Relating structure to function in the beta-propeller domain of dipeptidyl peptidase IV. Point mutations that influence adenosine deaminase binding, antibody binding and enzyme activity

Dipeptidylpeptidase--IV, a key enzyme for the degradation of incretins and neuropeptides: activity and expression in the liver of lean and obese rats

Given the scarcity of donors, moderately fatty livers (FLs) are currently being considered as possible grafts for orthotopic liver transplantation (OLT), notwithstanding their poor tolerance to conventional cold preservation. The behaviour of parenchymal and sinusoidal liver cells during transplantation is being studied worldwide. Much less attention has been paid to the biliary tree, although this is considered the Achille's heel even of normal liver transplantation. To evaluate the response of the biliary compartment of FLs to the various phases of OLT reliable markers are necessary. Previously we demonstrated that Alkaline Phosphatase was scarcely active in bile canaliculi of FLs and thus ruled it out as a marker. As an alternative, dipeptidylpeptidase-IV (DPP-IV), was investigated. This ecto-peptidase plays an important role in glucose metabolism, rapidly inactivating insulin secreting hormones (incretins) that are important regulators of glucose metabolism. DPP-IV inhibitors are indeed used to treat Type II diabetes. Neuropeptides regulating bile transport and composition are further important substrates of DPP-IV in the enterohepatic axis. DPP-IV activity was investigated with an azo-coupling method in the liver of fatty Zucker rats (fa/fa), using as controls lean Zucker (fa/+) and normal Wistar rats. Protein expression was studied by immunofluorescence with the monoclonal antibody (clone 5E8). In Wistar rat liver, DPP-IV activity and expression were high in the whole biliary tree, and moderate in sinusoid endothelial cells, in agreement with the literature. Main substrates of DPP-IV in hepatocytes and cholangiocytes could be incretins GLP-1 and GIP, and neuropeptides such as vasoactive intestinal peptide (VIP) and substance P, suggesting that these substances are inactivated or modified through the biliary route. In lean Zucker rat liver the enzyme reaction and protein expression patterns were similar to those of Wistar rat. In obese rat liver the patterns of DPP-IV activity and expression in hepatocytes reflected the morphological alterations induced by steatosis as lipid-rich hepatocytes had scarce activity, located either in deformed bile canaliculi or in the sinusoidal and lateral domains of the plasma membrane. These findings suggest that bile canaliculi in steatotic cells have an impaired capacity to inactivate incretins and neuropeptides. Incretin and/or neuropeptide deregulation is indeed thought to play important roles in obesity and insulin-resistance. No alteration in enzyme activity and expression was found in the upper segments of the biliary tree of obese respect to lean Zucker and Wistar rats. In conclusion, this research demonstrates that DPP-IV is a promising in situ marker of biliary functionality not only of normal but also of fatty rats. The approach, initially devised to investigate the behaviour of the liver during the various phases of transplantation, appears to have a much higher potentiality as it could be further exploited to investigate any pathological or stressful conditions involving the biliary tract (i.e., metabolic syndrome and cholestasis) and the response of the biliary tract to therapy and/or to surgery.

3D structure of the CD26-ADA complex obtained by cryo-EM and single particle analysis

The specific binding of adenosine deaminase to the multifunctional membrane glycoprotein dipeptidyl peptidase IV is thought to be immunologically relevant for certain regulatory and co-stimulatory processes. In this study we present the 3D structure of the complete CD26-ADA complex obtained by single particle cryo-EM at 22A resolution. ADA binding occurs at the outer edges of the beta-propeller of CD26. Docking calculations of available CD26 and ADA crystal data into the obtained EM density map revealed that the ADA-binding site is stretched across CD26 beta-propeller blades 4 and 5 involving the outermost distal hydrophobic amino acids L294 and V341 but not T440 and K441 as suggested by antibody binding. Though the docking of the ADA orientation appears less significant due to the lack of distinct surface features, non-ambiguous conclusions can be drawn in the combination with earlier indirect non-imaging methods affirming the crucial role of the ADA alpha2-helix for binding.

