Microsomal casein kinase II in endoplasmic reticulum- and Golgi apparatus-rich fractions of ROS 17/2.8 osteoblast-like cells: an enzyme that modifies osteopontin

Phosphorylated osteopontin promotes migration of human choriocarcinoma cells via a p70 S6 kinase-dependent pathway

This study examined the role of osteopontin (OPN), a phosphorylated secreted glycoprotein, in the promotion of trophoblastic cell migration, an early event in the embryo implantation process. Three human choriocarcinoma cell lines, namely JAR, BeWo, and JEG-3, were treated with variants of OPN differing in the extent of phosphorylation following sequential dephosphorylation with tartrate-resistant acid phosphatase (TRAP), and their migratory response was measured. The highly phosphorylated human milk form of OPN (OPN-1) strongly triggered migration in all three cell lines, whereas the less phosphorylated variants, OPN-2a and OPN-2b, failed to stimulate migration. JAR cell migration in response to OPN-1 was accompanied by a rapid rearrangement of actin filaments to the cellular membrane. Using broad spectrum protein kinase profiling, we identified p70 S6 kinase as a major signal transduction pathway activated by OPN-1 during the migratory response in JAR cells. Activation was blocked completely by rapamycin and LY294002, thus demonstrating that OPN-1-stimulated migration occurs through mTOR and PI3K pathways, respectively. Conversely, PD98059 did not affect the activation of p70 S6 kinase by OPN-1, therefore, this response does not involve the Ras/ MAPK signaling cascade. Together, these data show that the highly phosphorylated human OPN-1 can stimulate trophoblastic cell migration and provides evidence for the involvement of the PI3K/mTOR/p70 S6 kinase pathway in the JAR cells response. Because both OPN and TRAP are expressed in the uterus during early pregnancy, it is conceivable that extracellular phosphatases such as TRAP may modify OPN charge state and thus modulate cell migration.

Mechanical strain induces collagenase-3 (MMP-13) expression in MC3T3-E1 osteoblastic cells

Mechanical strain plays a crucial role in bone remodeling during growth and development and healing of bone besides systemic and local factors. One of the major factors involves in remodeling process is matrix metalloproteinases (MMPs) such as MMP-13 that has been shown to degrade the native interstitial collagens in several tissues. To study how mechanical strain affects extracellular matrix degradation by MMP-13, a biaxial strain was applied to MC3T3-E1 osteoblastic cells plated onto a collagen-coated flexible elastic membrane. The MMP-13 protein and mRNA expression were determined by Western blotting and reverse transcriptase-PCR, respectively. The zymographic activities of MMP-13 increased dramatically at 30 min, reached a peak by 2-fold at 1 h, and maintained up to 4 h. Moreover, the MMP-13 and c-fos mRNA expressed at 5 min, increased to 2.8- and 3-fold at 1 h, respectively, and gradually declined thereafter. Cycloheximide and actinomycin D did not inhibit the MMP-13 and c-fos mRNA expression, suggesting that such expression does not require de novo protein synthesis and not change their stabilities. To investigate which of the mitogen-activated protein kinase (MAPK) pathways involves in the expression of MMP-13, inhibitors such as PD98059, SB203580, and SP600125 were used. However, only PD98059 (an inhibitor of MEK1/2 activation) inhibited MMP-13 and c-fos gene expression; the result was further substantiated by transfecting with the dominant negative mutants of MEK1/2 (MEK K97R) and ERK2. Taken together, our results showed that mechanical strain induces the MMP-13 expression through MEK-ERK signaling pathway to regulate mechanical adaptation.

