Examination of ‘lipotoxicity’ in skeletal muscle of high‐fat fed and ob/ob mice

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Abstract
Excess lipid accumulation resulting from an elevated supply of plasma fatty acids is linked to the pathogenesis of the metabolic syndrome and heart disease. The term ‘lipotoxicity’ was coined to describe how lipid accumulation leads to cellular dysfunction and death in non‐adipose tissues including the heart, pancreas and liver. While lipotoxicity has been shown in cultured skeletal muscle cells, the degree of lipotoxicity in vivo and the functional consequences are unresolved. We studied three models of fatty acid overload in male mice: 5 h Intralipid® and heparin infusion, prolonged high fat feeding (HFF) and genetic obesity induced by leptin deficiency (ob/ob mice). Markers of apoptosis, proteolysis and autophagy were assessed as readouts of lipotoxicity. The Intralipid® infusion increased caspase 3 activity in skeletal muscle, demonstrating that enhancing fatty acid flux activates pro‐apoptotic pathways. HFF and genetic obesity increased tissue lipid content but did not influence apoptosis. Gene array analysis revealed that HFF reduced the expression of 31 pro‐apoptotic genes. Markers of autophagy (LC3β and beclin‐1 expression) were unaffected by HFF and were associated with enhanced Bcl2 protein expression. Proteolytic activity was similarly unaffected by HFF or in ob/ob mice. Thus, contrary to our previous findings in muscle culture in vitro and in other non‐adipose tissues in vivo, lipid overload did not induce apoptosis, autophagy or proteolysis in skeletal muscle. A broad transcriptional suppression of pro‐apoptotic proteins may explain this resistance to lipid‐induced cell death in skeletal muscle.

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