THC and CBD Induce Heme Oxygenase-1-Dependent Cell Death and Trigger Mitochondrial Dysfunction in Human Melanoma and Cutaneous Squamous Cell Carcinoma Cells

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Abstract
In the search for new therapeutic strategies for the treatment of skin cancer, cannabinoids have become the focus of scientific interest. The present study investigated the effects of the phytocannabinoids Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) on the viability, apoptosis, and mitochondrial function of human melanoma (A375) and cutaneous squamous cell carcinoma (SCC) cells (A431). Both cannabinoids caused a time- and concentration-dependent loss of viability and an upregulation of caspase-3/7 activity, associated with the induction of initiator caspases-8 and -9, PARP cleavage, and an increase in the autophagy marker LC3A/B-II. Inspired by the latest work on the dual role of heme oxygenase-1 (HO-1) in cell fate, the expression of this enzyme was examined and found to be upregulated at the mRNA and protein level by THC and CBD. Inhibition of HO-1 activity by tin protoporphyrin IX (SnPPIX) reduced the loss of viability caused by both cannabinoids, suggesting a cytotoxic rather than cytoprotective mediator role for this enzyme here. At the mitochondrial level, THC and CBD caused a reduction in membrane potential, a release of cytochrome c into the cytosol, and electron microscopically detectable mitochondrial damages. A more detailed functional analysis revealed an inhibition of mitochondrial oxygen consumption rate, accompanied by a decrease in various subunits of mitochondrial oxidative phosphorylation complexes. In conclusion, our data demonstrate a strong cytotoxic effect of THC and CBD on melanoma and cutaneous SCC cells involving mitochondrial apoptosis and mitochondrial dysfunction.

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