Inhibition of human p53 basal transcription by down-regulation of protein kinase Cdelta

Cell Fate Regulation upon DNA Damage: p53 Serine 46 Kinases Pave the Cell Death Road

Mild and massive DNA damage are differentially integrated into the cellular signaling networks and, in consequence, provoke different cell fate decisions. After mild damage, the tumor suppressor p53 directs the cellular response to cell cycle arrest, DNA repair, and cell survival, whereas upon severe damage, p53 drives the cell death response. One posttranslational modification of p53, phosphorylation at Serine 46, selectively occurs after severe DNA damage and is envisioned as a marker of the cell death response. However, the molecular mechanism of action of the p53 Ser46 phospho-isomer, the molecular timing of this phosphorylation event, and its activating effects on apoptosis and ferroptosis still await exploration. In this essay, the current body of evidence on the molecular function of this deadly p53 mark, its evolutionary conservation, and the regulation of the key players of this response, the p53 Serine 46 kinases, are reviewed and dissected.

Tumor suppressive role for kinases phosphorylating p53 in DNA damage-induced apoptosis

Tumor suppressor p53 plays an important role in cancer prevention. Under normal conditions, p53 is maintained at a low level. However, in response to various cellular stresses, p53 is stabilized and activated, which, in turn, initiates DNA repair, cell-cycle arrest, senescence and apoptosis. Post-translational modifications of p53 including phosphorylation, ubiquitination, and acetylation at multiple sites are important to regulate its activation and subsequent transcriptional gene expression. Particularly, phosphorylation of p53 plays a critical role in modulating its activation to induce apoptosis in cancer cells. In this context, previous studies show that several serine/threonine kinases regulate p53 phosphorylation and downstream gene expression. The molecular basis by which p53 and its kinases induce apoptosis for cancer prevention has been extensively studied. However, the relationship between p53 phosphorylation and its kinases and how the activity of kinases is controlled are still largely unclear; hence, they need to be investigated. In this review, we discuss various roles for p53 phosphorylation and its responsible kinases to induce apoptosis and a new therapeutic approach in a broad range of cancers.

A lincRNA-p21/miR-181 family feedback loop regulates microglial activation during systemic LPS- and MPTP- induced neuroinflammation

The role of microglial-mediated sustained neuroinflammation in the onset and progression of Parkinson’s disease (PD) is well established, but the mechanisms contributing to microglial activation remain unclear. LincRNA-p21, a well studied long intergenic noncoding RNA (lincRNA), plays pivotal roles in diverse biological processes and diseases. Its role in microglial activation and inflammation-induced neurotoxicity, however, has not yet been fully elucidated. Here, we report that lincRNA-p21 promotes microglial activation through a p53-dependent transcriptional pathway. We further demonstrate that lincRNA-p21 competitively binds to the miR-181 family and induces microglial activation through the miR-181/PKC-δ pathway. Moreover, PKC-δ induction further increases the expression of p53/lincRNA-p21 and thus forms a circuit. Taken together, our results suggest that p53/lincRNA-p21, together with miR-181/PKC-δ, form a double-negative feedback loop that facilitates sustained microglial activation and the deterioration of neurodegeneration.

Wild-type p53 controls the level of fibronectin expression in breast cancer cells

Aberrant fibronectin (FN) expression is associated with poor prognosis, cell adhesion, and cell motility in a variety of cancer cells. In this study, we investigated the relationship between p53 and FN expression in breast cancer cells. Basal FN expression was significantly decreased by treatment with the p53 activator III, RITA, in MCF7 breast cancer cells with wild-type p53. In addition, overexpression of wild-type p53 markedly decreased the level of FN expression in p53-mutant breast cancer cells. To examine the mechanism underlying the relationship between p53 and FN expression, we treated MCF7 breast cancer cells with the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). Our results showed that basal FN expression was increased by TPA treatment in a time-dependent manner. In contrast, the level of p53 expression was decreased by TPA treatment. However, the expression of FN and p53 was not altered by TPA in p53-mutant breast cancer cells. Furthermore, the alterations in FN and p53 expression in response to TPA were prevented by a specific MEK inhibitor, UO126. Finally, we demonstrated that TPA triggers degradation of p53 through the proteasomal pathway in MCF7 cells. TPA-induced FN expression was decreased by the proteasome inhibitor MG132. Under the same condition, p53 protein expression, but not mRNA expression, was reversed by MG132. Taken together, our data demonstrate that the level of FN expression is associated with the status and expression of p53 in breast cancer cells.

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.

Crosstalk between tumor suppressors p53 and PKCδ: Execution of the intrinsic apoptotic pathways

p53 and PKCδ are tumor suppressors that execute apoptotic mechanisms in response to various cellular stresses. p53 is a transcription factor that is frequently mutated in human cancers; it regulates apoptosis in transcription-dependent and -independent ways in response to genotoxic stresses. PKCδ is a serine/threonine protein kinase and mutated in human cancers. Available evidence shows that PKCδ activates p53 by direct and/or indirect mechanisms. Moreover, PKCδ is also implicated in the transcriptional regulation of p53 in response to DNA damage. Recent findings demonstrated that p53, in turn, binds onto the PKCδ promoter and induces its expression upon DNA damage to facilitate apoptosis. Both p53 and PKCδ are associated with the apoptotic mechanisms in the mitochondria by regulating Bcl-2 family proteins to provide mitochondrial outer membrane permeabilization. This review discusses the crosstalk between p53 and PKCδ in the context of apoptotic cell death and cancer therapy.

