Mechanism of Mer receptor tyrosine kinase inhibition of glomerular endothelial cell inflammation

Paeoniflorin alleviated STZ-induced diabetic retinopathy via regulation of the PDI/ADAM17/MerTK pathway

BACKGROUND

Diabetic retinopathy (DR) is a severe microvascular complication of diabetes and a leading cause of vision impairment in diabetic patients. The accumulation of apoptotic cells and inflammation are key pathological mechanisms in DR. The Mer tyrosine kinase (MerTK) receptor plays a critical role in maintaining retinal homeostasis. Proteolytic cleavage of MerTK by disintegrin and metalloproteinase-17 (ADAM17) disrupts MerTK-dependent clearance of apoptotic cells and diminishes its anti-inflammatory effects. Therefore, reducing the cleavage activity ADAM17's and promoting MerTK-dependent anti-inflammatory effects may represent potent strategy to alleviate DR.

METHODS

The DR mouse model was established using streptozotocin (STZ), and a high-glucose (HG)-induced in vitro model was developed using human retinal pigment epithelial (ARPE-19) cells. Relevant signaling molecules were analyzed through western blotting and immunohistochemistry.

RESULTS

Hyperglycemia promoted the accumulation of apoptotic cells and disrupted retinal microvascular growth. In both vivo and vitro model, MerTK expression was significantly reduced, while ADAM17 phosphorylation levels were markedly increased. In STZ-treated mice, protein disulfide isomerase (PDI) secretion initially rose but subsequently declined, whereas PDI secretion decreased under HG conditions. We then utilized paeoniflorin to increase the expression of this endogenous inhibitor of ADAM17. Results showed that paeoniflorin upregulated PDI production, suppressed ADAM17 expression, and enhanced MerTK phosphorylation in the eye tissues of STZ-induced mice. Additionally, paeoniflorin elevated the expression of suppressor of cytokine signaling 3 (SOCS3) and decreased the level of matrix metalloproteinase 9 (MMP9) both in vivo and in vitro.

CONCLUSION

Paeoniflorin may alleviate diabetic retinopathy by suppressing inflammation through modulation of the PDI/ADAM17/MerTK signaling pathway.

Osteopontin regulation of MerTK+ macrophages promotes Crohn's disease intestinal fibrosis

The pathogenesis of intestinal fibrosis in Crohn's disease (CD) remains unclear. Mer receptor tyrosine kinase (MerTK) is an immunosuppressive protein specifically expressed in macrophages. Osteopontin (OPN), also known as secreted phosphoprotein 1, contributes to inflammation and wound repair. This study investigates the potential profibrotic pathway in MerTK+ macrophages in order to provide a possible therapeutic target for intestinal fibrosis. MerTK expression in the inflamed and stenotic bowels was evaluated. The MerTK/ERK/TGF-β1 pathway was overactivated in the fibrotic intestinal tissues of patients with CD. This pathway was induced by epithelial cell apoptosis, resulting in activated fibroblasts with increased TGF-β1 secretion. OPN upregulated TGF production by altering ERK1/2 phosphorylation, as evidenced by OPN or MerTK knockdown and OPN overexpression in vitro. MerTK inhibitor UNC2025 alleviated intestinal fibrosis in mouse colitis models, suggesting a potential therapeutic target for intestinal fibrosis in patients with CD.

Ulinastatin promotes macrophage efferocytosis and ameliorates lung inflammation via the ERK5/Mer signaling pathway

Acute lung injury (ALI) is a pneumonic response characterized by neutrophil infiltration. Macrophage efferocytosis is the process whereby macrophages remove apoptotic cells, and is required for ALI inflammation to subside. The glycoprotein ulinastatin (UTI) has an anti‐inflammatory effect during the acute stages of ALI, but its effect on efferocytosis and the subinflammatory stage of ALI is unclear. Extracellular signal‐regulated kinase 5 (ERK5) is a key protein in efferocytosis, and we thus hypothesized that it may be activated by UTI to regulate efferocytosis and the resolution of pneumonia. To test this hypothesis, here we monitored phagocytosis of macrophages through in vivo and in vitro experiments. Pulmonary edema, neutrophil infiltration, protein exudation, and inflammatory factor regression were observed on days 1, 3, 5, and 7 in vivo. RAW264.7 cells were pretreated with different concentrations of UTI and ERK5 inhibitors, and the expression of tyrosine‐protein kinase Mer (Mer) protein on macrophage membrane was detected. UTI increased the phagocytosis of apoptotic neutrophils by macrophages in vitro and in vivo, and promoted the resolution of pneumonia. The protein expression of ERK5 and Mer increased with UTI concentration, while the expression of Mer was down‐regulated by ERK5 inhibitors. Therefore, our results suggest that UTI enhances efferocytosis and reduces lung inflammation and injury through the ERK5/Mer signaling pathway, which may be one of the targets of UTI in the treatment of lung injury.

