Lipid mediators and protein kinase C for intracellular signalling

Retracted: PKC-delta and PKD activate MAPK signal pathway in mechano-transcription of colonic smooth muscle cells

Retraction: [PKC-delta and PKD activate MAPK signal pathway in mechano-transcription of colonic smooth muscle cells, Z. Yang, K. He, T. Wang, et al. Neurogastroenterology & Motility 2023; e14623 (https://onlinelibrary.wiley.com/doi/full/10.1111/nmo.14623)]. The above article, published online on June 6, 2023 in Wiley Online Library (wileyonlinelibrary.com), has been retracted by agreement between the authors, the Journal Editor in Chief, Maura Corsetti, and John Wiley & Sons Ltd. The retraction has been agreed due to unat[1]tributed overlap between this article and the abstract published in Gastroenterology: Li F, Sarna SK and Shi XP. Roles of PKCs and PKD in Mechanotranscription in Colonic Smooth Muscle Cells: Inhibition of Mechanotranscription as a Potential Treatment for Motility Dysfunction in Obstructive Disorders. In: 2012 Digestive Disease Week Abstract Supplement; May 19-22, San Diego, CA. Abstract 120 (https://www.gastrojournal.org/article/S0016-5085(12)60115-2/pdf).

Protein Kinase C at the Crossroad of Mutations, Cancer, Targeted Therapy and Immune Response

The frequent PKC dysregulations observed in many tumors have made these enzymes natural targets for anticancer applications. Nevertheless, this considerable interest in the development of PKC modulators has not led to the expected therapeutic benefits, likely due to the complex biological activities regulated by PKC isoenzymes, often playing ambiguous and protective functions, further driven by the occurrence of mutations. The structure, regulation and functions of PKCs have been extensively covered in other publications. Herein, we focused on PKC alterations mostly associated with complete functional loss. We also addressed the modest yet encouraging results obtained targeting PKC in selected malignancies and the more frequent negative clinical outcomes. The reported observations advocate the need for more selective molecules and a better understanding of the involved pathways. Furthermore, we underlined the most relevant immune mechanisms controlled by PKC isoforms potentially impacting the immune checkpoint inhibitor blockade-mediated immune recovery. We believe that a comprehensive examination of the molecular features of the tumor microenvironment might improve clinical outcomes by tailoring PKC modulation. This approach can be further supported by the identification of potential response biomarkers, which may indicate patients who may benefit from the manipulation of distinctive PKC isoforms.

A cumulative Bayesian network meta-analysis on the comparative efficacy of pharmacotherapies for mania over the last 40 years

RATIONALE

Mania (or manic episodes) is a common symptom of bipolar disorder and is frequently accompanied by hyperactivity and delusions; given the cost and resources available, there is a paucity of evidence for direct comparison of different drugs.

OBJECTIVES

We aimed to provide evidence-based recommendations on the efficacy of overall currently used pharmacological treatments for patients with acute bipolar mania.

METHOD

We conducted a systematic review and network meta-analysis (NMA) using a Bayesian network frame. We searched the primary literature databases without language restrictions until Dec 18, 2021, for reports of randomized controlled trials (RCTs) of suspected antimanic drugs used as monotherapy for patients with acute bipolar mania, with the primary outcomes being efficacy (mean difference (MD), standardized mean difference (SMD) in the change of mania score).

RESULTS

Eighty-seven studies were included in which 18,724 manic participants (mean age = 34.6 years, with 50.36% males) were allocated at random to one of 25 active medication drug therapies or placebo, resulting in 87 direct comparisons on 192 data points. Tamoxifen (- 22·00 [- 26·00 to - 18·00]) had the best efficacy over the placebo. Meanwhile, risperidone (- 6·60 [- 8·40 to - 4·90]) was substantially more effective than placebo in treating acute mania. Carbamazepine, haloperidol, ziprasidone, cariprazine, olanzapine, quetiapine, aripiprazole, lithium, paliperidone, asenapine, and divalproex were noticeably more effective than placebo.

