Protein kinase C epsilon is a predictive biomarker of aggressive breast cancer and a validated target for RNA interference anticancer therapy

Protein kinase Cα and ε small-molecule targeted therapeutics: a new roadmap to two Holy Grails in drug discovery?

Protein kinase (PK)Calpha and epsilon are rational targets for cancer therapy. However, targeted experimental therapeutics that inhibit PKCalpha or epsilon are unavailable. The authors established recently that covalent modification of an active-site cysteine in human PKCepsilon, Cys452, by small molecules, for example 2-mercaptoethanolamine, is necessary and sufficient to render PKCepsilon kinase-dead. Cys452 is conserved in only eleven human protein kinase genes, including PKCalpha. Therefore, the design of small molecules that bind PKC active sites with an electrophile substituent positioned proximal to the Cys452 side chain may lead to targeted therapeutics that selectively inhibit PKCepsilon, PKCalpha or other PKC isozymes.

High-throughput screens identify microRNAs essential for HER2 positive breast cancer cell growth

MicroRNAs (miRNAs) are non-coding RNAs regulating gene expression post-transcriptionally. We have characterized the role of miRNAs in regulating the human epidermal growth factor receptor 2 (HER2)-pathway in breast cancer. We performed miRNA gain-of-function assays by screening two HER2 amplified cell lines (KPL-4 and JIMT-1) with a miRNA mimic library consisting of 810 human miRNAs. The levels of HER2, phospho-AKT, phospho-ERK1/2, cell proliferation (Ki67) and apoptosis (cPARP) were analyzed with reverse-phase protein arrays. Rank product analyses identified 38 miRNAs (q < 0.05) as inhibitors of HER2 signaling and cell growth, the most effective being miR-491-5p, miR-634, miR-637 and miR-342-5p. We also characterized miRNAs directly targeting HER2 and identified seven novel miRNAs (miR-552, miR-541, miR-193a-5p, miR-453, miR-134, miR-498, and miR-331-3p) as direct regulators of the HER2 3'UTR. We demonstrated the clinical relevance of the miRNAs and identified miR-342-5p and miR-744* as significantly down-regulated in HER2-positive breast tumors as compared to HER2-negative tumors from two cohorts of breast cancer patients (101 and 1302 cases). miR-342-5p specifically inhibited HER2-positive cell growth, as it had no effect on the growth of HER2-negative control cells in vitro. Furthermore, higher expression of miR-342-5p was associated with better survival in both breast cancer patient cohorts. In conclusion, we have identified miRNAs which are efficient negative regulators of the HER2 pathway that may play a role in vivo during breast cancer progression. These results give mechanistic insights in HER2 regulation which may open potential new strategies towards prevention and therapeutic inhibition of HER2-positive breast cancer.

Modulation of pancreatic tumor potential by overexpression of protein kinase C β1

OBJECTIVE

This study aimed to investigate whether the overexpression of protein kinase C β1 (PKCβ1) is able to modulate the malignant phenotype displayed by the human ductal pancreatic carcinoma cell line PANC1.

METHODS

PKCβ1 overexpression was achieved using a stable transfection approach. PANC1-PKCβ1 and control cells were analyzed both in vitro and in vivo.

RESULTS

PANC1-PKCβ1 cells displayed a lower growth capacity associated with the down-regulation of the MEK/ERK pathway and cyclin expression. Furthermore, PKCβ1 overexpression was associated with an enhancement of cell adhesion to fibronectin and with reduced migratory and invasive phenotypes. In agreement with these results, PANC1-PKCβ1 cells showed an impaired ability to secrete proteolytic enzymes. We also found that PKCβ1 overexpressing cells were more resistant to cell death induced by serum deprivation, an event associated with G0/G1 arrest and the modulation of PI3K/Akt and NF-κB pathways. Most notably, the overexpression of PKCβ1 completely abolished the ability of PANC1 cells to induce tumors in nude mice.

CONCLUSIONS

Our results established an important role for PKCβ1 in PANC1 cells suggesting it would act as a suppressor of tumorigenic behavior in pancreatic cancer.

PKC ε Phosphorylates and Mediates the Cell Membrane Localization of RhoA

Protein kinase Cε (PKCε) signals through RhoA to modulate cell invasion and motility. In this study, the multifaceted interaction between PKCε and RhoA was defined. Phosphopeptide mapping revealed that PKCε phosphorylates RhoA at T127 and S188. Recombinant PKCε bound to recombinant RhoA in the absence of ATP indicating that the association between PKCε and RhoA does not require an active ATP-docked PKCε conformation. Activation of PKCε resulted in a dramatic coordinated translocation of PKCε and RhoA from the cytoplasm to the cell membrane using time-lapse fluorescence microscopy. Stoichiometric FRET analysis revealed that the molecular interaction between PKCε and RhoA is a biphasic event, an initial peak at the cytoplasm and a gradual prolonged increase at the cell membrane for the entire time-course (12.5 minutes). These results suggest that the PKCε-RhoA complex is assembled in the cytoplasm and subsequently recruited to the cell membrane. Kinase inactive (K437R) PKCε is able to recruit RhoA to the cell membrane indicating that the association between PKCε and RhoA is proximal to the active catalytic site and perhaps independent of a PKCε-RhoA phosphorylation event. This work demonstrates, for the first time, that PKCε phosphorylates and modulates the cell membrane translocation of RhoA.

Expression pattern of protein kinase C ϵ during mouse embryogenesis

BACKGROUND

Protein kinase C epsilon (PKCϵ) belongs to the novel PKC subfamily, which consists of diacylglycerol dependent- and calcium independent-PKCs. Previous studies have shown that PKCϵ is important in different contexts, such as wound healing or cancer. In this study, we contribute to expand the knowledge on PKCϵ by reporting its expression pattern during murine midgestation using the LacZ reporter gene and immunostaining procedures.

RESULTS

Sites showing highest PKCϵ expression were heart at ealier stages, and ganglia in older embryos. Other stained domains included somites, bone, stomach, kidney, and blood vessels.

CONCLUSIONS

The seemingly strong expression of PKCϵ in heart and ganglia shown in this study suggests a important role of this isoform in the vascular and nervous systems during mouse development. However, functional redundancy with other PKCs during midgestation within these domains and others reported here possibly exists since PKCϵ deficient mice do not display obvious embryonic developmental defects.

Protein kinase C epsilon and genetic networks in osteosarcoma metastasis

Osteosarcoma (OS) is the most common primary malignant tumor of the bone, and pulmonary metastasis is the most frequent cause of OS mortality. The aim of this study was to discover and characterize genetic networks differentially expressed in metastatic OS. Expression profiling of OS tumors, and subsequent supervised network analysis, was performed to discover genetic networks differentially activated or organized in metastatic OS compared to localized OS. Broad trends among the profiles of metastatic tumors include aberrant activity of intracellular organization and translation networks, as well as disorganization of metabolic networks. The differentially activated PRKCε-RASGRP3-GNB2 network, which interacts with the disorganized DLG2 hub, was also found to be differentially expressed among OS cell lines with differing metastatic capacity in xenograft models. PRKCε transcript was more abundant in some metastatic OS tumors; however the difference was not significant overall. In functional studies, PRKCε was not found to be involved in migration of M132 OS cells, but its protein expression was induced in M112 OS cells following IGF-1 stimulation.

