TLR-induced innate immunity and inflammation are mediated by signaling cascades leading to activation of the MAPK family of Ser/Thr protein kinases, including p38 MAPK, which controls cytokine release during innate and adoptive immune responses. Failure to terminate such inflammatory reactions may lead to detrimental systemic effects, including septic shock and autoimmunity. In this study, we provide genetic evidence of a critical and nonredundant role of MAPK phosphatase (MKP)-1 in the negative control of MAPK-regulated inflammatory reactions in vivo. MKP-1−/− mice are hyperresponsive to low-dose LPS-induced toxicity and exhibit significantly increased serum TNF-α, IL-6, IL-12, MCP-1, IFN-γ, and IL-10 levels after systemic administration of LPS. Furthermore, absence of MKP-1 increases systemic levels of proinflammatory cytokines and exacerbates disease development in a mouse model of rheumatoid arthritis. When activated through TLR2, TLR3, TLR4, TLR5, and TLR9, bone marrow-derived MKP-1−/− macrophages exhibit increased cytokine production and elevated expression of the differentiation markers B7.2 (CD86) and CD40. MKP-1-deficient macrophages also show enhanced constitutive and TLR-induced activation of p38 MAPK. Based on these findings, we propose that MKP-1 is an essential component of the intracellular homeostasis that controls the threshold and magnitude of p38 MAPK activation in macrophages, and inflammatory conditions accentuate the significance of this regulatory function.

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is expressed in different tissues and cells, including pancreas and lymphocytes, and can induce apoptosis in various tumor cells but not in most normal cells. The specific roles of TRAIL in health and disease remain unclear. Here we show by cDNA array analyses that TRAIL gene expression is upregulated in pancreatic islets during the development of autoimmune type 1 diabetes in nonobese diabetic (NOD) mice and in Min6 islet beta-cells activated by TNF-alpha + interferon-gamma. However, stimulation of freshly isolated pancreatic islets or Min6 cells with TRAIL did not induce their apoptosis. TRAIL blockade exacerbates the onset of type 1 diabetes in NOD.Scid recipients of transferred diabetogenic T-cells and in cyclophosphamide-treated NOD mice. TRAIL inhibits the proliferation of NOD diabetogenic T-cells by suppressing interleukin (IL)-2 production and cell cycle progression, and this inhibition can be rescued in the presence of exogenous IL-2. cDNA array and Western blot analyses indicate that TRAIL upregulates the expression of the cdk inhibitor p27(kip1). Our data suggest that TRAIL is an important immune regulator of the development of type 1 diabetes.

IntroductionFolate is an essential cosubstrate of many biochemical reactions, such as the de novo synthesis of purines and pyrimidines, methionine, and deoxythymidylate monophosphate. 1,2 Mammals cannot synthesize folate; therefore, inadequate dietary supply or folate malabsorption results in defective DNA synthesis. One of the first manifestations of a folate deficiency is in the rapidly proliferating cells of the hematopoietic system, leading to pancytopenia and anemia of the megaloblastic type. Patients with megaloblastic anemia demonstrate ineffective erythropoiesis by harboring large immature red blood cell (RBC) precursors that fail to survive to the postmitotic, terminal stages, undergoing premature apoptosis. 2 Three mammalian folate transporter systems have been described to date in a variety of tissues: (1) the bidirectional reduced folate carrier 1 (RFC1), also known as SLC19A1, 3,4 (2) the glycosyl-phosphatidylinositol-anchored folate receptors (FOLR1, FOLR2, and FOLR4) and one secreted receptor in humans without a mouse homolog (FOLR3), 5 and (3) the human proton coupled folate transporter (PCFT). 6-8 The RFC1 transporter is expressed ubiquitously, including the brush-border membrane of epithelial cells in the small intestine. 9 Although RFC1 is necessary for folate transport in erythroid cells, its involvement in intestinal folate uptake has not been confirmed. 6 Inactivation of RFC1 in mice by homologous recombination led to either embryonic lethality or defective erythropoiesis in pups born to mothers who were supplemented with 1 mg daily subcutaneous doses of folic acid. 10,11 Because this transporter functions at a neutral pH optimum whereas the majority of intestinal folate transport occurs in an acidic luminal milieu, RFC1 is an unlikely candidate for an intestinal folate transporter. 7 The role of FOLR1 in folate absorption and transport has also been demonstrated, where a 60% to 70% reduction was observed in plasma folate of FOLR1 Ϫ/Ϫ mice fed low folate and normal chow. 12 FOLR1 also regulates folate homeostasis via endocytotic mechanisms during embryonic development, and mice rendered null for this receptor display severe morphogenetic abnormalities and die in utero unless provided supraphysiologic concentrations of either folinic acid or 5-methyltetrahydrofolate. 5,13 The PCFT transporter is highly expressed in tissues involved in folate and heme transport, including the duodenum and liver. 6,14 Initially, PCFT was identified as a low-affinity, pHindependent heme transporter 14 and then later described to function as a low pH-dependent folate transporter in intestinal cells. 6 The latter role of the transporter was confirmed by the identification of loss-of-function mutations in the human PCFT gene in persons diagnosed with hereditary folate malabsorption syndrome. 6 Studies have also indicated that PCFT facilitates folate transport during folate receptor-mediated endocytosis, where FOLR1 binds folate, and its export into the cytosol is driven by PCFT activity as the vesicle undergoes en...

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IGF-I regulates islet beta-cell growth, survival, and metabolism and protects against type 1 diabetes (T1D). However, the therapeutic efficacy of free IGF-I may be limited by its biological half-life in vivo. We investigated whether prolongation of its half-life as an IGF-I/IGF binding protein (IGFBP)-3 complex affords increased protection against T1D and whether this occurs by influencing T cell function and/or islet beta-cell growth and survival. Administration of IGF-I either alone or as an IGF-I/IGFBP-3 complex reduced the severity of insulitis and delayed the onset of T1D in nonobese diabetic mice, but IGF-I/IGFBP-3 was significantly more effective. Protection from T1D elicited by IGF-I/IGFBP-3 was mediated by up-regulated CCL4 and down-regulated CCL3 gene expression in pancreatic draining lymph nodes, activation of the phosphatidylinositol 3-kinase and Akt/protein kinase B signaling pathway of beta-cells, reduced beta-cell apoptosis, and stimulation of beta-cell replication. Reduced beta-cell apoptosis resulted from elevated Bcl-2 and Bcl-X(L) activity and diminished caspase-9 activity, indicating a novel role for a mitochondrial-dependent pathway of beta-cell death. Thus, IGF-I/IGFBP-3 affords more efficient protection from insulitis, beta-cell destruction, and T1D than IGF-I, and this complex may represent an efficacious therapeutic treatment for the prevention of T1D.