J Cachexia Sarcopenia Muscle (2011) 2:181–188 oxidative damage and increased metabolic potential, with an increased life span by up to 37% and 75%, respectively. These models demonstrate a crucial role for SOD1 shown through its significant contribution to oxidative damage and muscle degeneration. It is also not surprising that with a decline in O2− dismutation by SOD, to convert O2− to H2O2, the gene expression and activity of the H2O2 scavenging antioxidant enzyme GPx would also decrease. Consequently, these important cellular systems would indeed contribute to significant changes in redox-sensitive signalling pathways in cancer cachexia. SOD, not only has a crucial role in eliminating O2− accumulation, but also plays an important role in intracellular redox-sensitive signalling and regula- tion of oxidative systems [13]. As ROS have been described as important mediators of redox-sensitive intra- cellular signalling, so too are the antioxidants that regulate them. It is well-known that with the generation of intracellular O2−, SOD1 functions to dismute O2− to H2O2 [14]. H2O2 in particular, has been shown to be involved in numerous signalling cascades, and therefore, its cellular regulation, via SOD, has the potential to influence a number of important cellular pathways [24]. The changes in SOD1 function would indeed have critical consequences to cellular function that is most certainly redox-related. A circulatory protein, PIF was first described as a causative agent in cachexia when it was discovered in mice expressing the MAC16 tumour [25]. As this glycoprotein was initially discovered in the sera of mice bearing the cachectic MAC16 tumour, but not in the sera of the mice bearing the non-cachectic MAC13 tumours, it was regarded as an important factor in the development of cancer cachexia [25]. The accelerated loss of skeletal muscle protein in cachexia has been attributed to Ub-proteasome pathway activation [15, 26], and PIF has been shown to induce this pathway of skeletal muscle atrophy [26]. Although the PIF/Ub pathway indeed plays an important role in the development of cancer cachexia [15], a number of additional mechanisms are most likely involved [27]. Preproteasomal mechanisms, mediators, receptor binding, signalling pathways and activation of specific transcription factors are all important considerations [3] and ROS have been implicated in these cellular processes. Collectively, the results of this study suggest influential signalling involving O2− and antioxidant enzymes in cancer cachectic skeletal muscle. However, the signalling pathway (s) leading to the decrease in the NOX2 subunits and antioxidant enzyme expression, together with decreased antioxidant enzyme activity in cachectic skeletal muscle remains undefined. In response to ROS, cells activate the expression of a number of genes via transcription factor regulation, leading to modifications in the gene expression of important proteins, including antioxidant enzymes [13] 187 and those involved in muscle protein synthesis and regeneration [28]. Furthermore, it is possible to speculate that the downregulation of the NOX2 enzyme system is a compensatory response to O2− accumulation, induced primarily by SOD and GPx antioxidant system dysfunction and is therefore a regulated response to O2− build-up in the cellular system. However, with the knowledge that ROS can cause damage to cellular proteins [29], it is possible that the decrease in the NOX2 enzyme subunits and antioxidant enzymes is a result of protein oxidation. With this in mind, it would be important to investigate these oxidative systems and the oxidative status of cachectic skeletal muscle during the progression of the disease. 5 Conclusion It is evident from cachectic studies that show the presence of oxidised proteins and attenuation following antioxidant administration that ROS play an important role in the development of cachexia. Although the exact mechanisms are poorly defined, experimental research in the NOX enzyme systems have indicated a number of important roles for ROS, in addition to direct oxidative tissue damage, for its involvement in redox-sensitive signalling pathways. While studies have implicated the involvement of ROS in the pathogenesis of cachexia and have suggested a role for NOX, this study suggests that the increase in O2− in cachectic skeletal muscle is a result of antioxidant dysfunc- tion. However, what remains unclear is whether this result is a regulated response in the cellular system or an important contributor to the changes observed in skeletal muscle physiology. Furthermore, with the well-established role for the Ub-proteasome pathway in cancer cachexia, the increase in O2− levels in our study provides evidence of a signalling role for O2− in this pathway of skeletal muscle atrophy in cancer cachexia. While these results, along with the additional findings of this study indeed demonstrate complex changes in cachectic skeletal muscle, this multi- factoral condition coupled with a multifunctional system, further demonstrates the complexity of skeletal muscle response(s) to cancer induction. It is therefore important to understand further the role that these oxidative and antioxidative systems play in the skeletal muscle system and development of cancer cachexia. Acknowledgements This research was supported by Deakin University and Victoria University. Melanie Sullivan-Gunn was the recipient of a Victoria University postgraduate scholarship. The authors would like to thank the staff of the Deakin University Building Lp Animal House for their help and support with the animal study. The authors of this manuscript certify that they comply with the ethical guidelines for authorship and publishing in the Journal of Cachexia, Sarcopenia and Muscle [30].