Studies on the protein tyrosine phosphatase activity of tartrate-resistant acid phosphatase

Loss of Vlk in Prx1+ Cells Delays the Initial Steps of Endochondral Bone Formation and Fracture Repair in the Limb

Vertebrate lonesome kinase (Vlk) is a secreted tyrosine kinase important for normal skeletogenesis during embryonic development. Vlk null mice (Vlk^-/- ) are born with severe craniofacial and limb skeletal defects and die shortly after birth. We used a conditional deletion model to remove Vlk in limb bud mesenchyme (Vlk-Prx1 cKO) to assess the specific requirement for Vlk expression by skeletal progenitor cells during endochondral ossification, and an inducible global deletion model (Vlk-Ubq iKO) to address the role of Vlk during fracture repair. Deletion of Vlk with Prx1-Cre recapitulated the limb skeletal phenotype of the Vlk^-/- mice and enabled us to study the postnatal skeleton as Vlk-Prx1 cKO mice survived to adulthood. In Vlk-Prx1 cKO adult mice, limbs remained shorter with decreased trabecular and cortical bone volumes. Both Vlk-Prx1 cKO and Vlk-Ubq iKO mice had a delayed fracture repair response but eventually formed bridging calluses. Furthermore, levels of phosphorylated osteopontin (OPN) were decreased in tibias of Vlk-Ubq iKO, establishing OPN as a Vlk substrate in bone. In summary, our data indicate that Vlk produced by skeletal progenitor cells influences the timing and extent of chondrogenesis during endochondral bone formation and fracture repair. © 2022 American Society for Bone and Mineral Research (ASBMR).

Human CPPED1 belongs to calcineurin-like metallophosphoesterase superfamily and dephosphorylates PI3K-AKT pathway component PAK4

Protein kinases and phosphatases regulate cellular processes by reversible phosphorylation and dephosphorylation events. CPPED1 is a recently identified serine/threonine protein phosphatase that dephosphorylates AKT1 of the PI3K-AKT signalling pathway. We previously showed that CPPED1 levels are down-regulated in the human placenta during spontaneous term birth. In this study, based on sequence comparisons, we propose that CPPED1 is a member of the class III phosphodiesterase (PDE) subfamily within the calcineurin-like metallophosphoesterase (MPE) superfamily rather than a member of the phosphoprotein phosphatase (PPP) or metal-dependent protein phosphatase (PPM) protein families. We used a human proteome microarray to identify 36 proteins that putatively interact with CPPED1. Of these, GRB2, PAK4 and PIK3R2 are known to regulate the PI3K-AKT pathway. We further confirmed CPPED1 interactions with PAK4 and PIK3R2 by coimmunoprecipitation analyses. We characterized the effect of CPPED1 on phosphorylation of PAK4 and PIK3R2 in vitro by mass spectrometry. CPPED1 dephosphorylated specific serine residues in PAK4, while phosphorylation levels in PIK3R2 remained unchanged. Our findings indicate that CPPED1 may regulate PI3K-AKT pathway activity at multiple levels. Higher CPPED1 levels may inhibit PI3K-AKT pathway maintaining pregnancy. Consequences of decreased CPPED1 expression during labour remain to be elucidated.

A simple and robust reporter gene assay for measuring the bioactivity of anti-RANKL therapeutic antibodies

The mechanism sketch of the reporter gene assay for the bioactivity determination of anti-RANKL mAbs.

Synthesis and evaluation of novel purple acid phosphatase inhibitors

Transgenic studies in animals have demonstrated a direct association between the level of expression of purple acid phosphatase (PAP; also known as tartrate-resistant acid phosphatase) and the progression of osteoporosis. Consequently, PAP has emerged as a promising target for the development of novel therapeutic agents to treat this debilitating disorder. PAPs are binuclear hydrolases that catalyse the hydrolysis of phosphorylated substrates under acidic to neutral conditions. A series of phenyltriazole carboxylic acids, prepared by the reactions of azide derivatives with propiolic acid through copper(i)-catalysed azide-alkyne cycloaddition click reactions, has been assessed for their inhibitory effect on the catalytic activity of pig and red kidney bean PAPs. The binding mode of most of these compounds is purely uncompetitive with K _iuc values as low as ∼23 μM for the mammalian enzyme. Molecular modelling has been used to examine the binding modes of these triazole compounds in the presence of a substrate in the active site of the enzyme in order to rationalise their activities and to design more potent and specific derivatives.

