Several therapeutic agents and industrial chemicals induce liver tumors in rodents through the activation of the peroxisome proliferator-activated receptor alpha (PPARα). The cellular and molecular events by which PPARα activators induce rodent hepatocarcinogenesis has been extensively studied and elucidated. This review summarizes the weight of evidence relevant to the hypothesized mode of action (MOA) for PPARα activator-induced rodent hepatocarcinogenesis and identifies gaps in our knowledge of this MOA. Chemical-specific and mechanistic data support concordance of temporal and dose-response relationships for the key events associated with many PPARα activators including a phthalate ester plasticizer di(2-ethylhexyl) phthalate (DEHP) and the drug gemfibrozil. While biologically plausible in humans, the hypothesized key events in the rodent MOA, for PPARα activators, are unlikely to induce liver tumors in humans because of toxicodynamic and biological differences in responses. This conclusion is based on minimal or no effects observed on growth pathways, hepatocellular proliferation and liver tumors in humans and/or species (including hamsters, guinea pigs and cynomolgous monkeys) that are more appropriate human surrogates than mice and rats at overlapping dose levels. Overall, the panel concluded that significant quantitative differences in PPARα activator-induced effects related to liver cancer formation exist between rodents and humans. On the basis of these quantitative differences, most of the workgroup felt that the rodent MOA is "not relevant to humans" with the remaining members concluding that the MOA is "unlikely to be relevant to humans". The two groups differed in their level of confidence based on perceived limitations of the quantitative and mechanistic knowledge of the species differences, which for some panel members strongly supports but cannot preclude the absence of effects under unlikely exposure scenarios.

Cross talk between p53 and interferon-regulated pathways is implicated in the induction of gene expression by biologic and genotoxic stresses. We demonstrate that the interferon-stimulated gene ISG15 is induced by p53 and that p53 is required for optimal gene induction by double-stranded RNA (dsRNA), but not interferon. Interestingly, virus induces ISG15 in the absence of p53, suggesting that virus and dsRNA employ distinct signaling pathways.To promote host survival, cells respond to viral challenge by activating both protective and cell-death pathways. Interferon (IFN) is induced in virus-infected cells and functions in a paracrine manner to exert a protective effect on surrounding cells. IFN can also induce apoptosis in an antiviral strategy that sacrifices the infected cell to prevent viral spread (15, 37). Independent of IFN, virus infection or double-stranded RNA (dsRNA) directly activates a subset of interferon-stimulated genes (ISGs) (11,29). Virus-or dsRNA-induced gene expression occurs through a pathway distinct from the IFN-activated Jak-Stat pathway (3, 12). The interferon regulatory factors (IRFs) 1 and 3 appear to play central roles in virus-or dsRNAinduced gene expression, targeting interferon-stimulated response element (ISRE)-like elements in the promoters of responsive genes (26). In addition, the dsRNA-activated protein kinase, PKR, phosphorylates the NF-B inhibitor, IB (21), leading to activation of NF-B, which is required for the induction of IFN-␤ and some ISGs by virus and dsRNA (8,22,41). By bypassing the requirement for IFN induction, virusinduced gene products are thought to confer immediate protection to the infected cell (11).The tumor suppressor p53 can effect a protective or a suicidal response to genotoxic stress (1, 2). An accumulation of evidence for cross talk between p53 and the IFN system has implicated p53 in the host response to viral challenge. For example, the transcription factor IRF1, which both is induced by IFN and functions in the regulation of IFN and ISGs (26), cooperates with p53 in the induction of WAF-1 and is essential for radiation-induced cell-cycle arrest (36). The IFN-stimulated gene ISG15 was identified in screens for p53 and radiation-induced genes (17, 27) and thus resembles WAF-1 in being regulated by both IFN and p53 (32). Finally, the IFNregulated PKR functions in ISG induction by dsRNA (22, 41), interacts with p53, and enhances its transactivating activity (9, 10). Taken together, these studies suggest a role for p53 in the regulation of IFN-or virus-induced genes; however, direct evidence of a p53-dependent step is lacking.ISG15 is independently induced in response to IFN and virus (3,29) and was identified in a screen for p53-induced genes (17, 27); therefore, we first examined the direct regulation of ISG15 by p53. ISG15 expression was measured by Western blot in HeLa cells stably transfected with a temperature-sensitive p53 mutant (7). No ISG15 protein was detected in the presence of mutant p53; however, ISG15 expression was induced within 6 h o...

Interferon (IFN)-stimulated gene (15 kDa) (ISG15) is a ubiquitin-like protein that forms covalent conjugates with cellular proteins. ISG15 is induced by IFN, microbial challenge, and p53, suggesting that it represents a genetic response that is shared among diverse stress stimuli. To investigate the regulation of this posttranslational modification pathway by a genotoxic chemotherapeutic agent, we examined ISG15 induction and conjugation in cells treated with the topoisomerase I (topoI) poison, camptothecin (CPT). CPT induced ISG15mRNA, and induction required protein synthesis and a functional p53 protein. However, IFN and the Jak-Stat components of the IFN signaling pathway were dispensable for CPT induction of ISG15. CPT induced free ISG15 and conjugates in a dose-dependent and time-dependent manner. A single 55-kDa protein was the prominent CPT-induced ISG15 conjugate and localized to the nuclear compartment. CPT-induced ISG15 conjugates were distinct from those induced by IFN; however, CPT treatment dramatically enhanced ISG15 conjugation in response to IFN. These findings provide the first evidence of a stimulus-specific induction of discrete ISG15 conjugate species and demonstrate that treatment with a combination of cancer therapeutic agents can cooperate to enhance ISG15 conjugation. Identification of the specific ISG15 conjugates induced by chemotherapeutic agents may reveal novel molecular targets.

Cardiovascular disease (CVD) is the leading global cause of death, and treatments that further reduce CV risk remain an unmet medical need. Epidemiological studies have consistently identified low high-density lipoprotein cholesterol (HDL-C) as an independent risk factor for CVD, making HDL elevation a potential clinical target for improved CVD resolution. Endothelial lipase (EL) is a circulating enzyme that regulates HDL turnover by hydrolyzing HDL phospholipids and driving HDL particle clearance. Using MEDI5884, a first-in-class, EL-neutralizing, monoclonal antibody, we tested the hypothesis that pharmacological inhibition of EL would increase HDL-C by enhancing HDL stability. In nonhuman primates, MEDI5884 treatment resulted in lasting, dose-dependent elevations in HDL-C and circulating phospholipids, confirming the mechanism of EL action. We then showed that a favorable lipoprotein profile of elevated HDL-C and reduced low-density lipoprotein cholesterol (LDL-C) could be achieved by combining MEDI5884 with a PCSK9 inhibitor. Last, when tested in healthy human volunteers, MEDI5884 not only raised HDL-C but also increased HDL particle numbers and average HDL size while enhancing HDL functionality, reinforcing EL neutralization as a viable clinical approach aimed at reducing CV risk.