Mouse protein kinase C-delta, the major isoform expressed in mouse hemopoietic cells: sequence of the cDNA, expression patterns, and characterization of the protein

Fluorescence-quenching screening of protein kinase C ligands with an environmentally sensitive fluorophore

A novel fluorescence-quenching screening method for protein kinase C (PKC) ligands was developed utilizing solvatochromic fluorophores. Solvatochromic dyes, highly sensitive to the presence or the absence of competitive ligands in their binding to the C1b domain of PKCδ (δC1b), were combined with a known pharmacophoric moiety of 1,2-diacylglycerol (DAG) lactones, PKC ligands. Addition of δC1b to the fluorescent compounds caused a gradual increase in the fluorescent intensity in proportion to the increase of δC1b. As a competitive ligand was added to the complex of δC1b domain and fluorescent compounds, a gradual decrease in the fluorescent intensity was observed. The relative binding affinities of known ligands were successfully determined by this fluorescent method and corresponded well to the K(i) values measured by a radioisotope method. These results indicate that washing, which is a laborious step in binding evaluations, is not required for this environmentally sensitive fluorophore based system. Screening with the system was performed for 2560 preselected library compounds with possible pharmacophores, and some lead compounds were found. This fluorescence-based method could be applied widely to known ligand-receptor combinations.

Phosphorylation of paxillin at threonine 538 by PKCdelta regulates LFA1-mediated adhesion of lymphoid cells

We investigated the PKCdelta-mediated phosphorylation of paxillin within its LIM4 domain and the involvement of this phosphorylation in activation of LFA-1 integrins of the Baf3 pro-B lymphocytic cell line. Using phosphorylated-threonine-specific antibodies, phosphorylated amino acid analysis and paxillin phosphorylation mutants, we demonstrated that TPA, the pharmacological analog of the endogenous second messenger diacyl glycerol, stimulates paxillin phosphorylation at threonine 538 (T538). The TPA-responsive PKC isoform PKCdelta directly binds paxillin in a yeast two-hybrid assay and phosphorylates paxillin at T538 in vitro and also co-immunoprecipitates with paxillin and mediates phosphorylation of this residue in vivo. Recombinant wild-type paxillin, its phospho-inhibitory T538A or phospho-mimetic T538E mutants were expressed in the cells simultaneously with siRNA silencing of the endogenous paxillin. These experiments suggest that phosphorylation of paxillin T538 contributes to dissolution of the actin cytoskeleton, redistribution of LFA-1 integrins and an increase in their affinity. We also show that phosphorylation of T538 is involved in the activation of LFA-1 integrins by TPA.

Nuclear trafficking of pro-apoptotic kinases in response to DNA damage

The cellular response to genotoxic stress includes cell-cycle arrest, activation of DNA repair and induction of apoptosis. However, the signals that determine cell fate are largely unknown. Recent studies have shown that several pro-apoptotic kinases, including protein kinase C (PKC)delta, Abelson murine leukemia viral oncogene homolog 1 (c-Abl) and dual-specificity tyrosine-phosphorylation-regulated kinase 2 (DYRK2), undergo nuclear-cytoplasmic shuttling in response to DNA damage. Importantly, whereas precise regulation for the shuttling of these kinases remains uncertain, this mechanism has consequences for induction of apoptosis and implies that proper localization is central to the function of pro-apoptotic kinases. This review highlights recent progress demonstrating that the nuclear targeting of kinases is a novel and essential regulatory mechanism that directly influences the induction of apoptosis in response to DNA damage. The potential implications for novel therapies are also discussed.

PKCdelta signaling: mechanisms of DNA damage response and apoptosis

The cellular response to genotoxic stress that damages DNA includes cell cycle arrest, activation of DNA repair, and in the event of irreparable damage, induction of apoptosis. However, the signals that determine cell fate, that is, survival or apoptosis, are largely unknown. The delta isoform of protein kinase C (PKCdelta) has been implicated in many important cellular processes, including regulation of apoptotic cell death. The available information supports a model in which certain sensors of DNA lesions activate PKCdelta. This activation is triggered in part by tyrosine phosphorylation of PKCdelta by c-Abl tyrosine kinase. PKCdelta is further proteolytically activated by caspase-3. The cleaved catalytic fragment of PKCdelta translocates to the nucleus and induces apoptosis. Importantly, accumulating data have revealed the nuclear targets for PKCdelta in the induction of apoptosis. A pro-apoptotic function of activated PKCdelta is mediated by at least several downstream effectors known to be associated with the elicitation of apoptosis. Recent findings also demonstrated that PKCdelta is involved in cell cycle-specific activation and induction of apoptotic cell death. Moreover, previous studies have shown that PKCdelta regulates transcription by phosphorylating various transcription factors, including the p53 tumor suppressor that is critical for cell cycle arrest and apoptosis in response to DNA damage. These findings collectively support a pivotal role for PKCdelta in the induction of apoptosis with significant impact. This review is focused on the current views regarding the regulation of cell fate by PKCdelta signaling in response to DNA damage.

Potential role of apoptosis in development of the cystinotic phenotype

Much still remains unclear about the proximal biochemical effects of mutations on development of the phenotype in inborn errors of metabolism. Cystinosis is an example of this phenomenon. We have recently shown that cystinotic cells undergo apoptosis at a two- to fourfold higher rate than controls. Cystinotic cells pre-treated with cysteamine, normalizing cystine content, display a four- to fivefold decrease in apoptosis, while normal cells pre-treated with cystine dimethylester, increasing lysosomal cystine, exhibit a fivefold increase in apoptosis. We speculate that cystine exits the lysosomal compartment during early apoptosis and affects apoptotic proteins in the cytosol, causing an inappropriate commitment to proceed to cell death. The resulting chronic hypocellularity could account for all the characteristics of the nephropathic cystinotic phenotype. The milder variants of cystinosis may result from modifying mutations within an apoptotic protein, ablating the proapoptotic effects of cystine. Failure of the mouse knockout for cystinosis to show renal involvement may be the result of differences in apoptotic processes between man and mouse. Apoptosis is a major final common pathway for many disease states. Therefore, a better understanding of the effect of lysosomal cystine on apoptosis may help to clarify development of other diseases.

Transcriptional and post-transcriptional regulation of the PKC delta gene by etoposide in L1210 murine leukemia cells: implication of PKC delta autoregulation

Protein kinase C delta (PKC delta) plays an important role in the regulation of apoptosis in response to diverse anticancer agents. PKC delta is cleaved irreversibly to a catalytically active fragment in response to apoptotic stimuli; however, little information is available about the regulation of PKC delta gene expression. In this study, we found that the amount of steady-state PKC delta mRNA and protein was increased by etoposide in mouse L1210 leukemia cells. The transcriptional rate of the PKC delta gene and the stability of PKC delta mRNA were increased by treatment with etoposide, resulting in the accumulation of PKC delta protein. Rottlerin inhibited etoposide-induced PKC delta gene expression significantly, while Go6976, LY294002 and PD98059 had no effect. Further, both stable and adenovirus-mediated expression of a dominant negative PKC delta(KR) abrogated etoposide-induced PKC delta expression. Etoposide-stimulated PKC delta transcription but not PKC delta mRNA stability was blocked completely by pretreatment with rottlerin. Our data reveal a novel mechanism whereby PKC delta gene is regulated at the transcriptional and post-transcriptional level in the L1210 leukemia 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.