Complex coordinated extracellular metabolism: Acid phosphatases activate diluted human leukocyte proteins to generate energy flow as NADPH from purine nucleotide ribose

Complex metabolism is thought to occur exclusively in the crowded intracellular environment. Here we report that diluted enzymes from lysed human leukocytes produce extracellular energy. Our findings involve two pathways: the purine nucleotide catabolic pathway and the pentose phosphate pathway, which function together to generate energy as NADPH. Glucose6P fuel for NADPH production is generated from structural ribose of purine ribonucleoside monophosphates, ADP, and ADP-ribose. NADPH drives glutathione reductase to reduce an oxidized glutathione disulfide-glutathione redox couple. Acid phosphatases initiate ribose5P salvage from purine ribonucleoside monophosphates, and transaldolase controls the direction of carbon chain flow through the nonoxidative branch of the pentose phosphate pathway. These metabolic control points are regulated by pH. Biologically, this energy conserving metabolism could function in perturbed extracellular spaces.

•

Extracellular metabolism occurs in dilute human leukocyte cytosolic protein solution.

•

Extracellular metabolism is activated by acid phosphatases.

•

Ribose5P salvage from purine nucleotides and its metabolism produces NADPH.

•

Extracellular NADPH generating metabolism influences the redox environment.

•

Extracellular metabolism could be a component of inflammation and wound healing.

The extended human PTPome: a growing tyrosine phosphatase family

Tyr phosphatases are, by definition, enzymes that dephosphorylate phospho-Tyr (pTyr) from proteins. This activity is found in several structurally diverse protein families, including the protein Tyr phosphatase (PTP), arsenate reductase, rhodanese, haloacid dehalogenase (HAD) and His phosphatase (HP) families. Most of these families include members with substrate specificity for non-pTyr substrates, such as phospho-Ser/phospho-Thr, phosphoinositides, phosphorylated carbohydrates, mRNAs, or inorganic moieties. A Cys is essential for catalysis in PTPs, rhodanese and arsenate reductase enzymes, whereas this work is performed by an Asp in HAD phosphatases and by a His in HPs, via a catalytic mechanism shared by all of the different families. The category that contains most Tyr phosphatases is the PTP family, which, although it received its name from this activity, includes Ser, Thr, inositide, carbohydrate and RNA phosphatases, as well as some inactive pseudophosphatase proteins. Here, we propose an extended collection of human Tyr phosphatases, which we call the extended human PTPome. The addition of new members (SACs, paladin, INPP4s, TMEM55s, SSU72, and acid phosphatases) to the currently categorized PTP group of enzymes means that the extended human PTPome contains up to 125 proteins, of which ~ 40 are selective for pTyr. We set criteria to ascribe proteins to the extended PTPome, and summarize the more important features of the new PTPome members in the context of their phosphatase activity and their relationship with human disease.

MALDI mass sequencing and biochemical characterization of Setaria cervi protein tyrosine phosphatase

A 30-kDa acid phosphatase with protein tyrosine phosphatase activity was identified in Setaria cervi (ScPTP). The enzyme was purified to homogeneity using three-step column chromatography. Matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) analysis of purified ScPTP yielded a total of eight peptides matching most closely to phosphoprotein phosphatase of Ricinus communis (RcPP). A hydrophilicity plot of RcPP revealed the presence of these peptides in the hydrophilic region, suggesting their antigenic nature. The substrate specificity of ScPTP with ortho-phospho-L-tyrosine and inhibition with sodium orthovanadate and ammonium molybdate affirmed it as a protein tyrosine phosphatase. ScPTP was also found to be tartrate resistant. The Km and Vmax were 6.60 mM and 83.3 μM/ml/min, respectively, with pNPP and 8.0 mM and 111 μM/ml/min, respectively, with ortho-phospho-L-tyrosine as the substrate. The Ki value with sodium orthovanadate was calculated to be 16.10 mM. Active site modification with DEPC, EDAC and pHMB suggested the presence of histidine, cysteine and aspartate at its active site. Thus, on the basis of MALDI-TOF and biochemical studies, it was confirmed that purified acid phosphatase is a PTP.

ACP5 (Uteroferrin): phylogeny of an ancient and conserved gene expressed in the endometrium of mammals

Type 5 acid phosphatase (ACP5; also known as tartrate-resistant acid phosphatase or uteroferrin) is a metalloprotein secreted by the endometrial glandular epithelium of pigs, mares, sheep, and water buffalo. In this paper, we describe the phylogenetic distribution of endometrial expression of ACP5 and demonstrate that endometrial expression arose early in evolution (i.e., before divergence of prototherian and therian mammals ~166 million years ago). To determine expression of ACP5 in the pregnant endometrium, RNA was isolated from rhesus, mouse, rat, dog, sheep, cow, horse, armadillo, opossum, and duck-billed platypus. Results from RT-PCR and RNA-Seq experiments confirmed that ACP5 is expressed in all species examined. ACP5 was also demonstrated immunochemically in endometrium of rhesus, marmoset, sheep, cow, goat, and opossum. Alignment of inferred amino acid sequences shows a high conservation of ACP5 throughout speciation, with species-specific differences most extensive in the N-terminal and C-terminal regions of the protein. Analysis by Selecton indicated that most of the sites in ACP5 are undergoing purifying selection, and no sites undergoing positive selection were found. In conclusion, endometrial expression of ACP5 is a common feature in all orders of mammals and has been subjected to purifying selection. Expression of ACP5 in the uterus predates the divergence of therians and prototherians. ACP5 is an evolutionary conserved gene that likely exerts a common function important for pregnancy in mammals using a wide range of reproductive strategies.

