PKCdelta protects human breast tumor MCF-7 cells against tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis

Rottlerin and genistein inhibit neuroblastoma cell proliferation and invasion through EF2K suppression and related protein pathways

Neuroblastoma is one of the most common solid tumors in children younger than 1 year of age, with poor prognosis and survival rates. Therefore, novel molecular targets and therapeutic strategies are needed to prolong patient survival. For this purpose, we investigated the effects of rottlerin and genistein separately and in combination on neuroblastoma cells (SH-SY5Y, Kelly). First, the effects of rottlerin and genistein were investigated on cell proliferation. Different rottlerin (1-50 µM) and genistein (5-150 µM) doses were used as experimental groups compared to the control (DMSO/vehicle). The IC50 dose was found to be 5 µM for rottlerin and 30 µM for genistein (P < 0.0001). Other analyses, such as colony formation assays, annexin V/propidium iodide staining, matrigel invasion assays, and Western blot analysis, were performed with these doses and their combinations. To assess statistical significance, statistical analysis was conducted using the one-way ANOVA with the post hoc Tukey test. Our results showed that IC50 doses of rottlerin and genistein induced a significant reduction in cell proliferation, colony formation, and invasion in neuroblastoma cells (P < 0.0001). The combination of these doses increased the levels of inhibition of cell proliferation and invasion while decreasing the level of apoptosis (P 0.0001). Furthermore, these agents caused G1-cell cycle arrest in these cells. Our western blot data showed that rottlerin and genistein treatments markedly inhibit elongation factor 2 kinase (EF2K) and other pro-tumorigenic, metastatic proteins in neuroblastoma cells. These agents probably showed their anti-proliferative, anti-metastatic, and pro-apoptotic effects through EF2K downregulation. Our results suggested that rottlerin and genistein have inhibitory effects on cancer cell proliferation, invasion, and cell cycle and induce apoptosis in both cell lines. Combined treatment with rottlerin and genistein may be a viable approach and beneficial to neuroblastoma patients as the combined effect significantly suppresses the above-mentioned pathways.

Multiple subcellular localizations and functions of protein kinase Cδ in liver cancer

Protein kinase Cδ (PKCδ) is a member of the PKC family, and its implications have been reported in various biological and cancerous processes, including cell proliferation, cell death, tumor suppression, and tumor progression. In liver cancer cells, accumulating reports show the bi-functional regulation of PKCδ in cell death and survival. PKCδ function is defined by various factors, such as phosphorylation, catalytic domain cleavage, and subcellular localization. PKCδ has multiple intracellular distribution patterns, ranging from the cytosol to the nucleus. We recently found a unique extracellular localization of PKCδ in liver cancer and its growth factor-like function in liver cancer cells. In this review, we first discuss the structural features of PKCδ and then focus on the functional diversity of PKCδ based on its subcellular localization, such as the nucleus, cell surface, and extracellular space. These findings improve our knowledge of PKCδ involvement in the progression of liver cancer.

Protein kinase D2 contributes to TNF-α-induced epithelial mesenchymal transition and invasion via the PI3K/GSK-3β/β-catenin pathway in hepatocellular carcinoma

Although protein kinase D (PKD) has been shown to contribute to invasion and metastasis in several types of cancer, the role of PKD in the epithelial mesenchymal transition (EMT) of hepatocellular carcinoma (HCC) has remained unclear. We found that PKD2 is up-regulated in HCC and is correlated with the metastasis of HCC. PKD2 positively regulated TNF-α-induced EMT and metastasis of HCC. Mechanistic studies revealed TNF-α-induced PKD2 activation is mediated by the formation of a TNFR1/TRAF2 complex. PKD2 bound directly to the p110α and p85 subunits of PI3K and promoted the PI3K/Akt/GSK-3β signaling cascade to stimulate EMT. In conclusion, our results have uncovered a novel role for the regulation of EMT and suggest inhibition of PKD2 as a potential therapeutic strategy for HCC.

