Cancer cachexia is a syndrome of wasting of adipose tissue and skeletal muscle caused partly by chronic, systemic inflammation. Conjugated linoleic acids (CLA) are a group of dietary fatty acids that have anti-inflammatory properties, which are mainly attributed to the cis-9, trans-11 (c9t11-) CLA isomer. The first objective was to determine whether addition of a c9t11-CLA-rich oil (0.6% c9t11, 0.1% t10c12) in diet attenuate skeletal muscle wasting and adipose atrophy in a mouse model of cancer cachexia, male CD2F1 mice inoculated with one million colon-26 adenocarcinoma (C26) cells. The loss of body weight, muscle and adipose tissue mass caused by tumors were not rescued by supplementation with 1% of the c9t11-CLA-rich oil. In quadriceps muscle, c9t11-CLA-rich oil exacerbated tumor-induced gene expression of inflammatory markers tumor necrosis factor alpha (TNFα), interleukin (IL)-6 receptor and the E3 ligase muscle ring finger -1 (MuRF-1) involved in muscle proteolysis. In epididymal adipose tissue, tumor-driven delipidation and atrophy was aggravated by the c9,t11-CLA-rich oil, demonstrated by further reduced adipocyte size and lower adiponectin expression. These data suggest that addition of c9t11-CLA-rich oil in diet did not ameliorate wasting in mice with cancer cachexia. Instead, it increased expression of inflammatory markers in the muscle and increased adipose delipidation.
The second objective was to understand the effect of cancer cachexia on heart contractile function in the same mouse model. Earlier research has reported alterations in cardiac muscle metabolism in tumor-bearing animals and cancer patients, but it remains unclear whether cancer cachexia causes heart failure. Heart contractile function was measured by echocardiography in vivo on day 14 after tumor inoculation, and all mice were sacrificed on day 17 for other analysis. Mice from Tumor group had an impaired heart function compared to mice from No Tumor group. In hearts of Tumor mice compared to No Tumor group, there was marked fibrosis demonstrated by Masson’s trichrome staining and transmission electron microscopy revealed disrupted myocardial ultrastructure. Gene expression of the regulator of cardiac muscle contraction troponin I was significantly reduced. Moreover, both mRNA and protein levels of myosin heavy chain (MHC) were altered whereby MHCα (adult isoform) was decreased and MHCβ (fetal isoform) was increased indicating reactivation of the fetal gene expression pattern. These data suggest diminished heart function in mice with cancer cachexia, and this impaired function was associated with increased fibrosis, disrupted myocardial structure and altered composition of contractile proteins of cardiac muscle.
The third objective was to investigate the underlying mechanisms for decreased heart function. In cachectic mice bearing C26 tumors compared to mice without tumors, there was an induced gene expression pattern for cardiac remodeling. Echocardiography identified a decreased cardiac wall thickness. RT-PCR and western blotting revealed a decreased amount of the major cardiac myofibrillar proteins MHC and troponin I, induced expression of atrogenes (MuRF-1 and Atrogin-1), and increased amount of ubiquitinated proteins. These data provide evidence for cardiac muscle atrophy through the ubiquitin-proteasome mediated proteolysis in mice with cancer cachexia. Regulatory signaling pathways might involve p44/42 MAPK and need to be confirmed in future research.