Leukotactin-1-induced ERK activation is mediated via Gi/Go protein/PLC/PKC delta/Ras cascades in HOS cells

Recently cloned leukotactin-1 (Lkn-1) that belongs to CC chemokine family has not been characterized. To understand the intracellular events following Lkn-1 binding to CCR1, we investigated the activities of signaling molecules in response to Lkn-1 in human osteogenic sarcoma cells expressing CCR1. Lkn-1-stimulated cells showed elevated phosphorylation of extracellular signal-related kinases (ERK1/2) with a distinct time course. ERK activation was peaked in 30 min and 12 h showing biphasic activation of ERK. Pertussis toxin, an inhibitor of G(i)/G(o) protein, and phospholipase C (PLC) inhibitor blocked Lkn-1-induced activation of ERK. Protein kinase C delta (PKC delta) specific inhibitor rottlerin inhibited ERK activation in Lkn-1-stimulated cells. The activities of PLC and PKC delta were also enhanced by Lkn-1 stimulation. Dominant negative Ras inhibited activation of ERK. Immediate early response genes such as c-fos and c-myc were induced by Lkn-1 stimulation. Lkn-1 affected the cell cycle progression by cyclin D(3) induction. These results suggest that Lkn-1 activates the ERK pathway by transducing the signal through G(i)/G(o) protein, PLC, PKC delta and Ras, and it may play a role for cell proliferation, differentiation, and regulation of gene expression for other cellular processes.

The 5' untranslated region of protein kinase Cdelta directs translation by an internal ribosome entry segment that is most active in densely growing cells and during apoptosis

Protein kinase Cdelta (PKCdelta) is a member of the PKC family of phospholipid-dependent serine/threonine kinases and is involved in cell proliferation, apoptosis, and differentiation. Previous studies have suggested that different PKC isoforms might be translationally regulated. We report here that the 395-nt-long 5' untranslated region (5' UTR) of PKCdelta is predicted to form very stable secondary structures with free energies (deltaG values) of around -170 kcal/mol. The 5' UTR of PKCdelta can significantly repress luciferase translation in rabbit reticulocyte lysate but does not repress luciferase translation in a number of transiently transfected cell lines. By using a bicistronic luciferase reporter, we show that the 5' UTR of PKCdelta contains a functional internal ribosome entry segment (IRES). The activity of the PKCdelta IRES is greatest in densely growing cells and during apoptosis, when total protein synthesis and levels of full-length eukaryotic initiation factor 4G are reduced. However, the IRES activity of the 5' UTR of PKCdelta is not enhanced during serum starvation, another condition shown to inhibit cap-dependent translation, suggesting that its potency is dependent on specific cellular conditions. Accumulating data suggest that PKCdelta has a function as proliferating cells reach high density and in early and later events of apoptosis. Our studies suggest a mechanism whereby PKCdelta synthesis can be maintained under these conditions when cap-dependent translation is inhibited.

The 5' UTR of protein kinase C epsilon confers translational regulation in vitro and in vivo

We have examined translational regulation conferred by the 5' untranslated region (UTR) of PKCepsilon on expression of the luciferase reporter gene in vitro, using rabbit reticulocyte lysates and in vivo, in contact-inhibiting mouse Swiss 3T3 fibroblasts and non-contact-inhibiting Swiss 3T6 fibroblasts. In rabbit reticulocyte lysates, the 5' UTR of PKCepsilon significantly represses translation. In 3T3 and 3T6 cells, the 5' UTR of PKCepsilon reduces luciferase activity, but not to the same extent as it does in vitro. In rabbit reticulocyte lysate, the degree of repression mediated by different PKCepsilon 5' UTR-deletion constructs correlates with the free energy (DeltaG) of their predicted secondary structures. However, in cells, secondary structure is not the only determinant of repression; an internal region of the 5' UTR is both necessary and sufficient for repression. Mutation of an upstream AUG (uAUG) motif in this region partially relieves repression. We conclude that the 5' UTR of PKCepsilon can mediate translational regulation and that translation inhibition in vivo involves the uAUG motif. Our findings also suggest that there are factors present in fibroblasts, but not in rabbit reticulocyte lysates that substantially overcome the repressive qualities of the long, structured 5' UTR. Thus, we have identified a potential new level of regulation of PKC in mammalian cells.

Expression and localization of protein kinase C theta isoform in mouse testis

Protein kinase C (PKC) is encoded by a complex of a gene family, and its multiple isoforms are expressed in various mammalian tissues. The objective of this study was to investigate the expression and localization of a PKC theta isoform in mouse testis. PKC theta displays the highest homology to PKC delta, lacks the Ca2+-binding C2 domain and, thus, belongs to the subfamily of Ca2+-independent PKC enzymes which also includes the delta, epsilon, zeta and eta isoforms. We analyzed the PKC theta mRNA and protein by Northern blotting, in situ hybridization, and immunohistochemistry. In testes of normal mice, signals of PKC theta isoform expression were detected specifically in the interstitial cells of testes. The expression of PKC theta isoform was also detected in testes of germ cell-deficient W/W(v) mice. These results suggest that PKC theta isoform has the specific biological functions in the interstitial cells of testis.

Crystal structure of a phosphatidylinositol 3-phosphate-specific membrane-targeting motif, the FYVE domain of Vps27p

Phosphatidylinositol 3-phosphate regulates membrane trafficking and signaling pathways by interacting with the FYVE domains of target proteins. The 1.15 A structure of the Vps27p FYVE domain reveals two antiparallel beta sheets and an alpha helix stabilized by two Zn2+-binding clusters. The core secondary structures are similar to a rabphilin-3A Zn2+-binding domain and to the C1 and LIM domains. Phosphatidylinositol 3-phosphate binds to a pocket formed by the (R/K)(R/K)HHCR motif. A lattice contact shows how anionic ligands can interact with the phosphatidylinositol 3-phosphate-binding site. The tip of the FYVE domain has basic and hydrophobic surfaces positioned so that nonspecific interactions with the phospholipid bilayer can abet specific binding to phosphatidylinositol 3-phosphate.

Protein kinase C delta

The protein kinase C (PKC) family consists of 11 isoenzymes that, due to structural and enzymatic differences, can be subdivided into three groups: The Ca(2+)-dependent, diacylglycerol (DAG)-activated cPKCs (conventional PKCs: alpha, beta 1, beta 2, gamma); the Ca(2+)-independent, DAG-activated nPKCs (novel PKCs: delta, epsilon, eta, theta, mu), and the Ca(2+)-dependent, DAG non-responsive aPKCs (atypical PKCs: zeta, lambda/iota). PKC mu is a novel PKC, but with some special structural and enzymatic properties.

Novel roles of specific isoforms of protein kinase C in activation of the c-fos serum response element

Protein kinase C (PKC) is a multigene family of enzymes consisting of at least 11 isoforms. It has been implicated in the induction of c-fos and other immediate response genes by various mitogens. The serum response element (SRE) in the c-fos promoter is necessary and sufficient for induction of transcription of c-fos by serum, growth factors, and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). It forms a complex with the ternary complex factor (TCF) and with a dimer of the serum response factor (SRF). TCF is the target of several signal transduction pathways and SRF is the target of the rhoA pathway. In this study we generated dominant-negative and constitutively active mutants of PKC-alpha, PKC-delta, PKC-epsilon, and PKC-zeta to determine the roles of individual isoforms of PKC in activation of the SRE. Transient-transfection assays with NIH 3T3 cells, using an SRE-driven luciferase reporter plasmid, indicated that PKC-alpha and PKC-epsilon, but not PKC-delta or PKC-zeta, mediate SRE activation. TPA-induced activation of the SRE was partially inhibited by dominant negative c-Raf, ERK1, or ERK2, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of Elk-1. TPA-induced activation of the SRE was also partially inhibited by a dominant-negative MEKK1. Furthermore, TPA treatment of serum-starved NIH 3T3 cells led to phosphorylation of SEK1, and constitutively active mutants of PKC-alpha and PKC-epsilon activated the transactivation domain of c-Jun, a major substrate of JNK. Constitutively active mutants of PKC-alpha and PKC-epsilon could also induce a mutant c-fos promoter which lacks the TCF binding site, and they also induce transactivation activity of the SRF. Furthermore, rhoA-mediated SRE activation was blocked by dominant negative mutants of PKC-alpha or PKC-epsilon. Taken together, these findings indicate that PKC-alpha and PKC-epsilon can enhance the activities of at least three signaling pathways that converge on the SRE: c-Raf-MEK1-ERK-TCF, MEKK1-SEK1-JNK-TCF, and rhoA-SRF. Thus, specific isoforms of PKC may play a role in integrating networks of signal transduction pathways that control gene expression.