The Marburg virus surface protein GP is phosphorylated at its ectodomain

Marburg virus, a filovirus, contains only one transmembrane protein (GP) which is responsible for receptor recognition on target cells. GP, a type I membrane protein of approximately 220 kDa, is acylated and highly glycosylated carrying N- and O-linked sugar side chains. GP is transported through the exocytotic pathway toward the plasma membrane where budding of virions takes place. In the trans-Golgi network, GP is proteolytically activated by the prohormone convertase furin into two subunits GP(1) and GP(2). In the present paper, we provide evidence that GP undergoes an additional posttranslational modification; it is phosphorylated at its ectodomain. Phosphorylation takes place at serine residues between amino acid 260 and 273. The respective serines are located in conserved recognition sites for luminal protein kinases (protein kinase CK II and Golgi casein kinase). Consistent with this data, it was found that GP was phosphorylated in the Golgi apparatus of the expressing HeLa cells before cleavage of the molecule. GP is the first example of a viral glycoprotein with a phosphorylated ectodomain.

Sialyltransferase isoforms are phosphorylated in the cis-medial Golgi on serine and threonine residues in their luminal sequences

ST6Gal-I (alpha2,6-sialyltransferase) is expressed as two isoforms, STTyr and STCys, which exhibit differences in catalytic activity, trafficking through the secretory pathway, and proteolytic processing and secretion. We have found that the ST6Gal-I isoforms are phosphorylated on luminal Ser and Thr residues. Immunoprecipitation of 35S- and 32P-labeled proteins expressed in COS-1 cells suggests that the STTyr isoform is phosphorylated to a greater extent than the STCys isoform. Analysis of domain deletion mutants revealed that STTyr is phosphorylated on stem and catalytic domain amino acids, whereas STCys is phosphorylated on catalytic domain amino acids. An endoplasmic reticulum retained/retrieved chimeric Iip33-ST protein demonstrates drastically lower phosphorylation than does the wild type STTyr isoform. This suggests that the bulk of the ST6Gal-I phosphorylation is occurring in the Golgi. Treatment of cells with the ionophore monensin does not significantly block phosphorylation of the STTyr isoform, suggesting that phosphorylation is occurring in the cis-medial Golgi prior to the monensin block. This study demonstrates the presence of kinase activities in the cis-medial Golgi and the substantial phosphorylation of the luminal sequences of a glycosyltransferase.

Modulation of osteopontin post-translational state by 1, 25-(OH)2-vitamin D3. Dependence on Ca2+ influx

In osteoblastic ROS 17/2.8 cells, 1,25-(OH)2-vitamin D3 stimulates transcription of the extracellular matrix phosphoprotein osteopontin (OPN). We now show post-translational regulation of OPN production by 1,25-(OH)2D3. Prior to transcriptional up-regulation of OPN, 1, 25-(OH)2D3 induces a shift in OPN isoelectric point (pI) from 4.6 to 5.1. Loading equal amounts of OPN recovered from ROS 17/2.8 cells exposed to 1,25-(OH)2D3 or carrier for 3 h reveals that the pI shift represents reduced phosphorylation. Trypsin cleavage patterns of OPN produced after 1,25-(OH)2D3 treatment indicates phosphorylation changes in the resulting peptides. Using structural analogs to 1, 25-(OH)2D3, we found that analog AT (25-(OH)-16-ene-23-yne-D3), which triggers Ca2+ influx but does not bind to the vitamin D receptor, mimicked the OPN pI shift, whereas analog BT (1, 25-(OH)2-22-ene-24-cyclopropyl-D3), which binds to the vitamin D receptor without triggering Ca2+ influx, did not. Likewise, inclusion of the Ca2+ channel blocker nifedipine blocks the charge conversion of OPN. Isolation of OPN from rat femurs and tibiae demonstrates the existence of two OPN charge forms in vivo. We conclude that 1,25-(OH)2D3 regulates OPN not only at the transcriptional level, but also modulates OPN phosphorylation state. The latter involves a short term (<3 h) treatment and is associated with membrane-initiated Ca2+ influx.

