TRAIL conjugated to nanoparticles exhibits increased anti-tumor activities in glioma cells and glioma stem cells in vitro and in vivo.

Glioblastomas (GBM) are characterized by resistance to chemotherapy and radiotherapy, and therefore, alternative therapeutic approaches are needed. TRAIL induces apoptosis in cancer but not in normal cells and is considered to be a promising anti-tumor agent. However, its short in vivo half-life and lack of efficient administration modes are serious impediments to its therapeutic efficacy. Nanoparticles (NP) have been used as effective delivery tools for various anticancer drugs. TRAIL was conjugated to magnetic ferric oxide NP by binding the TRAIL primary amino groups to activated double bonds on the surface of the NP. The effect of NP-TRAIL was examined on the apoptosis of glioma cells and self-renewal of glioma stem cells (GSCs). In addition, the ability of the NP-TRAIL to track U251 cell-derived glioma xenografts and to affect cell apoptosis, tumor volume, and survival among xenografted rats was also examined. Conjugation of TRAIL to NP increased its apoptotic activity against different human glioma cells and GSCs, as compared with free recombinant TRAIL. Combined treatment with NP-TRAIL and γ-radiation or bortezomib sensitized TRAIL-resistant GSCs to NP-TRAIL. Using rhodamine-labeled NP and U251 glioma cell-derived xenografts, we demonstrated that the NP-TRAIL were found in the tumor site and induced a significant increase in glioma cell apoptosis, a decrease in tumor volume, and increased animal survival. In summary, conjugation of TRAIL to NP increased its apoptotic activity both in vitro and in vivo. Therefore, NP-TRAIL represents a targeted anticancer agent with more efficient action for the treatment of GBM and the eradication of GSCs.

The regulatory domain of protein kinase C delta positively regulates insulin receptor signaling.

Protein kinase C delta (PKCdelta) is induced by insulin to rapidly associate with insulin receptor (IR) and upregulates insulin signaling. We utilized specific JM and CT receptor domains and chimeras of PKCalpha and PKCdelta regulatory and catalytic domains to elucidate which components of PKCdelta are responsible for positive regulatory effects of PKCdelta on IR signaling. Studies were performed on L6 and L8 skeletal muscle myoblasts and myotubes. PKCdelta was preferentially bound to the JM domain of IR, and insulin stimulation increased this binding. Both PKCdelta/alpha and PKCalpha/delta chimeras (regulatory/catalytic) were bound preferentially to the JM but not to the CT domain of IR. Although IR-PKCdelta binding was higher in cells expressing either the PKCdelta/alpha or PKCalpha/delta chimera than in control cells, upregulation of IR signaling was observed only in PKCdelta/alpha cells. Thus, in response to insulin increases in tyrosine phosphorylation of IR and insulin receptor substrate-1, downstream signaling to protein kinase B and glycogen synthase kinase 3 (GSK3) and glucose uptake were greater in cells overexpressing PKCdelta/alpha and the PKCdelta/delta domains than in cells expressing the PKCalpha/delta domains. Basal binding of Src to PKCdelta was higher in both PKCdelta/alpha- and PKCalpha/delta-expressing cells compared to control. Binding of Src to IR was decreased in PKCalpha/delta cells but remained elevated in the PKCdelta/alpha cells in response to insulin. Finally, insulin increased Src activity in PKCdelta/alpha-expressing cells but decreased it in PKCalpha/delta-expressing cells. Thus, the regulatory domain of PKCdelta via interaction with Src appears to determine the role of PKCdelta as a positive regulator of IR signaling in skeletal muscle.

Phosphorylation of protein kinase Cdelta on distinct tyrosine residues induces sustained activation of Erk1/2 via down-regulation of MKP-1: role in the apoptotic effect of etoposide.

The mechanism underlying the important role of protein kinase Cdelta (PKCdelta) in the apoptotic effect of etoposide in glioma cells is incompletely understood. Here, we examined the role of PKCdelta in the activation of Erk1/2 by etoposide. We found that etoposide induced persistent activation of Erk1/2 and nuclear translocation of phospho-Erk1/2. MEK1 inhibitors decreased the apoptotic effect of etoposide, whereas inhibitors of p38 and JNK did not. The activation of Erk1/2 by etoposide was downstream of PKCdelta since the phosphorylation of Erk1/2 was inhibited by a PKCdelta-KD mutant and PKCdelta small interfering RNA. We recently reported that phosphorylation of PKCdelta on tyrosines 64 and 187 was essential for the apoptotic effect of etoposide. Using PKCdeltatyrosine mutants, we found that the phosphorylation of PKCdeltaon these tyrosine residues, but not on tyrosine 155, was also essential for the activation of Erk1/2 by etoposide. In contrast, nuclear translocation of PKCdelta was independent of its tyrosine phosphorylation and not necessary for the phosphorylation of Erk1/2. Etoposide induced down-regulation of kinase phosphatase-1 (MKP-1), which correlated with persistent phosphorylation of Erk1/2 and was dependent on the tyrosine phosphorylation of PKCdelta. Moreover, silencing of MKP-1 increased the phosphorylation of Erk1/2 and the apoptotic effect of etoposide. Etoposide induced polyubiquitylation and degradation of MKP-1 that was dependent on PKCdelta and on its tyrosine phosphorylation. These results indicate that distinct phosphorylation of PKCdeltaon tyrosines 64 and 187 specifically activates the Erk1/2 pathway by the down-regulation of MKP-1, resulting in the persistent phosphorylation of Erk1/2 and cell apoptosis.

