Modulation of zeta-protein kinase C by cyclic AMP in PC12 cells occurs through phosphorylation by protein kinase A

β-Adrenergic receptors stimulate interleukin-6 production through Epac-dependent activation of PKCδ/p38 MAPK signalling in neonatal mouse cardiac fibroblasts

BACKGROUND AND PURPOSE IL-6 plays crucial roles in cardiac hypertrophy, cardiac fibrosis and heart failure. Activation of β-adrenoceptors induced IL-6 production in neonatal mouse cardiac fibroblasts (NMCFs) through a G(s) /adenylate cyclase/cAMP/p38 MAPK pathway but independent of PKA. However, how cAMP activates p38 MAPK is still not defined. In this study, we have assessed the role of the exchange protein directly activated by cAMP (Epac) and PKCδ in p38 MAPK activation and IL-6 production by stimulated by the β-adrenoceptor agonist isoprenaline in NMCFs. EXPERIMENTAL APPROACH The IL-6 concentration in cell culture supernatants was measured by ELISA. The levels of phosphorylated and total p38 MAPK and PKCδ were determined by Western blot analysis. The translocation of PKCδ was determined by immunoblotting the soluble and particulate fractions. Expression of Epac1 or PKCδ was knocked down by the corresponding, adenovirus-mediated, small hairpin RNA (shRNA). RESULTS In NMCFs, activation of β-adrenoceptors enhanced PKCδ phosphorylation and translocation. Furthermore, knock-down of the PKCδ isoform using an adenovirus-mediated shRNA markedly down-regulated IL-6 induction by NMCFs stimulated with isoprenaline. Moreover, knock-down of Epac1 confirmed that Epac1 was upstream of PKCδ in IL-6 production. Additionally, both Epac1 and PKCδ mediated the p38 MAPK activation induced by isoprenaline. CONCLUSIONS AND IMPLICATIONS β-Adrenoceptor agonists activate a cAMP/Epac/PKCδ/p38 MAPK pathway to produce IL-6 in NMCFs. This study identifies Epac as the link between cAMP and p38 MAPK signalling pathways and demonstrates that PKCδ can function as a novel downstream effector of this β-adrenoceptor/cAMP/Epac pathway.

Long QT2 mutation on the Kv11.1 ion channel inhibits current activity by ablating a protein kinase Cα consensus site

Mutations that inhibit Kv11.1 ion channel activity contribute to abnormalities of cardiac repolarization that can lead to long QT2 (LQT2) cardiac arrhythmias and sudden death. However, for most of these mutations, nothing is known about the molecular mechanism linking Kv11.1 malfunction to cardiac death. We have previously demonstrated that disease-related mutations that create consensus sites for kinases on ion channels can dramatically change ion channel activity. Here, we show that a LQT2-associated mutation can inhibit Kv11.1 ion channel activity by perturbing a consensus site for the Ser/Thr protein kinase C α (PKCα). We first reveal by mass spectrometry analysis that Ser890 of the Kv11.1 ion channel is phosphorylated. Then, we demonstrate by a phospho-detection immunoassay combined with genetic manipulation that PKCα phosphorylates Ser890. Furthermore, we show that Ser890 phosphorylation is associated with an increase in Kv11.1 membrane density with alteration of recovery from inactivation. In addition, a newly discovered and as yet uncharacterized LQT2-associated nonsynonymous single nucleotide polymorphism 2660 G→A within the human ether-á-go-go-related gene 1 coding sequence, which replaces arginine 887 with a histidine residue (R887H), strongly inhibits PKCα-dependent phosphorylation of residue Ser890 on Kv11.1, and ultimately inhibits surface expression and current density. Taken together, our data provide a functional link between this channel mutation and LQT2.

Par-4: a new activator of myosin phosphatase

We show here for the first time that the pro-apoptotic protein Par-4 binds to and activates myosin phosphatase (MP). During agonist stimulation, Par-4 facilitates ZIPK targeting and inhibitory phosphorylation of MP, however, phosphorylation of Par-4 is required for MP inhibition. Our model presents Par-4 as an amplifier of the MP activity range.

Myosin phosphatase (MP) is a key regulator of myosin light chain (LC20) phosphorylation, a process essential for motility, apoptosis, and smooth muscle contractility. Although MP inhibition is well studied, little is known about MP activation. We have recently demonstrated that prostate apoptosis response (Par)-4 modulates vascular smooth muscle contractility. Here, we test the hypothesis that Par-4 regulates MP activity directly. We show, by proximity ligation assays, surface plasmon resonance and coimmunoprecipitation, that Par-4 interacts with the targeting subunit of MP, MYPT1. Binding is mediated by the leucine zippers of MYPT1 and Par-4 and reduced by Par-4 phosphorylation. Overexpression of Par-4 leads to increased phosphatase activity of immunoprecipitated MP, whereas small interfering RNA knockdown of endogenous Par-4 significantly decreases MP activity and increases MYPT1 phosphorylation. LC20 phosphorylation assays demonstrate that overexpression of Par-4 reduces LC20 phosphorylation. In contrast, a phosphorylation site mutant, but not wild-type Par-4, interferes with zipper-interacting protein kinase (ZIPK)-mediated MP inhibition. We conclude from our results Par-4 operates through a “padlock” model in which binding of Par-4 to MYPT1 activates MP by blocking access to the inhibitory phosphorylation sites, and inhibitory phosphorylation of MYPT1 by ZIPK requires “unlocking” of Par-4 by phosphorylation and displacement of Par-4 from the MP complex.

Rapid direct excitation and long-lasting enhancement of NMDA response by group I metabotropic glutamate receptor activation of hypothalamic melanin-concentrating hormone neurons

The effect of group I metabotropic glutamate receptor (mGluR1 and mGluR5) activation on identified melanin-concentrating hormone (MCH) neurons was studied using patch-clamp recording in hypothalamic slices from green fluorescent protein-expressing transgenic mice. S-3,5-dihydroxyphenylglycine (DHPG), a selective group I mGluR agonist, depolarized MCH cells and increased spike frequency. The mGluR-mediated depolarization was not blocked with tetrodotoxin but was significantly reduced by replacement of extracellular Na+ with Tris, by Ni2+ or the Na+/Ca2+ exchanger blocker KB-R7943, or with BAPTA in the pipette, consistent with a mechanism based on activation of the Na+/Ca2+ exchanger. DHPG also decreased potassium currents. DHPG-induced depolarization was reduced by either mGluR1 or mGluR5 antagonists, suggesting involvement of both receptor subtypes. DHPG-induced depolarization desensitized; blockade of mGluR1 prevented the desensitization. Group I mGluR activation enhanced NMDA-evoked currents; this enhancement was remarkably long lasting and could be blocked by protein kinase A or C blockers. DHPG potentiated electrically evoked NMDA receptor-mediated postsynaptic currents, and mGluR5 antagonists blocked this action. Group I mGluRs increased spontaneous EPSCs in MCH neurons, possibly by stimulation of nearby mGluR-expressing hypocretin neurons. We found no tonic activation of mGluRs. However, electrical stimulation produced a slow inward current, which could be blocked by group I mGluR antagonists, suggesting high, but not low, levels of synaptically released glutamate activated mGluRs. Together, group I mGluRs increase MCH neuron activity by multiple presynaptic and postsynaptic mechanisms, suggesting mGluRs may therefore play a role in hypothalamic signaling relating to MCH neuron modulation of food intake and energy metabolism.