Casein kinase 2 phosphorylation of recombinant rat osteopontin enhances adhesion of osteoclasts but not osteoblasts

Osteopontin (OP) is a highly phosphorylated bone matrix protein and contains the RGD cell-binding motif, which mediates cell adhesion through integrin receptors that include alpha(v)beta3. Casein kinase 2 (CK2) is a factor-independent serine/threonine kinase, which may be the predominant physiologically relevant kinase for OP phosphorylation. This study was designed to examine the effects of unphosphorylated recombinant rat OP, and CK2-phosphorylated OP (P-OP), on the adhesion and function of mouse osteoclasts (OC) and osteoblast-like cells (UMR 201-10B and UMR 106-06) in vitro. OP significantly increased OC adhesion compared to plastic alone, and cell attachment was further increased at least twofold on OP phosphorylated with CK2. Attachment was dependent on the integrity of the RGD domain and was completely abolished in the presence of 1 mM RGD peptide. Neither CK2 phosphorylation of mutant OP, in which the RGD was converted to RGE or RAD, nor protein kinase C (PKC) phosphorylation of wild-type OP enhanced OC attachment. An antibody to the beta3 integrin subunit, but not anti-mouse CD44 antibody, specifically blocked the proportion of attachment due to phosphorylation of OP. Actin ring formation in OC was increased by plating cells onto OP, with no further increase by phosphorylation. Both OP and CK2-phosphorylated OP enhanced attachment of the two osteoblastic cell lines, compared to plastic, but in contrast to OCs, there was no significant difference with phosphorylation. Osteoblast attachment was totally blocked by 1 mM RGD peptide, but was not influenced by the beta3 integrin antibody. Plating of UMR 201-10B cells onto OP further increased retinoic acid-induced alkaline phosphatase expression. The results suggest that specific phosphorylation of OP is important for interaction with OCs, compared with osteoblastic cells, and that alternative integrins may be important in the interaction between osteoblastic cells and OP compared with OCs.

Phosphorylation of the proteins of the extracellular matrix of mineralized tissues by casein kinase-like activity

The extracellular matrix of the connective tissue contains non-collagenous proteins (NCP) which are acidic in character. The NCP of mineralizing systems (bone, dentin) differ from those of the non-mineralizing systems (skin, tendon) in that the mineralized tissue NCP are frequently phosphorylated. The phosphorylated proteins have been implicated in various aspects of the mineralization process. Thus, it is of interest to consider the mechanism and regulation of phosphorylation of the major matrix NCP. The majority of the phosphorylation takes place at Ser or Thr residues embedded within acidic sequences, and therefore are targets for casein kinase I (CK1) or casein kinase II (CK2)-like kinases. CK1 and CK2 are distantly related members of the protein kinase family. They are ubiquitous, constitutively active, second-messenger-independent kinases. CK1 is found in a variety of isoforms, all homologous to the alpha-subunit of the protein kinase family. It acts as a monomer. The active form of CK2 is a tetrameric holoenzyme, with 2 alpha catalytic subunits and 2 beta regulatory subunits. The CK2 alpha has activity alone, but the holoenzyme is four- to five-fold that activity. CK2 can use either ATP or GTP as the phosphate donor, but CK1 can use only ATP. The CK2 activity which phosphorylates the mineralized tissue NCP appears to be localized to membrane-associated cell fractions, and is present in the endoplasmic reticulum and Golgi compartments in osteoblasts, where phosphorylation of the secreted proteins appears to take place as co- and post-translational processes. Data indicate that both alpha and beta subunits of the membrane-associated CK2 are isoforms of the cytosolic CK2 in the same cells. The CK1 has not been specifically localized. Studies of dephosphorylated NCP such as phosphophoryn (PP) have shown that CK1 will not phosphorylate dephosphorylated dPP unless prior phosphorylation with CK2 has been carried out. In turn, CK2 activity may be initiated only after an initial phosphorylation of one of the messenger-dependent kinases. Thus, the phosphorylation reactions in mineralized tissues may be a tightly regulated hierarchical or sequential cascade of intracellular phosphorylation events.