Long bone osteoclasts display an augmented osteoclast phenotype compared to calvarial osteoclasts

Osteoclasts are multinucleated cells specialized in degrading bone and characterized by high expression of the enzymes tartrate-resistant acid phosphatase (TRAP) and cathepsin K (CtsK). Recent studies show that osteoclasts exhibit phenotypic differences depending on their anatomical site of action. Using immunohistochemistry, RT-qPCR, FPLC chromatography and immunoblotting, we compared TRAP expression in calvaria and long bone. TRAP protein and enzyme activity levels were higher in long bones compared to calvaria. In addition, proteolytic processing of TRAP was more extensive in long bones than calvaria which correlated with higher cysteine proteinase activity and protein expression of CtsK. These two types of bones also exhibited a differential expression of monomeric TRAP and CtsK isoforms. Analysis of CtsK(-/-) mice revealed that CtsK is involved in proteolytic processing of TRAP in calvaria. Moreover, long bone osteoclasts exhibited higher expression of not only TRAP and CtsK but also of the membrane markers CD68 and CD163. The results suggest that long bone osteoclasts display an augmented osteoclastic phenotype with stronger expression of both membranous and secreted osteoclast proteins.

Hydrolysis of dinitrobenzamide phosphate prodrugs: the role of alkaline phosphatase

Phosphate prodrugs which undergo hydrolysis in vivo have been used to improve the solubility and pharmacokinetic properties of a number of drugs. Dinitrobenzamide mustards (DNBM) are examples of such drugs. We investigated the ability of purified alkaline phosphatase isoforms to dephosphorylate three DNBM phosphate prodrugs. In addition, the relative rate of dephosphorylation of these phosphate prodrugs in a number of tissues was determined. These phosphate prodrugs are indeed substrates for alkaline phosphatase, with time dependent formation of the hydrolysis product. Intestinal alkaline phosphatase (IAP) and placental alkaline phosphatase (PLAP) had the highest activity for these substrates and compound P2 was the most rapidly metabolised. Similarly, compound P2 had the shortest half life in mouse serum (t1/2 = 1.15 h) compared with P1 (t1/2 = 13.34 h) and P3 (t1/2 = 4.4 h). However, serum has very low dephosphorylase activity for these substrates compared with intestine and liver homogenates. In addition, there is little or no difference in the relative rate of dephosphorylation of each of the three compounds in mouse tissues in contrast to the pattern observed with purified alkaline phosphatase and mouse serum. Hence additional phosphatase enzymes may be involved in the metabolism of phosphate prodrugs in vivo.

Role for an essential tyrosine in peptide amidation

The catalytic core of the peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL) domain of peptidylglycine alpha-amidating monooxygenase was investigated with respect to its ability to function as a ureidoglycolate lyase and the identity and role of its bound metal ions. The purified PAL catalytic core (PALcc) contains molar equivalents of calcium and zinc along with substoichiometric amounts of iron and functions as a ureidoglycolate lyase. Limiting iron availability in the cells synthesizing PALcc reduces the specific activity of the enzyme produced. Concentrated samples of native PALcc have an absorption maximum at 560 nm, suggestive of a phenolate-Fe(III) charge transfer complex. An essential role for a Tyr residue was confirmed by elimination of PAL activity following site-directed mutagenesis. Purified PALcc in which the only conserved Tyr residue (Tyr(654)) was mutated to Phe was secreted normally, but was catalytically inactive and lacked bound iron and bound zinc. Our data demonstrate an essential role for Tyr(654) and suggest that it serves as an Fe(III) ligand in an essential iron-zinc bimetallic site.

Endocytosis of tartrate-resistant acid phosphatase by osteoblast-like cells is followed by inactivation of the enzyme

Tartrate-resistant acid phosphatase (TRACP) is generally used as a marker of osteoclasts. Yet, other bone-associated cells, such as osteoblasts and osteocytes, may also express activity of this enzyme. Osteoblasts containing TRACP activity are seen particularly in the vicinity of areas of bone resorption, suggesting that osteoclasts somehow induce TRACP activity in osteoblasts. In a recent study, we found that osteoblast-like cells appeared to have the capacity to endocytose TRACP released by osteoclast precursors. In the present study, we investigated the endocytosis of TRACP in more detail as well as the fate of the endocytosed enzyme. We found that incubation of osteoblast-like cells with TRACP-coated beads resulted in attachment of a high number of beads to the cells. After culturing osteoblast-like cells with medium conditioned by blood monocytes that contain TRACP, activity of the enzyme was found in the cells. Following replacement of the medium by normal medium that did not contain TRACP, a decrease in the level of TRACP activity in osteoblast-like cells occurred. Our data strongly suggest that osteoblast-like cells recognize TRACP released by osteoclast precursors and that upon endocytosis inactivation of the enzyme occurs. We propose that uptake of the enzyme is important for the control of enzyme activity, thereby preventing degradation of matrix constituents.

