Involvement of protein kinase C delta (PKCdelta) in phorbol ester-induced apoptosis in LNCaP prostate cancer cells. Lack of proteolytic cleavage of PKCdelta

Protein kinase C signaling "in" and "to" the nucleus: Master kinases in transcriptional regulation

PKC is a multifunctional family of Ser-Thr kinases widely implicated in the regulation of fundamental cellular functions, including proliferation, polarity, motility, and differentiation. Notwithstanding their primary cytoplasmic localization and stringent activation by cell surface receptors, PKC isozymes impel prominent nuclear signaling ultimately impacting gene expression. While transcriptional regulation may be wielded by nuclear PKCs, it most often relies on cytoplasmic phosphorylation events that result in nuclear shuttling of PKC downstream effectors, including transcription factors. As expected from the unique coupling of PKC isozymes to signaling effector pathways, glaring disparities in gene activation/repression are observed upon targeting individual PKC family members. Notably, specific PKCs control the expression and activation of transcription factors implicated in cell cycle/mitogenesis, epithelial-to-mesenchymal transition and immune function. Additionally, PKCs isozymes tightly regulate transcription factors involved in stepwise differentiation of pluripotent stem cells toward specific epithelial, mesenchymal, and hematopoietic cell lineages. Aberrant PKC expression and/or activation in pathological conditions, such as in cancer, leads to profound alterations in gene expression, leading to an extensive rewiring of transcriptional networks associated with mitogenesis, invasiveness, stemness, and tumor microenvironment dysregulation. In this review, we outline the current understanding of PKC signaling "in" and "to" the nucleus, with significant focus on established paradigms of PKC-mediated transcriptional control. Dissecting these complexities would allow the identification of relevant molecular targets implicated in a wide spectrum of diseases.

Role of alternatively spliced, pro-survival Protein Kinase C delta VIII (PKCδVIII) in ovarian cancer

Ovarian cancer is the deadliest malignant disease in women. Protein Kinase C delta (PRKCD; PKCδ) is serine/threonine kinase extensively linked to various cancers. In humans, PKCδ is alternatively spliced to PKCδI and PKCδVIII. However, the specific function of PKCδ splice variants in ovarian cancer has not been elucidated yet. Hence, we evaluated their expression in human ovarian cancer cell lines (OCC): SKOV3 and TOV112D, along with the normal T80 ovarian cells. Our results demonstrate a marked increase in PKCδVIII in OCC compared to normal ovarian cells. Therefore, we elucidated the role of PKCδVIII and the underlying mechanism of its expression in OCC. Using overexpression and knockdown studies, we demonstrate that PKCδVIII increases cellular survival and migration in OCC. Further, overexpression of PKCδVIII in T80 cells resulted in increased expression of Bcl2 and knockdown of PKCδVIII in OCC decreased Bcl2 expression. Using co-immunoprecipitations and immunocytochemistry, we demonstrate nuclear localization of PKCδVIII in OCC and further show increased association of PKCδVIII with Bcl2 and Bcl-xL in OCC. Using PKCδ splicing minigene, mutagenesis, siRNA and antisense oligonucleotides, we demonstrate that increased levels of alternatively spliced PKCδVIII in OCC is regulated by splice factor SRSF2. Finally, we verified that PKCδVIII levels are elevated in samples of human ovarian cancer tissue. The data presented here demonstrate that the alternatively spliced, signaling kinase PKCδVIII is a viable target to develop therapeutics to combat progression of ovarian cancer.

Anti-cancer activities of Schedule E1 drugs used in ayurvedic formulations

Schedule E1 is an important part of Drugs and Cosmetics Act (Government of India) that comprises the list of poisonous drugs from plant, animal and mineral origins to be consumed under medical supervision. Ayurveda, the world's oldest medicinal system has a list of drugs represented in schedule E1 that are used since thousands of years. This review reports the anti-cancer activities of fifteen toxic ayurvedic drugs from plant origin represented in Drugs and Cosmetics Act, 1940. The information was collected from the various authentic sources, compiled and summarised. The plant extracts, formulations, phytoconstituents and other preparations of these drugs have shown effective activities against mammary carcinoma, neuroblastoma, non-small cell lung carcinoma, lymphocytic leukaemia, colorectal adenocarcinoma, Ehrlich ascites carcinoma, prostate adenocarcinoma, glioblastoma asterocytoma and other malignancies. They have various mechanisms of action including Bax upregulation, Bcl2 downregulation, induction of cell cycle arrest at S phase, G2/M phase, inhibition of vascular endothelial growth factors, inhibition of Akt/mTOR signalling etc. Certain traditional ayurvedic preparations containing these plants are reported beneficial and the possibilities of these drugs as the alternative and adjuvant therapeutic agents in the current cancer care have been discussed. The studies suggest that these drugs could be utilised in future for the critical care of malignancies.

Anti-tumor activity of resveratrol against gastric cancer: a review of recent advances with an emphasis on molecular pathways

Gastric cancer (GC) is one of the most common cancers with high malignancy. In spite of the great development in diagnostic tools and application of anti-tumor drugs, we have not witnessed a significant increase in the survival time of patients with GC. Multiple studies have revealed that Wnt, Nrf2, MAPK, and PI3K/Akt signaling pathways are involved in GC invasion. Besides, long non-coding RNAs and microRNAs function as upstream mediators in GC malignancy. GC cells have acquired resistance to currently applied anti-tumor drugs. Besides, combination therapy is associated with higher anti-tumor activity. Resveratrol (Res) is a non-flavonoid polyphenol with high anti-tumor activity used in treatment of various cancers. A number of studies have demonstrated the potential of Res in regulation of molecular pathways involved in cancer malignancy. At the present review, we show that Res targets a variety of signaling pathways to induce apoptotic cell death and simultaneously, to inhibit the migration and metastasis of GC cells.

Activated PKB/GSK-3β synergizes with PKC-δ signaling in attenuating myocardial ischemia/reperfusion injury via potentiation of NRF2 activity: Therapeutic efficacy of dihydrotanshinone-I

Disrupted redox status primarily contributes to myocardial ischemia/reperfusion injury (MIRI). NRF2, the endogenous antioxidant regulator, might provide therapeutic benefits. Dihydrotanshinone-I (DT) is an active component in Salvia miltiorrhiza with NRF2 induction potency. This study seeks to validate functional links between NRF2 and cardioprotection of DT and to investigate the molecular mechanism particularly emphasizing on NRF2 cytoplasmic/nuclear translocation. DT potently induced NRF2 nuclear accumulation, ameliorating post-reperfusion injuries via redox alterations. Abrogated cardioprotection in NRF2-deficient mice and cardiomyocytes strongly supports NRF2-dependent cardioprotection of DT. Mechanistically, DT phosphorylated NRF2 at Ser40, rendering its nuclear-import by dissociating from KEAP1 and inhibiting degradation. Importantly, we identified PKC-δ-(Thr505) phosphorylation as primary upstream event triggering NRF2-(Ser40) phosphorylation. Knockdown of PKC-δ dramatically retained NRF2 in cytoplasm, convincing its pivotal role in mediating NRF2 nuclear-import. NRF2 activity was further enhanced by activated PKB/GSK-3β signaling via nuclear-export signal blockage independent of PKC-δ activation. By demonstrating independent modulation of PKC-δ and PKB/GSK-3β/Fyn signaling, we highlight the ability of DT to exploit both nuclear import and export regulation of NRF2 in treating reperfusion injury harboring redox homeostasis alterations. Coactivation of PKC and PKB phenocopied cardioprotection of DT in vitro and in vivo, further supporting the potential applicability of this rationale.