PKC signaling in glioblastoma

Glioblastoma Multiforme (GBM) is the most aggressive brain tumor characterized by intratumoral heterogeneity at cytopathological, genomic and transcriptional levels. Despite the efforts to develop new therapeutic strategies the median survival of GBM patients is 12-14 months. Results from large-scale gene expression profile studies confirmed that the genetic alterations in GBM affect pathways controlling cell cycle progression, cellular proliferation and survival and invasion ability, which may explain the difficulty to treat GBM patients. One of the signaling pathways that contribute to the aggressive behavior of glioma cells is the protein kinase C (PKC) pathway. PKC is a family of serine/threonine-specific protein kinases organized into three groups according the activating domains. Due to the variability of actions controlled by PKC isoforms, its contribution to the development of GBM is poorly understood. This review intends to highlight the contribution of PKC isoforms to proliferation, survival and invasive ability of glioma cells.

Plumbagin, a medicinal plant (Plumbago zeylanica)-derived 1,4-naphthoquinone, inhibits growth and metastasis of human prostate cancer PC-3M-luciferase cells in an orthotopic xenograft mouse model

We present here first time that Plumbagin (PL), a medicinal plant-derived 1,4-naphthoquinone, inhibits the growth and metastasis of human prostate cancer (PCa) cells in an orthotopic xenograft mouse model. In this study, human PCa PC-3M-luciferase cells (2 × 10(6)) were injected into the prostate of athymic nude mice. Three days post cell implantation, mice were treated with PL (2 mg/kg body wt. i.p. five days in a week) for 8 weeks. Growth and metastasis of PC-3M-luciferase cells was examined weekly by bioluminescence imaging of live mice. PL-treatment significantly (p = 0.0008) inhibited the growth of orthotopic xenograft tumors. Results demonstrated a significant inhibition of metastasis into liver (p = 0.037), but inhibition of metastasis into the lungs (p = 0.60) and lymph nodes (p = 0.27) was not observed to be significant. These results were further confirmed by histopathology of these organs. Results of histopathology demonstrated a significant inhibition of metastasis into lymph nodes (p = 0.034) and lungs (p = 0.028), and a trend to significance in liver (p = 0.075). None of the mice in the PL-treatment group showed PCa metastasis into the liver, but these mice had small metastasis foci into the lymph nodes and lungs. However, control mice had large metastatic foci into the lymph nodes, lungs, and liver. PL-caused inhibition of the growth and metastasis of PC-3M cells accompanies inhibition of the expression of: 1) PKCε, pStat3Tyr705, and pStat3Ser727, 2) Stat3 downstream target genes (survivin and Bcl(xL)), 3) proliferative markers Ki-67 and PCNA, 4) metastatic marker MMP9, MMP2, and uPA, and 5) angiogenesis markers CD31 and VEGF. Taken together, these results suggest that PL inhibits tumor growth and metastasis of human PCa PC3-M-luciferase cells, which could be used as a therapeutic agent for the prevention and treatment of human PCa.

Protein kinase Cϵ activity induces anti-inflammatory and anti-apoptotic genes via an ERK1/2- and NF-κB-dependent pathway to enhance vascular protection

Vascular endothelial injury predisposes to endothelial dysfunction and atherogenesis. We have investigated the hypothesis that PKCϵ (protein kinase Cϵ) is an important upstream regulator of cytoprotective pathways in vascular ECs (endothelial cells). Depletion of PKCϵ in human ECs reduced expression of the cytoprotective genes A1, A20 and Bcl-2. Conversely, constitutively active PKCϵ expressed in human ECs increased mRNA and protein levels of these cytoprotective genes, with up-regulation dependent upon ERK1/2 (extracellular-signal-regulated kinase 1/2) activation. Furthermore, inhibition of NF-κB (nuclear factor κB) by the pharmacological antagonist BAY 11-7085 or an IκB (inhibitor of NF-κB) SuperRepressor prevented cytoprotective gene induction. Activation of PKCϵ enhanced p65 NF-κB DNA binding and elevated NF-κB transcriptional activity. Importantly, although NF-κB activation by PKCϵ induced cytoprotective genes, it did not up-regulate pro-inflammatory NF-κB targets [E-selectin, VCAM-1 (vascular cell adhesion molecule 1) and ICAM-1 (intercellular adhesion molecule 1)]. Indeed, PKCϵ exhibited cytoprotective and anti-inflammatory actions, including inhibition of TNFα (tumour necrosis factor α)-induced JNK (c-Jun N-terminal kinase) phosphorylation and ICAM-1 up-regulation, a response attenuated by depletion of A20. Thus we conclude that PKCϵ plays an essential role in endothelial homoeostasis, acting as an upstream co-ordinator of gene expression through activation of ERK1/2, inhibition of JNK and diversion of the NF-κB pathway to cytoprotective gene induction, and propose that PKCϵ represents a novel therapeutic target for endothelial dysfunction.

Plumbagin inhibits prostate cancer development in TRAMP mice via targeting PKCε, Stat3 and neuroendocrine markers

Plumbagin (PL), 5-hydroxy-2-methyl-1,4-naphthoquinone, is a quinoid constituent isolated from the roots of the medicinal plant Plumbago zeylanica L. (also known as chitrak). PL has also been found in Juglans regia (English Walnut), Juglans cinerea (whitenut) and Juglans nigra (blacknut). The roots of P. zeylanica have been used in Indian and Chinese systems of medicine for more than 2500 years for the treatment of various types of ailments. We were the first to report that PL inhibits the growth and invasion of hormone refractory prostate cancer (PCa) cells [Aziz,M.H. et al. (2008) Plumbagin, a medicinal plant-derived naphthoquinone, is a novel inhibitor of the growth and invasion of hormone-refractory prostate cancer. Cancer Res., 68, 9024-9032.]. Now, we present that PL inhibits in vivo PCa development in the transgenic adenocarcinoma of mouse prostate (TRAMP). PL treatment (2 mg/kg body weight i.p. in 0.2 ml phosphate-buffered saline, 5 days a week) to FVB-TRAMP resulted in a significant (P < 0.01) decrease in prostate tumor size and urogenital apparatus weights at 13 and 20 weeks. Histopathological analysis revealed that PL treatment inhibited progression of prostatic intraepithelial neoplasia (PIN) to poorly differentiated carcinoma (PDC). No animal exhibited diffuse tumor formation in PL-treated group at 13 weeks, whereas 75% of the vehicle-treated mice elicited diffuse PIN and large PDC at this stage. At 20 weeks, 25% of the PL-treated animals demonstrated diffuse PIN and 75% developed small PDC, whereas 100% of the vehicle-treated mice showed large PDC. PL treatment inhibited expression of protein kinase C epsilon (PKCε), signal transducers and activators of transcription 3 phosphorylation, proliferating cell nuclear antigen and neuroendocrine markers (synaptophysin and chromogranin-A) in excised prostate tumor tissues. Taken together, these results further suggest PL could be a novel chemopreventive agent against PCa.