Localisation of high acid phosphotyrosine phosphatase activity in afferent arterioles and glomeruli of human kidney

Endothelial cells contain a variety of specific protein tyrosine phosphatases and an acid phosphatase differing from other known phosphatases. The highest activity of this acid phosphatase with artificial or unspecific substrates is present in the afferent arterioles and glomeruli of human kidney, and the activity is inhibited by nephrotoxic fluoride concentrations, suggesting that it plays a role in circulatory regulation. Here the activity was characterised with physiological substrates. An incubation mixture containing phosphotyrosine or phosphoserine was stable at pH 5 when phosphate-precipitating lead was chelated with tartrate. The activities were studied in frozen sections. Only phosphotyrosine was hydrolysed by some cells. High activity of tartrate-resistant phosphotyrosine phosphatase was present in lymphocytes, endothelial cells of afferent arterioles, and glomerular mesangial cells of kidney, decidual cells, and alveolar macrophages. In lymphocytes the activity was fluoride-resistant and vanadate-sensitive, in other cells fluoride- and vanadate-sensitive. In decidual cells and alveolar macrophages, the activity is due to specific osteoclastic/macrophagic tartrate-resistant acid phosphatase, in lymphocytes to specific protein tyrosine phosphatases, and in endothelial and mesangial cells to a protein tyrosine phosphatase-like acid phosphatase. The results suggest that in endothelial cells of the afferent arterioles, mesangial cells, and lymphocytes the cellular activities are regulated by high constitutive phosphotyrosine phosphatase activity and this may be related to the exceptional cyclosporin A sensitivity of these cells.

Serum tartrate-resistant acid phosphatase 5b in monitoring bisphosphonate treatment with clodronate: a comparison with urinary N-terminal telopeptide of type I collagen and serum type I procollagen amino-terminal propeptide

Osteoclastic tartrate-resistant acid phosphatase activity in serum (S-TRACP 5b) was measured in postmenopausal women ( n =59, mean age 56.1 years) with vertebral osteopenia before and during 2-year treatment with an 800-mg daily dose of clodronate, with a non-amino bisphosphonate. Changes in TRACP 5b were compared with those in urinary excretion of type I collagen amino-terminal telopeptide (U-NTX), corrected for creatinine excretion, a well-established marker of bone resorption, and to serum type I procollagen amino-terminal propeptide (S-PINP), a marker of bone formation. Marker changes 1 year after start of treatment were correlated with changes in bone mineral density (BMD). The least significant change (LSC) for each marker and BMD was calculated from values for subjects receiving placebo. Responders to treatment were those exhibiting a change larger than LSC. In response to clodronate treatment S-TRACP 5b (mean change up to -18%) decreased less than did U-NTX (up to -51%) or S-PINP (up to -46%). Marker changes correlated with changes in lumbar spine and trochanter BMD. The most efficient marker for finding responders to treatment was S-PINP, which changed more than the LSC (32%) in 72% of the subjects at the 1-year time point and in 79% at the 2-year time point. S-TRACP 5b change exceeded the LSC (27%) in 40% and 34% of the subjects at each time point, while U-NTX change exceeded the LSC (55%) in 55% and 40%, respectively. We conclude that, in terms of the proportion of subjects exhibiting any change exceeding the LSC, S-TRACP 5b did not appear to be superior to U-NTX and S-PINP in the follow-up of clodronate treatment. The reason may lie in the mechanism of action of clodronate, which rather than reducing the number of TRACP 5b-secreting osteoclasts, reduces the activity of bone proteolytic enzymes and thus the rate of bone organic matrix degradation. This is seen in decreased amounts of type I collagen breakdown products (U-NTX), and through coupling of bone resorption with bone formation, in a decrease in circulating levels of the marker that reflects new collagen formation (S-PINP).

Eccentric localization of osteocytes expressing enzymatic activities, protein, and mRNA signals for type 5 tartrate-resistant acid phosphatase (TRAP)

Enzymatic activity of type 5 tartrate-resistant acid phosphatase (TRAP) has been regarded as one of the reliable markers for osteoclasts and their precursors. The presence of TRAP activity in osteocytes near the bone resorbing surface has also been pointed out in some reports. However, the significance of TRAP reactions in osteocytes remains controversial and, in fact, there is no agreement as to whether the histochemical enzyme reactions in osteocytes represent the TRAP enzyme generated by the respective osteocytes or is a mere diffusion artifact of the reaction products derived from the nearby osteoclasts. Current histochemical, immunohistochemical, and in situ hybridization studies of rat and canine bones confirmed TRAP enzyme activity, TRAP immunoreactivity, and the expression of Trap mRNA signals in osteocytes located close to the bone-resorbing surface. TRAP/Trap- positive osteocytes thus identified were confined to the areas no further than 200 microm from the bone-resorbing surface and showed apparent upregulation of TRAP/Trap expression toward the active osteoclasts. Spatial and temporal patterns of TRAP/Trap expression in the osteocytes should serve as a valuable parameter for further analyses of biological interactions between the osteocytes and the osteoclasts associated with bone remodeling.