Functional proteomic analysis reveals roles for PKCδ in regulation of cell survival and cell death: Implications for cancer pathogenesis and therapy

Protein Kinase C-δ (PKCδ), regulates a broad group of biological functions and disease processes, including well-defined roles in immune function, cell survival and apoptosis. PKCδ primarily regulates apoptosis in normal tissues and non-transformed cells, and genetic disruption of the PRKCD gene in mice is protective in many diseases and tissue damage models. However pro-survival/pro-proliferative functions have also been described in some transformed cells and in mouse models of cancer. Recent evidence suggests that the contribution of PKCδ to specific cancers may depend in part on the oncogenic context of the tumor, consistent with its paradoxical role in cell survival and cell death. Here we will discuss what is currently known about biological functions of PKCδ and potential paradigms for PKCδ function in cancer. To further understand mechanisms of regulation by PKCδ, and to gain insight into the plasticity of PKCδ signaling, we have used functional proteomics to identify pathways that are dependent on PKCδ. Understanding how these distinct functions of PKCδ are regulated will be critical for the logical design of therapeutics to target this pathway.

Missing Selectivity of Targeted 4β-Phorbol Prodrugs Expected to be Potential Chemotherapeutics

Targeting cytotoxic 4β-phorbol esters toward cancer tissue was attempted by conjugating a 4β-pborbol derivative with substrates for the proteases prostate-specific antigen (PSA) and prostate-specific membrane antigen (PSMA) expressed in cancer tissue. The hydrophilic peptide moiety was hypothesized to prevent penetration of the prodrugs into cells and prevent interaction with PKC. Cleavage of the peptide in cancer tumors was envisioned to release lipophilic cytotoxins, which subsequently penetrate into cancer cells. The 4β-phorbol esters were prepared from 4β-phorbol isolated from Croton tiglium seeds, while the peptides were prepared by solid-phase synthesis. Cellular assays revealed activation of PKC by the prodrugs and efficient killing of both peptidase positive as well as peptidase negative cells. Consequently no selectivity for enzyme expressing cells was found.

Interplay between miRNAs and Genes Associated with Cell Proliferation in Endometrial Cancer

Endometrial cancer develops as a result of abnormal cell growth associated with uncontrolled cell proliferation, excessive activation of signaling pathways and miRNA activity. The aim of this study was to determine the expression profile of genes associated with cell proliferation and to assess which miRNAs can participate in the regulation of their expression. The study enrolled 40 patients with endometrial cancer and 10 patients without neoplastic changes. The expression profile of genes associated with cell proliferation and the expression profile of miRNAs were assessed using microarrays. RT-qPCR was performed to validate mRNA microarray results. The mirTAR tool was used to identify miRNAs that regulate the activity of genes associated with cell proliferation. Decreased expression of IGF1 and MYLK, as well as SOD2 overexpression, were observed in endometrial cancer using both mRNA microarrays and RT-qPCR. Microarray analysis showed low levels of NES and PRKCA, but this was only partially validated using RT-qPCR. Reduced activity of MYLK may be caused by increased miR-200c, miR-155 and miR-200b expression. Cell proliferation is disturbed in endometrial cancer, which may be associated with an overexpression of miR-200a, miR-200c, and miR-155, making it a potential diagnostic marker.

Mitochondrial targeting of TR3 is involved in TPA induced apoptosis in breast cancer cells

TPA is considered to be a tumor promoting molecule that induces the expression of COX-2 protein. However, it is contradictory to find that TPA can induce tumor cell apoptosis and exert antitumor activity. Therefore, the role of TPA in tumorigenesis and development has not yet been elucidated. Here we show that TPA can promote the apoptosis of breast cancer cells and increase the ratio of Bax/Bcl-2. It is suggested that TPA may induce apoptosis of breast cancer cells through mitochondrial apoptosis pathway. Further studies showed that TPA could cause mitochondrial dysfunction and trigger mitochondrial apoptotic pathway. In mechanism, the mitochondrial targeting of TR3 is involved in TPA induced apoptosis in breast cancer cells. In conclusion, our findings suggest that TPA can play a role in inhibiting cancer by inducing apoptosis and TR3 is expected to be a new target for cancer treatment.

Anticancer activity of the protein kinase C modulator HMI-1a3 in 2D and 3D cell culture models of androgen-responsive and androgen-unresponsive prostate cancer

Prostate cancer is one of the most common cancers in men. Although it has a relatively high 5‐year survival rate, development of resistance to standard androgen‐deprivation therapy is a significant clinical problem. Therefore, novel therapeutic strategies are urgently needed. The protein kinase C (PKC) family is a putative prostate cancer drug target, but so far no PKC‐targeting drugs are available for clinical use. By contrast to the standard approach of developing PKC inhibitors, we have developed isophthalate derivatives as PKC agonists. In this study, we have characterized the effects of the most potent isophthalate, 5‐(hydroxymethyl)isophthalate 1a3 (HMI‐1a3), on three prostate cancer cell lines (LNCaP, DU145, and PC3) using both 2D and 3D cell culture models. In 2D cell culture, HMI‐1a3 reduced cell viability or proliferation in all cell lines as determined by the metabolic activity of the cells (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl‐tetrazolium bromide assay) and thymidine incorporation. However, the mechanism of action in LNCaP cells was different to that in DU145 or PC3 cells. In LNCaP cells, HMI‐1a3 induced a PKC‐dependent activation of caspase 3/7, indicating an apoptotic response, whereas in DU145 and PC3 cells, it induced senescence, which was independent of PKC. This was observed as typical senescent morphology, increased β‐galactosidase activity, and upregulation of the senescence marker p21 and downregulation of E2F transcription factor 1. Using a multicellular spheroid model, we further showed that HMI‐1a3 affects the growth of LNCaP and DU145 cells in a 3D culture, emphasizing its potential as a lead compound for cancer drug development.

Discovery of a small-molecule protein kinase Cδ-selective activator with promising application in colon cancer therapy

Protein kinase C (PKC) isozymes play major roles in human diseases, including cancer. Yet, the poor understanding of isozymes-specific functions and the limited availability of selective pharmacological modulators of PKC isozymes have limited the clinical translation of PKC-targeting agents. Here, we report the first small-molecule PKCδ-selective activator, the 7α-acetoxy-6β-benzoyloxy-12-O-benzoylroyleanone (Roy-Bz), which binds to the PKCδ-C1-domain. Roy-Bz potently inhibited the proliferation of colon cancer cells by inducing a PKCδ-dependent mitochondrial apoptotic pathway involving caspase-3 activation. In HCT116 colon cancer cells, Roy-Bz specifically triggered the translocation of PKCδ but not other phorbol ester responsive PKCs. Roy-Bz caused a marked inhibition in migration of HCT116 cells in a PKCδ-dependent manner. Additionally, the impairment of colonosphere growth and formation, associated with depletion of stemness markers, indicate that Roy-Bz also targets drug-resistant cancer stem cells, preventing tumor dissemination and recurrence. Notably, in xenograft mouse models, Roy-Bz showed a PKCδ-dependent antitumor effect, through anti-proliferative, pro-apoptotic, and anti-angiogenic activities. Besides, Roy-Bz was non-genotoxic, and in vivo it had no apparent toxic side effects. Collectively, our findings reveal a novel promising anticancer drug candidate. Most importantly, Roy-Bz opens the way to a new era on PKC biology and pharmacology, contributing to the potential redefinition of the structural requirements of isozyme-selective agents, and to the re-establishment of PKC isozymes as feasible therapeutic targets in human diseases.