CONCLUSIONS

Overall, tamoxifen appears to be the most effective of the currently known pharmaceutical therapy available to treat acute mania or manic episodes; however, this conclusion is restricted by the scale of RCTs conducted, and risperidone was found to be the most effective medication among antipsychotics. Carbamazepine, haloperidol, ziprasidone, cariprazine, olanzapine, quetiapine, aripiprazole, lithium, paliperidone, asenapine, and divalproex were noticeably effective in treating acute mania or manic episodes.

Role and Mechanism of PKC-δ for Cardiovascular Disease: Current Status and Perspective

Protein kinase C (PKC) is a protein kinase with important cellular functions. PKC-δ, a member of the novel PKC subfamily, has been well-documented over the years. Activation of PKC-δ plays an important regulatory role in myocardial ischemia/reperfusion (IRI) injury and myocardial fibrosis, and its activity and expression levels can regulate pathological cardiovascular diseases such as atherosclerosis, hypertension, cardiac hypertrophy, and heart failure. This article aims to review the structure and function of PKC-δ, summarize the current research regarding its activation mechanism and its role in cardiovascular disease, and provide novel insight into further research on the role of PKC-δ in cardiovascular diseases.

Down-regulation of superoxide dismutase 1 by PMA is involved in cell fate determination and mediated via protein kinase D2 in myeloid leukemia cells

Myeloid leukemia cells maintain a high intracellular ROS level and use redox signals for survival. The metabolism of ROS also affects cell fate, including cell death and differentiation. Superoxide dismutases (SODs) are major antioxidant enzymes that have high levels of expression in myeloid leukemia cells. However, the role of SODs in the regulation of myeloid leukemia cells' biological function is still unclear. To investigate the function of SODs in myeloid leukemia cell death and differentiation, we used myeloid leukemia cell lines K562, MEG-01, TF-1, and HEL cells for this study. We found that PMA-induced megakaryocytic differentiation in myeloid leukemia cells is accompanied by cell death and SOD1 down-regulation, while SOD2 expression is not affected. The role of SOD1 is verified when ATN-224, a SOD1 specific inhibitor, inhibits cell proliferation and promotes cell death in myeloid leukemia cells without PMA treatment. Moreover, inhibition or silencing of SODs further increases cell death and decreases polyploidization induced by PMA while they were partially reversed by SOD1 overexpression. Thus, SOD1 expression is required for myeloid leukemia cell fate determination. In addition, the knockdown of PKD2 reduces cell death and promotes polyploidization induced by PMA. PMA/PKD2-mediated necrosis via PARP cleavage involves both SOD1-dependent and -independent pathways. Finally, ATN-224 enhanced the inhibition of cell proliferation by Ara-C. Taken together, the results demonstrate that SOD1 regulates cell death and differentiation in myeloid leukemia cells. ATN-224 may be beneficial for myeloid leukemia therapy.

Screening of cell cycle fusion proteins to identify kinase signaling networks

Kinase signaling networks are well-established mediators of cell cycle transitions. However, how kinases interact with the ubiquitin proteasome system (UPS) to elicit protein turnover is not fully understood. We sought a means of identifying kinase-substrate interactions to better understand signaling pathways controlling protein degradation. Our prior studies used a luciferase fusion protein to uncover kinase networks controlling protein turnover. In this study, we utilized a similar approach to identify pathways controlling the cell cycle protein p27(Kip1). We generated a p27(Kip1)-luciferase fusion and expressed it in cells incubated with compounds from a library of pharmacologically active compounds. We then compared the relative effects of the compounds on p27(Kip1)-luciferase fusion stabilization. This was combined with in silico kinome profiling to identify potential kinases inhibited by each compound. This approach effectively uncovered known kinases regulating p27(Kip1) turnover. Collectively, our studies suggest that this parallel screening approach is robust and can be applied to fully understand kinase-ubiquitin pathway interactions.