The localization of protein kinase Cdelta in different subcellular sites affects its proapoptotic and antiapoptotic functions and the activation of distinct downstream signaling pathways.

Protein kinase Cdelta (PKCdelta) regulates cell apoptosis and survival in diverse cellular systems. PKCdelta translocates to different subcellular sites in response to apoptotic stimuli; however, the role of its subcellular localization in its proapoptotic and antiapoptotic functions is just beginning to be understood. Here, we used a PKCdelta constitutively active mutant targeted to the cytosol, nucleus, mitochondria, and endoplasmic reticulum (ER) and examined whether the subcellular localization of PKCdelta affects its apoptotic and survival functions. PKCdelta-Cyto, PKCdelta-Mito, and PKCdelta-Nuc induced cell apoptosis, whereas no apoptosis was observed with the PKCdelta-ER. PKCdelta-Cyto and PKCdelta-Mito underwent cleavage, whereas no cleavage was observed in the PKCdelta-Nuc and PKCdelta-ER. Similarly, caspase-3 activity was increased in cells overexpressing PKCdelta-Cyto and PKCdelta-Mito. In contrast to the apoptotic effects of the PKCdelta-Cyto, PKCdelta-Mito, and PKCdelta-Nuc, the PKCdelta-ER protected the cells from tumor necrosis factor-related apoptosis-inducing ligand-induced and etoposide-induced apoptosis. Moreover, overexpression of a PKCdelta kinase-dead mutant targeted to the ER abrogated the protective effect of the endogenous PKCdelta and increased tumor necrosis factor-related apoptosis-inducing ligand-induced apoptosis. The localization of PKCdelta differentially affected the activation of downstream signaling pathways. PKCdelta-Cyto increased the phosphorylation of p38 and decreased the phosphorylation of AKT and the expression of X-linked inhibitor of apoptosis protein, whereas PKCdelta-Nuc increased c-Jun NH(2)-terminal kinase phosphorylation. Moreover, p38 phosphorylation and the decrease in X-linked inhibitor of apoptosis protein expression played a role in the apoptotic effect of PKCdelta-Cyto, whereas c-Jun NH(2)-terminal kinase activation mediated the apoptotic effect of PKCdelta-Nuc. Our results indicate that the subcellular localization of PKCdelta plays important roles in its proapoptotic and antiapoptotic functions and in the activation of downstream signaling pathways.

PKCepsilon induces astrocytic differentiation of multipotential neural precursor cells.

In this study, we examined the role of PKC in the differentiation of multipotential neural precursor cells (NPCs). We found that the NPCs expressed PKCalpha,beta2,delta,epsilon,zeta and low levels of PKCgamma. The PKC activator, PMA, selectively increased the number of astrocytes, whereas it decreased the generation of neurons and oligodendrocytes. Similarly, overexpression of PKCepsilon increased the differentiation of astrocytes and a PKCepsilonKD mutant abolished PMA effect. PMA phosphorylates PKCepsilon on serine 729. Using a PKCepsilonS729A mutant, we found that the phosphorylation of PKCepsilon on serine 729 was essential for the differentiation of astrocytes induced by PMA. To delineate the mechanisms involved in PMA and PKCepsilon effects, we examined the expression of Notch1, which has been associated with astrocytic differentiation. We found that PMA and PKCepsilon induced a large increase in Notch1 expression and the PKCepsilonS729A mutant abolished PMA effect. Moreover, the PKCepsilonS729A mutant also inhibited the effect of CNTF on astrocytic differentiation and Notch 1 expression. Finally, Notch1 mediated the effect of PMA on astrocytic differentiation, since the gamma-secretase inhibitor L-685,458, and Notch1 silencing abolished PMA effect. Our data suggest an important role of PKCepsilon in astrocytic differentiation and implicate Notch1 as a possible mediator of this effect.