Acyl-CoA thioesterase 7 is involved in cell cycle progression via regulation of PKCζ-p53-p21 signaling pathway

Acyl-CoA thioesterase 7 (ACOT7) is a major isoform of the ACOT family that catalyzes hydrolysis of fatty acyl-CoAs to free fatty acids and CoA-SH. However, canonical and non-canonical functions of ACOT7 remain to be discovered. In this study, for the first time, ACOT7 was shown to be responsive to genotoxic stresses such as ionizing radiation (IR) and the anti-cancer drug doxorubicin in time- and dose-dependent manners. ACOT7 knockdown induced cytostasis via activation of the p53-p21 signaling pathway without a DNA damage response. PKCζ was specifically involved in ACOT7 depletion-mediated cell cycle arrest as an upstream molecule of the p53-p21 signaling pathway in MCF7 human breast carcinoma and A549 human lung carcinoma cells. Of the other members of the ACOT family, including ACOT1, 4, 8, 9, 11, 12, and 13 that were expressed in human, ACOT4, 8, and 12 were responsive to genotoxic stresses. However, none of those had a role in cytostasis via activation of the PKCζ-p53-p21 signaling pathway. Analysis of the ACOT7 prognostic value revealed that low ACOT7 levels prolonged overall survival periods in breast and lung cancer patients. Furthermore, ACOT7 mRNA levels were higher in lung cancer patient tissues compared to normal tissues. We also observed a synergistic effect of ACOT7 depletion in combination with either IR or doxorubicin on cell proliferation in breast and lung cancer cells. Together, our data suggest that a low level of ACOT7 may be involved, at least in part, in the prevention of human breast and lung cancer development via regulation of cell cycle progression.

FBXO25 regulates MAPK signaling pathway through inhibition of ERK1/2 phosphorylation

The FBXO25 mediates degradation of ELK-1 and thus inhibits transcriptional activation of immediate early genes (iEG). Here we show that FBXO25 regulates yet another node of this signaling pathway, by decreasing MAPK/ERK activity. We show that induction of FBXO25 reduced ERK1/2 phosphorylation independently of MEK1/2. Accordingly, in HAP1 FBXO25 knockout cells (FBXO25^KO), we observed that upon PMA treatment ERK1/2 was more active than in parental cells. An increase in cell proliferation under receptor mediated activation of the ERK signaling pathway in FBXO25^KO cells was also observed. Taken together we show that FBXO25 functions as a negative regulator of MAPK signaling though the reduction of ERK1/2 activation.

Role of keratin 24 in human epidermal keratinocytes

Keratin 24 (K24) is a new kind of keratin genes, which encodes a novel keratin protein, K24 that bears high similarity to the type I keratins and displays a unique expression profile. However, the role of K24 is incompletely understood. In our study, we investigated the localization of K24 within the epidermis and possible functions. Keratin 24 was found to be modestly overexpressed in senescent keratinocytes and was mainly restricted to the upper stratum spinosum of epidermis. The protein was required for terminal differentiation upon CaCl2-induced differentiation. In vitro results showed that increased K24 in keratinocytes dramatically changed the differentiation of primary keratinocytes. It also inhibited cell survival by G1/S phase cell cycle arrest and induced senescence, autophagy and apoptosis of keratinocytes. In addition, K24 activated PKCδ signal pathway involving in cellular survival. In summary, K24 may be suggested as a potential differentiation marker and anti-proliferative factor in the epidermis.

Protein kinase C in cancer: The top five unanswered questions

Few kinases have been studied as extensively as protein kinase C (PKC), particularly in the context of cancer. As major cellular targets for the phorbol ester tumor promoters and diacylglycerol (DAG), a second messenger generated by stimulation of membrane receptors, PKC isozymes play major roles in the control of signaling pathways associated with proliferation, migration, invasion, tumorigenesis, and metastasis. However, despite decades of research, fundamental questions remain to be answered or are the subject of intense controversy. Primary among these unresolved issues are the role of PKC isozymes as either tumor promoter or tumor suppressor kinases and the incomplete understanding on isozyme-specific substrates and effectors. The involvement of PKC isozymes in cancer progression needs to be reassessed in the context of specific oncogenic and tumor suppressing alterations. In addition, there are still major hurdles in addressing isozyme-specific function due to the limited specificity of most pharmacological PKC modulators and the lack of validated predictive biomarkers for response, which impacts the translation of these agents to the clinic. In this review we focus on key controversial issues and upcoming challenges, with the expectation that understanding the intricacies of PKC function will help fulfill the yet unsuccessful promise of targeting PKCs for cancer therapeutics.

OSU-2S/Sorafenib Synergistic Antitumor Combination against Hepatocellular Carcinoma: The Role of PKCδ/p53

Background: Sorafenib (Nexavar^®) is an FDA-approved systemic therapy for advanced hepatocellular carcinoma (HCC). However, the low efficacy and adverse effects at high doses limit the clinical application of sorafenib and strongly recommend its combination with other agents aiming at ameliorating its drawbacks. OSU-2S, a PKCδ activator, was selected as a potential candidate anticancer agent to be combined with sorafenib to promote the anti-cancer activity through synergistic interaction. Methods: The antitumor effects of sorafenib, OSU-2S and their combination were assessed by MTT assay, caspase activation, Western blotting, migration/invasion assays in four different HCC cell lines. The synergistic interactions were determined by Calcusyn analysis. PKCδ knockdown was used to elucidate the role of PKCδ activation as a mechanism for the synergy. The knockdown/over-expression of p53 was used to explain the differential sensitivity of HCC cell lines to sorafenib and/or OSU-2S. Results: OSU-2S synergistically enhanced the anti-proliferative effects of sorafenib in the four used HCC cell lines with combination indices <1. This effect was accompanied by parallel increases in caspase 3/7 activity, PARP cleavage, PKCδ activation and inhibition of HCC cell migration/invasion. In addition, PKCδ knockdown abolished the synergy between sorafenib and OSU-2S. Furthermore, p53 restoration in Hep3B cells through the over-expression rendered them more sensitive to both agents while p53 knockdown from HepG2 cells increased their resistance to both agents. Conclusion: OSU-2S augments the anti-proliferative effect of sorafenib in HCC cell lines, in part, through the activation of PKCδ. The p53 status in HCC cells predicts their sensitivity toward both sorafenib and OSU-2S. The proposed combination represents a therapeutically relevant approach that can lead to a new HCC therapeutic protocol.

Multifunctional roles of PKCδ: Opportunities for targeted therapy in human disease

The serine-threonine protein kinase, protein kinase C-δ (PKCδ), is emerging as a bi-functional regulator of cell death and proliferation. Studies in PKCδ-/- mice have confirmed a pro-apoptotic role for this kinase in response to DNA damage and a tumor promoter role in some oncogenic contexts. In non-transformed cells, inhibition of PKCδ suppresses the release of cytochrome c and caspase activation, indicating a function upstream of apoptotic pathways. Data from PKCδ-/- mice demonstrate a role for PKCδ in the execution of DNA damage-induced and physiologic apoptosis. This has led to the important finding that inhibitors of PKCδ can be used therapeutically to reduce irradiation and chemotherapy-induced toxicity. By contrast, PKCδ is a tumor promoter in mouse models of mammary gland and lung cancer, and increased PKCδ expression is a negative prognostic indicator in Her2+ and other subtypes of human breast cancer. Understanding how these distinct functions of PKCδ are regulated is critical for the design of therapeutics to target this pathway. This review will discuss what is currently known about biological roles of PKCδ and prospects for targeting PKCδ in human disease.

Protein kinase C and cancer: what we know and what we do not

Since their discovery in the late 1970s, protein kinase C (PKC) isozymes represent one of the most extensively studied signaling kinases. PKCs signal through multiple pathways and control the expression of genes relevant for cell cycle progression, tumorigenesis and metastatic dissemination. Despite the vast amount of information concerning the mechanisms that control PKC activation and function in cellular models, the relevance of individual PKC isozymes in the progression of human cancer is still a matter of controversy. Although the expression of PKC isozymes is altered in multiple cancer types, the causal relationship between such changes and the initiation and progression of the disease remains poorly defined. Animal models developed in the last years helped to better understand the involvement of individual PKCs in various cancer types and in the context of specific oncogenic alterations. Unraveling the enormous complexity in the mechanisms by which PKC isozymes have an impact on tumorigenesis and metastasis is key for reassessing their potential as pharmacological targets for cancer treatment.