The Role of Protein Kinase C Signaling in Activated DRA Transcription

Expression of human MHC HLA-DRA class II gene can be up-regulated in B cells by Ig cross-linking as well as by phorbol esters such as 12-O-tetradecanoyl phorbol 13-acetate (TPA). Induced DRA expression involves activation of restricted protein kinase C (PKC) isoforms, resulting in activated activator protein-1-dependent transcription. In this report expression profiles and activation of PKC were analyzed in human Raji B lymphoblastoid cells. Transient transfection analysis with target plasmids containing either DRA promoter (wild-type or mutated) or TPA response elements demonstrated that pretreatment with the selective PKC inhibitor GF 109203X repressed TPA-mediated activation. Western analysis performed on cellular fractions of resting cells and of TPA-activated cells revealed abundant expression of classical PKC-α (cPKC-α), cPKC-βII, and atypical PKC-ζ isoforms and identified a sustained translocation of cPKC-α and cPKC-βII from the cytosolic compartment to membranes. As expected, the distribution of atypical PKC-ζ was unaffected by TPA treatment and displayed an even distribution between cytosol and membranes. This finding was confirmed by immunofluorescence microscopy. The TPA-mediated translocation of cPKC-α and cPKC-βII was not influenced by pretreatment with GF 109203X. Finally, functional activation and translocation of PKC were investigated with a selective in vitro kinase assay. Together, these results show that activated HLA-DRA expression in response to TPA treatment is strictly dependent on PKC activation acting on the X2 box of the DRA promoter and that selective inhibition of PKC enzymatic activity does not influence subcellular localization of expressed PKC isoenzymes. Thus, the translocation event per se occurs independently of PKC activation in these cells.

Three distinct mechanisms for translocation and activation of the delta subspecies of protein kinase C

We expressed delta subspecies of protein kinase C (delta-PKC) fused with green fluorescent protein (GFP) in CHO-K1 cells and observed the movement of this fusion protein in living cells after three different stimulations. The delta-PKC-GFP fusion protein had enzymological characteristics very similar to those of the native delta-PKC and was present throughout the cytoplasm in CHO-K1 cells. ATP at 1 mM caused a transient translocation of delta-PKC-GFP to the plasma membrane approximately 30 s after the stimulation and a sequent retranslocation to the cytoplasm within 3 min. A tumor-promoting phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA; 1 microM), induced a slower translocation of delta-PKC-GFP, and the translocation was unidirectional. Concomitantly, the kinase activity of delta-PKC-GFP was increased by these two stimulations, when the kinase activity of the immunoprecipitated delta-PKC-GFP was measured in vitro in the absence of PKC activators such as phosphatidylserine and diacylglycerol. Hydrogen peroxide (H2O2; 5 mM) failed to translocate delta-PKC-GFP but increased its kinase activity more than threefold. delta-PKC-GFP was strongly tyrosine phosphorylated when treated with H2O2 but was tyrosine phosphorylated not at all by ATP stimulation and only slightly by TPA treatment. Both TPA and ATP induced the translocation of delta-PKC-GFP even after treatment with H2O2. Simultaneous treatment with TPA and H2O2 further activated delta-PKC-GFP up to more than fivefold. TPA treatment of cells overexpressing delta-PKC-GFP led to an increase in the number of cells in G2/M phase and of dikaryons, while stimulation with H2O2 increased the number of cells in S phase and induced no significant change in cell morphology. These results indicate that at least three different mechanisms are involved in the translocation and activation of delta-PKC.

New perspectives on PKCtheta, a member of the novel subfamily of protein kinase C

Members of the protein kinase C (PKC) family of serine/threonine protein kinases have been implicated in numerous cellular responses in a large variety of cell types. Expression patterns of individual members and differences in their cofactor requirements and potential substrate specificity suggest that each isoenzyme may be involved in specific regulatory processes. The PKCtheta isoenzyme exhibits a relatively restricted expression pattern with high protein levels found predominantly in hematopoietic cells and skeletal muscle. PKCtheta was found to be expressed in T, but not B lymphocytes, and to colocalize with the T-cell antigen receptor (TCR) at the site of contact between the antigen-responding T cell and the antigen-presenting cell (APC). Colocalization of PKCtheta with the TCR was selective for this isoenzyme and occurred only upon antigen-mediated responses leading to T-cell activation and proliferation. PKCtheta was found to be involved in the regulation of transcriptional activation of early-activation genes, predominantly AP-1, and its cellular distribution and activation were found to be regulated by the 14-3-3 protein. Other findings indicated that PKCtheta can associate with the HIV negative factor (Nef) protein, suggesting that altered regulation of PKCtheta by Nef may contribute to the T-cell impairments that are characteristic of infection by HIV. PKCtheta is expressed at relatively high levels in skeletal muscle, where it is suggested to play a role in signal transduction in both the developing and mature neuromuscular junction. In addition, PKCtheta appears to be involved in the insulin-mediated response of intact skeletal muscle, as well as in experimentally induced insulin resistance of skeletal muscle. Further studies suggest that PKCtheta is expressed in endothelial cells and is involved in multiple processes essential for angiogenesis and wound healing, including the regulation of cell cycle progression, formation and maintenance of actin cytoskeleton, and formation of capillary tubes. Here, we review recent progress in the study of PKCtheta and discuss its potential role in various cellular responses.

Protein kinase C beta modulates thrombin-induced Ca2+ signaling and endothelial permeability increase

We investigated the function of the Ca2+-dependent protein kinase C (PKC) beta1 in the regulation of endothelial barrier property. Human dermal microvascular endothelial cells (HMEC-1) were transduced with full-length PKCbeta1 antisense (AS) cDNA or control pLNCX vector to generate stable cell lines (HMEC-AS and HMEC-pLNCX, respectively). Analyses indicated that HMEC-AS expressed the antisense PKCbeta1 transcript with decreased PKCbeta protein level (without a change in PKCalpha or PKCepsilon). The baseline transendothelial 125I-albumin clearance rates of HMEC-1, HMEC-pLNCX, and HMEC-AS were 5.0+/-0.5 x 10(-2), 6.8+/-0.4 x 10(-2), and 6.9+/-0.6 x 10(-2) microl/min, respectively. Activation of HMEC-1 and HMEC-pLNCX with phorbol 12-myristate 13-acetate (PMA) increased the rates to the respective 14.5+/-1.7 x 10(-2) microl/min and 16.9+/-2.8 x 10(-2) microl/min (corresponding to 191% and 149% increases over baseline). However, in HMEC-AS, PMA increased the rate to 9.8+/-1.0 x 10(-2) microl/min (42%). When HMEC-1 and HMEC-pLNCX were activated with thrombin, the rates increased to 10.8+/-1.4 x 10(-2) and 14.0+/-1.9 x 10(-2) microl/min, respectively (116% and 106%). In contrast, thrombin stimulation of HMEC-AS more than doubled the increase to 27.2+/-3.5 x 10(-2) microl/min (294%). Furthermore, the thrombin-induced peak increase in the [Ca2+]i in HMEC-AS was greater than in control cells. Fluorescence-activated cell sorter analysis of thrombin receptor expression indicated that the augmented thrombin-induced responses were not attributable to altered receptor density in HMEC-AS. These results indicate that PKCbeta functions in a negative feedback manner to inactivate thrombin-generated signals and thereby modulates the endothelial permeability increase. Because decreased PKCbeta expression significantly reduced the PMA-induced permeability increase, PKCbeta may downregulate thrombin receptor function upstream of PKC activation (i.e., Ca2+).

