Multiple biological responses activated by nuclear protein kinase C

Impaired Plakophilin-2 in obesity breaks cell cycle dynamics to breed adipocyte senescence

Plakophilin-2 (PKP2) is a key component of desmosomes, which, when defective, is known to promote the fibro-fatty infiltration of heart muscle. Less attention has been given to its role in adipose tissue. We report here that levels of PKP2 steadily increase during fat cell differentiation, and are compromised if adipocytes are exposed to a pro-inflammatory milieu. Accordingly, expression of PKP2 in subcutaneous adipose tissue diminishes in patients with obesity, and normalizes upon mild-to-intense weight loss. We further show defective PKP2 in adipocytes to break cell cycle dynamics and yield premature senescence, a key rheostat for stress-induced adipose tissue dysfunction. Conversely, restoring PKP2 in inflamed adipocytes rewires E2F signaling towards the re-activation of cell cycle and decreased senescence. Our findings connect the expression of PKP2 in fat cells to the physiopathology of obesity, as well as uncover a previously unknown defect in cell cycle and adipocyte senescence due to impaired PKP2.

Activation of PKCβII through nuclear trafficking guided by βγ subunits of trimeric G protein and 14-3-3ε

AIMS

Conventional members of protein kinase C (PKC) family, including PKCβII, are constitutively phosphorylated on three major motifs and located in the cytosol in a primed state. In response to cellular stimuli, PKCβII is activated through inducible phosphorylation and Mdm2-mediated ubiquitination. In this study, we aimed to identify the activation mechanism of PKCβII, focusing on the signaling cascade that regulate the phosphorylation and ubiquitination.

MATERIALS AND METHODS

Loss-of-function approaches and mutants of PDK1/PKCβII that display different regulatory properties were used to identify the cellular components and processes responsible for endocytosis.

KEY FINDINGS

Phorbol 12-myristate 13-acetate (PMA)-induced phosphorylation and ubiquitination of PKCβII, which are needed for its translocation to the plasma membrane, required the presence of both Gβγ and 14-3-3ε. Gβγ and 14-3-3ε mediated the constitutive phosphorylation of PKCβII by scaffolding PI3K and PDK1 in the cytosol, which is an inactive but required state for the activation of PKCβII by subsequent signals. In response to PMA treatment, the signaling complex translocated to the nucleus with dissociation of PI3K from it. Thereafter, PDK1 stably interacted with 14-3-3ε and was dephosphorylated; PKCβII interacted with Mdm2 along with Gβγ, leading to its ubiquitination at two lysine residues on its C-tail. Finally, PDK1/14-3-3ε and ubiquitinated PKCβII translocated to the plasma membrane.

SIGNIFICANCE

As PKCβII mediates a wide range of cellular functions and plays important roles in the pathogenesis of various diseases, our results will provide clues to understand the pathogenesis of PKCβII-related disorders and facilitate their treatment.

Tricyclodecan-9-yl-Xanthogenate (D609): Mechanism of Action and Pharmacological Applications

Tricyclodecan-9-yl xanthogenate (D609) is a synthetic tricyclic compound possessing a xanthate group. This xanthogenate compound is known for its diverse pharmacological properties. Over the last three decades, many studies have reported the biological activities of D609, including antioxidant, antiapoptotic, anticholinergic, anti-tumor, anti-inflammatory, anti-viral, anti-proliferative, and neuroprotective activities. Its mechanism of action is extensively attributed to its ability to cause the competitive inhibition of phosphatidylcholine (PC)-specific phospholipase C (PC-PLC) and sphingomyelin synthase (SMS). The inhibition of PCPLC or SMS affects secondary messengers with a lipidic nature, i.e., 1,2-diacylglycerol (DAG) and ceramide. Various in vitro/in vivo studies suggest that PCPLC and SMS inhibition regulate the cell cycle, block cellular proliferation, and induce differentiation. D609 acts as a pro-inflammatory cytokine antagonist and diminishes Aβ-stimulated toxicity. PCPLC enzymatic activity essentially requires Zn²⁺, and D609 might act as a potential chelator of Zn²⁺, thereby blocking PCPLC enzymatic activity. D609 also demonstrates promising results in reducing atherosclerotic plaque formation, post-stroke cerebral infarction, and cancer progression. The present compilation provides a comprehensive mechanistic insight into D609, including its chemistry, mechanism of action, and regulation of various pharmacological activities.

Activating adenosine A1 receptor accelerates PC12 cell injury via ADORA1/PKC/KATP pathway after intermittent hypoxia exposure

Obstructive sleep apnea hypopnea syndrome (OSAHS) is associated with the neurocognitive deficits as a result of the neuronal cell injury. Previous studies have shown that adenosine A1 receptor (ADORA1) played an important role against hypoxia exposure, such as controlling the metabolic recovery in rat hippocampal slices and increasing the resistance in the combined effects of hypoxia and hypercapnia. However, little is known about whether ADORA1 takes part in the course of neuronal cell injury after intermittent hypoxia exposure which was the main pathological characteristic of OSAHS. The present study is performed to explore the underlying mechanism of neuronal cell injury which was induced by intermittent hypoxia exposure in PC12 cells. In our research, we find that the stimulation of the ADORA1 by CCPA accelerated the injury of PC12 cells as well as upregulated the expression of PKC, inwardly rectifying potassium channel 6.2(Kir6.2) and sulfonylurea receptor 1(SUR1) while inhibition of the ADORA1 by DPCPX alleviated the injury of PC12 cells as well as downregulated the expression of PKC, Kir6.2, and SUR1. Moreover, inhibition of the PKC by CHE, also mitigated the injury of PC12 cells, suppressed the Kir6.2 and SUR1 expressions induced by PKC. Taken together, our findings indicate that ADORA1 accelerated PC12 cells injury after intermittent hypoxia exposure via ADORA1/PKC/K_ATP signaling pathway.

A Phase II trial of tandutinib (MLN 518) in combination with bevacizumab for patients with recurrent glioblastoma

AIM

A Phase II trial of bevacizumab plus tandutinib.

METHODS

We enrolled 41 recurrent, bevacizumab-naive glioblastoma patients for a trial of bevacizumab plus tandutinib. Median age was 55 and 71% were male. Treatment consisted of tandutinib 500 mg two-times a day (b.i.d.) and bevacizumab 10 mg/kg every 2 weeks starting day 15. Of 37 (90%) evaluable, nine (24%) had partial response.

RESULTS & CONCLUSION

Median overall and progression-free survival was 11 and 4.1 months; progression-free survival at 6 months was 23%. All patients suffered treatment-related toxicities; common grade ≥3 toxicities were hypertension (17.1%), muscle weakness (17.1%), lymphopenia (14.6%) and hypophosphatemia (9.8%). Four of six with grade ≥3 tandutinib-related myasthenic-like muscle weakness had electromyography-proven neuromuscular junction pathology. Tandutinib with bevacizumab was as effective but more toxic than bevacizumab monotherapy.

A phase II trial of enzastaurin (LY317615) in combination with bevacizumab in adults with recurrent malignant gliomas

We evaluated the efficacy of combination enzastaurin (LY317615) and bevacizumab for recurrent malignant gliomas and explored serologic correlates. We enrolled 81 patients with glioblastomas (GBM, n = 40) and anaplastic gliomas (AG, n = 41). Patients received enzastaurin as a loading dose of 1125 mg, followed by 500 or 875 mg daily for patients on non-enzyme-inducing or enzyme-inducing antiepileptics, respectively. Patients received bevacizumab 10 mg/kg intravenously biweekly. Clinical evaluations were repeated every 4 weeks. Magnetic resonance imaging was obtained at baseline and every 8 weeks from treatment onset. Phosphorylated glycogen synthase kinase (GSK)-3 levels from peripheral blood mononuclear cells (PBMCs) were checked with each MRI. Median overall survival was 7.5 and 12.4 months for glioblastomas and anaplastic glioma cohorts, with median progression-free survivals of 2.0 and 4.4 months, respectively. Of GBM patients, 3/40 (7.5 %) were not evaluable, while 8/37 (22 %) had partial or complete response and 20/37 (54 %) had stable disease for 2+ months. Of the 39 evaluable AG patients, 18 (46 %) had an objective response, and 16 (41 %) had stable disease for 2+ months. The most common grade 3+ toxicities were lymphopenia (15 %), hypophosphatemia (8.8 %) and thrombotic events (7.5 %). Two (2.5 %) GBM patients died suddenly; another death (1.3 %) occurred from intractable seizures. Phosphorylated GSK-3 levels from PBMCs did not correlate with treatment response. A minimally important improvement in health-related quality of life was self-reported in 7-9/24 (29.2-37.5 %). Early response based on Levin criteria was significantly associated with significantly longer progression free survival for glioblastomas. Enzastaurin (LY317615) in combination with bevacizumab for recurrent malignant gliomas is well-tolerated, with response and progression-free survival similar to bevacizumab monotherapy.

