Substrate specificity of Gaucher spleen phosphoprotein phosphatase

Cathepsin K regulates localization and secretion of Tartrate-Resistant Acid Phosphatase (TRAP) in TRAP-overexpressing MDA-MB-231 breast cancer cells

Background

Tartrate–resistant acid phosphatase (TRAP/ ACP5) belongs to the binuclear metallophosphatase family and is present in two isoforms. The primary translation product is an uncleaved TRAP 5a isoform with low phosphatase activity. TRAP 5a can be post-translationally processed to a cleaved TRAP 5b isoform with high phosphatase activity by e.g. cysteine proteinases, such as Cathepsin K (CtsK). The relevance of the phosphatase activity of TRAP 5b has been demonstrated for proliferation, migration and invasion of cancer cells. TRAP-overexpressing MDA-MB-231 breast cancer cells displayed higher levels of TRAP 5a and efficient processing of TRAP 5a to TRAP 5b protein, but no changes in levels of CtsK when compared to mock-transfected cells. In TRAP-overexpressing cells colocalization of TRAP 5a and proCtsK was augmented, providing a plausible mechanism for generation of TRAP 5b. CtsK expression has been associated with cancer progression and has been pharmacologically targeted in several clinical studies.

Results

In the current study, CtsK inhibition with MK-0822/Odanacatib did not abrogate the formation of TRAP 5b, but reversibly increased the intracellular levels of a N-terminal fragment of TRAP 5b and reduced secretion of TRAP 5a reversibly. However, MK-0822 treatment neither altered intracellular TRAP activity nor TRAP-dependent cell migration, suggesting involvement of additional proteases in proteolytic processing of TRAP 5a. Notwithstanding, CtsK was shown to be colocalized with TRAP and to be involved in the regulation of secretion of TRAP 5a in a breast cancer cell line, while it still was not essential for processing of TRAP 5a to TRAP 5b isoform.

Conclusion

In cancer cells multiple proteases are involved in cleaving TRAP 5a to high-activity phosphatase TRAP 5b. However, CtsK-inhibiting treatment was able to reduce secretion TRAP 5a from TRAP-overexpressing cancer cells.

Biology and clinical significance of tartrate-resistant acid phosphatases: new perspectives on an old enzyme

Type 5 tartrate-resistant acid phosphatase (TRAP) has been a clinically relevant biomarker for about 50 years. It has always been a reliable and specific cytochemical marker for hairy cell leukemia and for differentiated cells of monocytic lineage. Only recently has the test for serum TRAP activity been accepted as sensitive and specific enough for clinical use as a marker of osteoclasts and bone resorption. This has come about through steady advances in knowledge about TRAP enzymology, structure, function, and molecular regulation and a consequent appreciation that TRAP isoforms 5a and 5b have very different clinical significance. As a measure of osteoclast number and bone resorption, TRAP 5b has diagnostic and prognostic applications in osteoporosis, cancers with bone metastasis, chronic renal failure, and perhaps other metabolic and pathologic bone diseases. Serum TRAP 5a, on the other hand, has no relationship to bone metabolism but seems instead to be a measure of activated macrophages and chronic inflammation. Exploration of the real clinical usefulness of serum TRAP 5a for diagnosis and disease management in a wide variety of chronic inflammatory diseases is only now beginning. This perspective traces the important basic scientific developments that have led up to the refinement of serum TRAP isoform immunoassays and their validation as biomarkers of disease. Many unanswered questions remain, providing a wealth of opportunity for continued research of this multifaceted enzyme.

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.

Purification and properties of the native form of the purple acid phosphatase from bovine spleen

The purple acid phosphatase (PAP) from bovine spleen has been shown to exist as a single ca. 36-kDa polypeptide in intact spleen tissue. The previously isolated microheterogeneous complex of 15-kDa and 23- or 21-kDa subunits appears to arise from proteolytic cleavage of an exposed, highly variable loop in the polypeptide chain. Small amounts of a single polypeptide form, presumed to be the native form of the enzyme, have been obtained; this has permitted its optical and EPR spectra and fundamental kinetic properties to be determined. The most notable difference between the native and two-subunit forms of PAP is a ca. 3-fold higher enzymatic activity for the latter, which is due to a simple increase in Vmax. The two forms are very similar spectroscopically and chemically and appear to differ only in the loss of a highly antigenic ca. five amino acid segment of the polypeptide between positions 155 and 160 but not in NH2-terminal sequence or in carbohydrate content. Analysis of published sequence data suggests that the existence of an exposed highly antigenic loop at positions corresponding to 155-161 of the spleen PAP sequence is a relatively general feature of PAP's. Trypsin and chymotrypsin cleave both bovine spleen PAP and uteroferrin, apparently in this region, with significant enhancement of enzymatic activity.

