TRAIL-Based High Throughput Screening Reveals a Link between TRAIL-Mediated Apoptosis and Glutathione Reductase, a Key Component of Oxidative Stress Response

A high throughput screen for compounds that induce TRAIL-mediated apoptosis identified ML100 as an active chemical probe, which potentiated TRAIL activity in prostate carcinoma PPC-1 and melanoma MDA-MB-435 cells. Follow-up in silico modeling and profiling in cell-based assays allowed us to identify NSC130362, pharmacophore analog of ML100 that induced 65-95% cytotoxicity in cancer cells and did not affect the viability of human primary hepatocytes. In agreement with the activation of the apoptotic pathway, both ML100 and NSC130362 synergistically with TRAIL induced caspase-3/7 activity in MDA-MB-435 cells. Subsequent affinity chromatography and inhibition studies convincingly demonstrated that glutathione reductase (GSR), a key component of the oxidative stress response, is a target of NSC130362. In accordance with the role of GSR in the TRAIL pathway, GSR gene silencing potentiated TRAIL activity in MDA-MB-435 cells but not in human hepatocytes. Inhibition of GSR activity resulted in the induction of oxidative stress, as was evidenced by an increase in intracellular reactive oxygen species (ROS) and peroxidation of mitochondrial membrane after NSC130362 treatment in MDA-MB-435 cells but not in human hepatocytes. The antioxidant reduced glutathione (GSH) fully protected MDA-MB-435 cells from cell lysis induced by NSC130362 and TRAIL, thereby further confirming the interplay between GSR and TRAIL. As a consequence of activation of oxidative stress, combined treatment of different oxidative stress inducers and NSC130362 promoted cell death in a variety of cancer cells but not in hepatocytes in cell-based assays and in in vivo, in a mouse tumor xenograft model.

Loss of caspase-8 in hepatocytes accelerates the onset of liver regeneration in mice through premature nuclear factor kappa B activation

The cytokine tumor necrosis factor alpha (TNF-α; TNF) plays a critical role early in liver regeneration following partial hepatectomy (PH). TNF stimulates at least three different pathways leading to nuclear factor kappa B (NF-κB) activation, apoptosis signaling by way of caspase-8 (Casp8), and activation of cJun N-terminal kinases (JNK). The present study aimed to better define the role of Casp8 during liver regeneration. We performed PH in mice lacking Casp8 specifically in hepatocytes (Casp8(Δhepa) ) and determined their liver regeneration capacity by measuring liver mass restoration and kinetics of cell cycle progression. Casp8(Δhepa) mice showed an accelerated onset of DNA synthesis after PH, delayed hepatocyte mitosis, but overall normal liver mass restoration. Analysis of immediate TNF-dependent signaling pathways revealed that loss of Casp8 prevents proteolytic cleavage of the receptor-interacting protein 1 (RIP1) in hepatocytes and subsequently triggers premature activation of NF-κB and JNK/cJun related signals. In order to define the role of NF-κB in this setting we blocked NF-κB activation in Casp8(Δhepa) mice by concomitant inactivation of the NF-κB essential modulator (NEMO) in hepatocytes. Lack of NEMO largely reverted aberrant DNA synthesis in Casp8(Δhepa) mice but resulted in incomplete termination of the regeneration process and hepatomegaly.

CONCLUSION

Casp8 comprises a nonapoptotic function during liver regeneration by balancing RIP1, NF-κB, and JNK activation. While loss of Casp8 triggers NF-κB activation and thus improves liver regeneration, combined loss of Casp8 and NEMO impairs a controlled regenerative response and drives hepatomegaly.

