Characterization of Activators and Inhibitors of Protein Kinase Cμ

Abstract

In order to investigate regulatory mechanisms and to identify potential substrates of a novel member of the protein kinase C (PKC) family, PKCp, specific antibodies have been raised against unique aminoand carboxy-terminal regions. PKCp kinase activity was studied upon immunoprecipitation from stably transfected cell lines as well as from the A549 carcinoma cell line expressing the endogeneous PKCp gene. Cell fractionation revealed that PKCp is predominantly found in the particulate fraction, suggesting an association with the membrane or membrane-bound structures. In vitro kinase assays with immunoprecipitated PKCp demonstrated a Ca2+ independent enhancement of constitutive autophosphorylation activity by phosphatidylserine. Despite a limited in vitro phorbol ester response, an apparent phorbol ester activation of PKCp was observed when cell cultures, instead of immunoprecipitated enzyme, were treated with either phorbol 12-myristate 13-acetate or 1,2 dioleoyl-sn-glycerol. Both in vitro autophosphorylation and substrate phosphorylation of myelin basic protein and histone I11 were enhanced under these conditions. However, long-term treatment with the phorbol ester did not result in downregulation of PKCp protein levels and kinase activity. Studies with several protein kinase inhibitors revealed a novel sensitivity profile of PKCp, with no inhibition by calphostin C, reduced sensitivity to staurosporine but, compared to other PKCs, an approximately 60-fold higher sensitivity to the selective PKA inhibitor H89. Together, the data presented here show that localization of PKCp and regulation of its kinase activity differ from that of other PKCs suggesting a novel function of PKCp in intracellular signal pathways.Keywords. Protein kinase Cp; activators, of protein kinase Cp; inhibitors of protein kinase Cp.Protein kinases C (PKC) are important regulatory enzymes involved in multiple cellular responses. They are typically activated by lipid second messengers such as diacylglycerol in response to various extracellular agonists like hormones, neurotransmitters, growth factors and cytokines [ l , 21. Molecular cloning of various PKC isoforms has established that PKC is a multigene family [3 -111. All isozymes share a characteristic conserved catalytic domain in the carboxy-terminal region and an amino-terminal regulatory site (Cl). To date, 11 members have been identified, which can be grouped into three major classes according to their different activation conditions. The first group, the conventional PKCs (cPKCa, p l , p2 and y ) require Ca2' to be activated in the presence of phosphatidylserine [3] whereas the second group, the novel PKCs (nPKCs) lack the C2 domain of cPKCs and are Ca2+ independent [4-6, 8, 91. Common features within the C l domain of cPKCs and nPKCs are a conserved pseudosubstrate site and two adjacent aminoterminal cysteine clusters that are involved in phorbol ester binding [2]. Members of the third subgroup have been termed atypical PKCs (aPKCC, and A) because they lack the C2 region and contain o...

Inhibition of protein kinase C μ by various inhibitors. Inhibition from protein kinase c isoenzymes

“…Due to these properties PKCIa might have been classified as a member of the nPKC subgroup of the PKC family. However, PKC~t significantly differs in some structural features from the other PKC isotypes known so far [5]. With a sequence of 912 amino acids PKC~t is larger than the other PKCs.…”

“…After 30 min at 4°C the precipitated proteins were pelleted, redissolved in sample buffer, separated by SDS-polyacrylamide gel electrophoresis, and visualized by autoradiography of the gels. To analyze in vivo PKCI.t autophosphorylation NIH3T3 PKCI.t overexpressing transfectants were established essentially as previously described [5]. PKQt expression was induced by incubating 60% confluent growing transfectants 3 h in phosphate-free DMEM medium adding 1 ~tM CdC12 and 300 ~tCi 32p-labelled orthophosphate (Amersham) in a final volume of 2 ml.…”

“…Previously inhibitors G6 6976 and G6 6983 were added for a 20 min period. Cells were scraped off, then subjected to immunoprecipitation, SDS-PAGE and autoradiography as described [6]. The concentration of the inhibitor at which 50% suppression of PKCIx activity could be observed (ICs0) was determined from data shown in Fig.…”

“…Referring to its structural similarities to the PKC family, such as the: combination of amino terminal cysteine clusters a carboxy telminal kinase domain, and its phospholipid dependency, the kiaase was termed PKC/x [5]. Upon overexpression in various cell lines and immunoprecipitation, the novel PKC isoenzyme *( orresponding author.…”

“…E-mail: Franz-Josef. Johannes@Po.uni-Stuttgart.de Ab,Sreviations: PKC, protein kinaseC; TPA, 12-O-tetradecanoylphorbo]-13-acetate; PS, phosphatidylserine; PdBu, phorbol 12,13-dibutyrate could be partially characterized and was found to be calciumindependent but phorbol ester responsive [6]. Due to these properties PKCIa might have been classified as a member of the nPKC subgroup of the PKC family.…”