Identification of Activated Protein Kinase Cα (PKCα) in the Urine of Orthotopic Bladder Cancer Xenograft Model as a Potential Biomarker for the Diagnosis of Bladder Cancer

Bladder cancer has a high recurrence rate; therefore, frequent and effective monitoring is essential for disease management. Cystoscopy is considered the gold standard for the diagnosis and continuous monitoring of bladder cancer. However, cystoscopy is invasive and relatively expensive. Thus, there is a need for non-invasive, relatively inexpensive urinary biomarker-based diagnoses of bladder cancer. This study aimed to investigate the presence of activated protein kinase Cα (PKCα) in urine samples and the possibility of PKCα as a urinary biomarker for bladder cancer diagnosis. Activated PKCα was found to be present at higher levels in bladder cancer tissues than in normal bladder tissues. Furthermore, high levels of activated PKCα were observed in urine samples collected from orthotopic xenograft mice carrying human bladder cancer cells compared to urine samples from normal mice. These results suggest that activated PKCα can be used as a urinary biomarker to diagnose bladder cancer. To the best of our knowledge, this is the first report describing the presence of activated PKCα in the urine of orthotopic xenograft mice.

Protein Kinase C α-Responsive Gene Carrier for Cancer-Specific Transgene Expression and Cancer Therapy

The presence of intracellular signal transduction and its abnormal activities in many cancers has potential for medical and pharmaceutical applications. We recently developed a protein kinase C α (PKCα)-responsive gene carrier for cancer-specific gene delivery. Here, we demonstrate an in-depth analysis of cellular signal-responsive gene carrier and the impact of its selective transgene expression in response to malfunctioning intracellular signaling in cancer cells. We prepared a novel gene carrier consisting of a linear polyethylenimine (LPEI) main chain grafted to a cationic PKCα-specific substrate (FKKQGSFAKKK-NH₂). The LPEI-peptide conjugate formed a nanosized polyplex with pDNA and mediated efficient cellular uptake and endosomal escape. This polyplex also led to successful transgene expression which responded to the target PKCα in various cancer cells and exhibited a 10-100-fold higher efficiency compared to the control group. In xenograft tumor models, the LPEI-peptide conjugate promoted transgene expression showing a clear-cut response to PKCα. Furthermore, when a plasmid containing a therapeutic gene, human caspase-8 (pcDNA-hcasp8), was used, the LPEI-peptide conjugate had significant cancer-suppressive effects and extended animal survival. Collectively, these results reveal that our method has great potential for cancer-specific gene delivery and therapy.

A Reporter System Evaluates Tumorigenesis, Metastasis, β-catenin/MMP Regulation, and Druggability

