Dipeptidyl Peptidase-4 Inhibition and the Treatment of Type 2 Diabetes
Dipeptidyl peptidase (DPP)-4 is a complex enzyme that exists as a membrane-anchored cell surface peptidase that transmits intracellular signals via a short intracellular tail and as a second smaller soluble form present in the circulation. DPP-4 cleaves a large number of chemokines and peptide hormones in vitro, but comparatively fewer peptides have been identified as endogenous physiological substrates for DPP-4 in vivo. Both glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) are endogenous physiological substrates for DPP-4, and chemical inhibition of DPP-4 activity, or genetic inactivation of DPP-4 in rodents, results in increased levels of intact bioactive GIP and GLP-1. Furthermore, mice and rats with genetic inactivation or inhibition of DPP-4 exhibit improved glucose tolerance, elevated levels of GLP-1 and GIP, and resistance to diet-induced obesity and hyperglycemia. Sustained DPP-4 inhibition lowers blood glucose via stimulation of insulin and inhibition of glucagon secretion and is associated with preservation of ß-cell mass in preclinical studies. Although DPP-4 cleaves dozens of regulatory peptides and chemokines in vitro, studies of mice with genetic inactivation of incretin receptors demonstrate that GIP and GLP-1 receptor-dependent pathways represent the dominant mechanisms transducing the glucoregulatory actions of DPP-4 inhibitors in vivo. The available preclinical data suggests that highly selective DPP-4 inhibition represents an effective and safe strategy for the therapy of type 2 diabetes.

DPP-4 is a widely expressed cell surface peptidase that exhibits a complex biology encompassing cell membrane-associated activation of intracellular signal transduction pathways, cell-cell interaction, and enzymatic activity exhibited by both the membrane-anchored and soluble forms of the enzyme. DPP-4, also originally known as the lymphocyte cell surface marker CD26, or as the adenosine deaminase (ADA)-binding protein, is a 766-amino acid serine protease that preferentially cleaves peptide hormones containing a position two alanine or proline. The human gene encoding DPP has been localized to chromosome 2 locus 2q24.3. The majority of the DPP-4 protein is extracellular, with a hydrophobic transmembrane sequence (amino acids 7-28) anchoring the protein in the cell membrane, followed by a very short six-amino acid intracellular sequence. DPP-4 is found on the cell surface as a glycosylated homodimer; however, glycosylation does not appear to be essential for enzymatic activity or binding of ADA. The catalytic region encompasses amino acids 511-766 and is also present in a soluble form of DPP-4 (sDPP-4), which is comprised of the majority of the extracellular DPP-4 protein (amino acids 39-766). sDPP-4 is capable of exhibiting enzymatic activity and interacting with the mannose-6-phosphate/insulin-like growth factor II receptor (M6P-IGFIIR) on specific cell types. The wide tissue distribution of DPP-4 on numerous cell types and in different vascular beds and its presence as a soluble active enzyme in the circulation ensures that DPP-4-mediated proteolysis is a common event in most tissue compartments.

DPP-4 is a member of a complex gene family (Figure 1), many members of which also cleave structurally related peptides. The DPP-4-related enzymes (Figure 1) include seprase; fibroblast activation protein α; DPP-6, -8, and -9; attractin; N-acetylated-α-linked acidic dipeptidases I, II, and L; quiescent cell proline dipeptidase; thymus-specific serine protease; and DPP-4ß. ADA immunoaffinity chromatography, which selectively binds and sequesters DPP-4, removed the majority (95%) of DPP-4-like enzymatic activity present in human plasma, thereby identifying DPP-4 as the predominant enzyme responsible for X-Pro or X-Ala cleavage in human serum. The multiple members of the DPP-4 family mandate a careful assessment of the selectivity and specificity of any agent used to inhibit DPP-4 activity.



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Family of DPP-4-related proteases and their substrate specificities. For the majority of enzymes, the biological roles and identity of endogenous substrates remains poorly understood. APP, aminopeptidase P; FAP, fibroblast activation protein; PEP, prolyl endopeptidase.