GPR120 Gene expression and activity in subcutaneous and intramuscular adipose tissues of Angus crossbred steers.

We hypothesized that media long-chain fatty acids (LCFA) would more greatly depress cyclic adenosine monophosphate (cAMP), glycerol, and free fatty acid (FFA) concentrations in subcutaneous (s.c.) adipose tissue than in intramuscular (i.m.) adipose tissue via G protein-coupled receptor 120 (GPR120). The GPR120 receptor binds to LCFA, which reduces cAMP production, thereby causing a depression in lipolysis. Fresh ex vivo explants of s.c. and i.m. adipose tissue from the fifth to eighth longissimus thoracic rib muscle section of 8, 22-mo-old Angus crossbred steers were transferred immediately to 6-well culture plates containing 3 mL of Krebs-Henseleit buffer/Hepes/5 mM glucose. Samples were preincubated with 0.5 mM theophylline plus 10 µM forskolin for 30 min, after which increasing concentrations of acetate or propionate (volatile fatty acids, VFA) (0, 1, 5, and 10 mM) in the absence or presence of 100 µM oleate (18:1n-9) or 100 µM palmitate (16:0) (LCFA) were added to the incubation media and incubated an additional 30 min. Main effects of adipose tissue depot (i.m. vs. s.c) and VFA (acetate vs. propionate) for adipose tissue concentrations of forskolin-stimulated cAMP were P = 0.747 and P = 0.106, respectively. The addition of LCFA to the media depressed adipose tissue concentrations of cAMP (P = 0.006) (LCFA main effects). The Tissue × VFA × LCFA interaction was not significant for any dependent variable (P ≥ 0.872). Therefore, concentrations of cAMP, glycerol, and FFA were analyzed separately for i.m. and s.c. adipose tissue by split-plot analysis. Concentrations of cAMP, glycerol, or FFA in i.m. and s.c. adipose tissue were not affected by increasing concentrations of VFA (P ≥ 0.497). Media LCFA had no effect on i.m. adipose tissue cAMP (P = 0.570) or glycerol (P = 0.470) but depressed i.m. adipose tissue FFA (P < 0.001). In s.c. adipose tissue, LCFA decreased concentrations of cAMP (P = 0.042) and glycerol (P = 0.038), but increased FFA concentration (P = 0.026). Expression of GPR120 (P = 0.804) and stearoyl-CoA desaturase (P = 0.538) was not different between s.c. adipose tissue and i.m. adipose tissue. The binding of VFA to the GPR43 receptor depresses cAMP production, thereby attenuating lipolysis, but GPR43 mRNA was undetectable in those adipose tissue samples. These results provide evidence for functional GPR120 receptors in s.c. adipose tissue but question the role of GPR43 in the accumulation of adipose tissue lipids in growing steers.

We measured the mRNA abundance and activity of the fatty acid receptor, G protein-coupled receptor 120 (GPR120) in bovine subcutaneous and intramuscular (marbling) adipose tissue. The GPR120 receptor binds to long-chain fatty acids, which reduces cyclic adenosine monophosphate (cAMP) production, thereby decreasing lipolysis. The mRNA amount of GPR120 was similar between subcutaneous and intramuscular adipose tissues. In subcutaneous and intramuscular adipose tissue incubated in vitro, the fatty acids oleic acid and palmitic acid (the most abundant fatty acids in bovine adipose tissue) strongly depressed the production of cAMP and glycerol in subcutaneous adipose tissue and decreased the concentration of free fatty acids in intramuscular adipose tissue (all measured with commercial kits). This indicates that elevations in adipose tissue or plasma fatty acids may promote fat accumulation by decreasing the breakdown of stored lipids via GPR120. The volatile fatty acids acetate and propionate, which bind to G protein-coupled receptor 43 (GPR43) had no effect on cAMP, glycerol, or free fatty acids. This questions the role of GPR43 in the accumulation of adipose tissue lipids in growing steers.