Staurosporine, but not Ro 31-8220, induces interleukin 2 production and synergizes with interleukin 1alpha in EL4 thymoma cells

Differential inhibitor sensitivity between human kinases VRK1 and VRK2

Human vaccinia-related kinases (VRK1 and VRK2) are atypical active Ser-Thr kinases implicated in control of cell cycle entry, apoptosis and autophagy, and affect signalling by mitogen activated protein kinases (MAPK). The specific structural differences in VRK catalytic sites make them suitable candidates for development of specific inhibitors. In this work we have determined the sensitivity of VRK1 and VRK2 to kinase inhibitors, currently used in biological assays or in preclinical studies, in order to discriminate between the two proteins as well as with respect to the vaccinia virus B1R kinase. Both VRK proteins and vaccinia B1R are poorly inhibited by inhibitors of different types targeting Src, MEK1, B-Raf, JNK, p38, CK1, ATM, CHK1/2 and DNA-PK, and most of them have no effect even at 100 µM. Despite their low sensitivity, some of these inhibitors in the low micromolar range are able to discriminate between VRK1, VRK2 and B1R. VRK1 is more sensitive to staurosporine, RO-31-8220 and TDZD8. VRK2 is more sensitive to roscovitine, RO 31–8220, Cdk1 inhibitor, AZD7762, and IC261. Vaccinia virus B1R is more sensitive to staurosporine, KU55933, and RO 31–8220, but not to IC261. Thus, the three kinases present a different pattern of sensitivity to kinase inhibitors. This differential response to known inhibitors can provide a structural framework for VRK1 or VRK2 specific inhibitors with low or no cross-inhibition. The development of highly specific VRK1 inhibitors might be of potential clinical use in those cancers where these kinases identify a clinical subtype with a poorer prognosis, as is the case of VRK1 in breast cancer.

Multiple kinases regulate mafA expression in the pancreatic beta cell line MIN6

MafA is a basic leucine zipper transcription factor expressed within the beta cells of the pancreas and is required to maintain normal glucose homeostasis as it is involved in various aspects of beta cell biology. MafA protein levels are known to increase in response to high glucose through mechanisms that have yet to be fully characterized. We investigated whether discrete intracellular signaling events control mafA expression. We found that the general kinase inhibitor staurosporine induces mafA expression without altering the stability of the protein. Inhibition of the MAP-kinase JNK mimics the effects of staurosporine on the expression of mafA. Calmodulin kinase and calcium signaling are also important in stimulating mafA expression by high glucose. However, staurosporine, JNK, and calmodulin kinase have different effects on the induction of insulin expression. These data reveal that MafA levels are tightly controlled by the coordinated action of multiple kinase pathways.

Phorbol 12-myristate 13-acetate-induced ectodomain shedding and phosphorylation of the human meprinbeta metalloprotease

Shedding of proteins localized at the cell surface is an important regulatory step in the function of many of these proteins. Human meprin (N-benzoyl-l-tyrosyl-p-aminobenzoic acid hydrolase, PPH, EC 3.4.24.18) a zinc-metalloendopeptidase of the astacin family is an oligomeric protein complex of alpha- and beta-subunits and is expressed abundantly in the intestine and kidney as well as in leukocytes of the lamina propria and in cancer cells. In transfected cells intracellular proteolytic removal of the membrane anchor results in the secretion of the meprin alpha-subunit. In rats and mice, the beta-subunit exists in a membrane-anchored form. In contrast, human meprinbeta is constitutively converted into a secretable form. We now show that phorbol 12-myristate 13-acetate (PMA) stimulates an increased release of hmeprinbeta from transfected COS-1 cells, whereas hmeprinalpha secretion is not influenced. This stimulatory effect is inhibited by the protein kinase C (PKC) inhibitor staurosporine, suggesting that activation of PKC mediates PMA-induced hmeprinbeta shedding. The use of different protease inhibitors shows that two different metalloprotease activities are responsible for the constitutive and the PMA-stimulated hmeprinbeta shedding. We identified tumor necrosis factor alpha-converting enzyme (TACE or ADAM17) as the protease that mediates the PMA-induced release. We also demonstrate that hmeprinbeta is phosphorylated by PMA treatment on Ser687 within a PKC consensus sequence in the cytosolic domain of the protein. This phosphorylation of hmeprinbeta is not, however, implicated in the enhanced secretion by PMA treatment.