Cross Talk Mechanism among EMT, ROS, and Histone Acetylation in Phorbol Ester-Treated Human Breast Cancer MCF-7 Cells

Epithelial-mesenchymal transition (EMT) plays a pivotal role in the progression of cancer, and some transcription factors including Slug and Snail are known to be involved in EMT processes. It has been well established that the excess production of reactive oxygen species (ROS) and epigenetics such as DNA methylation and histone modifications participate in carcinogenesis; however, the cross talk mechanism among EMT, ROS, and epigenetics remains unclear. In the present study, we demonstrated that the treatment of human breast cancer MCF-7 cells with phorbol ester (TPA), a protein kinase C activator, significantly induced cell proliferation and migration, and these were accompanied by the significant induction of Slug expression. Moreover, the TPA-elicited induction of Slug expression was regulated by histone H3 acetylation and NADPH oxidase (NOX) 2-derived ROS signaling, indicating that ROS and histone acetylation are involved in TPA-elicited EMT processes. We herein determined the cross talk mechanism among EMT, ROS, and histone acetylation, and our results provide an insight into the progression of cancer metastasis.

New Potential Pharmacological Functions of Chinese Herbal Medicines via Regulation of Autophagy

Autophagy is a universal catabolic cellular process for quality control of cytoplasm and maintenance of cellular homeostasis upon nutrient deprivation and environmental stimulus. It involves the lysosomal degradation of cellular components such as misfolded proteins or damaged organelles. Defects in autophagy are implicated in the pathogenesis of diseases including cancers, myopathy, neurodegenerations, infections and cardiovascular diseases. In the recent decade, traditional drugs with new clinical applications are not only commonly found in Western medicines, but also highlighted in Chinese herbal medicines (CHM). For instance, pharmacological studies have revealed that active components or fractions from Chaihu (Radix bupleuri), Hu Zhang (Rhizoma polygoni cuspidati), Donglingcao (Rabdosia rubesens), Hou po (Cortex magnoliae officinalis) and Chuan xiong (Rhizoma chuanxiong) modulate cancers, neurodegeneration and cardiovascular disease via autophagy. These findings shed light on the potential new applications and formulation of CHM decoctions via regulation of autophagy. This article reviews the roles of autophagy in the pharmacological actions of CHM and discusses their new potential clinical applications in various human diseases.

Protein Kinase C (PKC) Isozymes and Cancer

Protein kinase C (PKC) is a family of phospholipid-dependent serine/threonine kinases, which can be further classified into three PKC isozymes subfamilies: conventional or classic, novel or nonclassic, and atypical. PKC isozymes are known to be involved in cell proliferation, survival, invasion, migration, apoptosis, angiogenesis, and drug resistance. Because of their key roles in cell signaling, PKC isozymes also have the potential to be promising therapeutic targets for several diseases, such as cardiovascular diseases, immune and inflammatory diseases, neurological diseases, metabolic disorders, and multiple types of cancer. This review primarily focuses on the activation, mechanism, and function of PKC isozymes during cancer development and progression.

Rottlerin induces Wnt co-receptor LRP6 degradation and suppresses both Wnt/β-catenin and mTORC1 signaling in prostate and breast cancer cells

