pak1 PBI is an agonist of GPR and acts as
PBI-4050 is an agonist of GPR40 and acts as an antagonist or inverse agonist of GPR84. It cannot be excluded that other targets besides GPR40 and GPR84 could be implicated in the mechanism of action of PBI-4050 and could be explored in future studies. However, the present study, and in particular the receptor KO models, strongly supports GPR40 and GPR84 as major mediators in pathologic fibrotic pathways and as the targets of the antifibrotic effects of PBI-4050. Our data show that PBI-4050 significantly attenuated fibrosis in a variety of injury contexts, as evidenced with the antifibrotic activity observed in kidney, liver, lung, heart, pancreas, and skin fibrosis models. Given our findings with both Gpr40 and Gpr84 KO mice, both receptors appear to be involved in the fibrotic pathways. Considering their expression along the nephron, within the glomerulus, and in numerous bone marrow–derived cell types, it is likely that GPR40 and GPR84 modulate profibrotic, inflammatory, and epithelial-mesenchymal transition processes. Therefore, GPR40 may partially protect against development of fibrosis, whereas GPR84 may induce the promotion and stimulation of fibrosis, as observed with the significant increase in fibrosis in Gpr40−/− mice and reduction of fibrosis in Gpr84−/− mice. The dual modulator PBI-4050 reinforces the involvement of both GPR40 and GPR84 in multiple models of fibrosis. In the context of the adenine-induced CKD model, treatment of Gpr40 and Gpr84 KO mice with PBI-4050 suggests its antifibrosis effects to be mainly mediated by GPR40 activation, whereas inhibition of GPR84 mostly accounts for reduction of cystic lesions. The relative role of each receptor may vary depending on the type of insult, pathology, and organ. Future work will aim to elucidate the precise intracellular signaling pathways used by both GPR40 and GPR84 to regulate fibrotic events in the pathogenesis of disease.
The inhibition of fibrotic and inflammatory markers by PBI-4050, found in human proximal tubule epithelial cells, podocytes, and primary fibroblasts (Figure 3), suggests that the attenuation of fibrosis that we have shown in various rodent fibrosis models could translate to human disease. Moreover, significant clinical activity observed in recent phase 2 clinical trials in type 2 diabetes subjects with metabolic syndrome and in idiopathic pulmonary fibrosis patients confirm translation of pharmacologic activity of PBI-4050 in humans. Of interest, PBI-4050 was well tolerated and demonstrated a good safety profile in these two early-phase clinical trials.
Introduction Professional phagocytic neutrophils are key effector pak1 in inflammation, forming the first line of host defense against invading microbes [1,2]. Many basic functions of neutrophils including granule mobilization/secretion, migration and the release of reactive oxygen species (ROS) through the NADPH-oxidase, are regulated by surface expressed chemoattractant receptors belonging to the family of G-protein coupled receptors (GPCRs) . Among the large number of receptors belonging to the GPCR family, neutrophils express e.g., two formyl peptide receptors (FPR1 and FPR2), the receptor for platelet activating factor (PAFR), those that recognize interleukin 8 (IL-8; CXCR1/2), complement fragment 5a (C5aR) and the danger signal ATP (P2Y2R) as well as some members of the free fatty acid receptor group (FFARs) that has in common that they recognize free fatty acids (FFAs) of different carbon chain lengths [, , ]. During the past decade, an increasing number of orphan GPCRs have been de-orphanized, and these include the recently described FFARs sensing metabolic intermediates in the form of free fatty acids with different carbon chain lengths [7,8]. FFAs were traditionally believed to exert their metabolic responses only through interactions with intracellular targets such as peroxisome proliferator-activated receptors , but it is now evident that FFAs facilitate their effects mainly through the group of GPCRs that together are termed FFARs. This receptor group includes FFA1R (earlier known as GPR40), FFA2R (earlier known as GPR43) and FFA3R (earlier known as GPR41) that have in common that they all bind short-chain fatty acids (SCFAs). Also, FFA4R (earlier known as GPR120) which binds both medium and long-chain fatty and n-3 polyunsaturated fatty acids (PUFA), and GPR84 (also known as EX33) sensing medium chain fatty acid (MCFAs) [6,8,10] belong to the family of FFARs. The levels of circulating FFAs has been shown to be elevated in several conditions associated with the metabolic disease syndrome e.g., liver cirrhosis and steatohepatitis, a link that highlights their role in metabolism [10,11]. Very recently, a direct connection was identified between the metabolic status and the function of the immune system , and in accordance with this, FFARs have gained increased interest also for potential roles as modulators of immune cell functions and the outcome of inflammatory reactions. In line with this, by employing the recently identified FFA2R selective agonist Compound 1 (Cmp1) and antagonist CATPB, we and others have provided data on the functional expression and modulation of the short chain fatty acid binding FFA2R expressed in human neutrophils [4,12]. In this study, we show that the activation/deactivation pattern of FFA2R is in some aspects different from that of the formyl peptide receptors (FPRs), the most studied pattern recognition receptors expressed in phagocytes and the prototypical neutrophil GPCRs . These findings have now been expanded to include GPR84, a FFAR that was de-orphanized in 2006 and that has been shown to recognize MCFAs with chain lengths of 9– 14 carbon atoms . Compared to the extensively studied FPRs and the recent characterized FFA2R, very little is known about the activation pattern and regulation mechanisms of GPR84 in neutrophils except for the fact that the receptor is highly expressed by peripheral blood leukocytes . Basic characterization of the activation pattern of GPR84, has been hindered by the lack of potent and selective molecular tools, a problem still common and shared with many other GPCRs. Molecular tools that selectively target GPR84 have, however, recently become available.