Induction of Free Fatty Acid‐mediated Angiopoietin‐Like Protein‐4 Gene Expression is Independent of PPARδ Activity in Skeletal Muscle Cells In Vitro

Abstract

Angiopoietin‐like protein‐4 (ANGPTL4) inhibits lipoprotein lipase activity in skeletal muscle. Free fatty acids (FFAs) activate ANGPTL4 expression, yet the method by which FFAs induce ANGPTL4 in skeletal muscle is not well‐known. The purpose of this study was to examine the effect of FFAs on peroxisome proliferator‐activated receptor δ (PPARδ)‐mediated ANGPTL4 expression in C2C12 cells. Chronic exposure of mono‐ (MUFA) and poly‐unsaturated fatty acids (PUFA), but not saturated fatty acids (SFA), significantly induced ANGPTL4 mRNA expression in C2C12 cells (MUFA: 4.5±1.1, p=0.02; PUFA: 5.6±1.6, p=0.02; SFA: 0.4±0.1, p=0.4 respectively). To analyze the mechanism of FFA‐induced PPARδ activation of ANGPTL4, mRNA expression of PPARδ, which is believed to induce ANGPTL4 activity, was measured. However, there was no significant difference in PPARδ expression after FFA treatments. Next, we used the PPARδ agonist (GW0742) and antagonist (GSK0660) to modulate PPARδ activity independent of FFA treatment and examined ANGPTL4 expression. GW0742 significantly increased ANGPTL4 in undifferentiated and differentiated C2C12 cells (12.6±3.9, p=0.01 and 2.8±0.2, p<0.001 respectively), while GSK0660 significantly suppressed basal ANGPTL4 expression (0.2±0.1, p=0.05 and 0.2±0.03, p<0.001 respectively). Yet, in the presence of FFA and GSK0660, ANGPTL4 gene expression significantly increased despite the absence of PPARδ activity in both undifferentiated and differentiated C2C12 cells (7.1±2.5, p=0.03 and 3.8±0.4, p<0.001). Taken together, these results indicate that FFAs may activate ANGPTL4 in a PPARδ independent manner, and a revised mechanism of FFA‐induced skeletal muscle ANGPTL4 expression may be warranted.