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Porcine leptin inhibits protein breakdown and stimulates fatty acid oxidation in C₂C₁₂ myotubes¹

Growth Biology Laboratory, USDA-ARS, Beltsville, MD 20705

² Correspondence: BARC-East, Bldg. 200, Rm. 201 (phone: 301-504-5958; fax: 301-504-8623; E-mail: tramsay{at}anri.barc.usda.gov).

This study evaluated the potential mechanism(s) by which leptin treatment inhibits loss of muscle mass with fasting. Cultures of C₂C₁₂ myoblasts were differentiated into myotubes with 5% (vol/vol) horse serum in Dulbecco’s modified Eagle’s medium/F12. These myotubes were used to assess ³H-tyrosine incorporation and release following incubation with recombinant porcine leptin (0 to 500 ng/mL). Protein synthesis in myotubes, as measured by ³H-tyrosine incorporation, was not affected by leptin treatment (P > 0.05). Protein breakdown in C₂C₁₂ myotubes, as measured by ³H-tyrosine release, was inhibited by leptin treatment. A leptin concentration of 0.5 ng/mL was sufficient to inhibit ³H-tyrosine release by 3.5% (P < 0.05); 50 ng/mL produced a maximal inhibition of 10.2% (P < 0.05). Dexamethasone (1 µM) was used to maximally stimulate protein breakdown. Leptin (50 ng/mL leptin) decreased dexamethasone-induced ³H-tyrosine release by 32% (P < 0.05). The inhibition of ³H-tyrosine release in C₂C₁₂ myotubes suggests that leptin produces a protein-sparing effect in vitro by inhibiting protein breakdown. Fatty acid metabolism also was investigated because fatty acids are a major energy source for muscle during periods of reduced intake, as occurs with leptin treatment. Acute (4 h) and chronic (24 h) exposures to porcine leptin (0 to 500 ng/mL) were used to evaluate ¹⁴C-palmitate oxidation. Acute leptin treatment had no effect (P > 0.05) on palmitate metabolism. Chronic leptin exposure resulted in up to a 26% increase in palmitate oxidation (P < 0.05). The stimulation of fatty acid oxidation with chronic leptin treatment suggests that leptin spares other energy sources in muscle from oxidation during periods of a leptin-induced decrease in feed intake.

Key Words: Fatty Acid Metabolism • Leptin • Protein Breakdown • Protein Synthesis

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