32 Depot-Specific Transcriptome and Adipocyte Metabolism in Intramuscular and Subcutaneous Adipose Tissue in Beef Cattle

Abstract

Abstract Intramuscular adipose tissue (IMAT; marbling) deposition is the primary determination of beef quality grade in the U.S. While finishing diets increase marbling, they also increase subcutaneous adipose tissue (SAT) accumulation, leading to excessive fattening and economic losses due to ‘waste fat’. Our objective was to investigate the IMAT and SAT-specific transcriptional profile and adipocyte function in vitro. A better understanding of the mechanisms underlying IMAT and SAT-specific characteristics will help to identify molecular and cellular molecules for developing targeted strategies to enhance marbling while limiting overall fatness in beef animals. A section of the Longissimus dorsi muscle from 9-11th ribs was collected to dissect IMAT and adjacent SAT (backfat) from six beef animals harvested at the Texas Tech University Meat Lab. The SAT and IMAT RNA was extracted and sequenced in an Illumina NovaSeq 6000 targeting a minimum of 30 million reads per sample. Transcriptome analysis showed more than 1,800 differently expressed genes (DEGs) in IMAT and SAT. There was a downregulation of genes associated with adipogenesis, lipolysis regulation, insulin sensitivity, and fatty acid metabolism (e.g., LEP, INSIG1, and PLIN4) in IMAT vs. SAT, while pathways of insulin resistance and ß-adrenergic signaling were activated. To elucidate whether the differences between IMAT and SAT transcriptional profiles were translated into changes in the adipocyte function, preadipocytes were isolated, cultured, and differentiated into adipocytes in vitro for 14 followed by functional analyses of adipogenic capacity, insulin sensitivity, and lipolytic responses. Statistical analysis was performed in GraphPad 9. Even though preadipocytes were more abundant in IMAT than in SAT (flow cytometry quantification of CD45-CD31- cell), IMAT preadipocyte proliferation rates were decreased compared with SAT. Additionally, SAT adipocytes were larger than IMAT. These findings suggest an enhancement in SAT adipogenesis/lipogenesis compared with IMAT. Metabolically, SAT adipocytes released greater glycerol concentrations compared with IMAT upon ß-adrenergic stimulation of lipolysis with isoproterenol and tended to have greater insulin-stimulated glucose uptake compared with IMAT. Altogether, these findings indicate that IMAT is unable to accumulate lipids as efficiently as SAT, and these differences may be underlined by a decreased preadipocyte hyperplasia, insulin resistance, and potential dysregulation of ß-adrenergic receptors, all of these similarly suggested by the transcriptome analysis. Our study revealed molecular and cellular differences between IMAT and SAT that may partly explain why it is common to observe increased SAT fatness and limited marbling beef cattle.