Vascular adhesion protein-1 defines a unique subpopulation of human hematopoietic stem cells and regulates their proliferation.

Although the development of hematopoietic stem cells (HSC) has been studied in great detail, their heterogeneity and relationships to different cell lineages remain incompletely understood. Moreover, the role of Vascular Adhesion Protein-1 in bone marrow hematopoiesis has remained unknown. Here we show that VAP-1, an adhesin and a primary amine oxidase producing hydrogen peroxide, is expressed on a subset of human HSC and bone marrow vasculature forming a hematogenic niche. Bulk and single-cell RNAseq analyses reveal that VAP-1+ HSC represent a transcriptionally unique small subset of differentiated and proliferating HSC, while VAP-1- HSC are the most primitive HSC. VAP-1 generated hydrogen peroxide acts via the p53 signaling pathway to regulate HSC proliferation. HSC expansion and differentiation into colony-forming units are enhanced by inhibition of VAP-1. Contribution of VAP-1 to HSC proliferation was confirmed with mice deficient of VAP-1, mice expressing mutated VAP-1 and using an enzyme inhibitor. In conclusion, VAP-1 expression allows the characterization and prospective isolation of a new subset of human HSC. Since VAP-1 serves as a check point-like inhibitor in HSC differentiation, the use of VAP-1 inhibitors enables the expansion of HSC.

Robo4 contributes to the turnover of Peyer's patch B cells.

All leukocytes can get entrance into the draining lymph nodes via the afferent lymphatics but only lymphoid cells can leave the nodes. The molecular mechanisms behind this phenomenon have remained unknown. We employed genome wide microarray analyses of the subcapsular sinus and lymphatic sinus (LS) endothelial cells and found Robo4 to be selectively expressed on LS lymphatics. Further analyses showed high Robo4 expression in lymphatic vessels of Peyer's patches, which only have efferent lymphatic vessels. In functional assays, Robo4-deficient animals showed accumulation of naïve  B cells (CD19+/CD62Lhi/CD44lo) in Peyer's patches, whereas no difference was seen within other lymphocyte subtypes. Short-term lymphocyte homing via high endothelial venules to peripheral and mesenteric lymph nodes and Peyer's patches was also slightly impaired in Robo4 knockout animals. These results show for the first time, selective expression of Robo4 in the efferent arm of the lymphatics and its role in controlling the turnover of a subset of B lymphocytes from Peyer's patches.

Gene-expression profiling of different arms of lymphatic vasculature identifies candidates for manipulation of cell traffic.

Afferent lymphatic vessels bring antigens and diverse populations of leukocytes to draining lymph nodes, whereas efferent lymphatics allow only lymphocytes and antigens to leave the nodes. Despite the fundamental importance of afferent vs. efferent lymphatics in immune response and cancer spread, the molecular characteristics of these different arms of the lymphatic vasculature are largely unknown. The objective of this work was to explore molecular differences behind the distinct functions of afferent and efferent lymphatic vessels, and find possible molecules mediating lymphocyte traffic. We used laser-capture microdissection and cell sorting to isolate lymphatic endothelial cells (LECs) from the subcapsular sinus (SS, afferent) and lymphatic sinus (LS, efferent) for transcriptional analyses. The results reveal marked differences between afferent and efferent LECs and identify molecules on lymphatic vessels. Further characterizations of Siglec-1 (CD169) and macrophage scavenger receptor 1 (MSR1/CD204), show that they are discriminatively expressed on lymphatic endothelium of the SS but not on lymphatic vasculature of the LS. In contrast, endomucin (EMCN) is present on the LS endothelium and not on lymphatic endothelium of the SS. Moreover, both murine and human MSR1 on lymphatic endothelium of the SS bind lymphocytes and in in vivo studies MSR1 regulates entrance of lymphocytes from the SS to the lymph node parenchyma. In conclusion, this paper reports surprisingly distinct molecular profiles for afferent and efferent lymphatics and a function for MSR1. These results may open avenues to explore some of the now-identified molecules as targets to manipulate the function of lymphatic vessels.

The endothelial protein PLVAP in lymphatics controls the entry of lymphocytes and antigens into lymph nodes.