Protein kinase C in the immune system: from signalling to chromatin regulation

Protein kinase C (PKC) form a key family of enzymes involved in signalling pathways that specifically phosphorylates substrates at serine/threonine residues. Phosphorylation by PKC is important in regulating a variety of cellular events such as cell proliferation and the regulation of gene expression. In the immune system, PKCs are involved in regulating signal transduction pathways important for both innate and adaptive immunity, ultimately resulting in the expression of key immune genes. PKCs act as mediators during immune cell signalling through the immunological synapse. PKCs are traditionally known to be cytoplasmic signal transducers and are well embedded in the signalling pathways of cells to mediate the cells' response to a stimulus from the plasma membrane to the nucleus. PKCs are also found to transduce signals within the nucleus, a process that is distinct from the cytoplasmic signalling pathway. There is now growing evidence suggesting that PKC can directly regulate gene expression programmes through a non-traditional role as nuclear kinases. In this review, we will focus on the role of PKCs as key cytoplasmic signal transducers in immune cell signalling, as well as its role in nuclear signal transduction. We will also highlight recent evidence for its newly discovered regulatory role in the nucleus as a chromatin-associated kinase.

Cell signaling through protein kinase C oxidation and activation

Due to the growing importance of cellular signaling mediated by reactive oxygen species (ROS), proteins that are reversibly modulated by these reactant molecules are of high interest. In this context, protein kinases and phosphatases, which act coordinately in the regulation of signal transduction through the phosphorylation and dephosphorylation of target proteins, have been described to be key elements in ROS-mediated signaling events. The major mechanism by which these proteins may be modified by oxidation involves the presence of key redox-sensitive cysteine residues. Protein kinase C (PKC) is involved in a variety of cellular signaling pathways. These proteins have been shown to contain a unique structural feature that is susceptible to oxidative modification. A large number of scientific studies have highlighted the importance of ROS as a second messenger in numerous cellular processes, including cell proliferation, gene expression, adhesion, differentiation, senescence, and apoptosis. In this context, the goal of this review is to discuss the mechanisms by which PKCs are modulated by ROS and how these processes are involved in the cellular response.

Differential role of PKC-induced c-Jun in HTLV-1 LTR activation by 12-O-tetradecanoylphorbol-13-acetate in different human T-cell lines

We have previously shown that TPA activates HTLV-1 LTR in Jurkat T-cells by inducing the binding of Sp1-p53 complex to the Sp1 site residing within the Ets responsive region 1 (ERR-1) of the LTR and that this activation is inhibited by PKCalpha and PKCepsilon. However, in H9 T-cells TPA has been noted to activate the LTR in two consecutive stages. The first stage is activation is mediated by PKCetta and requires the three 21 bp TRE repeats. The second activation mode resembles that of Jurkat cells, except that it is inhibited by PKCdelta. The present study revealed that the first LTR activation in H9 cells resulted from PKCetta-induced elevation of non-phosphorylated c-Jun which bound to the AP-1 site residing within each TRE. In contrast, this TRE-dependent activation did not occur in Jurkat cells, since there was no elevation of non-phosphorylated c-Jun in these cells. However, we found that PKCalpha and PKCepsilon, in Jurkat cells, and PKCetta and PKCdelta, in H9 cells, increased the level of phosphorylated c-Jun that interacted with the Sp1-p53 complex. This interaction prevented the Sp1-p53 binding to ERR-1 and blocked, thereby, the ERR-1-mediated LTR activation. Therefore, this PKC-inhibited LTR activation started in both cell types after depletion of the relevant PKCs by their downregulation. In view of these variable activating mechanisms we assume that there might be additional undiscovered yet modes of HTLV-1 LTR activation which vary in different cell types. Moreover, in line with this presumption we speculate that in HTLV-1 carriers the LTR of the latent provirus may also be reactivated by different mechanisms that vary between its different host T-lymphocyte subclones. Since this reactivation may initiate the ATL process, understanding of these mechanisms is essential for establishing strategies to block the possibility of reactivating the latent virus as preventive means for ATL development in carriers.

Protein kinases and phosphatases in the control of cell fate

Protein phosphorylation controls many aspects of cell fate and is often deregulated in pathological conditions. Several recent findings have provided an intriguing insight into the spatial regulation of protein phosphorylation across different subcellular compartments and how this can be finely orchestrated by specific kinases and phosphatases. In this review, the focus will be placed on (i) the phosphoinositide 3-kinase (PI3K) pathway, specifically on the kinases Akt and mTOR and on the phosphatases PP2a and PTEN, and on (ii) the PKC family of serine/threonine kinases. We will look at general aspects of cell physiology controlled by these kinases and phosphatases, highlighting the signalling pathways that drive cell division, proliferation, and apoptosis.

Low molecular weight heparin inhibits melanoma cell adhesion and migration through a PKCa/JNK signaling pathway inducing actin cytoskeleton changes

Low molecular weight heparin (LMWH) has significant antimetastatic capabilities and affects cancer progression in humans through, not fully defined mechanisms. Here we evaluated its activity at the intracellular level and how it is correlated with melanoma cell adhesion and migration. LMWH inhibited M5 and A375 melanoma cell adhesion and migration in a dose-dependent manner (p⩽0.01). Treatment of M5 melanoma cells with LMWH caused a marked down regulation of constitutive as well as the FN-induced phosphorylation (p⩽0.01) of protein kinase C alpha (PKCa). This was associated with a profound decrease in the cytoplasmic pPKCa (p⩽0.05) and a simultaneous enhancement of nuclear pPKCa localization (p⩽0.01). A significant decrease in the levels of pJNK (p⩽0.01), which is a downstream effector of PKCa, was also demonstrated in the LMWH-treated cells. Furthermore, LMWH-treated cells had disorganized actin stress fibers correlated to a strong decrease in cell-substratum interface area (p⩽0.05) and altered morphology. The decrease in the activation of PKCa, which is an important regulator of cell motility, was directly correlated to the reduced ability of the LMWH-treated melanoma cells to adhere onto and migrate towards the fibronectin (FN) substrate (p⩽0.01). The lineage activation of PKCa-JNK/p38 and their correlation to M5 cell adhesion was confirmed with the utilization of specific inhibitors. In conclusion, LMWH through the downregulation of pPKCa and redistribution to nuclear region attenuates JNK activation, which in turn induces cytoskeleton changes correlated to M5 cell decreased adhesion/migration. This may provide clues for the pharmacological targeting of melanoma.