Purple acid phosphatase from bovine spleen. Interactions at the active site in relation to the reaction mechanism

Oxidation of the reduced (pink) phosphate-free bovine spleen acid phosphatase with 1.5 mol H2O2 or sodium peroxodisulfate/mol, in the presence of Mes or Bistris pH 5, leads to a species with an absorption maximum at 558 nm. Addition of acetate or oxidation in the presence of acetate buffer engenders a species with a maximum at 550 nm. Addition of phosphate to both species shifts the maximum immediately to 540 nm; this is the species also found after preparation from the spleen. The assumption that these species represent strongly bidentate-binding hydroxo, acetato and phosphato complexes of the Fe(III)-Fe(III) system is supported by replacement reactions with other ligating oxoanions followed by their typical spectral shifts. These oxoanion complexes cannot be dissociated by gel filtration; this is possible only after reduction to the Fe(II)-Fe(III) system. The oxidized species without EPR signals below g values of 2 still reveals 5% activity which cannot be reduced to zero even in the presence of higher concentrations of peroxodisulfate. The pH optimum of the reaction with alpha-naphthyl phosphate shifts from 5.9 to 5.3 in the oxidized species. The apparent pK values around 4.5 as derived from the pH dependence of activity, of the EPR spectra, and the spectral shifts of the phosphate-saturated reduced and oxidized species are assigned to an aquo/hydroxo equilibrium at the Fe(III) or an equilibrium, where the phosphato ligand is replaced by a hydroxo ligand. A reaction mechanism is proposed in which a hydroxo ligand at the chromophoric Fe(III) attacks the phosphoric acid ester group only when that is monoprotonated and pre-oriented by electrostatic interaction with the nonchromophoric metal ion. Binding and inhibition studies with the oxoanions indicate that they compete with the catalytically active hydroxo group of the reduced and oxidized enzyme with nearly the same inhibition constants. Catalysis is not affected by the oxoanions which replace the additional mu-hydroxo ligand in the 558-nm-absorbing Fe(III)-Fe(III) species. In contrast to hemerythrin and ribonucleotide reductase, a binuclear iron center is proposed for the purple acid phosphatase, which is bridged by a carboxylato and two aquo/hydroxo groups, but without a mu-oxo bridge.

Tartrate-resistant, purple acid phosphatase in Gaucher cells of the spleen. Immuno- and cytochemical analysis

Bioptic material from the spleen of a three-year-old boy with a type 1 Gaucher disease was studied by immuno- and cytochemical methods with special regard to the macrophage-derived Gaucher cells. These cells were positive with PAS and Prussian blue staining, and were immuno-positive with the monoclonal 25 F9 antibody, specific to mature, non-inflammatory macrophages. Large Gaucher cells and their postulated small precursor cells revealed strong tartrate-resistant acid phosphatase (TRAP) and unspecific carboxylate esterase activities. Using a polyclonal antibody to bovine spleen purple phosphatase, a lysosomal TRAP from splenic macrophages, the TRAP of the Gaucher cells could be identified to belong to this group of iron-containing, purple acid phosphatases immunocytochemically. The origin of splenic Gaucher cells from blood monocytes and their further development are discussed.

Histochemical investigations on the localization of the purple acid phosphatase in the bovine spleen

The localization of the purple tartrate-resistant, iron-containing acid phosphatase in the bovine spleen was studied by enzyme histochemistry at the light and electron microscopic levels as well as by immunohistochemistry. The purple phosphatase was localized only in lysosome-like-organelles of cells belonging to the reticulo-phagocytic system. The same cells were identified as containing large iron(III)-deposits as ferritin in homogeneously granular accumulations and freely in the cytoplasm, or as hemosiderin in siderosomes. The phagocytosing cells containing purple phosphatase and ferritin often had close contact with clusters of aged and deformed erythrocytes. A possible catabolic role of the purple enzyme as a phosphatase degrading phosphoproteins of the erythrocyte membrane and the cytoskeleton was assumed.

Hydrolysis of phosphoproteins and inositol phosphates by cell surface phosphatase of Leishmania donovani