Cancer invasion, metastasis, and therapy resistance are the crucial phenomena in cancer malignancy. The high expression of matrix metalloproteinase 9 (MMP9) is a biomarker as well as a causal factor of cancer invasiveness and metastatic activity. However, a regulatory mechanism underlying MMP9 expression in cancer is not clarified yet. In addition, a new strategy for anticancer drug discovery is becoming an important clue. In the present study, we aimed (i) to develop a novel reporter system evaluating tumorigenesis, invasiveness, metastasis, and druggability with a combination of three-dimensional tumoroid model and Mmp9 promoter and (ii) to examine pharmacological actions of anticancer medications using this reporter system. High expression and genetic amplification of MMP9 were found in colon cancer cases. We found that proximal promoter sequences of MMP9 in murine and human contained conserved binding sites for transcription factors β-catenin/TCF/LEF, glucocorticoid receptor (GR), and nuclear factor kappa-B (NF-κB). The murine Mmp9 promoter (-569 to +19) was markedly activated in metastatic colon cancer cells and additionally activated by tumoroid formation and by β-catenin signaling stimulator lithium chloride. The Mmp9 promoter-driven fluorescent reporter cells enabled the monitoring of activities of MMP9/gelatinase, tumorigenesis, invasion, and metastasis in syngeneic transplantation experiments. We also demonstrated pharmacological actions as follows: dexamethasone and hydrocortisone, steroidal medications binding to GR, inhibited the Mmp9 promoter but did not inhibit tumorigenesis. On the contrary, antimetabolite 5-fluorouracil, a gold standard for colon cancer chemotherapy, inhibited tumoroid formation but did not inhibit Mmp9 promoter activity. Notably, antimalaria medication artesunate inhibited both tumorigenesis and the Mmp9 promoter in vitro, potentially through inhibition of β-catenin/TCF/LEF signaling. Thus, this novel reporter system enabled monitoring tumorigenesis, invasiveness, metastasis, key regulatory signalings such as β-catenin/MMP9 axis, and druggability. Impact Statement Cancer invasion and metastasis have been shown to be driven by matrix metalloproteinase 9 (MMP9), whose expression mechanism is not clarified yet. In addition, a new strategy for anticancer drug discovery is becoming important. We established a novel reporter system evaluating tumorigenesis, invasiveness, metastasis, and druggability with a combination of three-dimensional (3D) tumoroid model and Mmp9 promoter. Using this reporter system, we demonstrated pharmacological actions of anticancer medications such as antimetabolite 5-fluorouracil (5-FU) and antimalaria medication artesunate (ART), which inhibited both tumorigenesis and β-catenin/MMP regulatory signaling. Our study impacts the translational fields of oncology, drug discovery, and organoid model.

Role of amino acid residues surrounding the phosphorylation site in peptide substrates of G protein-coupled receptor kinase 2 (GRK2)

A series of amino acid substitutions was made in a previously identified β-tubulin-derived GRK2 substrate peptide (⁴⁰⁴DEMEFTEAESNMN⁴¹⁶) to examine the role of amino acid residues surrounding the phosphorylation site. Anionic amino acid residues surrounding the phosphorylation site played an important role in the affinity for GRK2. Compared to the original peptide, a modified peptide (Ac-EEMEFSEAEANMN-NH₂) exhibited markedly higher affinity for GRK2, but very low affinity for GRK5, suggesting that it can be a sensitive and selective peptide for GRK2.

Bryostatin-1 Restores Blood Brain Barrier Integrity following Blast-Induced Traumatic Brain Injury

Recent wars in Iraq and Afghanistan have accounted for an estimated 270,000 blast exposures among military personnel. Blast traumatic brain injury (TBI) is the 'signature injury' of modern warfare. Blood brain barrier (BBB) disruption following blast TBI can lead to long-term and diffuse neuroinflammation. In this study, we investigate for the first time the role of bryostatin-1, a specific protein kinase C (PKC) modulator, in ameliorating BBB breakdown. Thirty seven Sprague-Dawley rats were used for this study. We utilized a clinically relevant and validated blast model to expose animals to moderate blast exposure. Groups included: control, single blast exposure, and single blast exposure + bryostatin-1. Bryostatin-1 was administered i.p. 2.5 mg/kg after blast exposure. Evan's blue, immunohistochemistry, and western blot analysis were performed to assess injury. Evan's blue binds to albumin and is a marker for BBB disruption. The single blast exposure caused an increase in permeability compared to control (t = 4.808, p < 0.05), and a reduction back toward control levels when bryostatin-1 was administered (t = 5.113, p < 0.01). Three important PKC isozymes, PKCα, PKCδ, and PKCε, were co-localized primarily with endothelial cells but not astrocytes. Bryostatin-1 administration reduced toxic PKCα levels back toward control levels (t = 4.559, p < 0.01) and increased the neuroprotective isozyme PKCε (t = 6.102, p < 0.01). Bryostatin-1 caused a significant increase in the tight junction proteins VE-cadherin, ZO-1, and occludin through modulation of PKC activity. Bryostatin-1 ultimately decreased BBB breakdown potentially due to modulation of PKC isozymes. Future work will examine the role of bryostatin-1 in preventing chronic neurodegeneration following repetitive neurotrauma.

