Regulation of apoptosis during homeostasis and disease in the intestinal epithelium

A single epithelial layer serves as the interface between the organism and the contents of the gastrointestinal tract, underlining the importance of regulating cellular viability despite an onslaught of pathogens, toxins, waste by-products, and cytokines. A balance between cellular proliferation and apoptosis is necessary to maintain this critical barrier. Recent findings have begun to explain the mechanisms by which intestinal epithelial cells are able to survive in such an environment and how loss of normal regulatory processes may lead to inflammatory bowel disease (IBD) and predispose to inflammation-associated neoplasia. This review focuses on the regulation of physiological apoptosis in development and homeostasis and on pathological apoptosis in intestinal disease, inflammation, and neoplasia, identifying remaining questions and areas of needed investigation.

DNA microarray analysis of gene expression in eutopic endometrium from patients with deep endometriosis using laser capture microdissection

OBJECTIVE

To investigate differentially expressed genes in epithelial and stromal cells of eutopic endometrium from patients with deep endometriosis and women with normal pelvic cavities using laser capture microdissection and complementary DNA microarrays.

DESIGN

Prospective study.

SETTING

University hospital.

PATIENT(S)

Patients with deep endometriosis and fertile women who underwent laparoscopic tubal ligation or reversal of tubal sterilization.

INTERVENTION(S)

Endometrial tissue biopsies during the late proliferative phase and early, mid-, and late secretory phases.

MAIN OUTCOME MEASURE(S)

Genes that were regulated with a change greater than threefold were selected as differentially expressed genes. Validation was performed with real-time reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULT(S)

Microarray analysis identified up-regulation during the late secretory phase (patients with endometriosis vs. controls) of several genes in two important signaling pathways: RAS/RAF/MAPK and PI3K. This included the genes RON, SOS, 14-3-3 protein eta, and uPAR in epithelial cells and KSR and PI3K p85 regulatory subunit alpha in stromal cells; real-time RT-PCR analysis validated up-regulation of all six genes.

CONCLUSION(S)

The RAS/RAF/MAPK and PI3K pathways may be involved in initial development of endometriosis.

KSR Regulation of the Raf‐MEK‐ERK Cascade

Kinase suppressor of Ras (KSR) is a conserved component of the Ras pathway that functions as a molecular scaffold to enhance signaling between the core kinase components of the ERK cascade--Raf, MEK, and ERK. KSR interacts constitutively with MEK and translocates from the cytosol to the plasma membrane on Ras activation. At the membrane, KSR coordinates the assembly of a multiprotein complex containing Raf, MEK, and ERK and facilitates signal transmission from Raf to MEK and ERK. In this chapter, we will describe methods for assessing KSR function in response to Ras pathway activation. Protocols will be included that examine the ERK scaffolding activity and subcellular localization of KSR.

Coordinating ERK/MAPK signalling through scaffolds and inhibitors

The pathway from Ras through Raf and MEK (MAPK and ERK kinase) to ERK/MAPK (extracellular signal-regulated kinase/mitogen-activated protein kinase) regulates many fundamental cellular processes. Recently, a number of scaffolding proteins and endogenous inhibitors have been identified, and their important roles in regulating signalling through this pathway are now emerging. Some scaffolds augment the signal flux, but also mediate crosstalk with other pathways; certain adaptors target MEK-ERK/MAPK complexes to subcellular localizations; others provide regulated inhibition. Computational modelling indicates that, together, these modulators can determine the dynamic biological behaviour of the pathway.

An overview of apoptosis and the prevention of colorectal cancer

Colorectal cancer arises as a result of the accumulation of genetic errors many of which affect the control of apoptosis. Effective chemoprevention strategies for colorectal cancer must rectify these genetic defects. Mutation of apc is often the initiating genetic lesion in colorectal cancers that develop along the chromosomal instability pathway. Depending on the cellular context, loss of apc activates the Wnt signalling pathway causing immediate widespread apoptosis of colorectal epithelial cells and defects in differentiation and cell migration. Only cells that are inherently resistant to apoptosis survive this initial wave of apoptosis. These surviving cells constitute the epithelial population that develop into adenomas. Two gene targets of the Wnt signalling pathway are of particular relevance to apoptosis. Although controversial, survivin may function to inhibit apoptosis. MYC has two outputs in normal cells, the induction of apoptosis and proliferation. These opposing functions work so that MYC can only induce cell proliferation in cells if apoptosis is disabled. p53 couples apoptosis to mitogenic signals and survival pathways. Under some circumstances, NF-kappaB can act as an inhibitor of apoptosis possibly through increased expression of bcl-x(L). Tumours that evolve by the microsatellite instability pathway often have mutations in the proapoptotic gene bax. Colonic adenomas express cyclo-oxygenase-2 (COX-2) and may be targets of chemoprevention before the development of malignancy. However, the recent discovery that coxibs increase the risk of serious cardiovascular events limits their use as chemopreventive agents. Nevertheless, aspirin remains a drug of great interest as it is already known to reduce the risk of colorectal cancer by up to 50%. The balance of evidence shows that high vegetable fibre diets can prevent colorectal cancer, probably via the fermentation of butyrate enhancing the apoptotic response to DNA damage.