Tyrosine 311 is phosphorylated by c-Abl and promotes the apoptotic effect of PKCdelta in glioma cells.

In this study we characterized the phosphorylation of tyrosine 311 and its role in the apoptotic function of PKCdelta in glioma cells. We found that c-Abl phosphorylated PKCdelta on tyrosine 311 in response to H2O2 and that this phosphorylation contributed to the apoptotic effect of H2O2. In contrast, Src, Lyn, and Yes were not involved in the phosphorylation of tyrosine 311 by H2O2. A phosphomimetic PKCdelta mutant, in which tyrosine 311 was mutated to glutamic acid (PKCdeltaY311E), induced a large degree of cell apoptosis. Overexpression of the PKCdeltaY311E mutant induced the phosphorylation of p38 and inhibition of p38 abolished the apoptotic effect of the PKCdelta mutant. These results suggest an important role of tyrosine 311 in the apoptotic function of PKCdelta and implicate c-Abl as the kinase that phosphorylates this tyrosine.

Protein kinase C-epsilon regulates the apoptosis and survival of glioma cells.

In this study, we examined the role of protein kinase C (PKC)-epsilon in the apoptosis and survival of glioma cells using tumor necrosis factor-related apoptosis inducing ligand (TRAIL)-stimulated cells and silencing of PKCepsilon expression. Treatment of glioma cells with TRAIL induced activation, caspase-dependent cleavage, and down-regulation of PKCepsilon within 3 to 5 hours of treatment. Overexpression of PKCepsilon inhibited the apoptosis induced by TRAIL, acting downstream of caspase 8 and upstream of Bid cleavage and cytochrome c release from the mitochondria. A caspase-resistant PKCepsilon mutant (D383A) was more protective than PKCepsilon, suggesting that both the cleavage of PKCepsilon and its down-regulation contributed to the apoptotic effect of TRAIL. To further study the role of PKCepsilon in glioma cell apoptosis, we employed short interfering RNAs directed against the mRNA of PKCepsilon and found that silencing of PKCepsilon expression induced apoptosis of various glioma cell lines and primary glioma cultures. To delineate the molecular mechanisms involved in the apoptosis induced by silencing of PKCepsilon, we examined the expression and phosphorylation of various apoptosis-related proteins. We found that knockdown of PKCepsilon did not affect the expression of Bcl2 and Bax or the phosphorylation and expression of Erk1/2, c-Jun-NH2-kinase, p38, or STAT, whereas it selectively reduced the expression of AKT. Similarly, TRAIL reduced the expression of AKT in glioma cells and this decrease was abolished in cells overexpressing PKCepsilon. Our results suggest that the cleavage of PKCepsilon and its down-regulation play important roles in the apoptotic effect of TRAIL. Moreover, PKCepsilon regulates AKT expression and is essential for the survival of glioma cells.

Roles of tyrosine phosphorylation and cleavage of protein kinase Cdelta in its protective effect against tumor necrosis factor-related apoptosis inducing ligand-induced apoptosis.

Protein kinase Cdelta (PKCdelta) regulates cell apoptosis in a cell- and stimulus-specific manner. Here, we studied the role of PKCdelta in the apoptotic effect of TRAIL in glioma cells. We found that transfection of the cells with a PKCdelta kinase-dead mutant (K376R) or with a small interfering RNA targeting the PKCdelta mRNA increased the apoptotic effect of tumor necrosis factor-related apoptosis inducing ligand (TRAIL), whereas overexpression of PKCdelta decreased it. PKCdelta acted downstream of caspase 8 and upstream of cytochrome c release from the mitochondria. TRAIL induced cleavage of PKCdelta within 2-3 h of treatment, which was abolished by caspase 3, 8, and 9 inhibitors. The cleavage of PKCdelta was essential for its protective effect because overexpression of a caspase-resistant mutant (PKCdeltaD327A) did not protect glioma cells from TRAIL-induced apoptosis but rather increased it. TRAIL induced translocation of PKCdelta to the perinuclear region and the endoplasmic reticulum and phosphorylation of PKCdelta on tyrosine 155. Using a PKCdeltaY155F mutant, we found that the phosphorylation of PKCdelta on tyrosine 155 was essential for the cleavage of PKCdelta in response to TRAIL and for its translocation to the endoplasmic reticulum. In addition, phosphorylation of PKCdelta on tyrosine 155 was necessary for the activation of AKT in response to TRAIL. Our results indicate that PKCdelta protects glioma cells from the apoptosis induced by TRAIL and implicate the phosphorylation of PKCdelta on tyrosine 155 and its cleavage as essential factors in the anti-apoptotic effect of PKCdelta.