A G protein-associated ERK pathway is involved in LPS-induced proliferation and a PTK-associated p38 MAPK pathway is involved in LPS-induced differentiation in resting B cells

We, previously, showed that PKC-dependent ERK/p38 MAPK activation was inhibited by treating the resting B cell line 38B9 with an anti-MHC class II antibody. Further studies in this work demonstrated that PKA was involved in lipopolysaccharide (LPS)-induced proliferation of the cells, such that the PKC inhibitor activated PKA with concomitant LPS-induced proliferation but not IgG secretion. Consequently, the PKA inhibitor down-regulated ERK and p38 MAPK, and decreased cell proliferation. In addition, the treatment of LPS-stimulated 38B9 cells with PTK inhibitor reduced PKC- and PKA-dependent p38 MAPK activation and reduced the level of IgG secretion rather than the level of proliferation. However, the treatment of LPS-stimulated 38B9 cells with pertussis toxin (PTX), an inhibitor for the G protein-coupled receptor, inhibited the activation of both PKC- and PKA-dependent ERK and significantly reduced LPS-induced proliferation but not IgG secretion. Furthermore, ERK and p38 MAPK inhibitors reduced LPS-induced proliferation and differentiation, respectively, in 38B9 cells in a dose-dependent manner. These results suggest that LPS-induced proliferation of resting B cells is mainly mediated through a G protein-associated PKA/PKC-dependent ERK pathway and that a PTK-associated PKC/PKA-dependent p38 MAPK pathway is mostly involved in LPS-induced differentiation of the resting B cells.

Differential response of normal, dedifferentiated and transformed thyroid cell lines to cisplatin treatment

The effects of cisplatin (cisPt) on the extra cellular signal-regulated kinase (ERK) and the protein kinase B (PKB/Akt), known to play important roles in promoting cell survival and in down regulating apoptosis, were investigated in thyroid cell lines. The cytotoxic effect of cisPt was highest in normal PC-Cl3 cells, intermediate in dedifferentiated PC-E1A and PC-raf cells and lowest in fully transformed and tumorigenic PC-E1Araf cells. CisPt provoked ERK phosphorylation; such phosphorylation was unaltered by Gö6976, a conventional PKC inhibitor, whilst blocked by low doses (0.1 microM) or high doses (10 microM) of GF109203X, an inhibitor of all PKC isozymes, in PC-Cl3 and in PC-E1Araf cells, respectively. In PC-E1Araf, but not in PC-Cl3 cells, the cisPt-provoked ERK phosphorylation was also blocked by a myristoylated PKC-zeta pseudo substrate peptide (PS-zeta). The cytotoxic effects of cisPt increased when cells were pre-incubated with the mitogen-activated protein kinase (MEK) inhibitor PD98059. CisPt provoked the phosphorylation of PKB/Akt and this effect was blocked by LY294002, a PI3K inhibitor. In PC-Cl3 cells pre-incubated with LY294002 the effects of cisPt on ERK phosphorylation and cell mortality resulted unaffected; conversely, LY294002 reduced the ERK phosphorylation and increased cisPt cytotoxity of in PC-E1Araf cells. Furthermore, in PC-E1Araf cells pre-incubated with LY294002 and PS-zeta ERK phosphorylation was abolished and cisPt cytotoxicity was highest. Altogether results highlight a role for PKCs in the upstream regulation of ERK pathway facing the cell response to cisPt treatments. Understanding the mechanisms by which cells process cisPt provides important insights for designing more efficient platinum-based drugs.

Differential functions of PKC-delta and PKC-zeta in cisplatin response of normal and transformed thyroid cells

We investigated the effects of cisplatin (cisPt) in normal PC Cl3 and in transformed and tumourigenic PC E1Araf cells. cisPt cytotoxicity was higher in PC Cl3 than in PC E1Araf cells. In both cell lines, cisPt provoked the ERK1/2 phosphorylation; this was unaltered by Gö6976, a conventional PKC inhibitor, whilst it was blocked by low doses (0.1 microM) or high doses (10 microM) of GF109203X, an inhibitor of all PKC isozymes, in PC Cl3 and in PC E1Araf cells, respectively. In PC E1Araf, the cisPt-provoked ERK phosphorylation was also blocked by the use of a myristoylated PKC-zeta pseudosubstrate peptide. Conversely, in PC Cl3 the cisPt-provoked ERK phosphorylation was blocked by the use of rottlerin, a PKC-delta inhibitor. Results show that cisPt activates both PKC (the -delta and the -zeta isozymes in PC Cl3 and in PC E1Araf cells, respectively) and ERK in association with prolonged survival of thyroid cell lines.

Characterization of a Madin-Darby canine kidney cell line stably expressing TRPV5

To provide a cell model for studying specifically the regulation of Ca2+ entry by the epithelial calcium channel transient receptor potential-vanilloid-5 (TRPV5), green fluorescent protein (GFP)-tagged TRPV5 was expressed stably in Madin-Darby canine kidney type I (MDCK) cells. The localization of GFP-TRPV5 in this cell line showed an intracellular granular distribution. Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on plastic supports was threefold higher than in non-transfected cells. Moreover, apical Ca2+ uptake in GFP-TRPV5-MDCK cells cultured on permeable supports was eightfold higher than basolateral Ca2+ uptake, indicating that GFP-TRPV5 is expressed predominantly in the apical membrane. Patch-clamp analysis showed the presence of typical electrophysiological features of GFP-TRPV5, such as inwardly rectifying currents, inhibition by divalent cations and Ca2+-dependent inactivation. Moreover, the TRPV5 inhibitor ruthenium red completely inhibited Ca2+ uptake in GFP-TRPV5-MDCK cells, whereas Ca2+ uptake in non-transfected cells was not inhibited. The characterized GFP-TRPV5-MDCK cell line was used to assess the regulation of TRPV5. The protein kinase C activator phorbol 12-myristate 13-acetate and the cAMP-elevating compounds forskolin/3-isobutyl-1-methylxanthine, 8-Br-cAMP and PGE2 stimulated TRPV5 activity in GFP-TRPV5-MDCK cells by 121+/-7, 79+/-5, 55+/-4 and 61+/-7%, respectively. These compounds did not affect Ca2+ uptake in non-transfected cells. In conclusion, the GFP-TRPV5-MDCK cell line provides a model to specifically study the regulation of TRPV5 activity.

