Abstract
We recently identified that AMP-activated protein kinase (AMPK) α2 phosphorylation at S491 is mediated by p21-activated kinase 4 (PAK4), leading to muscular and systemic insulin resistance. This study examined how muscle PAK4 deletion affects atrophy in male mice and its link to human sarcopenia. Dexamethasone treatment increased the mRNA and protein levels of PAK4, which was partially the result of glucocorticoid response elements activation in the promoter of the Pak4 gene. Muscle-specific Pak4 knockout mice were protected from both dexamethasone- and denervation-induced muscle atrophy. Likewise, treatment with a proteolysis-targeting chimera (PROTAC) targeting PAK4 also mitigated muscle atrophy. PAK4 inhibition alleviated mitochondrial dysfunction and enhanced the expression of biogenesis-related genes via AMPK activation with reduced AMPKα2-S491 phosphorylation. Notably, muscle overexpression of phospho-deficient AMPKα2(S491A) mutant preserved mass in dexamethasone-treated mice, whereas constitutively phosphorylated AMPKα2(S491D) mutant abolished PAK4 PROTAC's antiatrophy effect. In humans, sarcopenic muscle exhibited higher levels of PAK4 protein and AMPKα2-S491 phosphorylation compared with non-sarcopenia controls, with an inverse correlation to sarcopenic index and grip strength. These findings reveal a novel AMPK phosphorylation-dependent mechanism by which PAK4 regulates mitochondrial function and muscle mass, offering new therapeutic avenues for combating muscle atrophy in chronic disease and aging. Clinical trial registration: Not applicable.