Involvement of protein kinase C in the transcriptional regulation of 12-O-tetradecanoylphorbol-13-acetate-inducible genes modulated by AP-1 or non-AP-1 transacting factors

Mechanobiology and developmental control

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

Mechanosensitive mechanisms in transcriptional regulation

Transcriptional regulation contributes to the maintenance of pluripotency, self-renewal and differentiation in embryonic cells and in stem cells. Therefore, control of gene expression at the level of transcription is crucial for embryonic development, as well as for organogenesis, functional adaptation, and regeneration in adult tissues and organs. In the past, most work has focused on how transcriptional regulation results from the complex interplay between chemical cues, adhesion signals, transcription factors and their co-regulators during development. However, chemical signaling alone is not sufficient to explain how three-dimensional (3D) tissues and organs are constructed and maintained through the spatiotemporal control of transcriptional activities. Accumulated evidence indicates that mechanical cues, which include physical forces (e.g. tension, compression or shear stress), alterations in extracellular matrix (ECM) mechanics and changes in cell shape, are transmitted to the nucleus directly or indirectly to orchestrate transcriptional activities that are crucial for embryogenesis and organogenesis. In this Commentary, we review how the mechanical control of gene transcription contributes to the maintenance of pluripotency, determination of cell fate, pattern formation and organogenesis, as well as how it is involved in the control of cell and tissue function throughout embryogenesis and adult life. A deeper understanding of these mechanosensitive transcriptional control mechanisms should lead to new approaches to tissue engineering and regenerative medicine.

Phosphorylation of Activation Transcription Factor-2 at Serine 121 by Protein Kinase C Controls c-Jun-mediated Activation of Transcription

Activation transcription factor-2 (ATF-2) is phosphorylated by various protein kinases, such as JNK/p38/ERK, calmodulin kinase IV, protein kinase A, and protein kinase C (PKC), in response to a variety of stimuli. However, the role of the phosphorylation of ATF-2 by PKC in vivo in the transcriptional control of genes that include the activation protein-1 (AP-1)/cyclic AMP-response element remains to be defined. Using antibodies against the phosphorylated serine residue (Ser(P)) at position 121 of ATF-2, we have demonstrated that PKC phosphorylates ATF-2 at Ser-121 and that phosphorylation of Ser-121 (to yield ATF-2pS121) becomes detectable at the late stage of the response of HeLa cells to 12-O-tetradecanoylphorbol-13-acetate (TPA) and is maintained for more than 2 h. By contrast, phosphorylation of ATF-2 at threonine residues 69 and 71 (Thr-69/71, to yield ATF-2pT69/71) and at Ser-340 and Ser-367 (to yield ATF-2pS340 and ATF-2pS367) is detectable as an immediate early response. Unlike levels of ATF-2pT69/71 and ATF-2pS340, the level of ATF-2pS121 increases in the nuclei of HeLa cells in response to TPA. A serine-to-alanine mutation at position 121 of ATF-2 represses the c-Jun-dependent transcription of AP-1/cyclic AMP-response element reporter genes and also the p300-mediated activation of a Gal4-reporter gene in response to TPA. Our results suggest that the phosphorylation of ATF-2 at Ser-121 plays a key role in the c-Jun-mediated activation of transcription that occurs in response to TPA.

Mechanisms of apoptosis-induction by rottlerin: therapeutic implications for B-CLL

Constitutively activated signaling pathways contribute to the apoptosis-defect of B-CLL cells. Protein kinase C-delta is a permanently activated kinase and a putative downstream target of phosphatidylinositol-3 kinase in B-CLL. Blockade of protein kinase C-delta (PKC-delta) by the highly specific inhibitor rottlerin induces apoptosis in chronic lymphocytic leukaemia (CLL) cells. By co-culturing bone marrow stromal and CLL cells, we determined that the proapoptotic effect of rottlerin is not abolished in the presence of survival factors, indicating that a targeted therapy against PKC-delta might be a powerful approach for the treatment of CLL patients. The downstream events following rottlerin treatment engage mitochondrial and non-mitochondrial pathways and ultimately activate caspases that execute the apoptotic cell death. Herein we report that the inhibition of PKC-delta decreases the expression of the important antiapoptotic proteins Mcl-1 and XIAP accompanied by a loss of the mitochondrial membrane potential Deltapsi. In addition, we discovered that ZAP-70-expressing cells are significantly more susceptible to rottlerin-induced cell death than ZAP-70 negative cells. We finally observed that rottlerin can augment cell toxicity induced by standard chemotherapeutic drugs. Conclusively, PKC-delta is a promising new target in the combat against CLL.

