G protein–coupled receptors (GPCRs) continue to be important discovery targets for the treatment of type 2 diabetes mellitus (T2DM). Many GPCRs are directly involved in the development of insulin resistance and β-cell dysfunction, and in the etiology of inflammation that can lead to obesity-induced T2DM. This review summarizes the current literature describing a number of well-validated GPCR targets, but also outlines several new and promising targets for drug discovery. We highlight the importance of understanding the role of these receptors in the disease pathology, and their basic pharmacology, which will pave the way to the development of novel pharmacological probes that will enable these targets to fulfill their promise for the treatment of these metabolic disorders.

Here we describe the pharmacological properties of a series of clinically relevant chemoattractant receptor-homologous molecule expressed on T-helper type 2 (CRTh2) receptor antagonists including fevipiprant (NVP-QAW039 or QAW039) which is currently in development for treatment of allergic diseases. [³H]-QAW039 displayed high affinity for the human CRTh2 receptor (1.14 ± 0.44 nM) expressed in CHO-cells, the binding being reversible and competitive with the native agonist PGD₂. The binding kinetics of QAW039 determined directly using [³H]-QAW039 revealed mean kinetic k_on and k_off values for QAW039 of 4.5x10⁷ M^-1min^-1 and 0.048 min^-1 respectively. Importantly, the kinetic off-rate (k_off) of QAW039 (t_1/2 = 14.4 min) was >7 fold slower than the slowest reference compound tested AZD-1981. In functional studies QAW039 behaved as an insurmountable antagonist of PGD₂ stimulated [³⁵S]-GTPγS activation and its effects were not fully reversed by increasing concentrations of PGD₂ following an initial 15min incubation period. This behavior is consistent with its relatively slow dissociation from the human CRTh2 receptor. In contrast the inhibitory effect of the other ligands tested was fully reversed by the 15 min time point, whereas QAW039's effects persisted for >180 min. All CRTh2 antagonists tested inhibited PGD₂ stimulated human eosinophil shape change but importantly QAW039 retained its potency in the whole blood shape change assay relative to the isolated shape change assay, potentially reflective of its relatively slower off-rate from the CRTh2 receptor. QAW039 was also a potent inhibitor of PGD₂ induced cytokine release in human Th2 cells. Slow CRTh2 antagonist dissociation could provide increased receptor coverage in the face of pathological PGD₂ concentrations, which may be clinically relevant.

Monocyte-like cell lines (MCLCs), including THP-1, HL-60 and U-937 cells, are used routinely as surrogates for isolated human peripheral blood mononuclear cells (PBMCs). To systematically evaluate these immortalised cells and PBMCs as model systems to study inflammation relevant to the pathogenesis of type II diabetes and immuno-metabolism, we compared mRNA expression of inflammation-relevant genes, cell surface expression of cluster of differentiation (CD) markers, and chemotactic responses to inflammatory stimuli. Messenger RNA expression analysis suggested most genes were present at similar levels across all undifferentiated cells, though notably, IDO1, which encodes for indoleamine 2,3-dioxygenase and catabolises tryptophan to kynureninase (shown to be elevated in serum from diabetic patients), was not expressed in any PMA-treated MCLC, but present in GM-CSF-treated PBMCs. There was little overall difference in the pattern of expression of CD markers across all cells, though absolute expression levels varied considerably and the correlation between MCLCs and PBMCs was improved upon MCLC differentiation. Functionally, THP-1 and PBMCs migrated in response to chemoattractants in a transwell assay, with varying sensitivity to MCP-1, MIP-1α and LTB-4. However, despite similar gene and CD expression profiles, U-937 cells were functionally impaired as no migration was observed to any chemoattractant. Our analysis reveals that the MCLCs examined only partly replicate the genotypic and phenotypic properties of human PBMCs. To overcome such issues a universal differentiation protocol should be implemented for these cell lines, similar to those already used with isolated monocytes. Although not perfect, in our hands the THP-1 cells represent the closest, simplified surrogate model of PBMCs for study of inflammatory cell migration.