Glucocorticoid receptor activation inhibits p53-induced apoptosis of MCF10Amyc cells via induction of protein kinase Cε

Glucocorticoid receptor (GR) is a ligand-dependent transcription factor that can promote apoptosis or survival in a cell-specific manner. Activated GR has been reported to inhibit apoptosis in mammary epithelial cells and breast cancer cells by increasing pro-survival gene expression. In this study, activated GR inhibited p53-dependent apoptosis in MCF10A cells and human mammary epithelial cells that overexpress the MYC oncogene. Specifically, GR agonists hydrocortisone or dexamethasone inhibited p53-dependent apoptosis induced by cisplatin, ionizing radiation, or the MDM2 antagonist Nutlin-3. In contrast, the GR antagonist RU486 sensitized the cells to apoptosis by these agents. Apoptosis inhibition was associated with maintenance of mitochondrial membrane potential, diminished caspase-3 and -7 activation, and increased expression at both the mRNA and protein level of the anti-apoptotic PKC family member PKCε. Knockdown of PKCε via siRNA targeting reversed the protective effect of dexamethasone and restored apoptosis sensitivity. These data provide evidence that activated GR can inhibit p53-dependent apoptosis through induction of the anti-apoptotic factor PKCε.

Quantification of PKC family genes in sporadic breast cancer by qRT-PCR: evidence that PKCι/λ overexpression is an independent prognostic factor

Drugs targeting protein kinase C (PKC) show promising therapeutic activity. However, little is known about the expression patterns of the 11 PKC genes in human tumors, and the clinical significance of most PKC genes is unknown. We used qRT-PCR assays to quantify mRNA levels of the 11 PKC genes in 458 breast tumors from patients with known clinical/pathological status and long-term outcome. The proportion of tumors in which the expression of the different genes was altered varied widely, from 9.6% for PKN2 to 40.2% for PKCι/λ. In breast tumors, overexpression was the main alteration observed for PKCι/λ (33.4%), PKCδ (29.5%) and PKCζ (9.6%), whereas underexpression was the main alteration observed for PKCα (27.3%), PKCε (11.6%), PKCη (8.7%) and PKN2 (8.1%). Both overexpression and underexpression were observed for PKCβ (underexpression 15.5%, overexpression 13.8%), PKCθ (underexpression 14.8%, overexpression 10.0%) and PKN1 (underexpression 6.6%, overexpression 7.4%). Several links were found between different PKC genes; and also between the expression patterns of PKC genes and several classical pathological and clinical parameters. PKCι/λ alone was found to have prognostic significance (p = 0.043), whereas PKCα showed a trend towards an influence on relapse-free survival (p = 0.052). PKCι/λ retained its prognostic significance in Cox multivariate regression analysis (p = 0.031). These results reveal very complex expression patterns of PKC genes in breast tumors, and suggest that their expression should be considered together when evaluating anti-tumoral drugs. PKCι/λ seems to be the most promising therapeutic target in breast cancer.

Localization of PKCη in cell membranes as a predictor for breast cancer response to treatment

BACKGROUND

Successful treatment of breast cancer is frequently limited by the resistance of tumors to chemotherapy. Recent studies suggested a role for protein kinase C (PKC) in the resistance to chemotherapy. Here we used retrospective analysis of breast cancer biopsies of neoadjuvantly treated patients to investigate the correlation of PKC expression with aggressiveness and resistance to chemotherapy.

PATIENTS AND METHODS

Our cohort (n = 25) included patients with advanced and aggressive breast cancers, who underwent neoadjuvant therapy with the CAF regimen (cyclophosphamide, doxorubicin, fluorouracil). Core biopsies (pre-chemotherapy) and surgical biopsies of primary tumors and lymph node metastases (post-chemotherapy) were scored for PKCeta (PKCh) and PKCepsilon (PKCe) expression in the cytoplasm, cell membrane, nuclear membrane, and the nucleus.

RESULTS

Our results showed increased expression of PKCh (not PKCe) in the cytoplasm and cell membranes of post-chemotherapy biopsies (p = 0.03). PKCh presence in cell membranes, indicating activation, was in correlation with poor survival (p = 0.007).

CONCLUSION

PKCh staining in cell and nuclear membranes is an indicator for poor survival and a predictor for the effectiveness of neoadjuvant treatment. Other avenues of treatment should be considered for these patients. PKCh presents a target for therapy where inhibition of its activity and/or translocation to membranes could interfere with the resistance to chemotherapy.

Non-small cell lung carcinoma cell motility, rac activation and metastatic dissemination are mediated by protein kinase C epsilon

Background

Protein kinase C (PKC) ε, a key signaling transducer implicated in mitogenesis, survival, and cancer progression, is overexpressed in human primary non-small cell lung cancer (NSCLC). The role of PKCε in lung cancer metastasis has not yet been established.

Principal Findings

Here we show that RNAi-mediated knockdown of PKCε in H358, H1299, H322, and A549 NSCLC impairs activation of the small GTPase Rac1 in response to phorbol 12-myristate 13-acetate (PMA), serum, or epidermal growth factor (EGF). PKCε depletion markedly impaired the ability of NSCLC cells to form membrane ruffles and migrate. Similar results were observed by pharmacological inhibition of PKCε with εV1-2, a specific PKCε inhibitor. PKCε was also required for invasiveness of NSCLC cells and modulated the secretion of extracellular matrix proteases and protease inhibitors. Finally, we found that PKCε-depleted NSCLC cells fail to disseminate to lungs in a mouse model of metastasis.

Conclusions

Our results implicate PKCε as a key mediator of Rac signaling and motility of lung cancer cells, highlighting its potential as a therapeutic target.

Curcumin sensitizes chemotherapeutic drugs via modulation of PKC, telomerase, NF-κB and HDAC in breast cancer

Background: Several tumor markers are overexpressed in breast cancer. Chemotherapy in breast cancer fails due to resistance to chemotherapeutic drugs. A phytochemical such as curcumin can be used in a therapeutic modality as it elicits anti-tumor effects. Methods: Action of curcumin on the expression of several tumor markers, such as protein kinase C, telomerase, NF-κB and histone deacetylase in MCF-7 (ER positive), MDA-MB-231 (ER negative), MCF-12F (control) and also in mice mammary tumors were investigated. Results: Curcumin downregulated the expression of tumor markers both in vitro and in vivo and sensitized tumor cells to the chemotherapeutic drugs cyclophosphamide and paclitaxel. Discussion: Curcumin may be of considerable value in synergistic therapy of cancer such that the drug dose level could be minimized reducing the associated toxicity.

The expression and role of protein kinase C (PKC) epsilon in clear cell renal cell carcinoma

Protein kinase C epsilon (PKCε), an oncogene overexpressed in several human cancers, is involved in cell proliferation, migration, invasion, and survival. However, its roles in clear cell renal cell carcinoma (RCC) are unclear. This study aimed to investigate the functions of PKCε in RCC, especially in clear cell RCC, to determine the possibility of using it as a therapeutic target. By immunohistochemistry, we found that the expression of PKCε was up-regulated in RCCs and was associated with tumor Fuhrman grade and T stage in clear cell RCCs. Clone formation, wound healing, and Borden assays showed that down-regulating PKCε by RNA interference resulted in inhibition of the growth, migration, and invasion of clear cell RCC cell line 769P and, more importantly, sensitized cells to chemotherapeutic drugs as indicated by enhanced activity of caspase-3 in PKCε siRNA-transfected cells. These results indicate that the overexpression of PKCε is associated with an aggressive phenotype of clear cell RCC and may be a potential therapeutic target for this disease.