PKCδ stabilizes TAp63 to promote cell apoptosis

PKCδ and p63 are respectively reported to play important roles in cell apoptosis. But there is no report on interaction between them in regulation of apoptosis. In the present study, we found that PKCδ can directly associate and up-regulate TA isoforms of p63 (TAp63) proteins via increasing their stability. PKCδ kinase activity and Thr157 site in TAp63 are crucial for this PKCδ-induced accumulation of TAp63. PKCδ can also enhance TAp63-mediated transcription and cell apoptosis. Taken together, our data indicate that PKCδ phosphorylates TAp63 proteins at Thr157 to stabilize them and promote cell apoptosis.

Cancer-associated protein kinase C mutations reveal kinase's role as tumor suppressor

Protein kinase C (PKC) isozymes have remained elusive cancer targets despite the unambiguous tumor promoting function of their potent ligands, phorbol esters, and the prevalence of their mutations. We analyzed 8% of PKC mutations identified in human cancers and found that, surprisingly, most were loss of function and none were activating. Loss-of-function mutations occurred in all PKC subgroups and impeded second-messenger binding, phosphorylation, or catalysis. Correction of a loss-of-function PKCβ mutation by CRISPR-mediated genome editing in a patient-derived colon cancer cell line suppressed anchorage-independent growth and reduced tumor growth in a xenograft model. Hemizygous deletion promoted anchorage-independent growth, revealing that PKCβ is haploinsufficient for tumor suppression. Several mutations were dominant negative, suppressing global PKC signaling output, and bioinformatic analysis suggested that PKC mutations cooperate with co-occurring mutations in cancer drivers. These data establish that PKC isozymes generally function as tumor suppressors, indicating that therapies should focus on restoring, not inhibiting, PKC activity.

Translocation of protein kinase C δ contributes to the moderately high glucose-, but not hypoxia-induced proliferation in primary cultured human retinal endothelial cells

Diabetic retinopathy is one of the most common complications in patients with diabetes and affects ~75% of them within 15 years of the onset of the disease. Activation of protein kinase C (PKC) is a key feature of diabetes mellitus and may be involved in the pathogenesis of diabetic retinopathy. The present study aimed to examine the translocation of protein kinase C (PKC) isoforms, which are triggered by high an moderately high glucose levels as well as hypoxic conditions. The underlying cell mechanisms of PKC translocation in primary cultured human retinal endothelial cells (HRECs) were also investigated. The expression levels of PKC isoforms were assessed using western blot analysis. Cell proliferation was determined using the MTT assay and DNA synthesis was assessed by bromodeoxyuridine incorporation. Translocation of PKC isoforms was examined by western blot analysis and immunofluorescence. The expression of PKC α, βI, βII, δ and ε was detected, while PKC ζ was not detected in HRECs. The results of the present study were consistent with the findings of a previous study by our group, reporting that moderately high glucose levels and hypoxia, but not high glucose levels, significantly increased cell proliferation. It was demonstrated that the PKC δ isoform was translocated from the cytosol to the membrane only under moderately high glucose conditions, while PKC α and ε isoforms were translocated from the cytosol to the membrane at high glucose conditions. In addition, PKC βI was translocated under all three conditions. Translocation of PKC βII was comparable among all groups. Furthermore, rottlerin, an inhibitor of PKC δ, blocked cell proliferation, which was induced by moderately high glucose levels, but not by hypoxia. Ro32-0432, an inhibitor of PKC α, βI and ε, did not significantly affect proliferation of HRECs in all treatment groups. In conclusion, the present study suggested that PKC α, βI, βII, δ and ε were expressed in primary cultured HRECs, whereas PKC ζ was not. Cell proliferation induced by moderately high glucose concentrations was associated with translocation of the PKC δ isoform; however, hypoxic conditions did not induce translocation.

Effects of prostaglandin E2 on p53 mRNA transcription and p53 mutagenesis during T-cell-independent human B-cell clonal expansion

Within T-cell-dependent germinal centers, p53 gene transcription is repressed by Bcl-6 and is thus less vulnerable to mutation. Malignant lymphomas within inflamed extranodal sites exhibit a relatively high incidence of p53 mutations. The latter might originate from normal B-cell clones manifesting activation-induced cytosine deaminase (AID) and up-regulated p53 following T-cell-independent (TI) stimulation. We here examine p53 gene transcription in such TI clones, with a focus on modulatory effects of prostaglandin E2 (PGE2), and evaluate progeny for p53 mutations. Resting IgM(+)IgD(+)CD27(-) B cells from human tonsils were labeled with CFSE and stimulated in vitro with complement-coated antigen surrogate, IL-4, and BAFF ± exogenous PGE2 (50 nM) or an analog specific for the EP2 PGE2 receptor. We use flow cytometry to measure p53 and AID protein within variably divided blasts, qRT-PCR of p53 mRNA from cultures with or without actinomycin D to monitor mRNA transcription/stability, and single-cell p53 RT-PCR/sequencing to assess progeny for p53 mutations. We report that EP2 signaling triggers increased p53 gene transcriptional activity in AID(+) cycling blasts (P<0.01). Progeny exhibit p53 mutations at a frequency (8.5 × 10(-4)) greater than the baseline error rate (<0.8 × 10(-4)). We conclude that, devoid of the repressive influences of Bcl-6, dividing B lymphoblasts in inflamed tissues should display heightened p53 transcription and increased risk of p53 mutagenesis.

