JNK Deficiency Enhances Fatty Acid Utilization and Diverts Glucose From Oxidation to Glycogen Storage in Cultured Myotubes

Abstract

Although germ‐line deletion of c‐Jun NH2‐terminal kinase (JNK) improves overall insulin sensitivity in mice, those studies could not reveal the underlying molecular mechanism and the tissue site(s) in which reduced JNK activity elicits the observed phenotype. Given its importance in nonesterified fatty acids (NEFA) and glucose utilization, we hypothesized that the insulin‐sensitive phenotype associated with Jnk deletion originates from loss of JNK function in skeletal muscle. Short hairpin RNA (shRNA)–mediated gene silencing was used to identify the functions of JNK subtypes in regulating energy metabolism and metabolic responses to elevated concentrations of NEFA in C2C12 myotubes, a cellular model of skeletal muscle. We show for the first time that cellular JNK2‐ and JNK1/JNK2‐deficiency divert glucose from oxidation to glycogenesis due to increased glycogen synthase (GS) activity and induction of Pdk4. We further show that JNK2‐ and JNK1/JNK2‐deficiency profoundly increase cellular NEFA oxidation, and their conversion to phospholipids and triglyceride. The increased NEFA utilization was coupled to increased expressions of selective NEFA handling genes including Cd36, Acsl4, and Chka, and enhanced palmitic acid (PA)‐dependent suppression of acetyl‐CoA carboxylase (Acc). In JNK‐intact cells, PA inhibited insulin signaling and glycogenesis. Although silencing Jnk1 and/or Jnk2 prevented PA‐induced inhibition of insulin signaling, it did not completely block decreased insulin‐mediated glycogenesis, thus indicating JNK‐independent pathways in the suppression of glycogenesis by PA. Muscle‐specific inhibition of JNK2 (or total JNK) improves the capacity of NEFA utilization and glycogenesis, and is a potential therapeutic target for improving systemic insulin sensitivity in type 2 diabetes (T2D).

JNK regulation of hepatic manifestations of the metabolic syndrome

“…This finding suggests the possibility that promotion of steatosis by JNK1 is mediated through an effect other than that on insulin resistance, as loss of JNK2 failed to affect lipid accumulation despite significantly improving insulin sensitivity. The absence of an effect on hepatic lipid accumulation in vivo is in contrast to findings in adipocytes and muscle cells in which JNK2 inhibition increased β-oxidation and lipolysis, respectively [52,53]. This discrepancy may reflect an artifact of the in vitro studies in the non-hepatic cells or the tissue-specific nature of JNK regulation of lipid metabolism.…”

“…An adenovirus-mediated knockdown of JNK1 in mouse liver not only improved insulin sensitivity but also upregulated genes involved in glycolysis, triglyceride secretion and β-oxidation [51]. Recent studies have also demonstrated that blocking JNK function increases fatty acid oxidation in muscle [52], so the possibility that JNK inhibition has a beneficial effect on steatosis by increasing fatty acid utilization in the liver or other organs needs further investigation.…”

α‐Lipoic acid up‐regulates expression of peroxisome proliferator‐activated receptor b in skeletal muscle: involvement of the JNK signaling pathway

“…The next step of our study was to investigate how α‐LA could up‐regulate PPARβ expression. Because it was known that α‐LA could affect the JNK pathway (12) and that expression regulation of genes involved in fatty acid metabolism depends on both the activity of JNK and PPARβ pathway (13), we decided to examine the activity of the JNK pathway. Based on Western blot analysis, the JNK pathway appears to be activated in C2C12 cells in basal conditions ( Fig.…”

“…More specifically, α‐LA has been shown to lower in C2C12 myotubes the activation of the JNK pathway (12). It is noteworthy that this pathway is involved in the down‐regulation of genes that regulate fatty acid oxidation (13). Moreover, transcription of fatty acid oxidation genes is known to be controlled by the peroxisome proliferator‐activated receptor (PPAR)b, a transcription factor highly expressed in the skeletal muscle of physically trained individuals and rodents (14).…”

cJun NH2-terminal kinase 1 (JNK1): roles in metabolic regulation of insulin resistance

“…In obese mice, muscle could therefore be a site of JNK1 function, regulating insulin sensitivity and glucose homeostasis. Indeed, shRNA-mediated knock-down of JNK in cultured myotubes causes increased fatty acid oxidation and increased conversion of glucose to the storage form glycogen [60]. …”