Background: Cancer-associated cachexia (CAC) is a multifactorial syndrome characterised by progressive loss of muscle mass with limited Food and Drug Administration treatments. Although emerging evidence suggests that l-leucine and β-hydroxy-β-methyl butyrate (HMB) have potential for treating CAC, the role of α-ketoisocaproate (KIC), a metabolite of l-leucine, remains unclear. Therefore, this study explored the use of KIC as a therapeutic agent for CAC-induced muscle atrophy by targeting myostatin.

Methods: We evaluated the effect of KIC on muscle atrophy using BALB/c mice and C2C12 myotubes as models of C26- and 4T1-induced CAC. Male and female mice were injected with C26 and 4T1 cells, respectively. Grip strength was measured weekly, and mice were sacrificed 4 weeks post-injection for muscle collection. C2C12 myotubes were treated with conditioned media (CM) derived from C26 or 4T1 cells.

Results: KIC suppressed mRNA expression of myostatin, a key regulator of muscle atrophy, more effectively than did l-leucine (-26.37 ± 4.11%, p < 0.01). KIC enhanced protein turnover in C2C12 myotubes and maintained 50% cell viability at high concentrations (KIC: 4.68 mM, HMB: 3.11 mM). Following CM treatment, KIC suppressed MuRF1 and MAFbx expression in a myostatin-dependent manner, thereby reducing their polyubiquitination. KIC restored Akt-FoxO3a phosphorylation, leading to improved myotube diameter (+63.8 ± 25.71%, p < 0.05) and fusion index (+51.9 ± 22.6%, p < 0.05). Immunofluorescence and nuclear fractionation revealed that KIC reduced FoxO3a nuclear accumulation. CM reduced p-Akt-FoxO3a interaction, which was rescued by KIC. In vivo, KIC administration increased body weight (11.11 ± 8.53%), grip strength (24.76 ± 10.58%), and skeletal muscle mass (p < 0.001) in C26 tumour-bearing mice. Protein expression of myostatin in the tibialis anterior (TA) muscle (-23.57 ± 12.22%, p < 0.05) and serum (-52.11 ± 3.56%, p < 0.001) was lower in KIC-treated mice (n = 12) compared with that in the controls. KIC increased the mean fibre cross-sectional area in TA (24.51 ± 14.14%, p < 0.01). In 4T1 tumour-bearing mice, KIC improved body weight (13.10 ± 10.76%) and grip strength (7.42 ± 4.33%) (p < 0.001, n = 10). Serum myostatin levels (-57.43 ± 9.46%, p < 0.001) and skeletal muscle weight were reduced in KIC-treated mice (n = 10).

Conclusion: Our findings demonstrate that KIC improves muscle function in CAC-induced muscle atrophy by regulating myostatin expression in skeletal muscle via the Akt-FoxO3a pathway. Thus, KIC has been proposed as a potential therapeutic agent against CAC.