Cooperative kinetics of human prostatic acid phosphatase

Recessive Mutations in ACP4 Cause Amelogenesis Imperfecta

Amelogenesis imperfecta (AI) is an innate disorder that affects the formation and mineralization of the tooth enamel. When diagnosed with AI, one's teeth can be hypoplastic (thin enamel), hypomature (normal enamel thickness but discolored and softer than normal enamel), hypocalcified (normal enamel thickness but extremely weak), or mixed conditions of the above. Numerous studies have revealed the genes that are involved in causing AI. Recently, ACP4 (acid phosphatase 4) was newly found as a gene causing hypoplastic AI, and it was suggested that mutant forms of ACP4 might affect access to the catalytic core or the ability to form a homodimer. In this study, a Korean and a Turkish family with hypoplastic AI were recruited, and their exome sequences were analyzed. Biallelic mutations were revealed in ACP4: paternal (NM_033068: c.419C>T, p.(Pro140Leu)) and maternal (c.262C>A, p.(Arg88Ser)) mutations in family 1 and a paternal (c.713C>T, p.(Ser238Leu)) mutation and de novo (c.350A>G, p.(Gln117Arg)) mutation in the maternal allele in family 2. Mutations were analyzed by cloning, mutagenesis, immunofluorescence, immunoprecipitation, and acid phosphatase activity test. Comparison between the wild-type and mutant ACP4s showed a decreased amount of protein expression from the mutant forms, a decreased ability to form a homodimer, and a decreased acid phosphatase activity level. We believe that these findings will not only expand the mutational spectrum of ACP4 but also increase our understanding of the mechanism of ACP4 function during normal and pathologic amelogenesis.

Mechanisms of Osteoblastic Bone Metastasis in Prostate Cancer: Role of Prostatic Acid Phosphatase

Prostate cancer (PCa) preferentially metastasizes to bone, leading to complications including severe pain, fractures, spinal cord compression, bone marrow suppression, and a mortality of ∼70%. In spite of recent advances in chemo-, hormonal, and radiation therapies, bone-metastatic, castrate-resistant PCa is incurable. PCa is somewhat unique among the solid tumors in its tendency to produce osteoblastic lesions composed of hypermineralized bone with multiple layers of poorly organized type I collagen fibrils that have reduced mechanical strength. Many of the signaling pathways that control normal bone homeostasis are at play in pathologic PCa bone metastases, including the receptor activator of nuclear factor-κB/receptor activator of nuclear factor-κB ligand/osteoprotegerin system. A number of PCa-derived soluble factors have been shown to induce the dysfunctional osteoblastic phenotype. However, therapies directed at these osteoblastic-stimulating proteins have yielded disappointing clinical results to date. One of the soluble factors expressed by PCa cells, particularly in bone metastases, is prostatic acid phosphatase (PAP). Human PAP is a prostate epithelium-specific secretory protein that was the first tumor marker ever described. Biologically, PAP exhibits both phosphatase activity and ecto-5′-nucleotidase activity, generating extracellular phosphate and adenosine as the final products. Accumulating evidence indicates that PAP plays a causal role in the osteoblastic phenotype and aberrant bone mineralization seen in bone-metastatic, castrate-resistant PCa. Targeting PAP may represent a therapeutic approach to improve morbidity and mortality from PCa osteoblastic bone metastases.

Cellular prostatic acid phosphatase, a PTEN-functional homologue in prostate epithelia, functions as a prostate-specific tumor suppressor

The inactivation of tumor suppressor genes (TSGs) plays a vital role in the progression of human cancers. Nevertheless, those ubiquitous TSGs have been shown with limited roles in various stages of diverse carcinogenesis. Investigation on identifying unique TSG, especially for early stage of carcinogenesis, is imperative. As such, the search for organ-specific TSGs has emerged as a major strategy in cancer research. Prostate cancer (PCa) has the highest incidence in solid tumors in US males. Cellular prostatic acid phosphatase (cPAcP) is a prostate-specific differentiation antigen. Despite intensive studies over the past several decades on PAcP as a PCa biomarker, the role of cPAcP as a PCa-specific tumor suppressor has only recently been emerged and validated. The mechanism underlying the pivotal role of cPAcP as a prostate-specific TSG is, in part, due to its function as a protein tyrosine phosphatase (PTP) as well as a phosphoinositide phosphatase (PIP), an apparent functional homologue to phosphatase and tensin homolog (PTEN) in PCa cells. This review is focused on discussing the function of this authentic prostate-specific tumor suppressor and the mechanism behind the loss of cPAcP expression leading to prostate carcinogenesis. We review other phosphatases' roles as TSGs which regulate oncogenic PI3K signaling in PCa and discuss the functional similarity between cPAcP and PTEN in prostate carcinogenesis.