Effect of peptide content on the regulation of transgene expression by protein kinase Cα-responsive linear polyethylenimine-peptide conjugates

We examined a series of linear polyethylenimine (LPEI)-based nanocarriers that activate transgene expression in response to cancer-specific protein kinase Cα (PKCα). Eight types of LPEI-peptide conjugate differing in peptide content and number were synthesized using click chemistry. The conjugates could form polyplexes with pDNA through electrostatic interaction, but the degree of pDNA condensation, sizes, and surface charges of the resulting polyplexes depended on the pendant-peptide content and number. None of the polyplexes showed significant cytotoxicity toward human hepatoma cells (HepG2). Furthermore, pendant peptide content and number markedly affected transgene activation in response to PKCα. To achieve an all-or-none response to PKCα, we determined the optimum peptide content and number in LPEI-peptide conjugates as ≈6 mol% and ≈40 peptides/conjugate.

Peptide substrates for G protein-coupled receptor kinase 2

G protein-coupled receptor kinases (GRKs) control the signaling and activation of G protein-coupled receptors through phosphorylation. In this study, consensus substrate motifs for GRK2 were identified from the sequences of GRK2 protein substrates, and 17 candidate peptides were synthesized to identify peptide substrates with high affinity for GRK2. GRK2 appears to require an acidic amino acid at the -2, -3, or -4 positions and its consensus phosphorylation site motifs were identified as (D/E)X1-3(S/T), (D/E)X1-3(S/T)(D/E), or (D/E)X0-2(D/E)(S/T). Among the 17 peptide substrates examined, a 13-amino-acid peptide fragment of β-tubulin (DEMEFTEAESNMN) showed the highest affinity for GRK2 (Km, 33.9 μM; Vmax, 0.35 pmol min(-1) mg(-1)), but very low affinity for GRK5. This peptide may be a useful tool for investigating cellular signaling pathways regulated by GRK2.

Protein Kinase C (PKC) Isozymes and Cancer

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

Histidinylated poly-L-lysine-based vectors for cancer-specific gene expression via enhancing the endosomal escape

In this work, we synthesized a series of poly-L-lysine (PLL)-based polymers for gene delivery, by modifying the PLL with both cationic peptide and histidine. The peptide moieties serve as cationic centers for polyplex formation, and also as substrates for protein kinase Cα (PKCα), which is specifically activated in many types of cancer cells, to achieve cancer-specific gene expression. The histidine groups serve as buffering moieties to increase the ability of the plasmid DNA (pDNA)-polymer complex (polyplex) to escape the endosome and thus to promote expression of the pDNA in the transfected cells. The facile synthesis of the polymers proceeded by modifying the PLL with side-group-protected peptide and protected histidine, followed by deprotection of the functional groups. The synthesized polymers showed significant buffering capacity over the neutral to acidic pH range and showed less cytotoxicity in vitro compared with histidine-unmodified polymers. The polyplexes successfully showed PKCα-responsive gene expression immediately after their introduction into cancer cells and the gene expression continued for at least 24 h. These PLL-based carriers thus show promise for cancer-targeted gene therapy.

A protein kinase assay based on FRET between quantum dots and fluorescently-labeled peptides

A novel protein kinase assay was developed, based on FRET between QDs and fluorescently-labeled substrate peptides. The negatively charged QDs recognize the change in net charge of the peptide upon phosphorylation. Despite its simple mechanism, this assay is sensitive and robust enough to be applied to the evaluation of protein kinase inhibitors.