PMA increases M3 muscarinic receptor levels and decreases retinal cells proliferation through a change in the levels of cell-cycle regulatory proteins

Protein kinase C (PKC) pathway plays important roles in different phenomena in nervous system development. Our previous data demonstrated that phorbol 12-myristate 13-acetate (PMA) treatment, a PKC activator, for 48 h decreases retinal cells proliferation by a mechanism mediated by muscarinic receptor activation, involving a decrease in M1 receptors levels. The aim of this work was to analyze how PMA interferes in the levels of cell cycle control proteins p53, p21 and cyclin D1 and also to investigate its influence on M3 receptor levels. Our results show that PMA (50 ng/mL) produces a significant increase in p21 and p53 levels, decreases cyclin D1 levels, and also enhances M3 receptors levels in cell cultures. Evaluating the postnatal retinal tissue development until 30 days, we observed that tissue differentiation is accompanied by an increase in M3 and p21 levels. Based on our results we suggest that PMA treatment is promoting a change in muscarinic receptors expression mimicking the pattern observed during tissue differentiation, indicating that PMA is probably accelerating the cholinergic differentiation in rat retinal cell cultures.

High glucose induces apoptosis in embryonic neural progenitor cells by a pathway involving protein PKCδ

Diabetic-induced neural tube defects in embryos are caused by apoptosis of neural progenitor cells (NPCs); however, the underlying mechanisms are poorly understood. The present study is aimed to investigate the specific cellular proteins that may be involved in apoptosis of NPCs. We show here that hyperglycemia-induced apoptosis of NPCs was through a PKCδ-dependent mechanism. Tyrosine phosphorylation of PKCδ was required for PKCδ binding to c-Abl in the cytoplasm, and inhibition of c-Abl by STI571 or knock-down of c-Abl by RNAi decreased the phosphorylation of PKCδ. Moreover, translocation of PKCδ and c-Abl complex from the cytoplasm to the nucleus, was blocked by down-regulation of PKCδ or c-Abl. Furthermore, we found that interaction of PKCδ and c-Abl played a crucial role in p53 accumulation in the nucleus, which was linked to the apoptosis of NPCs in response to high glucose.

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.

The cell death machinery governed by the p53 tumor suppressor in response to DNA damage

The cellular response to genotoxic stress that damages DNA includes cell cycle arrest, activation of DNA repair, and in the event of irreparable damage, induction of apoptosis. However, the signals that determine cell fate, that is, survival or apoptosis, are largely unclear. The tumor suppressor p53 has been implicated in many important cellular processes, including regulation of apoptotic cell death. When cells encounter genotoxic stress, certain sensors for DNA lesions eventually stabilize and activate p53. Subsequently, p53 exerts its tumor suppressor function by transactivating numerous target genes. Active p53 is subjected to a complex and diverse array of covalent post-translational modifications, which selectively influence the expression of p53 target genes. In this regard, the molecular basis for how p53 induces apoptosis has been extensively studied; however, the relative contribution of each downstream effector is still to be explored. Moreover, little is known about precise mechanisms by which modified p53 is capable of apoptosis induction. A thorough understanding for the whole picture of p53 modification in apoptosis will be extremely valuable in the development of highly effective and specific therapies for cancer patients. This review is focused on the current views regarding the regulation of cell fate by p53 in the apoptotic response to DNA damage.

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.

Regulating the activity of class II transactivator by posttranslational modifications: exploring the possibilities

First identified as the master regulator of major histocompatibility complex II transcription, class II transactivator (CIITA) has since been implicated in a host of pathologies by modulating the transcription of multiple different genes. How CIITA caters to cell- and tissue-specific transcriptional needs is hotly debated and investigated. One of the possible mechanisms underlying spatiotemporal control of CIITA transcriptional activity is the posttranslational modification (PTM) machinery that refines certain amino acid residues of CIITA and hence alters its activity in response to specific cellular and environmental cues. This review discusses our current understanding of the PTM map of CIITA, how these modifications fine-tune its activity, and how the study of this area may lead to potential therapeutic strategies.

Involvement of PKC signal pathways in oridonin-induced autophagy in HeLa cells: a protective mechanism against apoptosis

Our previous studies showed that oridonin could induce both apoptosis and autophagy in HeLa cells, and this autophagy might be a protective mechanism against apoptosis. In this study, the roles of PKC signal pathways in oridonin-induced HeLa cell autophagy and apoptosis were further investigated. We found that inhibition of PKC significantly reduced oridonin-induced autophagy whereas markedly increased apoptosis, while pretreatment with PKC activator caused opposite results. Subsequently, the oridonin-induced autophagy was also suppressed by Raf-1 or JNK inhibition accompanied by the increase of apoptosis, but it was not affected by ERK or p38 inhibition. In addition, oridonin-induced protein levels of Raf-1, JNK and p-JNK were sharply downregulated by PKC inhibitor, and they were enhanced by PKC activator. Taken together, these results demonstrate that PKC enhances oridonin-induced autophagy against apoptosis through regulating its downstream factors Raf-1 and JNK in HeLa cells.