Dimer ion formation and intermolecular fragmentation of 1,2-diacylglycerols revealed by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry for more comprehensive lipid analysis

RATIONALE

The ionization of neutral diacylglycerols (DAGs) by electrospray ionization mass spectrometry (ESI-MS) is challenging compared with other lipid classes which possess ionic head group conjugations. Although ESI-MS is the method of choice in lipidomic analysis, it is questionable whether all lipid classes can be efficiently ionized by this method. Actually, various lipids were not efficiently detected (due to poor ionization) in many studies which claimed to comprehensively describe lipid profiles. Since neutral lipids are precursors for the biosynthesis of most other lipid classes, the necessity for improved or alternative ionization and identification schemes becomes obvious.

METHODS

We identified the 1,2-diacylglycerol (DAG) dimer ion formation in the gas phase by ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) in negative electrospray ionization ((-)ESI) mode. The geometry of the dimer ion was investigated by accurate density functional theory (DFT) calculations at the B3LYP/6-311+G(d)//B3LYP/LANL2DZ level of theory. Fragmentation of the dimer ions of many investigated DAGs has been achieved via collision-induced dissociation (CID) experiments with several elevated collision energies (0-12 eV).

RESULTS

We revealed the possibility to ionize neutral DAGs as dimer ions in the negative ESI mode. Quantum mechanical calculations revealed a polar head-to-head intermolecular interaction between one charged DAG and one DAG neutral. This represents an energy minimum structure for the DAG dimer ions. We could furthermore detect CID fragmentation product ions that can only result from intermolecular reactions in this head-to-head conformation (SN2 nucleophilic substitution reactions inside the dimer DAG ion).

CONCLUSIONS

Here, we present for the first time the opportunity to ionize and identify DAGs as dimer ions. This new finding provides a new alternative for investigations of important diacylglycerol lipids and provides the opportunity to obtain complementary and more comprehensive results in future lipidomic studies.

Tamoxifen use for the management of mania: a review of current preclinical evidence

RATIONALE

Preliminary data on the efficacy of tamoxifen in reducing manic symptoms of bipolar disorder (BD) suggest that this agent may be a potential treatment for the management of this psychiatric disorder. However, the antimanic properties of tamoxifen have not been fully elucidated, hampering the development and/or use of mood-stabilising drugs that may share its same therapeutic mechanisms of action. Notably, we may gain a greater understanding of the neurobiological and therapeutic properties of tamoxifen by using suitable animal models of mania.

OBJECTIVES

Here, we review the preclinical studies that have evaluated the effects of tamoxifen to provide an overview of the current progress in our understanding of its antimanic actions, highlighting the critical role of protein kinase C (PKC) as a therapeutic target for the treatment of BD.

CONCLUSIONS

To date, this field has struggled to make significant progress, and the organisation of an explicit battery of tests is a valuable tool for assessing a number of prominent facets of BD, which may provide a greater understanding of the entire scope of this disease.

Impaired T cell protein kinase C delta activation decreases ERK pathway signaling in idiopathic and hydralazine-induced lupus

T cells from patients with lupus or treated with the lupus-inducing drug hydralazine have defective ERK phosphorylation. The reason for the impaired signal transduction is unknown but important to elucidate, because decreased T cell ERK pathway signaling causes a lupus-like disease in animal models by decreasing DNA methyltransferase expression, leading to DNA hypomethylation and overexpression of methylation-sensitive genes with subsequent autoreactivity and autoimmunity. We therefore analyzed the PMA stimulated ERK pathway phosphorylation cascade in CD4(+) T cells from patients with lupus and in hydralazine-treated cells. The defect in these cells localized to protein kinase C (PKC)delta. Pharmacologic inhibition of PKCdelta or transfection with a dominant negative PKCdelta mutant caused demethylation of the TNFSF7 (CD70) promoter and CD70 overexpression similar to lupus and hydralazine-treated T cells. These results suggest that defective T cell PKCdelta activation may contribute to the development of idiopathic and hydralazine-induced lupus through effects on T cell DNA methylation.

