THE KEY ROLE OF MITOCHONDRIA IN HUMAN MELANOMA CELLS: A TARGET OF DELTA-TOCOTRIENOL ANTI-CANCER ACTIVITY AND A STEMNESS MARKER

Malignant melanoma is one of the most aggressive and treatment resistant cancers. Treatment options include surgery, chemotherapy, immunotherapy and targeted therapies. Unfortunately, the large heterogeneity of melanomas and the presence of stem cells within the tumor mass are the main causes of treatment resistance. Natural compounds have attracted great interest in cancer research for their capability to induce paraptosis, a non-canonical cell death characterized by endoplasmic reticulum (ER) dilatation, mitochondrial swelling, reactive oxygen species (ROS) production and Ca++ overload. Accumulating evidence supported the anticancer effect of tocotrienols (TTs); aim of this study was to investigate whether δ-TT might induce paraptosis in A375 and BLM human melanoma cells. We demonstrated that δ-TT induces paraptosis in melanoma cells, causing ER dilatation, mitochondria swelling and the activation of MAPKs. This effect was accompanied by an impairment of mitochondrial function, resulting in a reduction in OXPHOS levels, oxygen consumption, mitochondrial activity and energy depletion. This dysfunction leads to mitochondrial ROS production and increase in Ca++ levels. We then focused our attention on A375 stem cells. Cancer stem cells (CSCs) are a small tumor subpopulation responsible for chemotherapy resistance and they have been reported to be coupled with metabolic alterations. Melanoma SCs deriving from the A375 cell line are characterized by the expression of the ABCG2 transporter and δ-TT treatment counteracts A375 SCs growth. In this work we observed that melanoma SCs are characterized by higher mitochondrial mass, increased OXPHOS, increased PGC1α expression levels and increased mitochondrial fusion markers. We then analyzed whether δ-TT might affect stem cell mitochondria and we observed that δ-TT significantly impairs mitochondrial protein levels. Finally, preliminary data demonstrate that δ-TT affects mitochondria also in vemurafenib-resistant A375 cells. Taken together, all these data demonstrate that 1) δ-TT induces paraptotic cell death in melanoma cells, involving mitochondrial dysfunction, ROS production and Ca++ overload; 2) mitochondrial mass could be considered a novel marker to identify melanoma SCs; 3) δ-TT targets melanoma SCs affecting mitochondrial functions 4) δ-TT impairs mitochondria in vemurafenib-resistant A375 cells.