High nuclear protein kinase Cθ expression may correlate with disease recurrence and poor survival in oral squamous cell carcinoma

Protein kinase Cs play important roles in many biological processes and tumorigenesis. This study examined the expression of protein kinase Cθ and assessed its significance in patients with oral squamous cell carcinoma. Immunohistochemical staining was carried out to investigate the expression of protein kinase Cθ in 59 cases of oral squamous cell carcinoma. The results were correlated with clinical characteristics and outcome of patients. Diffuse cytoplasmic protein kinase Cθ was identified in 53 (89.8%) of the 59 oral squamous cell carcinoma cases, and the expression was not statistically associated with any clinicopathologic parameter. Twenty (40.7%) of the 59 oral squamous cell carcinoma cases exhibited nuclear expression of protein kinase Cθ with different grade of intensity. χ(2) analysis indicated that high nuclear protein kinase Cθ expression correlated significantly with shorter 24-month survival (P = .043) and disease recurrence (P = .019). The Kaplan-Meier method also showed that high nuclear expression of protein kinase Cθ was significantly associated with poor overall survival (P = .034) and shorter time to recurrence (P = .003). Univariate analysis revealed that high nuclear protein kinase Cθ expression (P = .046; hazard ratio, 2.2), tumor size less than 2 cm (P = .049; hazard ratio, 4.7), lymph node metastasis (P = .003; hazard ratio, 3.0), and higher stage (P = .002; hazard ratio, 8.7) were each associated with shorter overall survival. We identified the aberrant nuclear expression of protein kinase Cθ in oral squamous cell carcinoma. High nuclear protein kinase Cθ expression may correlate with disease recurrence and poor survival in patients with oral squamous cell carcinoma.

PKC isoforms interact with and phosphorylate DNMT1

Background

DNA methyltransferase 1 (DNMT1) has been shown to be phosphorylated on multiple serine and threonine residues, based on cell type and physiological conditions. Although recent studies have suggested that protein kinase C (PKC) may be involved, the individual contribution of PKC isoforms in their ability to phosphorylate DNMT1 remains unknown. The PKC family consists of at least 12 isoforms that possess distinct differences in structure, substrate requirement, expression and localization.

Results

Here we show that PKCα, βI, βII, δ, γ, η, ζ and μ preferentially phosphorylate the N-terminal domain of human DNMT1. No such phosphorylation of DNMT1 was observed with PKCε. Using PKCζ as a prototype model, we also found that PKC physically interacts with and phosphorylates DNMT1. In vitro phosphorylation assays conducted with recombinant fragments of DNMT1 showed that PKCζ preferentially phosphorylated the N-terminal region of DNMT1. The interaction of PKCζ with DNMT1 was confirmed by GST pull-down and co-immunoprecipitation experiments. Co-localization experiments by fluorescent microscopy further showed that endogenous PKCζ and DNMT1 were present in the same molecular complex. Endogenous PKCζ activity was also detected when DNMT1 was immunoprecipitated from HEK-293 cells. Overexpression of both PKCζ and DNMT1 in HEK-293 cells, but not of either alone, reduced the methylation status of genes distributed across the genome. Moreover, in vitro phosphorylation of DNMT1 by PKCζ reduced its methytransferase activity.

Conclusions

Our results indicate that phosphorylation of human DNMT1 by PKC is isoform-specific and provides the first evidence of cooperation between PKCζ and DNMT1 in the control of the DNA methylation patterns of the genome.

Chromatin-associated protein kinase C-θ regulates an inducible gene expression program and microRNAs in human T lymphocytes

Studies in yeast demonstrate that signaling kinases have a surprisingly active role in the nucleus, where they tether to chromatin and modulate gene expression programs. Despite these seminal studies, the nuclear mechanism of how signaling kinases control transcription of mammalian genes is in its infancy. Here, we provide evidence for a hitherto unknown function of protein kinase C-theta (PKC-θ), which physically associates with the regulatory regions of inducible immune response genes in human T cells. Chromatin-anchored PKC-θ forms an active nuclear complex by interacting with RNA polymerase II, the histone kinase MSK-1, and the adaptor molecule 14-3-3ζ. ChIP-on-chip reveals that PKC-θ binds to promoters and transcribed regions of genes, as well as to microRNA promoters that are crucial for cytokine regulation. Our results provide a molecular explanation for the role of PKC-θ not only in normal T cell function, but also in circumstances of its ectopic expression in cancer.

Protein kinase C mRNA and protein expressions in hypobaric hypoxia-induced cardiac hypertrophy in rats

AIM

Protein kinase C (PKC), cloned as a serine/threonine kinase, plays key roles in diverse intracellular signalling processes and in cardiovascular remodelling during pressure overload or volume overload. We looked for correlations between changes in PKC isoforms (levels and/or subcellular distributions) and cardiac remodelling during experimental hypobaric hypoxic environment (HHE)-induced pulmonary hypertension.

METHODS

To study the PKC system in the heart during HHE, 148 male Wistar rats were housed for up to 21 days in a chamber at the equivalent of 5500 m altitude level (10% O(2)).

RESULTS

At 14 or more days of exposure to HHE, pulmonary arterial pressure (PAP) was significantly increased. In the right ventricle (RV): (1) the expression of PKC-alpha protein in the cytosolic and membrane fractions was increased at 3-14 days and at 5-7 days of exposure respectively; (ii) the cytosolic expression of PKC-delta protein was increased at 1-5, 14 and 21 days of exposure; (3) the membrane expressions of the proteins were decreased at 14-21 (PKC-betaII), 14-21 (PKC-gamma), and 0.5-5 and 21 (PKC-epsilon) days of exposure; (4) the expression of the active form of PKC-alpha protein on the plasma membrane was increased at 3 days of exposure (based on semiquantitative analysis of the immunohistochemistry). In the left ventricle, the expressions of the PKC mRNAs, and of their cytosolic and membrane proteins, were almost unchanged. The above changes in PKC-alpha, which were strongly evident in the RV, occurred alongside the increase in PAP.

CONCLUSION

PKC-alpha may help to modulate the right ventricular hypertrophy caused by pulmonary hypertension in HHE.

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.

Novel shuttling domain in a regulator (RSC1A1) of transporter SGLT1 steers cell cycle-dependent nuclear location

The gene product of RSC1A1, RS1, participates in the regulation of the Na(+)-D-glucose cotransporter SGLT1. RS1 inhibits release of SGLT1 from the trans Golgi network. In subconfluent LLC-PK(1) cells, RS1 migrates into the nucleus and modulates transcription of SGLT1, whereas most confluent cells do not contain RS1 in the nuclei. We showed that confluence-dependent nuclear location of RS1 is because of different phases of the cell cycle and identified a RS1 nuclear shuttling domain (RNS) with an associated protein kinase C (PKC) phosphorylation site (RNS-PKC) that mediates cell cycle-dependent nuclear location. RNS-PKC contains a novel non-conventional nuclear localization signal interacting with importin beta1, a nuclear export signal mediating export via protein CRM1 and a Ca(2+)-dependent calmodulin binding site. PKC and calmodulin compete for binding to RNS-PKC. Mutagenesis experiments and analyses of the phosphorylation status suggest the following sequences of events. Subconfluent cells without and with synchronization to the G2/M phase contain non-phosphorylated RNS-PKC that mediates nuclear import of RS1 but not its export. During confluence or synchronization of subconfluent cells to the G2/M phase, phosphorylation of RNS-PKC mediates rapid nuclear export of RS1.

A(1) adenosine receptor-mediated PKC and p42/p44 MAPK signaling in mouse coronary artery smooth muscle cells

The A(1) adenosine receptor (A(1)AR) is coupled to G(i)/G(o) proteins, but the downstream signaling pathways in smooth muscle cells are unclear. This study was performed in coronary artery smooth muscle cells (CASMCs) isolated from the mouse heart [A(1)AR wild type (A(1)WT) and A(1)AR knockout (A(1)KO)] to delineate A(1)AR signaling through the PKC pathway. In A(1)WT cells, treatment with (2S)-N(6)-(2-endo-norbornyl)adenosine (ENBA; 10(-5)M) increased A(1)AR expression by 150%, which was inhibited significantly by the A(1)AR antagonist 1,3-dipropyl-8-cyclopentylxanthine (10(-6)M), but not in A(1)KO CASMCs. PKC isoforms were identified by Western blot analysis in the cytosolic and membrane fractions of cell homogenates of CASMCs. In A(1)WT and A(1)KO cells, significant levels of basal PKC-alpha were detected in the cytosolic fraction. Treatment with the A(1)AR agonist ENBA (10(-5)M) translocated PKC-alpha from the cytosolic to membrane fraction significantly in A(1)WT but not A(1)KO cells. Phospholipase C isoforms (betaI, betaIII, and gamma(1)) were analyzed using specific antibodies where ENBA treatment led to the increased expression of PLC-betaIII in A(1)WT CASMCs while having no effect in A(1)KO CASMCs. In A(1)WT cells, ENBA increased PKC-alpha expression and p42/p44 MAPK (ERK1/2) phospohorylation by 135% and 145%, respectively. These effects of ENBA were blocked by Gö-6976 (PKC-alpha inhibitor) and PD-98059 (p42/p44 MAPK inhibitor). We conclude that A(1)AR stimulation by ENBA activates the PKC-alpha signaling pathway, leading to p42/p44 MAPK phosphorylation in CASMCs.