Allosteric modulation by protein kinase Cε leads to modified responses of EGF receptor towards tyrosine kinase inhibitors

Recently, we described a novel function of over-expressed protein kinase Cε (PKCε) as a negative allosteric modulator of EGFR signalling in several head and neck squamous carcinoma (HNSCC) cell lines. Extending this work, here we present several lines of evidence for the potency of PKCε to differently modulate the efficacy of EGFR tyrosine kinase inhibitors (TKIs) such as gefitinib and lapatinib. Using the HNSCC cell line FaDu as a model, we demonstrate by co-immunoprecipitation the physical association of over-expressed PKCε with the EGFR which is stabilised by gefitinib and leads to an increase in gefitinib-induced inhibition of EGFR downstream signalling and elevated EGFR-ErbB2 heterodimerisation. Cell cycle and Western blot analysis revealed that the gefitinib-induced apoptosis was enhanced whereas the pro-apoptotic effect of lapatinib that requires another EGFR conformation was reduced by PKCε. Our findings suggest that due to elevated expression PKCε may associate with the EGFR resulting in conformational changes and different allosteric modulation of the EGFR behaviour towards TKIs. This surprising capacity indicates PKCε as a novel predictive marker protein in molecular cancer therapy with EGFR tyrosine kinase inhibitors.

Cisplatin increases B-cell-lymphoma-2 expression via activation of protein kinase C and Akt2 in endometrial cancer cells

Human carcinomas often show resistance to cisplatin and Bcl-2 is associated with resistance to cisplatin. However, Bcl-2 regulation on cisplatin treatment in human cancers is unknown. Here, we show a novel mechanism by which cisplatin treatment promotes resistance by increasing the expression of Bcl-2 mRNA. Bcl-2 mRNA and protein expression was increased in cisplatin-resistant endometrial cancer cell lines (KLE and HEC-1-A), but not in cisplatin-sensitive cell line (Ishikawa). Cisplatin-mediated increase in Bcl-2 expression was blocked by combination with either actinomycin D or cycloheximide. In addition, Bcl-2 inhibition by HA14-1 led to increased cisplatin-induced apoptosis in KLE and HEC-1-A, whereas Bcl-2 overexpression in Ishikawa led to decreased cisplatin-induced apoptosis. Inhibition of protein kinase C (PKC) activity prevented cisplatin-dependant increase in Bcl-2 mRNA, and induced apoptosis in KLE cells. Furthermore, PKC inhibition was associated with decreased Akt and NF-κB activity. Cells stably expressing shRNA for Akt isoforms revealed that Akt2 was involved in cisplatin-dependant increase in Bcl-2 and apoptosis. Overexpression of Akt2 in Akt2-deficient cells led to increased Bcl-2 expression on cisplatin treatment. Our data suggest a novel regulation pathway of Bcl-2 by cisplatin, via the activation of PKC and Akt2, which has a profound impact on resistance to cisplatin-induced apoptosis in endometrial cancer cells.

RTVP-1 expression is regulated by SRF downstream of protein kinase C and contributes to the effect of SRF on glioma cell migration

Gliomas are characterized by increased infiltration into the surrounding normal brain tissue. We recently reported that RTVP-1 is highly expressed in gliomas and plays a role in the migration of these cells, however the regulation of RTVP-1 expression in these cells is not yet described. In this study we examined the role of PKC in the regulation of RTVP-1 expression and found that PMA and overexpression of PKCα and PKCε increased the expression of RTVP-1, whereas PKCδ exerted an opposite effect. Using the MatInspector software, we identified a SRF binding site on the RTVP-1 promoter. Chromatin immunoprecipitation (ChIP) assay revealed that SRF binds to the RTVP-1 promoter in U87 cells, and that this binding was significantly increased in response to serum addition. Moreover, silencing of SRF blocked the induction of RTVP-1 expression in response to serum. We found that overexpression of PKCα and PKCε increased the activity of the RTVP-1 promoter and the binding of SRF to the promoter. In contrast, overexpression of PKCδ blocked the increase in RTVP-1 expression in response to serum and the inhibitory effect of PKCδ was abrogated in cells expressing a SRFT160A mutant. SRF regulated the migration of glioma cells and its effect was partially mediated by RTVP-1. We conclude that RTVP-1 is a PKC-regulated gene and that this regulation is at least partly mediated by SRF. Moreover, RTVP-1 plays a role in the effect of SRF on glioma cell migration.

Protein kinase Cα suppresses the expression of STC1 in MDA-MB-231 breast cancer cells

Several protein kinase C (PKC) isoforms have been shown to influence different cellular processes that may contribute to the malignancy of breast cancer cells. To obtain insight into mechanisms mediating the PKC effects, global gene expression was analyzed in MDA-MB-231 breast cancer cells in which PKCα, PKCδ or PKCε had been down-regulated with siRNA. Gene set enrichment analyses revealed that hypoxia-induced genes were enriched among genes that increased in PKCα-down-regulated cells. The STC1 mRNA, encoding stanniocalcin 1, was particularly up-regulated following depletion of PKCα and was also induced by hypoxia. Both hypoxia and PKCα down-regulation also led to increased STC1 protein levels. The results demonstrate that PKCα suppresses the expression of STC1 in breast cancer cells.

Protein kinase C-epsilon activation induces mitochondrial dysfunction and fragmentation in renal proximal tubules

PKC-ε activation mediates protection from ischemia-reperfusion injury in the myocardium. Mitochondria are a subcellular target of these protective mechanisms of PKC-ε. Previously, we have shown that PKC-ε activation is involved in mitochondrial dysfunction in oxidant-injured renal proximal tubular cells (RPTC; Nowak G, Bakajsova D, Clifton GL Am J Physiol Renal Physiol 286: F307-F316, 2004). The goal of this study was to examine the role of PKC-ε activation in mitochondrial dysfunction and to identify mitochondrial targets of PKC-ε in RPTC. The constitutively active and inactive mutants of PKC-ε were overexpressed in primary cultures of RPTC using the adenoviral technique. Increases in active PKC-ε levels were accompanied by PKC-ε translocation to mitochondria. Sustained PKC-ε activation resulted in decreases in state 3 respiration, electron transport rate, ATP production, ATP content, and activities of complexes I and IV and F(0)F(1)-ATPase. Furthermore, PKC-ε activation increased mitochondrial membrane potential and oxidant production and induced mitochondrial fragmentation and RPTC death. Accumulation of the dynamin-related protein in mitochondria preceded mitochondrial fragmentation. Antioxidants blocked PKC-ε-induced increases in the oxidant production but did not prevent mitochondrial fragmentation and cell death. The inactive PKC-ε mutant had no effect on mitochondrial functions, morphology, oxidant production, and RPTC viability. We conclude that active PKC-ε targets complexes I and IV and F(0)F(1)-ATPase in RPTC. PKC-ε activation mediates mitochondrial dysfunction, hyperpolarization, and fragmentation. It also induces oxidant generation and cell death, but oxidative stress is not the mechanism of RPTC death. These results show that in contrast to protective effects of PKC-ε activation in cardiomyocytes, sustained PKC-ε activation is detrimental to mitochondrial function and viability in RPTC.

Protein kinase C: an attractive target for cancer therapy

Apoptosis plays an important role during all stages of carcinogenesis and the development of chemoresistance in tumor cells may be due to their selective defects in the intracellular signaling proteins, central to apoptotic pathways. Consequently, many studies have focused on rendering the chemotherapy more effective in order to prevent chemoresistance and pre-clinical and clinical data has suggested that protein kinase C (PKC) may represent an attractive target for cancer therapy. Therefore, a complete understanding of how PKC regulates apoptosis and chemoresistance may lead to obtaining a PKC-based therapy that is able to reduce drug dosages and to prevent the development of chemoresistance.