Alpha2(I) collagen gene regulation by protein kinase C signaling in human dermal fibroblasts

We investigated the mechanisms by which protein kinase C (PKC) regulates the expression of the α2(I) collagen gene in normal dermal fibroblasts. Reduction of PKC-α activity by treatment with Gö697-6 or by overexpression of a dominant negative (DN) mutant form decreased α2(I) collagen gene expression. This decrease required a sequence element in the collagen promoter that contains Sp1/Sp3 binding sites. Reduction of PKC-δ activity by rottlerin or overexpression of DN PKC-δ also decreased α2(I) collagen gene expression. This effect required a separate sequence element containing Sp1/Sp3-binding sites and an Ets-binding site. In both cases, point mutations within the response elements abrogated the response to PKC inhibition. Forced overexpression of Sp1 rescued the PKC inhibitor-mediated reduction in collagen protein expression. A DNA affinity precipitation assay revealed that inhibition of PKC-δ by rottlerin increased the binding activity of endogenous Fli1 and decreased that of Ets1. On the other hand, TGF-β1, which increased the expression of PKC-δ, had the opposite effect, increasing the binding activity of Ets1 and decreasing that of Fli1. Our results suggest that PKC-δ is involved in the regulation of the α2(I) collagen gene in the presence or absence of TGF-β. Alteration of the balance of Ets1 and Fli1 may be a novel mechanism regulating α2(I) collagen expression.

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

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

Regulation of airway smooth muscle cyclin D1 transcription by protein kinase C-delta

The precise mechanism by which protein kinase C-delta (PKCdelta) inhibits cell cycle progression is not known. We investigated the regulation of cyclin D1 transcription by PKCdelta in primary bovine airway smooth muscle cells. Overexpression of the active catalytic subunit of PKCdelta attenuated platelet-derived growth factor (PDGF)-mediated transcription from the cyclin D1 promoter, whereas overexpression of a dominant-negative PKCdelta increased promoter activity. A PKCdelta-specific pseudosubstrate increased cyclin D1 protein abundance. To determine the transcriptional mechanism by which PKCdelta negatively regulates cyclin D1 expression, we transiently transfected cells with cDNAs encoding cyclin D1 promoter 5' deletions and site mutations in the context of a -66 promoter fragment. We found that the -57 to -52 CRE/ATF2 site functions as a basal level and PDGF enhancer, whereas the -39 to -30 nuclear factor-kappaB site functions as a basal level suppressor. Further, PDGF and PKCdelta responsiveness of the cyclin D1 promoter was maintained following 5' deletion to the Ets-containing -22 minimal promoter. Finally, using electrophoretic mobility gel shift and reporter assays, we determined that PKCdelta inhibits CRE/ATF2 binding and transactivation, activates nuclear factor-kappaB binding and transactivation, and attenuates Ets transactivation. These data suggest that PKCdelta attenuates cyclin D1 promoter activity via the regulation of three distinct cis-acting regulatory elements.

Induced proliferation of human MRC-5 cells by nitrogen oxides via direct and indirect activation of MEKK1, JNK, and p38 signals

Nitrogen oxides (NOx) are important indoor air pollutants and an occupational hazard. Many studies demonstrated that NOx causes lung tissue damage based on the oxidation properties and the free-radical potentials of these gases. In this study we found that NOx delivered as a NO gas-saturated solution induced proliferation of human lung fibroblast MCR-5 cells as evidenced by increasing cell number and S phase distribution. Western data showed that NOx increased the expressions of c-Fos, c-Jun, and signaling kinases including MEKK1, JNK1, and p38 (with induction fold of 3.3, 2.8, and 3.2, respectively) in the cells 12 h after treatment. The levels of phospho-MEKK1 and phospho-JNK1 were also increased. The application of iNOS inhibitor, NAME, partially blocked the activation of MEKK4 and JNK1. These data suggested that JNK and p38 signaling kinases are activated partly by endogenous NO that are generated from NOx-activated iNOS in MRC-5 cells. Therefore, the NOx-induced cell proliferation via activation of MEKK1, JNK1, and p38 might contribute to lung tissue damage caused by NOx pollutants.

Protein kinase Cdelta-mediated signal to ornithine decarboxylase induction is independent of skin tumor suppression

Protein Kinase Cdelta (PKCdelta), a Ca(2+)-independent, phospholipid-dependent serine/threonine kinase, is among the PKC isoforms expressed in mouse epidermis. We reported that FVB/N transgenic mice that overexpress ( approximately eightfold) PKCdelta protein in basal epidermal cells are resistant to skin tumor formation by the 7,12-dimethylbenz(a)anthracene (DMBA)-initiation and 12-O-tetradecanoylphorbol-13-acetate (TPA)-promotion protocol. However, despite being resistant to skin tumor promotion by TPA, PKCdelta transgenic mice elicited a 3-4-fold increase in TPA-induced epidermal ODC activity and putrescine levels than their wild-type littermates. PKCdelta was observed to be the key component of the TPA signal transduction pathways to the induction of mouse epidermal ODC activity. To determine if TPA-induced ODC activity and associated putrescine levels in PKCdelta transgenic mice contributed to PKCdelta-mediated suppression of skin tumor promotion by TPA, the irreversible inhibitor of ODC, alpha-difluoromethylornithine (DFMO), was used. PKCdelta transgenic mice and their wild-type littermates were initiated with 100 nmol DMBA and then promoted twice weekly with 5 nmol TPA. The experimental group was given 0.5% DFMO in their drinking water, while the control group was given tap water. After 25 weeks, the number of papillomas (>2 mm) per mouse was counted. The DFMO treatment did not affect the skin tumor multiplicity of PKCdelta transgenic mice. These results indicate that PKCdelta-induced ODC activity is not involved in PKCdelta-mediated tumor suppression. Thus, the signaling pathways via PKCdelta to epidermal ODC induction and skin tumor suppression appear to be independent.