Repetitive, negligible force reaching in rats induces pathological overloading of upper extremity bones

Work-related repetitive motion disorders are costly. Immunohistochemical changes in bones resulting from repetitive reaching and grasping in 17 rats were examined. After 3-6 weeks, numbers of ED1+ macrophages and osteoclasts increased at periosteal surfaces of sites of muscle and interosseous membrane attachment and metaphyses of reach and nonreach forelimbs. These findings indicate pathological overloading leading to inflammation and subsequent bone resorption.

INTRODUCTION

Sixty-five percent of all occupational illnesses in U.S. private industry are attributed to musculoskeletal disorders arising from the performance of repeated motion, yet the precise mechanisms of tissue pathophysiology have yet to be determined for work-related musculoskeletal disorders. This study investigates changes in upper extremity bone tissues resulting from performance of a voluntary highly repetitive, negligible force reaching and grasping task in rats.

MATERIALS AND METHODS

Seventeen rats reached an average of 8.3 times/minute for 45-mg food pellets for 2 h/day, 3 days/week for up to 12 weeks. Seven rats served as normal or trained controls. Radius, ulna, humerus, and scapula were collected bilaterally as follows: radius and ulna at 0, 3, 4, 5, 6, and 12 weeks and humerus and scapula at 0, 4, and 6 weeks. Bones were examined for ED1-immunoreactive mononuclear cells and osteoclasts. Double-labeling immunohistochemistry was performed for ED1 (monocyte/macrophage lineage cell marker) and TRACP (osteoclast marker) to confirm that ED1+ multinucleated cells were osteoclasts. Differences in the number of ED1+ cells over time were analyzed by ANOVA.

RESULTS

Between 3 and 6 weeks of task performance, the number of ED1+ mononuclear cells and osteoclasts increased significantly at the periosteal surfaces of the distal radius and ulna of the reach and nonreach limbs compared with control rats. These cells also increased at periosteal surfaces of humerus and scapula of both forelimbs by 4-6 weeks. These cellular increases were greatest at muscle attachments and metaphyseal regions, but they were also present at some interosseous membrane attachments. The number of ED1+ cells decreased to control levels in radius and ulna by 12 weeks.

CONCLUSIONS

Increases in ED1+ mononuclear cells and osteoclasts indicate that highly repetitive, negligible force reaching causes pathological overloading of bone leading to inflammation and osteolysis of periosteal bone tissues.

TRACP as an osteopontin phosphatase

TRACP is synthesized as a latent proenzyme requiring proteolytic processing to attain maximal phosphatase activity. Excision of an exposed loop domain abolishes the interaction between the loop residue Asp146 and a ligand to the redox-sensitive iron of the active site, most likely Asn91, providing a mechanism for the enzyme repression. Both cathepsin K and L efficiently cleave in the loop domain and activate the latent enzyme, and we propose that cathepsin K acts as a physiological activator of TRACP in osteoclasts, whereas cathepsin L might fulfill a similar role in different types of macrophages. Considering the rather broad substrate specificity of TRACP, a tight regulation of its activity in the cell appears warranted. Besides proteolytic cleavage, the enzyme should need a specific local environment with a slightly acidic pH and reducing equivalents to keep the enzyme fully active. Cellular subcompartments where these required conditions prevail are potential subcellular site(s) of TRACP action. Of bone phosphoproteins shown to be substrates for TRACP, both osteopontin and bone sialoprotein are colocalized with TRACP in the resorption lacuna of the osteoclasts, and dephosphorylation of OPN impair its ability to promote adhesion as well as migration of osteoclasts in vitro. A role for TRACP as an osteopontin phosphatase in bone is therefore suggested. The expression of TRACP as well as OPN in other tissues with possible interactions between the two could suggest a more general function for TRACP as a regulator of OPN phosphorylation and bioactivity.

Acid phosphatases as markers of bone metabolism

Various biochemical markers have been used to assess bone metabolism and to monitor the effects of treatments. Tartrate resistant acid phosphatase (TRAP; EC 3.1.3.2) has often been used to assess bone absorption. Although osteoclasts contain abundant TRAP and they are responsible for bone resorption, the total TRAP activities in the serum measured by colorimetric methods little reflect the bone turnover. TRAP 5 is further separated into 5a and 5b by electrophoresis. Type 5b is considered to be derived from the osteoclast, and therefore attempts are being made to measure exclusively serum TRAP 5b by kinetic methods, immunological methods, and chromatographic methods including ion-exchange and heparin column chromatography.