Coupling to protein kinases A and C of adenosine A2B receptors involved in the facilitation of noradrenaline release in the prostatic portion of rat vas deferens

In the prostatic portion of rat vas deferens, the non-selective adenosine receptor agonist NECA (0.1-30 microM), but not the A(2A) agonist CGS 21680 (0.001-10 microM), caused a facilitation of electrically evoked noradrenaline release (up to 43 +/- 4%), when inhibitory adenosine A(1) receptors were blocked. NECA-elicited facilitation of noradrenaline release was prevented by the A(2B) receptor-antagonist MRS 1754, enhanced by preventing cyclic-AMP degradation with rolipram, abolished by the protein kinase A inhibitors H-89, KT 5720 and cyclic-AMPS-Rp and attenuated by the protein kinase C inhibitors Ro 32-0432 and calphostin C. The adenosine uptake inhibitor NBTI also elicited a facilitation of noradrenaline release; an effect that was abolished by adenosine deaminase and attenuated by MRS 1754, by inhibitors of the extracellular nucleotide metabolism and by blockade of alpha(1)-adrenoceptors and P2X receptors with prazosin and NF023, respectively. It was concluded that adenosine A(2B) receptors are involved in a facilitation of noradrenaline release in the prostatic portion of rat vas deferens that can be activated by adenosine formed by extracellular catabolism of nucleotides. The receptors seem to be coupled to the adenylyl cyclase-protein kinase A pathway but activation of the protein kinase C by protein kinase A, may also contribute to the adenosine A(2B) receptor-mediated facilitation of noradrenaline release.

Modulation of mutagenicity by phosphorylation of mutagen-metabolizing enzymes

In this Minireview, we discuss our findings on phosphorylation of cytochromes P450 (CYP) and influence of this modification on metabolic toxification and/or detoxification of a variety of mutagens. We show that phosphorylation drastically interferes with the mutagenicity of several classes of compounds which are of high human relevance (cytostatic drugs of the cyclophosphamide type, aromatic amines/amides, and nitrosamines). We illustrate this by describing the consequences of the stimulation of protein kinase A (with the example of CYP2B1 and CYP2E1), stimulation of protein kinase C, and inhibition of protein phosphatases PP1 and PP2A (with the example of CYP1A1 and CYP1A2). We discuss a possible mechanism governing these phosphorylation events.

PKC-zeta is required for angiotensin II-induced activation of ERK and synthesis of C-FOS in MCF-7 cells

We examined the signalling pathways responsible for the Ang II induction of growth in MCF-7 human breast cancer cells. Ang II in MCF-7 cells induced: (a) the translocation from the cytosol to membrane and nucleus of atypical protein kinase C-zeta (PKC-zeta) but not of PKC-alpha, -delta, - epsilon and -eta; (b) the expression of c-fos mRNA and protein; (c) the phosphorylation of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). All these effects were due to the activation of the Ang II type I receptor (AT1) since they were blocked by the AT1 antagonist losartan. The Ang II-stimulated ERK1/2 phosphorylation was blocked by (a) high doses of staurosporine, inhibitor of PKC-zeta, and by a synthetic myristoylated peptide with sequences based on the endogenous PKC-zeta pseudosubstrate region (zeta-PS); (b) PD098059, a mitogen-activated protein kinase kinase inhibitor (MAPKK/MEK); and, moreover, (c) the inhibitors of phosphoinositide 3-kinases (PI3K), LY294002 and wortmannin, thus indicating that PI3K may act upstream of ERK1/2. The Ang II-evoked c-fos induction was blocked only by high doses of staurosporine and by zeta-PS whilst PD098059, LY294002 and wortmannin were ineffective, thus indicating that c-fos induction is not due to ERK1/2 activity. When the epidermal growth factor-receptor (EGFR) tyrosine kinase activity was inhibited by the use of its inhibitor AG1478, Ang II was still able to induce ERK1/2 phosphorylation and c-fos expression, therefore proving that the transactivation of EGFR was not required for these Ang II effects in MCF-7 cells. The previously reported proliferation of MCF-7 cells induced by Ang II was blocked by PD098059 and by wortmannin in a dose-dependent manner, thereby indicating that in MCF-7 cells the PI3K and ERK pathways mediate the mitogenic signalling of AT1. Our results suggest that in MCF-7 cells Ang II activates multiple signalling pathways involving PKC-zeta, PI3K and MAPK; of these pathways only PKC-zeta appears responsible for the induction of c-fos.

Protein kinase A and protein kinase C signaling pathway interaction in phenylethanolamine N-methyltransferase gene regulation

The protein kinase A (PKA) and protein kinase C (PKC) signaling pathways appear to interact in regulating phenylethanolamine N-methyltransferase (PNMT) promoter-driven gene transcription in PC12 cells. Forskolin treatment of cells transfected with the rat PNMT promoter-luciferase reporter gene construct pGL3RP893 increased promoter activity approximately two-fold whereas phorbol-12-myristate-13 acetate (PMA) treatment had no effect. However, simultaneous forskolin and PMA treatment synergistically activated the PNMT promoter approximately four-fold, suggesting that PKC stimulation requires prior induction of the PKA pathway. Consistent with this possibility the adenylate cyclase inhibitor MDL12,330A, and the PKA inhibitor H-89 prevented PNMT promoter stimulation by the combination of forskolin and PMA. PKA and PKC regulation seems to be mediated in part by Egr-1 and Sp1 through their consensus elements in the PNMT promoter. Forskolin and PMA treatment of PC12 cells increased Egr-1 protein and phosphorylated Egr-1/DNA-binding complex formation to the same extent but only increased phosphorylated Sp1/DNA binding complex formation without altering Sp1 protein levels. Mutation of the - 165 bp Egr-1 and - 48 bp Sp1 sites, respectively, attenuated and abolished combined forskolin and PMA-mediated promoter activation. PNMT promoter analysis further showed that synergistic stimulation by PKA and PKC involves DNA sequences between - 442 and - 392 bp, and potentially a GCM binding element lying within this region.