Regulation of ornithine decarboxylase in B16 mouse melanoma cells: synergistic activation of melanogenesis by alphaMSH and ornithine decarboxylase inhibition

Ornithine decarboxylase (ODC) is the rate-limiting enzyme in the biosynthesis of polyamines, a family of cationic compounds required for optimal cell proliferation and differentiation. Within mammalian melanocytes, the expression of genes regulating cell growth and/or differentiation can be controlled by alpha-melanocyte-stimulating hormone (alphaMSH) and other melanogenesis modulating agents. In the B16 mouse melanoma model, alphaMSH stimulates melanogenesis by upmodulation of tyrosinase (tyr) activity, whereas the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) inhibits melanin synthesis. Therefore, we analyzed the regulation of ODC by these agents, as related to changes in the melanogenic pathway. Treatment of B16 cells with TPA or alphaMSH rapidly stimulated ODC activity. The effect was stronger for TPA and appeared mainly posttranslational. Irreversible inhibition of ODC with the active site-directed inhibitor alpha-difluoromethylornithine (DFMO) did not block TPA-mediated inhibition of tyr. Conversely, prolonged treatment of B16 cells with DFMO stimulated tyr activity by a posttranslational mechanism, probably requiring polyamine depletion. Combination treatment with alphaMSH and DFMO synergistically activated tyr. Therefore, ODC induction is not involved in the melanogenic response of B16 cells to alphaMSH. Rather, increased intracellular concentrations of polyamines following ODC induction might constitute a feedback mechanism to limit melanogenesis activation by alphaMSH.

Relation of the induction of epidermal ornithine decarboxylase and hyperplasia to the different skin tumor-promotion susceptibilities of protein kinase C alpha, -delta and -epsilon transgenic mice

To define the in vivo role of individual PKC isoforms in mouse skin carcinogenesis, we previously characterized FVB/n transgenic mice that over-expressed epitope-tagged PKC delta (T7-PKC delta) or PKC epsilon (T7-PKC epsilon) isoforms under the regulation of the human K14 promoter. In continuation of our prior PKC isoform specificity studies, we now report the generation of FVB/n transgenic mice with K14-regulated, epitope-tagged PKC alpha (T7-PKC alpha). T7-PKC alpha transgenic mice (line 115) express 8-fold more PKC alpha protein than wild-type mice. Using high-resolution immunogold cytochemistry, we determined that transgenic over-expression of T7-PKC alpha did not alter the subcellular localization of PKC alpha but that the density of PKC alpha staining increased. PKC alpha localized primarily to the cytoskeleton (tonofilaments, tight junctions) and cell membranes, with modest but definite nuclear labeling also identified. Also, PKC alpha over-expression did not alter the immunoreactive protein levels of other PKC isoforms (delta, epsilon, eta, zeta, mu) in the epidermis. Skin tumor-promotion susceptibility was compared among all 3 lines of T7-PKC transgenic mice (alpha, delta and epsilon). While T7-PKC alpha had no effect on skin tumor promotion by TPA, T7-PKC delta reduced papilloma burden by 76% compared to wild-type controls. T7-PKC epsilon further reduced papilloma burden to 93% compared to wild-type controls but still resulted in the development of squamous-cell carcinoma. To find potential mechanisms of PKC-associated differences in tumor promotion, the induction of known downstream effectors of tumor promotion, ornithine decarboxylase (ODC) activity and epidermal hyperplasia, was determined. Despite long-term papilloma inhibition in both PKC delta and PKC epsilon transgenic mice, the induction of ODC by TPA was not attenuated in PKC delta and epsilon mouse lines. Both PKC transgenic and wild-type mice exhibited sustained hyperplasia after repeated TPA treatments. However, TPA-induced epidermal hyperplasia in T7-PKC epsilon mice was significantly increased (52%) compared with T7-PKC alpha, T7-PKC delta and wild-type mice. TPA-induced ODC activity and the resultant accumulation of polyamines may play different roles (e.g., induction of apoptosis vs. proliferation) in the pathways leading to the induction of cancer in PKC alpha, PKC delta and PKC epsilon transgenic mice.