In the lymphatic sinuses of draining lymph nodes, soluble lymph-borne antigens enter the reticular conduits in a size-selective manner and lymphocytes transmigrate to the parenchyma. The molecular mechanisms that control these processes are unknown. Here we unexpectedly found that PLVAP, a prototypic endothelial protein of blood vessels, was synthesized in the sinus-lining lymphatic endothelial cells covering the distal conduits. In PLVAP-deficient mice, both small antigens and large antigens entered the conduit system, and the transmigration of lymphocytes through the sinus floor was augmented. Mechanistically, the filtering function of the lymphatic sinus endothelium was dependent on diaphragms formed by PLVAP fibrils in transendothelial channels. Thus, in the lymphatic sinus, PLVAP forms a physical sieve that regulates the parenchymal entry of lymphocytes and soluble antigens.

Flavopiridol inhibits lipopolysaccharide-induced TNF-α production through inactivation of nuclear factor-κB and mitogen-activated protein kinases in the MyD88-dependent pathway.

Flavopiridol is a cyclin-dependent kinase inhibitor and inhibits the growth of various cancer cells. The effect of flavopiridol on lipopolysaccharide (LPS)-induced proinflammatory mediator production was examined in RAW 264.7 macrophage-like cells. Flavopiridol significantly reduced the production of tumor necrosis factor-α and, to a lesser extent, nitric oxide in LPS-stimulated cells. Flavopiridol inhibited the activation of nuclear factor-κB and IκB kinase in response to LPS. Flavopiridol also inhibited the activation of a series of mitogen-activated protein kinases, such as p38, stress-activated protein kinase/c-Jun N-terminal kinase and extracellular signal-regulated kinase 1/2 in response to LPS. However, flavopiridol did not alter the expression of tumor necrosis factor receptor-associated factor 6, myeloid differentiation factor 88 (MyD88) or CD14/toll-like receptor (TLR) 4. Flavopiridol inhibited nitric oxide production induced by a MyD88-dependent TLR2 ligand, but not a MyD88-independent TLR3 ligand. Further, flavopiridol did not alter the phosphorylation of interferon regulatory factor 3 in the MyD88-independent pathway. Therefore, it was suggested that flavopiridol exclusively inhibited the activation of nuclear factor-κB and mitogen-activated protein kinases in the MyD88-dependent pathway. Flavopiridol might be useful for the prevention of LPS-induced inflammatory response.

ONO 3403, a synthetic serine protease inhibitor, inhibits lipopolysaccharide-induced tumor necrosis factor-{alpha} and nitric oxide production and protects mice from lethal endotoxic shock.

ONO 3403, a new synthetic serine protease inhibitor, is a derivative of camostat mesilate and has a higher protease-inhibitory activity. The effect of ONO 3403 on lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF)-α and nitric oxide (NO) production in RAW 264.7 macrophage-like cells was examined. ONO 3403 significantly inhibited LPS-induced TNF-α production at a lower concentration than camostat mesilate. It also inhibited LPS-induced NO production. Their inhibition was responsible for the reduced mRNA expression of TNF-α and inducible NO synthase. In LPS-stimulated cells, ONO 3403 prevented the augmentation of MyD88 expression and inhibited the phosphorylation of IκB-α, stress-activated protein kinase (SAPK) and IRF-3, and the production of interferon-ß. ONO 3403 abolished the elevation of the extracellular serine protease activity in response to LPS. Further, it reduced the circulating TNF-α level, hepatic injury and mortality in mice receiving an injection of D-galactosamine and LPS. ONO 3403 was suggested to inhibit LPS-induced inflammatory responses via inactivation of MyD88-dependent and independent pathways.

Retinoblastoma protein-interacting zinc finger 1, a tumor suppressor, augments lipopolysaccharide-induced proinflammatory cytokine production via enhancing nuclear factor-kappaB activation.

