Mitochondrial energy metabolism is negatively regulated by cannabinoid receptor 1 in intact human epidermis

Epidermal energy metabolism is relevant to skin physiology, ageing and photodamage. While selected hormones stimulate epidermal keratinocyte mitochondrial activity, its negative regulation remains unknown. In several cell types, cannabinoid receptor 1 (CB₁) is expressed both on the cell membrane (cmCB₁) and on the mitochondrial outer membrane (mtCB₁), where its stimulation directly suppresses mitochondrial functions. In the current pilot study, we investigated if CB₁ is a negative regulator of human epidermal energy metabolism under physiological conditions. Using organ-cultured full-thickness human skin specimens of healthy individuals, we showed that antagonizing the homeostatic CB₁ signalling by the administration of the CB₁ inverse agonist AM251 increased respiratory chain complex I and II/IV activity. The effect was CB₁-dependent, since the CB₁-selective agonist arachidonyl-2'-chloroethylamide could prevent the effect. Moreover, the phenomenon was also reproduced by siRNA-mediated down-regulation of CB₁. As revealed by the unaltered expression of several relevant markers (TFAM, VDAC1, MTCO1 and NDUFS4), modulation of CB₁ signalling had no effect on the epidermal mitochondrial mass. Next, by using immunoelectron microscopy, we found that human epidermal keratinocytes express both cmCB₁ and mtCB₁. Finally, by using equipotent extracellularly restricted (hemopressin) as well as cell-permeable (AM251) inverse agonists, we found that mitochondrial activity is most likely exclusively regulated by mtCB₁. Thus, our data identify mtCB₁ as a novel negative regulator of keratinocyte mitochondrial activity in intact human epidermis, and raise the question, whether topical therapeutic interventions capable of selectively activating mtCB₁ can reduce excessive mitochondrial ROS production resulting from dysregulated mitochondrial activity during skin ageing or photodamage.