Interactions of serine proteases with cultured fibroblasts

Protease nexin 1 inhibits hedgehog signaling in prostate adenocarcinoma

Prostate adenocarcinoma (CaP) patients are classified into low-, intermediate-, and high-risk groups that reflect relative survival categories. While there are accepted treatment regimens for low- and high-risk patients, intermediate-risk patients pose a clinical dilemma, as treatment outcomes are highly variable for these individuals. A better understanding of the factors that regulate the progression of CaP is required to delineate risk. For example, aberrant activation of the Hedgehog (Hh) pathway is implicated in CaP progression. Here, we identify the serine protease inhibitor protease nexin 1 (PN1) as a negative regulator of Hh signaling in prostate. Using human CaP cell lines and a mouse xenograft model of CaP, we demonstrate that PN1 regulates Hh signaling by decreasing protein levels of the Hh ligand Sonic (SHH) and its downstream effectors. Furthermore, we show that SHH expression enhanced tumor growth while overexpression of PN1 inhibited tumor growth and angiogenesis in mice. Finally, using comparative genome hybridization, we found that genetic alterations in Hh pathway genes correlated with worse clinical outcomes in intermediate-risk CaP patients, indicating the importance of this pathway in CaP.

A domain level interaction network of amyloid precursor protein and Abeta of Alzheimer's disease

The primary constituent of the amyloid plaque, beta-amyloid (Abeta), is thought to be the causal "toxic moiety" of Alzheimer's disease. However, despite much work focused on both Abeta and its parent protein, amyloid precursor protein (APP), the functional roles of APP and its cleavage products remain to be fully elucidated. Protein-protein interaction networks can provide insight into protein function, however, high-throughput data often report false positives and are in frequent disagreement with low-throughput experiments. Moreover, the complexity of the CNS is likely to be under represented in such databases. Therefore, we curated the published work characterizing both APP and Abeta to create a protein interaction network of APP and its proteolytic cleavage products, with annotation, where possible, to the level of APP binding domain and isoform. This is the first time that an interactome has been refined to domain level, essential for the interpretation of APP due to the presence of multiple isoforms and processed fragments. Gene ontology and network analysis were used to identify potentially novel functional relationships among interacting proteins.

Increases in pericellular proteolysis at developing neuromuscular junctions in culture

To determine whether localized changes in pericellular proteolysis contribute to synapse formation, we examined the degradative actions of developing Xenopus laevis nerve and muscle cells on films of extracellular matrix proteins adsorbed to the glass surface of a tissue culture chamber. Skeletal myocytes, growing neurites, and fibroblasts all removed fluorescent fibronectin and laminin from the culture substratum at regions of close cell-surface contact. In addition, however, motor neurites also displayed a particularly enhanced rate of gelatin elimination at developing neuromuscular junctions. It has already been shown (a) that there is a similar remodeling of organized muscle basal lamina proteoglycan accumulations along the path of nerve-muscle contact and (b) that this is the earliest detectable biochemical change specific to developing neuromuscular junctions. Our observations thus suggest that the establishment of motoneuron-muscle contact leads to a further activation of pericellular proteinases along both the pre- and the postsynaptic surfaces of the developing junction. We therefore consider whether site-specific proteinase-activation cascades could contribute to the inductive signals that direct synaptic differentiation.

Prenatal and postnatal expression of mRNA coding for rat prothrombin

The levels of prothrombin mRNA in prenatal and postnatal rat tissues were analyzed in order to determine tissue distribution of prothrombin expression and to determine if increases in liver prothrombin mRNA during development correlated with previously documented developmental increases in plasma prothrombin levels. Maternal tissues were also analyzed in order to determine if prothrombin mRNA levels varied due to gestational or postpartum influences. Northern analysis demonstrated that rat liver prothrombin mRNA levels increased several-fold late in gestation and reached maximal levels by 13 days after birth. Prothrombin mRNA was also expressed in diaphragm, stomach, intestine, kidney, spleen and adrenal tissues during development. In maternal tissues during pregnancy, prothrombin mRNA was expressed in liver, diaphragm, stomach, uterus and placenta. Prothrombin mRNA levels in each of these tissues that were positive by Northern analysis were quantitated by solution hybridization analysis. Between gestational day 18 and postnatal day 13, liver prothrombin mRNA levels increased from approx. 600 to 2100 molecules per cell (a 3.5-fold increase). In maternal liver during pregnancy, between day 18 and day 22, prothrombin mRNA levels increased from approx. 1800 to 2100 molecules per cell. Immediately after delivery, maternal liver prothrombin mRNA levels decreased to approx. 50% of preparturition levels. Prothrombin mRNA levels in placental tissue ranged from approx. 100 to 250 molecules per cell. In other fetal, postnatal and maternal tissues, prothrombin mRNA expression was less than 100 molecules per cell. These results demonstrate that the level and tissue-type expression of prothrombin mRNA varies in response to prenatal and postnatal influences.