Study of the Role of the Tyrosine Kinase Receptor MerTK in the Development of Kidney Ischemia-Reperfusion Injury in RCS Rats

Renal ischaemia reperfusion (I/R) triggers a cascade of events including oxidative stress, apoptotic body and microparticle (MP) formation as well as an acute inflammatory process that may contribute to organ failure. Macrophages are recruited to phagocytose cell debris and MPs. The tyrosine kinase receptor MerTK is a major player in the phagocytosis process. Experimental models of renal I/R events are of major importance for identifying I/R key players and for elaborating novel therapeutical approaches. A major aim of our study was to investigate possible involvement of MerTK in renal I/R. We performed our study on both natural mutant rats for MerTK (referred to as RCS) and on wild type rats referred to as WT. I/R was established by of bilateral clamping of the renal pedicles for 30' followed by three days of reperfusion. Plasma samples were analysed for creatinine, aspartate aminotransferase (ASAT), lactate dehydrogenase (LDH), kidney injury molecule -1 (KIM-1), and neutrophil gelatinase-associated lipocalin (NGAL) levels and for MPs. Kidney tissue damage and CD68-positive cell requirement were analysed by histochemistry. monocyte chemoattractant protein-1 (MCP-1), myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS), and histone 3A (H3A) levels in kidney tissue lysates were analysed by western blotting. The phagocytic activity of blood-isolated monocytes collected from RCS or WT towards annexin-V positive bodies derived from cultured renal cell was assessed by fluorescence-activated single cell sorting (FACS) and confocal microscopy analyses. The renal I/R model for RCS rat described for the first time here paves the way for further investigations of MerTK-dependent events in renal tissue injury and repair mechanisms.

NSC828779 Alleviates Renal Tubulointerstitial Lesions Involving Interleukin-36 Signaling in Mice

Renal tubulointerstitial lesions (TILs), a common pathologic hallmark of chronic kidney disease that evolves to end-stage renal disease, is characterized by progressive inflammation and pronounced fibrosis of the kidney. However, current therapeutic approaches to treat these lesions remain largely ineffectual. Previously, we demonstrated that elevated IL-36α levels in human renal tissue and urine are implicated in impaired renal function, and IL-36 signaling enhances activation of NLRP3 inflammasome in a mouse model of TILs. Recently, we synthesized NSC828779, a salicylanilide derivative (protected by U.S. patents with US 8975255 B2 and US 9162993 B2), which inhibits activation of NF-κB signaling with high immunomodulatory potency and low IC50, and we hypothesized that it would be a potential drug candidate for renal TILs. The current study validated the therapeutic effects of NSC828779 on TILs using a mouse model of unilateral ureteral obstruction (UUO) and relevant cell models, including renal tubular epithelial cells under mechanically induced constant pressure. Treatment with NSC828779 improved renal lesions, as demonstrated by dramatically reduced severity of renal inflammation and fibrosis and decreased urinary cytokine levels in UUO mice. This small molecule specifically inhibits the IL-36α/NLRP3 inflammasome pathway. Based on these results, the beneficial outcome represents synergistic suppression of both the IL-36α-activated MAPK/NLRP3 inflammasome and STAT3- and Smad2/3-dependent fibrogenic signaling. NSC828779 appears justified as a new drug candidate to treat renal progressive inflammation and fibrosis.

The Akt-mTORC1 pathway mediates Axl receptor tyrosine kinase-induced mesangial cell proliferation

Glomerulonephritis (GN), an important pathologic feature of many renal diseases, is frequently characterized by mesangial cell proliferation. We and others have previously shown that the TAM family receptor tyrosine kinases Axl, Mer, and Tyro-3 contribute to cell survival, proliferation, migration, and clearance of apoptotic cells (ACs); that Axl contributes to GN by promoting mesangial cell proliferation; and that small molecule inhibition of Axl ameliorates nephrotoxic serum-induced GN in mice. We now show that stimulation of renal mesangial cell Axl causes a modest increase in intracellular Ca²⁺ and activates NF-κB, mTOR, and the mTOR-containing mTORC1 complex, which phosphorylates the ribosomal protein S6. Axl-induction of Akt activation is upstream of NF-κB and mTOR activation, which are mutually codependent. Axl-induced NF-κB activation leads to Bcl-xl up-regulation. Axl is more important than Mer at mediating AC phagocytosis by mesangial cells, but less important than Mer at mediating phagocytosis of ACs by peritoneal macrophages. Taken together, our data suggest the possibility that Axl mediates mesangial cell phagocytosis of ACs and promotes mesangial cell proliferation by activating NF-κB and mTORC1.