Retinoic acid (RA) receptor transcriptional activation correlates with inhibition of 12-O-tetradecanoylphorbol-13-acetate-induced ornithine decarboxylase (ODC) activity by retinoids: a potential role for trans-RA-induced ZBP-89 in ODC inhibition

Evaluation of retinoic acid receptor (RAR) subtype-selective alpha and gamma agonists and antagonists and a retinoid X receptor (RXR) class-selective agonist for efficacy at inhibiting both induction of ornithine decarboxylase (ODC) by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis and rat tracheal epithelial cells and the appearance of papillomas in mouse epidermis treated in the 2-stage tumor initiation-promotion model indicated that (i) RXR class-selective transcriptional agonists, such as MM11246, were not involved in ODC inhibition; (ii) RAR-selective agonists that induce gene transcription from RA-responsive elements (RAREs) were active at low concentrations; (iii) RAR-selective antagonists that bind RARs and inhibit AP-1 activation on the collagenase promoter but do not activate RAREs to induce gene transcription were less effective inhibitors; and (iv) RARgamma-selective retinoid agonists were more effective inhibitors of TPA-induced ODC activity than RARalpha-selective agonists. These results suggest that RARE activation has a more important role in inhibition of ODC activity than RXR activation or AP-1 inhibition and that RARgamma-selective agonists would be the most useful inhibitors of epithelial cell proliferation induced by tumor promoters. The natural retinoid all-trans-RA induced expression of transcription factor ZBP-89, which represses activation of the GC box in the ODC promoter by the transcription factor Sp1.

Development of brain injury in mice by Angiostrongylus cantonensis infection is associated with the induction of transcription factor NF-kappaB, nuclear protooncogenes, and protein tyrosine phosphorylation

Eosinophilic meningitis or meningoencephalitis caused by Angiostrongylus cantonensis is endemic to the Pacific area of Asia, especially Taiwan, Thailand, and Japan. Although eosinophilia is an important clinical manifestation of A. cantonensis infection, the role of eosinophils in the progress of the infection remains to be elucidated. In this experiment, we showed that A. cantonensis-caused eosinoplia and inflammation might lead to the induction of NF-kappaB and protooncogene expression via activation of the tyrosine phosphorylation signal pathway. After mice were infected daily with 30 third-stage larvae of A. cantonensis by oral adminstration for 6 weeks, no significant differences PKC-alpha, MEK-1, ERK-2, JNK, and p38 protein expression were found between the control and infected mice. However, the protein tyrosine phosphorylation levels, NF-kappaB, and iNOS protein products were significantly increased by 3.5-, 3.3-, and 6.3-fold, respectively, after 3 weeks of A. cantonensis infection. The same pattern was found for c-Myc, c-Jun, and c-Fos proteins, which were elevated by 3.2-, 2.3-, and 3.4-fold, respectively, compared to control animals after 3 weeks. The expression potency of these proteins started increasing in week 1, reaching maximal induction in week 3, and then declining in week 5 after A. cantonensis infection. Another consistent result was noted in the pathological observations, including eosinophilia, leukocyte infiltration, granulomatous reactions, and time responses in brain tissues of infected mice. These data suggest that the development of brain injury by eosinophlia of A. cantonensis infection is associated with NF-kappaB and/or nuclear protooncogenes expression, which is activated by the tyrosine phosphorylation pathway.

Tumor necrosis factor-alpha-induced activating protein-1 activity is modulated by nitric oxide-mediated protein kinase G activation

We tested the hypothesis that protein kinase (PK)G activation in response to nitric oxide ((*)NO) mediates tumor necrosis factor (TNF)-alpha-induced activation of the transcription factor activating protein-1 (AP-1) in pulmonary microvessel endothelial monolayers (PEM). The DNA-binding activity of AP-1 was assessed using the electrophoretic mobility shift assay. TNF treatment (1,000 U/ml) for 4 h induced a significant increase in DNA binding of AP-1. The effects of TNF were prevented by the superoxide radical scavenger superoxide dismutase (SOD) (100 U/ml), the (*)NO synthase inhibitor aminoguanidine (100 microM), the guanylate cyclase inhibitor ODQ (100 microM), and the PKG inhibitors KT5823 (1 microM) and 8-bromo-cyclic guanosine monophosphate (cGMP)-thioate (100 microM). Spermine-NO (1 microM) and L-arginine (400 microM) prevented the aminoguanidine-induced ablation of AP-1 activation in response to TNF. Phosphorylation of H-Arg-Lys-Ile-Ser-Ala-Ser-Glu-Phe-Asp-Arg-Pro-Leu-Arg-OH (BPDEtide), a specific substrate for PKG, measured the activity of cGMP-dependent protein kinase (PKG). TNF for 0.5 h induced an increase in PKG activity that was prevented by aminoguanidine, ODQ, KT5823, and 8-bromo-cGMP-thioate; however, SOD had no effect. The PKG agonist 8-bromo-cGMP (100 microM), when given alone, increased PKG activity but induced significant DNA-binding activity of AP-1 only when given in the ODQ + TNF Group. SIN-1 (1 mM, a peroxynitrite agonist) increased DNA-binding activity of AP-1. SOD prevented SIN-1-induced AP-1 activation, a response similar to that of the SOD + TNF Group. PEM were transfected with the chloramphenicol acetyltransferase (CAT) reporter plasmid pBLCAT2, which contains a regulation sequence responsive to AP-1. The pharmacologic profile of TNF-induced CAT activity was identical to TNF-induced DNA binding by AP-1. Thus, TNF-induced AP-1-dependent gene transcription is modulated by (*)NO-dependent mediated activation of PKG.