The involvement of retinoblastoma protein-interacting zinc finger 1 (RIZ1), a tumor suppressor, in lipopolysaccharide (LPS)-induced inflammatory responses was investigated by using RAW 264.7 macrophage-like cells. LPS significantly augmented the expression of RIZ1 and the augmentation was mediated by the activation of nuclear factor (NF)-kappaB and Akt. The silencing of RIZ1 with the siRNA led to the inactivation of NF-kappaB in response to LPS. Moreover, the RIZ1 silencing caused the down-regulation of p53 activation and a p53 pharmacological inhibitor attenuated the RIZ1 expression. LPS-induced tumor necrosis factor-alpha and interleukin-6 production was prevented by RIZ1 siRNA or a p53 pharmacological inhibitor. Therefore, RIZ1 was suggested to augment LPS-induced NF-kappaB activation in collaboration with p53 and enhance the production of proinflammatory cytokines in response to LPS.

Retinoblastoma protein-interacting zinc finger 1 (RIZ1) participates in RANKL-induced osteoclast formation via regulation of NFATc1 expression.

The role of retinoblastoma protein-interacting zinc finger 1 (RIZ1) in receptor activator of NF-kappaB ligand (RANKL)-induced osteoclast formation was examined in mouse RAW 264.7 macrophage-like cells. The expression of RIZ1 was significantly augmented by RANKL-treated cells. Silencing of RIZ1 with the siRNA significantly reduced the appearance of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells as osteoclasts in RANKL-treated cells. The expression of nuclear factor of activated T cell 1 (NFATc1) as the terminal transcription factor of osteoclast formation was prevented by RIZ1 siRNA. It was suggested that that RIZ1 might participate in RANKL-induced osteoclast formation through the regulation of NFATc1 expression.

Tumor necrosis factor-a augments lipopolysaccharide-induced suppressor of cytokine signalling 3 (SOCS-3) protein expression by preventing the degradation.

The regulatory role of tumour necrosis factor-a (TNF-a) on the expression of suppressor of cytokine signalling 3 (SOCS-3) in response to lipopolysaccharide (LPS) was examined using peritoneal macrophages from TNF-a-deficient mice. The LPS-induced SOCS-3 expression was markedly augmented in macrophages from wild-type mice whereas such augmentation was not seen in the cells from TNF-a-deficient mice. However, there was no significant difference in the level of SOCS-3 messenger RNA expression between macrophages from wild-type mice and those from TNF-a-deficient mice. The addition of exogenous TNF-a augmented the LPS-induced SOCS-3 expression in macrophages from TNF-a-deficient mice. The pulse chase analysis suggested augmented degradation of LPS-induced SOCS-3 protein in macrophages from TNF-a-deficient mice. Moreover, MG 132, a 26S proteasome inhibitor, sustained the LPS-induced SOCS-3 expression in those cells. The tyrosine phosphorylation of SOCS-3 was definitely induced in LPS-stimulated macrophages from TNF-a-deficient mice but not wild-type mice. A tyrosine phosphatase inhibitor enhanced the tyrosine phosphorylation of SOCS-3 in wild-type mice and accelerated the degradation. Therefore, it was suggested that TNF-a prevented the degradation of SOCS-3 protein via inhibition of the tyrosine phosphorylation in LPS-stimulated macrophages.

The inhibition of lipopolysaccharide-induced tumor necrosis factor-alpha and nitric oxide production by Clostridium perfringens alpha-toxin and its relation to alpha-toxin-induced intracellular ceramide generation.

The effect of Clostridium perfringens alpha-toxin on production of tumor necrosis factor (TNF)-alpha and nitric oxide (NO) in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells was studied. The pretreatment of wild type alpha-toxin, but not the inactive mutant, significantly decreased LPS-induced TNF-alpha and NO production. alpha-Toxin inhibited the expression of TNF-alpha and an inducible type of NO synthase protein and mRNA. Furthermore, it inhibited the phosphorylation of IkappaB-alpha and p65 NF-kappaB subunit, and the NF-kappaB luciferase reporter gene activity in LPS-stimulated cells. The pretreatment of alpha-toxin increased the level of intracellular ceramide. Taken together, Clostridium perfringens alpha-toxin pretreatment was suggested to inhibit LPS-induced TNF-alpha and NO production through the inhibition of NF-kappaB activation. The relationship between alpha-toxin-induced intracellular ceramide generation and the NF-kappaB inhibition is discussed.