Transcription from the tartrate-resistant acid phosphatase promoter is negatively regulated by the Myc oncoprotein

TRAP, a characteristic marker of osteoclast differentiation, is an enzyme that plays an active role in the process of bone resorption. Despite the importance of TRAP in osteoclast biology, the components involved in the transcriptional regulation of this gene are largely unknown. This study investigated the regulation of TRAP transcription by the Myc oncoprotein in three different cell types. A series of nested TRAP promoter deletion constructs were cotransfected into P388D1 murine macrophages and C3H10T1/2 murine embryonic fibroblasts along with a backbone plasmid control or expression plasmids containing v-Myc, c-Myc, or an inactive v-Myc protein construct (delta84/NLS). Both v-Myc and c-Myc negatively regulated transcription from the TRAP promoter in P388D1 and C3H10T1/2 cells, 90% and 50%, respective to cell type and amount of endogenous Myc protein, and delta84/NLS had no effect. The functional Myc-responsive element(s) within the TRAP promoter was localized to a region between -436 and +1 bp, which contains two putative Myc-inhibitory binding sites coincident with an initiator element (Inr) at -116 bp and -18 bp. Conversely, in the HD-11EM chicken v-Myc transformed preosteoclast cell line, the full-length TRAP promoter transcription was increased when endogenous v-Myc levels were decreased in response to pretreatment of these cells with 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. This report provides the first evidence of the specific regulation of TRAP at the transcriptional level by Myc, a transcription factor that is normally expressed at relatively high levels in preosteoclasts and other myelomonocytic cells and suggests that Myc plays an active role in suppressing the transcription of a mature osteoclast selective gene.

Acid phosphatases

Acid phosphatases (APs) are a family of enzymes that are widespread in nature, and can be found in many animal and plant species. Mystery surrounds the precise functional role of these molecular facilitators, despite much research. Yet, paradoxically, human APs have had considerable impact as tools of clinical investigation and intervention. One particular example is tartrate resistant acid phosphatase, which is detected in the serum in raised amounts accompanying pathological bone resorption. This article seeks to explore the identity and diversity of APs, and to demonstrate the relation between APs, human disease, and clinical diagnosis.

Iron lactate-induced osteopenia in male Sprague-Dawley rats

Osteopenia was induced in rats fed a diet containing 50,000 ppm (5%) iron lactate for 2 or 4 weeks. Blood chemistry, urinalysis, and bone histomorphometry of the proximal tibial metaphysis were performed. Urinary pyridinoline and deoxypyridinoline and the osteoclast number per bone surface were selected for the measurement of dynamic resorption. The osteoclast surface, eroded surface, and osteoblast surface increased at both ends of the exposure periods, and bone resorption and formation both increased. The bone volume, trabecular thickness, and trabecular number decreased, and the secondary spongiosa of proximal metaphysis showed a marked bone loss. However, no mineralization defect was observed. At the end of the 2-week exposure period, biomarkers of osteoclasts and osteoblasts had increased the most, and the osteoblast surface, osteoclast surface, and osteoclast number per bone surface increased with prolonged exposure. The pathological changes of the bone lesion in iron lactate-overloaded rats were similar to those in rats of the osteoporotic model, because they consisted of changes reflecting the increase of bone resorption and formation without an osteomalacic change. However, the decline of serum parathyroid hormone (PTH) levels was different from that of the osteoporosis model rat. We concluded iron-induced bone lesions probably differ from those of low turnover bone diseases.

Heparin column analysis of serum type 5 tartrate-resistant acid phosphatase isoforms

The objective of the present study was to develop a specific method for the separation of tartrate-resistant acid phosphatase (TRAP) derived exclusively from osteoclasts. Heparin column-bound TRAP in human serum was separated into three peaks of TRAP activity when eluted with a linear gradient of sodium chloride. The last peak corresponded to TRAP 5b which was first named according to its electrophoretic mobility [Clin. Chem. 24 (1978) 309] and was considered to be derived from osteoclasts [J. Bone Miner. Res. 13 (1998) 683]. The second peak was found to be TRAP 5a. The height of the last peak varied from age to age.