A novel consensus motif in fibronectin mediates dipeptidyl peptidase IV adhesion and metastasis

Lung endothelial dipeptidyl peptidase IV (DPPIV/CD26) is a vascular address for cancer cells decorated with cell-surface polymeric fibronectin (poly-FN). Here, we identified the DPPIV-binding sites in FN and examined the effect of binding site peptides on DPPIV/poly-FN adhesion and metastasis. Using proteolytic fragments and maltose-binding protein fusion proteins that together span full-length FN, we found DPPIV-binding sites in type III repeats 13, 14, and 15 (FNIII13, -14, and -15, respectively). DPPIV binding was mediated by the consensus motif T(I/L)TGLX(P/R)G(T/V)X and was confirmed by swapping this motif in FNIII13, -14, and -15 with the corresponding region in FNIII12, which did not bind DPPIV. DPPIV binding was lost in swapped FNIII13, -14, and -15 and gained in swapped FNIII12 (FNIII12(14)). Peptides containing the DPPIV-binding domain of FNIII14 blocked DPPIV/poly-FN adhesion and impeded pulmonary metastasis. This study adds to the classes of cell-surface adhesion receptors for FN and will help in the further characterization of the functional implications of the DPPIV/poly-FN adhesion in metastasis and possibly in cell-mediated immunity involving DPPIV-expressing lymphocytes.

Cloning and characterization of dipeptidyl peptidase 10, a new member of an emerging subgroup of serine proteases

Two dipeptidyl peptidase IV (DPPIV, DPP4)-related proteins, DPP8 and DPP9, have been identified recently [Abbott, Yu, Woollatt, Sutherland, McCaughan, and Gorrell (2000) Eur. J. Biochem. 267, 6140-6150; Olsen and Wagtmann (2002) Gene 299, 185-193; Qi, Akinsanya, Riviere, and Junien (2002) Patent application WO0231134]. In the present study, we describe the cloning of DPP10, a novel 796-amino-acid protein, with significant sequence identity to DPP4 (32%) and DPP6 (51%) respectively. We propose that DPP10 is a new member of the S9B serine proteases subfamily. The DPP10 gene is located on the long arm of chromosome 2 (2q12.3-2q14.2), close to the DPP4 (2q24.3) and FAP (2q23) genes. The active-site serine residue is replaced by a glycine residue in DPP10, resulting in the loss of DPP activity. The serine residue is also replaced in DPP6, which lacks peptidase activity. DPP8 and DPP9 share an identical active site with DPP4 (Gly-Trp-Ser-Tyr-Gly). In contrast with the previous results suggesting that DPP9 is inactive, we show that DPP9 is a DPP, hydrolysing Ala-Pro-(7-amino-4-methyl-coumarin) with similar pH-specificity and protease-inhibitor-sensitivity to those of DPP4 and DPP8. Northern-blot analysis shows that whereas DPP8 and DPP9 are widely expressed, DPP10 is expressed mainly in the brain and pancreas. DPP6, which has the highest amino acid identity with DPP10, has been shown previously [Nadal, Ozaita, Amarillo, de Miera, Ma, Mo, Goldberg, Misumi, Ikehara, Neubert et al. (2003) Neuron 37, 449-461] to associate with A-type K(+) channel subunits, modulating their transport and function in somatodendritic compartments of neurons. It is possible that DPP10 is involved in similar functions in the brain. Elucidation of the physiological or pathophysiological role of DPP8, DPP9 and DPP10 and characterization of their structure-function relationships will add impetus to the development of inhibitor molecules for pharmacological or therapeutic use.

Fluorogenic substrate [Ala-Pro]2-cresyl violet but not Ala-Pro-rhodamine 110 is cleaved specifically by DPPIV activity: a study in living Jurkat cells and CD26/DPPIV-transfected Jurkat cells