CANCER-SPECIFIC GENE CARRIERS RESPONDING TO CANCER MICROENVIRONMENT: ACIDOSIS AND HYPER-ACTIVATED PROTEIN KINASES

Protein kinase (PK)-responsive gene carriers modified with polyethylene glycol (PEG) chains using an acid-labile linker were developed. These carriers were obtained by modifying the PEG chains and substrate peptides for the PKs (PKA or PKCα) on the branched polyethyleneimine main chain. Polyplexes formed from these carriers and plasmid DNA (pDNA) were stably dispersed under neutral pH medium. The polyplexes were also taken up by cells on the release of the PEG chains under the slightly acidic extracellular pH associated with cancer cells. The polyplexes taken up by cells resulted in gene expression when the substrate peptides were phosphorylated by the intracellular PKs to release pDNA from the polyplexes. These novel gene carriers are expected to be promising for cancer-specific gene therapy via intravenous administration.

Branched polyethylenimine-based PKCα-responsive gene carriers

We examined in vitro performance of the branched polyethylenimine (bPEI)-based gene carriers which respond to cancer-specific activation of protein kinase Cα (PKCα) to express plasmid DNA. The carriers were synthesized straightforward by using amide bond formation between a peptide terminal carboxyl and a primary amine group of bPEI. To examine the effect of the peptide contents in the carrier, we prepared several carriers with various peptide contents. The obtained polymers form polyplexes with tighter condensation of plasmid DNA than our previous gene carriers. After internalization of the polyplexes via endocytosis, the polyplexes effectively escaped from the endosome into cytosol. Then, the polyplexes showed a clear-cut response to PKCα to release plasmid DNA for gene expression. We determined the optimum contents of the peptides in carriers as 5 mol% to achieve the clear-cut response to PKCα.

Gene carrier showing all-or-none response to cancer cell signaling

In this work we designed a novel nano carrier, a linear polyethylenimine (LPEI)-peptide conjugate, for cancer-specific expression of transgenes. The conjugate was easily synthesized by using a click chemistry scheme orthogonal to the reactive side groups of the peptide, which is the substrate of protein kinase Cα (PKCα). Polyplexes of the conjugates with plasmid DNA (pDNA) were intact and stably dispersed even in the presence of cell lysate. Despite this stability, the polyplexes readily dissociated upon phosphorylation of the grafted peptides by PKCα. Because of its endosomal escape ability and adequate susceptibility to PKCα, the polyplexes showed an all-or-none type response to PKCα activity in transgene expression in vitro. The polyplexes achieved cancer tissue-specific transgene expression even for a tumor with a relatively low PKCα activity. Thus the LPEI-peptide conjugate has high potential as a nanocarrier for cancer-targeted gene therapy.

Peptide substrates for Rho-associated kinase 2 (Rho-kinase 2/ROCK2)

Peptide substrates sensitive for a certain protein kinase could be important for new-drug development and to understand the mechanism of diseases. Rho-associated kinase (Rho-kinase/ROCK) is a serine/threonine kinase, and plays an important part in cardiovascular disease, migration and invasion of tumor cells, and in neurological disorders. The purpose of this study was to find substrates with high affinity and sensitivity for ROCK2. We synthesized 136 peptide substrates from protein substrates for ROCK2 with different lengths and charged peptides. Incorporation of (32)P [counts per minute (CPM)] for each peptide substrate was determined by the radiolabel assay using [γ-(32)P]ATP. When the top five peptide substrates showing high CPMs (R4, R22, R133, R134, and R135) were phosphorylated by other enzymes (PKA, PKCα, and ERK1), R22, R133, and R135 displayed the highest CPM level for ROCK2 compared with other enzymes, whereas R4 and R134 showed similar CPM levels for ROCK2 and PKCα. We hypothesize that R22, R133, and R135 can be useful peptide substrates for ROCK2.

Bio and nanotechnological strategies for tumor-targeted gene therapy

Gene therapy is a new medical approach for the treatment of tumors. For safe and efficient gene therapy, therapeutic genes need to be delivered efficiently into the target tumor cells. Development of gene delivery systems to specifically recognize and target tumor cells and to distinguish them from normal cells, especially in the same tissue or organ, is one of the most important issues regarding the present gene delivery methodologies. The enhanced permeability and retention (EPR) effect using the characteristics of angiogenic tumor blood vessels, as well as gene delivery systems recognizing hyperactivated receptors or intracellular signals, is broadly applied to tumor-targeted gene therapy. In addition, bacterial vectors can be a useful means for targeting hypoxic or anoxic regions of a tumor.

