Of the 600+ known proteases identified to date in mammals, a significant percentage is involved or implicated in pathogenic and cancer processes. The dipeptidyl peptidase IV (DPIV) gene family, comprising four enzyme members [DPIV (EC 3.4.14.5), fibroblast activation protein, DP8 and DP9] and two nonenzyme members [DP6 (DPL1) and DP10 (DPL2)], are interesting in this regard because of their multiple diverse functions, varying patterns of distribution/localization and subtle, but significant, differences in structure/substrate recognition. In addition, their engagement in cell biological processes involves both enzymatic and nonenzymatic capabilities. This article examines, in detail, our current understanding of the biological involvement of this unique enzyme family and their overall potential as therapeutic targets.

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Dipeptidyl peptidase IV (DPPIV) is an atypical serine protease that modifies the biological activities of certain chemokines and neuropeptides. In addition, human DPPIV, also known as the T‐cell activation antigen CD26, binds adenosine deaminase (ADA) to the T‐cell surface, thus protecting the T‐cell from adenosine‐mediated inhibition of proliferation. Mutations were engineered into DPPIV (five point, 16 single point and six deletion mutations) to examine the binding of ADA and 19 monoclonal antibodies. Deletions of C‐terminal residues from the 738‐residue extracellular portion of DPPIV showed that the 214 residues C‐terminal to Ser552 were not required for ADA binding and that peptidase activity could be ablated by deletion of 20 residues from the C‐terminus. Point mutations at either of two locations, Leu294 and Val341, ablated ADA binding. Binding by six anti‐DPPIV antibodies that inhibited ADA binding was found to require Leu340 to Arg343 and Thr440/Lys441 but not the 214 residues C‐terminal to Ser552. The 13 other antibodies studied bound to a truncated DPPIV consisting of amino acids 1–356. Therefore, the binding sites on DPPIV of ADA and antibodies that inhibit ADA binding are discontinuous and overlapping. Moreover, the 47 and 97 residue spacing of amino acids in these binding sites concords with their location on a β propeller fold consisting of repeated β sheets of about 50 amino acids.

The Src homology 2 (SH2) domain-containing protein Grb7 and the erbB2 receptor tyrosine kinase are overexpressed in a subset of human breast cancers. They also co-immunoprecipitate from cell lysates and associate directly in vitro. Whereas the Grb7 SH2 domain binds strongly to erbB2, the SH2 domain of Grb14, a protein closely related to Grb7, does not. We have investigated the preferred binding site of Grb7 within the erbB2 intracellular domain and the SH2 domain residues that determine the high affinity of Grb7 compared with Grb14 for this site. Phosphopeptide competition and site-directed mutagenesis revealed that Tyr-1139 of erbB2 is the major binding site for the Grb7 SH2 domain, indicating an overlap in binding specificity between the Grb7 and Grb2 SH2 domains. Substituting individual amino acids in the Grb14 SH2 domain with the corresponding residues from Grb7 demonstrated that a Gln to Leu change at the ␤D6 position imparted high affinity erbB2 interaction, paralleled by a marked increase in affinity for the Tyr-1139 phosphopeptide. The reverse switch at the ␤D6 position abrogated Grb7 binding to erbB2. This residue therefore represents an important determinant of SH2 domain specificity within the Grb7 family.

The hallmarks of chronic liver diseases are chronic inflammation, cellular damage, regeneration and fibrosis. An appreciation of intrahepatic molecular expression patterns in normal and diseased liver provides clues for understanding pathogenic pathways whilst studies of the structure and function of molecules implicated in liver disease provide insights into their potential as therapeutic targets. We have examined the expression, function, molecular structure and structure-function relationships of type IV dipeptidyl aminopeptidases. In particular, the roles of CD26/DPPIV in T-cell proliferation and chemotaxis and of fibroblast activation protein in human cirrhosis are discussed. We have investigated the pathogenesis of liver disease by characterising patterns of cytokine and growth factor expression in experimental and human cirrhosis. We have quite recently expanded this approach to use differential gene expression analyses to elucidate overall pathways of gene activation and suppression in human cirrhosis. In addition, our detailed molecular and cellular studies of the mechanisms of spontaneous liver transplant tolerance have generated novel insights into this process. This review touches on these diverse aspects of liver function and disease.

Abstract:Hydrogen peroxide (H 2 O 2 ) can act as an intracellular messenger by oxidizing sulfhydryl groups in cysteines that can be oxidized at neutral pH. The oxidizing agents H 2 O 2 and pyrroloquinoline quinone and the large thiol reagents N-ethylmaleimide and 4-(hydroxymercuri) benzoate each inhibited dipeptidyl peptidase (DP) activity in the intracellular DPIV-related proteins DP8 and DP9 at pH 7.5. In contrast, these treatments did not alter activity in DPIV and fibroblast activation protein. Peptidase inhibition was completely reversed by 2-mercaptoethanol or reduced glutathione. Alkylation of DP8 by the small thiol reagent iodoacetamide prevented inhibition by H 2 O 2, N-ethylmaleimide or pyrroloquinoline quinone. Two cysteines were reactive per peptidase monomer. We exploited these properties to highly purify DP8 by thiol affinity chromatography. Homology modelling of DP8 and DP9 was consistent with the proposal that the mechanism involves decreased protein flexibility caused by intramolecular disulfide bonding. These novel data show that DP8 and DP9 are reversibly inactivated by oxidants at neutral pH and suggest that DP8 and DP9 are H 2 O 2 sensing proteins.

Background: Group IIA secreted phospholipase A 2 (hGIIA) is a bifunctional protein that regulates arachidonic acid metabolism by both catalysis-dependent and catalysis-independent mechanisms. Results: Selective inhibition of the catalysis-independent signaling function perturbs a hGIIA-vimentin interaction in rheumatoid synoviocytes. Conclusion:The signaling and catalytic functions of hGIIA are pharmacologically separable. Significance: Functionally selective inhibitors of hGIIA may provide new avenues for investigation and treatment of immunemediated inflammation.