PKCtheta is required for the activation of human T lymphocytes induced by CD43 engagement

The turnover of phosphoinositides leading to PKC activation constitutes one of the principal axes of intracellular signaling. In T lymphocytes, the enhanced and prolonged PKC activation resulting from the engagement of the TcR and co-receptor molecules ensures a productive T cell response. The CD43 co-receptor promotes activation and proliferation, by inducing IL-2 secretion and CD69 expression. CD43 engagement has been shown to promote phosphoinositide turnover and DAG production. Moreover, PKC activation was found to be required for the activation of the MAP kinase pathway in response to CD43 ligation. Here we show that CD43 engagement led to the membrane translocation and enzymatic activity of specific PKC isoenzymes: cPKC (alpha/beta), nPKC (epsilon and theta;), aPKC (zeta) and PKCmu. We also show that activation of PKCtheta; resulting from CD43 ligation induced CD69 expression through an ERK-dependent pathway leading to AP-1, NF-kappaB activation and an ERK independent pathway promoting NFAT activation. Together, these data suggest that PKCtheta; plays a critical role in the co-stimulatory functions of CD43 in human T cells.

Redundancy of biological regulation as the basis of emergence of multidrug resistance

Active efflux of xenobiotics is a major mechanism of cell adaptation to environmental stress. The ATP-dependent transmembrane transporter P-glycoprotein (Pgp) confers long-term cell survival in the presence of different toxins, including anticancer drugs (this concept is referred to as multidrug resistance, or MDR). The vital importance of this mechanism for cell survival dictates the reliability and promptness of its acquisition. To fulfill this requirement, the MDR1 gene that encodes Pgp in humans must be readily upregulated in cells that express low to null levels of MDR1 mRNA prior to stress. The MDR1 gene and a stable MDR phenotype can be induced after short-term exposure of cells to a variety of cues. This effect is implemented by activation of MDR1 transcription and mRNA stabilization. The MDR1 message abundance is regulated by mechanisms generally involved in stress response, namely activation of phospholipase C, protein kinase C and mitogen-activated protein kinase cascades, mobilization of intracellular Ca2+, and nuclear factor kappa B activation. Furthermore, the proximal MDR1 promoter sites critical for induction are not unique for the MDR1 gene; they are common regulatory elements in eukaryotic promoters. Moreover, MDR1 induction can result from activation of (an) intermediate gene(s) whose product(s), in turn, directly activate(s) the MDR1 promoter and/or cause(s) mRNA stabilization. Redundancy of signal transduction and transcriptional mechanisms is the basis for the virtually ubiquitous inducibility of the MDR1 gene. Thus, the complex network of MDR1 regulation ensures rapid emergence of pleiotropic resistance in cells.

In vivo expression of classic PKC isoforms in the rat dental follicle as related to tooth eruption

Activation of protein kinase C (PKC) can upregulate tooth eruption molecules such as osteoprotegerin (OPG) and vascular endothelial growth factor (VEGF). In this study, we examined the in vivo gene expression of classic isoforms of PKC in the dental follicle of postnatal rats. The expression level of PKC-alpha was significantly reduced at day 3 followed by a gradual return to day 1 level, a profile similar to OPG expression. The expression of PKC-beta was the lowest at day 1 followed by elevated levels from day 3 to day 11. Expression of PKC-beta is positively correlated with the expression of overall VEGF and VEGF120. The expression level of PKC-gamma was relatively steady in the postnatal days. Injection of a PKC activator, phorbol 12-myristate 13-acetate (PMA), at late postnatal days, slightly accelerated first mandibular molar eruption. This study suggests that PKC isoforms may be involved in the regulation of tooth eruption.