Endothelial dysfunction and cardiovascular risk in lupus nephritis: New roles for old players?

In systemic lupus erythematosus (SLE) patients, most of the clinical manifestation share a vascular component triggered by endothelial dysfunction. Endothelial cells (ECs) activation occurs both on the arterial and venous side, and the high vascular density of kidneys accounts for the detrimental outcomes of SLE through lupus nephritis (LN). Kidney damage, in turn, exerts a negative feedback on the cardiovascular (CV) system aggravating risk factors for CV diseases such as hypertension, stroke and coronary syndrome among others. Despite the intensive investigation on SLE and LN, the role of endothelial dysfunction, as well as the underlying mechanisms, remains to be fully understood, with no specifically targeted pharmacological treatment. It is not known, in fact, if the activation pathway(s) in venous ECs are similar to the one in arterial ECs and doubts persist on the shared manifestation of microcirculation compared to macrocirculation. In this work, we aim to review the recent literature about the role of endothelial activation and dysfunction in the development of CV complications in SLE and LN patients. We, therefore, focus on arteriovenous similarities and differences and on specific pathways of great vessels compared to capillaries. Critically summarising the available data is of pivotal importance for both basic researchers and clinicians in order to develop and test new pharmacological approaches in the treatment of basic components of SLE and LN.

Mer regulates microglial/macrophage M1/M2 polarization and alleviates neuroinflammation following traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide. Microglial/macrophage activation and neuroinflammation are key cellular events following TBI, but the regulatory and functional mechanisms are still not well understood. Myeloid-epithelial-reproductive tyrosine kinase (Mer), a member of the Tyro-Axl-Mer (TAM) family of receptor tyrosine kinases, regulates multiple features of microglial/macrophage physiology. However, its function in regulating the innate immune response and microglial/macrophage M1/M2 polarization in TBI has not been addressed. The present study aimed to evaluate the role of Mer in regulating microglial/macrophage M1/M2 polarization and neuroinflammation following TBI.

METHODS

The controlled cortical impact (CCI) mouse model was employed. Mer siRNA was intracerebroventricularly administered, and recombinant protein S (PS) was intravenously applied for intervention. The neurobehavioral assessments, RT-PCR, Western blot, magnetic-activated cell sorting, immunohistochemistry and confocal microscopy analysis, Nissl and Fluoro-Jade B staining, brain water content measurement, and contusion volume assessment were performed.

RESULTS

Mer is upregulated and regulates microglial/macrophage M1/M2 polarization and neuroinflammation in the acute stage of TBI. Mechanistically, Mer activates the signal transducer and activator of transcription 1 (STAT1)/suppressor of cytokine signaling 1/3 (SOCS1/3) pathway. Inhibition of Mer markedly decreases microglial/macrophage M2-like polarization while increases M1-like polarization, which exacerbates the secondary brain damage and sensorimotor deficits after TBI. Recombinant PS exerts beneficial effects in TBI mice through Mer activation.

CONCLUSIONS

Mer is an important regulator of microglial/macrophage M1/M2 polarization and neuroinflammation, and may be considered as a potential target for therapeutic intervention in TBI.

Compound K inhibits priming and mitochondria-associated activating signals of NLRP3 inflammasome in renal tubulointerstitial lesions

BACKGROUND

Renal tubulointerstitial lesions (TILs), a key pathological hallmark for chronic kidney disease to progress to end-stage renal disease, feature renal tubular atrophy, interstitial mononuclear leukocyte infiltration and fibrosis in the kidney. Our study tested the renoprotective and therapeutic effects of compound K (CK), as described in our US patent (US7932057B2), on renal TILs using a mouse unilateral ureteral obstruction (UUO) model.

METHODS

Renal pathology was performed and renal draining lymph nodes were subjected to flow cytometry analysis. Mechanism-based experiments included the analysis of mitochondrial dysfunction, a model of tubular epithelial cells (TECs) under mechanically induced constant pressure (MICP) and tandem mass tags (TMT)-based proteomics analysis.

RESULTS

Administration of CK ameliorated renal TILs by reducing urine levels of proinflammatory cytokines, and preventing mononuclear leukocyte infiltration and fibrosis in the kidney. The beneficial effects clearly correlated with its inhibition of: (i) NF-κB-associated priming and the mitochondria-associated activating signals of the NLRP3 inflammasome; (ii) STAT3 signalling, which in part prevents NLRP3 inflammasome activation; and (iii) the TGF-β-dependent Smad2/Smad3 fibrotic pathway, in renal tissues, renal TECs under MICP and/or activated macrophages, the latter as a major inflammatory player contributing to renal TILs. Meanwhile, TMT-based proteomics analysis revealed downregulated renal NLRP3 inflammasome activation-associated signalling pathways in CK-treated UUO mice.

