miR-200c regulates induction of apoptosis through CD95 by targeting FAP-1.

Tumor progression shares many characteristics with the process of epithelial-to-mesenchymal transition (EMT). Cells that have undergone an EMT are known to have an increased resistance to apoptosis. CD95/Fas is an apoptosis-inducing receptor expressed on many tissues and tumor cells. During tumor progression CD95 is frequently downregulated, and tumor cells lose apoptosis sensitivity. miR-200 microRNAs repress both the EMT-inducing ZEB1 and ZEB2 transcription factors. We now demonstrate that miR-200c sensitizes cells to apoptosis mediated by CD95. We have identified the apoptosis inhibitor FAP-1 as a target for miR-200c. FAP-1 was demonstrated to be responsible for the reduced sensitivity to CD95-mediated apoptosis in cells with inhibited miR-200. The identification of FAP-1 as an miR-200c target provides a molecular mechanism to explain both the downregulation of CD95 expression and the reduction in sensitivity of cells to CD95-mediated apoptosis that is observed in the context of reduced miR-200 expression during tumor progression.

Nonapoptotic functions of FADD-binding death receptors and their signaling molecules.

Death receptors (DRs) are surface receptors that when triggered have the capacity to induce apoptosis in cells by forming the death-inducing signaling complex (DISC). The first protein recruited to form the DISC is the adaptor protein FADD/Mort1. Some members of the DR family, CD95 and the TRAIL receptors DR4 and DR5, directly bind FADD, whereas others, such as TNF receptor I and DR3, initially bind another adaptor protein, TRADD, which then recruits FADD. While all DRs can activate both apoptotic and non-apoptotic pathways, it has been widely assumed that the main physiological role of FADD-binding death receptors is to trigger apoptosis. However, recent work has ascribed multiple non-apoptotic activities to these receptors and/or the signaling components of the DISC.

Let-7 prevents early cancer progression by suppressing expression of the embryonic gene HMGA2.

The microRNA let-7 regulates late embryonic development by suppressing expression of a number of genes such as c-myc and RAS as well as the embryonic gene high mobility group, A2 (HMGA2). We now demonstrate that HMGA2 is more efficiently targeted by let-7 than RAS. Its expression inversely correlates with the expression of let-7 in the NCI60 cells lines, and the expression of RAS does not change when amounts of let-7 that efficiently silence expression of HMGA2 are introduced into tumor cells. We did not find a difference in the expression of HMGA2 between primary ovarian cancer samples and matching metastases, suggesting that the expression of HMGA2 represents an early event during cancer progression. The late repression of HMGA2 by let-7 during embryonic development, and the early reexpression of HMGA2 during cancer development, is in line with the hypothesis that cancer development represents a case of reverse embryogenesis.

Let-7 expression defines two differentiation stages of cancer.

The early phases of carcinogenesis resemble embryonic development, often involving the reexpression of embryonic mesenchymal genes. The NCI60 panel of human tumor cell lines can genetically be subdivided into two superclusters (SCs) that correspond to CD95 Type I and II cells. SC1 cells are characterized by a mesenchymal and SC2 cells by an epithelial gene signature, suggesting that SC1 cells represent less differentiated, advanced stages of cancer. miRNAs are small 20- to 22-nucleotide-long noncoding RNAs that inhibit gene expression at the posttranscriptional level. By performing miRNA expression analysis on 10 Type I and 10 Type II cells, we have determined that SC1 cells express low and SC2 cells high levels of the miRNA let-7, respectively, suggesting that let-7 is a marker for less advanced cancers. Expression of the let-7 target high-mobility group A2 (HMGA2), an early embryonic gene, but not of classical epithelial or mesenchymal markers such as E-cadherin or vimentin, inversely correlated with let-7 expression in SC1 and SC2 cells. Using ovarian cancer as a model, we demonstrate that expression of let-7 and HMGA2 is a better predictor of prognosis than classical markers such as E-cadherin, vimentin, and Snail. These data identify loss of let-7 expression as a marker for less differentiated cancer.

The role of receptor internalization in CD95 signaling.

Activation of the cell surface CD95 receptor triggers a cascade of signaling events, including assembly of the death-inducing signaling complex (DISC), that culminate in cellular apoptosis. In this study, we demonstrate a general requirement of receptor internalization for CD95 ligand-mediated DISC amplification, caspase activation and apoptosis in type I cells. Recruitment of DISC components to the activated receptor predominantly occurs after the receptor has moved into an endosomal compartment and blockade of CD95 internalization impairs DISC formation and apoptosis. In contrast, CD95 ligand stimulation of cells unable to internalize CD95 results in activation of proliferative Erk and NF-kappaB signaling pathways. Hence, the subcellular localization and internalization pathways of CD95 play important roles in controlling activation of distinct signaling cascades to determine divergent cellular fates.

Identification of SNF1/AMP kinase-related kinase as an NF-kappaB-regulated anti-apoptotic kinase involved in CD95-induced motility and invasiveness.

The death receptor CD95 (APO-1/Fas) induces apoptosis in many tissues. However, in apoptosis-resistant tumor cells, stimulation of CD95 induces up-regulation of a defined number of mostly anti-apoptotic genes, resulting in increased motility and invasiveness of tumor cells. The majority of these genes are known NF-kappaB target genes. We have identified one of the CD95-regulated genes as the serine/threonine kinase (SNF1/AMP kinase-related kinase (SNARK)), which is induced in response to various forms of metabolic stress. We demonstrate that up-regulation of SNARK in response to CD95 ligand and tumor necrosis factor alpha depends on activation of NF-kappaB. Overexpression of SNARK rendered tumor cells more resistant, whereas a kinase-inactive mutant of SNARK sensitized cells to CD95-mediated apoptosis. Furthermore, small interfering RNA-mediated knockdown of SNARK increased the sensitivity of tumor cells to CD95 ligand- and TRAIL-induced apoptosis. Importantly, cells with reduced expression of SNARK also showed reduced motility and invasiveness in response to CD95 engagement. SNARK therefore represents an NF-kappaB-regulated anti-apoptotic gene that contributes to the tumor-promoting activity of CD95 in apoptosis-resistant tumor cells.