Suppression of plasma free fatty acids upregulates peroxisome proliferator-activated receptor (PPAR) and and PPAR coactivator 1 in human skeletal muscle, but not lipid regulatory genes

Fatty acids are an important ligand for peroxisome proliferator-activated receptor (PPAR) activation and transcriptional regulation of metabolic genes. To examine whether reduced plasma free fatty acid (FFA) availability affects the mRNA content of proteins involved in fuel metabolism in vivo, the skeletal muscle mRNA content of various transcription factors, transcriptional coactivators and genes encoding for lipid regulatory proteins were examined before and after 3 h of cycle exercise with (NA) and without (CON) pre-exercise ingestion of nicotinic acid (NA). NA resulted in a marked (3to 6-fold) increase (P,0·05) in PPAR , PPAR and PPAR coactivator 1 (PGC1 ) mRNA, but was without effect on nuclear respiratory factor-1 and Forkhead transcription factor, fatty acid transcolase/CD36, carnitine palmitoyl transferase 1, hormone sensitive lipase (HSL) and pyruvate dehydrogenase kinase 4. Exercise in CON was associated with increased (P,0·05) PPAR , PPAR and PGC1 mRNA, which was similar in magnitude to levels observed with NA at rest. Exercise was generally without effect on the mRNA content of lipid regulatory proteins in CON and did not affect the mRNA content of the measured subset of transcription factors, transcriptional co-activators and lipid regulatory proteins during NA. To determine the possible mechanisms by which NA might affect PGC1 expression, we measured p38 MAP kinase (MAPK) and plasma epinephrine. Phosphorylation of p38 MAPK was increased (P,0·05) by NA treatment at rest, and this correlated (r2=0·84, P,0·01) with increased PGC1 . Despite this close relationship, increasing p38 MAPK in human primary myotubes was without effect on PGC1 mRNA content. Plasma epinephrine was elevated (P,0·05) by NA at rest (CON: 0·27±0·06, NA: 0·72±0·11 nM) and throughout exercise. Incubating human primary myotubes with epinephrine increased PGC1 independently of changes in p38 MAPK phosphorylation. Hence, despite the fact that NA ingestion decreased FFA availability, it promoted the induction of PPAR / and PGC1 gene expression to a similar degree as prolonged exercise. We suggest that the increase in PGC1 may be due to the elevated plasma epinephrine levels. Despite these changes in transcription factors/coactivators, the mRNA content of lipid regulatory proteins was generally unaffected by plasma FFA availability. Journal of Molecular Endocrinology (2004) 33, 533–544