Characterization of a prostate-specific tyrosine phosphatase by mutagenesis and expression in human prostate cancer cells

The cellular form of human prostatic acid phosphatase (PAcP) is a neutral protein-tyrosine phosphatase (PTP) and may play a key role in regulating the growth and androgen responsiveness of prostate cancer cells. The functional role of the enzyme is at least due in part to its dephosphorylation of c-ErbB-2, an in vivo substrate of the enzyme. In this study, we investigated the molecular mechanism of phosphotyrosine dephosphorylation by cellular PAcP. We mutated several amino acid residues including one cysteine residue that was proposed to be involved in the PTP activity of the enzyme by serving as the phosphate acceptor. The cDNA constructs of mutant enzymes were transiently transfected into C-81 LNCaP and PC-3 human prostate cancer cells that lack the endogenous PAcP expression. The phosphotyrosine level of ErbB-2 in these transfected cells was subsequently analyzed. Our results demonstrated that the phosphotyrosine level of ErbB-2 in cells expressing H12A or D258A mutant PAcP is similar to that in control cells without PAcP expression, suggesting that these mutants are incapable of dephosphorylating ErbB-2. In contrast, cells expressing C183A, C281A, or wild-type PAcP had a decreased phosphotyrosine level of ErbB-2, compared with the control cells. Similar results were obtained from in vitro dephosphorylation of immunoprecipitated ErbB-2 by these mutant enzymes. Furthermore, transient expression of C183A, C281A, or the wild-type enzyme, but not H12A or D258A, decreased the growth rate of C-81 LNCaP cells. The data collectively indicate that His-12 and Asp-258, but not Cys-183 or Cys-281, are required for the PTP activity of PAcP.

Meaningful relationships: the regulation of the Ras/Raf/MEK/ERK pathway by protein interactions

The Ras/Raf/MEK (mitogen-activated protein kinase/ERK kinase)/ERK (extracellular-signal-regulated kinase) pathway is at the heart of signalling networks that govern proliferation, differentiation and cell survival. Although the basic regulatory steps have been elucidated, many features of this pathway are only beginning to emerge. This review focuses on the role of protein-protein interactions in the regulation of this pathway, and how they contribute to co-ordinate activation steps, subcellular redistribution, substrate phosphorylation and cross-talk with other signalling pathways.

Electrophoretic study of tartrate-resistant acid phosphatase isoforms in endstage renal disease and rheumatoid arthritis

The objective of this study was to identify the isoform, type-5a or type-5b, responsible for increased tartrate-resistant acid phosphatase (TRAP) activity in endstage renal disease (ESRD) and TRAP protein in rheumatoid arthritis (RA). We studied 24 sera each from healthy, ESRD and RA subjects. Type-5 TRAP activity and protein were quantitated by immunoassays. Isoform expression was determined by computerized imaging of non-denaturing polyacrylamide gels (PAGE) stained for TRAP activity. Other biochemical markers included: intact parathyroid hormone (iPTH), total and bone-specific alkaline phosphatase (TAP, BAP), N-telopeptides of type-I collagen (NTx), and free pyridinoline (Pyd). Isoform 5a was normal in both ESRD and RA. Isoform 5b was elevated in ESRD only. Serum TRAP activity correlated with both isoforms 5a and 5b in RA, but only with 5b in ESRD. TRAP protein assays did not correlate with PAGE assays for 5a or 5b. TRAP activity, but not protein, correlated with BAP and NTx in RA sera. Both TRAP activity and protein correlated with iPTH, TAP and Pyd in ESRD sera. Increased TRAP activity in ESRD was due to increased osteoclastic isoform 5b and related to bone turnover. Increased TRAP protein in RA was suspected, but not proven, to be isoform 5a and not related to bone turnover. Heterogeneity of serum TRAP and preferential expression of isoforms has clinical significance in different diseases including ESRD and RA.

Biology of tartrate-resistant acid phosphatase

Tartrate-resistant acid phosphatase (TRAP) is a member of the ubiquitously expressed enzyme family of the acid phosphatases. Nearly 30 years ago, TRAP became known to hematologists as cytochemical marker enzyme of hairy cell leukemia. Physiologically, TRAP is primarily a cytochemical marker of macrophages, osteoclasts and dendritic cells. TRAP is localized intracellularly in the lysosomal compartment. Recent data suggest also secretion of TRAP by some cell types, in particular by osteoclasts. Human, mouse and rat TRAP are biochemically well characterized. While the complete genomic sequence of TRAP has been elucidated, only limited information on the genetic details of the gene and its regulation is available. It appears that the intracellular iron content is involved in the regulation of the enzyme. The physiological substrates for this enzyme have not been identified yet and consequently the functional role of TRAP remains completely unknown, though some hypotheses have been forwarded, e.g. involvement in bone resorption and iron homeostasis (transport, metabolism). Taken together, research on the biology of TRAP has been intensive and has led to considerable progress on a number of fronts, including the cloning of the gene. Further studies are, however, still required to determine the role of TRAP in vivo.

