METRNL decreases during adipogenesis and inhibits adipocyte differentiation leading to adipocyte hypertrophy in humans.

BACKGROUND: Meteorin-like (METRNL) is a recently described circulating protein shown to be highly expressed in white adipose tissue and to beneficially affect energy metabolism in mice. OBJECTIVE: We systematically evaluated the role of METRNL for human adipogenesis and its association with obesity, browning and hyperinsulinemia in children. In addition, we assessed the functional relevance of METRNL for human adipogenesis. RESULTS: METRNL expression decreased during human adipocyte differentiation in vitro. Coherently, METRNL expression was lower in isolated adipocytes compared with stromal vascular fraction (SVF) cells in human samples. Withdrawal of the peroxisome proliferator-activated receptor-γ (PPARγ) agonist rosiglitazone from adipogenic media partially preserved the METRNL downregulation during adipogenesis. METRNL expression was higher in adipocytes of obese compared with lean children and correlated with adipocyte size, whereas in SVF METRNL expression correlated with proliferation capacity. Concordantly, overexpression of METRNL inhibited human adipocyte differentiation as shown by decreased lipogenesis and lower expression of PPARγ and markers of adipogenesis, whereas experimental downregulation promoted adipogenesis. Proliferation, in contrast, was advanced by METRNL overexpression. These interactions with adipose tissue dynamics may contribute to the clinically observed body mass index-independent association of METRNL expression with hyperinsulinemia and adipose tissue inflammation in human samples. METRNL was not associated with UCP1 expression or induction of browning in white adipocytes. CONCLUSIONS: Taken together, the downregulation of METRNL during adipogenesis and functional induction of increased proliferation in SVF cells with concomitant inhibition of adipocyte differentiation may result in hypertrophic AT accumulation. This may also explain our observations of increased METRNL expression in adipocytes but not SVF cells in obese children compared with lean children and the subsequent hyperinsulinemia.

Adipocyte C1QTNF5 expression is BMI-dependently related to early adipose tissue dysfunction and systemic CTRP5 serum levels in obese children.

BACKGROUND/OBJECTIVES: The recently identified adipocytokine C1QTNF5 (encodes for CTRP5) has been demonstrated to inhibit pro-metabolic insulin signaling in adipocytes. We hypothesized that adipocyte C1QTNF5 expression in subcutaneous (sc) adipose tissue (AT) would correlate with the degree of obesity, systemic CTRP5 serum levels, and early AT and metabolic dysfunction in children. SUBJECTS/METHODS: Sc AT samples were obtained from 33 healthy Caucasian lean children aged 10.06±4.84 years and 42 overweight and obese children aged 13.34±3.12 years. C1QTNF5 expression in sc AT as well as in investigated cell lines was assessed by quantitative real-time PCR. Systemic CTRP5 levels were assessed by ELISA. RESULTS: C1QTNF5 expression in sc adipocytes increased with body mass index (BMI) standard deviation score (SDS; R=0.48, P<0.001), body fat percentage (R=0.4, P=0.004), adipocyte number (R=0.69, P<0.001) and systemic CTRP5 serum levels (R=0.28, P=0.025) whereas expression in the stromal vascular fraction (SVF) was inversely correlated with BMI SDS (R=-0.24, P=0.04). Multiple regression analysis confirmed that BMI SDS was the strongest independent predictor for C1QTNF5 expression in sc adipocytes. SVF C1QTNF5 levels strongly correlated with SVF CD31 expression (R=0.54, P<0.001) indicating expression by endothelial cells. Primary human endothelial cells demonstrated stronger expression compared with human Simpson-Golahbi-Behmel syndrome pre-adipocytes and adipocytes. Adipocyte C1QTNF5 expression levels were BMI-dependently related to fasting insulin (R=0.3, P=0.03) and leptin serum levels (R=0.5, P<0.001). Sc AT samples containing crown-like structures (CLS) demonstrated increased adipocyte C1QTNF5 expression compared to CLS-negative samples (P=0.03). Functionally, tumor necrosis factor (TNF)α caused a fourfold induction of C1QTNF5 in human adipocytes (P<0.001) and a 50% reduction in primary human endothelial cells (P<0.001). CONCLUSIONS: In children adipocyte C1QTNF5 expression is already strongly related to the degree of obesity and is associated with obesity-related AT alterations, systemic CTRP5 serum levels as well as circulating markers of metabolic disease and is positively regulated by TNFα in vitro.