Characterization of human sublingual-gland protein kinase by phosphorylation of a peptide related to secreted proteins

Phosphoproteins in human saliva include proline-rich proteins, statherins, histatin 1 and cystatin SA-III. The presence of phosphate in these proteins is necessary for various functions in the mouth including calcium binding, inhibition of precipitation of calcium phosphate, inhibition of growth of hydroxyapatite crystals and adherence to hydroxyapatite. To elucidate the process of phosphorylation of these proteins, the phosphorylation of a peptide (APRP8) with an amino acid sequence identical to one of the phosphorylated sites in acidic proline-rich proteins by a kinase from the human sublingual gland was investigated. The kinase, which was highly labile, was purified 58-fold by fractionation of sublingual gland homogenate and gel filtration, but the enzyme was inactivated when further purification by chromatographic techniques commonly used for protein kinases was attempted. To compare the enzyme with other kinases, and to obtain information that could be used in its further purification, a characterization was undertaken. The enzyme required 10 mM Mg2+ for optimum activity, it had a KM of 0.09 mM for ATP and the KM for the peptide substrate APRP8 was 0.42 mM. It was not activated by cAMP or calmodulin, characteristics that are shared with casein kinases and mammary gland kinase. The sublingual kinase as well as casein kinase 2 were inhibited by heparin, but in other respects the two kinases had different properties. While casein kinase 2 is activated by polylysine and has optimal activity in 150 mM KCl, sublingual kinase was inhibited by polylysine and the addition of KCl. Moreover, casein kinase 2 can utilize both ATP and GTP as phosphoryl donors, but GTP was not a substrate for sublingual kinase. The sublingual kinase shared a substrate recognition sequence with mammary gland kinase, but, unlike that kinase, it could not utilize Ca2+ instead of Mg2+. While the sublingual kinase thus shared some properties with both casein kinase 2 and mammary gland kinase, distinct differences were also seen and the relationship to these enzymes remains to be determined. The characterization of the sublingual kinase will be useful in its further purification.

Identification of the phosphorylated sites of metabolically 32P-labeled osteopontin from cultured chicken osteoblasts

Osteopontin (OPN) is one of the major secretory phosphoproteins in both calcifying and non-calcifying tissues. Evidence has accumulated for the biological importance of the phosphoproteins and, in particular, the phosphate groups in bone formation, resorption, and calcification. The precise locations of the phosphate groups in the OPN molecule were determined by metabolically labeling OPN with 32P in cultured chicken osteoblasts, followed by purification to homogeneity. N-terminal sequencing showed a single sequence of WPVSKRQHAISA, consistent with that deduced from both cDNA, and previous amino acid sequencing of the protein isolated from chicken bone. Three 32P-labeled peptides were isolated by reverse-phase high performance liquid chromatography of thrombin-digested, 32P-labeled OPN. The N-terminal sequencing of each of these thrombin fragments gave single sequences as follows: WPVSKSRQHAIS, SHHTHRYHQDHVD, and ASKLRKAARKL, with approximate molecular masses of 5, 30, and 20 kDa. These data demonstrate that 32P was incorporated throughout the N- to C-terminal sequence of the protein. Thrombin specifically cleaved chicken OPN at two sites: between Arg-22 and Ser-23, which generated the 5-kDa N-terminal end fragment, and another between Lys-138 and Ala-139, which generated the 30- and 20-kDa fragments. To further define the exact locations of the phosphorylated amino acids and the surrounding amino acid sequences, OPN was digested with trypsin, which generated seven major 32P-labeled peptides whose amino acid sequences were determined. The phosphorylated peptide regions of osteopontin were identified as amino acids 8-18 (QHAIS*AS*S*EEK), 39-54 (LASQQTHYS*S*EENAD), 150-171 (LIEDDAT*AEVGDSQLAGLWLPK), 179-191 (ELAQHQSVENDSR), 194-205 (FDS*PEVGGDSK), 214-219 (ES*LASR), and 239-248 (HSIENNEVTR). The phosphorylated amino acid sites are followed by an asterisk (*). Of the seven identified phosphorylated peptide regions, three were localized on the N-terminal end of the osteopontin molecule (with five phosphorylated serines) and contained the sequence motifs that were phosphorylated by casein kinase II type(s), whereas the remaining four peptides are concentrated toward the C-terminal half of the molecule (with five phosphorylated residues) and contained recognition motifs for other kinases as well as casein kinase II.