Activation of Wnt/β-catenin signaling can result in up-regulation of mTORC1 signaling in cancer cells. The low density lipoprotein receptor-related protein-6 (LRP6) is an essential Wnt co-receptor for Wnt/β-catenin signaling. We found that rottlerin, a natural plant polyphenol, suppressed LRP6 expression and phosphorylation, and inhibited Wnt/β-catenin signaling in HEK293 cells. Furthermore, the inhibitory effects of rottlerin on LRP6 expression/phosphorylation and Wnt/β-catenin signaling were confirmed in human prostate cancer PC-3 and DU145 cells and breast cancer MDA-MB-231 and T-47D cells. Mechanistically, rottlerin promoted LRP6 degradation, but had no effects on LRP6 transcriptional activity. In addition, rottlerin-mediated LRP6 down-regulation was unrelated to activation of 5'-AMP-activated protein kinase (AMPK). Importantly, we also found that rottlerin inhibited mTORC1 signaling in prostate and breast cancer cells. Finally, we demonstrated that rottlerin was able to suppress the expression of cyclin D1 and survivin, two targets of both Wnt/β-catenin and mTORC1 signaling, in prostate and breast cancer cells, and displayed remarkable anticancer activity with IC(50) values between 0.7 and 1.7 μM for prostate cancer PC-3 and DU145 cells and breast cancer MDA-MB-231 and T-47D cells. The IC(50) values are comparable to those shown to suppress the activities of Wnt/β-catenin and mTORC1 signaling in prostate and breast cancer cells. Our data indicate that rottlerin is a novel LRP6 inhibitor and suppresses both Wnt/β-catenin and mTORC1 signaling in prostate and breast cancer cells, and that LRP6 represents a potential therapeutic target for cancers.

Alternative Pathways of Cancer Cell Death by Rottlerin: Apoptosis versus Autophagy

Since the ability of cancer cells to evade apoptosis often limits the efficacy of radiotherapy and chemotherapy, autophagy is emerging as an alternative target to promote cell death. Therefore, we wondered whether Rottlerin, a natural polyphenolic compound with antiproliferative effects in several cell types, can induce cell death in MCF-7 breast cancer cells. The MCF-7 cell line is a good model of chemo/radio resistance, being both apoptosis and autophagy resistant, due to deletion of caspase 3 gene, high expression of the antiapoptotic protein Bcl-2, and low expression of the autophagic Beclin-1 protein. The contribution of autophagy and apoptosis to the cytotoxic effects of Rottlerin was examined by light, fluorescence, and electron microscopic examination and by western blotting analysis of apoptotic and autophagic markers. By comparing caspases-3-deficient (MCF-7^3def) and caspases-3-transfected MCF-7 cells (MCF-7^3trans), we found that Rottlerin induced a noncanonical, Bcl-2-, Beclin 1-, Akt-, and ERK-independent autophagic death in the former- and the caspases-mediated apoptosis in the latter, in not starved conditions and in the absence of any other treatment. These findings suggest that Rottlerin could be cytotoxic for different cancer cell types, both apoptosis competent and apoptosis resistant.

Sphingosine suppresses mesothelioma cell proliferation by inhibiting PKC-δ and inducing cell cycle arrest at the G(0)/G(1) phase

BACKGROUND/AIMS

Sphingosine regulates cellular differentiation, cell growth, and apoptosis. The present study aimed at understanding sphingosine-regulated mesothelioma cell proliferation.

METHODS

Human malignant mesothelioma cells such as NCI-H28, NCI-H2052, NCI-H2452, and MSTO-211H cells were cultured. The siRNA to silence the protein kinase C (PKC)-δ-targeted gene was constructed and transfected into cells. MTT assay, cell cycle analysis using a flow cytometry, and cell-free PKC-δ assay were carried out.

RESULTS

For all the cell types sphingosine inhibited cell growth in a concentration (1-100 µM)-dependent manner. The sphingosine effect was not prevented by rottlerin, an inhibitor of protein kinase C-δ (PKC-δ); conversely, rottlerin further enhanced the sphingosine effect or rottlerin suppressed mesothelioma cell growth without sphingosine. In the cell-free PKC assay, sphingosine attenuated PKC-δ activity. Knocking-down PKC-δ induced cell cycle arrest at the G0/G1 phase and inhibited cell growth.

CONCLUSION

The results of the present study show that sphingosine suppressed mesothelioma cell proliferation by inhibiting PKC-δ, to induce cell cycle arrest at the G0/G1 phase.