Angiotensin II AT1 receptor stimulates Na+ -K+ATPase activity through a pathway involving PKC-zeta in rat thyroid cells

Angiotensin II (Ang II) receptor subtype 1, AT1, is expressed by the rat thyroid. A relationship between thyroid function and several components of the renin-angiotensin system has also been established, but the Ang II cellular effects in thyrocytes and its transduction signalling remain undefined. The aim of the present paper was to investigate the modulation of the activity of the Na(+)-K(+)ATPase by Ang II and its intracellular transduction pathway in PC-Cl3 cells, an established epithelial cell line derived from rat thyroid. Here we have demonstrated, by RT-PCR analysis, the expression of mRNA for the Ang II AT1 receptor in PC-Cl3 cells; mRNA for the Ang II AT2 receptor was not detected. Ang II was not able to affect the intracellular Ca(2+) concentration in fura-2-loaded cells, but it stimulated the translocation from the cytosol to the plasma membrane of atypical protein kinase C-zeta (PKC-zeta) and -iota (PKC-) isoforms with subsequent phosphorylation of the extracellular signal-regulated kinases 1 and 2 (ERK1 and 2). Translocated atypical PKCs displayed temporally different activations, the activation of PKC-zeta being the fastest. PC-Cl3 cells stimulated with increasing Ang II concentrations showed dose- and time-dependent activation of the Na(+)-K(+)ATPase activity, which paralleled the PKC-zeta translocation time course. Na(+)-K(+)ATPase activity modulation was dependent on PKC activation since the PKC antagonist staurosporine abolished the stimulatory effect of Ang II. The inhibition of the ERK kinases 1 and 2 (MEK1 and 2) by PD098059 (2'-amino-3'-methoxyflavone) failed to block the effect of Ang II on the Na(+)-K(+)ATPase activity. In conclusion, our results suggest that Ang II modulates Na(+)-K(+)ATPase activity in PC-Cl3 cells through the AT1 receptor via activation of atypical PKC-zeta while the Ang II-activated PKC- appears to have other as yet unknown functions.

Regulation of catalase enzyme activity by cell signaling molecules

Mitogenic cell proliferation requires a rapid and transient H2O2 generation, which is blocked by catalase or PKA activators. Previously, we observed that anemic HIV(+) individuals expressed acidic pIs of catalase in RBC with significantly high activities [Mol Cell Biochem 165: 77-81, 1996]. These findings led us to hypothesize that cell signaling molecules regulate catalase to control cell mitogenesis. To test the hypothesis, we determined (i) whether RBC counts correlate with their catalase activities, (ii) whether protein kinases and phosphatases alter catalase activity in vitro, and (iii) whether protein kinase activators increase catalase activity to suppress proliferation of cultured cells. The results indicated that RBC counts inversely correlated with RBC catalase activities in both HIV(+) (r: -0.6769, r2: 0.4582, n: 69 male, p < 0.0001) and HIV(-) (r: -0.3827, r2: 0.1464, n: 177 male, p < 0.0001) populations. Catalytic PKA, PKC and Casein Kinase II, but none of PKG, Ca2+/calmodulin kinase II and p34cdc/cyclinB, rapidly elevated catalase activity in vitro by up to 2-fold. Whereas a major CAT subunit (60 kDa) showed immunoreactive phosphoserine and phosphothreonine, the kinases- and gamma-32P-ATP-dependent phosphorylation occurred with a minor component (110 kDa). Among PKC isozymes examined, PKCzeta was the most effective modulator followed by PKCgamma, and protein phosphatase 1gamma and 2A decreased the catalase activity. PKA and PKCzeta activators of forskolin and okadaic acid increased catalase activity and 110 kDa expression in NIH3T3 cells up to 2.4-fold and suppressed the cell growth, showing an inverse correlation of the indices (r: -0.9286, r2: 0.8622, n: 18, p < 0.0001). Taken together, these results suggest for the first time that catalase is under the regulation of cell signaling molecules and capable of modulating mitogenic cell proliferation.

Differential transcriptional regulation by the alpha- and gamma-catalytic subunit isoforms of cAMP-dependent protein kinase

The C gamma and C alpha isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to C alpha, C gamma has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, C gamma-mediated gene transcription regulation was compared with that of C alpha in four cell lines using transient transfection/dual luciferase assays. As compared to C gamma, C alpha more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human alpha-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGH alpha-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, C gamma, but not C alpha, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGH alpha-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by C gamma. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both C alpha and C gamma (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK.KD) did not. These results suggest that both C alpha and C gamma can activate CRE-responsive genes; however, C alpha does so with better efficiency than C gamma. In contrast to C alpha, C gamma activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.

Activation of protein kinase A and atypical protein kinase C by A(2A) adenosine receptors antagonizes apoptosis due to serum deprivation in PC12 cells

We found in the present study that stimulation of A(2A) adenosine receptors (A(2A)-R) prevents apoptosis in PC12 cells. This A(2A)-protective effect was blocked by protein kinase A (PKA) inhibitors and was not observed in a PKA-deficient PC12 variant. Stimulation of PKA also prevented apoptosis, suggesting that PKA is required for the protective effect of A(2A)-R. A general PKC inhibitor, but not down-regulation of conventional and novel PKCs, readily blocked the protective effect of A(2A)-R stimulation and PKA activation, suggesting that atypical PKCs (aPKCs) serve a critical role downstream of PKA. Consistent with this hypothesis, stimulation of A(2A)-R or PKA enhanced nuclear aPKC activity. In addition, the A(2A)-protective effect was blocked by a specific inhibitor of one aPKC, PKCzeta, whereas overexpression of a dominant-positive PKCzeta enhanced survival. In contrast, inhibitors of MAP kinase and phosphatidylinositol 3-kinase did not modulate the A(2A)-protective effect. Dominant-negative Akt also did not alter the A(2A)-protective effect, whereas it significantly reduced the protective action of nerve growth factor. Collectively, these data suggest that aPKCs can function downstream of PKA to mediate the A(2A)-R-promoted survival of PC12 cells. Furthermore, the results indicate that different extracellular stimuli can employ distinct signaling pathways to protect against apoptosis induced by the same insult.