Leishmania donovani promastigotes contain intense tartrate-resistant cell surface acid phosphatase (ACP1) which blocks superoxide anion production by activated human neutrophils [A.T. Remaley et al. (1984) J. Biol. Chem, 259, 11173-11175]. An extensively purified preparation of ACP1 dephosphorylates several phosphoproteins which are phosphorylated at serine residues; these include: pyruvate kinase (Km 1.6 microM; Vmax 71.4 U (mg protein)-1), phosphorylase kinase (Km 0.076 microM; Vmax 5.4 U (mg protein)-1) and histones (Km 4.86 microM; Vmax 2.2 U (mg protein)-1). However, the specific activity of the leishmanial phosphatase on these phosphoproteins is very low as compared to other phosphoprotein phosphatases. The phosphatase activity of ACP1 was also low on phosphohistone phosphorylated at tyrosine residues. Phosphatidylinositol-4,5-diphosphate (PIP2) and inositoltriphosphate (IP3) were also tested as ACP1 substrates. PIP2 was hydrolyzed rapidly by ACP1. The rate of hydrolysis of PIP2 was higher at pH 6.8 (Km 2.35 microM; Vmax 107 X 10(3) U (mg protein)-1) than at pH 5.5 (Km 4.16 microM; Vmax 71 X 10(3) U (mg protein)-1). 32P-labeled IP3 was also a substrate for ACP1; the hydrolysis products consisted of a mixture of inositoldiphosphate and inositolmonophosphate. ACP1 and ten other phosphatases were tested for their ability to dephosphorylate proteins and to inhibit O2- production by stimulated human neutrophils. There was no correlation between the protein phosphatase activity of the acid- and alkaline phosphatases and their ability to block neutrophil O2- production. The results indicate that ACP1 probably blocks the production of reduced oxygen intermediates by a mechanism that does not involve dephosphorylation of phosphoproteins; however, the possibility that the parasite's phosphatase affects phagocyte metabolism by degrading PIP2 or IP3 should be considered.

Properties of an acid phosphatase from Legionella micdadei which blocks superoxide anion production by human neutrophils

The high-speed supernatant (100,000 g, 1 h) obtained after centrifuging a suspension of Legionella micdadei that had been freeze-thawed and sonicated contained (i) considerable acid phosphatase activity when assayed using 4-methylumbelliferyl phosphate (MUP) as the substrate, and a factor that blocked superoxide anion production by human neutrophils stimulated with f-Met-Leu-Phe. Chromatography of the extract on a hydroxylapatite column resolved two acids phosphatases (designated ACP1 and ACP2). Subsequent chromatography of ACP2 on a Sephadex G-150 column revealed coincident elution of phosphatase activity and neutrophil blocking activity. When heated at 45 degrees C for various periods of time, the phosphatase activity of the acid phosphatase preparation was lost at the same rate as the ability of the preparation to block superoxide anion production by neutrophils. Furthermore, preincubation of neutrophils and acid phosphatase together in the presence of a heteropolymolybdate complex that inhibits the phosphatase eliminated the effect of the L. micdadei phosphatase on neutrophil superoxide anion production. ACP2 had the following properties: pH optimum, 6.0; Km for MUP, 3.8 mM; isoelectric point, 4.5; substrate specificity, MUP greater than ADP greater than phosphoenolpyruvate greater than phosphothreonine greater than phosphoserine greater than phosphotyrosine; molecular weight (estimated by sucrose density gradient centrifugation and gel filtration chromatography), 71,000-86,000. These results indicate that a cell-associated phosphatase may play a role in the virulence of L. micdadei.

Gaucher disease: comparative study of acid phosphatase and glucocerebrosidase in normal and type-1 Gaucher tissues

Acid phosphatase activity was determined in serum, cultured fibroblasts, and peripheral blood lymphocytes of six splenectomized adult patients with non-neuropathic Gaucher disease in two Canadian families. Elevated levels of serum acid phosphatase activity (520-711% of normal) were found in four patients who also developed orthopedic complications associated with Gaucher disease, including intermittent bone pain, arthritis, collapse of femoral head, and pathological fractures. Serum acid phosphatase activity in two patients who do not have bone involvement were found to be within the normal range. Contrary to the serum enzyme, acid phosphatase activity in lymphocytes and cultured fibroblasts of all of the patients was within the normal range. Deficient glucocerebrosidase (7.5-15.5% of normal) and acid beta-glucosidase (13.8-27.8% of normal) activities were noted in all probands. Similarly, normal levels of fibroblast and lymphocyte acid phosphatase activity were found in Gaucher heterozygotes whose glucocerebrosidase activity was about 50% of normal. Acid polyacrylamide gel electrophoresis and acid phosphatase activity staining of the patients' sera showed that the elevated acid phosphatase is isozyme type 5 osteoclastic origin. The apparent Michaelis constant, Km, of fibroblast glucocerebrosidase for the natural substrate was 0.6 +/- 0.1 mM for controls and 0.6 +/- 0.2 mM for the patients. These data suggest that the assay of serum acid phosphatase activity for the presumptive diagnosis of Gaucher disease is not completely reliable and that the elevated level of serum acid phosphatase in Gaucher disease is most likely a secondary phenomenon which may be indicative of bone involvement in some patients with this disorder. It also demonstrates the clinical heterogeneity of type 1 Gaucher disease, even among full sibs of the same heterozygous parents.