Regulation of Chemokine Activity - A Focus on the Role of Dipeptidyl Peptidase IV/CD26

Abstract

Chemokines are small, chemotactic proteins that play a crucial role in leukocyte migration and are, therefore, essential for proper functioning of the immune system. Chemokines exert their chemotactic effect by activation of chemokine receptors, which are G protein-coupled receptors (GPCRs), and interaction with glycosaminoglycans (GAGs). Furthermore, the exact chemokine function is modulated at the level of posttranslational modifications. Among the different types of posttranslational modifications that were found to occur in vitro and in vivo, i.e., proteolysis, citrullination, glycosylation, and nitration, NH₂-terminal proteolysis of chemokines has been described most intensively. Since the NH₂-terminal chemokine domain mediates receptor interaction, NH₂-terminal modification by limited proteolysis or amino acid side chain modification can drastically affect their biological activity. An enzyme that has been shown to provoke NH₂-terminal proteolysis of various chemokines is dipeptidyl peptidase IV or CD26. This multifunctional protein is a serine protease that preferably cleaves dipeptides from the NH₂-terminal region of peptides and proteins with a proline or alanine residue in the penultimate position. Various chemokines possess such a proline or alanine residue, and CD26-truncated forms of these chemokines have been identified in cell culture supernatant as well as in body fluids. The effects of CD26-mediated proteolysis in the context of chemokines turned out to be highly complex. Depending on the chemokine ligand, loss of these two NH₂-terminal amino acids can result in either an increased or a decreased biological activity, enhanced receptor specificity, inactivation of the chemokine ligand, or generation of receptor antagonists. Since chemokines direct leukocyte migration in homeostatic as well as pathophysiologic conditions, CD26-mediated proteolytic processing of these chemotactic proteins may have significant consequences for appropriate functioning of the immune system. After introducing the chemokine family together with the GPCRs and GAGs, as main interaction partners of chemokines, and discussing the different forms of posttranslational modifications, this review will focus on the intriguing relationship of chemokines with the serine protease CD26.

Keywords: CD26; GPCR; chemokine; dipeptidyl peptidase IV; glycosaminoglycan; leukocyte migration; posttranslational modification; proteolysis.

Figure 1

The chemokine family and effect of CD26 on chemokine receptor–ligand interactions. Various chemokines can be subjected to proteolytic processing by the enzyme dipeptidyl peptidase IV or CD26. The effects of truncation by CD26 are indicated by colors. Red, CD26-mediated proteolysis negatively affects the interaction between chemokine and chemokine receptor. Green, CD26-mediated proteolysis has a positive effect on the interaction between chemokine and chemokine receptor. Blue, proteolytic processing by CD26 does not influence the interaction between chemokine and chemokine receptor. Brown, the implications of truncation by CD26 remain to be determined. *, in contrast to intact CCL4, CCL4(3–69) shows affinity for CCR1 and CCR2. **, also known as CXCR7. ***, the notation “ACKR5” is reserved for this receptor. NC, not cleaved by CD26.

Figure 2

Chemokine-induced signal transduction. Chemokine receptors are G protein-coupled receptors (GPCRs), implying that classical chemokine-induced signaling is G protein-dependent. Binding of a chemokine ligand induces a change in conformation of the GPCR, thereby facilitating exchange of guanosine diphosphate (GDP), which is bound by the α-subunit of the G protein during receptor inactivity, for guanosine triphosphate (GTP). Most chemokine receptors are coupled to G proteins that hold an inhibitory type of α subunit (Gα_i), implying that the newly formed Gα–GTP complex mediates inhibition of adenylyl cyclase, resulting in decreasing cyclic adenosine monophosphate (cAMP) concentrations. The βγ-subunit of the G protein (G_βγ), in turn, activates phospholipase Cβ (PLCβ), resulting in initiation of cleavage of phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). The latter facilitates calcium release from the endoplasmic reticulum. Calcium and PIP₂ cooperate in the activation of protein kinase C (PKC) and other calcium-sensitive protein kinases. In addition, G_βγ interacts with Ras, followed by activation of phosphatidylinositol-3-kinases (PI3K) and PIP₃. PIP₃ activates Rac, a GTPase, and interacts with protein kinase B (PKB), which are important for leukocyte migration and actin polymerization, respectively. In the end, modulation of actin-dependent processes regulates various leukocyte functions and initiates chemotaxis. In addition to G protein-dependent signaling, some chemokine receptors couple to arrestin after chemokine-binding and phosphorylation of the receptor by G protein-coupled receptor kinases (GRK). Arrestin mediates G protein uncoupling of the receptor and plays a role in receptor desensitization. In addition, arrestin interaction can promote receptor internalization to endosomes and ligand degradation, initiation of an additional round of cell signaling, or receptor recycling to the cell membrane.

Figure 3

Schematic structure of homodimeric CD26. Each CD26 monomer consists of an intracellular NH₂-terminal tail, a transmembrane region, a flexible part, a glycosylation-rich region, a cysteine-rich region, and a catalytic domain. Ser₆₃₀, Asp₇₀₈, and His₇₄₀ are involved in the catalytic process and are generally referred to as “the catalytic triad.” Structurally, the two termini of a CD26 monomer contribute to the formation of a β-propeller structure. An α/β-hydroxylase domain is covalently bound to the β-propeller structure. These structural features imply that CD26’s catalytic pocket is situated in a locked hole.