Rottlerin potentiates camptothecin-induced cytotoxicity in human hormone refractory prostate cancers through increased formation and stabilization of topoisomerase I-DNA cleavage complexes in a PKCδ-independent pathway

Combination therapy, which can optimize killing activity to cancers and minimize drug resistance, is a mainstream therapy against hormone-refractory prostate cancers (HRPCs). Rottlerin, a natural polyphenolic component, synergistically increased PC-3 (a HRPC cell line) apoptosis induced by camptothecin (a topoisomerase I inhibitor). Using siRNA technique to knockdown protein kinase C-δ (PKCδ), the data showed that rottlerin-mediated synergistic effect was PKCδ-independent, although rottlerin has been used as a PKCδ inhibitor. Rottlerin potentiated camptothecin-induced DNA fragmentation at S phase and ATM phosphorylation at Ser1981. The effect was correlated to apoptosis (r2 = 0.9). To detect upstream signals, the data showed that camptothecin acted on and stabilized topoisomerase I-DNA complex, leading to the formation of camptothecin-trapped cleavage complexes (TOP1cc). The effect was potentiated by rottlerin. To determine DNA repair capability, the time-related γH2A.X formation was examined after camptothecin removal. Consequently, rottlerin significantly inhibited camptothecin removal-mediated decline of γH2A.X formation at S phase, indicating the impairment of DNA repair activity in the presence of rottlerin. The combinatory treatment of camptothecin and rottlerin induced conformational change and activation of Bax and formation of truncated Bad, suggesting the contribution of mitochondria stress to apoptosis. In summary, the data suggest that rottlerin-mediated camptothecin sensitization is through the augmented stabilization of TOP1cc, leading to an increase of DNA damage stress and, possibly, an impairment of DNA repair capability. Subsequently, mitochondria-involved apoptosis is triggered through Bax activation and truncated Bad formation. The novel discovery may provide an anticancer approach of combinatory use between rottlerin and camptothecin for the treatment of HRPCs.

Rottlerin and cancer: novel evidence and mechanisms

Because cancers are caused by deregulation of hundreds of genes, an ideal anticancer agent should target multiple gene products or signaling pathways simultaneously. Recently, extensive research has addressed the chemotherapeutic potential of plant-derived compounds. Among the ever-increasing list of naturally occurring anticancer agents, Rottlerin appears to have great potentiality for being used in chemotherapy because it affects several cell machineries involved in survival, apoptosis, autophagy, and invasion. The underlying mechanisms that have been described are diverse, and the final, cell-specific, Rottlerin outcome appears to result from a combination of signaling pathways at multiple levels. This paper seeks to summarize the multifocal signal modulatory properties of Rottlerin, which merit to be further exploited for successful prevention and treatment of cancer.

TRAIL promotes caspase-dependent pro-inflammatory responses via PKCδ activation by vascular smooth muscle cells

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is best known for its selective cytotoxicity against transformed tumor cells. Most non-transformed primary cells and several cancer cell lines are not only resistant to death receptor-induced apoptosis, but also subject to inflammatory responses in a nuclear factor-κB (NF-κB)-dependent manner. Although the involvement of TRAIL in a variety of vascular disorders has been proposed, the exact molecular mechanisms are unclear. Here, we aimed to delineate the role of TRAIL in inflammatory vascular response. We also sought possible molecular mechanisms to identify potential targets for the prevention and treatment of post-angioplastic restenosis and atherosclerosis. Treatment with TRAIL increased the expression of intercellular adhesion molecule-1 by primary human vascular smooth muscle cells via protein kinase C (PKC)δ and NF-κB activation. Following detailed analysis using various PKCδ mutants, we determined that PKCδ activation was mediated by caspase-dependent proteolysis. The protective role of PKCδ was further confirmed in post-traumatic vascular remodeling in vivo. We propose that the TRAIL/TRAIL receptor system has a critical role in the pathogenesis of inflammatory vascular disorders by transducing pro-inflammatory signals via caspase-mediated PKCδ cleavage and subsequent NF-κB activation.

