Palmitate-Mediated Downregulation of Peroxisome Proliferator–Activated Receptor-γ Coactivator 1α in Skeletal Muscle Cells Involves MEK1/2 and Nuclear Factor-κB Activation

Abstract

The mechanisms by which elevated levels of free fatty acids cause insulin resistance are not well understood. Previous studies have reported that insulin-resistant states are characterized by a reduction in the expression of peroxisome proliferator-activated receptor-␥ coactivator (PGC)-1, a transcriptional activator that promotes oxidative capacity in skeletal muscle cells. However, little is known about the factors responsible for reduced PGC-1 expression. The expression of PGC-1 mRNA levels was assessed in C2C12 skeletal muscle cells exposed to palmitate either in the presence or in the absence of several inhibitors to study the biochemical pathways involved. We report that exposure of C2C12 skeletal muscle cells to 0.75 mmol/l palmitate, but not oleate, reduced PGC-1␣ mRNA levels (66%; P < 0.001), whereas PGC-1␤ expression was not affected. Palmitate led to mitogen-activated protein kinase (MAPK)-extracellular signal-related kinase (ERK) 1/2 (MEK1/2) activation. In addition, pharmacological inhibition of this pathway by coincubation of the palmitate-exposed cells with the MEK1/2 inhibitors PD98059 and U0126 prevented the downregulation of PGC-1␣. Furthermore, nuclear factor-B (NF-B) activation was also involved in palmitatemediated PGC-1␣ downregulation, since the NF-B inhibitor parthenolide prevented a decrease in PGC-1␣ expression. These findings indicate that palmitate reduces PGC-1␣ expression in skeletal muscle cells through a mechanism involving MAPK-ERK and NF-B activation.

Fatty Acid Incubation of Myotubes From Humans With Type 2 Diabetes Leads to Enhanced Release of β-Oxidation Products Because of Impaired Fatty Acid Oxidation

“…In the present study, gene expression of PGC1␣ was increased in response to fatty acids, but the relative response was blunted in diabetic myotubes compared with control cells. In C2C12 myotubes, PGC1␣ expression was enhanced after 4 h but reduced after 16 h of incubation with palmitic acid (35), whereas unsaturated fatty acids like oleic acid and linoleic acid enhanced PGC1␣ expression in human myotubes (36). Our data suggest that the reduced capacity for complete mitochondrial fatty acid oxidation observed for type 2 diabetes myotubes might be related to suboptimal responses in expression of PGC1␣, e.g., when the myotubes are exposed to fatty acids.…”

Erk regulation of pyruvate dehydrogenase flux through PDK4 modulates cell proliferation

“…Additional studies are required to elucidate the specific mechanism through which Erk regulates PDK4 mRNA, the levels of which have been shown to be controlled by numerous proteins and signaling pathways (Abbot et al 2005;Sugden and Holness 2006;Roche and Hiromasa 2007). Some transcriptional regulators of PDK4, such as retinoid X receptor and FOXO1, have previously been found to be negatively regulated by Erk signaling (Arnaud et al 2004;Coll et al 2006;Asada et al 2007;Burgermeister et al 2007;Macoritto et al 2008;Shankar et al 2008;Meng et al 2011). Importantly, previous studies implicating PI3K/Akt as a negative regulator of PDK4 expression were performed in myocytes (Puthanveetil et al 2010), which must generate significant energy (i.e., ATP) from glucose in response to insulin to function properly (Latronico et al 2004).…”

Activation of Peroxisome Proliferator–Activated Receptor β/δ Inhibits Lipopolysaccharide-Induced Cytokine Production in Adipocytes by Lowering Nuclear Factor-κB Activity via Extracellular Signal–Related Kinase 1/2

“…Measurements of mRNA. Levels of mRNA were assessed by the RT-PCR as previously described (21). Total RNA was isolated using the Ultraspec reagent (Biotecx, Houston, TX).…”