Negative regulation of multifunctional Ca2+/calmodulin-dependent protein kinases: physiological and pharmacological significance of protein phosphatases.

Multifunctional Ca2+/calmodulin-dependent protein kinases (CaMKs) play pivotal roles in intracellular Ca2+ signaling pathways. There is growing evidence that CaMKs are involved in the pathogenic mechanisms underlying various human diseases. In this review, we begin by briefly summarizing our knowledge of the involvement of CaMKs in the pathogenesis of various diseases suggested to be caused by the dysfunction/dysregulation or aberrant expression of CaMKs. It is widely known that the activities of CaMKs are strictly regulated by protein phosphorylation/dephosphorylation of specific phosphorylation sites. Since phosphorylation status is balanced by protein kinases and protein phosphatases, the mechanism of dephosphorylation/deactivation of CaMKs, corresponding to their 'switching off', is extremely important, as is the mechanism of phosphorylation/activation corresponding to their 'switching on'. Therefore, we focus on the regulation of multifunctional CaMKs by protein phosphatases. We summarize the current understanding of negative regulation of CaMKs by protein phosphatases. We also discuss the biochemical properties and physiological significance of a protein phosphatase that we designated as Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKP), and those of its homologue CaMKP-N. Pharmacological applications of CaMKP inhibitors are also discussed. These compounds may be useful not only for exploring the physiological functions of CaMKP/CaMKP-N, but also as novel chemotherapies for various diseases.

Identification and characterization of CaMKP-N, nuclear calmodulin-dependent protein kinase phosphatase.

Calmodulin-dependent protein kinase phosphatase (CaMKP) dephosphorylates and concomitantly deactivates multifunctional Ca(2+)/calmodulin-dependent protein kinases (CaMKs), such as CaMKI, CaMKII, and CaMKIV. In the present study, a nuclear CaMKP-related protein, CaMKP-N, was identified. This protein consisted of 757 amino acid residues with a calculated molecular weight of 84,176. Recombinant CaMKP-N dephosphorylated CaMKIV. The activity of CaMKP-N requires Mn(2+) ions and is stimulated by polycations. Transiently expressed CaMKP-N in COS-7 cells was localized in the nucleus. This finding together with previous reports regarding localization of CaMKs indicates that CaMKP-N dephosphorylates CaMKIV and nuclear CaMKII, whereas CaMKP dephosphorylates CaMKI and cytosolic CaMKII.

Substrate specificity of Ca(2+)/calmodulin-dependent protein kinase phosphatase: kinetic studies using synthetic phosphopeptides as model substrates.

Ca(2+)/calmodulin-dependent protein kinase phosphatase (CaMKPase) dephosphorylates and regulates multifunctional Ca(2+)/calmodulin-dependent protein kinases. In order to elucidate the mechanism of substrate recognition by CaMKPase, we chemically synthesized a variety of phosphopeptide analogs and carried out kinetic analysis using them as CaMKPase substrates. This is the first report using systematically synthesized phosphopeptides as substrates for kinetic studies on substrate specificities of protein Ser/Thr phosphatases. CaMKPase was shown to be a protein Ser/Thr phosphatase having a strong preference for a phospho-Thr residue. A Pro residue adjacent to the dephosphorylation site on the C-terminal side and acidic clusters around the dephosphorylation site had detrimental effects on dephosphorylation by CaMKPase. Deletion analysis of a model substrate peptide revealed that the minimal length of the substrate peptide was only 2 to 3 amino acid residues including the dephosphorylation site. The residues on the C-terminal side of the dephosphorylation site were not essential for dephosphorylation, whereas the residue adjacent to the dephosphorylation site on the N-terminal side was essential. Ala-scanning analysis suggested that CaMKPase did not recognize a specific motif around the dephosphorylation site. Myosin light chain phosphorylated by protein kinase C and Erk2 phosphorylated by MEK1 were poor substrates for CaMKPase, while a synthetic phosphopeptide corresponding to the sequence around the phosphorylation site of the former was not dephosphorylated by CaMKPase but that of the latter was fairly good substrate. These data suggest that substrate specificity of CaMKPase is determined by higher-order structure of the substrate protein rather than by the primary structure around its dephosphorylation site. Use of phosphopeptide substrates also revealed that poly-L-lysine, an activator for CaMKPase, activated the enzyme mainly through increase in the V(max) values.

Detection of a variety of Ser/Thr protein kinases using a synthetic peptide with multiple phosphorylation sites.