Tumor necrosis factor-alpha induces early-onset endothelial adhesivity by protein kinase Czeta-dependent activation of intercellular adhesion molecule-1

We tested the hypothesis that TNF-alpha induces early-onset endothelial adhesivity toward PMN by activating the constitutive endothelial cell surface ICAM-1, the beta2-integrin (CD11/CD18) counter-receptor. Stimulation of human pulmonary artery endothelial cells with TNF-alpha resulted in phosphorylation of ICAM-1 within 1 minute, a response that was sustained up to 15 minutes after TNF-alpha challenge. We observed that TNF-alpha induced 10-fold increase in PMN adhesion to endothelial cells in an ICAM-1-dependent manner and that this response paralleled the rapid time course of ICAM-1 phosphorylation. We also observed that the early-onset TNF-alpha-induced endothelial adhesivity was protein synthesis-independent and associated with cell surface ICAM-1 clustering. Pretreatment of cells with the pan-PKC inhibitor, chelerythrine, prevented the activation of endothelial adhesivity. As PKCzeta, an atypical PKC isoform abundantly expressed in endothelial cells, is implicated in signaling TNF-alpha-induced ICAM-1 gene transcription, we determined the possibility that PKCzeta was involved in mediating endothelial adhesivity through ICAM-1 expression. We observed that TNF-alpha stimulation of endothelial cells induced PKCzeta activation and its association with ICAM-1. Inhibition of PKCzeta by pharmacological and genetic approaches prevented the TNF-alpha-induced phosphorylation and the clustering of the cell surface ICAM-1 as well as activation of endothelial adhesivity. Thus, TNF-alpha induces early-onset, protein synthesis-independent expression of endothelial adhesivity by PKCzeta-dependent phosphorylation of cell surface ICAM-1 that precedes the de novo ICAM-1 synthesis. The rapid ICAM-1 expression represents a novel mechanism for promoting the stable adhesion of PMN to endothelial cells that is needed to facilitate the early-onset transendothelial migration of PMN.

Cell death induced by acute renal injury: a perspective on the contributions of apoptosis and necrosis

In humans and experimental models of renal ischemia, tubular cells in various nephron segments undergo necrotic and/or apoptotic cell death. Various factors, including nucleotide depletion, electrolyte imbalance, reactive oxygen species, endonucleases, disruption of mitochondrial integrity, and activation of various components of the apoptotic machinery, have been implicated in renal cell vulnerability. Several approaches to limit the injury and augment the regeneration process, including nucleotide repletion, administration of growth factors, reactive oxygen species scavengers, and inhibition of inducers and executioners of cell death, proved to be effective in animal models. Nevertheless, an effective approach to limit or prevent ischemic renal injury in humans remains elusive, primarily because of an incomplete understanding of the mechanisms of cellular injury. Elucidation of cell death pathways in animal models in the setting of renal injury and extrapolation of the findings to humans will aid in the design of potential therapeutic strategies. This review evaluates our understanding of the molecular signaling events in apoptotic and necrotic cell death and the contribution of various molecular components of these pathways to renal injury.

Increased translocation of cardiac protein kinase C beta2 accompanies mild cardiac hypertrophy in rats fed saturated fat

Signal transduction through protein kinase C (PKC) beta2 may modulate cardiac hypertrophy in pressure-overloaded rat myocardium. Because PKC beta2 can be activated by fatty acids and diacylglycerol, we hypothesized that altering the level and type of dietary fat might modulate cardiac PKC activation and stimulate hypertrophy in otherwise normal rat myocardium. Male Sprague-Dawley rats (n = 32) were randomly assigned to either a low fat [10% total energy intake (TEI)] or high fat diet (40% TEI) based on corn or coconut oil as a source of saturated or unsaturated fat. After 40 d of isoenergetic diet consumption, the heart/body weight ratio was slightly greater in rats fed saturated fat diets compared with those fed unsaturated fat (P = 0.05). Increased activation of PKC beta2, as evidenced by greater membrane translocation, was also observed in all rats fed saturated fat diets (P < 0.01). PKC alpha, beta1 and epsilon did not change. These results suggest that dietary fat type can alter PKC beta2 activation in the heart, and exert a mild hypertrophic effect on the heart.

