Regulation of the cell death program involves physical interactions between different members of the Bcl‐2 family that either promote or suppress apoptosis. The Bcl‐2 homolog, Bak, promotes apoptosis and binds anti‐apoptotic family members including Bcl‐2 and Bcl‐xL. We have identified a domain in Bak that is both necessary and sufficient for cytotoxic activity and binding to Bcl‐xL. Sequences similar to this domain were identified in Bax and Bip1, two other proteins that promote apoptosis and interact with Bcl‐xL, and were likewise critical for their capacity to kill cells and bind Bcl‐xL. Thus, the domain is of central importance in mediating the function of multiple cell death‐regulatory proteins that interact with Bcl‐2 family members.

BH3-only proteins function at a proximal point in a conserved cell death pathway by binding, through their BH3 domains, to other Bcl-2 family members and triggering mitochondrial events associated with apoptosis. Here, we describe a strongly pro-apoptotic BH3-only protein, designated Bbc3, whose expression increases in response to diverse apoptotic stimuli. bbc3 mRNA levels were induced by exposure to DNA-damaging agents and by wild-type p53, which mediates DNA damage-induced apoptosis. p53 transactivated bbc3 through consensus p53 binding sites within the bbc3 promoter region, indicating that bbc3 is a direct target of p53. Additionally, bbc3 mRNA was induced by p53-independent apoptotic stimuli, including dexamethasone treatment of thymocytes, and serum deprivation of tumor cells. Insulin-like growth factor-1 and epidermal growth factor, growth factors with broad anti-apoptotic activity, were each sufficient to suppress Bbc3 expression in serum-starved tumor cells. These results suggest that the transcriptional regulation of bbc3 contributes to the transduction of diverse cell death and survival signals.

Cells are eliminated in a variety of physiological settings by apoptosis, a genetically encoded process of cellular suicide. Apoptosis comprises an intrinsic cellular defence against tumorigenesis, which, when suppressed, may contribute to the development of malignancies. The bcl-2 oncogene, which is activated in follicular lymphomas, functions as a potent suppressor of apoptosis under diverse conditions. Here we describe the complementary DNA cloning and functional analysis of a new Bcl-2 homologue, Bak, which promotes cell death and counteracts the protection from apoptosis provided by Bcl-2. Moreover, enforced expression of Bak induces rapid and extensive apoptosis of serum-deprived fibroblasts. This raises the possibility that Bak is directly involved in activating the cell death machinery.

DNA damage and/or hyperproliferative signals activate the wild-type p53 tumor suppressor protein, which induces a G 1 cell cycle arrest or apoptosis. Although the mechanism of p53-mediated cell cycle arrest is fairly well defined, the p53-dependent pathway regulating apoptosis is poorly understood. Here we report the functional characterization of murine ei24 (also known as PIG8), a gene directly regulated by p53, whose overexpression negatively controls cell growth and induces apoptotic cell death. Ectopic ei24 expression markedly inhibits cell colony formation, induces the morphological features of apoptosis, and reduces the number of ␤-galactosidase-marked cells, which is efficiently blocked by coexpression of Bcl-X L . The ei24/PIG8 gene is localized on human chromosome 11q23, a region frequently altered in human cancers. These results suggest that ei24 may play an important role in negative cell growth control by functioning as an apoptotic effector of p53 tumor suppressor activities.

The type 17 cytokine receptors are thought to play critical roles in mucosal immunity. We have shown that intestinal epithelial expression of IL–17 receptor control the development of microbiota in the terminal ileum and conditional deletion of these receptors results in overgrowth of segmented filamentous bacteria. To study the roles of these receptors in response to an attaching and effacing pathogen we challenged intestinal specific Il17ra mice and Il17rc mice with Citrobacter rodentium (C. rodentium). Conditional deletion of Il17ra and l17rc Iresulted in increased bacterial growth in the colon as well as enhanced dissemination to the liver suggesting that intestinal epithelial IL–17 receptors expression is required for control of this infection. This was supported by unbiased RNAseq analysis of the colon that showed diminished expression of Pigr, Tnfsf13 and Nox1 in the absence of Il17ra. IgA has crucial role in mucosal immunity to protect from pathogenic bacteria. PIGR binds dimeric IgA and translocation into lumen, while TNFSF13 activates induce differentiation to plasma cells following–crass switch. C. rodentium specific IgA in stool and C. rodentium specific IgA producing cells in lamina propria were reduced in Il17ra mice. These results suggested IL-17 receptor signaling regulates transcytosis of C. rodentium specific IgA as well as secretion of bacteria specific IgA. On the other hands Nox1 can produce apical hydrogen peroxide. Nox1 null mice were more sensitive for C. rodentium infection. Furthermore Nox1 was expressed by IL–17A stimulation in colonic organoid. Taken together, IL–17 receptor signaling control to produce C. rodentium specific IgA through PIGR and TNFSF13 as well as hydrogen peroxide through NOX1.

Th17 cells producing cytokines, IL-17A and IL-22 are important key player in host defense. We have reported that intestinal epithelial expression of IL-17 receptors in gut epithelial cells control the development of microbiota. To study the roles of these receptors in response to an attaching and effacing pathogen we challenged Citobacter rodenium (C. rodentium) to intestinal epithelial specific IL-17RA or IL-22RA1 KO mice. Conditional deletion of Il17ra or Il22ra1 resulted in increased bacterial growth in the colon as well as enhanced dissemination to the spleen. It suggested both cytokine receptors are required for control of this infection. In addition this was supported by unbiased RNAseq analysis of the colonic epithelium that showed diminished expression of Pigr, Duox2 and Duoxa2 in the absence of IL-17RA signaling and reduced Bcl3 expression in the absence of IL-22ra1 expression. To elucidate whether production from epithelial cells effect bacteria killing we cultured crypt and stimulated IL-17A. It resulted in increasing Pigr, Duoxa2 and Duox2 were associated with increasing bacteria killing. Pigr binds dimeric IgA and translocation into lumen from lamina propria, while Duox2 and Duoxa2 make complex and produce hydrogen peroxide into lumen from apical epithelial cell membrane. These result suggested mucosal immunity to C. rodentium requires both IL-17RA and IL-22ra1 signaling and that IL-17RA regulates transcytosis of C. rodentium specific IgA as well as luminal hydrogen peroxide concentrations to control bacterial growth. In contrast, Il-22Ra1 regulates epithelial Bcl3 expression, which restrains TNFα and NF-κB signaling in the colonic epithelium.

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