Invertebrate p53-like mRNA isoforms are differentially expressed in mussel haemic neoplasia

Mussels of the genus Mytilus are widely used in environmental monitoring. They can develop a leukaemia-like disease, haemic neoplasia, which could be induced, in part, by environmental stressors. The molluscan p53 tumor suppressor gene family was previously shown to be involved in haemic neoplasia at the protein level. The purpose of this study was the quantification of molluscan p53-like isoforms at the mRNA level in mussels with haemic neoplasia compared to normal controls. The three isoforms monitored were a p53-like, a TAp63/73-like containing an intact transactivation (TA) domain, and an NH(2)-terminally truncated p63/73 isoform termed DeltaNp63/p73-like that lacks the full TA domain. Using a comprehensive data set of 62 individual Mytilus trossulus and reverse transcription real-time PCR, we found that both the p53 and the DeltaNp63/73 isoforms were up-regulated in neoplastic haemocytes compared to normal haemocytes (p<0.0001). In contrast, the mRNA levels of the non-truncated isoform TAp63/73 did not change significantly in mussels with the disease at alpha=0.01 (p=0.0141), in contrast to previous findings at the protein level. Correlations in mRNA levels between the truncated isoform and the full-length isoforms in normal haemocytes were lost in neoplastic haemocytes. The increase in mRNA concentration of the truncated DeltaNp63/73 isoform in molluscan haemic neoplasia is similar to observations in many human cancers and cell lines and underlines the phylogenetically ancient oncogenic role of this isoform.

Frequent epigenetic inactivation of hSRBC in gastric cancer and its implication in attenuated p53 response to stresses

hSRBC is a putative tumor suppressor located at 11p15.4, at which frequent genomic loss has been observed in several human malignancies. To explore the candidacy of hSRBC as a suppressor of gastric tumorigenesis, we analyzed the expression and mutation status of hSRBC in gastric tissues and cell lines. hSRBC transcript was expressed in all normal and benign tumor tissues examined, but undetectable or very low in 73% (11/15) cancer cell lines and 41% (46/111) primary tumors. Loss or reduction of hSRBC expression was tumor-specific and correlated with stage and grade of tumors. While allelic loss or somatic mutations of the gene were infrequent, its expression was restored in tumor cells by 5-aza-2'-deoxycytidine treatment and aberrant hypermethylation of 23 CpG sites in the promoter region showed a tight association with altered expression. Transient or stable expression of hSRBC led to a G(1) cell cycle arrest and apoptosis of tumor cells, and strongly suppresses colony forming ability and xenograft tumor growth. In addition, hSRBC elevated apoptotic sensitivity of tumor cells to genotoxic agents, such as 5-FU, etoposide and ultraviolet. Interestingly, hSRBC increased the protein stability of p53 and expression of p53 target genes, such as p21(Waf1), PUMA and NOXA, while hSRBC-mediated cell cycle arrest and apoptosis were abolished by blockade of p53 function. Our findings suggest that hSRBC is a novel tumor suppressor whose epigenetic inactivation contributes to the malignant progression of gastric tumors, in part, through attenuated p53 response to stresses.

Phorbol ester up-regulates phospholipase D1 but not phospholipase D2 expression through a PKC/Ras/ERK/NFkappaB-dependent pathway and enhances matrix metalloproteinase-9 secretion in colon cancer cells

Despite its importance in cell proliferation and tumorigenesis, very little is known about the molecular mechanism underlying the regulation of phospholipase D (PLD) expression. PLD isozymes are significantly co-overexpressed with cancer marker genes in colorectal carcinoma. Phorbol 12-myristate 13-acetate (PMA) treatment, as a mitogenic signal in colon cancer cells, selectively increases PLD1 expression in transcription and post-transcription. Moreover, experiments using intraperitoneal injection of PMA into mice showed selective PLD1 induction in the intestine and lung tissues, which suggests its physiological relevance in vivo. Therefore, we have undertaken a detailed analysis of the effects of PMA on the promoter activity of PLD genes. Protein kinase C inhibitors, but not a protein kinase A inhibitor, were found to suppress the up-regulation of PLD1 but not PLD2. Dominant-negative mutants of Ras, Raf, and MEK suppressed the induction and activity of PLD1. Moreover, depletion of the supposedly involved proteins reduced the endogenous PLD1 protein level. An important role for NFkappaB as a downstream target of ERK in PMA-induced PLD1 induction was also demonstrated using the inhibitor, small interfering RNA, chromatin immunoprecipitation assay, and site-specific mutagenesis. Furthermore, inhibitors of these signaling proteins and depletion of PLD1 suppressed PMA-induced matrix metalloproteinase-9 secretion and PLD1 induction. In conclusion, we demonstrate for the first time that induction of PLD1 through a protein kinase C/Ras/ERK/NFkappaB-dependent pathway is involved in the secretion of matrix metalloproteinase-9 in colorectal cancer cells.

Protein kinase C delta activates IkappaB-kinase alpha to induce the p53 tumor suppressor in response to oxidative stress

Protein kinase C delta (PKCdelta) functions as a redox-sensitive kinase in various cell types. Upon exposure to reactive oxygen species (ROS), it is activated by tyrosine phosphorylation, nuclear translocation and caspase-3-mediated cleavage. Activated PKCdelta is associated with cell cycle arrest or apoptosis, although its precise mechanism of action is unclear. Previous studies have demonstrated that the transcription factor, nuclear factor kappaB (NF-kappaB), functions as a redox-sensitive factor. ROS induce NF-kappaB signaling pathways including upstream IkappaB kinases (IKKs), although the mechanisms of ROS-induced activation of IKKs are unknown. Here we show that both PKCdelta and IKKalpha, but not IKKbeta, translocate to the nucleus in response to oxidative stress. The results also demonstrate that PKCdelta interacts with and activates IKKalpha. Importantly, our data suggest that, upon exposure to oxidative stress, PKCdelta-mediated IKKalpha activation does not contribute to NF-kappaB activation; instead, nuclear IKKalpha regulates the transcription activity of the p53 tumor suppressor by phosphorylation at Ser20. These findings collectively support a novel mechanism in which the PKCdelta-->IKKalpha signaling pathway contributes to ROS-induced activation of the p53 tumor suppressor.