Transgenic overexpression of PKCε in the mouse prostate induces preneoplastic lesions

It is well established that protein kinase C (PKC) isozymes play distinctive roles in mitogenic and survival signaling as well as in cancer progression. PKCε, the product of the PRKCE gene, is up-regulated in various types of cancers including prostate, lung and breast cancer. To address a potential role for PKCs in prostate cancer progression we generated three mouse transgenic lines expressing PKCα, PKCδ, or PKCε in the prostate epithelium under the control of the rat probasin (PB) promoter. Whereas PB-PKCε and PB-PKCδ mice did not show any evident phenotype, PB-PKCε mice developed prostate hyperplasia as well as prostate intraepithelial neoplasia (PIN) that displayed enhanced phospho-Akt, phospho-S6, and phospho-Stat3 levels, as well as enhanced resistance to apoptotic stimuli. PKCε overexpression was insufficient to drive neoplastic changes in the mouse prostate. Notably, overexpression of PKCε by adenoviral means in normal immortalized RWPE-1 prostate cells confers a growth advantage and hyperactivation of Erk and Akt. Our results argue for a causal link between PKCε overexpression and prostate cancer development.

Sustained inhibition of PKCα reduces intravasation and lung seeding during mammary tumor metastasis in an in vivo mouse model

Metastasis is the major reason for breast cancer-related deaths. Although there is a host of indirect evidence for a role of PKCα in primary breast cancer growth, its role in the molecular pathways leading to metastasis have not been comprehensively studied. By treating mice with αV5-3, a novel peptide inhibitor selective for PKCα, we were able to determine how PKCα regulates metastasis of mammary cancer cells using a syngeneic and orthotopic model. The primary tumor growth was not affected by αV5-3 treatment. However, the mortality rate was reduced and metastasis in the lung decreased by more than 90% in the αV5-3-treated mice relative to the control-treated mice. αV5-3 treatment reduced intravasation by reducing MMP-9 activities. αV5-3 treatment also reduced lung seeding of tumor cells and decreased cell migration, effects that were accompanied by a reduction in NFκB-activity and cell surface levels of the CXCL12 receptor, CXCR4. αV5-3 treatment caused no apparent toxicity in non-tumor bearing naïve mice. Rather, inhibiting PKCα protected against liver damage and increased the number of immune cells in tumor-bearing mice. Importantly, αV5-3 showed superior efficacy relative to anti-CXCR4 antibody in reducing metastasis, in vivo. Together, these data show that pharmacological inhibition of PKCα effectively reduces mammary cancer metastasis by targeting intravasation and lung seeding steps in the metastatic process and suggest that PKCα-specific inhibitors, such as αV5-3, can be used to study the mechanistic roles of PKCα specifically and may provide a safe and effective treatment for the prevention of lung metastasis of breast cancer patients.

PKCalpha expression is a marker for breast cancer aggressiveness

Background

Protein kinase C (PKC) isoforms are potential targets for breast cancer therapy. This study was designed to evaluate which PKC isoforms might be optimal targets for different breast cancer subtypes.

Results

In two cohorts of primary breast cancers, PKCα levels correlated to estrogen and progesterone receptor negativity, tumor grade, and proliferative activity, whereas PKCδ and PKCε did not correlate to clinicopathological parameters. Patients with PKCα-positive tumors showed poorer survival than patients with PKCα-negative tumors independently of other factors. Cell line studies demonstrated that PKCα levels are high in MDA-MB-231 and absent in T47D cells which proliferated slower than other cell lines. Furthermore, PKCα silencing reduced proliferation of MDA-MB-231 cells. PKCα inhibition or downregulation also reduced cell migration in vitro.

Conclusions

PKCα is a marker for poor prognosis of breast cancer and correlates to and is important for cell functions associated with breast cancer progression.

Protein kinase Cvarepsilon mediates Stat3Ser727 phosphorylation, Stat3-regulated gene expression, and cell invasion in various human cancer cell lines through integration with MAPK cascade (RAF-1, MEK1/2, and ERK1/2)

Protein kinase C epsilon (PKCε), a novel calcium-independent PKC isoform, has been shown to be a transforming oncogene. PKCε-mediated oncogenic activity is linked to its ability to promote cell survival. However, the mechanisms by which PKCε signals cell survival remain elusive. We found that signal transducers and activators of transcription 3 (Stat3), which is constitutively activated in a wide variety of human cancers, is a protein partner of PKCε. Stat3 has two conserved amino acid (Tyr705 and Ser727) residues, which are phosphorylated during Stat3 activation. PKCε interacts with Stat3α isoform which has Ser727 and not with Stat3β isoform which lacks Ser727. PKCε-Stat3 interaction and Stat3Ser727 phosphorylation was initially observed during induction of squamous cell carcinomas and in prostate cancer. Now we present that: 1) PKCε physically interacts with Stat3α isoform in various human cancer cells: skin melanomas (MeWo and WM266-4), gliomas (T98G and MO59K), bladder (RT-4 and UM-UC-3), colon (Caco-2), lung (H1650), pancreatic (PANC-1), and breast (MCF-7 and MDA:MB-231). 2) Inhibition of PKCε expression using specific siRNA inhibits Stat3Ser727 phosphorylation, Stat3-DNA binding, Stat3-regulated gene expression as well as cell invasion. 3) PKCε mediates Stat3Ser727 phosphorylation via integration with the MAPK cascade (RAF-1, MEK1/2, and ERK1/2). The results indicate that PKCε-mediated Stat3Ser727 phosphorylation is essential for constitutive activation of Stat3 and cell invasion in various human cancers.

Rapid modulation of the organic anion transporting polypeptide 2B1 (OATP2B1, SLCO2B1) function by protein kinase C-mediated internalization

Members of the organic anion transporting polypeptide (OATP) family are involved in various pharmacological, pathophysiological, and physiological processes, such as hepatic drug uptake, progress of cancer, or transport of hormones. Although variability in expression and function of OATPs has been investigated in detail, data concerning regulation are rather limited. Here, we report a novel mechanism for rapid regulation of OATP2B1 mediated by protein kinase C (PKC) resulting in significant changes of transport activity. PKC activation by the phorbol ester (phorbol 12-myristate 13-acetate, PMA) resulted in increased phosphorylation of OATP2B1 as well as reduced OATP2B1 transport activity with a decrease in V(max) of E(1)S uptake (288 +/- 21 (control) versus 165 +/- 16 pmol/min/mg of protein (PMA)). This effect was sensitive to the PKC inhibitor bisindolylmaleimide I (BIM-I). Confocal microscopy, fluorescence-based internalization assay, and live-cell imaging using green fluorescent protein-tagged OATP2B1 revealed that transport inhibition was due to internalization of the transporter. Furthermore, colocalization with LAMP-2 and chloroquine-sensitive degradation of OATP2B1 suggest that the internalized protein is targeted to a lysosomal degradation pathway. With regard to the underlying mechanism inhibition of caveolin/lipid raft-mediated endocytosis failed to prevent OATP2B1 internalization, whereas inhibition of clathrin-mediated processes blocked OATP2B1 sequestration. However, small interfering RNA-mediated clathrin knock-down affected general trafficking of OATP2B1 and resulted in intracellular accumulation in the absence of PMA. In conclusion, our data demonstrate that OATP2B1 function is regulated by PKC-mediated, clathrin-dependent internalization and followed by lysosomal degradation. Furthermore, internalization could be shown in an ex vivo placenta perfusion. Our findings represent a new, rapid mechanism in regulation of human OATPs.