1,25(OH)2-vitamin D3 affects the subcellular distribution of protein kinase C isoenzymes in muscle cells

Previous studies have shown the involvement of protein kinase C (PKC) in 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] regulation of DNA synthesis (long-term effect) and Ca2+ channel activity (short-term effect) in cultured myoblasts. Both events mediate stimulation of myoblast cell proliferation and growth by 1,25(OH)2D3. To characterise further the role of PKC in the hormone mode of action in muscle cells, the presence of PKC isoenzymes in chicken embryo myoblasts and changes in their total cell and subcellular levels after treatment (72 h and 5 min) with 1,25(OH)2D3 (1 nM), 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) and 1,2-dioctanoyl-rac-glycerol (DOG; 50 microM) were investigated. Western blot analysis provided evidence on the expression of PKC alpha, beta and delta isoforms in avian myoblasts. Two immunoreactive bands of 80 kDa (intact molecule) and 50 kDa (catalytic fragment) were detected for each isoenzyme. 1,25(OH)2D3 and DOG, which increased myoblast PKC activity parallel with the stimulation of DNA synthesis and culture growth and the phorbol ester TPA which induced the opposite changes, exerted differential effects on PKC isoenzymes. Long-term (72 h) treatment with 1,25(OH)2D3 and DOG did not change total PKC isoform levels but decreased the 80 kDa species and increased the release of the catalytic fragment of PKC delta and beta, whereas TPA augmented the total amounts of the three PKC isoforms, increasing the band of 80 kDa of PKC beta and delta and the 50 kDa species for PKC alpha. Subcellular distribution studies showed that the 80 kDa molecule is only present in the cytosolic fraction whereas in the particulate fractions the 50 kDa fragments are detected. Increased amounts of the catalytic fragments of PKC beta and delta both in the nucleus and membranes were observed after 72 h treatment with DOG while 1,25(OH)2D3 increases PKC beta in the nucleus and PKC delta in membranes. TPA induced the appearance of the 50 kDa species of PKC alpha in the nuclear and membrane fractions. The phorbol ester also decreased the catalytic fragments of PKC beta and delta in membranes. Increased levels of PKC beta, and to a lesser extent of PKC delta, in membranes and cytosol could be detected after short exposure (5 min) of myoblasts to 1,25(OH)2D3, DOG and TPA. In conclusion, the data indicate the operation in myoblasts of PKC signal transduction pathways mediated by the Ca(2+)-dependent PKCs alpha and beta and the Ca(2+)-independent PKC delta. Moreover, the results suggest that the beta and delta isoforms of PKC could play a role in the regulation of muscle cell metabolism by 1,25(OH)2D3.

Overexpression of Protein Kinase C Isoform  but not δ in Human Interleukin-3–Dependent Cells Suppresses Apoptosis and Induces bcl-2 Expression

Hematopoietic progenitor cells die by apoptosis after removal of the appropriate colony-stimulating factor (CSF). Recent pharmacologic data have implicated protein kinase C (PKC) in the suppression of apoptosis in interleukin-3 (IL-3) and granulocyte-macrophage (GM)-CSF–dependent human myeloid cells. Because IL-3 and GM-CSF induce increases in diacylglycerol without mobilizing intracellular Ca++, it seemed that one of the novel Ca++ independent isoforms of PKC was involved. We report here that overexpression of PKC in factor-dependent human TF-1 cells extends cell survival in the absence of cytokine. Overexpression of PKCδ does not have this effect. By 72 to 96 hours after cytokine withdrawal, the PKC transfectants remain distributed in all phases of the cell cycle, as shown by fluorescence-activated cell sorting (FACS) analysis, while little intact cellular DNA is detectable in vector or PKCδ transfectants. PKC induces bcl-2 protein expression fivefold to sixfold over the levels in empty vector transfectants, whereas the levels in PKCδ transfectants are similar to those in vector controls.

Overexpression of Protein Kinase C Isoform  but not δ in Human Interleukin-3–Dependent Cells Suppresses Apoptosis and Induces bcl-2 Expression

Hematopoietic progenitor cells die by apoptosis after removal of the appropriate colony-stimulating factor (CSF). Recent pharmacologic data have implicated protein kinase C (PKC) in the suppression of apoptosis in interleukin-3 (IL-3) and granulocyte-macrophage (GM)-CSF–dependent human myeloid cells. Because IL-3 and GM-CSF induce increases in diacylglycerol without mobilizing intracellular Ca++, it seemed that one of the novel Ca++ independent isoforms of PKC was involved. We report here that overexpression of PKC in factor-dependent human TF-1 cells extends cell survival in the absence of cytokine. Overexpression of PKCδ does not have this effect. By 72 to 96 hours after cytokine withdrawal, the PKC transfectants remain distributed in all phases of the cell cycle, as shown by fluorescence-activated cell sorting (FACS) analysis, while little intact cellular DNA is detectable in vector or PKCδ transfectants. PKC induces bcl-2 protein expression fivefold to sixfold over the levels in empty vector transfectants, whereas the levels in PKCδ transfectants are similar to those in vector controls.

Both the catalytic and regulatory domains of protein kinase C chimeras modulate the proliferative properties of NIH 3T3 cells

Protein kinase C (PKC) isozymes exhibit important differences in terms of their regulation and biological functions. Not only may some PKC isoforms be active and others not for a given response, but the actions of different isoforms may even be antagonistic. In NIH 3T3 cells, for example, PKCdelta arrests cell growth whereas PKCepsilon stimulates it. To probe the contribution of the regulatory and the catalytic domains of PKC isozymes to isozyme-specific responses, we prepared chimeras between the regulatory and the catalytic domains of PKCalpha, -delta, and -epsilon. These chimeras, which preserve the overall structure of the native PKC enzymes, were stably expressed in mouse fibroblasts. A major objective was to characterize the growth properties of the cells that overexpress the various PKC constructs. Our data demonstrate that both the regulatory and the catalytic domains play roles in cell proliferation. The regulatory domain of PKCepsilon enhanced cell growth in the absence or presence of phorbol 12-myristate 13-acetate (PMA), and, in the presence of PMA, all chimeras with the PKCepsilon regulatory domain also gave rise to colonies in soft agar; the role of the catalytic domain of PKCepsilon was evident in the PMA-treated cells that overexpressed the PKC chimera containing the delta regulatory and the epsilon catalytic domains (PKCdelta/epsilon). The important contribution of the PKCepsilon catalytic domain to the growth of PKCdelta/epsilon-expressing cells was also evident in terms of a significantly increased saturation density in the presence of PMA, their formation of foci upon PMA treatment, and the induction of anchorage-independent growth. Aside from the growth-promoting effect of PKCepsilon, we have shown that most chimeras with PKCalpha and -delta regulatory domains inhibit cell growth. These results underscore the complex contributions of the regulatory and catalytic domains to the overall behavior of PKC.

