Abstract 4786: A role for increased Fra-1 expression in cachexia: Novel mechanisms of increased apoptosis in C2C12 myotubes

Over 60% of patients with advanced cancer have cancer-induced cachexia, and the prevalence of weight loss reaches 86% in end-stage cancer. While cachexia is directly responsible for approximately 25% of cancer deaths and is inversely correlated with survival time, current treatments are limited and primarily palliative. Tumor necrosis factor (TNFα) is elevated in cancer patients, and is strongly associated with cachexia as it produces pleiotropic cachectic effects including apoptosis, inhibition of myogenesis and increased protein degradation. These effects are mediated in part by the pro-inflammatory transcription factor nuclear factor kappa B (NFκB) which upregulates components of the ubiquitin-proteasome pathway, and inhibits muscle cell differentiation through post-translational suppression of the muscle specific transcription factor, myogenic determination factor 1. We believe a critical role of AP-1 in cancer cachexia precedes NFκB-activated proteasome degradation. AP-1, and more specifically a switch to Jun:Fra-1 heterodimers, induces caspase 3 activity which is necessary to cleave large actomyosin complexes prior to further degradation by the ubiquitin-proteasome system. In C2C12 mouse muscle cells treated with TNFα, we see increased expression of the AP-1 subunits cJun, JunB and Fra-1 along with the expected decrease in myosin heavy chain (MyHC) expression. Inhibiting the mitogen activated protein kinase (MAPK) pathway with U0126 in TNFα-treated cells reduces the expression of these AP-1 subunits and partially restores expression of MyHC. When we overexpress the Fra-1 subunit in C2C12 cells, we see increased mRNA expression of FAS and caspases 1, 8 and 11 (ortholog of human caspase 4). Caspase 11-mediated activation of caspase 1 appears to activate IL-1β, a catabolism-inducing cytokine implicated in cachexia. Downstream of Fra-1 expression, we also see increased caspase 3 activity and an increase in the number of TUNEL-positive apoptotic cells. We hypothesize that TNFα signaling through the MAPK pathway leads to phosphorylation-dependent stabilization of Fra-1, and leads to a switch to Jun:Fra-1 heterodimer composition which, in turn, transcriptionally up-regulates FAS and caspases 1, 8 and 11. We think increased expression of FAS, and caspases 8 and 11 lead to FAS-mediated apoptosis and increased caspase 3 activity. We used high-throughput screening to identify AP-1 inhibitors, and then tested these inhibitors for their ability to restore MyHC expression in TNFα-treated cells. We have identified a novel compound, LS474, which specifically inhibits Fra-1 expression and restores MyHC expression. This suggests that the Fra-1 subunit of AP-1 plays an important role in mediating cachexia and is a good therapeutic target in the treatment of cancer cachexia. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4786. doi:10.1158/1538-7445.AM2011-4786