Protein kinase Cδ and caspase-3 modulate TRAIL-induced apoptosis in breast tumor cells

This report describes that protein kinase C delta (PKCδ) overexpression prevents TRAIL-induced apoptosis in breast tumor cells; however, the regulatory mechanism(s) involved in this phenomenon is(are) incompletely understood. In this study, we have shown that TRAIL-induced apoptosis was significantly inhibited in PKCδ overexpressing MCF-7 (MCF7/PKCδ) cells. Our data reveal that PKCδ inhibits caspase-8 activation, a first step in TRAIL-induced apoptosis, thus preventing TRAIL-induced apoptosis. Inhibition of PKCδ using rottlerin or PKCδ siRNA reverses the inhibitory effect of PKCδ on caspase-8 activation leading to TRAIL-induced apoptosis. To determine if caspase-3-induced PKCδ cleavage reverses its inhibition on caspase-8, we developed stable cell lines that either expresses wild-type PKCδ (MCF-7/cas-3/PKCδ) or caspase-3 cleavage-resistant PKCδ mutant (MCF-7/cas-3/PKCδ mut) utilizing MCF-7 cells expressing caspase-3. Cells that overexpress caspase-3 cleavage-resistant PKCδ mutant (MCF-7/cas-3/PKCδmut) significantly inhibited TRAIL-induced apoptosis when compared to wild-type PKCδ (MCF-7/cas-3/PKCδ) expressing cells. In MCF-7/cas-3/PKCδmut cells, TRAIL-induced caspase-8 activation was blocked leading to inhibition of apoptosis when compared to wild-type PKCδ (MCF-7/cas-3/PKCδ) expressing cells. Together, these results strongly suggest that overexpression of PKCδ inhibits caspase-8 activation leading to inhibition of TRAIL-induced apoptosis and its inhibition by rottlerin, siRNA, or cleavage by caspase-3 sensitizes cells to TRAIL-induced apoptosis. Clinically, PKCδ overexpressing tumors can be treated with a combination of PKCδ inhibitor(s) and TRAIL as a new treatment strategy.

Rottlerin inhibits ROS formation and prevents NFkappaB activation in MCF-7 and HT-29 cells

Rottlerin, a polyphenol isolated from Mallotus Philippinensis, has been recently used as a selective inhibitor of PKC δ, although it can inhibit many kinases and has several biological effects. Among them, we recently found that Rottlerin inhibits the Nuclear Factor κB (NFκB), activated by either phorbol esters or H₂O₂. Because of the redox sensitivity of NFκB and on the basis of Rottlerin antioxidant property, we hypothesized that Rottlerin could prevent NFκB activation acting as a free radicals scavenger, as other natural polyphenols. The current study confirms the antioxidant property of Rottlerin against the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) in vitro and against oxidative stress induced by H₂O₂ and by menadione in culture cells. We also demonstrate that Rottlerin prevents TNFα-dependent NFκB activation in MCF-7 cells and in HT-29 cells transfected with the NFκB-driven plasmid pBIIX-LUC, suggesting that Rottlerin can inhibit NFκB via several pathways and in several cell types.

Death receptor 5-recruited raft components contributes to the sensitivity of Jurkat leukemia cell lines to TRAIL-induced cell death

In the present study we demonstrated Jurkat leukemia cell lines of TIB152 and TIB153 with different sensitivities to recombinant soluble TRAIL cytotoxicity. TRAIL receptor death receptor 5 (DR5) was constitutively localized in the rafts in both cell lines. FADD, caspase-8, and PI3K-p85 subunit were recruited into DR5 lipid rafts of TIB152 but not in TIB153 cells. The expression and enzyme activity of acid sphingomyelinase, which digests sphingomyeline to produce ceramide and plays an essential role in lipid raft assembling, were higher in the rafts of TIB152 than in TIB153. These data provide evidences that DR5-recruited raft components contribute to the different sensitivity of Jurkat leukemia cell lines to TRAIL-induced cell death and may throw some light on the development of better therapeutic strategies for the cancer cells resistant to TRAIL treatment.