Changing J774A.1 cells to new medium perturbs multiple signaling pathways, including the modulation of protein kinase C by endogenous sphingoid bases

Sphingosine, sphinganine, and other long-chain (sphingoid) bases are highly bioactive intermediates of sphingolipid metabolism that have diverse effects when added to cells, including the inhibition of protein kinase C (PKC) as evaluated by both enzymatic activity and [3H]phorbol dibutyrate ([3H]PDBu) binding. Nonetheless, changes in endogenous sphingoid bases have not been proven to affect PKC or other signal transduction pathways. We have discovered recently that changing J774A.1 cells to new medium results in up to 10-fold increases in sphingoid bases (Smith, E. R., and Merrill, A. H., Jr. (1995) J. Biol. Chem. 270, 18749-18758); therefore, this system was used to elevate sphingosine and sphinganine and determine if PKC was affected. Incubation of J774A.1 cells in new medium for 30 min increased the levels of these endogenous sphingoid bases to approximately 0.5 nmol/mg of protein and decreased [3H]PDBu binding by 40-60%. Addition of NH4Cl, which suppresses the change in sphingosine, restored [3H]PDBu binding. Elevation of endogenous sphinganine by a second method (addition of fumonisin B1, an inhibitor of ceramide synthase) also reduced [3H]PDBu binding; therefore, elevations in sphingosine and sphinganine can both affect PKC. The elevation in sphingoid bases was also associated with an increase in the amount of PKC-delta (the major PKC isozyme in J774A. 1 cells) in the cytosol, as determined by activity assays and immunoblot analyses. Changing the culture medium affected other PKC isozymes, increased cellular levels of diacylglycerol, dihydroceramide, and ceramide, and altered the expression of two genes (the expression of JE was increased, and the induction of MnSOD by TNF-alpha was potentiated). Thus, changing the culture medium has numerous effects on J774A.1 cells, including the modulation of PKC by endogenous sphingoid bases.

The catalytic domain of protein kinase C-delta in reciprocal delta and epsilon chimeras mediates phorbol ester-induced macrophage differentiation of mouse promyelocytes

The overexpression of protein kinase C-delta (PKC-delta), but not PKC-epsilon, enables the mouse myeloid cell line 32D to differentiate into macrophages when treated with phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA). To determine the domain of PKC-delta that is responsible for this isotype-specific function, cDNAs that encode reciprocal chimeras of PKC-delta and -epsilon (PKC-delta epsilon and PKC-epsilon delta) were constructed by exchanging regulatory and kinase domains using polymerase chain reaction technology. Both chimeras were stably expressed in 32D cells using the pLTR expression vector and displayed protein kinase activity upon TPA treatment. TPA treatment of L epsilon delta, cells that overexpressed the PKC-epsilon delta chimera, induced a dramatically increased cell volume, surface adherence, surface expression of Mac-1 and Mac-3, lysozyme production, and phagocytosis. These are the characteristics of the macrophage phenotype found in TPA-treated 32D cells that overexpressed PKC-delta. In contrast, little effect was seen in L delta epsilon, 32D cells that overexpressed PKC-delta epsilon, with or without TPA treatment. A PKC inhibitor directed toward the catalytic domain of PKC, GF109203X, and a selective inhibitor of PKC-delta, Rottlerin, blocked the TPA-induced differentiation of PKC-epsilon delta-overexpressing 32D cells. These results demonstrate that the catalytic domain of PKC-delta contains the primary determinants for its activity in phorbol ester-induced macrophage differentiation.

Protein kinase C-delta mRNA is down-regulated transcriptionally and post-transcriptionally by 12-O-tetradecanoylphorbol-13-acetate

Activation of protein kinase C-delta (PKC-delta) by 12-O-tetradecanoylphorbol-13-acetate (TPA) is followed by a gradual decrease in detectable protein 12-24 h later in the mouse B lymphoma cell line A20. Down-regulation is associated with TPA-induced proteolysis and a 50-86% decrease in PKC-delta mRNA 0.5-24 h post-treatment which is due to both a 50% decrease in transcription and accelerated degradation of PKC-delta mRNA as determined using the pulse-chase method. Destabilization of PKC-delta mRNA is also observed when actinomycin D is added to cells pretreated with TPA for 2 h; however, addition of actinomycin D or cycloheximide prior to TPA treatment blocks destabilization. Addition of PKC inhibitors to TPA-treated cells also blocks destabilization of PKC-delta mRNA. Cells treated with TPA for 4 h contain an activity not found in control cells which destabilizes PKC-delta mRNA but not glyceraldehyde-3-phosphate dehydrogenase mRNA in vitro. Addition of TPA to control extracts fails to increase degradation of PKC-delta mRNA in vitro, suggesting that treatment of intact cells is required to induce the synthesis of a factor(s) that destabilizes PKC-delta mRNA. This factor(s) then acts along with transcriptional and post-translational regulatory mechanisms to down-regulate PKC-delta.

Protein kinase C delta specifically associates with phosphatidylinositol 3-kinase following cytokine stimulation

Phosphatidylinositol (PI) 3-kinase is activated as a result of cytokine-induced association of the enzyme with specific tyrosine-phosphorylated proteins. PI 3-kinase lipid products, PI 3, 4-P2 and PI 3,4,5-P3, have been shown, in vitro, to directly activate novel and atypical protein kinase C (PKC) isozymes. However, the mechanism by which PI 3-kinase may be involved in regulation of PKC isoforms in vivo is presently unknown. We investigated a possible relationship by looking for associations between these enzymes. We found that in a human erythroleukemia cell line, as well as in rabbit platelets, PI 3-kinase and PKCdelta associate in a specific manner that is modulated by cell activation. Granulocyte-macrophage colony-stimulating factor treatment of cells caused increased association of PKCdelta and PI 3-kinase as did treatment of platelets with platelet-activating factor. Results using two PI 3-kinase inhibitors, wortmannin and LY-294002, showed that the former inhibited this association, while the latter did not, suggesting that PI 3-kinase lipid products may not be a prerequisite for the PI 3-kinase/PKCdelta association. Our results also suggest that tyrosine phosphorylation of PKCdelta is not involved in its association with PI 3-kinase.

Characterization of IkappaB kinases. IkappaB-alpha is not phosphorylated by Raf-1 or protein kinase C isozymes, but is a casein kinase II substrate

The NF-kappaB transcription factor is activated by a wide variety of stimuli, including phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate. In its inactive state, NF-kappaB is sequestered in the cytoplasm tethered to an inhibitor protein, IkappaB. Activation comprises the rapid phosphorylation of IkappaB-alpha at N-terminal sites, which presumably marks IkappaB-alpha for proteolytic degradation and leads to release of NF-kappaB into the nucleus. In addition, IkappaB-alpha is constitutively phosphorylated at the C terminus, which may be a prerequisite for proper IkappaB function. Protein kinase C (PKC) is activated by 12-O-tetradecanoylphorbol-13-acetate and has been previously reported to phosphorylate IkappaB-alpha in vitro. As PKC has turned out to constitute a multigene family encoding isozymes with different biological functions, we have reinvestigated IkappaB-alpha phosphorylation by PKC using recombinant PKC isozymes expressed in insect cells. While crude PKC preparations were efficient IkappaB-alpha kinases, highly purified PKC isozymes completely failed to phosphorylate IkappaB-alpha. Biochemical separation of porcine spleen yielded at least two fractions with IkappaB-alpha kinase activity, both of which were devoid of detectable PKC isozymes. One peak contained both Raf-1 and casein kinase II (CKII). Purified Raf-1 does not phosphorylate IkappaB-alpha directly, but associates with CKII, which efficiently phosphorylates the C terminus of IkappaB-alpha. Two-dimensional phosphopeptide mapping and high pressure liquid chromatography-mass spectroscopy analysis showed that all IkappaB-alpha kinases induced phosphorylation at the same prominent sites in the C terminus. Our results clearly indicate that PKC isozymes alpha, beta, gamma, delta, epsilon, eta, and zeta as well as Raf-1 are not IkappaB-alpha kinases. They furthermore demonstrate that IkappaB-alpha is targeted by several kinases, one of which appears to be CKII.