Differential subcellular expression of protein kinase C betaII in breast cancer: correlation with breast cancer subtypes

Protein kinase C betaII (PKCβII) represents a novel potential target for anticancer therapies in breast cancer. In order to identify patient subgroups which might benefit from PKC-targeting therapies, we investigated the expression of PKCβII in human breast cancer cell lines and in a tissue microarray (TMA). We first screened breast cancer cell line representatives of breast cancer subtypes for PKCβII expression at the mRNA and at the protein levels. We analyzed a TMA comprising of tumors from 438 patients with a median followup of 15.4 years for PKCβII expression by immunohistochemistry along with other prognostic factors in breast cancer. Among a panel of human breast cancer cell lines, only MDA-MB-436, a triple negative basal cell line, showed overexpression for PKCβII both at the mRNA and at the protein levels. In breast cancer patients, cytoplasmic expression of PKCβII correlated positively with human epidermal growth factor receptor-2 (HER-2; P = 0.01) and Ki-67 (P = 0.016), while nuclear PKCβII correlated positively with estrogen receptor (ER; P = 0.016). The positive correlation of CK5/6 with cytoplasmic PKCβII (P = 0.033) lost statistical significance after adjusting for multiple comparisons (P = 0.198). Cytoplasmic PKCβII did not correlate with cyclooxygenase (COX-2; P = 0.925) and vascular endothelial growth factor (P = 1). There was no significant association between PKCβII staining and overall survival. Cytoplasmic PKCβII correlates with HER-2 and Ki-67, while nuclear PKCβII correlates with ER in breast cancer. Our study suggests the necessity for assessing the subcellular localization of PKCβII in breast cancer subtypes when evaluating the possible effectiveness of PKCβII-targeting agents.

The protein kinase C agonist PEP005 (ingenol 3-angelate) in the treatment of human cancer: a balance between efficacy and toxicity

The diterpene ester ingenol-3-angelate (referred to as PEP005) is derived from the plant Euphorbia peplus. Crude euphorbia extract causes local toxicity and transient inflammation when applied topically and has been used in the treatment of warts, skin keratoses and skin cancer. PEP005 is a broad range activator of the classical (α, β, γ) and novel (δ, ε, η, θ) protein kinase C isoenzymes. Direct pro-apoptotic effects of this drug have been demonstrated in several malignant cells, including melanoma cell lines and primary human acute myelogenous leukemia cells. At micromolar concentrations required to kill melanoma cells this agent causes PKC-independent secondary necrosis. In contrast, the killing of leukemic cells occurs in the nanomolar range, requires activation of protein kinase C δ (PKCδ) and is specifically associated with translocation of PKCδ from the cytoplasm to the nuclear membrane. However, in addition to this pro-apoptotic effect the agent seems to have immunostimulatory effects, including: (i) increased chemokine release by malignant cells; (ii) a general increase in proliferation and cytokine release by activated T cells, including T cells derived from patients with chemotherapy-induced lymphopenia; (iii) local infiltration of neutrophils after topical application with increased antibody-dependent cytotoxicity; and (iv) development of specific anti-cancer immune responses by CD8(+) T cells in animal models. Published studies mainly describe effects from in vitro investigations or after topical application of the agent, and careful evaluation of the toxicity after systemic administration is required before the possible use of this agent in the treatment of malignancies other than skin cancers.

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.

Preclinical development of a bifunctional cancer cell homing, PKCepsilon inhibitory peptide for the treatment of head and neck cancer

Head and neck squamous cell carcinoma (HNSCC) is the sixth most frequent cancer worldwide, comprising approximately 50% of all malignancies in some developing nations. Our recent work identified protein kinase Cepsilon (PKCepsilon) as a critical and causative player in establishing an aggressive phenotype in HNSCC. In this study, we investigated the specificity and efficacy of HN1-PKCepsilon, a novel bifunctional cancer cell homing, PKCepsilon inhibitory peptide, as a treatment for HNSCC. HN1-PKCepsilon peptide was designed by merging two separate technologies and synthesized as a capped peptide with two functional modules, HN1 (cancer cell homing) and PKCepsilon (specific PKCepsilon inhibitory), connected by a novel linker module. HN1-PKCepsilon preferentially internalized into UMSCC1 and UMSCC36 cells, two HNSCC cell lines, in comparison with oral epithelial cells: 82.1% positive for UMSCC1 and 86.5% positive for UMSCC36 compared with 1.2% positive for oral epithelial cells. In addition, HN1-PKCepsilon penetrated HNSCC cells in a dose- and time-dependent manner. Consistent with these in vitro observations, systemic injection of HN1-PKCepsilon resulted in selective delivery of HN1-PKCepsilon into UMSCC1 xenografts in nude mice. HN1-PKCepsilon blocked the translocation of active PKCepsilon in UMSCC1 cells, confirming HN1-PKCepsilon as a PKCepsilon inhibitor. HN1-PKCepsilon inhibited cell invasion by 72 +/- 2% (P < 0.001, n = 12) and cell motility by 56 +/- 2% (P < 0.001, n = 5) in UMSCC1 cells. Moreover, in vivo bioluminescence imaging showed that HN1-PKCepsilon significantly (83 +/- 1% inhibition; P < 0.02) retards the growth of UMSCC1 xenografts in nude mice. Our work indicates that the bifunctional HN1-PKCepsilon inhibitory peptide represents a promising novel therapeutic strategy for HNSCC.

Phorbol-12-myristate 13-acetate acting through protein kinase Cepsilon induces translocator protein (18-kDa) TSPO gene expression

Translocator protein (TSPO) is an 18-kDa cholesterol-binding protein that is expressed at high levels in steroid synthesizing and several cancer cells where it is involved in steroidogenesis and cell proliferation, respectively. The factors regulating Tspo expression are unknown. We analyzed Tspo transcriptional responses to the tumor promoter, phorbol-12-myristate 13-acetate (PMA), in cells with varying TSPO levels. PMA induced Tspo promoter activity and Tspo mRNA levels in TSPO-poor nonsteroidogenic cells (NIH-3T3 fibroblasts and COS-7 kidney) but not in TSPO-rich steroidogenic cells (MA-10 Leydig) with high basal Tspo transcriptional activity. The stimulatory effect of PMA was mediated by an 805-515-bp region upstream of the transcription start site. Electrophoretic mobility shift assay (EMSA) revealed that PMA induced binding of c-jun and GA-binding protein transcription factor (GABP-alpha) to their respective activator protein 1 (AP1) and v-ets erythroblastosis virus E26 oncogene homologue (Ets) sites in this region. Protein kinase C (PKC)-specific inhibitors blocked PMA induction of Tspo promoter activity with an inhibition profile suggestive of involvement of PKCepsilon. PKCepsilon expression correlated with TSPO content in the three cell lines. In NIH-3T3 cells, PKCepsilon overexpression induced Tspo promoter activity and mRNA levels and enhanced PMA-induced up regulation of c-jun and TSPO. In MA-10 cells, a PKCepsilon-specific translocation inhibitor peptide reduced basal Tspo promoter activity. PKCepsilon siRNA pool reduced PKCepsilon and TSPO levels in MA-10 cells indicating a role for PKCepsilon in regulating TSPO expression. Taken together, these data suggest that elevated TSPO expression in steroidogenic cells may be due to high constitutive expression of PKCepsilon that renders them unresponsive to further induction while PMA activation of PKCepsilon drives inducible TSPO expression in nonsteroidogenic cells, likely through AP1 and Ets.