Fluorogenic substrates [Ala-Pro](2)-cresyl violet and Ala-Pro-rhodamine 110 have been tested for microscopic detection of protease activity of dipeptidyl peptidase IV (DPPIV) in living cells. DPPIV activity is one of the many functions of the multifunctional or moonlighting protein CD26/DPPIV. As a model we used Jurkat cells, which are T-cells that lack CD26/DPPIV expression, and CD26/DPPIV-transfected Jurkat cells. Ala-Pro-rhodamine 110 is not fluorescent, but after proteolytic cleavage rhodamine 110 fluoresces. [Ala-Pro](2)-cresyl violet is fluorescent by itself but proteolytic cleavage into cresyl violet induces a shift to longer wavelengths. This phenomenon enables the simultaneous determination of local (intracellular) substrate and product concentrations, which is important for analysis of kinetics of the cleavage reaction. [Ala-Pro](2)-cresyl violet, but not Ala-Pro-rhodamine 110, appeared to be specific for DPPIV. When microscopic analysis is performed on living cells during the first minutes of the enzyme reaction, DPPIV activity can be precisely localized in cells with the use of [Ala-Pro](2)-cresyl violet. Fluorescent product is rapidly internalized into submembrane granules in transfected Jurkat cells and is redistributed intracellularly via internalization pathways that have been described for CD26/DPPIV. We conclude that [Ala-Pro](2)-cresyl violet is a good fluorogenic substrate to localize DPPIV activity in living cells when the correct wavelengths are used for excitation and emission and images are captured in the early stages of the enzyme reaction.

The multifunctional or moonlighting protein CD26/DPPIV

CD26/DPPIV can be considered a moonlighting protein because it is a multifunctional protein that exerts its different functions depending on cell type and intra- or extracellular conditions in which it is expressed. In the present review, we summarize all its known functions in relation to physiological and pathophysiological conditions. The protein is a proteolytic enzyme, receptor, costimulatory protein, and is involved in adhesion and apoptosis. The CD26/DPPIV protein plays a major role in immune response. Abnormal expression is found in the case of autoimmune diseases, HIV-related diseases and cancer. Natural substrates for CD26/DPPIV are involved in immunomodulation, psycho/neuronal modulation and physiological processes in general. Therefore, targeting of CD26/ DPPIV and especially its proteolytic activity has many therapeutic potentials. On the other hand, there are homologous proteins with overlapping proteolytic activity, which thus may prevent specific modulation of CD26/DPPIV. In conclusion, CD26/DPPIV is a protein present both in various cellular compartments and extracellularly where it exerts different functions and thus is a true moonlighting protein.

Intrahepatic expression of the hepatic stellate cell marker fibroblast activation protein correlates with the degree of fibrosis in hepatitis C virus infection

BACKGROUND

Activated hepatic stellate cells (HSCs), recognised by their alpha smooth muscle actin immunoreactivity, are primarily responsible for liver fibrosis. However, the presence of alpha smooth muscle actin positive HSCs is not always associated with the development of liver fibrosis. Recently, other markers of human HSCs including the gelatinase fibroblast activation protein (FAP) and glial fibrillary acidic protein have been identified.

AIMS

We examined the relationship between the expression of these HSC markers and the severity of liver injury in patients with chronic hepatitis C virus infection.

METHODS

Liver tissue from 27 patients was examined using immunohistochemistry. Linear correlation analysis was used to compare staining scores with the stage and grade of liver injury.

RESULTS-CONCLUSIONS

FAP expression, seen at the tissue-remodelling interface, was strongly and significantly correlated with the severity of liver fibrosis. A weaker correlation was seen between glial fibrillary acidic protein expression and fibrosis stage. This contrasted with the absence of a relationship between alpha smooth muscle actin and the fibrotic score. A correlation was also observed between FAP expression and necroinflammatory score. In summary, FAP expression identifies a HSC subpopulation at the tissue-remodelling interface that is related to the severity of liver fibrosis.

CD26: a multifunctional integral membrane and secreted protein of activated lymphocytes

CD26 has proved interesting in the fields of immunology, endocrinology, cancer biology and nutrition owing to its ubiquitous and unusual enzyme activity. This dipeptidyl aminopeptidase (DPP IV) activity generally inactivates but sometimes alters or enhances the biological activities of its peptide substrates, which include several chemokines. CD26 costimulates both the CD3 and the CD2 dependent T-cell activation and tyrosine phosphorylation of TCR/CD3 signal transduction pathway proteins. CD26 in vivo has integral membrane protein and soluble forms. Soluble CD26 is at significant levels in serum, these levels alter in many diseases and soluble CD26 can modulate in vitro T-cell proliferation. CD26, being an adenosine deaminase binding protein (ADAbp), functions as a receptor for ADA on lymphocytes. The focus of this review is the structure and function of CD26 and the influence of its ligand binding activity on T-cell proliferation and the T cell costimulatory activity of CD26.