Studies of human and mouse GPR as ascertained
Studies of human and mouse GPR84, as ascertained by mRNA levels in various tissues, have determined that GPR84 is highly expressed on bone marrow cells, splenic T and B cells, and circulating granulocytes/monocytes/macrophages. In the latter cells, mRNA expression of is up-regulated only under inflammatory conditions. GPR84 is also expressed in brain, heart, muscle, colon, thymus, spleen, kidney, liver, intestine, placenta, lung, and leukocytes., GPR84 may be a mediator of the relationship between obesity and diabetes. Indeed, as adipocytes release fatty acids in the presence of macrophages, increased GPR84 expression and stimulation prevent the release of regulating hormones/adiponectin. GPR84 is expressed in the gastric corpus mucosa, and this receptor can be an important luminal sensor of food intake; it is most likely expressed on enteroendocrine cells, where it stimulates the release of peptide hormones, including incretins glucagon-like peptides 1 and 2.,
Although both receptors have been associated with metabolic regulation and inflammation, they have not been previously linked to organ fibrosis. In this study, we demonstrate that PBI-4050, a synthetic ligand of GPR40 and GPR84, acts on cells involved in the fibrotic pathway: macrophages, fibroblasts, and epithelial cells. Moreover, PBI-4050 reduces fibrosis in animal models of kidney, lung, heart, liver, pancreas, and skin fibrosis. We also demonstrate that both receptors are modulated in models of fibrotic diseases and show that mice with a deletion in have increased renal interstitial fibrosis in response to ischemia, unilateral ureteral obstruction (UUO), and adenine-induced nephropathy models, whereas knockout mice have reduced kidney fibrosis in a model of adenine-induced nephropathy.
Materials and Methods
Results PBI-4050 (3-pentylbenzeneacetic pkg inhibitor sodium salt) (Figure 1A), a synthetic analog of decanoic acid, was designed to rigidify the fatty acid structure and reduce the β-oxidation of decanoate, the sodium salt of decanoic acid, a medium-chain fatty acid. As decanoic acid can activate the free fatty acid receptors GPR402, 25 and GPR84,4, 26, 27 the signaling properties of PBI-4050 on these receptors were verified.
Discussion The data presented herein clearly demonstrate the contribution of two fatty acid receptors, GPR40 and GPR84, in the regulation of cells involved in fibrosis. Although GPR40 is minimally detectable in immune cells, with the exception of monocytes, GPR84 is abundantly found in bone marrow, T cells, B cells, granulocytes, microglia, neutrophils, and monocytes/macrophages,38, 39, 40 and has been shown to work as a proinflammatory mediator. Herein, we report that GPR40 is also expressed in human and mouse epithelial cells (proximal tubule and cortical collecting duct), and GPR84 is expressed in human fibroblasts/myofibroblasts and human podocytes. To date, there have only been limited reports indicating that GPR40 is expressed and modulated in kidney,11, 12 and to our knowledge, there is no report that GPR84 is modulated in kidney diseases. Our data confirm the overexpression of GPR40 and GPR84 in different chronic and acute kidney injury models. Interestingly, although GPR84 is weakly expressed under normal conditions in renal tissue, the data indicate that it is overexpressed in various kidney disease models, whereas GPR40 is overexpressed after epithelial injury. Also, LPS-treated podocytes and macrophages, as well as TGF-β–treated fibroblasts and podocytes, show significant GPR84 up-regulation. Our data suggest that through the binding to GPR40 and/or GPR84, PBI-4050 reduces fibrosis via the regulation of macrophages, fibroblasts/myofibroblasts, and epithelial cells. PBI-4050 inhibits activation of fibroblasts to profibrotic myofibroblasts, as demonstrated by abrogation of α-SMA expression in fibroblasts, and subsequent accumulation of extracellular matrix protein deposition and fibrosis. Furthermore, PBI-4050 reduced macrophage activation and the expression of proinflammatory markers (monocyte chemoattractant protein-1, IL-8, and IL-6) and profibrotic markers (connective tissue growth factor and IL-6). PBI-4050 treatment decreases F4/80 immunostaining (marker of macrophages) in the kidneys and pancreas islets of db/db eNOS mice (M.-Z.Z. and R.C.H., unpublished data). Therefore, the protective actions of PBI-4050 via GPR40 and GPR84 may reside in modulating such infiltrating cells, and in reducing proliferation/activation of myofibroblasts and epithelial-mesenchymal transition in the injured tissue. Ma et al have shown that pretreatment of human renal proximal tubule epithelial (HK-2) cells with the GPR40 agonist GW9508 attenuated cisplatin-induced apoptosis, an effect that could also contribute to the protective action of PBI-4050 in our renal injury models.