Lipid Metabolism Gene Expression And Cellularity of Intramuscular Adipocytes Within The Longissimus Muscle of Angus- And Wagyu-Sired Cattle When Raised To A Similar Age or Body Weight

Background: This study investigates intramuscular (IM) adipocyte development and growth in the Longissimus muscle (LM) between Wagyu- and Angus-sired steers compared at a similar age and days on feed (DOF) endpoint or similar body weight (BW) endpoint by measuring IM adipocyte cell area and lipid metabolism gene expression. Methods: Angus-sired steers (AN, n=6) were compared with steers from two different Wagyu sires, selected for either growth or marbling, to be compared at a similar DOF (WA-GD, n=5 and WA-MD, n=5) in experiment 1 or BW (WA-GB, n=4 and WA-MB, n=5) in experiment 2, respectively. Results: In experiment 1, WA-MD steers had a greater percentage of IM fat in the LM compared with AN and WA-GD steers. In experiment 2, WA-MB steers had a greater percentage of IM fat in the LM compared with AN and WA-GB steers. The distribution of IM adipocyte area was unimodal at all biopsy collections, with IM adipocyte area becoming progressively larger as cattle age and BW increased (P≤0.01). Peroxisome proliferator activated receptor delta (PPARd) was upregulated earlier for WA-MD and WA-MB cattle compared with other steers at a similar age and BW (P≤0.02; treatment×biopsy interaction). An earlier upregulation of PPARd is believed to have then upregulated peroxisome proliferator activated receptor gamma (PPARg) at a lesser BW for WA-MB steers (P=0.09; treatment×biopsy interaction), while WA-MD steers had a greater (P≤0.04) overall mean PPARg expression compared with other steers. Glycerol-3-phosphate acyltransferase, lipin 1, and hormone sensitive lipase demonstrated expression patterns similar to PPARg and PPARd or CCAAT enhancer binding protein beta, which emphasizes their importance in marbling development and growth. Additionally, WA-MD and WA-MB steers often had a greater early expression of fatty acid transporters (fatty acid transport protein 1; P<0.02; treatment×biopsy interaction) and binding proteins (fatty acid binding protein 4) compared with other steers. With many lipolytic genes upregulated at harvest, acetyl-CoA carboxylase beta may be inhibiting fatty acid oxidation in the LM to allow greater IM fat accumulation.Conclusions: Cattle with a greater marbling propensity appear to upregulate adipogenesis at a lesser maturity through PPARd, PPARg, and possibly adipogenic regulating compounds in lysophosphatidic acid and diacylglycerol.