Introduction: Thermogenic activity in brown adipose tissue (BAT) induces fat loss and protects against cardiometabolic disorders as BAT takes up large amounts of triglyceride (TG)-derived fatty acids (FAs) for oxidation. Interestingly, this process shows a strong day-night rhythm (Van den Berg, Cell Rep 2018). The aim of the present study was to elucidate the molecular mechanisms underlying this diurnal rhythm.

Methods: BAT was collected from chow-fed male C57BL/6J mice at 3-hour intervals throughout a 24-hour period, and RNA-sequencing, lipidomics and ChIP-sequencing were performed. Oscillation was identified by JTK cycle. Knockout and overexpression models were used to study causal relationships in diurnal lipid handling by BAT.

Results: Out of the 13,549 expressed genes and 1,941 measured lipid species, 5,486 genes and 430 lipids were oscillating. Among the oscillating genes that were in synchrony with metabolic BAT activity, we could identify pronounced enrichment of genes involved in lipolysis, accompanied by oscillations of FA and monoacylglycerol content. This coincided with peak lipoprotein lipase (Lpl) expression, encoding the enzyme responsible for the liberation of FAs from lipoprotein-derived TGs, and was predicted to be driven by PPARγ activity. ChIP-sequencing indeed confirmed oscillated binding of PPARγ to genes involved in extracellular lipolysis, including Lpl. Of the known modulators of LPL activity, angiopoietin-like 4 (Angptl4) showed the largest diurnal amplitude opposite to Lpl. Subsequent kinetic experiments revealed that the diurnal rhythm in LPL activity and TG-derived FA uptake by BAT was flattened in Angptl4 engineered mice, with persistent high uptake in Angptl4 knockout mice and low uptake in Angptl4 overexpressing mice.

Conclusion: Our findings suggests involvement of PPARγ, and a crucial role for ANGPTL4 in mediating the day-night rhythm in BAT, and imply that time of day is essential when targeting LPL activity in BAT to improve metabolic health.