Protein kinase Cdelta supports survival of MDA-MB-231 breast cancer cells by suppressing the ERK1/2 pathway

Mechanisms that mediate apoptosis resistance are attractive therapeutic targets for cancer. Protein kinase Cdelta (PKCdelta) is considered a pro-apoptotic factor in many cell types. In breast cancer, however, it has shown both pro-survival and pro-apoptotic effects. Here, we report for the first time that down-regulation of PKCdelta per se leads to apoptosis of MDA-MB-231 cells. Inhibition of MEK1/2 by either PD98059 or U0126 suppressed the induction of apoptosis of PKCdelta-depleted MDA-MB-231 cells but did not support survival of MCF-7 or MDA-MB-468 cells. Basal ERK1/2 phosphorylation was substantially higher in MDA-MB-231 cells than in the other cell lines. PKCdelta depletion led to even higher ERK1/2 phosphorylation levels and also to lower expression levels of the ERK1/2 phosphatase MKP3. Depletion of MKP3 led to apoptosis and higher levels of ERK1/2 phosphorylation, suggesting that this may be a mechanism mediating the effect of PKCdelta down-regulation. However, PKCdelta silencing also induced increased MEK1/2 phosphorylation, indicating that PKCdelta regulates ERK1/2 phosphorylation both upstream and downstream. Moreover, PKCdelta silencing led to increased levels of the E3 ubiquitin ligase Nedd4, which is a potential regulator of MKP3, because down-regulation led to increased MKP3 levels. Our results highlight PKCdelta as a potential target for therapy of breast cancers with high activity of the ERK1/2 pathway.

Kalopanaxsaponin A inhibits PMA-induced invasion by reducing matrix metalloproteinase-9 via PI3K/Akt- and PKCdelta-mediated signaling in MCF-7 human breast cancer cells

Induction of matrix metalloproteinase (MMP)-9 is particularly important for the invasiveness of breast cancers. We investigated the inhibitory effect of kalopanaxsaponin A (KPS-A) on cell invasion and MMP-9 activation in phorbol 12-myristate 13-acetate (PMA)-treated MCF-7 human breast cancer cells. KPS-A inhibited PMA-induced cell proliferation and invasion. PMA-induced cell invasion was blocked in the presence of a primary antibody of MMP-9, and KPS-A suppressed the increased expression and/or secretion of MMP-9 and tissue inhibitor of metalloproteinase (TIMP)-1. Using specific inhibitors, we confirmed that PMA-induced cell invasion and MMP-9 expression is primarily regulated by nuclear factor-kappa B (NF-kappaB) activation via phosphatidylinositol 3-kinase (PI3K)/Akt and activator protein-1 (AP-1) activation via extracellular signal-regulated kinase (ERK)1/2. KPS-A decreased PMA-induced transcriptional activation of NF-kappaB and AP-1 and inhibited PMA-induced phosphorylation of ERK1/2 and Akt. Treatment with the protein kinase C (PKC)delta inhibitor rottlerin caused a marked decrease in PMA-induced MMP-9 secretion and cell invasion, as well as ERK/AP-1 activation, and KPS-A reduced PMA-induced membrane localization of PKCdelta. Furthermore, oral administration of KPS-A led to a substantial decrease in tumor volume and expression of proliferating cell nuclear antigen, MMP-9, TIMP-1 and PKCdelta in mice with MCF-7 breast cancer xenografts in the presence of 17beta-estradiol. These results suggest that KPS-A inhibits PMA-induced invasion by reducing MMP-9 activation, mainly via the PI3K/Akt/NF-kappaB and PKCdelta/ERK/AP-1 pathways in MCF-7 cells and blocks tumor growth and MMP-9-mediated invasiveness in mice with breast carcinoma. Therefore, KPS-A may be a promising anti-invasive agent with the advantage of oral dosing.