Protein kinase C epsilon is required for the induction of mitogen-activated protein kinase phosphatase-1 in lipopolysaccharide-stimulated macrophages

LPS induces in bone marrow macrophages the transient expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1). Because MKP-1 plays a crucial role in the attenuation of different MAPK cascades, we were interested in the characterization of the signaling mechanisms involved in the control of MKP-1 expression in LPS-stimulated macrophages. The induction of MKP-1 was blocked by genistein, a tyrosine kinase inhibitor, and by two different protein kinase C (PKC) inhibitors (GF109203X and calphostin C). We had previously shown that bone marrow macrophages express the isoforms PKC beta I, epsilon, and zeta. Of all these, only PKC beta I and epsilon are inhibited by GF109203X. The following arguments suggest that PKC epsilon is required selectively for the induction of MKP-1 by LPS. First, in macrophages exposed to prolonged treatment with PMA, MKP-1 induction by LPS correlates with the levels of expression of PKC epsilon but not with that of PKC beta I. Second, Gö6976, an inhibitor selective for conventional PKCs, including PKC beta I, does not alter MKP-1 induction by LPS. Last, antisense oligonucleotides that block the expression of PKC epsilon, but not those selective for PKC beta I or PKC zeta, inhibit MKP-1 induction and lead to an increase of extracellular-signal regulated kinase activity during the macrophage response to LPS. Finally, in macrophages stimulated with LPS we observed significant activation of PKC epsilon. In conclusion, our results demonstrate an important role for PKC epsilon in the induction of MKP-1 and the subsequent negative control of MAPK activity in macrophages.

The role of DOC-2/DAB2 protein phosphorylation in the inhibition of AP-1 activity. An underlying mechanism of its tumor-suppressive function in prostate cancer

DOC-2/DAB2, a novel phosphoprotein with signal-transducing capability, inhibits human prostatic cancer cells (Tseng, C.-P., Ely, B. D., Li, Y., Pong, R.-C., and Hsieh, J.-T. (1998) Endocrinology 139, 3542-3553). However, its mechanism of action is not understood completely. This study delineates the functional significance of DOC-2/DAB2 protein phosphorylation and demonstrates that in vivo activation of protein kinase C (PKC) by 12-O-tetradecanoylphorbol-13-acetate (TPA) induces DOC-2/DAB2 phosphorylation, including a serine residue at position 24. Mutation of Ser(24) to Ala reduced DOC-2/DAB2 phosphorylation by PKC. Using a synthetic Ser(24) peptide (APS(24)KKEKKKGSEKTD) or recombinant DOC-2/DAB2 as substrates, PKCbetaII, PKCgamma, and PKCdelta (but not casein kinase II) directly phosphorylated Ser(24) in vitro. This indicates that DOC-2/DAB2 is a PKC-specific substrate. Since expression of wild-type DOC-2/DAB2, but not the S24A mutant, inhibited TPA-induced AP-1 activity in prostatic epithelial cells, phosphorylation of Ser(24) appears to play a critical role in modulating TPA-induced AP-1 activity. Taken together, these data suggest that PKC-regulated phosphorylation of DOC-2/DAB2 protein may help its growth inhibitory function.

Macrophage Colony-Stimulating Factor Induces the Expression of Mitogen-Activated Protein Kinase Phosphatase-1 Through a Protein Kinase C-Dependent Pathway

M-CSF triggers the activation of extracellular signal-regulated protein kinases (ERK)-1/2. We show that inhibition of this pathway leads to the arrest of bone marrow macrophages at the G0/G1 phase of the cell cycle without inducing apoptosis. M-CSF induces the transient expression of mitogen-activated protein kinase phosphatase-1 (MKP-1), which correlates with the inactivation of ERK-1/2. Because the time course of ERK activation must be finely controlled to induce cell proliferation, we studied the mechanisms involved in the induction of MKP-1 by M-CSF. Activation of ERK-1/2 is not required for this event. Therefore, M-CSF activates ERK-1/2 and induces MKP-1 expression through different pathways. The use of two protein kinase C (PKC) inhibitors (GF109203X and calphostin C) revealed that M-CSF induces MKP-1 expression through a PKC-dependent pathway. We analyzed the expression of different PKC isoforms in bone marrow macrophages, and we only detected PKCβI, PKCε, and PKCζ. PKCζ is not inhibited by GF109203X/calphostin C. Of the other two isoforms, PKCε is the best candidate to mediate MKP-1 induction. Prolonged exposure to PMA slightly inhibits MKP-1 expression in response to M-CSF. In bone marrow macrophages, this treatment leads to a complete depletion of PKCβI, but only a partial down-regulation of PKCε. Moreover, no translocation of PKCβI or PKCζ from the cytosol to particulate fractions was detected in response to M-CSF, whereas PKCε was constitutively present at the membrane and underwent significant activation in M-CSF-stimulated macrophages. In conclusion, we remark the role of PKC, probably isoform ε, in the negative control of ERK-1/2 through the induction of their specific phosphatase.

