Inhibition of thermogenic pathways: a new target for paracetamol?

S. Bashir, W. A. Morgan. SCHOOL OF HEALTH, SPORT AND BIOSCIENCE, UNIVERSITY OF EAST LONDON, LONDON, United Kingdom.

Paracetamol has been known to reduce core body temperature of both febrile and non-febrile animals by a mechanism that is not fully understood. Historically the actions of paracetamol were attributed to the inhibition of central cyclo-oxygenase enzymes however given the weak inhibitory effects on COX-1 and COX-2 enzymes alternative targets have been sought including a possible novel COX-3 (1-2). Both reduction of pyresis and induction of hypothermia occur only when metabolic rate decreases and/or heat loss increases. In an attempt to determine if the hypothermic properties of paracetamol could be attributed to the impact on thermogenic pathways, studies were undertaken to assess the effect of paracetamol and two putative COX-3 inhibitors aminophenazone and phenazone on both 3T3-L1 and brown adipose tissue (BAT) mediated lipolysis, fatty acid oxidation (FAO), mitochondrial oxygen consumption rate (OCR) in both 3T3-L1 mouse adipocytes and isolated mitochondria. For lipolysis assay, 3T3-L1 adipocytes were treated with paracetamol and different lipolytic agents, primary brown adipocytes were isolated from male Wistar rats and treated with paracetamol and other agents and glycerol release was measured. Effect of paracetamol, aminophenazone and phenzaone (1-10 mM) on basal and stimulated OCR in 3T3-L1 adipocytes was examined using XFp Analyser. Effect of paracetamol on FAO using XFp FAO assay and on mitochondrial function using electron flow assay was then examined. The results were analysed using the analysis of variance (ANOVA), followed by Dunnett\'s or Bonferroni\'s Multiple Comparison Test. A *P < 0.05, ** P<0.01, *** P<0.001 was considered statistically significant. For lipolysis experiments, n=4 whereas for XFp experiments, n=3, expressed as means ± Standard deviations. Paracetamol caused a significant decrease both in basal and stimulated lipolysis at 1, 3 and 24 hours. Paracetamol and other agents significantly decreased glycerol release. In FAO assay, paracetamol significantly reduced basal OCR and after Oligomycin, FCCP and rotenone/Antimycin A. Paracetamol and other agents caused a significant (up to 50%) inhibition of OCR in 3T3-L1 cells without affecting cell viability. Paracetamol was also able to inhibit OCR similar to known ETC inhibitors. The failure to find a central target for the hypothermic effects of paracetamol and the ability of these hypothermic agents to inhibit lipolysis in adipocytes, FAO, mitochondrial OCR without a loss of cellular viability suggest that disruption of thermogenic pathways could partly explain the hypothermic properties of these compounds.

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