RESUMEN
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 (3- to 6-fold) increase (P<0.05) in PPARalpha, PPARdelta and PPAR coactivator 1alpha (PGC1alpha) 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) PPARalpha, PPARdelta and PGC1alpha 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 PGC1alpha 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 PGC1alpha. Despite this close relationship, increasing p38 MAPK in human primary myotubes was without effect on PGC1alpha 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 PGC1alpha independently of changes in p38 MAPK phosphorylation. Hence, despite the fact that NA ingestion decreased FFA availability, it promoted the induction of PPARalpha/delta and PGC1alpha gene expression to a similar degree as prolonged exercise. We suggest that the increase in PGC1alpha 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.
Asunto(s)
Ácidos Grasos/sangre , Proteínas de Choque Térmico/metabolismo , Hipolipemiantes/farmacología , Músculo Esquelético/fisiología , Niacina/farmacología , PPAR alfa/metabolismo , PPAR delta/metabolismo , Factores de Transcripción/metabolismo , Antígenos CD36/efectos de los fármacos , Antígenos CD36/genética , Calcimicina/farmacología , Carnitina O-Palmitoiltransferasa/efectos de los fármacos , Carnitina O-Palmitoiltransferasa/genética , Células Cultivadas , Inhibidores Enzimáticos/farmacología , Epinefrina/sangre , Prueba de Esfuerzo , Factores de Transcripción Forkhead , Proteínas de Choque Térmico/efectos de los fármacos , Proteínas de Choque Térmico/genética , Humanos , Imidazoles/farmacología , Lípidos/genética , Músculo Esquelético/citología , Músculo Esquelético/efectos de los fármacos , Proteínas Nucleares/efectos de los fármacos , Proteínas Nucleares/genética , PPAR alfa/efectos de los fármacos , PPAR alfa/genética , PPAR delta/efectos de los fármacos , PPAR delta/genética , Coactivador 1-alfa del Receptor Activado por Proliferadores de Peroxisomas gamma , Fosforilación , Proteínas Quinasas/efectos de los fármacos , Proteínas Quinasas/genética , Piridinas/farmacología , Esterol Esterasa/efectos de los fármacos , Esterol Esterasa/genética , Factores de Transcripción/efectos de los fármacos , Factores de Transcripción/genética , Regulación hacia Arriba , Proteínas Quinasas p38 Activadas por Mitógenos/antagonistas & inhibidores , Proteínas Quinasas p38 Activadas por Mitógenos/metabolismoRESUMEN
It has been hypothesized that epinephrine may stimulate interleukin (IL)-6 gene expression in skeletal muscle. The aim of the present study was to examine the effect of epinephrine on IL-6 gene expression within, and protein release from, skeletal muscle. We hypothesized that physiologic epinephrine would neither result in an increase in IL-6 mRNA nor protein release from skeletal muscle. Soleus muscle was excised from 4-week-old anesthetized Sprague Dawley rats and incubated in a Krebs buffer with the addition of either saline (CON), epinephrine, at concentrations of 1,000 nmol/L (EPI 1,000), 100 nmol/L (EPI 100), or 10 nmol/L (EPI 10), or the calcium ionophore, ionomycin (IONO), a positive control. After a 1-hour incubation, muscle was collected and extracted for RNA, reverse transcribed, and IL-6 gene expression was determined by real-time polymerase chain reaction (PCR). An aliquot of incubation medium was also collected and analyzed for IL-6 protein by enzyme-linked immunosorbent (ELISA). EPI 1,000 and IONO increased (P < .05) IL-6 mRNA, whereas EPI 100 and EPI 10 were without effect. IL-6 protein release from skeletal muscle was increased in IONO (P < .05), but not in CON or EPI at any concentration. These data demonstrate that while pharmacologic concentrations of epinephrine activate IL-6 mRNA, supraphysiologic and high-physiologic doses appear to have little, if any, effect on IL-6 gene transcription in skeletal muscle. In addition, ionomycin can stimulate IL-6 gene expression and protein release after only 1 hour of exposure.