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.

Angoline and chelerythrine, benzophenanthridine alkaloids that do not inhibit protein kinase C

Starting with an extract derived from the stem of Macleaya cordata (Papaveraceae) that was active in the process of inhibiting phorbol 12,13-dibutyrate binding to partially purified protein kinase C (PKC), the benzophenanthridine alkaloid angoline was isolated and identified. This discovery appeared in context, as a related benzophenanthridine alkaloid, chelerythrine, has been reported to mediate a variety of biological activities, including potent and selective inhibition of protein kinase C (PKC). However, in our studies, angoline was not observed to function as a potent inhibitor of PKC. Moreover, we were unable to confirm the reported inhibitory activity of chelerythrine. In a comprehensive series of studies performed with various PKC isozymes derived from a variety of mammalian species, neither chelerythrine nor angoline inhibited activity with high potency. To the contrary, chelerythrine stimulated PKC activity in the cytosolic fractions of rat and mouse brain in concentrations up to 100 microM. In addition, chelerythrine and angoline did not inhibit [3H]phorbol 12,13-dibutyrate binding to the regulatory domain of PKC at concentrations up to 40 microg/ml, and no significant alteration of PKC-alpha, -beta, or -gamma translocation was observed with human leukemia (HL-60) cells in culture. Further, chelerythrine did not inhibit 12-O-tetradecanoylphorbol 13-acetate-induced ornithine decarboxylase activity with cultured mouse 308 cells, but angoline was active in this capacity with an IC50 value of 1.0 microg/ml. A relatively large number of biological responses have been reported in studies conducted with chelerythrine, and alteration of PKC activity has been considered as a potential mechanism of action. In light of the current report, mechanisms independent of PKC inhibition should be considered as responsible for these effects.

Selenocompounds induce a redox modulation of protein kinase C in the cell, compartmentally independent from cytosolic glutathione: its role in inhibition of tumor promotion

Since selenite and other redox-active selenocompounds can modify protein kinase C (PKC) in the test tube, we have determined whether or not this redox regulation occurs inside the cell despite having high concentrations of GSH and the role of this regulation in the inhibition of tumor promotion. By using phorbol ester-promoted JB6 epidermal cell transformation assay, the concentrations of selenite, selenocystine, and selenodiglutathione which are optimal for chemopreventive activity were determined. At such concentrations (0.5 to 2 microM) in the cells treated with these agents, only a slight but transient decrease in PKC activity was observed when measured with a low (5 microM), but not with a high (100 microM) concentration of ATP. However, when the cells were serum starved or pretreated with 2-deoxyglucose, there was a pronounced but transient inactivation of PKC when assayed with both low and high concentrations of ATP. The inactivation was reversed in the cell by an endogenous mechanism or by treatment with thiol agents in the test tube. In spite of a substantial (90%) depletion of GSH in the cells by pretreatment with buthionine sulfoximine, there was no further increase in the redox modification of PKC by selenite as well as no change in the inhibitory effect of selenite on the phorbol ester-stimulated induction of ornithine decarboxylase, which is an intermediate marker related to cell transformation. While GSH is known to influence certain actions of selenium, it may not be required to mediate the effects of selenite tested in this study. The water-soluble cytosolic GSH did not interfere with the redox modification of PKC probably due to the shielding of the cysteine-rich region of the enzyme by a weak hydrophobic association with the membrane. Due to the presence of cofactors in the crude cell extracts, PKC was more sensitive to selenite than in the purified form and was inactivated by low concentrations of selenite (IC50 = 0.05 microM). This modification was reversed by thiol agents as well as by NADPH. A protein disulfide reductase, which can regenerate PKC, was present in the homogenate. Conceivably, selenite and other selenocompounds induce a redox modification of cellular PKC, compartmentally independent from the cytosolic GSH, but intimately connected to a NADPH-dependent reductase system, to mediate, at least in part, some of the cancer-preventive actions.