Protein kinase C delta is required for survival of cells expressing activated p21RAS

Inhibition of protein kinase C (PKC) activity in transformed cells and tumor cells containing activated p21(RAS) results in apoptosis. To investigate the pro-apoptotic pathway induced by the p21(RAS) oncoprotein, we first identified the specific PKC isozyme necessary to prevent apoptosis in the presence of activated p21(RAS). Dominant-negative mutants of PKC, short interfering RNA vectors, and PKC isozyme-specific chemical inhibitors directed against the PKCdelta isozyme demonstrated that PKCdelta plays a critical role in p21(RAS)-mediated apoptosis. An activating p21(RAS) mutation, or activation of the phosphatidylinositol 3-kinase (PI3K) Ras effector pathway, increased the levels of PKCdelta protein and activity in cells, whereas inhibition of p21(RAS) activity decreased the expression of the PKCdelta protein. Activation of the Akt survival pathway by oncogenic Ras required PKCdelta activity. Akt activity was dramatically decreased after PKCdelta suppression in cells containing activated p21(RAS). Conversely, constitutively activated Akt rescued cells from apoptosis induced by PKCdelta inhibition. Collectively, these findings demonstrate that p21(RAS), through its downstream effector PI3K, induces PKCdelta expression and that this increase in PKCdelta activity, acting through Akt, is required for cell survival. The p21(RAS) effector molecule responsible for the initiation of the apoptotic signal after suppression of PKCdelta activity was also determined to be PI3K. PI3K (p110(C)(AAX), where AA is aliphatic amino acid) was sufficient for induction of apoptosis after PKCdelta inhibition. Thus, the same p21(RAS) effector, PI3K, is responsible for delivering both a pro-apoptotic signal and a survival signal, the latter being mediated by PKCdelta and Akt. Selective suppression of PKCdelta activity and consequent induction of apoptosis is a potential strategy for targeting of tumor cells containing an activated p21(RAS).

Camptothecin-induced apoptosis is enhanced by Myc and involves PKCdelta signaling

The MYC oncogene is frequently deregulated in human tumors, indicative of a poor prognosis because of enhanced resistance to treatment. In such cases, the cellular sensitivity to chemotherapy could be restored by reactivation of Myc-driven apoptosis. We have analyzed apoptosis induced by the cytotoxic agents camptothecin (CPT) and paclitaxel (PTX) using Rat1 fibroblasts with different c-myc status and human Tet21N neuroblastoma cells with conditional MYCN expression. In these cell lines, the drug sensitivity was enhanced by Myc in line with previous reports showing that Myc sensitizes to apoptosis induction by many different apoptosis inducers. CPT-induced apoptosis involved cleavage and activation of proapoptotic Bid and Bax, induction of mitochondrial membrane depolarization, activation of caspase-9 and caspase-3, protein kinase c delta (PKCdelta) signaling and upregulation of p53. We also observed reduced transcriptional activity by Myc and other transcription factors in response to CPT. In contrast, the manner by which Myc potentiates the apoptosis induced by PTX differs from that of CPT and remains to be explored. In summary, our findings revealed that activation of PKCdelta in response to CPT treatment requires Myc and is important in CPT-mediated apoptosis signaling.

4-Hydroxynonenal signalling to apoptosis in isolated rat hepatocytes: the role of PKC-delta

4-Hydroxynonenal, a significant aldehyde end product of membrane lipid peroxidation with numerous biochemical activities, has consistently been detected in various human diseases. Concentrations actually detectable in vivo (0.1-5 microM) have been shown to up-regulate different genes and modulate various enzyme activities. In connection with the latter aspect, we show here that, in isolated rat hepatocytes, 1 microM 4-hydroxynonenal selectively activates protein kinase C-delta, involved in apoptosis of many cell types; it also induces very early activation of Jun N-terminal kinase, in parallel increasing activator protein-1 DNA-binding activity in a time-dependent manner and triggering apoptosis after only 120 min treatment. These phenomena are likely protein kinase C-delta-dependent, being significantly reduced or annulled by cell co-treatment with rottlerin, a selective inhibitor of protein kinase C-delta. We suggest that 4-hydroxynonenal may induce apoptosis through activation of protein kinase C-delta and of Jun N-terminal kinase, and consequent up-regulation of activator protein-1 DNA binding.