Cloning and characterisation of genes (pkc1 and pkcA) encoding protein kinase C homologues from Trichoderma reesei and Aspergillus niger

Oligonucleotides, designed on the basis of conserved flanking amino acid sequence segments within the catalytic domain of eukaryotic protein kinase C (PKC) proteins, were used as primers for polymerase chain reactions to amplify a 427-bp chromosomal DNA fragment from the filamentous fungus Trichoderma reesei. This fragment was then used to isolate genes encoding PKC homologues of T. reesei and Aspergillus niger (pkc1 and pkcA, respectively). The genes contain six (T. reesei) and eight (A. niger) introns, which exhibit notable conservation in position with those found in the corresponding Schizosaccharomyces pombe pkc1+ and Drosophila melanogaster dPKC53Ebr genes. A single 4.2-kb transcript was detected in Northern analyses. The deduced PKC1 (T.reesei, 126 kDa) and PKCA (A. niger, 122 kDa) amino acid sequences reveal domains homologous to the C1 and C3/C4 domains of PKC-related proteins, but lack typical Ca(2+)-binding (C2) domains. Both contain a large, extended N-terminus, which shares a high degree of similarity with the corresponding regions of Saccharomyces cerevisiae PKC1 and S. pombe pkc1+ and pkc2+ proteins, but which is not present in PKCs of Dictyostelium or higher eukaryotes. This extended region can be divided into three subdomains; the N-terminal one contains a hydrophobic helix-turn-helix motif, whereas the C-terminal one contains potential targets for proteolytic processing. A polyclonal antiserum raised against the pseudosubstrate-binding domain of PKC1 recognizes in T. reesei a 115-120 kDa protein in Western blots. Expression of pkc1 cDNA in insect cells directs the synthesis of a PKC1 protein of similar size. The T. reesei PKC1 protein was partially purified and some of its properties examined: it is stimulated about twofold by phospholipids or phorbol esters but is not stimulated by Ca2+. We conclude that these PKC proteins from filamentous fungi represent the Ca(2+)-insensitive fungal homologues of the nPKC family.

Antisense oligodeoxynucleotide inhibition of delta protein kinase C expression accelerates induced differentiation of murine erythroleukaemia cells

The potential regulatory role of delta protein kinase C (delta PKC) in murine erythroleukaemia cell differentiation was studied by using antisense oligodeoxynucleotides targeting the translation initiation region of mouse delta PKC mRNA. Cell treatment with antisense oligonucleotides, at a concentration of 20 microM, followed by hexamethylenebisacetamide induction, produced a specific 2-fold increase in the differentiation rate of both slowly and rapidly differentiating murine erythroleukaemia cell clones. Cell permeabilization by a cationic lipid resulted in a decrease of one order of magnitude in the amounts of antisense oligonucleotides necessary to elicit the maximal response, and accelerated the kinetics of the stimulatory effect. These changes in murine erythroleukaemia cell differentiation rates, observed in both cell clones, were associated with 60% and 50% decreases, respectively, in delta PKC immunoreactive protein in slowly and rapidly differentiating cells. The present results indicate strongly that basal levels of delta PKC in murine erythroleukaemia cells are essential in regulating the initial differentiation rate of these cells in response to chemical induction, and provide further evidence that this PKC isoform plays a fundamental role in maintaining the undifferentiated phenotype of murine erythroleukaemia cells.

Identification of the primary growth response gene, ST2/T1, as a gene whose expression is differentially regulated by different protein kinase C isozymes

Individual protein kinase C isozymes have been shown to play different roles in mediating proliferation, differentiation and transformation, but it is not known to what extent these effects involve induction of expression of particular genes. To explore the differential gene expression that might be induced by activation of different PKC isozymes, we stably transfected NIH 3T3 cells with expression vectors that encode the isozymes PKC-alpha, -beta II, -gamma, -delta, -epsilon, -sigma and -eta. Using differential display-reverse transcription-polymerase chain reaction we isolated a small cDNA that encodes a portion of the primary response gene, ST2 (also referred to as T1 or DER4), and we confirmed by RNA blot studies that ST2/T1 expression is differentially regulated by PKC isozymes. ST2/T1 mRNA is undetectable in the unstimulated parental NIH 3T3 cells that express only the alpha isozyme of PKC, but it can be induced by phorbol ester treatment. Clones that overexpress PKC-alpha, -delta or -epsilon similarly do not express ST2/T1 until they are stimulated with phorbol esters, which induces expression of ST2/T1 with kinetics similar to wild-type NIH 3T3 but to different extents. In contrast, ST2/T1 mRNA is already present in unstimulated cells that overexpress PKC-beta II, -gamma, -sigma and -eta, but phorbol ester greatly enhances ST2/T1 expression in these cells. These results suggest a differential role for PKC isozymes in mediating the ST2/T1 expression that is induced by growth stimuli.

Residues in the second cysteine-rich region of protein kinase C delta relevant to phorbol ester binding as revealed by site-directed mutagenesis

Phorbol esters bind with high affinity to protein kinase C (PKC) isozymes as well as to two novel receptors, n-chimaerin and Unc-13. The cysteine-rich regions present in these proteins were identified as the binding sites for the phorbol ester tumor promoters and the lipophilic second messenger sn-diacylglycerol. A 50-amino-acid peptide comprising the second cysteine-rich region of PKC delta, expressed in Escherichia coli as a glutathione S-transferase (GST)-fusion protein, bound [3H]phorbol 12,13-dibutyrate (PDBu) with high affinity (Kd = 0.8 nM). Using the cDNA of that cysteine-rich region as a template, a series of 37 point mutations was generated by site-directed mutagenesis, and the mutated proteins were analyzed quantitatively for binding of [3H]PDBu and, as appropriate, for binding of the ultrapotent analog [3H]bryostatin 1. Mutants displayed one of three patterns of behavior: phorbol ester binding was completely abolished, binding affinity was reduced, or binding was not significantly modified. As expected, five of the six cysteines as well as the two histidines involved in Zn2+ coordination are critical for the interaction of the protein with the phorbol esters. In addition, mutations in several positions, including phenylalanine 3, tyrosine 8, proline 11, leucines 20, 21 and 24, tryptophan 21, glutamine 27, and valine 38 drastically reduced the interaction with the ligands. The effect of these mutations can be rationalized from the three-dimensional (NMR) structure of the cysteine-rich region. In particular, the C-terminal portion of the protein does not appear to be essential, and the loop comprising amino acids 20 to 28 is implicated in the binding activity.

Anti-CD3-induced changes in protein kinase C isozymes expression in human CD4+ and CD8+ T lymphocytes

In order to determine whether there is a differential expression and activation of PKC isozymes between CD4+ and CD8+ T cells, peripheral blood mononuclear cells were stimulated with anti-CD3 monoclonal antibody (moAb) for various time intervals and the expression of calcium-dependent PKC isozymes (alpha, beta, gamma) and calcium-independent PKC isozymes (delta, epsilon, zeta) was analyzed with dual color flow cytometry, using anti-PKC isozyme antibodies and anti-CD4 or anti-CD8 antibodies. The basal fluorescence intensity of all PKC isozymes was comparable between CD4+ T cells and CD8+ T cells. Following activation with anti-CD3 moAb a marked increase in the fluorescence intensity of all PKC isozymes in both CD4+ and CD8+ T cells, albeit to a different extent and with different kinetics was observed. Among all PKC isozymes studied, the least striking changes were observed in PKC zeta isozyme and the most striking changes were observed in PKC-epsilon isozyme. Laser-based confocal microscopic studies confirmed that the increase in fluorescence intensity of PKC isozymes following anti-CD3 moAb stimulation, as measured by flow cytometry was accompanied by the translocation of PKC isozymes from cytosol to the plasma membrane. This study demonstrates a differential effect of anti-CD3 moAb on the expression of PKC isozymes between CD4+ and CD8+ T cells and suggests that flow cytometry can be used to study the translocation of PKC isozymes from cytosol to the plasma membrane.