Induction of cyclooxygenase-2 by staurosporine through the activation of nuclear factor for IL-6 (NF-IL6) and activator protein 2 (AP2) in an osteoblast-like cell line

The induction of cyclooxygenase-2 (COX-2) plays a crucial role in many physiological and pathological processes. The expression of the COX-2 gene is regulated by many extracellular stimuli, including growth factors, cytokines, and tumor promoters. Staurosporine, a potential anti-tumor drug, was found recently to up-regulate the expression of the COX-2 gene in the mouse osteoblast-like cell line MC3T3-E1. The ability of staurosporine to induce the expression of the COX-2 gene was investigated using luciferase reporters controlled by various COX-2 core promoter regions. Two cis-acting sites for activator protein 2 (AP2) and nuclear factor for IL-6 (NF-IL6), respectively, were identified as responsible for the staurosporine-mediated COX-2 up-regulation. Mutational analysis further verified that both NF-IL6 and AP2 are involved in this process. Further studies showed the stimulatory effect of staurosporine on luciferase activity to be both time- and concentration-dependent. Luciferase activity could be induced at as low as 5 nM staurosporine and reached a maximum at around 20 nM. At 50 nM, the stimulatory effect of staurosporine on luciferase activity reached a maximum at about 8 hr and fell rapidly following 10 hr of incubation. Interestingly, a selective protein kinase C inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl) maleimide (GF109203X), failed to stimulate luciferase activity under the same conditions. This finding implies that staurosporine-mediated COX-2 gene expression is specific and independent of protein kinase C activity.

Regulation of PDGFR-alpha in rat pulmonary myofibroblasts by staurosporine

Upregulation of the platelet-derived growth factor (PDGF) receptor-alpha (PDGFR-alpha) is a mechanism of myofibroblast hyperplasia during pulmonary fibrosis. We previously identified interleukin (IL)-1beta as a major inducer of the PDGFR-alpha in rat pulmonary myofibroblasts in vitro. In this study, we report that staurosporine, a broad-spectrum kinase inhibitor, upregulates PDGFR-alpha gene expression and protein. A variety of other kinase inhibitors did not induce PDGFR-alpha expression. Staurosporine did not act via an IL-1beta autocrine loop because the IL-1 receptor antagonist protein did not block staurosporine-induced PDGFR-alpha expression. Furthermore, staurosporine did not activate a variety of signaling molecules that were activated by IL-1beta, including nuclear factor-kappaB, extracellular signal-regulated kinase, and c-Jun NH2-terminal kinase. However, both staurosporine- and IL-1beta-induced phosphorylation of p38 mitogen-activated protein kinase and upregulation of PDGFR-alpha by these two agents was inhibited by the p38 inhibitor SB-203580. Finally, staurosporine inhibited basal and PDGF-stimulated mitogenesis over the same concentration range that induced PDGFR-alpha expression. Collectively, these data demonstrate that staurosporine is a useful tool for elucidating the signaling mechanisms that regulate PDGFR expression in lung connective tissue cells and possibly for evaluating the role of the PDGFR-alpha as a growth arrest-specific gene.

Cluster formation by protein kinase Ctheta during murine T cell activation: effect of age

Protein kinase Ctheta; (PKCtheta;) is thought to play an important role in T cell activation, in that exposure of cloned T cells to antigen-presenting cells bearing agonist peptides, but not antagonist peptides, leads to clustering of PKCtheta; molecules in the section of the T cell plasma membrane that is in contact with the APCs. To see whether aging affects this PKCtheta; clustering reaction in mouse T lymphocytes, we used immunofluorescence staining and confocal microscopy to observe the localization of PKCtheta; in CD4 and CD8 T lymphocytes activated by coincubation with anti-CD3 hybridoma cells. Aging led to a twofold decline in the proportion of both CD4 and CD8 T cells in which PKCtheta; underwent cluster formation. This decrease with age was not due to differences in the number of cell conjugates formed, nor to kinetic differences of PKCtheta; clustering, nor to the accumulation of memory T cells in old mice. There were no effects of aging on the levels or kinase activity of PKCtheta; in murine T cells. Our data suggest alterations in the upstream signals that regulate PKCtheta; translocation.