A novel peptide with multiple phosphorylation sites, which we designated as multide, was developed to detect a wide variety of protein kinases in crude cell extracts. Multide, KKRKSSLRRWSPLTPRQMSFDC, has been designed to contain consensus sequences for various Ser/Thr protein kinases including cAMP-dependent protein kinase, protein kinase C, MAP kinases, and Ca(2+)/calmodulin-dependent protein kinases in a single peptide. In-gel protein kinase assay using multide was found to be very useful for analyzing the activities of protein kinases that are altered in response to various extracellular stimuli. The substrate specificities of the protein kinases thus detected were further determined by using five multide analogs with different phosphorylation sites.

Phosphorylation and activation of Ca2+/calmodulin-dependent protein kinase phosphatase by Ca2+/calmodulin-dependent protein kinase II.

Ca2+/calmodulin-dependent protein kinase phosphatase (CaMKPase) is a protein phosphatase which dephosphorylates autophosphorylated Ca2+/calmodulin-dependent protein kinase II (CaMKII) and deactivates the enzyme (Ishida, A., Kameshita, I. and Fujisawa, H. (1998) J. Biol. Chem. 273, 1904-1910). In this study, a phosphorylation-dephosphorylation relationship between CaMKII and CaMKPase was examined. CaMKPase was not significantly phosphorylated by CaMKII under the standard phosphorylation conditions but was phosphorylated in the presence of poly-L-lysine, which is a potent activator of CaMKPase. The maximal extent of the phosphorylation was about 1 mol of phosphate per mol of the enzyme and the phosphorylation resulted in an about 2-fold increase in the enzyme activity. Thus, the activity of CaMKPase appears to be regulated through phosphorylation by its target enzyme, CaMKII.

Molecular cloning of Ca2+/calmodulin-dependent protein kinase phosphatase.

Calmodulin-dependent protein kinase (CaM-kinase) phosphatase dephosphorylates and concomitantly deactivates CaM-kinase II activated upon autophosphorylation, and CaM-kinases IV and I activated upon phosphorylation by CaM-kinase kinase [Ishida, I., Okuno, S., Kitani, T., Kameshita, I., and Fujisawa, H. (1998) Biochem. Biophys. Res. Commun. 253, 159-163], suggesting that CaM-kinase phosphatase plays important roles in the function of Ca2+ in the cell, because the three multifunctional CaM-kinases (CaM-kinases I, II, and IV) are thought to be the key enzymes in the Ca2+-signaling system. In the present study, cDNA for CaM-kinase phosphatase was cloned from a rat brain cDNA library. The coded protein consisted of 450 amino acids with a molecular weight of 49, 165. Western blot analysis showed the ubiquitous tissue distribution of CaM-kinase phosphatase. Immunocytochemical analysis revealed that CaM-kinase phosphatase is evenly distributed outside the nucleus in a cell.

MAP kinase-independent induction of proto-oncogene c-fos mRNA by hemin in human cells.

Treatment of HeLa cells or human skin fibroblast cells with hemin led to a time- and dose-dependent rapid induction of c-fos mRNA. This induction was absent in the cells treated with actinomycin D, indicating that the c-fos induction by hemin occurs at the level of transcription. Metalloporphyrins, including zinc-, cobalt-, and tin-protoporphyrin, ferric ion, and protoporphyrin also induced c-fos mRNA. Transient reporter assay with the reporter constructs of the human c-fos gene promoter up to -404 bp connected to the luciferase gene showed high activity but no induction by hemin, suggesting that cis-acting elements, including the serum response element located about -310 bp upstream of the human c-fos gene promoter, may not contribute to the heme-dependent induction. With in-gel assay of protein kinases, the activity of the mitogen-activated protein (MAP) kinases such as extracellular signal-regulated kinase 12 or p38 MAP kinase in hemin-treated HeLa cells was not stimulated. Stimulation of c-Jun N-terminal kinase by hemin was nil. Furthermore, PD58059 and SB203580, inhibitors for MAP kinases, did not affect the hemin-dependent c-fos induction. Of the inhibitors for protein kinases so far tested, KN-62, a specific inhibitor for calmodulin-dependent protein kinase II (CaMK II), inhibited the induction of c-fos mRNA by hemin. Phosphorylation of CaMK II in hemin-treated cells increased. With gel mobility assay, the DNA AP-1 binding activity transiently increased when treating HeLa cells with hemin. Therefore, induction of c-fos led to an activation of AP-1 in the presence of hemin. We suggest that calmodulin-dependent protein kinase II rather than the MAP kinase family regulates the induction of the human c-fos gene expression by hemin.

Synthesis of caged peptides using caged lysine: application to the synthesis of caged AIP, a highly specific inhibitor of calmodulin-dependent protein kinase II.

