Characterization of an interferon-stimulated response element (ISRE) in the Il23a promoter

We have demonstrated previously that IFN-γ plays a protective role in the initiation of chronic intestinal inflammation through attenuation of Toll-like receptor-mediated IL-23 induction in macrophages. Here, an interferon-stimulated response element (ISRE) is identified in a region of conserved nucleotide sequences in the Il23a promoter. This ISRE mediated, in part, Il23a promoter induction by LPS and inhibition of LPS-induced activity by IFN-γ. LPS and IFN-γ recruit interferon regulatory factors (IRFs) to the Il23a ISRE in murine bone marrow-derived macrophages (BMMs). Functionally, IRF-1 is a negative regulator of Il23a in LPS-stimulated BMMs. IRF-1(-/-) BMMs demonstrated enhanced LPS-induced Il23a expression compared with WT BMMs. Moreover, IRF-1 deficiency resulted in prolonged occupancy of RelA on the Il23a promoter. Consequently, IRF-1(-/-) mice were more susceptible to colonic injury by trinitrobenzenesulfonic acid, and IL-10/IRF-1 double-deficient (IL-10/IRF-1(-/-)) mice demonstrated more severe colonic inflammation compared with IL-10(-/-) mice. The severity of colitis in both models correlated with increased colonic IL-23. CD11b(+) lamina propria mononuclear cells, comprising predominantly macrophages, were identified as the major source of IL-23 in colitis-prone mice. Basal and heat-killed Escherichia coli-stimulated levels of Il23a were increased in IL-10/IRF-1(-/-) compared with WT and IL-10(-/-) colonic CD11b(+) lamina propria mononuclear cells. In conclusion, these experiments characterize IRF-ISRE interactions on the Il23a promoter, which have in vivo relevance as a homeostatic checkpoint in chronic intestinal inflammation.     

Activation of caspase-9, but not caspase-2 or caspase-8, is essential for heat-induced apoptosis in Jurkat cells

Exposure of cells to hyperthermia is known to induce apoptosis, although the underlying mechanisms are only partially understood. Here, we examine the molecular requirements necessary for heat-induced apoptosis using genetically modified Jurkat T-lymphocytes. Cells stably overexpressing Bcl-2/Bcl-x(L) or stably depleted of Apaf-1 were completely resistant to heat-induced apoptosis, implicating the involvement of the mitochondria-mediated pathway. Pretreatment of wild-type cells with the cell-permeable biotinylated general caspase inhibitor b-VAD-fmk (biotin-Val-Ala-Asp(OMe)-CH(2)F) both inhibited heat-induced apoptosis and affinity-labeled activated initiator caspase-2, -8, and -9. Despite this finding, however, cells engineered to be deficient in caspase-8, caspase-2, or the caspase-2 adaptor protein RAIDD (receptor-interacting protein (RIP)-associated Ich-1/CED homologous protein with death domain) remained susceptible to heat-induced apoptosis. Additionally, b-VAD-fmk failed to label any activated initiator caspase in Apaf-1-deficient cells exposed to hyperthermia. Cells lacking Apaf-1 or the pro-apoptotic BH3-only protein Bid exhibited lower levels of heat-induced Bak activation, cytochrome c release, and loss of mitochondrial membrane potential, although cleavage of Bid to truncated Bid (tBid) occurred downstream of caspase-9 activation. Combined, the data suggest that caspase-9 is the critical initiator caspase activated during heat-induced apoptosis and that tBid may function to promote cytochrome c release during this process as part of a feed-forward amplification loop.     

Diabetic liver injury from streptozotocin is regulated through the caspase-8 homolog cFLIP involving activation of JNK2 and intrahepatic immunocompetent cells

