Endocannabinoid metabolism : the impact of inflammatory factors and pharmacological inhibitors

Abstract: The endocannabinoid (eCB) system is an endogenous signaling system consisting of ligands (referred to as endocannabinoids, eCBs), receptors and metabolic enzymes. The eCB system is involved in homeostatic control of a variety of biological functions such as neuronal signaling, mood, appetite and pathological conditions such as pain, inflammation and tumour progression. The main eCBs N- arachidonoylethanolamine (AEA, anandamide) and 2-arachidonoylglycerol (2-AG) are synthesised upon stimuli when and where their action is demanded. The signaling is brief and the eCBs are quickly degraded. The enzyme primarily responsible for eCB degradation is fatty acid amide hydrolase (FAAH) for AEA and monoacylglycerol lipase (MAGL) for 2-AG. In addition, both substances are substrates for cyclooxygenase-2 (COX-2). COX-2 is upregulated in inflammation, pain and in several tumours including prostate cancers, but it is not known whether COX-2 contribute significantly to eCB metabolism under these conditions.Increasing endogenous levels of eCBs by inhibiting their degradation is exploited as a future therapy for pain conditions. One suggested therapeutic strategy is dual inhibition of enzymes FAAH and COX-2 to raise AEA levels. Paper I and II of this thesis investigates FAAH and COX inhibitory effects of: the major metabolites and enantiomers of derivatives (flu-AM1 and ibu-AM5) of the current clinically used NSAIDs ibuprofen and flurbiprofen. The metabolites 3 ́hydroxyibuprofen and 4 ́hydroxyflurbiprofen retained the FAAH and COX- inhibitory effects seen by the parent compounds although at lower potencies. Both enantiomers of flu-AM1 were equally potent as FAAH inhibitors and displayed a useful substrate selective COX-2 inhibition profile, favoring eCBs as substrates rather than arachidonic acid.Paper III explores the impact of COX-2 and the effect of (R)-flu-AM1 upon AEA levels and degradation in mouse leukemic macrophage RAW264.7 cells. Despite the high inhibitory potency in enzyme assays, neither (R)-flu-AM1 nor the combination of a FAAH inhibitor with flurbiprofen increased AEA levels in the intact cells to any great extent. This suggests that the eCB turnover in these cells is rather slow. Further, in paper IV, induction of COX-2 did not unmask an ability of this enzyme to “gate” the uptake of AEA analogous to that seen with FAAH.Paper IV and V focus upon the role of the eCB system in prostate cancer. The eCB system is altered in cancer and is linked to the progression and prognosis of prostate cancer. How and whereby this change occurs is unknown. This thesis explores the impact of the inflammatory factors TNFα, IL-6 and lactic acid induced low pH upon the mRNA levels of eCB related enzymes and the functional impact upon AEA degradation in human DU145 and rat AT-1 prostate cancer cells. TNFα treatment of DU145 and IL-6 and lactic acid induced low pH exposure of AT-1 changed the mRNA levels of 2-AG related enzymes leaving AEA rather unaffected other than for a substantial induction of COX-2 mRNA in DU145 cells. Thus, AEA homeostasis was not shifted in prostate cancer cell lines exposed to inflammatory factors. The results suggest that COX-2 does not gate the uptake of AEA and is a minor contributor to AEA degradation in intact cells.