N(alpha)-Fmoc-N(epsilon)-(2-nitrobenzyloxycarbonyl)-lysine has been prepared and used in the solid-phase synthesis of caged peptides. The synthesized caged AIP (cagedKcagedKALRRQEAVDAL) showed characteristics required for caged peptides including a significantly reduced inhibitory activity to calmodulin-dependent protein kinase II and instantaneous recovery of the activity with photo-irradiation.

Critical amino acid residues of AIP, a highly specific inhibitory peptide of calmodulin-dependent protein kinase II.

The importance of the individual amino acid residues of AIP (KKALRRQEAVDAL), a highly specific inhibitor of calmodulin-dependent protein kinase II (CaMKII), was studied. Replacement of Arg6, Gln7, or Ala9 by other amino acid residues produced a marked increase in the IC50 value. Leu4 and Val10 were also sensitive to replacement, but some hydrophobic amino acids could substitute for these residues. Although replacement of Ala3, Glu8, Ala12, and Leu13 by other residues produced no significant increase in the IC50, the substitution of Lys for Ala3 decreased the IC50. An AIP analog (KKKLRRQEAFDAY), in which Ala3 and Val10 were replaced with Lys and Phe, respectively, showed an IC50 value as low as 4 nM, suggesting that it is a useful tool for studying the physiological roles of CaMKII.

A novel protein phosphatase that dephosphorylates and regulates Ca2+/calmodulin-dependent protein kinase II.

A synthetic peptide corresponding to the autophosphorylation site of Ca2+/calmodulin-dependent protein kinase II (CaMKII) (residues 281-289) was conjugated to paramagnetic particles, and phosphorylated by a constitutively active CaMKII fragment. Using this phosphopeptide conjugate as a substrate, a calyculin A-insensitive, Mn(2+)-dependent, and poly-L-lysine-stimulated protein phosphatase activity was detected in the crude extract of rat brain. The protein phosphatase (designated as CaMKII phosphatase) (CaMKIIPase) was purified to near homogeneity from rat brain. CaMKIIPase showed apparent molecular weights of 54,000 and 65,000, on SDS-polyacrylamide gel electrophoresis and gel-filtration analysis, respectively. It was not inhibited by 100 nM calyculin A or 10 microM okadaic acid. Mn2+, but not Mg2+, was absolutely required for activity. CaMKIIPase was potently activated by polycations. Autophosphorylated CaMKII was dephosphorylated by CaMKIIPase, whereas phosphorylase kinase, mixed histones, myelin basic protein, and alpha-casein (which had been phosphorylated by cAMP-dependent protein kinase) and phosphorylase a (phosphorylated by phosphorylase kinase) were not significantly dephosphorylated. No other proteins than CaMKII in rat brain extract which had been phosphorylated by CaMKII were dephosphorylated. The stimulated Ca(2+)-independent activity of autophosphorylated CaMKII was reversed by the action of CaMKIIPase. Thus, CaMKIIPase appears to be a specialized protein phosphatase for the regulation of CaMKII.

Rapid adhesion and spread of non-adherent colon cancer Colo201 cells induced by the protein kinase inhibitors, K252a and KT5720 and suppression of the adhesion by the immunosuppressants FK506 and cyclosporin A.

We examined alterations in cell morphology and expression of adhesion molecules in response to a general protein kinase inhibitor K252a treatment of non-adherent colon adenocarcinoma Colo201 cells. K252a induced rapid cell adhesion and spreading with concomitant formation of actin stress fibers. A protein kinase A inhibitor KT5720 also induced cell adhesion, but the rate of spread was slower than that seen with K252a. These adhesions were mediated by integrin molecules since cell adhesion required Mg2+, Mn2+ or Ca2+, and was inhibited by monoclonal antibodies for integrins alpha2 and beta1. Indirect immunofluorescence microscopic observations revealed that integrin alpha2 and beta1 molecules in K252a-treated cells were concentrated at sites of focal adhesion, but expressions of integrin molecules were not modulated. Tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin increased during K252a- or KT5720-induced cell adhesion. Immunosuppressants FK506 and cyclosporin A suppressed the K252a-induced cell adhesion and abolished tyrosine phosphorylation of cellular proteins including FAK and paxillin. Furthermore, W7 and calmidazolium, inhibitors of calmodulin, also inhibited the cell adhesion. Based on findings that FK506 and cyclosporin A are inhibitors of the calcium calmodulin-dependent protein phosphatase, calcineurin, this phosphatase may regulate integrin-dependent cell adhesion and spread of Colo201 cells. This Colo201 cell model provides a pertinent system for studying molecules involved in signal transduction pathways and can shed light on mechanisms of metastasis and invasion of colon carcinoma cells.