Stimulation of atrial natriuretic peptide receptor/guanylyl cyclase- A signaling pathway antagonizes the activation of protein kinase C-alpha in murine Leydig cells

Atrial natriuretic peptide (ANP) regulates diverse physiological responses by binding to its specific guanylyl cyclase-A receptor (Npra) which synthesizes the intracellular second messenger cGMP. To understand the molecular mechanisms of cellular signaling of ANP, we have studied its effect on the enzymatic activity of overexpressed protein kinase C (PKC) in murine Leydig tumor (MA-10) cells which were transfected with PKC-alpha cDNA. Treatments with 12-O-tetradecanoylphorbol-13-acetate (TPA), angiotensin II (ANG II) and endothelin-1 (ET-1) stimulated the PKC activity by 4-5-fold in PKC-alpha cDNA transfected MA-10 cells. The pretreatment of PKC-alpha transfected cells with ANP significantly inhibited the TPA-, ANG II- and ET-1-stimulated PKC activity. The agonist-stimulated PKC activity was also inhibited in the presence of 8-bromo-cGMP, however, cAMP had no effect on stimulatory PKC activity. The exposure of cells to Npra- antagonist A71915, which blocks the production of cGMP, significantly reduced the inhibitory effect of ANP on agonist-stimulated PKC activity and accumulation of intracellular cGMP in MA-10 cells. Similarly, inhibition of cGMP-dependent protein kinase by KT5823, restored the stimulatory levels of PKC activity in the presence of ANP. These results provide direct evidence that ANP antagonizes the agonist-stimulated PKC activity in MA-10 cells, involving the specific receptor Npra, its second messenger cGMP and cGMP-dependent protein kinase. Together, these findings implicate that ANP may act as a negative mediator of 'cross-talk' between PKC-alpha and Npra signaling pathway in MA-10 cells.

Expression of guanylyl cyclase-A/atrial natriuretic peptide receptor blocks the activation of protein kinase C in vascular smooth muscle cells. Role of cGMP and cGMP-dependent protein kinase

To understand the molecular mechanisms of cellular signaling of atrial natriuretic peptide (ANP), we have studied its effect on the enzymatic activity of endogenous and overexpressed protein kinase C (PKC) in rat thoracic aortic vascular smooth muscle (RTASM) cells. Angiotensin II (ANG II), endothelin-1 (ET-1), and 12-O-tetradecanoylphorbol 13-acetate (TPA) stimulated fourfold to fivefold PKC activity in PKC-alpha cDNA-transfected RTASM cells. However, pretreatment of these cells with ANP significantly inhibited the agonist-stimulated PKC activity in a dose-dependent manner. The inhibitory effect of ANP was more effective if cells were transfected with both PKC-alpha and guanylyl cyclase-A/atrial natriuretic peptide receptor (Npra) cDNAs. The agonist-stimulated PKC activity was also inhibited if RTASM cells were pretreated with cGMP analog 8-bromo-cGMP; however, the treatment of cells with a cAMP analog, dibutyryl-cAMP, did not show any discernible effect. The pretreatment of cells with Npra antagonist A-71915, significantly blocked the production of cGMP as well as the inhibitory effect of ANP on PKC activity. To further examine whether the antagonistic action of ANP and 8-bromo-cGMP on agonist-stimulated PKC activity were mediated through cGMP-dependent protein kinase (PKG), cells were treated with ANP or 8-bromo-cGMP and activators of PKC in the presence of KT-5823, a specific inhibitor of PKG. The treatment of cells with KT-5823 significantly attenuated the inhibitory effects of both ANP and 8-bromo-cGMP on agonist-stimulated PKC activity. The results from these studies provide strong evidence that ANP antagonizes the activation of PKC in RTASM cells, involving guanylyl cyclase-A receptor Npra and second messenger cGMP. Our data further support the notion that ANP acts as a negative mediator of signaling cross-talks between Npra and PKC in a cGMP-dependent manner, probably involving cGMP-dependent protein kinase in this process.

Phorbol 12-myristate 13-acetate inhibits epidermal growth factor signalling in human keratinocytes, leading to decreased ornithine decarboxylase activity