The endemic occurrence of obesity and the associated risk factors that constitute the metabolic syndrome have been predicted to lead to a dramatic increase in chronic liver disease. Non-alcoholic steatohepatitis (NASH) has become the most frequent liver disease in countries with a high prevalence of obesity. In addition, hepatic steatosis and insulin resistance have been implicated in disease progression of other liver diseases, including chronic viral hepatitis and hepatocellular carcinoma. The molecular mechanisms underlying the link between insulin signaling and hepatocellular injury are only partly understood. We have explored the role of the antiapoptotic caspase-8 homolog cellular FLICE-inhibitory protein (cFLIP) on liver cell survival in a diabetic model with hypoinsulinemic diabetes in order to delineate the role of insulin signaling on hepatocellular survival. cFLIP regulates cellular injury from apoptosis signaling pathways, and loss of cFLIP was previously shown to promote injury from activated TNF and CD95/Apo-1 receptors. In mice lacking cFLIP in hepatocytes (flip^−/−), loss of insulin following streptozotocin treatment resulted in caspase- and c-Jun N-terminal kinase (JNK)-dependent liver injury after 21 days. Substitution of insulin, inhibition of JNK using the SP600125 compound in vivo or genetic deletion of the mitogen-activated protein kinase (MAPK)9 (JNK2) in all tissues abolished the injurious effect. Strikingly, the difference in injury between wild-type and cFLIP-deficient mice occurred only in vivo and was accompanied by liver-infiltrating inflammatory cells with a trend toward increased amounts of NK1.1-positive cells and secretion of proinflammatory cytokines. Transfer of bone marrow from rag-1-deficient mice that are depleted from B and T lymphocytes prevented liver injury in flip^−/− mice. These findings support a direct role of insulin on cellular survival by alternating the activation of injurious MAPK, caspases and the recruitment of inflammatory cells to the liver. Thus, increasing resistance to insulin signaling pathways in hepatocytes appears to be an important factor in the initiation and progression of chronic liver disease.     

Human beta-interferon gene expression is regulated by an inducible enhancer element

We have localized the regulatory sequence required for viral or poly(I)-poly(C) activation of human beta-interferon gene expression to a region located between -37 and -77 from the mRNA cap site. This sequence has the characteristics of an inducible enhancer element: it can act upstream or downstream of the beta-interferon gene regardless of its orientation, and at distances up to approximately 1 kilobase from its normal location. Moreover, this element can confer inducibility on a heterologous promoter. Further analysis has identified a minimal regulatory element of 14 base pairs within this enhancer. Sequences closely related to this element are present five times within the 5'-flanking regions of both the alpha- and beta-interferon genes. The number of these minimal regulatory elements required for maximal beta-interferon gene expression appears to differ in different cell lines.     

Head involution defective (Hid)-triggered apoptosis requires caspase-8 but not FADD (Fas-associated death domain) and is regulated by Erk in mammalian cells

The molecular machinery of apoptosis is evolutionarily conserved with some exceptions. One such example is the Drosophila proapoptotic gene Head involution defective (Hid), whose mammalian homologue is not known. Hid is apoptotic to mammalian cells, and we have examined the mechanism by which Hid induces death. We demonstrate for the first time a role for the extracellular signal-related kinase-1/2 (Erk-1/2) in the regulation of Hid function in mammalian cells. Bcl-2 and an inhibitor of caspase-9 blocked apoptosis, indicative of a role for the mitochondrion in this pathway, and we provide evidence for a role for caspase-8 in Hid-induced apoptosis. Thus, apoptosis was blocked by an inhibitor of caspase-8, deletion of caspase-8 rendered cells resistant to Hid-induced apoptosis, and Hid associated with caspase-8 in cell lysates. The Fas-associated death domain (FADD) was dispensable for the apoptotic function of Hid, indicating that Hid does not require extracellular death receptor signaling for the activation of caspase-8. In activated T cells, the cytokine interleukin-2 blocked caspase-8 processing and apoptosis, suggesting that survival cues from trophic factors may target a Hid-like intermediate present in mammalian cells. Thus, this study shows that Hid engages with conserved components of cellular death machinery and suggests that apoptotic paradigms characterized by FADD-independent activation of caspase-8 may involve a Hid-like molecule in mammalian cells.     

tk Enzyme expression in differentiating muscle cells is regulated through an internal segment of the cellular tk gene.

Thymidine kinase (tk) enzyme expression is shut down when cultured skeletal muscle cells terminally differentiate. This regulation is mediated by a rapid and specific decline in the abundance of cellular tk mRNA. tk-deficient mouse myoblasts were transformed to the tk-positive phenotype by using both the cellular tk gene of the chicken and the herpesvirus tk gene. Myoblasts transformed with the cellular tk gene effectively regulate tk enzyme activity upon terminal differentiation. Conversely, myoblasts transformed with the herpesvirus tk gene continue to express tk enzyme activity in postreplicative muscle cells. A regulated pattern of expression is retained when the promoter of the cellular tk gene is replaced by the promoter of the herpesvirus tk gene. Moreover, the cellular tk gene is appropriately regulated during terminal muscle differentiation when its 3' terminus is removed and replaced by the terminus of the viral tk gene. Thus, the element of the cellular tk gene sufficient to specify its regulation is entirely intragenic.