Role of caveolin 1, E-cadherin, Enolase 2 and PKCalpha on resistance to methotrexate in human HT29 colon cancer cells

Background

Methotrexate is one of the earliest cytotoxic drugs used in cancer therapy, and despite the isolation of multiple other folate antagonists, methotrexate maintains its significant role as a treatment for different types of cancer and other disorders. The usefulness of treatment with methotrexate is limited by the development of drug resistance, which may be acquired through different ways. To get insights into the mechanisms associated with drug resistance and sensitization we performed a functional analysis of genes deregulated in methotrexate resistant cells, either due to its co-amplification with the dhfr gene or as a result of a transcriptome screening using microarrays.

Methods

Gene expression levels were compared between triplicate samples from either HT29 sensitive cells and resistant to 10^-5 M MTX by hybridization to the GeneChip^® HG U133 PLUS 2.0 from Affymetrix. After normalization, a list of 3-fold differentially expressed genes with a p-value < 0.05 including multiple testing correction (Benjamini and Hochberg false discovery rate) was generated. RT-Real-time PCR was used to validate the expression levels of selected genes and copy-number was determined by qPCR. Functional validations were performed either by siRNAs or by transfection of an expression plasmid.

Results

Genes adjacent to the dhfr locus and included in the 5q14 amplicon were overexpressed in HT29 MTX-resistant cells. Treatment with siRNAs against those genes caused a slight reduction in cell viability in both HT29 sensitive and resistant cells. On the other hand, microarray analysis of HT29 and HT29 MTX resistant cells unveiled overexpression of caveolin 1, enolase 2 and PKCα genes in resistant cells without concomitant copy number gain. siRNAs against these three genes effectively reduced cell viability and caused a decreased MTX resistance capacity. Moreover, overexpression of E-cadherin, which was found underexpressed in MTX-resistant cells, also sensitized the cells toward the chemotherapeutic agent. Combined treatments targeting siRNA inhibition of caveolin 1 and overexpression of E-cadherin markedly reduced cell viability in both sensitive and MTX-resistant HT29 cells.

Conclusion

We provide functional evidences indicating that caveolin 1 and E-cadherin, deregulated in MTX resistant cells, may play a critical role in cell survival and may constitute potential targets for coadjuvant therapy.

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 protein kinase C (PKC) family of proteins in cytokine signaling in hematopoiesis

The members of the protein kinase C (PKC) family of proteins play important roles in signaling for various growth factors, cytokines, and hormones. Extensive work over the years has led to the identification of three major groups of PKC isoforms. These include the classic PKCs (PKCalpha, PKCbeta(I), PKCbeta(II), PKCgamma), the novel PKCs (PKCdelta, PKCepsilon, PKCeta, PKCmu, PKCtheta), and the atypical PKCs (PKCzeta, PKCiota/lambda). All these PKC subtypes have been shown to participate in the generation of signals for important cellular processes and to mediate diverse and, in some cases, opposing biologic responses. There is emerging evidence that these kinases also play key functional roles in the regulation of cell growth, apoptosis, and differentiation of hematopoietic cells. In this review, both the engagement of the various PKC members in cytokine and growth factor signaling and their role in the regulation of hematopoiesis are discussed.

The different facets of protein kinases C: old and new players in neuronal signal transduction pathways

Signal transduction pathways are crucial for cell-to-cell communication. Various molecular cascades allow the translation of distinct stimuli, targeting the cell, into a language that the cell itself is able to understand, thus elaborating specific responses. Within this context, a strategic role is played by protein kinases which catalyze the phosphorylation of specific substrates. The serine/threonine protein kinase C (PKC) enzymes family (at least 10 isoforms) is implicated in the transduction of signals coupled to receptor-mediated hydrolysis of membrane phospholipids. Within this molecular pathway, protein-protein interactions play a critical role in directing the distinct activated PKCs towards selective subcellular compartments, in order to guarantee spatio-temporal and localized cellular responses. A space-specific modulation of biochemical events is particularly important during learning. Among the various mechanisms, the modulation of mRNA decay appears to be an efficient post-transcriptional way of controlling gene expression during learning, allowing changes to take place in selected neuronal regions, in particular at synaptic level. To this regard, recent studies have pointed out that PKC activation is also involved in a novel signalling cascade leading to the stabilization of specific mRNAs. This review will especially focus the attention on the implication of PKC in memory trace formation and how alterations within this molecular cascade may have consequences on physiological and pathological neuronal aging (i.e. Alzheimer's disease).

Diacylglycerol, phosphatidic acid, and the converting enzyme, diacylglycerol kinase, in the nucleus

There exists phosphoinositide (PI) cycle in the nucleus, which is operated differentially from the classical PI cycle at the plasma membrane. Evidence has been accumulated that nuclear PIs and the related enzymes are closely involved in a variety of nuclear processes, although the details remain to be elucidated. In this mini review, some components of PI cycle, i.e., diacylglycerol, phosphatidic acid, and the converting enzyme, diacylglycerol kinase, in the nucleus are discussed with focusing on the lipid metabolism, cell cycle regulation, and animal models.

Nuclear protein kinase C

Protein kinase C (PKC) isozymes constitute a family of ubiquitous phosphotransferases which act as key transducers in many agonist-induced signaling cascades. To date, at least 11 different PKC isotypes have been identified and are believed to play distinct regulatory roles. PKC isoforms are physiologically activated by a number of lipid cofactors. PKC is thought to reside in the cytoplasm in an inactive conformation and to translocate to the plasma membrane or cytoplasmic organelles upon cell activation by different stimuli. However, a sizable body of evidence collected over the last 20 years has shown PKC to be capable of translocating to the nucleus. Furthermore, PKC isoforms are resident within the nucleus. Studies from independent laboratories have to led to the identification of quite a few nuclear proteins which are PKC substrates and to the characterization of nuclear PKC-binding proteins which may be critical for finely tuning PKC function in this cell microenvironment. Several lines of evidence suggest that nuclear PKC isozymes are involved in the regulation of biological processes as important as cell proliferation and differentiation, gene expression, neoplastic transformation, and apoptosis. In this review, we shall highlight the most intriguing and updated findings about the functions of nuclear PKC isozymes.

The role of protein kinase C-alpha in hematologic malignancies

In recent years advances in histopathological and molecular understanding of hematologic malignancies have led to the development of drugs which selectively target proteins associated with hematologic tumorigenesis. One such targeted agent is the antisense oligonucleotide aprinocarsen, which specifically inhibits the signaling protein, protein kinase C-alpha (PKC-alpha). Although PKC-alpha has been associated with tumorigenesis, its role and expression levels in patients with hematologic malignancies are not well understood. We here review studies investigating the expression and role of PKC-alpha in hematologic malignancies. Such a review may offer new insights on how to develop strategies in identifying patients that might best benefit from PKC-alpha inhibition.