Low affinity binding of phorbol esters to protein kinase C and its recombinant cysteine-rich region in the absence of phospholipids

Binding of phorbol esters to protein kinase C (PKC) has been regarded as dependent on phospholipids, with phosphatidylserine being the most effective for reconstituting binding. By using a purified single cysteine-rich region from PKC delta expressed in Escherichia coli we were able to demonstrate that specific binding of [3H]phorbol 12,13-dibutyrate to the receptor still takes place in the absence of the phospholipid cofactor. However, [3H]phorbol 12,13-dibutyrate bound to the cysteine-rich region with 80-fold lower affinity in the absence than in the presence of 100 micrograms/ml phosphatidylserine. Similar results were observed with the intact recombinant PKC delta isolated from insect cells. When different phorbol derivatives were examined, distinct structure-activity relations for the cysteine-rich region were found in the presence and absence of phospholipid. Our results have potential implications for PKC translocation, for inhibitor design, and for PKC structural determination.

Crystal structure of the cys2 activator-binding domain of protein kinase C delta in complex with phorbol ester

Protein kinase Cs (PKCs) are a ubiquitous family of regulatory enzymes that associate with membranes and are activated by diacylglycerol or tumor-promoting agonists such as phorbol esters. The structure of the second activator-binding domain of PKC delta has been determined in complex with phorbol 13-acetate, which binds in a groove between two pulled-apart beta strands at the tip of the domain. The C3, C4, and C20 phorbol oxygens form hydrogen bonds with main-chain groups whose orientation is controlled by a set of highly conserved residues. Phorbol binding caps the groove and forms a contiguous hydrophobic surface covering one-third of the domain, explaining how the activator promotes insertion of PKC into membranes.

Phorbol ester and diacylglycerol activation of native protein kinase C species from various tissues

The characteristics of PKC activation induced by a number of compounds were investigated using PKCs, partially-purified from sources with a naturally high abundance of certain Ca2+ dependent PKC isoforms. Native isoforms were used rather than PKC isoforms expressed from a baculovirus system to assess the effect of tissue specific factors on activity. However, some data using recombinant PKC alpha were included for comparison. The presence of specific PKC isoforms in different tissues was determined using Western blot analysis. Protein kinase C alpha, beta 1, delta, epsilon, and zeta/iota were all present in rat midbrain cytosolic extract, PKC alpha, beta 1, delta, and zeta/iota were present in spleen cytosol, and PKC alpha and zeta/iota were present in COS 7 cell cytosol. The predominance of alpha and beta activities in COS 7 and spleen extracts respectively was confirmed by enzymic assay. The PKC activity assay was configured such that the Ca2+ dependence of the PKC activity induced by different PKC activators could be determined. Phorbol 12,13-dibutyrate (PDBu) was virtually equipotent on the Ca(2+)-dependent PKC activity from midbrain and spleen and slightly less potent on that from COS 7 cells. In the absence of Ca2+, PDBu was considerably less potent overall (as, indeed, were the other PKC activators) and was less potent on COS 7 cell PKC than on those from midbrain or spleen. Mezerein was more potent than PDBu at inducing PKC activity in COS 7 cell extracts in either the absence or presence of Ca2+ whereas in the presence of Ca2+, mezerein was slightly less potent on midbrain and spleen than PDBu and equipotent in the absence of Ca2+. Maximum values for Ca(2+)-independent activation by mezerein indicated that this activator was particularly effective in recruiting Ca(2+)-dependent PKC isoform activity in a Ca2+ free environment. The greater potency of mezerein on PKC alpha was confirmed using PKC alpha and beta further purified from rat spleen by hydroxylapatite (HAP) chromatography. The effects of both PDBu and mezerein were investigated using anterior pituitary tissue where a particularly high potency of mezerein in the absence of Ca2+ was noted. The diacylglycerol, 1,2-dioctanoyl-sn-glycerol (DOG), appeared to cause little or no activation of native Ca(2+)-dependent isoforms in Ca2+ free conditions unlike another longer chain diacylglycerol, 1,2-dioleoyl-sn-glycerol. Also DOG activated midbrain PKCs more potently than PKCs from spleen or COS 7 cells (or lung and pituitary tissue) in the presence of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of the cysteine-rich region of the Caenorhabditis elegans protein Unc-13 as a high affinity phorbol ester receptor. Analysis of ligand-binding interactions, lipid cofactor requirements, and inhibitor sensitivity

The Caenorhabditis elegans Unc-13 protein is a novel member of the phorbol ester receptor family having a single cysteine-rich region with high homology to those present in protein kinase C (PKC) isozymes and the chimaerins. We expressed the cysteine-rich region of Unc-13 in Escherichia coli and quantitatively analyzed its interactions with phorbol esters and related analogs, its phospholipid requirements, and its inhibitor sensitivity. [3H]Phorbol 12,13-dibutyrate [3H]PDBu bound with high affinity to the cysteine-rich region of Unc-13 (Kd = 1.3 +/- 0.2 nM). This affinity is similar to that of other single cysteine-rich regions from PKC isozymes as well as n-chimaerin. As also described for PKC isozymes and n-chimaerin, Unc-13 bound diacylglycerol with an affinity about 2 orders of magnitude weaker than [3H]PDBu. Structure-activity analysis revealed significant but modest differences between recombinant cysteine-rich regions of Unc-13 and PKC delta. In addition, Unc-13 required slightly higher concentrations of phospholipid for reconstitution of [3H]PDBu binding. Calphostin C, a compound described as a selective inhibitor of PKC, was also able to inhibit [3H]PDBu binding to Unc-13, suggesting that this inhibitor is not able to distinguish between different classes of phorbol ester receptors. In conclusion, although our results revealed some differences in ligand and lipid cofactor sensitivities, Unc-13 represents a high affinity cellular target for the phorbol esters as well as for the lipid second messenger diacylglycerol, at least in C. elegans. The use of phorbol esters or some "specific" antagonists of PKC does not distinguish between cellular pathways involving different PKC isozymes or novel phorbol ester receptors such as n-chimaerin or Unc-13.

Subnanomolar concentration of VIP induces the nuclear translocation of protein kinase C in neonatal rat cortical astrocytes

At subnanomolar concentrations, vasoactive intestinal peptide (VIP) can act as an astroglial mitogen and as a secretagogue for neurotrophic substances released from glia (Brenneman et al.: J Neurosci Res 25:386-394, 1990). Here we report that treatment with subnanomolar (0.1 nM) VIP, that does not produce an increase in intracellular cAMP levels, induced the translocation of protein kinase C (PKC) from the cytoplasm to the nucleus in neonatal cortical astrocytes, as revealed by immunohistochemistry, Western blot analysis, and measurements of the enzyme activity. Western blot analysis of subcellular fractions, using PKC isotype-specific antisera, showed PKC alpha as well as the two novel PKC isotypes, delta and zeta immunoreactivities, whereas PKC beta or gamma immunoreactivities were not detected. PKC alpha was associated predominantly with the cytosolic compartment, while PKC delta was found in the plasma membrane and in nuclear fractions. In contrast, PKC zeta was distributed ubiquitously within the major subcellular fractions. Treatment of the cells with 0.1 nM VIP caused a marked increase in nuclear PKC alpha and, to a lesser extent, PKC delta and PKC zeta immunoreactivities. Western blot analysis showed that a low (1 nM) concentration of phorbol, 12-myristate, 13 acetate also caused the subcellular redistribution of PKC immunoreactivities from the cytoplasm to the nuclear fraction, similar to VIP treatment. Exposure of astrocytes to high concentrations (1 microM) of phorbol, 12-myristate, 13 acetate resulted in the down-regulation of PKC alpha and PKC delta, while distribution of PKC zeta immunoreactivities were only slightly altered.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of elevated levels of protein kinase C-zeta mRNA and protein with murine B-lymphocytic neoplasia

