PRAP1 aggravates palmitate-induced insulin resistance in skeletal muscle cells through a CD36-dependent mechanism

Proline-rich acidic protein 1 (PRAP1) is involved in intestinal lipid handling, but its role in skeletal muscle insulin resistance is unknown. We examined circulating PRAP1 in normal diet- and high-fat diet-fed mice and evaluated PRAP1 expression and secretion in palmitate-treated intestinal epithelial cells, together with its functional effects in differentiated C2C12 myotubes. PRAP1 expression and secretion were assessed following palmitate exposure, while C2C12 myotubes were treated with recombinant PRAP1 and palmitate followed by insulin stimulation. CD36 expression, fatty acid uptake, intracellular ceramide, and insulin signaling were evaluated, and the involvement of CD36 was examined pharmacologically. Serum PRAP1 levels were increased in high-fat diet-fed mice, and palmitate increased PRAP1 expression and secretion from intestinal epithelial cells. PRAP1 alone minimally affected insulin signaling but markedly exacerbated the palmitate-induced impairment of IRS-1/Akt signaling in C2C12 myotubes. This effect was accompanied by increased CD36 expression, fatty acid uptake, and ceramide accumulation. CD36 inhibition reduced fatty acid uptake and ceramide accumulation and restored insulin-stimulated IRS-1/Akt phosphorylation. These findings identify PRAP1 as a conditional amplifier of lipotoxic insulin resistance and suggest that PRAP1 may contribute to obesity-associated skeletal muscle insulin resistance through CD36-dependent increases in fatty acid uptake and ceramide accumulation.