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

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

Use of a novel method to find substrates of protein kinase C delta identifies M2 pyruvate kinase

Protein kinase C (PKC) family members have been implicated in numerous cellular processes. However, identifying the substrates of each PKC isozyme remains a challenge. Here, we describe a method using two-dimensional (2D) isoelectric focusing gel electrophoresis to identify substrates of delta PKC (deltaPKC) in MCF-7 breast carcinoma cells. We show that M2 pyruvate kinase is a substrate of deltaPKC, and further characterize the interaction between M2 pyruvate kinase and deltaPKC in MCF-7 cells by immunoprecipitation. deltaPKC activation in vitro or in cells did not appear to alter the enzyme activity or polymerization of M2 pyruvate kinase.

Genome-wide transcriptional profiling in human squamous cell carcinoma of the skin identifies unique tumor-associated signatures

The elucidation of specific genetic changes associated with human cancer pathogenesis has focused efforts to relate such changes to the neoplastic phenotype. To further our understanding of the genetic basis of human squamous cell carcinoma (SCC) of the skin, this study used a genome-wide (12 627 sequences) approach to determine transcriptional signatures in lesional and nonlesional sites from five SCC patients. Several novel genes involving the p53 pathway, anti-apoptotic pathways, signal transduction, structural loss and DNA replication, including BCL2A1, MUC4, PTPN11 (SHP2) and FGF9, are upregulated in SCC and could warrant further study regarding their role in disease pathogenesis. SCC pathology is likely combinatorial in nature involving the compounded changes from several cellular processes.

Involvement of protein kinase C delta in the alteration of mitochondrial mass in human cells under oxidative stress

Alteration of mitochondrial mass of human 143B osteosarcoma cells upon exposure to hydrogen peroxide (H(2)O(2)) was investigated. We found that mitochondrial mass and the intracellular level of H(2)O(2) were increased by exogenous H(2)O(2), which was accompanied with up-regulation of functional PKCdelta. To investigate the role of PKCdelta in H(2)O(2)-induced increase of mitochondrial mass, we treated 143B cells with PKCdelta activator, bistratene A, and PKCdelta inhibitor, rottlerin, respectively. The results show that bistratene A caused an increase of mitochondrial mass and that the H(2)O(2)-induced increase of mitochondrial mass was completely suppressed by rottlerin. Furthermore, we found that activation of PKCdelta by bistratene A increased the intracellular levels of H(2)O(2) and MnSOD protein expression. By contrast, suppression of PKCdelta by rottlerin decreased the intracellular levels of H(2)O(2) and MnSOD protein expression. Moreover, we noted that MnSOD expression was highly correlated with the expression of p53, which was controlled by PKCdelta. Finally, we demonstrated that PKCdelta was overexpressed in skin fibroblasts of patients with MERRF syndrome. Taken together, we conclude that PKCdelta is involved in the regulation of mitochondrial mass and intracellular H(2)O(2) in human cells and may play a key role in the overproliferation of mitochondria in the affected tissues of patients with mitochondrial diseases such as MERRF syndrome.

hTERT phosphorylation by PKC is essential for telomerase holoprotein integrity and enzyme activity in head neck cancer cells

Telomerase activity is suppressed in normal somatic tissues but is activated in most cancer cells. We have previously found that all six telomerase subunit proteins, including hTERT and hsp90 are needed for full enzyme activity. Telomerase activity has been reported to be upregulated by protein kinase C (PKC), but the mechanism is not clear. In this study, we examined how PKC regulates telomerase activity in head and neck cancer cells. PKC inhibitor, bisindolylmaleimide I (BIS), inhibited telomerase activity but had no effect on the expressions of telomerase core subunits. RNA interference (RNAi) and in vitro phosphorylation studies revealed that PKC isoforms α, β, δ, ε, ζ specifically involved in telomerase regulation, and the phosphorylation target was on hTERT. Treatment with the hsp-90 inhibitor novobiocin dissociated hsp90 and hTERT as revealed by immunoprecipitation and immunoblot analysis and reduced telomerase activity. Treatment with the PKC activator SC-10 restored the association of hsp90 and hTERT and reactivate telomerase, suggesting that hTERT phosphorylation by PKC is essential for telomerase holoenzyme integrity and function. Analysis on clinical normal and tumour tissues reveal that the expressions of PKC α, β, δ, ε, ζ were higher in the tumour tissues, correlated with telomerase activity. Disruption of PKC phosphorylation by BIS significantly increased chemosensitivity to cisplatin. In conclusion, PKC isoenzymes α, β, δ, ε, ζ regulate telomerase activity in head and neck cancer cells by phosphorylating hTERT. This phosphorylation is essential for telomerase holoenzyme assembly, leading to telomerase activation and oncogenesis. Manipulation of telomerase activity by PKC inhibitors is worth exploring as an adjuvant therapeutic approach.

Protein kinase C (PKC) family in cancer progression

PKC family consist of a number of serine-threonine kinases which are divided into three groups based on their activating factors. PKCs have been linked to carcinogenesis since PKC activators can act as tumor promoters. Furthermore, functional studies have suggested that PKCs play a role in the carcinogenesis and maintenance of malignant phenotype. Potentiation of malignant phenotype may be mediated by activation of selective PKC isoenzymes or through altered isoenzyme expression profile compared to the originating tissue. Activation of PKCalpha and beta isoenzymes have often been linked to malignant phenotype while PKCdelta is thought to mediate anti-cancer effects. This review will focus on the regulation and significance of PKC isoenzymes to cancer progression.