Role of cAMP-response element-binding protein phosphorylation in hepatic apoptosis under protein kinase C alpha suppression during sepsis

Previous studies have shown that a decrease in protein kinase C (PKC) alpha levels contributes to hepatic failure and/or apoptosis during sepsis, and suppression of PKCalpha plays a critical role in triggering caspase-dependent apoptosis, which can modulate expression of Bcl-xL. However, the underlying molecular mechanism remains uncertain. In the present study, we examined whether a decrease in the nuclear PKCalpha levels causes hepatic apoptosis via modulation of cAMP-response element-binding protein (CREB) or nuclear factor-kappaB (NFkappaB), the crucial factors regulating the expression of prosurvival Bcl-xL. For polymicrobial sepsis induction, a cecal ligation and puncture model was used; at 9 or 18 h after CLP, experiments were terminated, referring as early or late sepsis, respectively. Additionally, PKCalpha was suppressed by stable transfection of antisense PKCalpha plasmid into a Clone-9 rat hepatic epithelial cell. The results showed that the nuclear PKCalpha was significantly decreased in the liver during sepsis, which was accompanied by decreases in phospho-CREB content, DNA-binding activity of CREB, and Bcl-xL expression. Likewise, the binding activity of NFkappaB increased significantly, which was associated with a decrease in cytosolic inhibitory-kappaBalpha content. The in vitro suppression of PKCalpha also resulted in decreases in the phospho-CREB content and DNA-binding activity, which were accompanied by down-regulation of Bcl-xL and apoptosis, but no significant alteration in NFkappaB-binding activity. The in vivo and in vitro results suggest that the suppression of PKCalpha results in a decreased CREB phosphorylation and subsequent down-regulation of Bcl-xL, which may contribute to the hepatic apoptosis during sepsis.

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

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) induces apoptosis in a number of tumorogenic or transformed cells, yet is relatively non-toxic to most normal cells, therefore, it is a promising agent for cancer therapy. However, some cancer cell lines were resistant to TRAIL cytoxicity, including MCF-7 breast cancer cells. The mechanism is not clear. Here, we report that protein kinase C delta (PKCdelta) protects MCF-7 cells from the recombinant soluble TRAIL (rsTRAIL)- mediated apoptosis. It was demonstrated that rottlerin, a PKCdelta inhibitor, sensitized MCF-7 cells to rsTRAIL cytoxicity. Combination of rottlerin and rsTRAIL inhibited PKCdelta translocation from the cytosol to membrane, and PKCdelta kinase activity on the cell membrane was kept pace with the change of PKCdelta expression. Moreover, inhibition of PKCdelta by interference RNA could facilitate apoptosis of MCF-7 cells induced by rsTRAIL. Further experiments on the signal machinery showed that rottlerin increased the sensitivity of MCF-7 cells to rsTRAIL by suppressing the transcription activity of NF-kappaB, and enhancing the caspase-processing to generate executive apoptotic signals. These findings indicate that PKCdelta functions as a survival factor protecting MCF-7 cells from the apoptosis induced by rsTRAIL.

Erythroid cell differentiation is characterized by nuclear matrix localization and phosphorylation of protein kinases C (PKC) alpha, delta, and zeta

Protein kinases C (PKC) zeta expression and phosphorylation at nuclear level during dimethyl sulfoxide (DMSO)-induced differentiation in Friend erythroleukemia cells have been previously reported, suggesting a possible role of this PKC isoform in the DMSO-related signaling. In order to shed more light on this tantalizing topic, we investigated PKC intracellular and sub-cellular localization and activity during DMSO-induced erythroid differentiation. Results indicated that at least PKC alpha, zeta, and delta are strongly and temporally involved in the DMSO-induced differentiation signals since their expression and phosphorylation, though at different extents, were observed during treatments. Intriguingly, while PKC alpha and zeta associate to the nuclear matrix during the differentiation event, PKC delta appears to be residentially associated to the nuclear matrix. Furthermore, an evident downregulation of the beta-globin gene transcription (differentiation hallmark) was detected upon a progressive inhibition of these PKC isoforms by means of specific inhibitors, indicating, therefore, that PKC alpha, zeta, and delta phosphorylation play a crucial role in the control of erythroid differentiation.

HSP25 inhibits radiation-induced apoptosis through reduction of PKCδ-mediated ROS production

Since radiation-induced caspase-dependent apoptosis and ROS generation were partially prevented by HSP25 overexpression, similar to the treatment of control cells with antioxidant agents such as DPI and tiron, questions arise whether radiation-mediated ROS generation contributes to the apoptotic cell death, and also whether HSP25 overexpression can reduce ROS mediated apoptotic cell death. In the present study, radiation-induced cytochrome c release from mitochondria and activation of caspases accompanied by a decrease of mitochondrial membrane potential in Jurkat T cells were shown to be inhibited by mitochondrial complex I inhibitor rotenone, suggesting that mitochondrial ROS might be important in radiation-induced caspase-dependent apoptosis. When HSP25 was overexpressed, effects similar to the treatment of cells with the antioxidants were obtained, indicating that HSP25 suppressed radiation-induced mitochondrial alteration that resulted in apoptosis. Furthermore, activation of p38 MAP kinase by radiation was associated with radiation-induced cell death and ROS production and PKCdelta was an upstream molecule for p38 MAP kinase activation, ROS generation and subsequent caspase-dependent apoptotic events. However, in the HSP25 overexpressed cells, the above-described effects were blocked. In fact, radiation-induced membrane translocation of PKCdelta and tyrosine phosphorylation were inhibited by HSP25. Based on the above data, we suggest that HSP25 downregulates PKCdelta, which is a key molecule for radiation-induced ROS generation and mitochondrial-mediated caspase-dependent apoptotic events.

The role of protein kinase C-alpha (PKC-alpha) in melanoma

In the 1980s, protein kinase C (PKC) was identified as a contributing factor in skin tumorigenesis. As drugs targeting specifically PKC have become available, the intent of this review was to assess the role of PKC, in particular of PKC-alpha in melanoma or other skin cancers. We reviewed and summarized published studies examining the role of PKC-alpha in the development of melanoma or other skin cancers. Most studies to date have been cell-culture based. In models of melanoma, PKC-alpha activation is typically associated with increased tumour cell proliferation, invasiveness and decreased differentiation, suggesting that PKC-alpha inhibitors, such as aprinocarsen, an antisense oligonucleotide directed against PKC-alpha, may be appropriate in the treatment of skin malignancies. Because of the recent developments on selective or specific PKC-alpha inhibitors, including aprinocarsen, there is a growing need to conduct further translational research, especially in melanoma patients, to identify the patient population that might benefit most from PKC-alpha targeted therapy.

Translocation of diacylglycerol kinase theta from cytosol to plasma membrane in response to activation of G protein-coupled receptors and protein kinase C

Diacylglycerol kinase (DGK) phosphorylates the second messenger diacylglycerol (DAG) to phosphatidic acid. We previously identified DGK as one of nine mammalian DGK isoforms and reported on its regulation by interaction with RhoA and by translocation to the plasma membrane in response to noradrenaline. Here, we have investigated how the localization of DGK, fused to green fluorescent protein, is controlled upon activation of G protein-coupled receptors in A431 cells. Extracellular ATP, bradykinin, or thrombin induced DGK translocation from the cytoplasm to the plasma membrane within 2-6 min. This translocation, independent of DGK activity, was preceded by protein kinase C (PKC) translocation and was blocked by PKC inhibitors. Conversely, activation of PKC by 12-O-tetradecanoylphorbol-13-acetate induced DGK translocation. Membrane-permeable DAG (dioctanoylglycerol) also induced DGK translocation but in a PKC (staurosporin)-independent fashion. Mutations in the cysteine-rich domains of DGK abrogated its hormone- and DAG-induced translocation, suggesting that these domains are essential for DAG binding and DGK recruitment to the membrane. We show that DGK interacts selectively with and is phosphorylated by PKCepsilon and -eta and that peptide agonist-induced selective activation of PKCepsilon directly leads to DGK translocation. Our data are consistent with the concept that hormone-induced PKC activation regulates the intracellular localization of DGK, which may be important in the negative regulation of PKCepsilon and/or PKCeta activity.

