Cannabinoïde Signaling in the Skin : Therapeutic Potential of the “C(ut)annabinoid” System
Kinga Fanni Tóth, Dorottya Ádám, Tamás Bíró, and Attila Oláh
Molecules 2019, 24(5), 918;
Keywords : acne; atopic dermatitis; cannabinoid; fibrosis; hair growth; inflammation; itch; psoriasis;
skin; tumor; wound healing
1.1. The Barrier and Beyond : Novel Aspects of Cutaneous (Patho)physiology
The skin is a vital organ that fulfills multiple roles. Besides being a complex protective barrier against a wide-variety of environmental challenges [1–3], it is an active neuroendocrinoimmuno organ, which produces several hormones, plays an important role in thermoregulation, and is involved in the detection of various environmental signals, as well as in their translation/transmission to the nervous and immune systems [3–5]. Indeed, functional expression of olfactory [6,7], photo [8,9], and taste receptors [10–12 —among others—has recently been proven in different non-neuronal cells of the integumentary system. The complex protection provided by the skin is based on a fine-tuned barrier system, which includes the cutaneous physicochemical, immunological andmicrobiological barriers. The development of this complex barrier requires active and tightly regulated cooperation, and therefore appropriate Molecules 2019, 24, 918; doi:10.3390/molecules24050918 www.mdpi.com/journal/molecules communication of several cell types, including numerous “professional” immune cells (e.g., Langerhans cells, dendritic cells, macrophages, mast cells, various T cell populations), and other cell types (e.g., keratinocytes, fibroblasts, melanocytes, sebocytes, adipocytes) [1–3,13–18]. Moreover, cells of the human skin express a wide-array of pathogen- and danger-associated molecular pattern recognizing receptors, and are capable of producing several anti-microbial peptides and lipids, as well as pro- and anti-inflammatory cytokines and chemokines, by which they can initiate and regulate local immune responses [1,2,4,16–25]. Obviously, these interactions are under the tight control of several signaling systems, among which the current review aims to focus on a remarkably multifaceted one, namely the cutaneous cannabinoid (“c[ut]annabinoid”) system.
1.2. (Endo)cannabinoid Signaling and its most Important Interactions
The endocannabinoid system (ECS) is a complex, evolutionarily conserved [26–30] homeostatic signaling network. It comprises endogenous ligands (endocannabinoids [eCB], e.g., anandamide [AEA]), eCB-responsive receptors (e.g., CB1 and CB2 cannabinoid receptors), and a complex enzyme and transporter apparatus. These molecules are involved in the synthesis (e.g., N-acyl phosphatidylethanolamine-specific phospholipase D [NAPE-PLD], diacylglycerol lipase [DAGL]- and -, protein tyrosine phosphatase non-receptor type 22 [PTPN22]), cellular uptake and release (i.e., the putative endocannabinoid membrane transporter(s) [EMT]), inter- and intracellular transport (e.g., fatty acid binding proteins), and degradation (e.g., fatty acid amide hydrolase [FAAH], monoacylglycerol lipase [MAGL]) of eCBs (Figure 1) [31–50]. Importantly, depending on the definition, several other endogenous molecules can be classified as “cannabinoid-like” or “cannabinoid-related” (e.g., palmitoylethanolamine [PEA], oleoylethanolamide [OEA]) beyond the “classical” eCBs [31–47,51]. Besides eCBs and related endogenous mediators, the Cannabinaceae-derived “classical” (e.g., the psychotropic ()-trans-D9-tetrahydrocannabinol [THC] or the non-psychotropic ()-cannabidiol [CBD]) and other plants-derived “non-classical” (e.g., the CB2-selective agonist -caryophyllene, or the liverwort-derived ()-cis-perrottetinene [()-cis-PET]) phytocannabinoids (pCBs) represent another important, and ever growing group of cannabinoids [31–47,52]. To date, more than 500 biologically active components were identified in the plants of the Cannabis genus, among which more than 100 were classified as pCBs. Moreover, as mentioned above, several other plants were already shown to produce molecules with cannabinoid activity [30,32,47,52]. It is suggested that consumption of cannabimimetic food components might have played a role in hominid evolution, and production of cannabimimetic food seems to be a promising future nutraceutical strategy .
Depending on their concentration, eCBs and pCBs are able to activate/antagonize/inhibit a remarkably wide-variety of cellular targets including several metabotropic (e.g., CB1 or CB2), ionotropic (certain transient receptor potential [TRP] ion channels) and nuclear (peroxisome proliferator-activated receptors [PPARs]) receptors, various enzymes, and transporters [31–47,53–56] (Figure 1). Importantly, each ligand can be characterized by a unique, molecular fingerprint, and in some cases, they can even exert opposing biological actions on the same target molecule (Figure 2a). Indeed, it was nicely shown in several biochemical studies that THC was a partial CB1 agonist, whereas CBD was an antagonist/inverse agonist of the receptor . Keeping this in mind it is easy to understand why CBD is co-administered with THC in the oromucosal spray Sativex®, where the intent is to prevent the onset of potential psychotropic side effects rooting from the THC-induced activation of CB1 expressed in the central nervous system . Intriguingly, despite solid experimental and clinical evidence proving that CBD is able to antagonize CB1, it is very important to emphasize that it can context-dependently behave as a functional CB1 activator as well. Indeed, by inhibiting FAAH and/or EMT, its administration can lead to an elevation of the local eCB-tone, and hence to an indirectly increased CB1 activity in certain systems [59,60].