Suppression of apoptosis in the protein kinase Cdelta null mouse in vivo

Protein kinase C (PKC) delta is an essential regulator of mitochondrial dependent apoptosis in epithelial cells. We have used the PKCdelta(-/-) mouse to ask if loss of PKCdelta protects salivary glands against gamma-irradiation-induced apoptosis in vivo and to explore the mechanism underlying protection from apoptosis. We show that gamma-irradiation in vivo results in a robust induction of apoptosis in the parotid glands of wild type mice, whereas apoptosis is suppressed by greater than 60% in the parotid glands of PKCdelta(-/-) mice. Primary parotid cells from PKCdelta(-/-) mice are defective in mitochondrial dependent apoptosis as indicated by suppression of etoposide-induced cytochrome c release, poly(ADP-ribose) polymerase cleavage, and caspase-3 activation. Notably, apoptotic responsiveness can be restored by re-introduction of PKCdelta by adenoviral transduction. Etoposide and gamma-irradiation-induced activation of p53 is similar in primary parotid cells and parotid glands from PKCdelta(+/+) and PKCdelta(-/-) mice, indicating that PKCdelta functions downstream of the DNA damage response. In contrast, activation of the c-Jun amino-terminal kinase is reduced in primary parotid cells from PKCdelta(-/-) cells and in parotid C5 cells, which express a dominant inhibitory mutant of PKCdelta. Similarly, c-Jun amino-terminal kinase activation is suppressed in vivo in gamma-irradiated parotid glands from PKCdelta(-/-) mice. These studies indicate an essential role for PKCdelta downstream of the p53 response and upstream of the c-Jun amino-terminal kinase activation in DNA damage-induced apoptosis in vivo and in vitro.

PKCdelta-mediated regulation of FLIP expression in human colon cancer cells

FLICE-like inhibitory protein (FLIP), a naturally occurring caspase-inhibitory protein that lacks the critical cysteine domain necessary for catalytic activity, is a negative regulator of Fas-induced apoptosis. Decreased FLIP levels sensitize tumor cells to Fas- and TRAIL-mediated apoptosis; however, the cellular mechanisms regulating FLIP expression have not been defined. Here, we examined the roles of the PKC and NF-kappaB pathway in the regulation of FLIP in human colon cancers. FLIP mRNA levels were increased in Caco-2 cells by treatment with PMA; actinomycin D completely inhibited the induction of FLIP by PMA, indicating transcriptional regulation. PKC inhibitors Gö6983 and Ro-31-8220 blocked PMA-stimulated FLIP expression. Pretreatment with the PKCdelta-selective inhibitor rottlerin or transfection with PKCdelta siRNA inhibited PMA-induced FLIP expression, which identifies a role for PKCdelta in FLIP induction. Treatment with the proteasome inhibitor, MG132, or the NF-kappaB inhibitor (e.g., PDTC and gliotoxin), or overexpression of the superrepressor of IkappaB-alpha inhibited PMA-induced upregulation of FLIP. Moreover, PMA-induced NF-kappaB transactivation was blocked by GF109203x. In conclusion, our results demonstrate a critical role for PKCdelta/NF-kappaB in the regulation of FLIP in human colon cancer cells.

Protein kinase C delta regulates Ser46 phosphorylation of p53 tumor suppressor in the apoptotic response to DNA damage

The p53 tumor suppressor is activated in the cellular response to genotoxic stress. Transactivation of p53 target genes dictates cell cycle arrest and DNA repair or induction of apoptosis; however, a molecular mechanism responsible for these distinct functions remains unclear. Recent studies revealed that phosphorylation of p53 on Ser(46) was associated with induction of p53AIP1 expression, resulting in the commitment of the cell fate into apoptotic cell death. Moreover, upon exposure to genotoxic stress, p53DINP1 was expressed and recruited a kinase(s) to p53 that specifically phosphorylated Ser(46). Here, we show that the pro-apoptotic kinase, protein kinase C delta (PKCdelta), is involved in phosphorylation of p53 on Ser(46). PKCdelta-mediated phosphorylation is required for the interaction of PKCdelta with p53. The results also demonstrate that p53DINP1 associates with PKCdelta upon exposure to genotoxic agents. Consistent with these results, PKCdelta potentiates p53-dependent apoptosis by Ser(46) phosphorylation in response to genotoxic stress. These findings indicate that PKCdelta regulates p53 to induce apoptotic cell death in the cellular response to DNA damage.

Protein kinase C delta induces apoptosis of vascular smooth muscle cells through induction of the tumor suppressor p53 by both p38-dependent and p38-independent mechanisms

Apoptotic death of vascular smooth muscle cells (SMCs) is a prominent feature of blood vessel remodeling. In the present study, we examined the novel PKC isoform protein kinase C delta (PKCdelta) and its role in vascular SMC apoptosis. In A10 SMCs, overexpression of PKCdelta was sufficient to induce apoptosis, whereas inhibition of PKCdelta diminished H2O2-induced apoptosis. Moreover, evidence is provided that the tumor suppressor p53 is an essential mediator of PKCdelta-induced apoptosis in SMCs. Activation of PKCdelta led to accumulation as well as phosphorylation of p53 in SMCs; this induction correlated with apoptosis. Furthermore, blocking p53 induction with small interference RNA or targeted gene deletion prevented PKCdelta-induced apoptosis, whereas restoring p53 expression rescued the ability of PKCdelta to induce apoptosis in p53 null SMCs. We also establish that PKCdelta regulates p53 at both transcriptional and post-translational levels. Specifically, the transcriptional regulation required p38 MAPK, whereas the post-translational modification, at least for serine 46, did not involve MAPK. Additionally, PKCdelta, p38 MAPK, and p53 co-associate in cells under conditions favoring apoptosis. Together, our data suggest that SMC apoptosis proceeds through a pathway that involves PKCdelta, the intermediary p38 MAPK, and the downstream target tumor suppressor p53.

