The group of proton-sensing G-protein coupled receptors (GPCRs) consists of the four receptors GPR4, TDAG8 (GPR65), OGR1 (GPR68), and G2A (GPR132). These receptors are cellular sensors of acidification, a property that has been attributed to the presence of crucial histidine residues. However, the pH detection varies considerably among the group of proton-sensing GPCRs and ranges from pH of 5.5 to 7.8. While the proton-sensing GPCRs were initially considered to detect acidic cellular environments in the context of inflammation, recent observations have expanded our knowledge about their physiological and pathophysiological functions and many additional individual and unique features have been discovered that suggest a more differentiated role of these receptors in health and disease. It is known that all four receptors contribute to different aspects of tumor biology, cardiovascular physiology, and asthma. However, apart from their overlapping functions, they seem to have individual properties, and recent publications identify potential roles of individual GPCRs in mechanosensation, intestinal inflammation, oncoimmunological interactions, hematopoiesis, as well as inflammatory and neuropathic pain. Here, we put together the knowledge about the biological functions and structural features of the four proton-sensing GPCRs and discuss the biological role of each of the four receptors individually. We explore all currently known pharmacological modulators of the four receptors and highlight potential use. Finally, we point out knowledge gaps in the biological and pharmacological context of proton-sensing GPCRs that should be addressed by future studies.

Celecoxib, a selective cyclooxygenase‐2 (COX‐2) inhibitor, has recently been approved for the symptomatic treatment of arthritis. In some clinical studies, doses of 400 and 800 mg/day provided somewhat less efficacy compared with 200 mg/day, which suggests an early ceiling effect. Using the zymosan‐induced inflammation model in rats, we show that celecoxib significantly reduces paw swelling at 50 mg/kg but completely loses its anti‐inflammatory efficacy at doses ≥100 mg/kg. To evaluate the underlying mechanisms, we used rat renal mesangial cells as a cell culture model. In these cells, celecoxib (50 μM) increased the interleukin Iβ stimulated nuclear translocation and DNA binding of NF‐κB and facilitated the degradation of I‐κB. Consequently, COX‐2 and tumor necrosis factor α (TNF‐α) expression were increased. The up‐regulation of COX‐2 and TNF‐α also occurred in the spinal cord of rats treated with celecoxib (≥100 mg/kg), indicating that in vitro mechanisms were relevant in vivo. Clinically, the overexpression of COX‐2 might be less important because celecoxib inhibits COX‐2 enzymatically. However, the up‐regulation of TNF‐α and possibly other NF‐κB regulated proinflammatory genes might worsen the pathophysiological processes underlying chronic arthritis.

A major immunological response during neuroinflammation is the activation of microglia, which subsequently release proinflammatory mediators such as prostaglandin E₂ (PGE₂). Besides its proinflammatory properties, cyclooxygenase-2 (COX-2)-derived PGE₂ has been shown to exhibit anti-inflammatory effects on innate immune responses. Here, we investigated the role of microsomal PGE₂ synthase-1 (mPGES-1), which is functionally coupled to COX-2, in immune responses using a model of lipopolysaccharide (LPS)-induced spinal neuroinflammation. Interestingly, we found that activation of E-prostanoid (EP)2 and EP4 receptors, but not EP1, EP3, PGI₂ receptor (IP), thromboxane A₂ receptor (TP), PGD₂ receptor (DP), and PGF₂ receptor (FP), efficiently blocked LPS-induced tumor necrosis factor α (TNFα) synthesis and COX-2 and mPGES-1 induction as well as prostaglandin synthesis in spinal cultures. In vivo, spinal EP2 receptors were up-regulated in microglia in response to intrathecally injected LPS. Accordingly, LPS priming reduced spinal synthesis of TNFα, interleukin 1β (IL-1β), and prostaglandins in response to a second intrathecal LPS injection. Importantly, this reduction was only seen in wild-type but not in mPGES-1-deficient mice. Furthermore, intrathecal application of EP2 and EP4 agonists as well as genetic deletion of EP2 significantly reduced spinal TNFα and IL-1β synthesis in mPGES-1 knock-out mice after LPS priming. These data suggest that initial inflammation prepares the spinal cord for a negative feedback regulation by mPGES-1-derived PGE₂ followed by EP2 activation, which limits the synthesis of inflammatory mediators during chronic inflammation. Thus, our data suggest a role of mPGES-1-derived PGE₂ in resolution of neuroinflammation.

The UDP-glucose ceramide glycosyltransferase (UGCG) is a key enzyme in the sphingolipid metabolism by generating glucosylceramide (GlcCer), the precursor for all glycosphingolipids (GSL), which are essential for proper cell function. Interestingly, the UGCG is also overexpressed in several cancer types and correlates with multidrug resistance protein 1 (MDR1) gene expression. This membrane protein is responsible for efflux of toxic substances and protects cancer cells from cell damage through chemotherapeutic agents. Studies showed a connection between UGCG and MDR1 overexpression and multidrug resistance development, but the precise underlying mechanisms are unknown. Here, we give an overview about the UGCG and its connection to MDR1 in multidrug resistant cells. Furthermore, we focus on UGCG transcriptional regulation, the impact of UGCG on cellular signaling pathways and the effect of UGCG and MDR1 on the lipid composition of membranes and how this could influence multidrug resistance development. To our knowledge, this is the first review presenting an overview about UGCG with focus on the relationship to MDR1 in the process of multidrug resistance development.

Inflammation is a protective response to pathogens and injury. To be effective it needs to be resolved by endogenous mechanisms in order to avoid prolonged and excessive inflammation, which can become chronic. Specialized pro-resolving mediators (SPMs) are a group of lipids derived from omega-3 fatty acids, which can induce the resolution of inflammation. How SPMs exert their anti-inflammatory and pro-resolving effects is, however, not clear. Here, we show that SPMs such as protectins, maresins, and D-series resolvins function as biased positive allosteric modulators (PAM) of the prostaglandin E ₂ (PGE ₂ ) receptor EP4 through an intracellular binding site. They increase PGE ₂ -induced G _s -mediated formation of cAMP and thereby promote anti-inflammatory signaling of EP4. In addition, SPMs endow the endogenous EP4 receptor on macrophages with the ability to couple to G _i -type G-proteins, which converts the EP4 receptor on macrophages from an anti-phagocytotic receptor to one increasing phagocytosis, a central mechanism of the pro-resolving activity of synthetic SPMs. In the absence of the EP4 receptor, SPMs lose their anti-inflammatory and pro-resolving activity in vitro and in vivo. Our findings reveal an unusual mechanism of allosteric receptor modulation by lipids and provide a mechanism by which synthetic SPMs exert pro-resolving and anti-inflammatory effects, which may facilitate approaches to treat inflammation.