Role of estrogenic compounds (diethylstibestrol, 17beta-estradiol, and bisphenol A) in the phosphorylation of substrate by protein kinase Calpha

Estrogenic compounds can activate protein kinase C (PKC), which is a calcium and phospholipid-dependent serine/threonine kinase. In the present study, we investigated the role of 17beta-estradiol (E2), diethylstibestrol (DES), and bisphenol A (BPA) in the phosphorylation of substrate by PKCalpha using the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The level of phosphorylated peptide was low in the absence of phosphatidylserine (PS). Moreover, reduction of phosphorylation ratios was identified in the presence of diacylglycerol (DAG) and Ca(2+) or PS and Ca(2+) after adding E2, DES, and BPA. However, no change in phosphorylation ratios was found in the presence of DAG and PS. Addition of E2, DES, and BPA also had no influence on the phosphorylation reaction of substrate by cell or tissue lysate samples. Our study suggests that E2, DES, and BPA can bind to the C2 domain of PKCalpha but have no effects on the phosphorylation reaction of substrates in the presence of DAG and PS.

Plasma protein kinase C (PKC)alpha as a biomarker for the diagnosis of cancers

Protein kinase C (PKC)alpha plays a key role in the differentiation, proliferation and apoptosis of cancer cells, and its activity is higher in cancer cells than in normal cells. In the present study, we investigated the existence of activated PKCalpha in plasma and its possibility for cancer diagnosis. Plasma samples were prepared from xenograft mouse models of cancer and from normal mice. Phosphorylation ratios for a PKCalpha-specific peptide substrate (Alphatomega) were analyzed by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry and activated PKCalpha was identified by western blot analysis. Increased levels of activated PKCalpha were found in the plasma of cancer-bearing mice (U87, A549, A431, HuH-7 and B16 melanoma) compared with the levels found in control mice. Phosphorylation ratios for peptide substrate increased with an increase in tumor size. Moreover, the addition of Ro-31-7549, a highly specific inhibitor of PKCalpha, produced a concentration-dependent reduction of phosphorylation ratios, whereas the non-PKCalpha inhibitors, rottlerin and H-89, did not significantly effect phosphorylation ratios. In addition, the level of activated PKCalpha decreased after cancer resection but increased if the cancer recurred. From these results, we suggest that (i) activated PKCalpha in plasma can be a useful biomarker for the diagnosis of cancers and (ii) the level of activated PKCalpha can be monitored to assess the recurrence of cancer after surgical removal. To our knowledge, this is the first report demonstrating the existence of activated PKCalpha in plasma and its possibility for cancer diagnosis.

Protein kinase Calpha-responsive polymeric carrier: its application for gene delivery into human cancers

For cancer-targeting gene delivery, we applied a protein kinase C (PKC)alpha-responsive polymeric carrier to human cancers (U-87 MG [human glioblastoma-astrocytoma, epithelial-like cell line] and A549 [human lung adenocarcinoma epithelial cell line]). Two polymers, one a PKCalpha-responsive polymer (PPC[S]) containing the phosphorylation site serine, and the other a negative control polymer (PPC[A]), in which the serine was substituted with alanine, were synthesized. No cytotoxicity of the polymer was identified. When the complexes were transfected into cancer cells or tissues in which PKCalpha was hyper-activated, the luciferase expression from the PPC(S)/plasmid (pDNA) complex was higher than that from the PPC(A)/pDNA complex. These results show that the phosphorylation of complex by PKCalpha in cancer cells leads to high gene expression and that our system can be used as a human cancer cell-targeting gene delivery system.