hKSR-2 inhibits MEKK3-activated MAP kinase and NF-kappaB pathways in inflammation

Kinase suppressor of ras (KSR) and MEKK3 (MAP kinase kinase kinase) are integral members of the MAP kinase pathway. We have recently identified a new isoform of the KSR family named human kinase suppressor of ras-2 (hKSR-2), and demonstrated that hKSR-2 negatively regulates Cot, a MAP3K family member which is important in inflammation and oncogenesis [P.L. Channavajhala, L. Wu, J.W. Cuozzo, J.P. Hall, W. Liu, L.L. Lin, Y. Zhang, J. Biol. Chem. 278 (2003) 47089-47097]. In this report, we provide evidence that hKSR-2 also regulates the activity of MEKK3 (another MAP3K family member) in HEK-293T cells. We demonstrate that hKSR-2 is a negative regulator of MEKK3-mediated activation of MAP kinase (specifically ERK and JNK) and NF-kappaB pathways, and concurrently inhibits MEKK3-mediated interleukin-8 production. We find that while hKSR-2 blocks MEKK3 activation, it has little to no effect on other members of the MAP3K family, including MEKK4, TAK1, and Ras-Raf, suggesting that its effects are selective.

Expression of kinase suppressor of Ras1 enhances cisplatin-induced extracellular signal-regulated kinase activation and cisplatin sensitivity

Kinase suppressor of Ras1 (KSR1) interacts with several mitogen-activated protein (MAP) kinase pathway components, including Raf, MAP/extracellular signal-regulated kinase (ERK) kinase (MEK), and ERK, and acts as a positive regulator of the Ras signaling cascade. Previous studies have shown that exposure of cells to the anticancer agent cisplatin (cis-diamminedichloroplatinum, CDDP) is associated with changes in multiple signal transduction pathways, including c-Jun-NH2-kinase, ERK, and p38 pathways. Moreover, ERK activation has been linked to changes in cell survival following CDDP treatment. In this report, we have examined the effects of KSR1 expression on the sensitivity of cells to CDDP-induced apoptosis. Loss of KSR1 expression in mouse embryo fibroblasts (MEFs) derived from KSR1 knockout mice (KSR-/- MEF) is associated with decreased CDDP-induced ERK activation and increased resistance to CDDP-induced apoptosis compared with wild-type MEFs (KSR+/+ MEF). Furthermore, transduction of KSR-/- MEFs and MCF-7 breast cancer cells with wild-type KSR1 resulted in enhanced ERK activation following CDDP exposure and increased sensitivity to CDDP. In addition, inhibition of ERK activation by exposing MEFs to the MEK1/2-specific inhibitors PD98059 and U0126 protected both KSR+/+ and KSR-/- MEFs cells from CDDP-induced apoptosis. These results indicate that KSR1-mediated regulation of ERK activity represents a novel determinant of CDDP sensitivity of cancer cells.

Inflammatory bowel disease reveals the kinase activity of KSR1

Kinase suppressor of Ras-1 (KSR1) is a recently identified member of the EGFR-Ras-Raf-1-MAPK signaling pathway. A new study demonstrates that KSR1 protects intestinal epithelium from TNF-alpha-induced apoptosis, abrogating inflammatory bowel disease (IBD). Since its discovery, there has been disagreement as to whether KSR1 possesses intrinsic kinase activity. Using transgenic mouse models and genetically modified mouse colon epithelial cells, Polk and coworkers show that the kinase activity of KSR1 is off in normal colon epithelial cells, becoming activated only at the onset of IBD. They also provide strong evidence that KSR1 kinase activity is essential for anti-apoptotic protection of the intestinal epithelium. These new data in support of KSR1 as a kinase highlight an ongoing debate as to whether KSR1 does indeed serve as a specific kinase in transphosphorylating and transactivating c-Raf-1 toward MEK1.