Differential effects of contraction and PPAR agonists on the expression of fatty acid transporters in rat skeletal muscle

Abstract

We have examined over the course of a 1-week period the independent and combined effects of chronically increased muscle contraction and the peroxisome proliferator-activated receptor (PPAR)alpha and PPARgamma activators, Wy 14,643 and rosiglitazone, on the expression and plasmalemmal content of the fatty acid transporters, FAT/CD36 and FABPpm, as well as on the rate of fatty acid transport. In resting muscle, the activation of either PPARalpha or PPARgamma failed to induce the protein expression of FAT/CD36. PPARalpha activation also failed to induce the protein expression of FABPpm. In contrast, PPARgamma activation induced the expression of FABPpm protein (40%; P < 0.05). Chronic muscle contraction increased the protein expression of FAT/CD36 (approximately 50%; P < 0.05), whereas FABPpm was slightly increased (12%; P < 0.05). Neither PPARalpha nor PPARgamma activation altered the contraction-induced expression of FAT/CD36 or FABPpm. Changes in protein expression of FAT/CD36 or FABPpm, induced by either contractions or by administration of rosiglitazone, were largely attributable to increased transcription. The contraction-induced increments in FAT/CD36 were accompanied by parallel increments in plasmalemmal FAT/CD36 and in rates of fatty acid transport (P < 0.05). Up-regulation of FABPpm expression was, however, accompanied by a reduction in plasmalemmal FABPpm, which did not affect the rates of long chain fatty acid (LCFA) transport. These studies have shown that in skeletal muscle (i) neither PPARalpha nor PPARgamma activation alters FAT/CD36 expression, (ii) PPARgamma activation selectively up-regulates FABPpm expression and (iii) contraction-induced up-regulation of LCFA transport does not appear to occur via activation of either PPARalpha or PPARgamma.

Figure 1. Schematic representation of the experimental design

There were three experimental groups in which saline (sham), WY 14,643 or Rosiglitazone were infused continuously for 7 days, using an implanted mini-osmotic pump. In addition, in each animal, EDL muscles in one leg were induced to contract for up to 7 days while the contralateral EDL was not stimulated (rest). The animals were killed after 1,3,5 and 7 days of treatment.

Figure 2. FAT/CD36 mRNA (A) and FABPpm mRNA (B) in resting and chronically contracting muscles treated with the PPARα activator Wy 14,643 and the PPARγ activator rosiglitazone

Data are means ± s.e.m. In each treatment group, data from day 5 and day 7 were pooled and mRNA data were normalized to 285 ribosomal RNA. n = 6–8 muscles for each group. *P < 0.05, resting muscle versus chronically contracting muscle. **P < 0.05, resting muscle + rosiglitazone versus resting muscle no drug, and resting muscle + Wy 14,643.

Figure 3. FAT/CD36 protein in resting (A) and chronically contracting muscles (B) treated with the PPARα activator Wy 14,643 and the PPARγ activator rosiglitazone

Data are means ± s.e.m. n = 4 – 5 muscles for each group at each data point. *P < 0.05, chronic contraction versus day 0. **P < 0.05, chronic contraction + rosiglitazone versus day 0. ***P < 0.05, chronic contraction + Wy 14,643 versus day 0. †P < 0.05, chronic contraction + Wy 14,643 versus chronic contraction, and chronic contraction + Wy 14,643 versus chronic contraction + rosiglitazone.

Figure 4. FABPpm protein in resting (A) and chronically contracting muscles (B) treated with the PPARα activator Wy 14,643 and the PPARγ activator rosiglitazone

Data are means ± s.e.m. n = 4 – 5 muscles for each group at each data point. *P < 0.05, resting muscle + rosiglitazone versus day 0, and chronic contraction + rosiglitazone versus day 0. **P < 0.05, chronic contraction versus day 0. ***P < 0.05, chronic contraction + Wy 14,643 versus day 0. †P < 0.05, chronic contraction + rosiglitazone versus resting muscle and chronic contraction, and chronic contraction + rosiglitazone versus resting muscle + Wy 14,643 and chronic contraction + Wy 14,643.

Figure 5. Plasmalemmal FAT/CD36 (A) and FABPpm protein levels (B) and rates of palmitate transport into giant sarcolemmal vesicles (C) and in resting and chronically contracting muscles treated with the PPARα activator Wy 14,643 and the PPARγ activator rosiglitazone

Data are means ± s.e.m. n = 4 – 5 muscles for each group. *P < 0.05, resting muscle versus chronically contracting muscle (with and without Wy 14,643 and/or rosiglitazone). **P < 0.05, rosiglitazone-treated muscles versus all other treatments.