Several studies have suggested that murine and human keratinocytes respond differently to phorbol 12-myristate 13-acetate (PMA). Using an in vitro assay, we found that in contrast to its effect on murine skin, PMA did not induce ornithine decarboxylase (ODC) activity in human skin biopsies. To explore the signalling induced by PMA and to determine whether an in vitro culture system could be used to predict biological activity of retinoids in human keratinocytes, we studied a simian virus 40 (SV40)-transformed human keratinocyte cell line. Epidermal growth factor (EGF) stimulates ODC activity and increases the steady-state level of ODC mRNA in a dose- and time-dependent manner in these cells [Prystowsky, Clevenger and Zheng (1993) Exp. Dermatol. 2, 125-132]. In this report, 10(-10) M-10(-7) M PMA induced ODC mRNA and enzyme synthesis at 7 h, but did not significantly induce ODC activity and inhibited the EGF induction of ODC activity. To explore the mechanism whereby PMA interfered with EGF signalling, the effect of PMA on EGF binding to its cell-surface receptor was studied; acute treatment with PMA (within 7 h) decreased EGF binding to 41-57% of the baseline level. In contrast, chronic treatment with PMA (24 h) increased EGF binding to 156% of the baseline level and was associated with an increase in quantity of EGF receptor protein. Protein kinase C (PKC) activation correlated with the acute decrease in EGF binding following PMA treatment. In summary, PMA induced ODC mRNA and ODC enzyme synthesis, while steady-state levels of immunoprecipitable ODC enzyme protein and ODC activity were not increased, demonstrating possible increased turnover of ODC enzyme protein. Additionally, PMA inhibited the induction of ODC by EGF through decreased EGF binding, possibly mediated by PKC activation. Finally treatment of the keratinocytes with retinoids including etretinate, Ro13-7410, etarotene, Ro40-8757, 13-cisretinoic acid, and acitretin blocked the PMA induction of ODC mRNA, suggesting this in vitro model could be a valuable screening assay for predicting biological activity in humans.

Functional expression and characterization of the mouse epitope tag-protein kinase C isoforms, alpha, beta I, beta II, gamma, delta and epsilon

To simplify the detection of the exogenously produced protein kinase C (PKC) isoforms, we constructed the T7-Tag PKC alpha, betaI, betaII, gamma, delta and epsilon expression plasmids. T7-Tag sequences (AlaSerMetThrGlyGlyGlnGlnMetGlyArg) were inserted at the 5'-end of the translational initiation site. Transient transfection of T7-Tag PKC expression plasmids into CV-1 cells increased the levels of PKC protein, and PKC activity. T7-Tag-PKC epsilon, like native PKC epsilon, transactivated the transcription of a NF-kappa B reporter construct. These results indicate that the plasmids encoding T7-Tag PKC are functional and may be useful to define the role PKC isoforms play in cell proliferation, differentiation and tumor promotion.

Lack of 12-O-tetradecanoylphorbol-13-acetate responsiveness of ornithine decarboxylase introns which have AP-1 consensus sequences

The AP-1 consensus sequences (TGAGTCA) are the major 12-O-tetradecanoylphorbol113-acetate (TPA) responsive elements shared by several TPA inducible genes, such as c-sis, c-fos, c-myc, collagenase, stromelysin, hMTIIA and SV40. However, the role of AP-1 binding sites, which are present in the introns 3, 5, and 11 of ODC gene, in the regulation of TPA-induced ornithine decarboxylase (ODC) gene transcription are unknown. We determined the TPA responsiveness of the AP-1 sequences in the introns of ODC gene in CV-1 cells which induce ODC activity and mRNA in response to TPA treatment. ODC introns containing AP-1 sequences were inserted into the chloramphenicol acetyltransferase (CAT) reporter gene. Transient transfection of CV-1 cells with the intron-CAT constructs followed by TPA treatment did not induce CAT activity. However, when flanking regions of the AP-1 site in intron 3 were narrowed down to 74 bp, TPA induced CAT activity by 5- to 7-fold. The TPA-inducibility could be eliminated by mutation of the AP-1 site (TGAGTCA-->TGATGCCA or TGATGA) in 74 bp of intron 3. These results indicate that the AP-1 sequences in the intact ODC introns may not be responsive to TPA. The flanking sequences of the AP-1 site may be crucial to determine whether the AP-1 site is accessible to the TPA-induced transcriptional factor(s).

Regulation of rat ornithine decarboxylase promoter activity by binding of transcription factor Sp1

Ornithine decarboxylase (ODC) is the rate-limiting enzyme of polyamine biosynthesis. We investigated the transcriptional regulation of the rat ODC gene using transient expression assays. The 5'-flanking region (-1156 to +13) of the ODC gene was sufficient to mediate strong basal expression of a luciferase reporter. Sequences between -345 and -93 contributed to basal promoter activity. This region, containing five potential Sp1 binding sites, was analyzed by electrophoretic mobility shift assays. Three specific DNA-protein complexes were identified using H35 nuclear extracts and the -345/-93 ODC probe. Binding to all three was eliminated by competition with an oligonucleotide containing an Sp1 binding site, but not by a mutant Sp1 oligonucleotide. Preincubation with an antibody against Sp1 supershifted complexes associated with one or more of Sp1 binding sites 1-4 as well as with site 5. DNase I footprinting revealed two protected regions: PR-I (-92 to -130) and PR-II (-304 to -332). PR-I contains a putative binding site for Sp1 that was protected by recombinant Sp1 protein. Transfection studies in Schneider SL2 cells demonstrated that the ODC promoter is transactivated up to 350-fold by Sp1 and that this transactivation is dependent on the presence of Sp1 binding sites 1-4. Thus, although the ODC promoter binds multiple nuclear proteins, Sp1 or a related protein appears to be a critical determinant of ODC transcription, possibly through cooperative interactions between Sp1 and additional transcription factors.