The death ligand, TRAIL (tumor necrosis factor-related apoptosis-inducing ligand), has shown great promise for inducing apoptosis selectively in tumors. Although many tumor cells are resistant to TRAIL-induced apoptosis alone, they can often be sensitized by co-treatment with DNA-damaging agents such as etoposide. However, the molecular mechanism underlying this therapeutically important synergy is unknown. We explored the mechanism mediating TRAIL-DNA damage apoptotic synergy in human mesothelioma cells, a tumor type particularly refractory to existing therapies. We show that Bid, a cytoplasmic Bcl-2 homology domain 3-containing protein activated by caspase 8 in response to TRAIL ligation, is essential for TRAIL-etoposide apo-ptotic synergy and, furthermore, that exposure to DNA damage primes cells to induction of apoptosis by otherwise sublethal levels of activated Bid. Finally, we show that the extensive caspase 8 cleavage seen during TRAIL-etoposide synergy is a consequence and not a cause of the apoptotic cascade activated downstream of Bid. These data indicate that TRAIL-etoposide apoptotic synergy arises because DNA damage increases the inherent sensitivity of cells to levels of TRAIL-activated Bid that would otherwise be insufficient for apoptosis. Such studies indicate how the adroit combination of differing proapoptotic and sublethal signals can provide an effective strategy for treating refractory tumors.
"With the exception of cancers in which essential components of the extrinsic cell death pathway are mutated or silenced (resulting, for example, in inactivation of TRAIL receptors or caspase-8), co-drugging has been shown to increase the efficacy of TRAIL in cell lines, tumor models and patients (Ashkenazi and Herbst 2008). Sub-lethal doses of chemotherapy or ionizing radiation are known to sensitize resistant cancer cells to TRAIL-induced apoptosis via modulation of TRAIL receptors, DISC or mitochondrial components (Broaddus et al 2005, Di Pietro et al 2001, Ehrhardt et al 2008, Galligan et al 2005, Ganten et al 2004, Keane et al 1999, Morizot et al 2010). Drugs that inhibit histone deacetylases or the proteasome (which modulate gene expression and protein stability, respectively) have also been shown to be effective in overcoming resistance to TRAIL in cell lines and tumor models (Bagci-Onder et al 2012, Brooks et al 2005, Butler et al 2006, Frew et al 2008, Inoue et al 2004, Kabore et al 2006, Vanoosten et al 2005). "
[Show abstract][Hide abstract] ABSTRACT: TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) holds promise as an anti-cancer therapeutic but efficiently induces apoptosis in only a subset of tumor cell lines. Moreover, even in clonal populations of responsive lines, only a fraction of cells dies in response to TRAIL and individual cells exhibit cell-to-cell variability in the timing of cell death. Fractional killing in these cell populations appears to arise not from genetic differences among cells but rather from differences in gene expression states, fluctuations in protein levels and the extent to which TRAIL-induced death or survival pathways become activated. In this study, we ask how cell-to-cell variability manifests in cell types with different sensitivities to TRAIL, as well as how it changes when cells are exposed to combinations of drugs. We show that individual cells that survive treatment with TRAIL can regenerate the sensitivity and death-time distribution of the parental population, demonstrating that fractional killing is a stable property of cell populations. We also show that cell-to-cell variability in the timing and probability of apoptosis in response to treatment can be tuned using combinations of drugs that together increase apoptotic sensitivity compared to treatment with one drug alone. In the case of TRAIL, modulation of cell-to-cell variability by co-drugging appears to involve a reduction in the threshold for mitochondrial outer membrane permeabilization.
"This sub-network suggests that HCV may be able to repress the expressions of several key genes in a number of HCV pathways via miR-16, miR-215, or both. For example, miR-16 targets MAPK14 (p38 MAPK) and BID (also a miR-215 target), which are two key regulators in apoptosis-associated pathways [52,53]. The induction of apoptosis among virus-infected cells is an effective host mechanism against virus infection. "
[Show abstract][Hide abstract] ABSTRACT: Hepatitis C virus (HCV) is a major cause of chronic liver disease by infecting over 170 million people worldwide. Recent studies have shown that microRNAs (miRNAs), a class of small non-coding regulatory RNAs, are involved in the regulation of HCV infection, but their functions have not been systematically studied. We propose an integrative strategy for identifying the miRNA-mRNA regulatory modules that are associated with HCV infection. This strategy combines paired expression profiles of miRNAs and mRNAs and computational target predictions. A miRNA-mRNA regulatory module consists of a set of miRNAs and their targets, in which the miRNAs are predicted to coordinately regulate the level of the target mRNA.
We simultaneously profiled the expression of cellular miRNAs and mRNAs across 30 HCV positive or negative human liver biopsy samples using microarray technology. We constructed a miRNA-mRNA regulatory network, and using a graph theoretical approach, identified 38 miRNA-mRNA regulatory modules in the network that were associated with HCV infection. We evaluated the direct miRNA regulation of the mRNA levels of targets in regulatory modules using previously published miRNA transfection data. We analyzed the functional roles of individual modules at the systems level by integrating a large-scale protein interaction network. We found that various biological processes, including some HCV infection related canonical pathways, were regulated at the miRNA level during HCV infection.
Our regulatory modules provide a framework for future experimental analyses. This report demonstrates the utility of our approach to obtain new insights into post-transcriptional gene regulation at the miRNA level in complex human diseases.
"The BH3 family member Bid mediates the cross-talk between the extrinsic and intrinsic apoptotic pathways (Li et al., 1998). Earlier studies have shown that the overexpression of a dominant negative Bid construct inhibited apoptosis induced by both the Fas and TRAIL receptors in Jurkat cells (Werner et al., 2002), and Bid has been reported to regulate the synergy between TRAIL and epotoside in M28 and REN cells (Broaddus et al., 2005). We found that Bid is critical for FLIP silencing-induced processing of caspase 3 and cell death in the HCT116 and HT29 cells, indicating that mitochondrial activation is required for cell death to occur. "
[Show abstract][Hide abstract] ABSTRACT: Death receptors can directly (type I cells) or indirectly induce apoptosis by activating mitochondrial-regulated apoptosis (type II cells). The level of caspase 8 activation is thought to determine whether a cell is type I or II, with type II cells less efficient at activating this caspase following death receptor activation. FLICE-inhibitory protein (FLIP) blocks death receptor-mediated apoptosis by inhibiting caspase 8 activation; therefore, we assessed whether silencing FLIP could convert type II cells into type I. FLIP silencing-induced caspase 8 activation in Bax wild-type and null HCT116 colorectal cancer cells; however, complete caspase 3 processing and apoptosis were only observed in Bax wild-type cells. Bax-null cells were also more resistant to chemotherapy and tumor necrosis factor-related apoptosis inducing ligand and, unlike the Bax wild-type cells, were not sensitized to these agents by FLIP silencing. Further analyses indicated that release of second mitochondrial activator of caspases from mitochondria and subsequent inhibition of X-linked inhibitor of apoptosis protein (XIAP) was required to induce full caspase 3 processing and apoptosis following FLIP silencing. These results indicate that silencing FLIP does not necessarily bypass the requirement for mitochondrial involvement in type II cells. Furthermore, targeting FLIP and XIAP may represent a therapeutic strategy for the treatment of colorectal tumors with defects in mitochondrial-regulated apoptosis.
Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. The impact factor represents a rough estimation of the journal's impact factor and does not reflect the actual current impact factor. Publisher conditions are provided by RoMEO. Differing provisions from the publisher's actual policy or licence agreement may be applicable.