Introduction: Insulin-sensitive tissues such as liver and skeletal muscle modify their gene expression under conditions of obesity-induced insulin resistance, and some of these gene products may be released to maintain glucose homeostasis. This study aimed to identify mRNAs encoding liver and skeletal muscle peptides that have the potential to regulate β-cell function by binding to islet GPCRs, and to quantify changes in expression of these liver and muscle mRNAs in obese mice.
Methods: Livers and gastrocnemius muscles were extracted from C57BL/6 mice fed either a chow diet (CD:3% fat/4% sugar) or high-fat-high-sugar diet (HFHSD:20% fat/10% sugar) for 34 weeks and qPCR was used to quantify mRNAs encoding previously validated islet GPCR peptide ligands.
Results: GTTS and ITTs demonstrated overt insulin resistance on a HFHSD (GTT:CD 0 min(9.12±0.97 mmol/l glucose), 30 min(14.25±1.71), 120 min(9.35±1.24); HFHSD 0 min(9.46±0.81), 30 min(29.42±1.34), 120 min(12.54±1.74), P<0.001, n=4; ITT: CD 0 min(8.92±1.25 mmol/l glucose), 30 min(6.07±1.14), 60 min(6.57±1.07); HFHSD 0 min(9.18±0.22), 30 min(9.94±1.25), 60 min(10.34±0.52), P<0.05, n=4). Our qPCR screening demonstrated that 107 and 76 islet GPCR ligand mRNAs were expressed above trace levels in hepatocytes and myocytes respectively. Several of these mRNAs showed altered expression in CD and HFHSD hepatocytes and myocytes. In particular, Col3a1 and Cort mRNAs were up-regulated (control:1; upregulation(>1): 3.81±0.61- and 11.49±3.65-fold) in insulin resistant hepatocytes and myocytes, respectively, while other peptide mRNAs, including Pyy and Wnt5b, were down-regulated (downregulation: (<1): 0.006±0.001- and 0.46±0.12-fold) in HFHSD liver and muscle, respectively.
Discussion: This focused approach of GPCR ligand expressome screening has the potential to define novel cross-talk from hepatocytes and myocytes to islet β-cell GPCRs, with the future aim that such liver- and skeletal muscle-derived peptide ligands (or stable derivatives) may be developed as new therapies for diabetes.