A polysaccharide from Ganoderma atrum inhibits tumor growth by induction of apoptosis and activation of immune response in CT26-bearing mice

Ganoderma atrum is one species of edible and pharmaceutical mushroom with various biological activities. Recently, a novel polysaccharide, PSG-1, was purified from G. atrum. The antitumor activity and its mechanism of action were studied. In vitro, PSG-1 has little effect on inhibiting proliferation of CT26 tumor cells. However, the tumor size was significantly decreased in PSG-1-treated mice. The results showed that PSG-1 induced apoptosis in CT26 cells. Moreover, the intracellular cyclic AMP (cAMP) level and protein kinase A (PKA) activity were markedly increased in PSG-1-treated mice. In contrast, the contents of cyclic GMP and DAG and the PKC activity were decreased. Similarly, the expression of PKA protein was upregulated, while PKC protein expression in PSG-1-treated group was lowered. Additionally, PSG-1 increased the immune organ index and serum biochemistry parameter. In general, PSG-1 enhances the antitumor immune response, induces apoptosis in CT26-bearing mice, and could be a safe and effective adjuvant for tumor therapy or functional food.

12-O-tetradecanoylphorbol-13-acetate promotes breast cancer cell motility by increasing S100A14 level in a Kruppel-like transcription factor 4 (KLF4)-dependent manner

The S100 protein family represents the largest subgroup of calcium binding EF-hand type proteins. These proteins have been reported to be involved in a wide range of biological functions that are related to normal cell development and tumorigenesis. S100A14 is a recently identified member of the S100 protein family and differentially expressed in a number of different human malignancies. However, the transcriptional regulation of S100A14 and its role in breast cancer needs to be further investigated. Here, we determined that 12-O-tetradecanoylphorbol-13-acetate (TPA) up-regulated the expression of KLF4 and facilitated its binding directly to two conserved GC-rich DNA segments within the S100A14 promoter, which is essential for the transactivation of KLF4 induced S100A14 expression. Furthermore, stable silencing of KLF4 significantly suppressed breast cancer cell migration induced by TPA. Collectively, these results offer insights into the fact that TPA provokes cell motility through regulating the expression and function of S100A14 in a KLF4-dependent manner.

Protein kinase C delta mediated cytotoxicity of 6-Hydroxydopamine via sustained extracellular signal-regulated kinase 1/2 activation in PC12 cells

OBJECTIVES

The incidence of Parkinson's disease (PD) is increasing as the global population ages. 6-hydroxydopamine (6-OHDA) can induce PD-like neuropathology and biochemical changes in both in vitro and in vivo models. Therefore, clarification of the molecular mechanism of 6-OHDA-induced cell death might contribute to the understanding of the pathogenesis of PD.

METHODS

With this goal in mind, we investigated the role of protein kinase C delta (PKC delta) in 6-OHDA-dependent death using the pheochromocytoma cell line, PC12. Cells were treated with 6-OHDA to induce toxicity with or without pretreatment using rottlerin (a PKC delta inhibitor), bisindolylmaleimide I (a general PKC inhibitor), Gö6976 (a PKC inhibitor selective for calcium-dependent PKC isoforms), or phorbol-12-myristate-13-acetate (PMA, a PKC activator).

RESULTS

Phorbol-12-myristate-13-acetate decreased cell survival and increased the rate of apoptosis while rottlerin increased cell survival and decreased the rate of apoptosis. In contrast, neither bisindolylmaleimide I nor Gö6976 affected 6-OHDA-induced cell death. Western analysis demonstrated that phosphorylation of PKC delta on Thr 505 as well as extracellular signal-regulated kinase (ERK) phosphorylation increased after exposure to 6-OHDA. This increase in PKC delta phosphorylation was potentiated by PMA. However, rottlerin attenuated the 6-OHDA-stimulated increase in PKC delta and ERK phosphorylation.

CONCLUSION

These data suggest that PKC delta, rather than classic-type PKC (alpha, beta1, beta2, gamma), participates in 6-OHDA-induced neurotoxicity in PC12 cells, and PKC delta activity is required for subsequent ERK activation during cell death.

The expanding spectrum of the autoimmune lymphoproliferative syndromes

PURPOSE OF REVIEW

Several autoimmune lymphoproliferative syndromes have been described lately. We review here the main clinical and laboratory findings of these new disorders.

RECENT FINDINGS

The prototypical autoimmune lymphoproliferative syndrome (ALPS) has had its diagnostic criteria modified, somatic mutations in RAS genes were found to cause an ALPS-like syndrome in humans, and mutations in a gene encoding a protein kinase C (PRKCD) were discovered to cause a syndrome of lymphoproliferation, autoimmunity and natural killer cell defect.

SUMMARY

The recent discoveries shed light on the molecular pathways governing lymphocyte death, proliferation and immune tolerance in humans.

Early down-regulation of PKCδ as a pro-survival mechanism in Huntington's disease

A balance between cell survival and apoptosis is crucial to avoid neurodegeneration. Here, we analyzed whether the pro-apoptotic protein PKCδ, and the pro-survival PKCα and βII, were dysregulated in the brain of R6/1 mouse model of Huntington's disease (HD). Protein levels of the three PKCs examined were reduced in all the brain regions analyzed being PKCδ the most affected isoform. Interestingly, PKCδ protein levels were also decreased in the striatum and cortex of R6/2 and Hdh(Q111/Q111) mice, and in the putamen of HD patients. Nuclear PKCδ induces apoptosis, but we detected reduced PKCδ in both cytoplasmic and nuclear enriched fractions from R6/1 mouse striatum, cortex and hippocampus. In addition, we show that phosphorylation and ubiquitination of PKCδ are increased in 30-week-old R6/1 mouse brain. All together these results suggest a pro-survival role of reduced PKCδ levels in response to mutant huntingtin-induced toxicity. In fact, we show that over-expression of PKCδ increases mutant huntingtin-induced cell death in vitro, whereas over-expression of a PKCδ dominant negative form or silencing of endogenous PKCδ partially blocks mutant huntingtin-induced cell death. Finally, we show that the analysis of lamin B protein levels could be a good marker of PKCδ activity, but it is not involved in PKCδ-mediated cell death in mutant huntingtin-expressing cells. In conclusion, our results suggest that neurons increase the degradation of PKCδ as a compensatory pro-survival mechanism in response to mutant huntingtin-induced toxicity that can help to understand why cell death appears late in the disease.

Regulation of Transcriptional Networks by PKC Isozymes: Identification of c-Rel as a Key Transcription Factor for PKC-Regulated Genes

BACKGROUND

Activation of protein kinase C (PKC), a family of serine-threonine kinases widely implicated in cancer progression, has major impact on gene expression. In a recent genome-wide analysis of prostate cancer cells we identified distinctive gene expression profiles controlled by individual PKC isozymes and highlighted a prominent role for PKCδ in transcriptional activation.

PRINCIPAL FINDINGS

Here we carried out a thorough bioinformatics analysis to dissect transcriptional networks controlled by PKCα, PKCδ, and PKCε, the main diacylglycerol/phorbol ester PKCs expressed in prostate cancer cells. Despite the remarkable differences in the patterns of transcriptional responsive elements (REs) regulated by each PKC, we found that c-Rel represents the most frequent RE in promoters regulated by all three PKCs. In addition, promoters of PKCδ-regulated genes were particularly enriched with REs for CREB, NF-E2, RREB, SRF, Oct-1, Evi-1, and NF-κB. Most notably, by using transcription factor-specific RNAi we were able to identify subsets of PKCδ-regulated genes modulated by c-Rel and CREB. Furthermore, PKCδ-regulated genes condensed under the c-Rel transcriptional regulation display significant functional interconnections with biological processes such as angiogenesis, inflammatory response, and cell motility.