Protein kinase C epsilon: an oncogene and emerging tumor biomarker

Members of the protein kinase C (PKC) family have long been studied for their contributions to oncogenesis. Among the ten different isoforms of this family of serine/threonine kinases, protein kinase Cε (PKCε) is one of the best understood for its role as a transforming oncogene. In vitro, overexpression of PKCε has been demonstrated to increase proliferation, motility, and invasion of fibroblasts or immortalized epithelial cells. In addition, xenograft and transgenic animal models have clearly shown that overexpression of PKCε is tumorigenic resulting in metastatic disease. Perhaps most important in implicating the epsilon isoform in oncogenesis, PKCε has been found to be overexpressed in tumor-derived cell lines and histopathological tumor specimens from various organ sites. Combined, this body of work provides substantial evidence implicating PKCε as a transforming oncogene that plays a crucial role in establishing an aggressive metastatic phenotype. Reviewed here is the literature that has led to the current understanding of PKCε as an oncogene. Moreover, this review focuses on the PKCε-mediated signaling network for cell motility and explores the interaction of PKCε with three major PKCε signaling nodes: RhoA/C, Stat3 and Akt. Lastly, the emerging role of PKCε as a tumor biomarker is discussed.

Opposite effects of protein kinase C beta1 (PKCbeta1) and PKCepsilon in the metastatic potential of a breast cancer murine model

In this paper we investigated whether protein kinase C (PKC) beta1 and PKCepsilon, members of the classical and novel PKC family, respectively, induce phenotypic alterations that could be associated with tumor progression and metastatic dissemination in a murine model of breast cancer. Stable overexpression of PKCbeta1 in LM3 cells altered their ability to proliferate, adhere, and survive, and impaired their tumorigenicity and metastatic capacity. Moreover, PKCbeta1 induced the re-expression of fibronectin, an extracellular matrix glycoprotein which loss has been associated with the acquisition of a transformed phenotype in different cell models, and exerted an important inhibition on proteases production, effects that probably impact on LM3 invasiveness and dissemination. Conversely, PKCepsilon overexpression enhanced LM3 survival, anchorage-independent growth, and caused a significant increase in spontaneous lung metastasis. Our results suggest PKCbeta1 functions as an inhibitory protein for tumor growth and metastasis dissemination whereas PKCepsilon drives metastatic dissemination without affecting primary tumor growth.

Protein kinase C isoforms: Multi-functional regulators of cell life and death

The protein kinase C (PKC) family consists of 10 related serine/threonine protein kinases some of which are critical regulators of cell proliferation, survival and cell death. While early studies relied on broad spectrum chemical activators or inhibitors of this family, the generation of isoform specific tools has greatly facilitated our understanding of the contribution of specific PKC isoforms to cell proliferation and apoptosis. These studies suggest that PKC-alpha, PKC-epsilon, and the atypical PKC's, PKC-lambda/iota and PKC-zeta, preferentially function to promote cell proliferation and survival, while the novel isoform, PKC-delta is an important regulator of apoptosis. The essential role of this kinase family in both cell survival and apoptosis suggests that specific isoforms may function as molecular sensors, promoting cell survival or cell death depending on environmental cues. Given their central role in cell and tissue homeostasis, it is not surprising that the expression or activity of some of these kinases is altered in human diseases, particularly cancer.

Target validation to biomarker development: focus on RNA interference

With the growing number of putative molecular targets and increased economic pressure on companies developing novel drugs, particularly in the cancer area, the need to work on highly validated targets is essential. The use of biomarkers for proof of mechanism of action is becoming an important tool in validation efforts in the preclinical phase of drug development, helping to reduce the attrition rate of candidate drugs once they have entered the clinic. In this review, we highlight how RNA interference (RNAi) has become the method of choice to perform both target validation and identification in academia and industry. RNAi takes advantage of a naturally occurring mechanism whereby cells regulate the expression of genes at the post-transcriptional level, and it introduces a new era in loss-of-function experiments, allowing for the rapid measurement of the phenotype observed upon target expression abrogation. Design of both small-interfering RNA and short-hairpin RNA constructs and their delivery into cells have emerged as the most important aspects of this technology, and reduction or measurement of potential unwanted off-target effects must also be taken into consideration. A number of successes have already been described, and several oncology targets and biomarkers have been identified and validated with this technique.

Activation of PKC epsilon induces lactotroph proliferation through ERK1/2 in response to phorbol ester

The aim of this investigation was to contribute to current knowledge about intracellular mechanisms that are involved in lactotroph cell proliferation, by evaluating the role of PKCalpha, PKCepsilon and extracellular-signal regulated kinase (ERK) 1/2 in response to phorbol 12-myristate13-acetate (PMA). In primary pituitary cultures, the activation of protein kinase C (PKC) by PMA for 15 min stimulated lactotroph proliferation; whereas a prolonged activation for 3-8h diminished this proliferative effect. The use of PMA for 15 min-activated PKCepsilon and ERK1/2, whereas incubation with PMA for 3 h induced PKCalpha activation and attenuated the PMA-triggered phosphorylation of ERK1/2. The following inhibitors: PKCs (bisindolylmaleimide I), PKCepsilon (epsilonV1 peptide) and ERK1/2 (PD98059) prevented the mitogenic activity induced by PMA for 15 min. Lactotroph cells stimulated with PMA for 15 min showed a translocation of PKCepsilon to membrane compartment and nucleus. These results thus establish that PKCepsilon plays an essential role in the lactotroph proliferation induced by PMA by triggering signals that involve ERK1/2 activation.

Overexpression of protein kinase Calpha mRNA may be an independent prognostic marker for gastric carcinoma

BACKGROUND AND OBJECTIVES

The variability of the prognosis of gastric carcinoma drives extensive researches for novel prognostic markers. The aims of this study were to correlate the expression of protein kinase Calpha (PKCalpha) mRNA with clinicopathological parameters and to evaluate the significant value of PKCalpha in gastric carcinoma prognosis.

METHODS

PKCalpha mRNA levels were analyzed in tumor/non-tumor pairs of gastric tissues from surgical specimens of 41 patients with gastric carcinoma employing quantitative real-time polymerase chain reaction. Expression of PKCalpha in gastric carcinoma was also examined using immunohistochemistry.

RESULTS

PKCalpha mRNA expression was significantly upregulated in gastric carcinoma (P = 0.007). Overexpression of PKCalpha mRNA was correlated with distant metastasis (P = 0.040). Patients with high PKCalpha mRNA expression had a significantly poorer overall survival compared with patients with low PKCalpha mRNA expression (P = 0.0113). The uni-variate Cox regression analysis showed that high PKCalpha mRNA expression (P = 0.0363) and depth of invasion (P = 0.0443) were two significant prognostic markers for gastric carcinoma. In backward stepwise multi-variate analysis, PKCalpha mRNA overexpression was also proved to be an independent prognostic marker for gastric carcinoma (P = 0.0275).

CONCLUSIONS

Our results suggest that overexpression of PKCalpha mRNA has correlation with distant metastasis and may be an independent prognostic marker for gastric carcinoma.

