The Effect of Cancer Cachexia Progression on the Feeding Regulation of Skeletal Muscle Protein Turnover

Cancer cachexia is defined as the unintentional loss of skeletal muscle mass with or without fat loss that cannot be reversed by conventional nutritional support. Cachexia occurs in ~20% of cancer patients. More specifically, 50% of lung cancer patients, the most common cancer worldwide, develop cachexia. Cachexia occurs most often in lung and gastrointestinal cancers, whereas breast and prostate have the lowest rate of cachexia. Cancer-induced cachexia disrupts skeletal muscle protein turnover (decreasing protein synthesis and increasing protein degradation). Skeletal muscle’s capacity for protein synthesis is highly sensitive to local and systemic stimuli that are controlled by mTORC1 and AMPK signaling. During cachexia, altered protein turnover is thought to occur through suppressed anabolic signaling via mTORC1, coinciding with the chronic activation of AMPK. While progress has been made in understanding some of the mechanisms underlying the suppressed anabolic signaling in cachectic muscle, gaps still remain in our understanding of muscle’s ability to respond to anabolic stimulus prior to cachexia development. The purpose of this study was to determine if cachexia progression disrupts the feeding regulation of AMPK signaling and if gp130 signaling and muscle contraction could regulate this process. Specific aim 1 examined the feeding regulation of skeletal muscle protein synthesis in pre-cachectic tumor bearing mice. Feeding increased muscle protein synthesis, while lowering AMPK signaling in pre-cachectic tumor bearing mice. Importantly, pre-cachectic tumor bearing mice have overall suppressed muscle protein synthesis independent of the fast or fed condition. Muscle specific AMPK loss was sufficient to improve the fasting suppression of muscle mTORC1 and protein synthesis in pre-cachectic tumor bearing mice. Specific aim 2 examined if muscle gp130 signaling regulates the feeding regulation of AMPK during cancer cachexia progression. Muscle gp130 loss lowered the fasting induction of AMPK in pre-cachectic tumor bearing mice without improving protein synthesis. Muscle gp130 loss did not alter the feeding regulation of muscle Akt/mTORC1 signaling and protein synthesis. Specific Aim 3 examined if an acute bout of muscle contractions could improve the muscle protein synthesis response to feeding during the progression of cachexia. Pre-cachectic tumor bearing mice exhibit suppressed protein synthesis in response low frequency electrical stimulation, and the inability to synergistically induce protein synthesis in response to feeding and contraction. In summary, pre-cachectic tumor bearing mice have lowered Akt/mTORC1 signaling and protein synthesis. Feeding can induce Akt/mTORC1 and protein synthesis and AMPK regulates the fasting suppression of protein synthesis in pre-cachectic tumor bearing mice. While gp130 loss reduces AMPK it is not sufficient to improve protein synthesis in pre-cachectic tumor bearing mice. The added protein synthesis response to feeding and contraction is blunted in pre-cachectic tumor bearing mice. These findings provide novel insight into the regulation of Akt/mTORC1 signaling and protein synthesis in response to feeding. Additionally, these studies highlight gp130’s regulation of AMPK prior to cachexia development, and the blunted anabolic muscle response to feeding and contraction in pre-cachectic tumor bearing mice. By understanding these intracellular signaling processes and perturbations prior to cachexia development, we will be able to elucidate potential therapeutic targets and treatment options to manipulate and prevent cancer cachexia.