CONCLUSION/SIGNIFICANCE

Our study identified candidate transcription factors in the promoters of PKC regulated genes, in particular c-Rel was found as a key transcription factor in the control of PKCδ-regulated genes. The deconvolution of PKC-regulated transcriptional networks and their nodes may greatly help in the identification of PKC effectors and have significant therapeutics implications.

Loss-of-function of the protein kinase C δ (PKCδ) causes a B-cell lymphoproliferative syndrome in humans

Defective lymphocyte apoptosis results in chronic lymphadenopathy and/or splenomegaly associated with autoimmune phenomena. The prototype for human apoptosis disorders is the autoimmune lymphoproliferative syndrome (ALPS), which is caused by mutations in the FAS apoptotic pathway. Recently, patients with an ALPS-like disease called RAS-associated autoimmune leukoproliferative disorder, in which somatic mutations in NRAS or KRAS are found, also were described. Despite this progress, many patients with ALPS-like disease remain undefined genetically. We identified a homozygous, loss-of-function mutation in PRKCD (PKCδ) in a patient who presented with chronic lymphadenopathy, splenomegaly, autoantibodies, elevated immunoglobulins and natural killer dysfunction associated with chronic, low-grade Epstein-Barr virus infection. This mutation markedly decreased protein expression and resulted in ex vivo B-cell hyperproliferation, a phenotype similar to that of the PKCδ knockout mouse. Lymph nodes showed intense follicular hyperplasia, also mirroring the mouse model. Immunophenotyping of circulating lymphocytes demonstrated expansion of CD5+CD20+ B cells. Knockdown of PKCδ in normal mononuclear cells recapitulated the B-cell hyperproliferative phenotype in vitro. Reconstitution of PKCδ in patient-derived EBV-transformed B-cell lines partially restored phorbol-12-myristate-13-acetate-induced cell death. In summary, homozygous PRKCD mutation results in B-cell hyperproliferation and defective apoptosis with consequent lymphocyte accumulation and autoantibody production in humans, and disrupts natural killer cell function.

Role of protein kinase C δ in ER stress and apoptosis induced by oxidized LDL in human vascular smooth muscle cells

During atherogenesis, excess amounts of low-density lipoproteins (LDL) accumulate in the subendothelial space where they undergo oxidative modifications. Oxidized LDL (oxLDL) alter the fragile balance between survival and death of vascular smooth muscle cells (VSMC) thereby leading to plaque instability and finally to atherothrombotic events. As protein kinase C δ (PKCδ) is pro-apoptotic in many cell types, we investigated its potential role in the regulation of VSMC apoptosis induced by oxLDL. We found that human VSMC silenced for PKCδ exhibited a protection towards oxLDL-induced apoptosis. OxLDL triggered the activation of PKCδ as shown by its phosphorylation and nuclear translocation. PKCδ activation was dependent on the reactive oxygen species generated by oxLDL. Moreover, we demonstrated that PKCδ participates in oxLDL-induced endoplasmic reticulum (ER) stress-dependent apoptotic signaling mainly through the IRE1α/JNK pathway. Finally, the role of PKCδ in the development of atherosclerosis was supported by immunohistological analyses showing the colocalization of activated PKCδ with ER stress and lipid peroxidation markers in human atherosclerotic lesions. These findings highlight a role for PKCδ as a key regulator of oxLDL-induced ER stress-mediated apoptosis in VSMC, which may contribute to atherosclerotic plaque instability and rupture.

Activation of nuclear factor κB (NF-κB) in prostate cancer is mediated by protein kinase C epsilon (PKCepsilon)

Protein kinase C ε (PKCε) has emerged as an oncogenic kinase and plays important roles in cell survival, mitogenesis and invasion. PKCε is up-regulated in most epithelial cancers, including prostate, breast, and lung cancer. Here we report that PKCε is an essential mediator of NF-κB activation in prostate cancer cells. A strong correlation exists between PKCε overexpression and NF-κB activation status in prostate cancer cells. Moreover, transgenic overexpression of PKCε in the mouse prostate causes preneoplastic lesions that display significant NF-κB hyperactivation. PKCε RNAi depletion or inhibition in prostate cancer cells diminishes NF-κB translocation to the nucleus with subsequent impairment of both activation of NF-κB transcription and induction of NF-κB responsive genes in response to the proinflammatory cytokine tumor necrosis factor α (TNFα). On the other hand, PKCε overexpression in normal prostate cells enhances activation of the NF-κB pathway. A mechanistic analysis revealed that TNFα activates PKCε via a C1 domain/diacylglycerol-dependent mechanism that involves phosphatidylcholine-phospholipase C. Moreover, PKCε facilitates the assembly of the TNF receptor-I signaling complex to trigger NF-κB activation. Our studies identified a molecular link between PKCε and NF-κB that controls key responses implicated in prostate cancer progression.

Phorbol esters isolated from Jatropha meal induced apoptosis-mediated inhibition in proliferation of chang and Vero cell lines

The direct feeding of Jatropha meal containing phorbol esters (PEs) indicated mild to severe toxicity symptoms in various organs of different animals. However, limited information is available on cellular and molecular mechanism of toxicity caused by PEs present in Jatropha meal. Thus, the present study was conducted to determine the cytotoxic and mode of action of PEs isolated from Jatropha meal using human hepatocyte (Chang) and African green monkey kidney (Vero) cell lines. The results showed that isolated PEs inhibited cell proliferation in a dose-dependent manner in both cell lines with the CC₅₀ of 125.9 and 110.3 μg/mL, respectively. These values were compatible to that of phorbol 12-myristate 13-acetate (PMA) values as positive control i.e., 124.5 and 106.3 μg/mL respectively. Microscopic examination, flow cytometry and DNA fragmentation results confirmed cell death due to apoptosis upon treatment with PEs and PMA at CC₅₀ concentration for 24 h in both cell lines. The Western blot analysis revealed the overexpression of PKC-Δ and activation of caspase-3 proteins which could be involved in the mechanism of action of PEs and PMA. Consequently, the PEs isolated form Jatropha meal caused toxicity and induced apoptosis-mediated proliferation inhibition toward Chang and Vero cell lines involving over-expression of PKC-Δ and caspase-3 as their mode of actions.

Phorbol esters from Jatropha meal triggered apoptosis, activated PKC-δ, caspase-3 proteins and down-regulated the proto-oncogenes in MCF-7 and HeLa cancer cell lines

Jatropha meal produced from the kernel of Jatropha curcas Linn. grown in Malaysia contains phorbol esters (PEs). The potential benefits of PEs present in the meal as anticancer agent are still not well understood. Hence, this study was conducted to evaluate the cytotoxic effects and mode of actions of PEs isolated from Jatropha meal against breast (MCF-7) and cervical (HeLa) cancer cell lines. Isolated PEs inhibited cells proliferation in a dose-dependent manner of both MCF-7 and HeLa cell lines with the IC₅₀ of 128.6 ± 2.51 and 133.0 ± 1.96 µg PMA equivalents/mL respectively, while the values for the phorbol 12-myristate 13-acetate (PMA) as positive control were 114.7 ± 1.73 and 119.6 ± 3.73 µg/mL, respectively. Microscopic examination showed significant morphological changes that resemble apoptosis in both cell lines when treated with PEs and PMA at IC₅₀ concentration after 24 h. Flow cytometry analysis and DNA fragmentation results confirmed the apoptosis induction of PEs and PMA in both cell lines. The PEs isolated from Jatropha meal activated the PKC-δ and down-regulated the proto-oncogenes (c-Myc, c-Fos and c-Jun). These changes probably led to the activation of Caspase-3 protein and apoptosis cell death occurred in MCF-7 and HeLa cell lines upon 24 h treatment with PEs and PMA. Phorbol esters of Jatropha meal were found to be promising as an alternative to replace the chemotherapeutic drugs for cancer therapy.