Modulation of CREB and NF-kappaB signal transduction by cannabinol in activated thymocytes

Cannabinoid compounds inhibit the cAMP signalling cascade in leukocytes. One of these compounds, cannabinol (CBN) has been shown to inhibit interleukin-2 (IL-2) expression and the activation of cAMP response element binding protein (CREB) and nuclear factor for immunoglobulin kappa chain in B cells (NF-kappaB) following phorbol-12-myristate-13 acetate (PMA) plus ionomycin (Io) treatment of thymocytes. Therefore, the objective of the present studies was to determine the role of cAMP and protein kinase A (PKA) in the CBN-mediated inhibition of IL-2, CREB, and NF-kappaB in PMA/Io-activated thymocytes. The inhibition of CREB/ATF-1 phosphorylation, or cAMP response element (CRE) or kappaB DNA binding activity produced by CBN in PMA/Io-activated thymocytes, could not be reversed by DBcAMP costimulation. Furthermore, DBcAMP failed to reverse the concentration-dependent inhibition of IL-2 protein secretion by CBN. Pretreatment of thymocytes with H89 produced a modest inhibition of PMA/Io-induced CREB/ATF-1 phosphorylation and CRE DNA binding activity but H89 had no effect on protein binding to a kappaB motif. Additionally, H89 modestly inhibited PMA/Io-induced IL-2 secretion. In light of the modest involvement of the cAMP pathway in CBN-mediated inhibition of CREB and IL-2 in PMA/Io-activated thymocytes, PD098059 (PD), the MEK inhibitor, was utilized to determine the role of ERK MAP kinases in thymocytes. ERKs play a critical role in IL-2 production but not for CREB phsophorylation. Collectively, these findings suggest that CBN may modulate several signalling pathways in activated T cells.

MEK5, a new target of the atypical protein kinase C isoforms in mitogenic signaling

The MEK5-extracellular signal-regulated kinase (ERK5) tandem is a novel mitogen-activated protein kinase cassette critically involved in mitogenic activation by the epidermal growth factor (EGF). The atypical protein kinase C isoforms (aPKCs) have been shown to be required for cell growth and proliferation and have been reported to interact with the adapter protein p62 through a short stretch of acidic amino acids termed the aPKC interaction domain. This region is also present in MEK5, suggesting that it may be an aPKC-binding partner. Here we demonstrate that the aPKCs interact in an EGF-inducible manner with MEK5 and that this interaction is required and sufficient for the activation of MEK5 in response to EGF. Consistent with the role of the aPKCs in the MEK5-ERK5 pathway, we show that zetaPKC and lambda/iotaPKC activate the Jun promoter through the MEF2C element, a well-established target of ERK5. From all these results, we conclude that MEK5 is a critical target of the aPKCs during mitogenic signaling.

Differential redistribution of protein kinase C isoforms by cyclic AMP in HL60 cells

In this study we have analyzed the distribution of protein kinase C isoforms in cytosol, membrane, and nucleus in HL60 cells. Furthermore, we have studied the redistribution of these isoforms after cyclic AMP treatment. Protein kinase C localization and cyclic AMP-induced translocation was demonstrated by Western blot analysis. Cytosol, membrane and nucleus in HL60 cells expressed the abundance of protein kinase C alpha, betaI, betaII, delta, lambda, and zeta isoforms. After cyclic AMP treatment, the amount of protein kinase C betaI and zeta increased only in the nucleus, while protein kinase C delta increased in the three fractions tested. These effects were dependent on the cyclic AMP concentration and duration of action. Our results suggest the existence of cross-talk between the cyclic AMP system and protein kinase C in HL60 cells. Taking into account the processes regulated by protein kinase C, these findings also suggest that cyclic AMP plays a regulatory role in various cellular responses in HL60 cells, such as differentiation and gene expression. The increase observed in PKC delta was due to cyclic AMP-dependent protein kinase C activation, and the synthesis of enzyme was probably activated by the nucleotide.

Roles of atypical protein kinase C in lysophosphatidic acid-induced type II adenylyl cyclase activation in RAW 264.7 macrophages

1 Lysophosphatidic acid (LPA) has been widely studied as a naturally occurring and multifunctional phospholipid messenger in diverse tissue and cell types and shown to inhibit adenylyl cyclase (AC) by a G protein-mediated mechanism. 2 In type II AC-expressing mouse RAW 264.7 macrophages, we showed that LPA at 3-50 microM increased cyclic AMP formation in a concentration-dependent manner, the effect being additive with that of forskolin or cholera toxin, and synergistic with that of prostaglandin E1 (PGE1) or isoproterenol. 3 The potentiation effect of LPA was unaffected by the removal of serum or pertussis toxin treatment. 4 Both colchicine and cytochalasin B potentiated the cyclic AMP response to PGE1, the effect being additive to that of LPA. 5 On studying the regulation of type II AC by protein kinase C (PKC), phorbol 12-myristate-13 acetate (PMA) potentiated the PGE1-elicited cyclic AMP response, this effect being non-additive to that of LPA, suggesting that PKC activation was the common mechanism involved in AC potentiation by LPA and PMA. 6 PKC inhibitor Ro 31-8220, but not Go 6976, significantly inhibited the LPA-induced cyclic AMP potentiation. 7 The potentiation effect of LPA was unaffected by long-term treatment with PMA, which resulted in the down-regulation of PKCalpha, betaI, betaII and PKCdelta, but not PKCepsilon, mu, lambda and zeta. 8 By in situ kinase assay, we found a marked increase in atypical PKC activity after LPA treatment. 9 Taken together, we conclude that LPA can elicit a unique signalling cascade in RAW 264.7 macrophages and increase type II AC activity via the activation of atypical PKC.

Critical targets of protein kinase C in differentiation of tumour cells

The ultimate target of pharmacological research is to find new drugs for treating human diseases such as cancer. Agents causing differentiation and thus growth arrest should be particularly useful in this regard. A potential target for such anticancer therapy is the enzyme family protein kinase C (PKC), which is involved in the transduction of signals for cell proliferation, differentiation, and apoptosis. Our recent work showing the induction of differentiation in melanoma cells by an activator of one PKC isoform, PKCdelta, touches on several important areas of investigation, which will form the basis of this review: the role of individual isoforms of PKC, their downstream targets and their specific substrates, the mechanism of activation of specific genes involved in the differentiation process, and the molecular basis for the morphological changes associated with differentiation. The central role that PKC plays in these processes points to the need for a greater understanding of the signalling pathways utilized by individual isoforms of this family of enzymes.