Protein kinase C alpha-specific peptide substrate graft-type copolymer for cancer cell-specific gene regulation systems

We recently proposed a novel gene regulation system responding to specifically and abnormally activated intracellular enzymes in diseased cells. In the present study, we focused on protein kinase C (PKC)alpha, which is hyper-activated in most tumor cells, as a trigger for transgene regulation. We prepared cationic copolymers comprising hydrophilic and neutral polymers in main chains and cationic peptide substrates with different contents in side chains. Our copolymer with high peptide content (>3 mol%) condensed with pDNA more weakly than with poly(L-lysine) (pLL) having a similar molecular weight, but gene suppression was nearly identical to that of pLL, probably due to the steric hindrance of the main chains in our copolymer. Steric hindrance of the main chains barely affected the phosphorylation reaction of the pendant peptide. In cell and mouse experiments, higher gene expression was observed in complexes of pDNA with copolymers pended PKC alpha-specific substrate peptide than that in complexes with negative copolymers pended peptide substituted phosphorylation site of serine residues with alanine. These results indicate that our system can recognize intracellular PKC alpha as a trigger to regulate transgene expression, and may be useful for tumor gene therapy.

Regulation of Transgene Expression in Tumor Cells by Exploiting Endogenous Intracellular Signals

Recently, we have proposed a novel strategy for a cell-specific gene therapy system based on responses to intracellular signals. In this system, an intracellular signal that is specifically and abnormally activated in the diseased cells is used for the activation of transgene expression. In this study, we used protein kinase C (PKC)alpha as a trigger to activate transgene expression. We prepared a PKCalpha-responsive polymer conjugate [PPC(S)] and a negative control conjugate [PPC(A)], in which the phosphorylation site serine (Ser) was replaced with alanine (Ala). The phosphorylation for polymer/DNA complexes was determined with a radiolabel assay using [gamma-(32)P]ATP. PPC(S)/DNA complexes were phosphorylated by the addition of PKCalpha, but no phosphorylation of the PPC(A)/DNA complex was observed. Moreover, after microinjection of polymer/GFP-encoding DNA complexes into HepG2 cells at cation/anion (C/A) ratios of 0.5 to 2.0, significant expression of GFP was observed in all cases using PPC(S)/DNA complexes, but no GFP expression was observed in the negative control PPC(A)/DNA complex-microinjected cells at C/A ratios of 1.0 and 2.0. On the other hand, GFP expression from PPC(S)/DNA complexes was completely suppressed in cells pretreated with PKCalpha inhibitor (Ro31-7549). These results suggest that our gene regulation system can be used for tumor cell-specific expression of a transgene in response to PKCalpha activity.

Design of polymeric carriers for cancer-specific gene targeting: utilization of abnormal protein kinase Calpha activation in cancer cells

We succeeded in cancer cell specific gene expression by using a polyplex responsive to protein kinase Calpha, which is activated in various types of cancer cells.

Letter: correlation between phosphorylation ratios by matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis and enzyme kinetics

To identify the correlation between the phosphorylation ratios by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry (MALDI-ToF MS) analysis and enzyme kinetics (Km, Vmax, and Vmax/Km) is important to understand whether MALDI-TOF MS can be applied for monitoring the properties of peptides that are substrates of protein kinases. The correlation between phosphorylation ratios and enzyme kinetics was examined using peptides for protein kinase C (PKC) and for 60 kDa phosphoprotein, encoded by the cellular sarcoma gene (c-Src). Phosphorylation ratios, analyzed by MALDI-ToF MS, showed higher correlation coefficient (r = or > +0.7) for Vmax/Km compared with that (r = or < -/+0.6) for Km or Vmax. For ion modes, a higher correlation coefficient between phosphorylation ratios and Vmax/Km was identified in the positive mode (r = or > +0.7) compared to that in the negative mode (r = or < +0.5). These results suggest that MALDI-ToF MS is a useful tool to evaluate Vmax/Km of peptides for protein kinases.