Structure of Acid phosphatases

Acid phosphatases are enzymes that have been studied extensively due to the fact that their dysregulation is associated with pathophysiological conditions. This characteristic has been exploited for the development of diagnostic and therapeutic methods. As an example, prostatic acid phosphatase was the first marker for metastatic prostate cancer diagnosis and the dysregulation of tartrate resistant acid phosphatase is associated with abnormal bone resorption linked to osteoporosis. The pioneering crystallization studies on prostatic acid phosphatase and mammalian tartrate-resistant acid phosphatase conformed significant milestones towards the elucidation of the mechanisms followed by these enzymes (Schneider et al., EMBO J 12:2609-2615, 1993). Acid phosphatases are also found in nonmammalian species such as bacteria, fungi, parasites, and plants, and most of them share structural similarities with mammalian acid phosphatase enzymes. Acid phosphatase (EC 3.1.3.2) enzymes catalyze the hydrolysis of phosphate monoesters following the general equation. Phosphate monoester + H2O -->/<-- alcohol + phosphate. The general classification "acid phosphatase" relies only on the optimum acidic pH for the enzymatic activity in assay conditions using non-physiological substrates. These enzymes accept a wide range of substrates in vitro, ranging from small organic molecules to phosphoproteins, constituting a heterogeneous group of enzymes from the structural point of view. These structural differences account for the divergence in cofactor dependences and behavior against substrates, inhibitors, and activators. In this group only the tartrate-resistant acid phosphatase is a metallo-enzyme whereas the other members do not require metal-ion binding for their catalytic activity. In addition, tartrate-resistant acid phosphatase and erythrocytic acid phosphatase are not inhibited by L-(+)-tartrate ion while the prostatic acid phosphatase is tartrate-sensitive. This is an important difference that can be exploited in in vitro assays to differentiate between different kinds of phosphatase activity. The search for more sensitive and specific methods of detection in clinical laboratory applications led to the development of radioimmunoassays (RIA) for determination of prostatic acid phosphatase in serum. These methods permit the direct quantification of the enzyme regardless of its activity status. Therefore, an independent structural classification exists that helps to group these enzymes according to their structural features and mechanisms. Based on this we can distinguish the histidine acid phosphatases (Van Etten, Ann N Y Acad Sci 390:27-51, 1982), the low molecular weight protein tyrosine acid phosphatases and the metal-ion dependent phosphatases. A note of caution is worthwhile mentioning here. The nomenclature of acid phosphatases has not been particularly easy for those new to the subject. Unfortunately, the acronym PAP is very common in the literature about purple acid phosphatases and prostatic acid phosphatase. In addition, LPAP is the acronym chosen to refer to the lysophosphatidic acid phosphatase which is a different enzyme. It is important to bear in mind this distinction while reviewing the literature to avoid confusion.

A facile nanoparticle immunoassay for cancer biomarker discovery

BACKGROUND

Gold nanoparticles (AuNPs) scatter light intensely at or near their surface plasmon wavelength region. Using AuNPs coupled with dynamic light scattering (DLS) detection, we developed a facile nanoparticle immunoassay for serum protein biomarker detection and analysis. A serum sample was first mixed with a citrate-protected AuNP solution. Proteins from the serum were adsorbed to the AuNPs to form a protein corona on the nanoparticle surface. An antibody solution was then added to the assay solution to analyze the target proteins of interest that are present in the protein corona. The protein corona formation and the subsequent binding of antibody to the target proteins in the protein corona were detected by DLS.

RESULTS

Using this simple assay, we discovered multiple molecular aberrations associated with prostate cancer from both mice and human blood serum samples. From the mice serum study, we observed difference in the size of the protein corona and mouse IgG level between different mice groups (i.e., mice with aggressive or less aggressive prostate cancer, and normal healthy controls). Furthermore, it was found from both the mice model and the human serum sample study that the level of vascular endothelial growth factor (VEGF, a protein that is associated with tumor angiogenesis) adsorbed to the AuNPs is decreased in cancer samples compared to non-cancerous or less malignant cancer samples.