The identification and characterization of novel PKCϵ phosphorylation sites provide evidence for functional cross-talk within the PKC superfamily

PKCϵ (protein kinase Cϵ) is a phospholipid-dependent serine/threonine kinase that has been implicated in a broad array of cellular processes, including proliferation, survival, migration, invasion and transformation. Here we demonstrate that, in vitro, PKCϵ undergoes autophosphorylation at three novel sites, Ser234, Ser316 and Ser368, each of which is unique to this PKC isoform and is evolutionarily conserved. We show that these sites are phosphorylated over a range of mammalian cell lines in response to a number of different stimuli. Unexpectedly, we find that, in a cellular context, these phosphorylation events can be mediated in-trans by cPKC (classical PKC) isoforms. The functional significance of this cross-talk is illustrated through the observation that the cPKC-mediated phosphorylation of PKCϵ at residue Ser368 controls an established PKCϵ scaffold interaction. Thus our current findings identify three new phosphorylation sites that contribute to the isoform-specific function of PKCϵ and highlight a novel and direct means of cross-talk between different members of the PKC superfamily.

Protein kinase Cepsilon mediates polymeric fibronectin assembly on the surface of blood-borne rat breast cancer cells to promote pulmonary metastasis

Malignant breast cancer cells that have entered the blood circulation from primary mammary fat pad tumors or are grown in end-over-end suspension culture assemble a characteristic, multi-globular polymeric fibronectin (polyFn) coat on their surfaces. Surface polyFn is critical for pulmonary metastasis, presumably by facilitating lung vascular arrest via endothelial dipeptidylpeptidase IV (CD26). Here, we show that cell-surface polyFn assembly is initiated by the state of suspension, is dependent upon the synthesis and secretion of cellular Fn, and is augmented in a dose- and time-dependent manner by plasma Fn. PolyFn assembly is regulated by protein kinase Cepsilon (PKCepsilon), which translocates rapidly and in increasing amounts from the cytosol to the plasma membrane and is phosphorylated. PolyFn assembly is impeded by select inhibitors of this kinase, i.e. bisindolylmaleimide I, Ro-32-0432, Gö6983, and Rottlerin, by the phorbol 12-myristate 13-acetate-mediated and time-dependent loss of PKCepsilon protein and decreased plasma membrane translocation, and more specifically, by stable transfection of lung-metastatic MTF7L breast cancer cells with small interfering RNA-PKCepsilon and dominant-negative PKCepsilon constructs (e.g. RD-PKCepsilon). The inability to assemble a cell surface-associated polyFn coat by knockdown of endogenous Fn or PKCepsilon impedes cancer cells from metastasis to the lungs. The present studies identify a novel regulatory mechanism for polyFn assembly on blood-borne breast cancer cells and depict its effect on pulmonary metastasis.

Molecular epidemiologic features of inflammatory breast cancer: a comparison between Egyptian and US patients

BACKGROUND

Inflammatory breast cancer (IBC) is a lethal form of breast cancer with unknown etiology. A higher frequency of IBC and a more aggressive IBC phenotype was reported in Egypt than in the United States. This difference in disease frequency and presentation might be related to molecular epidemiologic factors.

METHODS

We used tumor blocks and demographic, epidemiologic, and clinical data of 48 IBC patients from Egypt and 12 patients from the United States. We counted tumor emboli in tumors before and after immunohistochemical staining with lymphatic vessel endothelial receptor-1 (LYVE-1), and measured the expression of RhoC GTPase protein in the two groups.

RESULTS

Erythema, edema, and peau d'orange were found in 77% of the Egyptian patients as compared with 29% found in the US patients (P=0.02). The number of tumor emboli was significantly higher in tumors from Egypt (mean+/-SD, 14.1+/-14.0) than in the tumors from the United States (5.0+/-4.0, P=0.01). The number of tumor emboli in LYVE-1 positive vessels was higher in tumors from Egypt (3.5+/-2.8) than tumors from the United States (1.6+/-0.5, P=0.15). We detected a high level of RhoC in 87% of the tumors from Egypt and 14% of the tumors from the United States (P=0.0003).

CONCLUSION

Patients from Egypt have a more aggressive form of IBC than those in the United States. Our analysis of IBC patients shows that distinct molecular phenotypes can be found when these two study populations are compared. Future studies should explore the epidemiologic and environmental exposures and the genetic factors that might lead to the different clinical and molecular features of IBC in patients from these two countries.

A Protein Kinase Cε-Anti-apoptotic Kinase Signaling Complex Protects Human Vascular Endothelial Cells against Apoptosis through Induction of Bcl-2

Endothelial cell apoptosis is associated with vascular injury and predisposes to atherogenesis. Endothelial cells express anti-apoptotic genes including Bcl-2, Bcl-XL and survivin, which also contribute to angiogenesis and vascular remodeling. We report a central role for protein kinase Cepsilon (PKCepsilon) in the regulation of Bcl-2 expression and cytoprotection of human vascular endothelium against apoptosis. Using myristoylated inhibitory peptides, a predominant role for PKCepsilon in vascular endothelial growth factor-mediated endothelial resistance to apoptosis was revealed. Immunoblotting of endothelial cells infected with an adenovirus expressing a constitutively active form of PKCepsilon (Adv-PKCepsilon-CA) or control Adv-beta-galactosidase demonstrated a 3-fold, PKCepsilon-dependent increase in Bcl-2 expression, with no significant change in Bcl-XL, Bad, Bak, or Bax. The induction of Bcl-2 inhibited apoptosis induced by serum starvation or etoposide, and PKCepsilon activation attenuated etoposide-induced caspase-3 cleavage. The functional role of Bcl-2 was confirmed with Bcl-2 antagonist HA-14-1. Inhibition of phosphoinositide 3-kinase attenuated vascular endothelial growth factor-induced protection against apoptosis, and this was rescued by overexpression of constitutively active PKCepsilon, suggesting PKCepsilon acts downstream of phosphoinositide 3-kinase. Co-immunoprecipitation studies demonstrated a physical interaction between PKCepsilon and Akt, which resulted in formation of a signaling complex, leading to optimal induction of Bcl-2. This study reveals a pivotal role for PKCepsilon in endothelial cell cytoprotection against apoptosis. We demonstrate that PKCepsilon forms a signaling complex and acts co-operatively with Akt to protect human vascular endothelial cells against apoptosis through induction of the anti-apoptotic protein Bcl-2 and inhibition of caspase-3 cleavage.

The therapeutic role of targeting protein kinase C in solid and hematologic malignancies

The protein kinase C (PKC) family, the most prominent target of tumor-promoting phorbol esters, is functionally linked to cell differentiation, growth, survival, migration and tumorigenesis and so mediates tumor cell proliferation, survival, multidrug resistance, invasion, metastasis and tumor angiogenesis. Therefore, targeting PKC isozymes may represent an attractive target for novel anticancer therapies. Recent preclinical and clinical studies using the macrocyclic bisindolylmaleimide enzastaurin or the N-benzylstaurosporine midostaurin demonstrate promising activity of PKC inhibitors in a variety of tumors, including diffuse large B-cell lymphoma, multiple myeloma and Waldenstroem's macroglobulinemia. However, our knowledge of PKCs in tumorigenesis is still only partial and each PKC isoform may contribute to tumorigenesis in a distinct way. Specifically, PKC isoforms have vastly different roles, which vary depending on expression levels of organ and tissue distribution, cell type, intracellular localization, protein-protein and lipid-protein interactions and the biologic environment. Although PKC activation generally positively affects tumor cell growth, motility, invasion and metastasis, recent reports show that many PKCs can also have negative effects. Therefore, it is necessary to further dissect the relative contribution of PKC isozymes in the development and progression of specific tumors in order to identify therapeutic opportunities, using either PKC inhibitors or PKC activators.