OBJECTIVE— Long-chain fatty acids (LCFAs) contribute to metabolic homeostasis in part via gene regulation. This study's objective was to identify novel LCFA target genes in human skeletal muscle cells (myotubes). RESEARCH DESIGN AND METHODS— In vitro methods included culture and treatment of human myotubes and C2C12 cells, gene array analysis, real-time RT-PCR, Western blotting, ELISA, chromatin immunoprecipitation, and RNA interference. Human subjects (two cohorts) were characterized by oral glucose tolerance test, hyperinsulinemic-euglycemic clamp, magnetic resonance imaging and spectroscopy, and standard blood analyses (glucose, insulin, C-peptide, and plasma lipids). RESULTS— We show here that ANGPTL4 (encoding angiopoietin-like protein 4) represents a prominent LCFA-responsive gene in human myotubes. LCFA activated peroxisome proliferator-activated receptor (PPAR)-δ, but not PPAR-α or -γ, and pharmacological activation of PPAR-δ markedly induced ANGPTL4 production and secretion. In C2C12 myocytes, knockdown of PPARD , but not of PPARG , blocked LCFA-mediated ANGPTL4 induction, and LCFA treatment resulted in PPAR-δ recruitment to the ANGPTL4 gene. In addition, pharmacological PPAR-δ activation induced LIPE (encoding hormone-sensitive lipase), and this response crucially depended on ANGPTL4, as revealed by ANGPTL4 knockdown. In a human cohort of 108 thoroughly phenotyped subjects, plasma ANGPTL4 positively correlated with fasting nonesterified fatty acids ( P = 0.0036) and adipose tissue lipolysis ( P = 0.0012). Moreover, in 38 myotube donors, plasma ANGPTL4 levels and adipose tissue lipolysis in vivo were reflected by basal myotube ANGPTL4 expression in vitro ( P = 0.02, both). CONCLUSIONS— ANGPTL4 is produced by human myotubes in response to LCFA via PPAR-δ, and muscle-derived ANGPTL4 seems to be of systemic relevance in humans.

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Trans-activator of transcription (Tat) is a cell penetrating peptide which can translocate and carry macromolecular cargoes through cell membranes. This study investigated the hypoglycemic activity of orally delivered insulin - Tat mixture in alloxan-induced diabetic mice. The mixtures of insulin and Tat at 1:1, 1:3 and 1:6 molar ratios were given orally at the insulin doses ranging from 1-200 IU/kg. The fasting blood glucose (FBG) levels were measured at initial, 1, 2, 4, 6, and 12 h after administration. At 1:3 molar ratio of the mixture and after 12 h of administration, insulin at 200 IU/kg showed the highest with prolonged hypoglycemic activity of 74.0±10.3% FBG reduction (2.18 folds of subcutaneously injected (SC) insulin). Free insulin administered orally did not show any hypoglycemic activity. The mixtures at the insulin doses of 100 and 50 IU/kg also showed potent FBG reduction of 73.8±8.2 and 71.3±16.9% at 12 h after administration (2.18 and 2.10 folds of SC insulin, respectively). After incubation with Mono-Mac-6 cells, only the -mixtures but not the free insulin showed intra-cellular insulin uptake, indicating the insulin penetration through the cell membranes via Tat. In simulated gastric fluid, the insulin content in the mixture was not found, demonstrating the degradation of insulin in the gastric environments. Insulin may be absorbed at upper gastrointestinal tract facilitated by Tat. The potent and prolonged hypoglycemic activity of insulin co-administered orally with Tat can be further developed as an effective oral insulin delivery system.