The endocannabinoid system : a translational study from Achilles tendinosis to cyclooxygenase

Abstract: The endogenous cannabinoids anandamide (arachidonoyl ethanolamide, AEA) and 2-arachidonoyl glycerol (2-AG) exert their effect by activating cannabinoid receptors (CB). These receptors mediate a broad range of physiological functions such as beneficial effects in pain and inflammation, although little is known about the expression of CB receptors in human pain conditions. AEA and 2-AG are short- lived molecules due to their rapid cellular accumulation and metabolism. The enzymes primarily responsible for their degradation are fatty acid amide hydrolase (FAAH) for AEA and monoacylglycerol lipase (MGL) for 2-AG. Inhibition of endocannabinoid metabolism is a potential approach for drug development, and there is a need for the identification of novel compounds with inhibitory effects upon FAAH and MGL.In Paper I of this thesis, the expression of CB1 receptors in human Achilles tendon was examined. We found expression of CB1 receptors in tenocytes, blood vessel wall as well as in the perineurium of the nerve. A semi-quantitative analysis showed an increase of CB1 receptors in painful human Achilles tendinosis.In papers II and III, termination of AEA signalling was investigated via inhibition of FAAH. In Paper II, Flu-AM1, an analogue of flurbiprofen, was investigated. The compound inhibited both FAAH and the oxygenation of 2-AG by cyclooxygenase-2. In Paper III the antifungal compound ketoconazole was shown to inhibit the cellular uptake of AEA in HepG2, CaCo-2 and C6 cell lines in a manner consistent with inhibition of FAAH.The role of FAAH in gating the cellular accumulation of AEA was investigated in Paper IV. FAAH has been shown to control the concentration gradient of AEA across the plasmamembrane in RBL2H3 cells, whereas no such effect is seen in other FAAH-expressing cell lines. To determine whether this effect is assay dependent or due to intrinsic differences between the cell lines, we assayed four cell lines with different levels of FAAH expression using the same methodology. We found that the sensitivity of FAAH uptake inhibition was not dependent on the expression level of FAAH, suggesting that factors other than FAAH are important for uptake.Paper V is focused on the inhibition of MGL. Prior to this study no selective inhibitors of the enzyme had been described. Thus, we screened a number of compounds for their inhibitory effect on MGL. Troglitazone was found to be an inhibitor of MGL, although its potency was dependent upon the enzyme assay used.