Opposing effects of PKCalpha and PKCepsilon on basolateral membrane dynamics in intestinal epithelia

PKC is a critical effector of plasma membrane dynamics, yet the mechanism and isoform-specific role of PKC are poorly understood. We recently showed that the phorbol ester PMA (100 nM) induces prompt activation of the novel isoform PKCepsilon followed by late activation of the conventional isoform PKCalpha in T84 intestinal epithelia. PMA also elicited biphasic effects on endocytosis, characterized by an initial stimulatory phase followed by an inhibitory phase. Activation of PKCepsilon was shown to be responsible for stimulation of basolateral endocytosis, but the role of PKCalpha was not defined. Here, we used detailed time-course analysis as well as selective activators and inhibitors of PKC isoforms to infer the action of PKCalpha on basolateral endocytosis. Inhibition of PKC by the selective conventional PKC inhibitor Gö-6976 (5 microM) completely blocked the late inhibitory phase and markedly prolonged the stimulatory phase of endocytosis measured by FITC-dextran uptake. The PKCepsilon-selective agonist carbachol (100 microM) induced prolonged stimulation of endocytosis devoid of an inhibitory phase. Actin disassembly caused by PMA was completely blocked by Gö-6850 but not by Gö-6976, implicating PKCepsilon as the key isoform responsible for actin disruption. The Ca2+ agonist thapsigargin (5 microM) induced early activation of PKC when added simultaneously with PMA. This early activation of PKCalpha blocked the ability of PMA to remodel basolateral F-actin and abolished the stimulatory phase of basolateral endocytosis. Activation of PKCalpha stabilizes F-actin and thereby opposes the effect of PKCepsilon on membrane remodeling in T84 cells.

Characterization of a protein kinase C gene in Sporothrix schenckii and its expression during the yeast-to-mycelium transition

The yeast-to-mycelium transition in Sporothrix schenckii has been shown to respond to protein kinase C (PKC) effectors, indicating the involvement of PKC in this regulation. In this study, we identified the presence of two pkcl-like genes in S. schenckii. Using fungal genomic DNA as template and primers targeted to conserved sequences in the Saccharomyces cerevisiae pkc1 gene, two partially overlapping extra long polymerase chain reaction (XL-PCR) products were obtained. These XL-PCR products were sequenced and found to encode part of the C3/C4 domains of two different PKC-like proteins. The presence of two different genes was confirmed by Southern blot analysis. These two genes were named pkcSs-1 and pkcSs-2. The sequence of the pkcSs-2 gene was completed and revealed an open reading frame of 3942 nucleotides interrupted by five introns. A transcript of 8.7 kb was detected in northern blot analysis of poly A+ RNA. The pkcSs-2 gene encodes a protein of 1194 amino acids and 132.84 kDa that contains the characteristic structure and domains of other fungal PKCs reported to date. Using reverse transcription-PCR (RT-PCR), the pkcSs-2 gene was found to be expressed at all intervals tested during the yeast-to-mycelium transition.

Emergence of multidrug resistance in leukemia cells during chemotherapy: mechanisms and prevention

Multifactorial resistance to extracellular stimuli is one of the major factors of tumor progression. Cells can acquire a multidrug resistant (MDR) phenotype in response to a wide variety of stress-inducing agents including chemotherapeutic drugs. In addition to the mechanisms expressed in the tumor prior to chemotherapy (presumably these mechanisms allowed tumor cells to escape the control of growth and differentiation), a complex phenotype of pleiotropic resistance is presented in the residual or recurrent tumor. This review analyzes the molecular mechanisms of MDR acquisition with the focus on hematopoietic malignancies. In particular, the chemotherapy-induced up-regulation of P-glycoprotein, a broad-specificity transmembrane efflux pump, is considered a major event in establishment of MDR in leukemia cells that were sensitive before drug exposure. The pharmacological and genetic approaches to prevent the acquisition of Pgp-mediated MDR during chemotherapy are discussed.

