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      Recombinant Forms of the Neurotrophic Factor Pigment Epithelium-Derived Factor Activate Cellular Metabolism and Inhibit Proliferation of the RAW Macrophage Cell Line

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          Recombinant forms of the neurotrophic factor pigment epithelium-derived factor (PEDF) activate metabolism of RAW macrophage cells while simultaneously inhibiting their proliferation. The recombinant forms (rPEDF) acted with EC<sub>50</sub>s of 0.1–1 n M while full-length native bovine PEDF was inactive. Urea, which is the buffer used to extract recombinant PEDF, stimulated RAW cell proliferation, the first report of an effect of urea on non-kidney cells. PEDF acted within 12 h and its effects persisted up to 72 h with continuous exposure. Although rPEDF had no direct action on glioma cell lines, it increased the amount of a soluble factor released by RAW cells which was capable of blocking glioma cell division. Thus PEDF may function as a neuroimmune modulator, affecting both neural and immune system cells.

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          Use of MTT colorimetric assay to measure cell activation.

          The MTT tetrazolium salt colorimetric assay previously described by Mosmann (1983, J. Immunol. Methods 65, 55) to measure cytotoxicity and cell proliferation was further explored to extend its application to the measurement of cell activation. The level of MTT cleavage by viable cells of various origins was found to be directly proportional to the number of cells but to increase as a non-linear function of time. This non-linear relationship was related to a time-linear cell death during MTT incubation. The cleavage of MTT by viable cells was found to follow first order kinetics and could be fitted to Michaelis' kinetics. Different cell types exhibited similar apparent Km values (1949 microM) and different apparent maximal velocities (V). The apparent V values determined for a given cell type under different experimental conditions were rigorously similar. This analysis of MTT cleavage by viable cells suggests that the colorimetric MTT test can be useful to quantify the activation level of cells, independently of proliferation.
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            Pigment epithelium-derived factor behaves like a noninhibitory serpin. Neurotrophic activity does not require the serpin reactive loop.

            Pigment epithelium-derived factor (PEDF), a neurite-promoting factor, has an amino acid primary structure that is related to members of the serine protease inhibitor (serpin) family. Controlled proteolysis of native PEDF (50 kDa) with either trypsin, chymotrypsin, elastase, or subtilisin yields in each case one major limited product of 46 kDa as analyzed by SDS-polyacrylamide gel electrophoresis. N-terminal sequence analysis of the isolated 46-kDa products indicates a favored cleavage region located toward the C-terminal end of PEDF. A proteolyzed PEDF protein reaction mixture reveals two overlapping sequences: that of the N terminus of intact PEDF and that of an internal region, consistent with cleavage of PEDF about position 382. These data indicate that PEDF protein has a globular conformation with one protease-sensitive exposed loop that contains the homologous serpin-reactive site. Cleavage within the reactive-site loop of PEDF does not cause a conformational change in the molecules (the stressed (S)-->relaxed (R) transition) and results in heat denaturation identical to its native counterpart. This lack of conformational change is also seen upon cleavage within the reactive-site loop of the noninhibitory serpin ovalbumin. Furthermore, the PEDF neurite-promoting function is not lost with cleavage of the exposed loop. Recombinant PEDF polypeptide fragments with larger truncations from the C-terminal end show neurotrophic activity. Our results clearly indicate that integrity of the PEDF homologous serpin reactive center is dispensable for neurotrophic activity. Thus, the PEDF induction of neurites must be mediated by a mechanism other than serine protease inhibition. Altogether our data indicate that PEDF belongs to the subgroup of noninhibitory serpins and that its N-terminal region confers a neurite-promoting activity to the protein. The neurotrophic active site of PEDF is separated from the serpin reactive-site loop, not only in the primary structure, but also in the folded protein structure.
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              Identification of pigment epithelium-derived factor in the interphotoreceptor matrix of bovine eyes.

              Pigment epithelium-derived factor (PEDF) is a neurotrophic protein and a member of the serine protease inhibitor superfamily. Here we describe the identification of PEDF in bovine eyes and optimization of its purification from this natural source. We have developed a polyclonal antibody to recombinant human PEDF, Ab-rPEDF, that immunoreacts in a specific, sensitive, and linear fashion with PEDF protein, and furthermore, blocks its neurotrophic activity. We show that Ab-rPEDF specifically recognizes a 49,500-M(r) polypeptide on Western transfers of a wash of the extracellular matrix between the retinal pigment epithelium and the neural retina-termed interphotoreceptor matrix (IPM). PEDF is present as approximately 1% of total soluble IPM protein. Starting with an IPM wash, PEDF protein is purified 164-fold to near homogeneity by ammonium sulfate fractionation and cation-exchange chromatography, with a recovery of 47%. The highly purified protein has an apparent M(r) of 49,500 +/- 1,500 as assessed by SDS-polyacrylamide gel electrophoresis, and a native pI of 7.0-7.7. It elutes as a single peak on gel-filtration chromatography with a retention time immediately behind that of ovalbumin (43,000 M(r)). N-glycosidase treatment indicates that each PEDF molecule has a 5% carbohydrate content attached to internal asparagine residue(s). Amino terminal sequence of the purified PEDF reveals removal of an amino-terminal peptide region for the mature protein. Purified PEDF has neurotrophic activity on human retinoblastoma cells, as previously observed for IPM. The neurotrophic activities of both PEDF and IPM are blocked by antiserum Ab-rPEDF. Altogether, PEDF is present in the bovine IPM as a soluble, extracellular, monomeric glycoprotein that by itself confers neurotrophic activity to the IPM. Thus, native PEDF isolated and purified as described here should prove useful for biochemical studies as well as other approaches.

                Author and article information

                S. Karger AG
                December 1999
                15 December 1999
                : 7
                : 1
                : 51-58
                aDepartment of Pharmacology, Howard University College of Medicine, Washington, D.C., and bMolecular Genetics Section, Clinical Neuroscience Branch, NINDS, NIH and cLaboratory of Retinal Cell and Molecular Biology, NEI, NIH, Bethesda, Md., USA
                26420 Neuroimmunomodulation 2000;7:51–58
                © 1999 S. Karger AG, Basel

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                Page count
                Figures: 8, References: 28, Pages: 8
                Original Paper


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