Role of protein kinase C (PKC) in agonist-induced mu-opioid receptor down-regulation: II. Activation and involvement of the alpha, epsilon, and zeta isoforms of PKC

Phosphorylation of specific amino acid residues is believed to be crucial for the agonist-induced regulation of several G protein-coupled receptors. This is especially true for the three types of opioid receptors (mu, delta, and kappa), which contain consensus sites for phosphorylation by numerous protein kinases. Protein kinase C (PKC) has been shown to catalyze the in vitro phosphorylation of mu- and delta-opioid receptors and to potentiate agonist-induced receptor desensitization. In this series of experiments, we continue our investigation of how opioid-activated PKC contributes to homologous receptor down-regulation and then expand our focus to include the exploration of the mechanism(s) by which mu-opioids produce PKC translocation in SH-SY5Y neuroblastoma cells. [D-Ala2,N-Me-Phe4,Gly-ol]enkephalin (DAMGO)-induced PKC translocation follows a time-dependent and biphasic pattern beginning 2 h after opioid addition, when a pronounced translocation of PKC to the plasma membrane occurs. When opioid exposure is lengthened to >12 h, both cytosolic and particulate PKC levels drop significantly below those of control-treated cells in a process we termed "reverse translocation." The opioid receptor antagonist naloxone, the PKC inhibitor chelerythrine, and the L-type calcium channel antagonist nimodipine attenuated opioid-mediated effects on PKC and mu-receptor down-regulation, suggesting that this is a process partially regulated by Ca2+-dependent PKC isoforms. However, chronic exposure to phorbol ester, which depletes the cells of diacylglycerol (DAG) and Ca2+-sensitive PKC isoforms, before DAMGO exposure, had no effect on opioid receptor down-regulation. In addition to expressing conventional (PKC-alpha) and novel (PKC-epsilon) isoforms, SH-SY5Y cells also contain a DAG- and Ca2+-independent, atypical PKC isozyme (PKC-zeta), which does not decrease in expression after prolonged DAMGO or phorbol ester treatment. This led us to investigate whether PKC-zeta is similarly sensitive to activation by mu-opioids. PKC-zeta translocates from the cytosol to the membrane with kinetics similar to those of PKC-alpha and epsilon in response to DAMGO but does not undergo reverse translocation after longer exposure times. Our evidence suggests that direct PKC activation by mu-opioid agonists is involved in the processes that result in mu-receptor down-regulation in human neuroblastoma cells and that conventional, novel, and atypical PKC isozymes are involved.

Effect of staurosporine on transcription factor NF-kappaB in human keratinocytes

Activation of the transcription factor NF-kappaB is known to be important in the regulated expression of a large number of pro-inflammatory genes including interleukin-8 (IL-8). Previously, we showed that the protein kinase inhibitor staurosporine potentiates IL-1-stimulated IL-8 production in human keratinocytes. Moreover, recent studies by other investigators demonstrated that staurosporine treatment alone results in a concentration-dependent increase in IL-8 mRNA and protein production. Therefore, in order to understand the mechanism underlying this observation, the effect of staurosporine on the activation of NF-kappaB was investigated. Electrophoretic mobility shift assays using an oligonucleotide containing the NF-kappaB consensus motif demonstrated that staurosporine treatment resulted in the activation of NF-kappaB by 15 min post-treatment. The ability of staurosporine to activate NF-kappaB was investigated further, using luciferase reporters under the control of the HIV-LTR or IL-8 core promoter transfected into human U937 cells. Stimulation with staurosporine resulted in a concentration-dependent induction of luciferase activity. In contrast, the very selective protein kinase C inhibitor 3-[8-[(dimethylamino)methyl]-6,7,8,9-tetrahydropyrido-[1,2-a]indol -10-yl]-4-(1-methyl-3-indolyl)-1H-pyrrole-2,5-dione hydrochloride (Ro32-0432) did not stimulate the activation of NF-kappaB, as measured in the luciferase reporter assay. The mechanism underlying NF-kappaB activation does not appear to involve the classical activation pathways in that staurosporine does not induce the disappearance of IkappaB family members. In conclusion, staurosporine appears to stimulate the activation of NF-kappaB in at least two cell types, and this effect appears to be independent of protein kinase C.