Preliminary study: Leucine Supplementation Exacerbates Muscle Wasting Independent of the Ubiquitin-Proteasome System

Abstract

Cancer Cachexia is a devastating syndrome that affects around 50-80% of cancer patients and is characterized by a rapid, drastic fat and muscle mass loss. The APCMin/+ mouse strain is a well-studied mouse model of human colorectal cancer and cancer cachexia. The branched-chain amino acid leucine is known to stimulate muscle growth/maintenance through activation of mTOR and protein synthesis. PURPOSE: To examine the effects of chronic leucine supplementation on cancer cachexia development in APCMin/+ mice. METHODS: 7 APCMin/+ mice (APC) and 11 wild-type (WT) were used for this study. The animals were assigned to the following groups: WT no leucine (WTNL, n=5), WT leucine (WTL, n=5), APCMin/+ no leucine (APCNL, n=5) and APCMin/+ leucine (APCL, n=2). Mice were given ad libitum access to food and water. Mice in the leucine groups received 1.5% leucine-rich water. Plantaris muscles and tibias were excised at 20 weeks of age. Tissue was immediately frozen for morphology and gene expression analysis using RT- qPCR. RESULTS: The number of polyps increased in APCMin/+ compared to WT (46.57 ± 2.44 vs 0.00 ± 0.00). The number of polyps < 1 mm was increased (14.33 ± 1.45 vs. 7.75 ± 2.05) in APCL compared to APCNL (p<.05). There was a main effect for APCMin/+ to have lower body mass than WT (p<.0001). There was a main effect of genotype to decrease plantaris weight/tibia length in APCMin/+ mice vs. WT mice (p<.0001) and a main effect of leucine to decrease plantaris weight/tibia length in APCMin/+ mice (p<.05), which appeared to be driven by the APC (interaction p=.0841). There was an ~8-fold increase in atrogin-1 gene expression in APCNL compared to WTNL (p<.05). Atrogin-1 gene expression was ~7-fold lower in APCL compared to APCNL (p<.05). There was a main effect of genotype to increase MuRF1 expression in APCMin/+ mice compared to WT (p<. 05) and a main effect of leucine to decrease MuRF1 expression (p<.05), which appeared to be driven by the APC genotype (interaction p=.0560). No difference was found in MyoD or Myogenin gene expression. CONCLUSION: The preliminary data suggest deletory effects of leucine in cancer cachexia, which need to be affirmed by further studies. Based on gene expression of the E3 ubiquitin ligases, this loss in muscle mass may be independent of protein degradation. Supported by the Arkansas Biosciences Institute