T cell expressed PKCtheta demonstrates cell-type selective function

T lymphocyte stimulation leading to interleukin-2 (IL-2) expression requires activation of protein kinase C (PKC); however, the relevant PKC isoform(s) have not yet been systematically defined. Here we examine seven major T cell expressed PKC isoforms (PKCalpha, delta, epsilon, zeta, nu, theta and iota) and identify PKCtheta to be essential for IL-2 expression (via the critical NF-AT and NF-kappaB enhancer) in Jurkat T cells. Employing a conditionally activated PKCtheta estrogen-receptor fusion mutant, a de novo synthesis-independent transactivation of JNK2 was established. Based on mRNA in situ hybridization to mouse whole body sections, PKCtheta was found to be highly expressed in lymphoid organs but also skeletal muscle and the nervous system. PKCtheta function appears to be cell-type specific, since its isoenzyme-selective function was not observed in ectopic expression studies, employing COS-1 or NIH3T3 cells. These results confirm PKCtheta to be the prime target for the activating effect of phorbol ester in T cell signaling and suggest that gene expression as well as gene function of PKCtheta is strictly controlled by the cell type.

Up-regulation of the Pit-2 phosphate transporter/retrovirus receptor by protein kinase C epsilon

The membrane receptors for the gibbon ape leukemia retrovirus and the amphotropic murine retrovirus serve normal cellular functions as sodium-dependent phosphate transporters (Pit-1 and Pit-2, respectively). Our earlier studies established that activation of protein kinase C (PKC) by treatment of cells with phorbol 12-myristate 13-acetate (PMA) enhanced sodium-dependent phosphate (Na/Pi) uptake. Studies now have been carried out to determine which type of Na/Pi transporter (Pit-1 or Pit-2) is regulated by PKC and which PKC isotypes are involved in the up-regulation of Na/Pi uptake by the Na/Pi transporter/viral receptor. It was found that the activation of short term (2-min) Na/Pi uptake by PMA is abolished when cells are infected with amphotropic murine retrovirus (binds Pit-2 receptor) but not with gibbon ape leukemia retrovirus (binds Pit-1 receptor), indicating that Pit-2 is the form of Na/Pi transporter/viral receptor regulated by PKC. The PKC-mediated activation of Pit-2 was blocked by pretreating cells with the pan-PKC inhibitor bisindolylmaleimide but not with the conventional PKC isotype inhibitor Gö 6976, suggesting that a novel PKC isotype is required to regulate Pit-2. Overexpression of PKCepsilon, but not of PKCalpha, -delta, or -zeta, was found to mimic the activation of Na/Pi uptake. To further establish that PKCepsilon is involved in the regulation of Pit-2, cells were treated with PKCepsilon-selective antisense oligonucleotides. Treatment with PKCepsilon antisense oligonucleotides decreased the PMA-induced activation of Na/Pi uptake. These results indicate that PMA-induced stimulation of Na/Pi uptake by Pit-2 is specifically mediated through activation of PKCepsilon.

Protein kinase Ctheta, a selective upstream regulator of JNK/SAPK and IL-2 promoter activation in Jurkat T cells

The predominant expression of protein kinase C (PKC) theta in T cells (J. Biol. Chem. 1993. 268: 4997-5004), its isoenzyme-specific ability to stimulate AP-1 transcriptional activity (Mol. Cell. Biol. 1996. 16: 1842-1850) and the recent discovery of its selective and antigen-dependent colocalization with the contact region between T cells and antigen-presenting cells (Nature 1997. 385: 83-89) suggest that, among the PKC family members, PKCtheta plays a specialized role in T cell activation. By investigating the downstream effectors of PKCtheta we now demonstrate a direct and isoenzyme-specific contribution of PKCtheta to c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) but not extracellular regulated kinase (ERK) activation. Expression of a constitutively active (CA) form of PKCtheta (but not CA-PKCalpha, epsilon and lambda/iota) resulted in strong activation of JNK/SAPK and expression of a dominant-negative form of PKCtheta interfered with the endogenous activation signal for JNK/SAPK. Importantly, Ca2+ ionophore and CA-PKCtheta (but not CA-PKCalpha, epsilon and lambda/iota) caused synergistic activation of the IL-2 promoter. Together, these data establish that PKCtheta is required for activation of JNK/SAPK signaling leading to IL-2 promoter transcription in T lymphocytes.