Phospholipid signalling pathways in Trypanosoma cruzi growth control

The role of phospholipids (PLs) in the signal transduction pathways that are activated by a mitogenic stimulus (foetal calf serum) in Trypanosoma cruzi epimastigotes (EPI) was investigated. Only phosphatidylinositol-bis-phosphate was significantly altered in this process. Other phosphoinositides, including major PLs such as phosphatidylcholine and phosphatidylethanolamine, were unaltered. Lysophosphatidic acid, reported to be the primary active substance in effects of serum in other systems, had no mitogenic activity when added to epimastigote cultures. Involvement of phosphoinositide-specific phospholipase C was established using the inhibitors ET-18-OCH3 and U73122, which prevented phosphatidylinositol-bis-phosphate hydrolysis; the latter compound decreased T. cruz proliferation. The intracellular signalling downstream to the phospholipase C was mediated by Ca2+/PL-dependent protein kinase and Ca2+/calmodulin-dependent protein kinase II, judging from the marked decrease in replication caused by the specific inhibitors staurosporine, derythro-sphingosine and KN-93. Previous reports have demonstrated a dual control of cell growth in EPI, whose proliferation is stimulated by the activation of a phospholipase C system and inhibited by activation of an adenylate cyclase system. Investigating this 'cross-talk' phenomenon, we observed that an increase in intracellular cAMP inhibited growth mediated by a cAMP-dependent protein kinase, but did not cause PL alterations, and also did not prevent the effect of serum on them.

Cyclic AMP-dependent activation of the proenkephalin gene requires phosphorylation of CREB at serine-133 and a Src-related kinase

The transcription factor CREB [cyclic AMP response element (CRE)-binding protein] is activated by several kinase pathways on phosphorylation of serine-133. Phosphorylation of CREB at serine-133 is required for the induction of target gene expression. The proenkephalin gene is a target of cyclic AMP-dependent agonists like forskolin, and its expression is driven by the enhancer element CRE-2. It has been shown that CREB binds CRE-2 in extracts from striatum and hypothalamus. However, these studies did not show a functional requirement for CREB serine-133 phosphorylation in CRE-2 function. We demonstrate that CREB binds CRE-2 in primary astrocyte cultures and that transcriptional activation of CRE-2 requires CREB phosphorylation at serine-133. In addition, it has recently been shown that, at least in some contexts, CREB phosphorylation is not sufficient to activate target gene expression and that another intracellular signal seems to be required. Therefore, we also sought to determine if another signaling event, in addition to CREB phosphorylation, might be involved in cyclic AMP-mediated induction of the proenkephalin gene. We have found that the inhibition of src-related nonreceptor tyrosine kinases blocks forskolin-induced proenkephalin gene expression without having any effect on serine-133-phosphorylated CREB levels and that constitutively activated src kinase can activate the proenkephalin promoter.

Identification of Src as a novel atypical protein kinase C-interacting protein

Atypical protein kinase C-zeta (PKC-zeta) participates in nerve growth factor (NGF) signaling and is required for NGF-induced differentiation of PC12 cells. The biological activity of PKC-zeta is likely mediated by interaction of PKC-zeta with specific proteins. Affinity column chromatography employing the PKC-zeta regulatory domain coupled to glutathione-agarose was used to search for proteins that bound PKC-zeta. Two proteins (59/60 kDa) were recovered from NGF-stimulated PC12 cell lysates that bound the matrix. Western blot analysis of pooled column fractions identified these proteins as tubulin and src, respectively. Using purified preparations of src and tubulin, PKC-zeta was shown to interact with both proteins using blot overlay. To demonstrate a functional interaction in vivo, PC12 cells expressing a temperature-sensitive v-src were shifted to the permissive temperature (37 degrees C), followed by immunoprecipitation. At the permissive temperature where src was active, PKC-zeta was tyrosine phosphorylated and coassociated with src in vivo; by comparison, at the nonpermissive temperature (40 degrees C) PKC-zeta was not tyrosine phosphorylated. Taken together, these findings support a novel role for the interaction of src and atypical PKC in vivo, which is dependent upon the activity of src and the tyrosine phosphorylation state of PKC-zeta.

Phosphorylation of proteins in chick ciliary ganglion under conditions that induce long-lasting changes in synaptic transmission: phosphoprotein targets for nitric oxide action

Production of nitric oxide and the activation of protein kinases are required for long-term potentiation of synaptic transmission at the giant synapses in chicken ciliary ganglion. In the present study, we investigated the ability of nitric oxide to regulate the phosphorylation of endogenous proteins under conditions that induced long-term potentiation in intact ciliary ganglion and the protein kinases responsible for the phosphorylation of these proteins in lysed ciliary ganglion. Using Calcium Green-1 we showed that the nitric oxide donor sodium nitroprusside did not change the intraterminal Ca2+ dynamics in ciliary ganglion. Two dimensional phosphopeptide analysis of 32Pi-labelled intact ciliary ganglion showed that the sodium nitroprusside (300 microM) increased the phosphorylation of several phosphopeptides (P50a, P50b and P41) derived from proteins at 50,000 and 41,000 mol. wts which we have called nitric oxide-responsive phosphoproteins. A similar stimulation of phosphorylation was achieved by 8-bromo-cyclic AMP (100 microM), which also induced long-term potentiation, but not by phorbol dibutyrate (2 microM) that does not induce long-term potentiation in ciliary ganglion. When subcellular fractions from lysed ciliary ganglion were labelled in vitro by [gamma-32P]ATP in the presence of purified cGMP-dependent, cAMP-dependent or Ca2+-phospholipid-dependent protein kinases, we identified cyclic GMP-dependent protein kinase substrates that gave rise to phosphopeptides co-migrating with P50a, P50b and P41 from 32Pi-labelled intact ciliary ganglion. P50a and P41 were derived from soluble proteins while P50b was derived from a membrane-associated protein. The proteins giving rise to P50a, P50b and P41 were also substrates for cyclic AMP-dependent protein kinase, but not for calcium and phospholipid-dependent protein kinase in vitro, suggesting that nitric oxide-responsive phosphoproteins are convergence points in information processing in vivo and their phosphorylation might represent an important mechanism in nitric oxide-mediated synaptic plasticity in ciliary ganglion.

Cleavage of zetaPKC but not lambda/iotaPKC by caspase-3 during UV-induced apoptosis

The stimulation of caspases is a critical event in apoptotic cell death. Several kinases critically involved in cell proliferation pathways have been shown to be cleaved by caspase-mediated mechanisms. Thus, the degradation of delta protein kinase C (PKC) and MEKK-1 by caspase-3 generates activated fragments corresponding to their catalytic domains, consistent with the observations that both enzymes are important for apoptosis. In contrast, other kinases reported to have anti-apoptotic properties, such as Raf-1 and Akt, are inactivated by proteolytic degradation by the caspase system. Since the atypical PKCs have been shown to play critical roles in cell survival, in the study reported here we have addressed the potential degradation of these PKCs by the caspase system in UV-irradiated HeLa cells. Herein we show that although zetaPKC and lambda/iotaPKC are both inhibited in UV-treated cells, only zetaPKC but not lambda/iotaPKC is cleaved by a caspase-mediated process. This cleavage generates a fragment that corresponds to its catalytic domain that is enzymatically inactive. The sequence where caspase-3 cleaves zetaPKC was mapped, and a mutant resistant to degradation was shown to protect cells from apoptosis more efficiently than the wild-type enzyme.