CONCLUSIONS

The present study, for the first time, presents the potent renoprotective and therapeutic effects of CK on renal TILs by targeting the NLRP3 inflammasome and STAT3 signalling.

The role of endothelial MERTK during the inflammatory response in lungs

As a key homeostasis regulator in mammals, the MERTK receptor tyrosine kinase is crucial for efferocytosis, a process that requires remodeling of the cell membrane and adjacent actin cytoskeleton. Membrane and cytoskeletal reorganization also occur in endothelial cells during inflammation, particularly during neutrophil transendothelial migration (TEM) and during changes in permeability. However, MERTK’s function in endothelial cells remains unclear. This study evaluated the contribution of endothelial MERTK to neutrophil TEM and endothelial barrier function. In vitro experiments using primary human pulmonary microvascular endothelial cells found that neutrophil TEM across the endothelial monolayers was enhanced when MERTK expression in endothelial cells was reduced by siRNA knockdown. Examination of endothelial barrier function revealed increased passage of dextran across the MERTK-depleted monolayers, suggesting that MERTK helps maintain endothelial barrier function. MERTK knockdown also altered adherens junction structure, decreased junction protein levels, and reduced basal Rac1 activity in endothelial cells, providing potential mechanisms of how MERTK regulates endothelial barrier function. To study MERTK’s function in vivo, inflammation in the lungs of global Mertk^-/- mice was examined during acute pneumonia. In response to P. aeruginosa, more neutrophils were recruited to the lungs of Mertk^-/- than wildtype mice. Vascular leakage of Evans blue dye into the lung tissue was also greater in Mertk^-/- mice. To analyze endothelial MERTK’s involvement in these processes, we generated inducible endothelial cell-specific (iEC) Mertk^-/- mice. When similarly challenged with P. aeruginosa, iEC Mertk^-/- mice demonstrated no difference in neutrophil TEM into the inflamed lungs or in vascular permeability compared to control mice. These results suggest that deletion of MERTK in human pulmonary microvascular endothelial cells in vitro and in all cells in vivo aggravates the inflammatory response. However, selective MERTK deletion in endothelial cells in vivo failed to replicate this response.

Myeloid SOCS3 Deficiency Regulates Angiogenesis via Enhanced Apoptotic Endothelial Cell Engulfment

Mononuclear phagocytes, such as macrophages and microglia, are key regulators of organ homeostasis including vascularization processes. Here, we investigated the role of the suppressor of cytokine signaling 3 (SOCS3) in myeloid cells as a regulator of mononuclear phagocyte function and their interaction with endothelial cells in the context of sprouting angiogenesis. As compared to SOCS3-sufficient counterparts, SOCS3-deficient microglia and macrophages displayed an increased phagocytic activity toward primary apoptotic endothelial cells, which was associated with an enhanced expression of the opsonin growth arrest-specific 6 (Gas6), a major prophagocytic molecule. Furthermore, we found that myeloid SOCS3 deficiency significantly reduced angiogenesis in an ex vivo mouse aortic ring assay, which could be reversed by the inhibition of the Gas6 receptor Mer. Together, SOCS3 in myeloid cells regulates the Gas6/Mer-dependent phagocytosis of endothelial cells, and thereby angiogenesis-related processes. Our findings provide novel insights into the complex crosstalk between mononuclear phagocytes and endothelial cells, and may therefore provide a new platform for the development of new antiangiogenic therapies.

Gas6/TAM Receptors in Systemic Lupus Erythematosus

Systemic lupus erythematosus (SLE) is a multiorgan autoimmune disease associated with impaired immune system regulation. The exact mechanisms of SLE development remain to be elucidated. TAM receptor tyrosine kinases (RTKs) are important for apoptotic cell clearance, immune homeostasis, and resolution of immune responses. TAM deficiency leads to lupus-like autoimmune diseases. Activation of TAM receptors leads to proteolytic cleavage of the receptors, generating soluble forms of TAM. Circulating TAM receptors have an immunoregulatory function and may also serve as biomarkers for disease prognosis. Here, we review the biological function and signaling of TAM RTKs in the development and pathogenesis of lupus and lupus nephritis. Targeting Gas6/TAM pathways may be of therapeutic benefit. A discussion of potential TAM activation and inhibition in the treatment of lupus and lupus nephritis is included.