Overexpression of protein kinase Cdelta enhances cisplatin-induced cytotoxicity correlated with p53 in gastric cancer cell line

OBJECTIVE

An important issue in cancer therapy is to investigate the mechanism for cellular sensitivity to anticancer agents such as cisplatin. Cisplatin is one of the DNA-damaging agents and several factors including p53 are related to the sensitivity to cisplatin in cancer. Protein kinase C (PKC) delta is known as a positive regulator for cisplatin-induced cell death. In our present study, we examined whether overexpression of PKCdelta and p53 increases the sensitivity of the human gastric cancer cell line, MKN28, which has a mutation of p53 gene, to cisplatin.

METHODS

Cell viability and DNA content were measured in MKN28 with adenovirus-mediated expression of PKCdelta and p53 after exposure to cisplatin. In addition, the active form of caspase-3 was detected by Western blotting.

RESULTS

Overexpression of exogenous PKCdelta did not induce cell death in MKN28 but inhibited cell growth at 1 microg/ml cisplatin as compared to that by cisplatin alone. Moreover, overexpression of both wild-type p53 and exogenous PKCdelta in MKN28 increased cisplatin-induced cell death in MKN28.

CONCLUSION

These results suggest that PKCdelta, in cooperation with p53, possibly regulates cisplatin-induced caspase-3-mediated cell death in gastric cancer.

Matrix Metalloproteinases 9 and 10 Inhibit Protein Kinase C–Potentiated, p53-Mediated Apoptosis

p53, a major sensor of DNA damage, is a transcription factor that, depending on its phosphorylation status, regulates the cell cycle, DNA repair, or apoptosis. The protein kinase C (PKC) family of isozymes is also implicated in cell cycle and programmed cell death (PCD) control and has recently been shown to influence p53 function. Using three human colon adenocarcinoma cell lines SW480, EB-1, and HCT116 that either lack p53 function and were engineered to express inducible wild-type p53 (wt p53), or that constitutively express wt p53, we show that phorbol ester-mediated PKC activation potentiates p53-induced PCD. Despite the effectiveness of PKC/p53 synergy in inducing SW480 tumor cell death, however, a fraction of the cells invariably survive. To address the putative mechanisms that underlie resistance to PKC/p53-induced cell death, we generated a phorbol 12-myristate 13-acetate/p53-resistant SW480 subline and compared the gene expression profile of resistant and parental cells by DNA microarray analysis. The results of these experiments show that PKC/p53-resistant cells express a higher level of several matrix metalloproteinases (MMP), including MMP-9, MMP-10, and MMP-12, and corresponding real-time PCR assays indicate that p53 is a negative regulator of MMP-9 gene expression. Using MMP inhibitors and MMP-specific small interfering RNA, we show that MMP function confers protection from PKC/p53-induced apoptosis and identify the protective MMPs as MMP-9 and MMP-10. Taken together, these observations provide evidence that MMPs are implicated in tumor cell resistance to the synergistic proapoptotic effect of PKC and p53.

Distinctive activation mechanisms and functions for protein kinase Cdelta

PKCdelta (protein kinase Cdelta) is a serine/threonine kinase that plays a key role in growth regulation and tissue remodelling. Traditional models of PKC activation have focused on lipid cofactors and anchoring proteins that localize the active conformation of PKCdelta to membranes, in close proximity with its target substrates. However, recent studies identify a distinct mode for PKCdelta activation involving tyrosine phosphorylation by Src family kinases. The tyrosine-phosphorylated form of PKCdelta (which accumulates in the soluble fraction of cells exposed to oxidant stress) displays lipid-independent kinase activity and is uniquely positioned to phosphorylate target substrates throughout the cell (not just on lipid membranes). This review summarizes (1) recent progress towards understanding structure-activity relationships for PKCdelta, with a particular focus on the stimuli that induce (and the distinct functional consequences that result from) tyrosine phosphorylation events in PKCdelta's regulatory, hinge and catalytic domains; (2) current concepts regarding the role of tyrosine phosphorylation as a mechanism to regulate PKCdelta localization and actions in mitochondrial and nuclear compartments; and (3) recent literature delineating distinct roles for PKCdelta (relative to other PKC isoforms) in transcriptional regulation, cell cycle progression and programmed cell death (including studies in PKCdelta-/- mice that implicate PKCdelta in immune function and cardiovascular remodelling). Collectively, these studies argue that the conventional model for PKCdelta activation must be broadened to allow for stimulus-specific differences in PKCdelta signalling during growth factor stimulation and oxidant stress.