Enhanced retinoid-induced apoptosis of MDA-MB-231 breast cancer cells by PKC inhibitors involves activation of ERK

Retinoids are vitamin A derivatives, which cause growth inhibition, differentiation and/or apoptosis in various cell types, including some breast cancer cells. In general, estrogen receptor (ER)-positive cells are retinoic acid (RA) sensitive, whereas ER-negative cells are resistant. In this report, we show that ER-negative MDA-MB-231 cells are strongly growth inhibited by retinoids in combination with a PKC inhibitor. While neither RA nor GF109203X (GF) has a significant growth inhibitory effect in these cells, RA+GF potently suppress proliferation. We found that RA+GF induce apoptosis, as shown by an increase in fragmented DNA, Annexin-V-positive cells and caspase-3 activation. Apoptosis was also induced by GF in combination with two synthetic retinoids. Expression of phosphorylated as well as total PKC was decreased by GF and this was potentiated by RA. In addition, treatment with GF caused a strong and sustained activation of ERK1/2 and p38-MAPK, as well as a weaker activation of JNK. Importantly, inhibition of ERK but not p38 or JNK suppressed apoptosis induced by RA+GF, indicating that activation of ERK is specifically required. In support of this novel finding, the ability of other PKC inhibitors to cause apoptosis in combination with RA correlates with ability to cause sustained activation of ERK.

Expression Levels of Protein Kinase C-α in Non–Small-Cell Lung Cancer

Current treatments of non-small-cell lung cancer (NSCLC) are inadequate and new therapies are being developed that target specific cellular signaling proteins associated with tumor growth. One potential target is protein kinase C (PKC)-alpha, a signaling molecule with an important role in cell regulation and proliferation. The present study examines the expression levels of PKC-alpha in NSCLC to better understand the distribution of PKC-alpha in NSCLC. We analyzed tumor specimens from an independent tumor tissue bank to determine PKC-alpha protein and messenger RNA gene expression in NSCLC. In addition, we used publicly available gene expression array data to further understand PKC-a-associated gene expression profiles in NSCLC. We found that PKC-alpha is highly expressed in < or = 20% of patients with NSCLC. We also found that PKC-alpha was preferentially expressed in adenocarcinoma compared with squamous cell carcinoma of the lung.

Protein kinase C alpha expression in breast and ovarian cancer

In recent years research has focused on the development of specific, targeted drugs to treat cancer. One approach has been to block intracellular signaling proteins, such as protein kinase C alpha (PKC-alpha). To help support the rationale for clinical studies of a PKC-alpha-targeted therapy in breast and ovarian cancers, we reviewed publications studying PKC-alpha expression in these tumors. Since these investigations were mostly performed in cell lines, we supplemented this review with some preliminary findings from studies examining PKC-alpha expression in tumor tissue biopsies obtained from patients with breast and ovarian cancer. Based on the reviewed publications using representative cell lines and our preliminary findings on tumor tissue of patients with breast cancer, we infer that PKC-alpha levels may especially be increased in breast cancer patients with low or negative estrogen receptor (ER) levels. Thus, clinical studies determining efficacy of selective or specific inhibitors of PKC-alpha should include determination of ER status in order to help answer whether blocking PKC-alpha in patients with low or absent ER can result in clinical benefit.

The role of protein kinase C-alpha (PKC-alpha) in malignancies of the gastrointestinal tract

Drugs specifically designed to block cellular signalling proteins are currently evaluated as a new way to treat gastrointestinal tumours. One such "new targeted agent" is aprinocarsen, an antisense oligonucleotide that specifically blocks the mRNA of protein kinase C-alpha (PKC-alpha). Blocking PKC-alpha, an important cellular signalling molecule associated with tumour growth, is anticipated to result in tumour cell arrest and achieve clinical benefits. However, it is not known which patients may benefit most from a specific inhibition of PKC-alpha. Past experience with other novel targeted agents suggests that expression of the target molecule is an important factor for the success of such a specific therapy. Therefore, reviewing the specific role of PKC-alpha in various gastrointestinal tumours may contribute to focus the clinical development of selective or specific PKC-alpha inhibitors, such as aprinocarsen, on those patients with a distinctive PKC-alpha expression pattern.

Protein kinase C delta is activated in mouse ovarian surface epithelial cancer cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

Interactions between the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and protein kinase C (PKC) signaling pathways are governed in cell and tissue-specific manners, albeit the physiological significance of which is unclear. This research sought to define the effects of TCDD on the PKC pathway using a mouse ovarian surface epithelial cancer cell line (ID8). Phorbol-12-myristate-13-acetate (PMA) potentiated (1 nM) TCDD-induced 7-ethoxyresorufin-O-deethylase (EROD) activity after 24h of treatment, and pre-treatment with (1 microM) of either a general PKC inhibitor (BisI) or PKCdelta-specific inhibitor (Rotterlin) abolished the potentiation indicating that activation of PKC enhances TCDD signal transduction. Western blot analysis revealed that unstimulated ID8 cells express PKCalpha, beta, epsilon, tau, lambda and RACK1. PKCgamma, eta, theta and DGKtheta were not detected. TCDD (1 nM) increased PKCdelta protein approximately eight-fold after 24h of treatment and this effect was dose-dependent (0.1-100 nM); other PKC isoforms and related signaling proteins tested were unaffected by TCDD treatment. Immunofluorescent microscopy revealed that TCDD (1 nM) promoted the subcellular redistribution of PKCdelta, from the cytoplasm and the nucleus to the perinuclear area after 2h of treatment, however, after 24h of treatment PKCdelta was observed in nuclear structures that resembled nucleoli. TCDD (1 nM) also increased total PKC and PKCdelta-specific kinase activities in biphasic time-responsive manners. Total PKC and PKCdelta-specific activities increased after 1-2h of treatment. Then TCDD increased the total PKC activity again after 12h of treatment, whereas, PKCdelta-specific activity resurged at 24h and remained elevated at 48 h after treatment. The results indicate that TCDD preferentially induces PKCdelta protein expression and phosphotransferase activity, and its membrane translocation, indicating a potential intracellular role for PKCdelta as an effector molecule for TCDD-mediated biological events in this ovarian cancer cell line.

The V5 Domain of Protein Kinase C Plays a Critical Role in Determining the Isoform-Specific Localization, Translocation, and Biological Function of Protein Kinase C-δ and -ε

The catalytic domain of overexpressed protein kinase C (PKC)-δ mediates phorbol 12-myristate 13-acetate (PMA)-induced differentiation or apoptosis in appropriate model cell lines. To define the portions of the catalytic domain that are critical for these isozyme-specific functions, we constructed reciprocal chimeras, PKC-δ/εV5 and -ε/δV5, by swapping the V5 domains of PKC-δ and -ε. PKC-δ/εV5 failed to mediate PMA-induced differentiation of 32D cells, showing the essential nature of the V5 domain for PKC-δ's functionality. The other chimera, PKC-ε/δV5, endowed inactive PKC-ε with nearly all PKC-δ's apoptotic ability, confirming the importance of PKC-δ in this function. Green fluorescent protein (GFP)-tagged PKC-δV5 and -ε/δV5 in A7r5 cells showed substantial basal nuclear localization, while GFP-tagged PKC-ε and -δ/εV5 showed significantly less, indicating that the V5 region of PKC-δ contains determinants critical to its nuclear distribution. PKC-ε/δV5-GFP showed much slower kinetics of translocation to membranes in response to PMA than parental PKC-ε, implicating the PKC-εV5 domain in membrane targeting. Thus, the V5 domain is critical in several of the isozyme-specific functions of PKC-δ and -ε.

