Modulation of Fatty Acid-Induced Skeletal Muscle Inflammation by the Peripheral Endocannabinoid System.
The endocannabinoid system (ECS) has classically been known as a major neuromodulatory system of the central nervous system, in which endocannabinoids act as retrograde neurotransmitters that inhibit synaptic activity. However, recent evidence has indicated that the ECS may also play a key role in the control of fuel and energy metabolism through its ability to modulate signalling events within peripheral tissues. There is, for example, mounting evidence that suggests that the ECS becomes dysregulated during obesity and that its pathological over-activation in tissues such as skeletal muscle may promote insulin resistance. There is also growing appreciation that the ECS may modulate the activity of other lipid-regulated signalling systems, which may arise as a consequence of endocannabinoids sharing structural attributes with their congeners that include, for example, fatty acids and their derivatives such as the eicosanoids and prostamides. In this regard, it has long been known that sustained increases in the circulating concentration of fatty acids, especially saturated fatty acids (SFAs), can deleteriously affect signalling processes that impact upon energy balance and fuel metabolism in tissues such as skeletal muscle. In particular, the chronic over-supply of SFAs (e.g. palmitate) has been shown to promote insulin resistance and induce proinflammatory NFκB signalling in skeletal muscle. However, whilst endocannabinoids, acting via specific cell surface cannabinoid (CB1 and CB2) receptors, have been shown to suppress insulin signalling in muscle it remains unknown whether they also stimulate or have the capacity to augment palmitate-induced proinflammatory signaling. Consequently, the aim of this project was to determine what impact endocannabinoids have upon fatty acid-induced NFB signalling in muscle cells and to establish whether cannabinoid receptors have any significant role to play in this process. To address this issue, the effects of the endogenous endocannabinoid ligands anandamide (AEA) and 2-arachadinoylglycerol (2-AG) on palmitate-induced NFκB-signalling and gene expression were investigated in L6 rat skeletal muscle cells. The findings reveal that whilst AEA and 2-AG did not induce NFκB signaling they were able to attenuate palmitate’s ability to promote activation of this pathway as judged by the increased cellular retention of IκBα and suppressed expression of NFκB-target genes that encode the cytokines IL6 and CINC1. Intriguingly, the ability of AEA and 2-AG to mitigate the pro-inflammatory effects of palmitate in muscle cells is unlikely to have been mediated via their target cannabinoid (CB) receptors. Unlike AEA or 2-AG, the application of pharmacological agonists targeting CB1 or CB2 was without effect on palmitate’s proinflammtory action. AEA and 2-AG have been reported to also interact with non-cannabinoid receptors (e.g. TRPV1), whose activation by these EC ligands cannot be excluded as a possible explanation for the anti-inflammatory effect that these lipid-derived molecules exhibit. The likelihood that AEA and 2-AG may have reduced palmitate’s proinflammatory potential by suppressing its uptake into muscle cells or alternatively increasing its partitioning into neutral lipid was not supported by studies testing these possibilities. Consequently, further work is needed to fully understand the underlying mechanisms by which these endocannabinoids attenuate palmitate-induced NFB signalling. Such work will not only advance our understanding of ECS function in muscle, but may potentially uncover new therapeutic targets for the treatment of low-grade tissue inflammation that is commonly seen during obesity.
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