Protein kinases of cultured osteoblasts: selectivity for the extracellular matrix proteins of bone and their catalytic competence for osteopontin

The enzyme activities of the major kinases found within the cytosolic and microsomal fractions of embryonic avian calvaria osteoblasts were assayed for their specificity for various noncollagenous extracellular matrix (ECM) proteins of bone. At least 6 proteins with M(r)'s of 66, 58, 50, 36, 30, and 22 kD out of more than 30 of the noncollagenous proteins of the bone ECM were phosphorylated by the kinase(s) found in both osteoblast cellular fractions. The purification and N-terminal sequence analysis of three of the above proteins, M(r)'s 66 and 58 kD (+50 kD), identified them as chicken bone sialoprotein (BSP) and osteopontin (OPN), respectively. Heparin, a specific inhibitor of factor-independent protein kinase (FIPK) activity, blocked the phosphorylation of all six ECM proteins by the microsomal kinase(s) but only inhibited the phosphorylation of the 66, 50, and 36 kD by the cytosolic enzyme(s). Casein kinase II (a known FIPK) showed a similar phosphorylation pattern of the same bone ECM proteins as the FIPK(s) found in osteoblast cell extracts, while purified cyclic adenosine monophosphate (cAMP)-dependent protein kinase did not phosphorylate any of the ECM proteins. Use of dephosphorylated casein showed that in comparison with casein kinase II, casein was a poor substrate for the FIPK found in the osteoblast cellular extracts. Further studies, using FIPK(s) of osteoblasts and purified chicken OPN or bacterially produced recombinant murine OPN as a substrate, showed that both species of OPN were excellent substrates for the FIPK(s) found in osteoblasts. The phosphorylation of the purified chicken and recombinant mouse OPNs were evaluated by quantitative analysis using commercially available protein kinases. cAMP-dependent kinase showed no phosphorylation of either protein, and cyclic guanodine monophosphate (cGMP)-dependent kinase and protein kinase C incorporated 1.2 and 0.5 mol phosphate/mol OPN, respectively. However, both chicken and mouse OPNs were significantly phosphorylated by casein kinase II (9.3 and 9.0 mol of phosphate/mol of OPN, respectively). These results demonstrate that the noncollagenous proteins of the bone ECM, and in particular OPN, are predominantly phosphorylated by FIPK(s), and this class of kinase is the major enzyme found within the microsomal fraction of osteoblasts.

Characterization and partial purification of microsomal casein kinase II from osteoblast-like cells: an enzyme that phosphorylates osteopontin and phosphophoryn

Microsomal casein kinase II (mCKII) is a membrane-bound enzyme present in the microsomal fractions of ROS 17/2.8 osteoblast-like cells. It phosphorylates acidic matrix phosphoproteins such as phosphophoryn and osteopontin. Addition of 1.0% Nonidet P-40 facilitates extraction of the optimum amount of detergent-solubilized and -activated enzyme from microsomal fractions. mCKII was partially purified over 3000-fold by sequential chromatography over DEAE-cellulose and heparin-agarose. SDS-polyacrylamide gels, showed that mCKII contained 43 kDa and 31 kDa polypeptides, corresponding to the alpha- and beta-subunits of the enzyme, respectively. The alpha subunit was identified by anti-CKII antiserum and the beta subunit, by its ability to undergo autophosphorylation. The enzyme was inhibited by 50% with 0.4 micrograms/ml heparin and stimulated by 100% with 1.0 mM spermine when casein was used as a substrate. The phosphorylation of phosphophoryn was reduced to 50% by 0.8 micrograms/ml heparin, but was increased to 2-2.5 fold by 5 to 15 mM spermine, which may be due to substrate-directed effects. Kinetic analysis showed that the apparent Km values for phosphophoryn (0.39 microM) and for osteopontin (2.1 microM) were lower than that for casein (21.3 microM). Vmax values of phosphophoryn and osteopontin were 2.2-fold and 4.6-fold higher than that of casein. Using the ratio Vmax/Km as a measure of kinetic specificity, osteopontin and phosphophoryn appear to be the more specific substrates than casein for mCKII. Thus, both proteins can be considered as physiological substrates for mCKII.