene in the red allele. The short insertion in the intron decreased the transcription activity in the model system and retained MybA1 transcripts with unspliced intron in the total RNA. From the experiments using deletion mutants of the 33 bp short insertion, 16 bp of the 3′ end in the insertion is a key structure for a defect in splicing of MybA1 transcripts. Thus, a weakly colored grape berry might be a result of the short insertion in the intron of a color regulatory gene. This is new evidence concerning the molecular mechanism of the fate of grape berry color. These findings are expected to contribute to the further understanding of the color variation in grape berries, which is correlated with the evolutional events occurring in the MybA1 gene of grapes