Expression of mRNA for protein kinase C (PKC)-alpha, -beta, -gamma, -delta, -epsilon, -zeta, and -eta has been shown, by polymerase chain reaction-generated isozyme-specific probes, to be cell-type -and differentiation-stage-specific in mouse hemopoietic cells. Recently, we cloned a 2.2-kb mouse PKC -zeta cDNA. In this study, we used the nearly full-length cDNA PKC-zeta probe to demonstrate that expression of PKC-zeta was significantly elevated in lymphocytic neoplasms at both the mRNA and protein levels. Normal brain, kidney, and liver contain 2.4- and 4.4-kb mRNAs, whereas normal lymphoid organs (spleen, thymus, and lymph nodes) express barely detectable amounts of PKC-zeta. These vanishingly small levels of PKC-zeta mRNA did not increase when polyclonal spleen B-cell proliferation and differentiation were induced in vivo with anti-immunoglobulin D antiserum or in vitro with lipopolysaccharide. In contrast, 2.4-kb transcripts of PKC-zeta are abundant in virtually all neoplastic B-lymphocytic cell lines. Furthermore, additional transcripts of a novel size, about 7 and 8 kb, were found in several mature B-cell lymphomas and plasma cell tumors. Western blot analysis of protein extracts from normal B cells and hemopoietic tumors confirmed that these quantitative differences in PKC-zeta mRNA also exist at the protein level. That is, only trace amounts of PKC-zeta protein were detectable in pro-B cells and pre-B cells, but abundant amounts of this isoform were found in protein extracts from most B-cell lymphomas and plasma cell tumors. These findings suggest that this atypical member of the PKC multigene family participate in the multistep process of malignant transformation of lymphocytes.

Expression and biochemical characterization of human protein kinase C-theta

In this study, the recently identified human protein kinase C-theta (PKC-theta) isoform has been biochemically characterized in detail. An antiserum raised against the unique V3 domain of PKC-theta identified an 80-kDa protein in all human T-cell lines tested, in erythroleukemia K562 cells and in histiocytic lymphoma U-937 cells, but not in a B-lymphoma line (Raji) or in several melanoma, carcinoma, schwanoma or astrocytoma lines, confirming, at the protein level, its predominant expression in hematopoietic cell lines, in particular T cells. Immunoreactive PKC-theta was detected almost exclusively in the cytosolic compartment of unstimulated Jurkat T cells. Stimulation with phorbol ester, however, caused rapid translocation to the membrane. In order to compare the properties of PKC-theta with a representative member of the Ca(2+)-dependent PKC enzymes, full-length cDNAs encoding PKC-theta or PKC-alpha were transiently expressed in COS-1 cells, and recombinant enzymes were partially purified via a six-histidine peptide tag. The catalytic activity of these PKC enzymes was assayed against distinct substrates in the absence and presence of known PKC cofactors. Significant differences were found with respect to activation requirements and substrate preferences between PKC-theta and PKC-alpha. Both enzymes were stimulated by phospholipid and phorbol ester, and were active towards a PKC-derived substrate peptide corresponding to the pseudosubstrate site of PKC. In contrast to PKC-alpha, however, full activation of PKC-theta did not require Ca2+, and its basal activity towards histone H1 was not stimulated by lipid cofactors. Additionally, a myelin-basic-protein-(MBP)-derived peptide, which was readily phosphorylated by PKC-alpha, was a poor substrate for PKC-theta. Similar to PKC-alpha, transient PKC-theta overexpression in murine EL4 thymoma cells caused an approximately 2.5-fold increase in the phorbol-12-myristate-13-acetate-induced transcriptional activation of an interleukin-2 promoter-reporter gene construct. The unique expression and functional properties of PKC-theta suggest that it may play a specialized role in T-cell signaling pathways.

Tyrosine phosphorylation and stimulation of protein kinase C delta from porcine spleen by src in vitro. Dependence on the activated state of protein kinase C delta

Native protein kinase C delta from porcine spleen is phosphorylated in vitro by the tyrosine kinase src and to a much smaller extent by fyn. The tyrosine phosphorylation of PKC delta is restricted to the activated state of the enzyme, i.e. it occurs only in the presence of an activator, such as TPA or bryostatin. Upon phosphorylation at tyrosine, the apparent molecular weight of PKC delta increases by 6 kDa. Phosphorylation by src induces a stimulation of PKC delta activity apparently exhibiting some substrate selectivity. Other PKC isoenzymes, such as cPKC (alpha, beta, gamma), are not phosphorylated by src or only to a very small extent. This phosphorylation is not dependent on TPA and does not cause an increase in activity and molecular weight of the enzyme.

Human immunodeficiency virus-1 recombinant gp120 induces changes in protein kinase C isozymes--a preliminary report

Human immunodeficiency virus 1 (HIV-1) and its purified proteins activate target cell functions. Because protein kinase C (PKC) plays a crucial role in signal transduction and there is a molecular heterogeneity of PKC, we compared the effect of recombinant HIV-1 gp120 and phorbol ester (PMA) on PKC isozymes in monocytic U937 cells, with isozyme-specific antibodies using flow cytometry. All PKC isozymes except PKC-gamma were present in U937 cells. Both PMA and HIV-1 gp120 increased levels of calcium-dependent and -independent PKC isozymes. The most striking change was observed in PKC-zeta isozymes levels. This study for the first time demonstrates that HIV-1 gp120 affects calcium-independent PKC isozymes in U937 cells.

Protein kinase C isoform expression and regulation in the developing rat heart

To determine whether age-dependent differences in cardiac responses to autonomic agonists could result from developmental changes in protein kinase C (PKC) isoform expression, we probed extracts from the fetal, neonatal, and adult heart as well as cultured neonatal and isolated adult ventricular myocytes with specific antisera to calcium-dependent (alpha and beta) and calcium-independent (delta, epsilon and zeta) isoforms of the enzyme. Although PKC-beta immunoreactivity could not be detected in cultured neonatal or isolated adult ventricular myocytes, adult and neonatal myocytes expressed multiple other isoforms of PKC. Our studies revealed an age-dependent decline in the immunoreactivity for three PKC isoforms. PKC-alpha was detected in extracts from the fetal and 2-day-old neonatal heart as well as cultured neonatal rat ventricular myocytes. Only faint PKC-alpha immunoreactivity was detected in extracts from the adult heart, and PKC-alpha was not detected in extracts from isolated adult ventricular myocytes, suggesting that PKC-alpha resides in nonmyocyte elements in the adult heart. PKC-delta also was detected in greater abundance in fetal and neonatal than in adult myocardial extracts. The decline in PKC-alpha and PKC-delta expression occurred during the first 2 postnatal weeks. PKC-zeta was detected in greatest abundance in extracts from the fetal heart. PKC-zeta expression declined markedly by the second postnatal day, and only faint PKC-zeta immunoreactivity was detected in extracts from adult myocardium. Failure to detect PKC-zeta in extracts from isolated adult ventricular myocytes suggests that PKC-zeta resides primarily in nonmyocyte elements in the adult heart. PKC-epsilon was detected in all preparations, but it was detected in greatest abundance in extracts from neonatal hearts. In vitro sympathetic innervation of previously noninnervated neonatal ventricular myocytes or in vivo chemical sympathectomy of the neonatal heart did not modulate PKC isoform expression, suggesting that sympathetic innervation does not significantly regulate PKC isoform expression. PKC-alpha partitioned to the soluble fraction of unstimulated myocytes and was selectively translocated to the particulate fraction by Ca2+. In contrast, a major portion of the novel PKC isoforms partitioned to the particulate fraction of unstimulated myocytes. The subcellular distribution of novel PKC isoforms was not influenced by Ca2+. 12-O-Tetradecanoylphorbol 13-acetate (TPA, 300 nmol/L) induced translocation of soluble PKC-alpha, PKC-delta, and PKC-epsilon to the particulate fraction at 30 minutes and complete (PKC-alpha and PKC-delta) or 80% (PKC-epsilon) downregulation at 24 hours. PKC-zeta was not affected by TPA.(ABSTRACT TRUNCATED AT 400 WORDS)