Regulation of Lipolysis and Adipose Tissue Signaling during Acute Endotoxin-Induced Inflammation: A Human Randomized Crossover Trial

Background Lipolysis is accelerated during the acute phase of inflammation, a process being regulated by pro-inflammatory cytokines (e.g. TNF-alpha), stress-hormones, and insulin. The intracellular mechanisms remain elusive and we therefore measured pro-and anti-lipolytic signaling pathways in adipocytes after in vivo endotoxin exposure. Methods Eight healthy, lean, male subjects were investigated using a randomized cross over trial with two interventions: i) bolus injection of saline (Placebo) and ii) bolus injection of lipopolysaccharide endotoxin (LPS). A H-3-palmitate tracer was used to measure palmitate rate of appearance (Ra-palmitate) and indirect calorimetry was performed to measure energy expenditures and lipid oxidation rates. A subcutaneous abdominal fat biopsy was obtained during both interventions and subjected to western blotting and qPCR quantifications. Results LPS caused a mean increase in serum free fatty acids (FFA) concentrations of 90% (CI-95%: 37-142, p = 0.005), a median increase in Ra-palmitate of 117% (CI-95%: 77-166, p<0.001), a mean increase in lipid oxidation of 49% (CI-95%: 1-96, p = 0.047), and a median increase in energy expenditure of 28% (CI-95%: 16-42, p = 0.001) compared with Placebo. These effects were associated with increased phosphorylation of hormone sensitive lipase (pHSL) at ser(650) in adipose tissue (p = 0.03), a trend towards elevated pHSL at ser(552) (p = 0.09) and cAMP-dependent protein kinase A (PKA) phosphorylation of perilipin 1 (PLIN1) (p = 0.09). Phosphatase and tensin homolog (PTEN) also tended to increase (p = 0.08) while phosphorylation of Akt at Thr(308) tended to decrease ( p = 0.09) during LPS compared with Placebo. There was no difference between protein or mRNA expression of ATGL, G0S2, and CGI-58. Conclusion LPS stimulated lipolysis in adipose tissue and is associated with increased pHSL and signs of increased PLIN1 phosphorylation combined with a trend toward decreased insulin signaling. The combination of these mechanisms appear to be the driving forces behind the increased lipolysis observed in the early stages of acute inflammation and sepsis.