Alterations in protein kinase C activity and processing during zinc-deficiency-induced cell death

Protein kinases C (PKCs) are a family of serine/threonine kinases that are critical for signal transduction pathways involved in growth, differentiation and cell death. All PKC isoforms have four conserved domains, C1-C4. The C1 domain contains cysteine-rich finger-like motifs, which bind two zinc atoms. The zinc-finger motifs modulate diacylglycerol binding; thus, intracellular zinc concentrations could influence the activity and localization of PKC family members. 3T3 cells were cultured in zinc-deficient or zinc-supplemented medium for up to 32 h. Cells cultured in zinc-deficient medium had decreased zinc content, lowered cytosolic classical PKC activity, increased caspase-3 processing and activity, and reduced cell number. Zinc-deficient cytosols had decreased activity and expression levels of PKC-alpha, whereas PKC-alpha phosphorylation was not altered. Inhibition of PKC-alpha with Gö6976 had no effect on cell number in the zinc-deficient group. Proteolysis of the novel PKC family member, PKC-delta, to its 40-kDa catalytic fragment occurred in cells cultured in the zinc-deficient medium. Occurrence of the PKC-delta fragment in mitochondria was co-incident with caspase-3 activation. Addition of the PKC-delta inhibitor, rottlerin, or zinc to deficient medium reduced or eliminated proteolysis of PKC-delta, activated caspase-3 and restored cell number. Inhibition of caspase-3 processing by Z-DQMD-FMK (Z-Asp-Gln-Met-Asp-fluoromethylketone) did not restore cell number in the zinc-deficient group, but resulted in processing of full-length PKC-delta to a 56-kDa fragment. These results support the concept that intracellular zinc concentrations influence PKC activity and processing, and that zinc-deficiency-induced apoptosis occurs in part through PKC-dependent pathways.

Suppression of cell migration by protein kinase Cδ

The ability of cancer cells to migrate is strongly correlated with malignant progression and metastasis. Survival signals that suppress apoptosis have also been linked to increased cell motility. We previously reported that suppression of protein kinase Cdelta (PKCdelta) provided survival signals in a rat fibroblast model system. These studies have been extended to human breast cancer cells with differential cell motilities and PKCdelta levels. BT-549 cells, which lack detectable expression of PKCdelta, migrate very efficiently, whereas MCF-7 cells, which express high levels of PKCdelta, migrate very poorly. Ectopic expression of PKCdelta suppressed cell migration in the BT-549 cells, and downregulation of PKCdelta enhanced cell migration in the MCF-7 cells. Downregulation of PKCdelta in the MCF-7 cells also led to increased secretion of the matrix metalloprotease MMP-9. The migration of mouse embryo fibroblasts (MEFs) from wild type and PKCdelta knockout mice was also examined and MEFs from PKCdelta knockout mice had a five-fold increase in cell migration relative to the wild-type MEFs. These data provide evidence that PKCdelta suppresses cell migration in both human breast cancer cells and in primary mouse fibroblasts, and indicate that the loss of PKCdelta in human cancers could contribute to both cell survival and metastasis.

PKCdelta requires p53 for suppression of the transformed phenotype in human colon cancer cells

We have previously demonstrated that the delta isoform of Protein Kinase C (PKCdelta) acts as a tumor suppressor in HCT116 human colon cancer cells, and that p21(waf1/cip1) is an essential downstream effector of PKCdelta. Our data suggested that p53 might also be involved in the suppression of the neoplastic phenotype induced by PKCdelta. Here we show that homozygous knockout of p53 renders the HCT116 cell line unresponsive to PKCdelta overexpression. Whereas reconstitution of p53 alone did not modify the morphology and growth properties of HCT116/p53null cells, overexpression of both p53 and PKCdelta induced a number of alterations indicating suppression of the transformed phenotype. Interestingly, PKCdelta was ineffective when overexpressed in HT29 cells, a human colon cancer line characterized by the Arg273His dominant-negative mutation of p53. Thus, our data indicate that wild-type p53 is an essential effector of PKCdelta in human colon cancer cells.

p21(Waf1/Cip1) and p53 are downstream effectors of protein kinase C delta in tumor suppression and differentiation in human colon cancer cells

We have previously demonstrated that the delta isoform of protein kinase C (PKCdelta) is importantly involved in cell growth inhibition and tumor suppression in colon cancer cells. To investigate further the activity and mechanism of action of PKCdelta, we have retrovirally transduced a PKCdelta cDNA in HCT116 human colon cancer cells. PKCdelta-overexpressing cells (HCT116/PKCdelta) were growth-inhibited, showed marked morphologic changes and underwent multinucleation and phenotypic changes characteristic of mitotic catastrophe. Compared to controls, HCT116/PKCdelta cells showed a highly attenuated tumorigenic profile and poor anchorage-independent growth. In addition, transfected cells established junction-coordinated intercellular communications, expressed cell surface microvilli and overexpressed the colon differentiation marker alkaline phosphatase. HCT116/PKCdelta cells also produced the 89 kDa, carboxy-terminal catalytic domain of PARP. In HCT116/PKCdelta cells, p21(Waf1/Cip1) and p53 were transiently upregulated for 48 hr after PKCdelta transduction. In a p21 null subline of HCT116 cells (HCT116/p21null), overexpression of PKCdelta did not affect tumorigenicity or differentiation, indicating that p21 is essential for the antitumorigenic activity of PKCdelta. Similarly, overexpression of PKCdelta caused no significant phenotypic changes in HCT116/E6 cells, an HCT116 subline in which the p53 protein is downregulated by the human papillomavirus E6 gene product. We conclude that overexpression of PKCdelta in human colon cancer cells induces multiple antineoplastic effects that depend on the activities of p21(Waf1/Cip1) and p53.