Regulation of multifunctional Ca2+/calmodulin-dependent protein kinases by Ca2+/calmodulin-dependent protein kinase phosphatase.

We have recently reported a novel protein phosphatase which dephosphorylates and thereby deactivates Ca2+/calmodulin-dependent protein kinase II (CaMKII) activated through autophosphorylation (Ishida, A., Kameshita, I., and Fujisawa, H. (1998) J. Biol. Chem. 273, 1904-1910). In the present study, we show that this protein phosphatase also catalyzed dephosphorylation of Ca2+/calmodulin-dependent protein kinases I (CaMKI) and IV (CaMKIV) which had been phosphorylated and activated by Ca2+/calmodulin-dependent protein kinase kinase alpha, resulting in reversible deactivation of the enzymes. The fairly high degree of the substrate specificity of this protein phosphatase suggests that the physiological significance of the phosphatase may be the regulation of the three multifunctional Ca2+/calmodulin-dependent protein kinases, CaMKI, CaMKII, and CaMKIV, which are the key enzymes in a Ca(2+)-signaling system in the cell.

A new peptide conjugate as a highly specific substrate for MAP kinase.

A synthetic peptide (APRTPGGRC) cross-linked to poly-L-lysine through a carboxy-terminal cysteinyl residue was found to be a highly specific substrate for mitogen-activated protein (MAP) kinases. This peptide conjugate exhibited a much lower Km value (74 microM) than the free peptide substrate (APRTPGGRR, Km > 1 mM) previously used as a specific substrate for MAP kinases. Unlike myelin basic protein, which has been often used as a substrate for MAP kinases, this conjugate did not serve as substrate for cAMP-dependent protein kinase, protein kinase C, or multifunctional calmodulin-dependent protein kinases. Using the peptide conjugate as a substrate, MAP kinase activities in crude cell extracts were directly determined by in vitro assay and specifically detected by in-gel assay.

Detection of protein phosphatase activities in sodium dodecyl sulfate-polyacrylamide gel using peptide substrates.

A method for detection of protein phosphatase activity toward phosphorylated oligopeptides in SDS-polyacrylamide gel was developed. A synthetic peptide (MHRQETVDC) corresponding to the autophosphorylation site of calmodulin-dependent protein kinase II (residue 281-289) was conjugated to poly-L-lysine and phosphorylated with [gamma-32P]ATP by the action of calmodulin-dependent protein kinase II, and the [32P]-phosphopeptide-polymer conjugate was included as a substrate for protein phosphatases in gels. When a crude extract from rat brain was electrophoresed on polyacrylamide gel containing the [32P]phosphopeptide conjugate, followed by treatment for in situ renaturation and autoradiography, three transparent bands corresponding to apparent molecular weights of 52,000, 58,000 and 74,000, resulting from the removal of the [32P]phosphate from the phosphopeptide conjugate included in the gel were observed, indicating the existence of at least three different phosphoprotein phosphatases catalyzing dephosphorylation of the phosphopeptide in the brain. Among the three, two bands corresponding to molecular weights of 52,000 and 58,000 were not clearly observed when other phosphopeptide-polymer conjugates such as C-syntide-2 and CAMKAKS peptide were included in gels, suggesting that site-specific protein phosphatases can be detected in crude tissue extracts by this in-gel protein phosphatase assay.

Detection of protein kinase activities toward oligopeptides in sodium dodecyl sulfate-polyacrylamide gel.

In the previous paper, we reported a sensitive method for detection of protein kinase activities in gels after SDS-polyacrylamide gel electrophoresis (Kameshita, I., and Fujisawa, H. (1989) Anal. Biochem. 183, 139-143). This method is useful for the detection of various protein kinase activities toward protein substrates included in gels, but inapplicable to oligopeptide substrates because most of the oligopeptides eluted from the gel matrix during electrophoresis. The present study describes a new procedure for the detection of protein kinase activities toward synthetic oligopeptides in the gel. The oligopeptides which were linked to amino acid polymers such as poly-L-lysine through their amino-terminal cysteinyl residue by a heterobifunctional reagent were efficiently retained in the gel matrix and served as substrates for the protein kinases. As little as 2.5 pg of the catalytic subunit of cAMP-dependent protein kinase was detected by this in-gel assay method using a synthetic peptide as a substrate. This technique can be used for selective and sensitive detection of various protein kinases in crude tissue extracts.