Activation of mGluRII induces LTD via activation of protein kinase A and protein kinase C in the dentate gyrus of the hippocampus in vitro

The involvement of metabotropic glutamate receptor group II (mGluRII) in the induction of long-term depression (LTD) was investigated in the medial perforant path of the rat dentate gyrus, a region with a very high density of mGluRII. Perfusion of either of two potent mGluRII agonists, (2S,1R,2R,3R)-2-(2S, 1'R, 2'R, 3'R)-2 (2' 3'-dicarboxycyclopropyl)glycine (DCG-IV) or (+)-2- aminobicyclo[3.1.0]hexane-2-6-dicarboxylic acid (LY354740) induced a reversible inhibition of the field EPSP followed, upon washout of the agonist, by LTD. The reversible inhibition was associated with a change in paired pulse depression, indicating an underlying presynaptic reduction in the probability of transmitter release, whereas the LTD was not associated with a change in paired pulse depression, indicating either a presynaptic reduction in the number of active release sites, or a postsynaptic change. Further evidence that the DCG-IV-induced LTD was generated by activation of mGluRII was the finding that the mGluRII antagonist (RS)-alpha-methylserine-O-phosphate monophenylphosphoryl ester (MSOPPE) prevented the induction of the LTD induced by DCG-IV. The DCG-IV-induced LTD showed mutual occlusion with LFS-induced LTD. The generation of the agonist-induced LTD required, in part, activation of N-methyl-D-aspartate receptors (NMDAR), as LTD induction was partially blocked in the presence of the NMDAR antagonist D-2-amino-5-phosphonopentanoate (AP5). Evidence for involvement of protein kinase C (PKC) and protein kinase (PKA) in the induction of LTD by activation of mGluRII was obtained by showing an inhibition of the DCG-IV-induced LTD by the PKC inhibitors Ro-31-8220 and bisindolylmaleimide I, and also by the PKA inhibitor H-89. The study demonstrates that activation of mGluRII induces LTD via activation the PKA and PKC pathways in the medial perforant path of the dentate gyrus.

Differential modulation of CYP2E1 activity by cAMP-dependent protein kinase upon Ser129 replacement

Many toxic compounds are activated by cytochrome P450 (CYP) 2E1 to reactive metabolites, which represents a potential hazard for cellular homeostasis. Therefore knowledge about CYP2E1 regulation could be of great biological importance. It has been shown that CYP2E1 is controlled transcriptionally and post-translationally by phosphorylation. In the present study we investigated the role of serine-129 (Ser129) in the protein kinase A (PKA) recognition sequence motif Arg-Arg-Phe-Ser129. To gain further insights into the possible relevance of Ser129 for CYP2E1 function, Ser129 was replaced by alanine (Ala) or glycine (Gly) by site-directed mutations of the cDNA coding for CYP2E1. The mutant cDNAs were transfected into Chinese hamster lung fibroblast V79 cells. Despite the mutation in the PKA phosphorylation motif, all strains produced catalytically active CYP2E1. However, there was a marked change in the substrate preference: The Gly129-containing strains hydroxylated p-nitrophenol (PNP) to a markedly higher extent than the wild-type cDNA-containing cells, while they demethylated N-nitrosodimethylamine (NDMA) to a markedly lower extent than the wild-type cells. All the strains activated NDMA to mutagenic products. Treatment with the membrane-permeating cAMP derivative db-cAMP reduced markedly both the PNP hydroxylase and the NDMA demethylase activities as well as the mutation frequency induced by NDMA in the Ser129-containing strain. This decrease in activity was not accompanied by a decrease in CYP2E1 content. In addition, the catalytic activities of CYP2E1 were decreased in microsomes from rat hepatocytes treated with db-cAMP. Also in this case, the decrease in activities was not accompanied by a decrease in enzyme protein. These findings argue that involvement of Ser129 and its phosphorylation is not in determining CYP2E1 protein level, but rather in controlling its catalytic activity. In contrast, in the strains containing Ala129 or Gly129, treatment with db-cAMP caused a marked increase in both PNP hydroxylase and NDMA demethylase. In these strains a similar db-cAMP-mediated increase was also observed in the mutation frequency, resulting from the treatment with the promutagen NDMA, which is activated by CYP2E1. Our results show that CYP2E1 in V79 cells responds in two separate ways to db-cAMP exposure depending on the amino acid residue present in the PKA recognition sequence. The enzyme is committed to a negative regulation by db-cAMP if Ser129 is the target amino acid for PKA, leading to a decrease in the metabolic activation to mutagenic and carcinogenic species. On the other hand, Ala129 or Gly129 substitution directed CYP2E1 toward a positive regulation by increasing its catalytic activities and metabolic activation to mutagenic intermediates in the presence of db-cAMP. We also obtained evidence that cAMP-mediated downregulation of wild-type (Ser129) CYP2E1 was not accompanied by its destruction but instead by its stabilization, which shows that Ser129 is not involved in CYP2E1 degradation but dictates requirements for its specific activities.

The cAMP-dependent protein kinase A and protein kinase C-beta pathways synergistically interact to activate HIV-1 transcription in latently infected cells of monocyte/macrophage lineage

The HIV-1 long terminal repeat (LTR) responds to a variety of cellular signal transduction pathways. We demonstrate that the cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC) signaling pathways synergize to increase HIV-1 LTR-mediated transcription and viral replication in a latently infected promonocytic cell line (U1). The LTR-mediated synergy induced by cholera toxin (Ctx), a potent activator of the cAMP-dependent PKA pathway, and the PKC activator phorbol 12-myristate 13-acetate (PMA) was abrogated by a PKC-beta-specific inhibitor (LY333531). In contrast, the LTR-mediated synergy induced by Ctx and TNF alpha was not affected by LY333531. The synergy induced by Ctx and TNF alpha was also abrogated by mutation of the cAMP-responsive downstream sequence elements (DSE) in the 5' untranslated leader region, whereas the DSE mutations did not affect the synergy induced by Ctx and PMA. These distinctions indicate that Ctx cooperates differently with TNF alpha and PMA to activate the HIV-1 LTR. Ctx and PMA synergistically activated AP-1- and NF-kappa B-dependent transcription, even though no cooperative binding of AP-1 or NF-kappa B was observed in gel shift assays. An extensive mutational analysis of the HIV-1 LTR that included the NF-kappa B and AP-1 binding sites revealed no distinct cis-acting element or region within the HIV-1 LTR that was required for the transcriptional synergy. Ctx and PMA also synergistically interact to activate the HTLV-1 LTR. These results indicate that the transcriptional synergy elicited by Ctx and PMA targets multiple functional elements and promoters, requires a cooperative interaction between the PKA and PKC-beta pathways, and differs mechanistically from the transcriptional synergy induced by Ctx and TNF alpha.