CONCLUSION

The molecular aberrations observed from this study may become new biomarkers for prostate cancer detection. The nanoparticle immunoassay reported here can be used as a convenient and general tool to screen and analyze serum proteins and to discover new biomarkers associated with cancer and other human diseases.

Human prostatic acid phosphatase, an authentic tyrosine phosphatase, dephosphorylates ErbB-2 and regulates prostate cancer cell growth

Cellular prostatic acid phosphatase (cPAcP), an authentic tyrosine phosphatase, is proposed to function as a negative growth regulator of prostate cancer (PCa) cells in part through its dephosphorylation of ErbB-2. Nevertheless, the direct interaction between cPAcP and ErbB-2 has not been shown nor the specific dephosphorylation site of ErbB-2 by cPAcP. In this report, our data show that the phosphorylation level of ErbB-2 primarily at Tyr(1221/2) correlates with the growth rate of both LNCaP and MDA PCa2b human PCa cells. Further, cPAcP reciprocally co-immunoprecipitated with ErbB-2 in a non-permissive growth condition. Expression of wild type cPAcP, but not inactive mutant, by cDNA in cPAcP-null LNCaP C-81 cells results in decreased tyrosine phosphorylation of ErbB-2 including Tyr(1221/2). Concurrently, Tyr(317) phosphorylation of p52(Shc), proliferating cell nuclear antigen expression, and cell growth are decreased in these cells. Conversely, decreased cPAcP expression by short hairpin RNA in LNCaP C-33 cells was associated with elevated phosphorylation of ErbB-2 initially at Tyr(1221/2). Its downstream p52(Shc), ERK1/2, Akt, Src, STAT-3, and STAT-5 were activated, and cell proliferation, proliferating cell nuclear antigen, and cyclin D1 expression were increased. Stable subclones of C-33 cells by small interfering PAcP had elevated Tyr(1221/2) phosphorylation of ErbB-2 and exhibited androgen-independent growth and increased tumorigenicity in xenograft female animals. In summary, our data together indicate that in prostate epithelia, cPAcP interacts with and dephosphorylates ErbB-2 primarily at Tyr(1221/2) and hence blocks downstream signaling, leading to reduced cell growth. In PCa cells, decreased cPAcP expression is associated with androgen-independent cell proliferation and tumorigenicity as seen in advanced hormone-refractory prostate carcinomas.

Cooperativity and saturation in biochemical networks: A saturable formalism using Taylor series approximations

Cooperative and saturable systems are common in molecular biology. Nevertheless, common canonical formalisms for kinetic modeling that are theoretically well justified do not have a saturable form. Modeling and fitting data from saturable systems are widely done using Hill-like equations. In practice, there is no theoretical justification for the generalized use of these equations, other than their ability to fit experimental data. Thus it is important to find a canonical formalism that is (a) theoretically well supported, (b) has a saturable functional form, and (c) can be justifiably applicable to any biochemical network. Here we derive such a formalism using Taylor approximations in a special transformation space defined by power-inverses and logarithms of power-inverses. This formalism is generalized for processes with n-variables, leading to a useful mathematical representation for molecular biology: the Saturable and Cooperative Formalism (SC formalism). This formalism provides an appropriate representation that can be used for modeling processes with cooperativity and saturation. We also show that the Hill equation can be seen as a special case within this formalism. Parameter estimation for the SC formalism requires information that is also necessary to build Power-Law models, Metabolic Control Analysis descriptions or (log)linear and Lin-log models. In addition, the saturation fraction of the relevant processes at the operating point needs to be considered. The practical use of the SC formalism for modeling is illustrated with a few examples. Similar models are built using different formalisms and compared to emphasize advantages and limitations of the different approaches.