Modulation by Sphingosine of Phosphorylation of Substrate Proteins by Protein Kinase C in Nuclei from Cow Mammary Gland

Protein kinase C (PKC) is an enzyme activated by diacylglycerols such as 1-oleoyl-2-acetyl-sn-glycerol (OAG), phospholipids (in particular phosphatidylserine; PS) and Ca2+, which regulate a wide variety of intracellular functions by phosphorylating multiple substrate proteins and enzymes. The effect of sphingosine, the backbone moiety of sphingolipids, on PKC activity and phosphorylation of endogenous proteins catalyzed by PKC was investigated in nuclei of cow mammary gland. Sphingosine inhibited nuclear PKC activity when lysine-rich histone was used as the substrate. The sphingosine inhibition of the PKC activity was reversed by the excess addition of PS, but not by OAG or Ca2+. Several nuclear proteins, including 56-kDa, 43-kDa, 38-kDa and 36-kDa proteins, were shown to be substrates for PKC. Of the substrate proteins, the 38-kDa and 36-kDa proteins were identified as annexin I, the Ca2+/phospholipid-binding protein; the 56-kDa and 43-kDa proteins have not yet been identified. Sphingosine inhibited phosphorylation of the 56-kDa protein and the 36-kDa annexin I, whereas it enhanced that of the 43-kDa protein. The 38-kDa annexin I species was unaffected by sphingosine. As with the PKC activity, inhibition by sphingosine of phosphorylation of the 56-kDa protein and 36-kDa annexin I was reversed by the excess addition of PS, but not by OAG or Ca2+. In addition, by the excess addition of PS and not by OAG or Ca2+, the sphingosine-enhanced phosphorylation of the 43-kDa protein was reversed and returned to near the level in the absence of sphingosine. It is suggested that sphingosine is involved in the regulation of PKC-dependent phosphorylation in the nucleus by modulating the association of PKC or its substrates, particularly annexin I, with membrane phospholipids in cow mammary gland.

Nuclear phosphoinositides and their functions

Phosphoinositides are minor components of biological membranes, which have emerged as essential regulators of a variety of cellular processes, both on the plasma membrane and on several intracellular organelles. The versatility of these lipids stems from their ability to function either as substrates for the generation of second messengers, as membrane-anchoring sites for cytosolic proteins or as regulators of the actin cytoskeleton. Despite a vast literature demonstrating the presence of phosphoinositides in the nucleus, only recently has the function(s) of the nuclear pool of these lipids and their soluble analogues, inositol polyphosphates, started to emerge. These compounds have been shown to serve as essential co-factors for several nuclear processes, including DNA repair, transcription regulation and RNA dynamics. In this light, phosphoinositides and inositol polyphosphates might represent high turnover activity switches for nuclear complexes responsible for these processes. The regulation of these large machineries would be linked to the phosphorylation state of the inositol ring and limited temporally and spatially based on the synthesis and degradation of these molecules.

Centaurin-alpha(1) associates with and is phosphorylated by isoforms of protein kinase C

Centaurin-alpha(1) is a member of the family of ADP-ribosylation factors (ARF) GTPase activating proteins (GAPs), although ARF GAP activity has not yet been demonstrated. The human homologue, centaurin-alpha(1) functionally complements the ARF GAP activity of Gcs1 in yeast. Although Gcs1 is involved in the formation of actin filaments in vivo, the function of centaurin remains elusive. We have identified a number of novel centaurin-alpha(1) binding partners; including CKIalpha and nucleolin. In this report, we have focused on the interaction of centaurin-alpha(1) with PKC. All groups of PKC associate directly through their cysteine rich domains. Centaurin-alpha(1) is also a substrate for all PKC classes and we have identified the two sites of phosphorylation. This is the first report of a kinase that phosphorylates centaurin-alpha(1).

Protein kinase C mu is down-regulated in androgen-independent prostate cancer

Progression to androgen independence (AI) is the main cause of death in prostate cancer. Our prior differential gene expression studies by microarray analysis in progressive prostate cancer cell line model identified dysregulation of protein kinase C mu (PKCmu) expression in prostate cancer. In this study, quantitative ribonuclease protection assay and immunoblot analysis demonstrate down regulation of PKCmu at transcription and translational level, respectively, in AI C4-2 cells compared to its parental androgen dependent (AD) LNCaP prostate cancer cells. Significantly lower PKCmu kinase activity was confirmed in C4-2 cells by in vitro kinase assay. Immunohistochemical studies of prostate cancer tissue from patient progressing to AI prostate cancer demonstrated that PKCmu expression is decreased in 100% of AI human prostate cancers. The consistent down regulation of PKCmu in cell line models and human prostate cancer tissues suggests a possible functionally significant role for PKCmu in progression to AI in prostate cancer.

Role of endogenous regucalcin in nuclear regulation of regenerating rat liver: suppression of the enhanced ribonucleic acid synthesis activity

The role of endogenous regucalcin in the regulation of ribonucleic acid (RNA) synthesis activity in the nucleus of normal and regenerating rat livers was investigated. Nuclear RNA synthesis was measured by the incorporation of [(3)H]-uridine 5'-triphosphate into the nuclear RNA in vitro. The presence of regucalcin (0.25 or 0.5 microM) in the reaction mixture caused a significant decrease in nuclear RNA synthesis of normal rat liver. alpha-Amanitin (10(-8)-10(-6) M), an inhibitor of RNA polymerase II and III, decreased significantly nuclear RNA synthesis activity. The effect of regucalcin (0.25 microM) in decreasing nuclear RNA synthesis activity was not seen in the presence of alpha-amanitin (10(-6) M). The calcium chloride (10 microM)-increased nuclear RNA synthesis activity was significantly suppressed by the addition of regucalcin (0.25 microM). RNA synthesis activity was significantly enhanced in the nuclei of regenating rat liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly inhibited in the presence of PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M). Western analysis of the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy showed a significant increase in regucalcin protein as compared with that of sham-operated rats. The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) in the reaction mixture caused a significant increase in nuclear RNA synthesis activity of normal rat liver. This increase was completely blocked by the addition of regucalcin (1.0 microM). The effect of anti-regucalcin monoclonal antibody (50 ng/ml) in increasing nuclear RNA synthesis activity was significantly enhanced in the nuclei of regenerating liver obtained at 24, 48, or 72 h after partial hepatectomy. This enhancement was significantly suppressed by the addition of alpha-amanitin (10(-6) M), PD98059 (10(-5) M), staurosporine (10(-6) M), or vanadate (10(-3) M) in the reaction mixture. The present study demonstrates that endogenous regucalcin has a suppressive effect on the enhancement of RNA synthesis activity in the nucleus of regenerating rat liver with proliferative cells.

Distinct protein kinase C isoforms mediate regulation of vascular endothelial growth factor expression by A2A adenosine receptor activation and phorbol esters in pheochromocytoma PC12 cells

Vascular endothelial growth factor (VEGF) stimulates angiogenesis during development and in disease. In pheochromocytoma (PC12) cells, VEGF expression is regulated by A(2A) adenosine receptor (A(2A)AR) activation. The present work examines the underlying signaling pathway. The adenylyl cyclase-protein kinase A cascade has no role in the down-regulation of VEGF mRNA induced by the A(2A)AR agonist, 2-[4-[(2-carboxyethyl)phenyl]ethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680). Conversely, 6-h exposure of cells to either phorbol 12-myristate 13-acetate (PMA) or protein kinase C (PKC) inhibitors mimicked the CGS21680-induced down-regulation. PMA activated PKCalpha, PKCepsilon, and PKCzeta, and CGS21680 activated PKCepsilon and PKCzeta as assessed by cellular translocation. By 6 h, PMA but not CGS21680 decreased PKCalpha and PKCepsilon expression. Neither compound affected PKCzeta levels. Following prolonged PMA treatment to down-regulate susceptible PKC isoforms, CGS21680 but not PMA inhibited the cobalt chloride induction of VEGF mRNA. The proteasome inhibitor, MG-132, abolished PMA- but not CGS21680-induced down-regulation of VEGF mRNA. Phorbol 12,13-diacetate reduced VEGF mRNA levels while down-regulating PKCepsilon but not PKCalpha expression. In cells expressing a dominant negative PKCzeta construct, CGS21680 was unable to reduce VEGF mRNA. Together, the findings suggest that phorbol ester-induced down-regulation of VEGF mRNA occurs as a result of a reduction of PKCepsilon activity, whereas that mediated by the A(2A)AR occurs following deactivation of PKCzeta.

