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Sphingosine kinase (Sphk) enzymes are important in intracellular sphingolipid metabolism as well as in the biosynthesis of sphingosine 1-phosphate (S1P), an extracellular lipid mediator. Here, we show that Sphk1 is expressed and is required for small intestinal tumor cell proliferation in Apc Min/؉ mice. Adenoma size but not incidence was dramatically reduced in Apc Min/؉ Sphk ؊/؊ mice. Concomitantly, epithelial cell proliferation in the polyps was significantly attenuated, suggesting that Sphk1 regulates adenoma progression. Although the S1P receptors (S1P1R, S1P2R, and S1P3R) are expressed, polyp incidence or size was unaltered in Apc Min/؉ S1p2r ؊/؊ , Apc Min/؉ S1p3r ؊/؊ , and Apc Min/؉ S1p1r ؉/؊ bigenic mice. These data suggest that extracellular S1P signaling via its receptors is not involved in adenoma cell proliferation. Interestingly, tissue sphingosine content was elevated in the adenomas of Apc Min/؉ Sphk1 ؊/؊ mice, whereas S1P levels were not significantly altered. Concomitantly, epithelial cell proliferation and the expression of the G 1 /S cell cycle regulator CDK4 and c-myc were diminished in the polyps of Apc Min/؉ Sphk1 ؊/؊ mice. In rat intestinal epithelial (RIE) cells in vitro, Sphk1 overexpression enhanced cell cycle traverse at the G 1 /S boundary. In addition, RIE cells treated with sphingosine but not C6-ceramide exhibited reduced cell proliferation, reduced retinoblastoma protein phosphorylation, and cyclin-dependent kinase 4 (Cdk4) expression. Our findings suggest that Sphk1 plays a critical role in intestinal tumor cell proliferation and that inhibitors of Sphk1 may be useful in the control of intestinal cancer.The concept that sphingolipid metabolism is an important source of signaling lipids has gained considerable acceptance. For example, sphingomyelin, an abundant membrane phospholipid, is metabolized into lipid mediators such as sphingosine, ceramide, ceramide 1-phosphate, and sphingosine 1-phosphate (S1P) (9). S1P, synthesized by the action of sphingosine kinase (Sphk), is secreted by cells and functions as an extracellular mediator by activating a family of G-protein-coupled receptors termed S1PR1-5 (13, 19, 30). The generality of S1P signaling in vertebrates is underscored by recent findings that S1PRs are needed for a multitude of physiological processes, including heart and vascular development, angiogenesis, and immune cell trafficking (12). However, the physiological and pathological significance of intracellular sphingolipid metabolism in vertebrates is virtually unknown. However, a recent study showed that the inhibition of S1P lyase resulted in alterations in tissue S1P levels, which influenced T-cell trafficking and immune responses (35). The importance of sphingolipid metabolism is underscored by its impact on cell death, stress responses, metabolism, and animal development in unicellular (Saccharomyces cerevisiae and Dictyostelium discoidium) and multicellular (Caenorhabditis elegans and Drosophila melanogaster) eukaryotes (4,5,10,21,26,28).Mammals express two functiona...

Myeloid-derived suppressor cells (MDSCs) are of myeloid origin and are able to suppress T cell responses. The role of MDSCs in autoimmune diseases remains controversial, and little is known about the function of MDSCs in autoimmune arthritis. In this study, we clarify that MDSCs play crucial roles in the regulation of proinflammatory immune response in a collagen-induced arthritis (CIA) mouse model. MDSCs accumulated in the spleens of mice with CIA when arthritis severity peaked. These MDSCs inhibited the proliferation of CD4+ T cells and their differentiation into Th17 cells in vitro. Moreover, MDSCs inhibited the production of IFN-γ, IL-2, TNF-α, and IL-6 by CD4+ T cells in vitro, whereas they promoted the production of IL-10. Adoptive transfer of MDSCs reduced the severity of CIA in vivo, which was accompanied by a decrease in the number of CD4+ T cells and Th17 cells in the draining lymph nodes. However, depletion of MDSCs abrogated the spontaneous improvement of CIA. In conclusion, MDSCs in CIA suppress the progression of CIA by inhibiting the proinflammatory immune response of CD4+ T cells. These observations suggest that MDSCs play crucial roles in the regulation of autoimmune arthritis, which could be exploited in new cell-based therapies for human rheumatoid arthritis.

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Rheumatoid arthritis (RA) is characterized by massive synovial proliferation, angiogenesis, subintimal infiltration of inflammatory cells and the production of cytokines such as TNF-α and IL-6. Allograft inflammatory factor-1 (AIF-1) has been identified in chronic rejection of rat cardiac allografts as well as tissue inflammation in various autoimmune diseases. AIF-1 is thought to play an important role in chronic immune inflammatory processes, especially those involving macrophages. In the current work, we examined the expression of AIF-1 in synovial tissues and measured AIF-1 in synovial fluid (SF) derived from patients with either RA or osteoarthritis (OA). We also examined the proliferation of synovial cells and induction of IL-6 following AIF-1 stimulation. Immunohistochemical staining showed that AIF-1 was strongly expressed in infiltrating mononuclear cells and synovial fibroblasts in RA compared with OA. Western blot analysis and semiquantitative RT-PCR analysis demonstrated that synovial expression of AIF-1 in RA was significantly greater than the expression in OA. AIF-1 induced the proliferation of cultured synovial cells in a dose-dependent manner and increased the IL-6 production of synovial fibroblasts and PBMC. The levels of AIF-1 protein were higher in synovial fluid from patients with RA compared with patients with OA (p < 0.05). Furthermore, the concentration of AIF-1 significantly correlated with the IL-6 concentration (r = 0.618, p < 0.01). These findings suggest that AIF-1 is closely associated with the pathogenesis of RA and is a novel member of the cytokine network involved in the immunological processes underlying RA.

Sphingosine-1-phosophate, generated from the phosphorylation of sphingosine by sphingosine kinase enzymes, is suggested to function as an intracellular second messenger for inflammatory mediators, including formyl peptide, C5a, and Fc. More recently, a role for sphingosine kinases during inflammation has also been proposed. Here we show that sphingosine kinase 1 knockout mice exhibit normal inflammatory cell recruitment during thioglycollate-induced peritonitis and that sphingosine kinase 1-null neutrophils respond normally to formyl peptide. In the collagen-induced arthritis model of rheumatoid arthritis, sphingosine kinase 1 knockout mice developed arthritis with normal incidence and severity. Our findings show that sphingosine kinase 1 is dispensable for inflammatory responses and support the need for more extensive studies of sphingosine kinases in inflammation.