Protein kinase cδ in apoptosis: a brief overview

Protein kinase C-delta (PKCδ), a member of the lipid-regulated serine/threonine PKC family, has been implicated in a wide range of important cellular processes. In the past decade, the critical role of PKCδ in the regulation of both intrinsic and extrinsic apoptosis pathways has been widely explored. In most cases, over-expression or activation of PKCδ results in the induction of apoptosis. The phosphorylations and multiple cell organelle translocations of PKCδ initiate apoptosis by targeting multiple downstream effectors. During apoptosis, PKCδ is proteolytically cleaved by caspase-3 to generate a constitutively activated catalytic fragment, which amplifies apoptosis cascades in nucleus and mitochondria. However, PKCδ also exerts its anti-apoptotic and pro-survival roles in some cases. Therefore, the complicated role of PKCδ in apoptosis appears to be stimulus and cell type dependent. This review is mainly focused on how PKCδ gets activated in diverse ways in response to apoptotic signals and how PKCδ targets different downstream regulators to sponsor or restrain apoptosis induction.

Green tea polyphenols precondition against cell death induced by oxygen-glucose deprivation via stimulation of laminin receptor, generation of reactive oxygen species, and activation of protein kinase Cε

As the development of synthetic drugs for the prevention of stroke has proven challenging, utilization of natural products capable of preconditioning neuronal cells against ischemia-induced cell death would be a highly useful complementary approach. In this study using an oxygen-glucose deprivation and reoxygenation (OGD/R) model in PC12 cells, we show that 2-day pretreatment with green tea polyphenols (GTPP) and their active ingredient, epigallocatechin-3-gallate (EGCG), protects cells from subsequent OGD/R-induced cell death. A synergistic interaction was observed between GTPP constituents, with unfractionated GTPP more potently preconditioning cells than EGCG. GTPP-induced preconditioning required the 67-kDa laminin receptor (67LR), to which EGCG binds with high affinity. 67LR also mediated the generation of reactive oxygen species (ROS) via activation of NADPH oxidase. An exogenous ROS-generating system bypassed 67LR to induce preconditioning, suggesting that sublethal levels of ROS are indeed an important mediator in GTPP-induced preconditioning. This role for ROS was further supported by the fact that antioxidants blocked GTPP-induced preconditioning. Additionally, ROS induced an activation and translocation of protein kinase C (PKC), particularly PKCε from the cytosol to the membrane/mitochondria, which was also blocked by antioxidants. The crucial role of PKC in GTPP-induced preconditioning was supported by use of its specific inhibitors. Preconditioning was increased by conditional overexpression of PKCε and decreased by its knock-out with siRNA. Collectively, these results suggest that GTPP stimulates 67LR and thereby induces NADPH oxidase-dependent generation of ROS, which in turn induces activation of PKC, particularly prosurvival isoenzyme PKCε, resulting in preconditioning against cell death induced by OGD/R.

Star-PAP control of BIK expression and apoptosis is regulated by nuclear PIPKIα and PKCδ signaling

BIK protein is an initiator of mitochondrial apoptosis, and BIK expression is induced by proapoptotic signals, including DNA damage. Here, we demonstrate that 3' end processing and expression of BIK mRNA are controlled by the nuclear PI4,5P(2)-regulated poly(A) polymerase Star-PAP downstream of DNA damage. Nuclear PKCδ is a key mediator of apoptosis, and DNA damage stimulates PKCδ association with the Star-PAP complex where PKCδ is required for Star-PAP-dependent BIK expression. PKCδ binds the PI4,5P(2)-generating enzyme PIPKIα, which is essential for PKCδ interaction with the Star-PAP complex, and PKCδ activity is directly stimulated by PI4,5P(2). Features in the BIK 3' UTR uniquely define Star-PAP specificity and may block canonical PAP activity toward BIK mRNA. This reveals a nuclear phosphoinositide signaling nexus where PIPKIα, PI4,5P(2), and PKCδ regulate Star-PAP control of BIK expression and induction of apoptosis. This pathway is distinct from the Star-PAP-mediated oxidative stress pathway indicating signal-specific regulation of mRNA 3' end processing.

Paclitaxel induces apoptosis through activation of nuclear protein kinase C-δ and subsequent activation of Golgi associated Cdk1 in human hormone refractory prostate cancer

PURPOSE

Emerging evidence shows that the translocation of apoptosis related factors on cellular organelles, such as mitochondria, endoplasmic reticulum, Golgi apparatus and nucleus, has a crucial role in the apoptotic process. We characterized the effect of paclitaxel (Sigma®) on Golgi involved apoptosis in human hormone refractory prostate cancer.

MATERIALS AND METHODS

FACScan™ flow cytometric analysis was used to determine cell cycle distribution and the subG1 (apoptosis) population. Protein expression and localization were detected by Western blot, confocal microscopic examination and the sucrose gradient separation technique.

RESULTS

Paclitaxel induced Golgi apparatus disassembly and interaction between Golgi complexes and mitochondria. Discontinuous sucrose gradient fractionation was used to determine and collect Golgi containing fractions. Data revealed that paclitaxel induced an increase of Cdk1 activity and DR5 expression on the Golgi complex that was associated with increased cleavage of caspase-8, a DR5 downstream factor, and caspase-3 into catalytically active fragments. Data were validated by confocal immunofluorescence microscopy. Golgi associated effects were inhibited by the Cdk1 inhibitor roscovitine (Sigma), suggesting a critical role for Golgi-Cdk1. Also, paclitaxel caused an increase of nuclear but not of Golgi associated PKC-δ activity. The selective PKC-δ inhibitor rottlerin (Sigma) completely inhibited the increase of Golgi-Cdk1 activity, suggesting that nuclear PKC-δ served as an upstream regulator of Golgi-Cdk1.

CONCLUSIONS

Data suggest that paclitaxel induces nuclear translocation and activation of PKC-δ, which in turn causes Golgi-Cdk1 activation, leading to Golgi associated DR5 up-regulation, and caspase-8 and 3 activation. Golgi mediated signaling cascades facilitate mitochondria involved apoptotic pathways and at least partly explain the anticancer activity of paclitaxel action.

HIV-1gp120 induces neuronal apoptosis through enhancement of 4-aminopyridine-senstive outward K+ currents

Human immunodeficiency virus type 1 (HIV-1)-associated dementia (HAD) usually occurs late in the course of HIV-1 infection and the mechanisms underlying HAD pathogenesis are not well understood. Accumulating evidence indicates that neuronal voltage-gated potassium (Kv) channels play an important role in memory processes and acquired neuronal channelopathies in HAD. To examine whether Kv channels are involved in HIV-1-associated neuronal injury, we studied the effects of HIV-1 glycoprotein 120 (gp120) on outward K+ currents in rat cortical neuronal cultures using whole-cell patch techniques. Exposure of cortical neurons to gp120 produced a dose-dependent enhancement of A-type transient outward K+ currents (IA). The gp120-induced increase of IA was attenuated by T140, a specific antagonist for chemokine receptor CXCR4, suggesting gp120 enhancement of neuronal IA via CXCR4. Pretreatment of neuronal cultures with a protein kinase C (PKC) inhibitor, GF109203X, inhibited the gp120-induced increase of IA. Biological significance of gp120 enhancement of IA was demonstrated by experimental results showing that gp120-induced neuronal apoptosis, as detected by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay and caspase-3 staining, was attenuated by either an IA blocker 4-aminopyridine or a specific CXCR4 antagonist T140. Taken together, these results suggest that gp120 may induce caspase-3 dependent neuronal apoptosis by enhancing IA via CXCR4-PKC signaling.