A diverse set of developmentally regulated proteoglycans is expressed in the rat central nervous system

Cellular interactions in neural development are influenced by various extracellular proteins, many of which bind glycosaminoglycans or proteoglycans. Precise functions of nervous system proteoglycans remain unknown, in part because neural proteoglycan composition is poorly understood. In this study, 25 putative proteoglycan core proteins were identified in subcellular fractions of rat brain. Levels of many of these varied considerably during development. Membrane-associated proteoglycans included two heparan sulfate proteoglycans (cores of 50 and 59 kd) that are covalently linked to glycosyl-phosphatidylinositol lipid, as well as several that appear to aggregate either with themselves or with copurifying proteins. These data indicate that brain proteoglycans exhibit the abundance, structural diversity, and developmental regulation that would be anticipated for molecules with diverse developmental functions.

Proteolytic regulation of neurite outgrowth from neuroblastoma cells by thrombin and protease nexin-1

This review summarizes studies on the reciprocal regulation of neuroblastoma neurite outgrowth by thrombin and protease nexin-1 (PN-1). PN-1 recently was shown to possess the same deduced amino acid sequence as the glial-derived neurite-promoting factor. The neurite outgrowth activity of PN-1 depends on its ability to inhibit thrombin. Thrombin not only blocks the neurite outgrowth activity of PN-1, but it also brings about neurite retraction in the presence of PN-1. Thrombin also produces neurite retraction in the absence of PN-1 and other regulatory factors. This suggests that its activity is due to a direct action on cells. The neurite retraction by thrombin depends on its proteolytic activity. It does not occur with the other serine proteases that have been tested, indicating that it is a specific effect and is not due to a general proteolytic effect that could detach neurites from the culture dish. Serum brings about neurite retraction in certain neuroblastoma cells and primary neuronal cultures; most of this activity is due to residual thrombin in the serum. Together, these results suggest that PN-1 and thrombin (or a thrombin-like protease) play a role in regulation of neurite outgrowth.

Monoclonal antibodies to protease nexin 1 that differentially block its inhibition of target proteases

Protease nexin 1 (PN-1) is a protease inhibitor secreted by cultured fibroblasts that forms complexes with certain serine proteases; the complexes bind back to the cells and are internalized and degraded. In the present studies, a panel of PN-1 monoclonal antibodies (mAbs) was isolated; none showed detectable cross-reactivity with four related plasma protease inhibitors. Four purified mAbs (mAbp1, mAbp6, mAbp9, and mAbp18) were tested for their ability to block the formation of complexes between PN-1 and target proteases. mAbp1, as well as a rabbit polyclonal anti-PN-1 IgG preparation, did not block formation of 125I-thrombin-PN-1 complexes. mAbp6, mAbp9, and mAbp18 blocked the formation of 125I-thrombin-PN-1 and 125I-urokinase-PN-1 complexes at stoichiometric concentrations of mAb and PN-1. Studies on their ability to block formation of 125I-trypsin-PN-1 complexes showed that mAbp18 also blocked this reaction at stoichiometric concentrations with PN-1 whereas mAbp6 and mAbp9 blocked less effectively. Thus, mAbp18 appears to bind at or close to the reactive center of PN-1. The blocking mAbs should be useful in studies to probe physiological functions of PN-1.

Effects of fibroblasts and endothelial cells on inactivation of target proteases by protease nexin-1, heparin cofactor II, and C1-inhibitor

Previous studies have shown that glycosaminoglycans in the extracellular matrix accelerate the inactivation of target proteases by certain protease inhibitors. It has been suggested that the ability of the matrix of certain cells to accelerate some inhibitors but not others might reflect the site of action of the inhibitors. Previous studies showed that fibroblasts accelerate the inactivation of thrombin by protease nexin-1, an inhibitor that appears to function at the surface of cells in extravascular tissues. The present experiments showed that endothelial cells also accelerate this reaction. The accelerative activity was accounted for by the extracellular matrix and was mostly due to heparan sulfate. Fibroblasts but not endothelial cells accelerated the inactivation of thrombin by heparin cofactor II, an abundant inhibitor in plasma. This is consistent with previous suggestions that heparin cofactor II inactivates thrombin when plasma is exposed to fibroblasts and smooth muscle cells. Neither fibroblasts nor endothelial cells accelerated the inactivation of C1s by plasma C1-inhibitor.

Localization of protease nexin-1 on the fibroblast extracellular matrix

Protease nexin-1 (PN-1) is a protease inhibitor that is secreted by fibroblasts and several other cultured cells. PN-1 forms complexes with certain serine proteases in the extracellular environment including thrombin, urokinase, and plasmin. The complexes then bind to the cells and are rapidly internalized and degraded. This report demonstrates that PN-1 is present on the surface of fibroblasts, bound to the extracellular matrix. Immunofluorescent studies showed that PN-1 colocalized with fibronectin on both intact cells and in preparations of extracellular matrix made from these cells. In contrast, PN-1 did not colocalize with the epidermal growth factor receptor, a plasma membrane marker. An enzyme-lined immunosorbent assay was developed which showed that the extracellular matrix contained at least 60-80% of the cellular immunoreactive PN-1. Extraction of the matrix with 2 M NaCl removed PN-1 in a form which reacted with 125I-thrombin to form complexes which were immunoprecipitated by anti-PN-1 IgG and were of identical size as complexes made from soluble PN-1 and 125I-thrombin. These data indicate that in addition to its role as a soluble protease inhibitor, PN-1 is also a component of the extracellular matrix and might control its proteolysis.