Protein kinase C zeta nuclear translocation mediates the occurrence of radioresistance in friend erythroleukemia cells

Friend erythroleukemia cells require high doses (15 Gy) of ionizing radiation to display a reduced rate of proliferation and an increased number of dead cells. Since ionizing radiation can activate several signaling pathways at the plasma membrane which can lead to the nuclear translocation of a number of proteins, we looked at the intranuclear signaling system activated by Protein Kinases C, being this family of enzymes involved in the regulation of cell growth and death. Our results show an early and dose-dependent increased activity of zeta and epsilon isoforms, although PKC zeta is the only isoform significantly active and translocated into the nuclear compartment upon low (1.5 Gy) and high (15 Gy) radiation doses. These observations are concomitant and consistent with an increase in the anti-apoptotic protein Bcl-2 level upon both radiation doses. Our results point at the involvement of the PKC pathway in the survival response to ionizing radiation of this peculiar cell line, offering PKC zeta for consideration as a possible target of pharmacological treatments aimed at amplifying the effect of such a genotoxic agent.

Molecular characterization of protein kinase C-alpha binding to lamin A

Previous results from our laboratory have identified lamin A as a protein kinase C (PKC)-binding protein. Here, we have identified the regions of PKC-alpha that are crucial for this binding. By means of overlay assays and fusion proteins made of glutathione-S-transferase (GST) fused to elements of rat PKC-alpha, we have established that binding occurs through both the V5 region and a portion of the C2 region (i.e., the calcium-dependent lipid binding (CaLB) domain) of the kinase. In particular, we have found that amino acid 200-217 of the CaLB domain are essential for binding lamin A, as a synthetic peptide corresponding to this stretch of amino acids prevented the interaction between the CaLB domain and lamin A. We also show that the presence of four lysine residues of the CaLB domain (K205, K209, K211, and K213) was essential for the binding. We have determined that binding of elements of PKC-alpha to lamin A does not require the presence of cofactors such as phosphatidylserine (PS) and Ca(2+). We have also found that the binding site of lamin A for the CaLB domain of PKC-alpha is localized in the carboxyl-terminus of the lamin, downstream of amino acid 499. Our findings may prove to be important to clarify the mechanisms regulating PKC function within the nucleus and may also lead to the synthesis of isozyme-specific drugs to attenuate or reverse PKC-dependent nuclear signaling pathways important for the pathogenesis of cancer.

Identification of PSF as a protein kinase Calpha-binding protein in the cell nucleus

Protein kinase C (PKC) isoforms are present in the cell nucleus in diverse cell lines and tissues. Since little is known about proteins interacting with PKC inside the cell nucleus, we used Neuro-2a neuroblastoma cells, in which PKCalpha is present in the nucleus, to screen for nuclear binding partners for PKC. Applying overlay assays, we detected several nuclear proteins which bind to PKCalpha. Specificity of binding was shown by its dependence on PKC activation by phorbol ester, calcium, and phosphatidylserine. The PKC-binding proteins were partially purified and analyzed by microsequencing and mass spectrometry. Four proteins could be identified: PTB-associated splicing factor (PSF), p68 RNA helicase, and the heterogeneous nuclear ribonucleoprotein (hnRNP) proteins A3 and L. In the case of PSF, binding to PKC could also be demonstrated in a GST-pull-down assay using GST-PKCalpha, expressed in insect cells. Phosphorylation experiments revealed that PSF is a weak in vitro substrate for PKCalpha.

Role of specific protein kinase C isozymes in mediating epidermal growth factor, thyrotropin-releasing hormone, and phorbol ester regulation of the rat prolactin promoter in GH4/GH4C1 pituitary cells

Epidermal growth factor (EGF) and TRH both produce enhanced prolactin (PRL) gene transcription and PRL secretion in GH4 rat pituitary tumor cell lines. These agents also activate protein kinase C (PKC) in these cells. Previous studies have implicated the PKCepsilon isozyme in mediating TRH-induced PRL secretion. However, indirect studies using phorbol ester down-regulation to investigate the role of PKC in EGF- and TRH-induced PRL gene transcription have been inconclusive. In the present study, we examined the role of multiple PKC isozymes on EGF- and TRH-induced activation of the PRL promoter by utilizing general and selective PKC inhibitors and by expression of genes for wild-type and kinase-negative forms of the PKC isozymes. Multiple nonselective PKC inhibitors, including staurosporine, bisindolylmaleimide I, and Calphostin C, inhibited both EGF and TRH induced rat PRL promoter activity. TRH effects were more sensitive to Calphostin C, a competitive inhibitor of diacylglycerol, whereas Go 6976, a selective inhibitor of Ca(2+)-dependent PKCs, produced a modest inhibition of EGF but no inhibition of TRH effects. Rottlerin, a specific inhibitor of the novel nPKCdelta isozyme, significantly blocked both EGF and TRH effects. Overexpression of genes encoding PKCs alpha, betaI, betaII, delta, gamma, and lambda failed to enhance either EGF or TRH responses, whereas overexpression of nPKCeta enhanced the EGF response. Neither stable nor transient overexpression of nPKCepsilon produced enhancement of EGF- or TRH-induced PRL promoter activity, suggesting that different processes regulate PRL transcription and hormone secretion. Expression of a kinase inactive nPKCdelta construct produced modest inhibition of EGF-mediated rPRL promoter activity. Taken together, these data provide evidence for a role of multiple PKC isozymes in mediating both EGF and TRH stimulated PRL gene transcription. Both EGF and TRH responses appear to require the novel isozyme, nPKCdelta, whereas nPKCeta may also be able to transmit the EGF response. Inhibitor data suggest that the EGF response may also involve Ca(2+)-dependent isozymes, whereas the TRH response appears to be more dependent on diacylglycerol.

Activation of Integrin-RACK1/PKCalpha signalling in human articular chondrocyte mechanotransduction

OBJECTIVE

The objective of this study was to examine PKC isozyme expression in human articular chondrocytes and assess roles for RACK1, a receptor for activated C kinase in the mechanotransduction process.

METHODS

Primary cultures of human articular chondrocytes and a human chondrocyte cell line were studied for expression of PKC isozymes and RACK1 by western blotting. Following mechanical stimulation of chondrocytes in vitro in the absence or presence of anti-integrin antibodies and RGD containing oligopeptides, subcellular localization of PKCalpha and association of RACK1 with PKCalpha and beta1 integrin was assessed.

RESULTS

Human articular chondrocytes express PKC isozymes alpha, gamma, delta, iota, and lambda. Following mechanical stimulation at 0.33Hz chondrocytes show a rapid, beta1 integrin dependent, translocation of PKCalpha to the cell membrane and increased association of RACK1 with PKCalpha and beta1 integrin.

CONCLUSIONS

RACK1 mediated translocation of activated PKCalpha to the cell membrane and modulation of integrin-associated signaling are likely to be important in regulation of downstream signaling cascades controlling chondrocyte responses to mechanical stimuli.

Binding of elements of protein kinase C-alpha regulatory domain to lamin B1

Previous results from our laboratory have demonstrated that lamin B1 is a protein kinase C (PKC)-binding protein. Here, we have identified the regions of PKC-alpha that are important for this binding. By means of overlay assays and fusion proteins made of glutathione-S-transferase (GST) fused to elements of the regulatory domain of rat PKC-alpha, we have established that binding occurs through both the V1 region and a portion of the C2 region (i.e., the calcium-dependent lipid binding [CaLB] domain) of the kinase. In particular, we have found that amino acids 200-217 of the CaLB domain are essential for binding lamin B1, as a synthetic peptide corresponding to this stretch of amino acids prevented the interaction between the CaLB domain of PKC-alpha and lamin B1. In agreement with the results of other investigators, we have determined that binding of regulatory elements of PKC-alpha to lamin B1 does not require the presence of cofactors such as PS and Ca(2+). We have also found that the binding site of lamin B1 for PKC-alpha is localized in the carboxyl-terminus of the lamin. Our findings may prove to be important in shedding more light on the mechanisms that regulate PKC functions within the nuclear compartment and may also lead to the synthesis of isozyme-specific pharmacological tools to attenuate or reverse PKC-dependent nuclear signalling pathways important for the pathogenesis of cancer.