Bioactive compounds and biological activities of Jatropha curcas L. kernel meal extract

Defatted Jatropha curcas L. (J. curcas) seed kernels contained a high percentage of crude protein (61.8%) and relatively little acid detergent fiber (4.8%) and neutral detergent fiber (9.7%). Spectrophotometric analysis of the methanolic extract showed the presence of phenolics, flavonoids and saponins with values of 3.9, 0.4 and 19.0 mg/g DM, respectively. High performance liquid chromatography (HPLC) analyses showed the presence of gallic acid and pyrogallol (phenolics), rutin and myricetin (flavonoids) and daidzein (isoflavonoid). The amount of phorbol esters in the methanolic extract estimated by HPLC was 3.0 ± 0.1 mg/g DM. Other metabolites detected by GC-MS include: 2-(hydroxymethyl)-2 nitro-1,3-propanediol, β-sitosterol, 2-furancarboxaldehyde, 5-(hydroxymethy) and acetic acid in the methanolic extract; 2-furancarboxaldehyde, 5-(hydroxymethy), acetic acid and furfural (2-furancarboxaldehyde) in the hot water extract. Methanolic and hot water extracts of kernel meal showed antimicrobial activity against both Gram positive and Gram negative pathogenic bacteria (inhibition range: 0-1.63 cm) at the concentrations of 1 and 1.5 mg/disc. Methanolic extract exhibited antioxidant activities that are higher than hot water extract and comparable to β-carotene. The extracts tended to scavenge the free radicals in the reduction of ferric ion (Fe(3+)) to ferrous ion (Fe(2+)). Cytotoxicity assay results indicated the potential of methanolic extract as a source of anticancer therapeutic agents toward breast cancer cells.

Control of protein kinase C activity, phorbol ester-induced cytoskeletal remodeling, and cell survival signals by the scaffolding protein SSeCKS/GRAVIN/AKAP12

The product of the SSeCKS/GRAVIN/AKAP12 gene ("SSeCKS") is a major protein kinase (PK) C substrate that exhibits tumor- and metastasis-suppressing activity likely through its ability to scaffold multiple signaling mediators such as PKC, PKA, cyclins, calmodulin, and Src. Although SSeCKS and PKCα bind phosphatidylserine, we demonstrate that phosphatidylserine-independent binding of PKC by SSeCKS is facilitated by two homologous SSeCKS motifs, EG(I/V)(T/S)XWXSFK(K/R)(M/L)VTP(K/R)K(K/R)X(K/R)XXXEXXXE(E/D) (amino acids 592-620 and 741-769). SSeCKS binding to PKCα decreased kinase activity and was dependent on the two PKC-binding motifs. SSeCKS scaffolding of PKC was increased in confluent cell cultures, correlating with significantly increased SSeCKS protein levels and decreased PKCα activity, suggesting a role for SSeCKS in suppressing PKC activation during contact inhibition. SSeCKS-null mouse embryo fibroblasts displayed increased relative basal and phorbol ester (phorbol 12-myristate 13-acetate)-induced PKC activity but were defective in phorbol 12-myristate 13-acetate-induced actin cytoskeletal reorganization and cell shape change; these responses could be rescued by the forced expression of full-length SSeCKS but not by an SSeCKS variant deleted of its PKC-binding domains. Finally, the PKC binding sites in SSeCKS were required to restore cell rounding and/or decreased apoptosis in phorbol ester-treated LNCaP, LNCaP-C4-2, and MAT-LyLu prostate cancer cells. Thus, PKC-mediated remodeling of the actin cytoskeleton is likely regulated by the ability of SSeCKS to control PKC signaling and activity through a direct scaffolding function.

Involvement of protein kinase Cdelta/extracellular signal-regulated kinase/poly(ADP-ribose) polymerase-1 (PARP-1) signaling pathway in histamine-induced up-regulation of histamine H1 receptor gene expression in HeLa cells

The histamine H(1) receptor (H1R) gene is up-regulated in patients with allergic rhinitis. However, the mechanism and reason underlying this up-regulation are still unknown. Recently, we reported that the H1R expression level is strongly correlated with the severity of allergic symptoms. Therefore, understanding the mechanism of this up-regulation will help to develop new anti-allergic drugs targeted for H1R gene expression. Here we studied the molecular mechanism of H1R up-regulation in HeLa cells that express H1R endogenously in response to histamine and phorbol 12-myristate 13-acetate (PMA). In HeLa cells, histamine stimulation caused up-regulation of H1R gene expression. Rottlerin, a PKCδ-selective inhibitor, inhibited up-regulation of H1R gene expression, but Go6976, an inhibitor of Ca(2+)-dependent PKCs, did not. Histamine or PMA stimulation resulted in PKCδ phosphorylation at Tyr(311) and Thr(505). Activation of PKCδ by H(2)O(2) resulted in H1R mRNA up-regulation. Overexpression of PKCδ enhanced up-regulation of H1R gene expression, and knockdown of the PKCδ gene suppressed this up-regulation. Histamine or PMA caused translocation PKCδ from the cytosol to the Golgi. U0126, an MEK inhibitor, and DPQ, a poly(ADP-ribose) polymerase-1 inhibitor, suppressed PMA-induced up-regulation of H1R gene expression. These results were confirmed by a luciferase assay using the H1R promoter. Phosphorylation of ERK and Raf-1 in response to PMA was also observed. However, real-time PCR analysis showed no inhibition of H1R mRNA up-regulation by a Raf-1 inhibitor. These results suggest the involvement of the PKCδ/ERK/poly(ADP-ribose) polymerase-1 signaling pathway in histamine- or PMA-induced up-regulation of H1R gene expression in HeLa cells.

A protein kinase C/protein kinase D pathway protects LNCaP prostate cancer cells from phorbol ester-induced apoptosis by promoting ERK1/2 and NF-{kappa}B activities

Phorbol esters such as phorbol 12-myristate 13-acetate (PMA) induce apoptosis in many tumor cells including the androgen-sensitive LNCaP prostate cancer cells. Although phorbol ester-induced apoptotic pathways have been well characterized, little is known of the pro-survival pathways modulated by these agents. We now provide experimental evidence to indicate that protein kinase D (PKD) promotes survival signals in LNCaP cells in response to PMA treatment. Knockdown of endogenous PKD1 or PKD2 decreased extracellular signal-regulated kinase (ERK) 1/2 and nuclear factor-kappaB (NF-κB)-dependent transcriptional activities and potentiated PMA-induced apoptosis, whereas overexpression of wild-type PKD1 enhanced ERK1/2 activity and suppressed PMA-induced apoptosis. PMA caused rapid activation, followed by progressive downregulation of endogenous PKD1 in a time- and concentration-dependent manner. The downregulation of PKD1 was dependent on the activity of protein kinase C (PKC), but not that of PKD. Selective depletion of endogenous PKC isoforms revealed that both PKCδ and PKCε were required for PKD1 activation and subsequent downregulation. Further analysis showed that the downregulation of PKD1 was mediated by a ubiquitin-proteasome degradation pathway, inhibition of which correlated to increased cell survival. In summary, our data indicate that PKD1 is activated and downregulated by PMA through a PKC-dependent ubiquitin-proteasome degradation pathway, and the activation of PKD1 or PKD2 counteracts PMA-induced apoptosis by promoting downstream ERK1/2 and NF-κB activities in LNCaP prostate cancer cells.