Complexities of TGF-β targeted cancer therapy

Many advanced tumors produce excessive amounts of Transforming Growth Factor-β (TGF-β) which, in normal epithelial cells, is a potent growth inhibitor. However, in oncogenically activated cells, the homeostatic action of TGF-β is often diverted along alternative pathways. Hence, TGF-β signaling elicits protective or tumor suppressive effects during the early growth-sensitive stages of tumorigenesis. However, later in tumor development when carcinoma cells become refractory to TGF-β-mediated growth inhibition, the tumor cell responds by stimulating pathways with tumor progressing effects. At late stages of malignancy, tumor progression is driven by TGF-β overload. The tumor microenvironment is a target of TGF-β action that stimulates tumor progression via pro-tumorigenic effects on vascular, immune, and fibroblastic cells. Bone is one of the richest sources of TGF-β in the body and a common site for dissemination of breast cancer metastases. Osteoclastic degradation of bone matrix, which accompanies establishment and growth of metastases, triggers further release of bone-derived TGF-β. This leads to a vicious positive feedback of tumor progression, driven by ever increasing levels of TGF-β released from both the tumor and bone matrix. It is for this reason, that pharmaceutical companies have developed therapeutic agents that block TGF-β signaling. Nonetheless, the choice of drug design and dosing strategy can affect the efficacy of TGF-β therapeutics. This review will describe pre-clinical and clinical data of four major classes of TGF-β inhibitor, namely i) ligand traps, ii) antisense oligonucleotides, iii) receptor kinase inhibitors and iv) peptide aptamers. Long term dosing strategies with TGF-β inhibitors may be ill-advised, since this class of drug has potentially highly pleiotropic activity, and development of drug resistance might potentiate tumor progression. Current paradigms for the use of TGF-β inhibitors in oncology have therefore moved towards the use of combinatorial therapies and short term dosing, with considerable promise for the clinic.

Loss of caspase-8 protects mice against inflammation-related hepatocarcinogenesis but induces non-apoptotic liver injury

BACKGROUND & AIMS

Disruption of the nuclear factor-κB (NF-κB) essential modulator (NEMO) in hepatocytes of mice (NEMO(Δhepa) mice) results in spontaneous liver apoptosis and chronic liver disease involving inflammation, steatosis, fibrosis, and development of hepatocellular carcinoma. Activation of caspase-8 (Casp8) initiates death receptor-mediated apoptosis. We investigated the pathogenic role of this protease in NEMO(Δhepa) mice or after induction of acute liver injury.

METHODS

We created mice with conditional deletion of Casp8 in hepatocytes (Casp8(Δhepa)) and Casp8(Δhepa)NEMO(Δhepa) double knockout mice. Acute liver injury was induced by Fas-activating antibodies, lipopolysaccharides, or concanavalin A. Spontaneous hepatocarcinogenesis was monitored by magnetic resonance imaging.

RESULTS

Hepatocyte-specific deletion of Casp8 protected mice from induction of apoptosis and liver injury by Fas or lipopolysaccharides but increased necrotic damage and reduced survival times of mice given concanavalin A. Casp8(Δhepa)NEMO(Δhepa) mice were protected against steatosis and hepatocarcinogenesis but had a separate, spontaneous phenotype that included massive liver necrosis, cholestasis, and biliary lesions. The common mechanism by which inactivation of Casp8 induces liver necrosis in both injury models involves the formation of protein complexes that included the adaptor protein Fas-associated protein with death domain and the kinases receptor-interacting protein (RIP) 1 and RIP3-these have been shown to be required for programmed necrosis. We demonstrated that hepatic RIP1 was proteolytically cleaved by Casp8, whereas Casp8 inhibition resulted in accumulation of RIP complexes and subsequent liver necrosis.

CONCLUSIONS

Inhibition of Casp8 protects mice from hepatocarcinogenesis following chronic liver injury mediated by apoptosis of hepatocytes but can activate RIP-mediated necrosis in an inflammatory environment.

Hepatocyte-specific NEMO deletion promotes NK/NKT cell- and TRAIL-dependent liver damage

Nuclear factor κB (NF-κB) is one of the main transcription factors involved in regulating apoptosis, inflammation, chronic liver disease, and cancer progression. The IKK complex mediates NF-κB activation and deletion of its regulatory subunit NEMO in hepatocytes (NEMO^Δhepa) triggers chronic inflammation and spontaneous hepatocellular carcinoma development. We show that NEMO^Δhepa mice were resistant to Fas-mediated apoptosis but hypersensitive to tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) as the result of a strong up-regulation of its receptor DR5 on hepatocytes. Additionally, natural killer (NK) cells, the main source of TRAIL, were activated in NEMO^Δhepa livers. Interestingly, depletion of the NK1.1⁺ cells promoted a significant reduction of liver inflammation and an improvement of liver histology in NEMO^Δhepa mice. Furthermore, hepatocyte-specific NEMO deletion strongly sensitized the liver to concanavalin A (ConA)–mediated injury. The critical role of the NK cell/TRAIL axis in NEMO^Δhepa livers during ConA hepatitis was further confirmed by selective NK cell depletion and adoptive transfer of TRAIL-deficient^−/− mononuclear cells. Our results uncover an essential mechanism of NEMO-mediated protection of the liver by preventing NK cell tissue damage via TRAIL/DR5 signaling. As this mechanism is important in human liver diseases, NEMO^Δhepa mice are an interesting tool to give insight into liver pathophysiology and to develop future therapeutic strategies.