Theoretical investigations of prostatic acid phosphatase

The phosphotyrosyl protein phosphatase activity of prostatic acid phosphatase (PAP) has been well established. It has also been suggested that PAP partly regulates the activity of growth factor receptors by dephosphorylating the autophosphorylysable tyrosines in them. We studied the binding of the peptides from epidermal growth factor receptor (EGFR) and its homolog (ErbB-2), corresponding to their autophosphorylation sites, to PAP using theoretical modeling and molecular dynamics (MD) simulation methods. Nine different peptides, each with a phosphotyrosine residue, were docked on human PAP. The binding energies of these peptide-PAP complexes were calculated theoretically and compared to experimentally obtained affinities. The peptide Ace--DNLpYYWD--NH2 from ErbB-2(1197-1203) showed the most favorable free energy of binding when estimated theoretically. The results demonstrate that the presence of another tyrosine residue proximate to C-terminal of autophosphorylysable Tyr enhances the binding affinity considerably. The presence of a bulky group instead prevents the binding, as is observed in case of peptide Ace--NLYpYWDQ--NH2 which failed to bind, both in theoretical calculations and experiments. Thus we demonstarted that PAP could potentially bind to EGFR and Erbb-2 and dephosphorylate them. Thus it could be involved in the regulation of the function of such receptors. In addition, complexes of a peptide from AngiotensinII and phosphotyrosine(pY) with human PAP were also modeled. The effects of different protonation states of the titratable active site residues on ligand (pY) binding have also been investigated. For a favorable binding His12 and Asp258 should be neutral, His257 should be positively charged and the phosphate group of the ligand should be in PO(4) (3-) state. Furthermore, the analysis of protein motion as observed during simulations suggests the loop-loop contact in the PAP dimer to be of importance in cooperativity.

Positive cooperativity in substrate binding of human prostatic acid phosphatase entrapped in AOT–isooctane–water reverse micelles

The kinetics of 1-naphthyl phosphate and phenyl phosphate hydrolysis, catalyzed by human prostatic acid phosphatase (PAP) entrapped in AOT-isooctane-water reverse micelles, has been studied over surfactant hydration degree (w0) range 5 to 35. Continuous spectrophotometric acid phosphatase assays, previously prepared, were employed. PAP was catalytically active over the whole w0 studied range. In order to determine steady-state reaction constants the experimental data were fitted to Hill rate equation. Positive cooperativity in substrate binding was observed, as it was earlier found in aqueous solutions. The extent of cooperativity (expressed as the value of the Hill cooperation coefficient h) increased from 1 to 4, when the micellar water-pool size was growing, at fixed enzyme concentration. In the plots of catalytic activity (kcat) versus w0, the maxima have been found at w0=10 (pH 5.6) and 23 (pH 3.8). It is suggested that catalytically active monomeric and dimeric PAP forms are entrapped in reverse micelles of w0=10 and 23, respectively.

Concentration-dependent dissociation/association of human prostatic acid phosphatase

The apparent molecular mass of human prostatic acid phosphatase (PAP) was estimated over a wide range of enzyme concentrations using equilibrium centrifugation in the "Airfuge" tabletop ultracentrifuge. We show that the average mass of all active PAP species steeply increases at enzyme concentrations around 100 nM. The data indicate that at lower concentrations, active monomer prevail, whereas at concentrations above 100 nM, PAP active dimers are formed. These findings were confirmed by measurements of fluorescence emission intensity as a function of enzyme concentration. A shift of the normalized PAP fluorescence intensity around 100 nM independently indicates that a major structural change of the PAP protein occurs in that range of concentrations. From these findings, we conclude that in dilute solutions, several active PAP species exist, which are involved in concentration-dependent dissociation/association equilibria.

Crystal structures of human prostatic acid phosphatase in complex with a phosphate ion and alpha-benzylaminobenzylphosphonic acid update the mechanistic picture and offer new insights into inhibitor design

The X-ray crystal structure of human prostatic acid phosphatase (PAP) in complex with a phosphate ion has been determined at 2.4 A resolution. This structure offers a snapshot of the final intermediate in the catalytic mechanism and does not support the role of Asp 258 as a proton donor in catalysis. A total of eight hydrogen bonds serve to strongly bind the phosphate ion within the active site. Bound PEG molecules from the crystallization matrix have allowed the identification of a channel within the molecule that likely plays a role in molecular recognition and in macromolecular substrate selectivity. Additionally, the structure of PAP in complex with a phosphate derivative, alpha-benzylaminobenzylphosphonic acid, a potent inhibitor (IC(50) = 4 nM), has been determined to 2.9 A resolution. This structure gives new insight into the determinants of binding hydrophobic ligands within the active site and allows us to explain PAP's preference for aromatic substrates.