The synthetic bryostatin analog Merle 23 dissects distinct mechanisms of bryostatin activity in the LNCaP human prostate cancer cell line

Bryostatin 1 has attracted considerable attention both as a cancer chemotherapeutic agent and for its unique activity. Although it functions, like phorbol esters, as a potent protein kinase C (PKC) activator, it paradoxically antagonizes many phorbol ester responses in cells. Because of its complex structure, little is known of its structure-function relations. Merle 23 is a synthetic derivative, differing from bryostatin 1 at only four positions. However, in U-937 human leukemia cells, Merle 23 behaves like a phorbol ester and not like bryostatin 1. Here, we characterize the behavior of Merle 23 in the human prostate cancer cell line LNCaP. In this system, bryostatin 1 and phorbol ester have contrasting activities, with the phorbol ester but not bryostatin 1 blocking cell proliferation or tumor necrosis factor alpha secretion, among other responses. We show that Merle 23 displays a highly complex pattern of activity in this system. Depending on the specific biological response or mechanistic change, it was bryostatin-like, phorbol ester-like, intermediate in its behavior, or more effective than either. The pattern of response, moreover, varied depending on the conditions. We conclude that the newly emerging bryostatin derivatives such as Merle 23 provide powerful tools to dissect subsets of bryostatin mechanism and response.

p23/Tmp21 associates with protein kinase Cdelta (PKCdelta) and modulates its apoptotic function

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKCδ, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKCδ C1b domain associates with p23 and identified two key residues (Asp(245) and Met(266)) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKCδ-dependent apoptosis and activation of PKCδ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKCδ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKCδ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKCδ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli.

Differential regulation of gene expression by protein kinase C isozymes as determined by genome-wide expression analysis

Protein kinase C (PKC) isozymes are key signal transducers involved in normal physiology and disease and have been widely implicated in cancer progression. Despite our extensive knowledge of the signaling pathways regulated by PKC isozymes and their effectors, there is essentially no information on how individual members of the PKC family regulate gene transcription. Here, we report the first PKC isozyme-specific analysis of global gene expression by microarray using RNAi depletion of diacylglycerol/phorbol ester-regulated PKCs. A thorough analysis of this microarray data revealed unique patterns of gene expression controlled by PKCα, PKCδ, and PKCε, which are remarkably different in cells growing in serum or in response to phorbol ester stimulation. PKCδ is the most relevant isoform in controlling the induction of genes by phorbol ester stimulation, whereas PKCε predominantly regulates gene expression in serum. We also established that two PKCδ-regulated genes, FOSL1 and BCL2A1, mediate the apoptotic effect of phorbol esters or the chemotherapeutic agent etoposide in prostate cancer cells. Our studies offer a unique opportunity for establishing novel transcriptional effectors for PKC isozymes and may have significant functional and therapeutic implications.

Differential sensitization of different prostate cancer cells to apoptosis

Although protein kinase C (PKC) plays an important role in sensitizing prostate cancer cells to apoptosis, and suppression of PKC is able to trigger an apoptotic crisis in cells harboring oncogenic ras, little is known about whether dyregulation of Ras effectors in prostate cancer cells, together with loss of PKC, is synthetically lethal. The current study aims at investigating whether prostate cancer cells with aberrant Ras effector signaling are sensitive to treatment with HMG (a PKC inhibitor) for the induction of apoptosis. We show that prostate cancer DU145 cells expressing a high level of JNK1 become susceptible to apoptosis after treatment with HMG, in which caspase 8 is activated and cytochrome c is released to the cytosol. In contrast, the addition of HMG sensitizes LNCaP or PC3 prostate cancer cells harboring an active Akt to apoptosis, in which ROS is upregulated to induce the UPR and GADD153 expression. The concurrent activation of JNK1 and Akt has an additive effect on apoptosis following PKC suppression. Thus, the data identify Akt and JNK1 as potential targets in prostate cancer cells for PKC inhibition-induced apoptosis.

Growth hormone-releasing hormone antagonist induces apoptosis of human endometrial cancer cells through PKCδ-mediated activation of p53/p21

The growth hormone-releasing hormone (GHRH) antagonists have been shown to inhibit growth of human cancer cells, but the underlying molecular mechanisms and their actions have not been fully investigated. In this study, we first showed that GHRH-R splice variant 1 (SV1) was expressed in two human endometrial cancer cell lines, Ishikawa and ECC-1. By using MTT assay, immunoblotting for cleaved caspase-3 and TUNEL assays, we found that cell growth inhibition and apoptosis were induced in GHRH antagonist, JMR-132-treated cells by activating PKCδ and could be inhibited by treatment with PKC inhibitor, GF109203X. In addition, activation and protein expression of p53 as well as the expression of its downstream effector, p21, were increased by JMR-132 treatment. Moreover, JMR-132-induced p53 and p21 expression were diminished by treatment with PKC inhibitor. Knockdown of endogenous p53 and p21 by siRNAs abolished the JMR-132-induced cell growth inhibition and apoptosis. This study demonstrates a novel mechanism in which GHRH antagonist-induced cell growth inhibition and apoptosis through PKCδ-mediated activation of p53/p21 in human endometrial cancer cells. These findings may suggest the feasibility of GHRH antagonists as a therapeutic approach for human cancer.

Regulation of prostate cancer cell survival by protein kinase Cepsilon involves bad phosphorylation and modulation of the TNFalpha/JNK pathway

Protein kinase Cepsilon (PKCepsilon), a diacyglycerol- and phorbol ester-responsive serine-threonine kinase, has been implicated in mitogenic and survival control, and it is markedly overexpressed in human tumors, including in prostate cancer. Although prostate cancer cells undergo apoptosis in response to phorbol ester stimulation via PKCdelta-mediated release of death factors, the involvement of PKCepsilon in this response is not known. PKCepsilon depletion by RNAi or expression of a dominant negative kinase-dead PKCepsilon mutant potentiated the apoptotic response of PMA and sensitized LNCaP cells to the death receptor ligand TNFalpha. On the other hand, overexpression of PKCepsilon by adenoviral means protected LNCaP cells against apoptotic stimuli. Interestingly, PKCepsilon RNAi depletion significantly enhanced the release of TNFalpha in response to PMA and greatly potentiated JNK activation by this cytokine. Further mechanistic analysis revealed that PMA fails to promote phosphorylation of Bad in Ser(112) in PKCepsilon-depleted LNCaP cells, whereas PKCepsilon overexpression greatly enhanced Bad phosphorylation. This effect was independent of Akt, ERK, or p90Rsk, well established kinases for Ser(112) in Bad. Moreover, expression of a S112A-Bad mutant potentiated PMA-induced apoptosis. Finally, we found that upon activation PKCepsilon accumulated in mitochondrial fractions in LNCaP cells and that Bad was a substrate of PKCepsilon in vitro. Our results established that PKCepsilon modulates survival in prostate cancer cells via multiple pathways.

ROCK mediates phorbol ester-induced apoptosis in prostate cancer cells via p21Cip1 up-regulation and JNK

It is established that androgen-dependent prostate cancer cells undergo apoptosis upon treatment with phorbol esters and related analogs, an effect primarily mediated by PKCdelta. Treatment of LNCaP prostate cancer cells with phorbol 12-myristate 13-acetate (PMA) causes a strong and sustained activation of RhoA and its downstream effector ROCK (Rho kinase) as well as the formation of stress fibers. These effects are impaired in cells subjected to PKCdelta RNA interference depletion. Functional studies revealed that expression of a dominant negative RhoA mutant or treatment with the ROCK inhibitor Y-27632 inhibits the apoptotic effect of PMA in LNCaP cells. Remarkably, the cytoskeleton inhibitors cytochalasin B and blebbistatin blocked not only PMA-induced apoptosis but also the activation of JNK, a mediator of the cell death effect by the phorbol ester. In addition, we found that up-regulation of the cell cycle inhibitor p21(Cip1) is required for PMA-induced apoptosis and that inhibitors of ROCK or the cytoskeleton organization prevent p21(Cip1) induction. Real time PCR analysis and reporter gene assay revealed that PMA induces p21(Cip1) transcriptionally in a ROCK- and cytoskeleton-dependent manner. p21(Cip1) promoter analysis revealed that PMA induction is dependent on Sp1 elements in the p21(Cip1) promoter but independent of p53. Taken together, our studies implicate ROCK-mediated up-regulation of p21(Cip1) and the cytoskeleton in PKCdelta-dependent apoptosis in prostate cancer cells.