CTRP13 ablation improves systemic glucose and lipid metabolism

OBJECTIVE: Tissue crosstalk mediated by secreted hormones underlies the integrative control of metabolism. We previously showed that CTRP13/C1QL3, a secreted protein of the C1q family, can improve glucose metabolism and insulin action in vitro and reduce food intake and body weight in mice when centrally delivered. A role for CTRP13 in regulating insulin secretion in isolated islets has also been demonstrated. It remains unclear, however, whether the effects of CTRP13 on cultured cells and in mice reflect the physiological function of the protein. Here, we use a loss-of-function mouse model to address whether CTRP13 is required for metabolic homeostasis. METHODS: WT and Ctrp13 knockout (KO) mice fed a standard chow or a high-fat diet were subjected to comprehensive metabolic phenotyping. Transcriptomic analyses were carried out on visceral and subcutaneous fat, liver, and skeletal muscle to identify pathways altered by CTRP13 deficiency. RNA-seq data was further integrated with the Metabolic Syndrome in Man (METSIM) cohort data. Adjusted regression analysis was used to demonstrate that genetic variation of CTRP13 expression accounts for a significant proportion of variance between differentially expressed genes (DEGs) in adipose tissue and metabolic traits in humans. RESULTS: Contrary to expectation, chow-fed Ctrp13-KO male mice had elevated physical activity, lower body weight, and improved lipid handling. On a high-fat diet (HFD), Ctrp13-KO mice of either sex were consistently more active and leaner. Loss of CTRP13 reduced hepatic glucose output and improved glucose tolerance, insulin sensitivity, and triglyceride clearance, though with notable sex differences. Consistent with the lean phenotype, transcriptomic analyses revealed a lower inflammatory profile in visceral fat and liver. Reduced hepatic steatosis was correlated with the suppression of lipid synthesis and enhanced lipid catabolism gene expression. Visceral fat had the largest number of DEGs and mediation analyses on the human orthologs of the DEGs suggested the potential causal contribution of CTRP13 to human metabolic syndrome. CONCLUSIONS: Our results suggest that CTRP13 is a negative metabolic regulator, and its deficiency improves systemic metabolic profiles. Our data also suggest the reduction in circulating human CTRP13 levels seen in obesity and diabetes may reflect a compensatory physiologic response to counteract insulin resistance.