Tartrate-resistant acid phosphatase 5b: a novel serum marker of bone resorption

Human serum contains two forms of tartrate-resistant acid phosphatase (TRAP), 5a and 5b. Of these, 5a contains sialic acid and 5b does not. We show here that antigenic properties and pH optimum of TRAP purified from human osteoclasts are identical to those of serum TRAP 5b and completely different from those of serum TRAP 5a, suggesting that 5b would be derived from osteoclasts and 5a from some other source. We developed a novel immunoassay specific for 5b using a monoclonal antibody O1A as capture antibody. O1A did not bind acid phosphatase derived from platelets and erythrocytes. Western analysis showed that O1A was specific for TRAP in both human bone and serum. We measured bound TRAP activity at pH 6.1, where 5b is highly active and 5a almost completely inactive. The immunoassay detected more than 90% of the initial TRAP 5b activity after 8-h incubation of serum samples at 25 degrees C and after 3 days incubation at 4 degrees C. Serum TRAP 5b activity decreased significantly after 6 months of hormone replacement therapy (HRT) of postmenopausal women compared with the change observed in postmenopausal women receiving placebo (p < 0.0001). Instead, no significant differences were observed between the changes in the placebo and HRT groups in total serum TRAP amount. These results show that serum TRAP 5b is a specific and sensitive marker for monitoring antiresorptive treatment. Instead, total serum TRAP cannot be used for that purpose. These findings may turn out to be a significant improvement in using serum TRAP as a resorption marker.

Intracellular fragmentation of bone resorption products by reactive oxygen species generated by osteoclastic tartrate-resistant acid phosphatase

Tartrate-resistant acid phosphatase (TRAP) is highly expressed in bone-resorbing osteoclasts and activated macrophages. It has been suggested that a redox-active iron in the binuclear iron center of TRAP could have the capacity to react with hydrogen peroxide to produce highly destructive reactive oxygen species (ROS). Here we show that TRAP can generate ROS in vitro and that cells over-expressing TRAP produce higher amounts of intracellular ROS than their parent cells. We further demonstrate that these ROS can be targeted to destroy collagen and other proteins. In resorbing osteoclasts, TRAP was found in transcytotic vesicles transporting matrix degradation products through the cell, suggesting that TRAP-facilitated fragmentation of endocytosed material takes place in a specific cellular compartment. These results suggest that bone matrix degradation occurs not only extracellularly in the resorption lacunae but also intracellularly in the transcytotic vesicles. We propose that proteins containing redox-active iron could represent a novel mechanism of physiological fragmentation of organic molecules. This mechanism could be important in tissue remodeling and as a defense mechanism of phagocytosing cells.

Three-dimensional structure of a mammalian purple acid phosphatase at 2.2 A resolution with a mu-(hydr)oxo bridged di-iron center

The crystal structure of purple acid phosphatase from rat bone has been determined by molecular replacement and the structure has been refined to 2.2 A resolution to an R -factor of 21.3 % (R -free 26.5 %). The core of the enzyme consists of two seven-stranded mixed beta-sheets, with each sheet flanked by solvent-exposed alpha-helices on one side. The two sheets pack towards each other forming a beta-sandwich. The di-iron center, located at the bottom of the active-site pocket at one edge of the beta-sandwich, contains a mu-hydroxo or mu-oxo bridge and both metal ions are observed in an almost perfect octahedral coordination geometry. The electron density map indicates that a mu-(hydr)oxo bridge is found in the metal center and that at least one solvent molecule is located in the first coordination sphere of one of the metal ions. The crystallographic study of rat purple acid phosphatase reveals that the mammalian enzymes are very similar in overall structure to the plant enzymes in spite of only 18 % overall sequence identity. In particular, coordination and geometry of the iron cluster is preserved in both enzymes and comparison of the active-sites suggests a common mechanism for the mammalian and plant enzymes. However, significant differences are found in the architecture of the substrate binding pocket.

Two-site immunoassays for osteoclastic tartrate-resistant acid phosphatase based on characterization of six monoclonal antibodies

Tartrate-resistant acid phosphatase (TRAP), an enzyme expressed in bone-resorbing osteoclasts, is secreted into the circulation during bone resorption. We used six monoclonal antibodies (MAbs) to optimize direct two-site fluoroimmunoassays for determining serum TRAP concentrations. Four of the MABs, 1F1, 2H1, 4E6, and 5C1, were raised against recombinant human TRAP, and the other two, O1A and J1B, against human bone TRAP. 2H1, J1B, and O1A appeared to be highly specific for TRAP. 1F1 and 4E6 were poor in recognizing bone TRAP and were not useful in the assay. 5C1, while having a good affinity for the bone enzyme, was not specific. Serum TRAP is relatively stable, because 7 days of storage of serum samples at 4 degreesC and -20 degreesC or five thawing-freezing cycles, did not change the TRAP concentration detected using the two-site assays. All studied assays detected an increase in serum TRAP concentrations of postmenopausal women compared with premenopausal women, the difference being highest with MAB pairs 2H1-5C1 and O1A-J1B. These results suggest that serum TRAP may be a useful bone resorption marker, and the MAB pairs 2H1-5C1 and O1A-J1B may be useful in determining the bone resorption rate.