Localization of atypical protein kinase C isoforms into lysosome-targeted endosomes through interaction with p62

An increasing number of independent studies indicate that the atypical protein kinase C (PKC) isoforms (aPKCs) are critically involved in the control of cell proliferation and survival. The aPKCs are targets of important lipid mediators such as ceramide and the products of the PI 3-kinase. In addition, the aPKCs have been shown to interact with Ras and with two novel proteins, LIP (lambda-interacting protein; a selective activator of lambda/iotaPKC) and the product of par-4 (a gene induced during apoptosis), which is an inhibitor of both lambda/iotaPKC and zetaPKC. LIP and Par-4 interact with the zinc finger domain of the aPKCs where the lipid mediators have been shown to bind. Here we report the identification of p62, a previously described phosphotyrosine-independent p56(lck) SH2-interacting protein, as a molecule that interacts potently with the V1 domain of lambda/iotaPKC and, albeit with lower affinity, with zetaPKC. We also show in this study that ectopically expressed p62 colocalizes perfectly with both lambda/iotaPKC and zetaPKC. Interestingly, the endogenous p62, like the ectopically expressed protein, displays a punctate vesicular pattern and clearly colocalizes with endogenous lambda/iotaPKC and endogenous zetaPKC. P62 colocalizes with Rab7 and partially with lamp-1 and limp-II as well as with the epidermal growth factor (EGF) receptor in activated cells, but not with Rab5 or the transferrin receptor. Of functional relevance, expression of dominant negative lambda/iotaPKC, but not of the wild-type enzyme, severely impairs the endocytic membrane transport of the EGF receptor with no effect on the transferrin receptor. These findings strongly suggest that the aPKCs are anchored by p62 in the lysosome-targeted endosomal compartment, which seems critical for the control of the growth factor receptor trafficking. This is particularly relevant in light of the role played by the aPKCs in mitogenic cell signaling events.

Ceramide-induced translocation of protein kinase C zeta in primary cultures of astrocytes

The present research was undertaken to study the possible involvement of the atypical protein kinase C (PKC) zeta in ceramide signal transduction in primary cultures of rat astrocytes. As shown by Western blot analysis, translocation of immunoreactive PKCzeta to the particulate fraction occurred upon exposure of astrocytes to cell-permeable ceramide analogs or to exogenous sphingomyelinase. The particulate fraction may correspond to a perinuclear area, as indicated by immunocytochemical techniques. Furthermore, treatment of cells with N-octanoylsphingosine led to an increased phosphorylation of PKCzeta. Results thus show that stimulation of PKCzeta may be one of the intracellular events triggered by activation of the sphingomyelin pathway.

Positioning atypical protein kinase C isoforms in the UV-induced apoptotic signaling cascade

Recent studies have documented the involvement of the atypical protein kinase C (aPKC) isoforms in important cellular functions such as cell proliferation and survival. Exposure of cells to a genotoxic stimulus that induces apoptosis, such as UV irradiation, leads to a profound inhibition of the atypical PKC activity in vivo. In this study, we addressed the relationship between this phenomenon and different proteins involved in the apoptotic response. We show that (i) the inhibition of the aPKC activity precedes UV-induced apoptosis; (ii) UV-induced aPKC inhibition and apoptosis are independent of p53; (iii) Bcl-2 proteins are potent modulators of aPKC activity; and (iv) the aPKCs are located upstream of the interleukin-converting enzyme-like protease system, which is required for the induction of apoptosis by both Par-4 (a selective aPKC inhibitor) and UV irradiation. We also demonstrate here that inhibition of aPKC activity leads to a decrease in mitogen-activated protein (MAP) kinase activity and simultaneously an increase in p38 activity. Both effects are critical for the induction of apoptosis in response to Par-4 expression and UV irradiation. Collectively, these results clarify the position of the aPKCs in the UV-induced apoptotic pathway and strongly suggest that MAP kinases play a role in this signaling cascade.

Receptor-mediated endocytosis of urokinase-type plasminogen activator is regulated by cAMP-dependent protein kinase

Internalization of the urokinase-type plasminogen activator (uPA) requires two receptors, the uPA receptor (uPAR) and the low density lipoprotein receptor-related protein (LRP)/alpha2-macroglobulin (alpha2M) receptor. Here, we address whether protein kinases are involved in the internalization of uPA by human melanoma cells. Initially, we found that the internalization of uPA was significantly inhibited by the serine/threonine protein kinase inhibitors staurosporine, K-252a and H-89, but not by the tyrosine kinase inhibitors, genistein and lavendustin A. Internalization of uPA was also inhibited by a pseudosubstrate peptide for cAMP-dependent protein kinase (PKA), but not by a pseudosubstrate peptide for protein kinase C. We confirmed a requirement for PKA-activity and implicated a specific isoform by using an antisense oligonucleotide against the regulatory subunit RI alpha of PKA which suppresses PKA-I activity. Exposure of cells to this oligonucleotide led to a specific, dose-dependent decrease in RI alpha protein and to a significant inhibition in the rate of uPA internalization. We further demonstrate that treatment of melanoma cells with either H-89 or PKA RI alpha antisense oligonucleotides also resulted in a decreased internalization of two other ligands of LRP, activated alpha2M and lactoferrin, indicating that PKA activity is associated with LRP. Finally, we demonstrate that PKA activity is also required for the internalization of transferrin, but not for the internalization of the epidermal growth factor or adenovirus 2, suggesting that in melanoma cells, PKA activity is not generally required for clathrin-mediated endocytosis, but is rather associated with specific internalization receptors.

Signal transduction from multiple Ras effectors

Ras proteins activate a signaling cascade through direct binding of the serine/threonine kinase Raf. They also activate additional signaling pathways that are essential for full biological activity. Candidate effectors for these pathways include RalGDS and phosphatidyl inositol 3' kinase, as well as several other Ras binding proteins the biochemical and biological properties of which are poorly understood.