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Mucosal melanoma is a rare malignant and highly aggressive tumor arising from pigmented cells, known as melanocytes. Melanosomes, organelles synthesized by melanocytes and implicated in melanin synthesis move along cell dendrites to transfer melanin pigments to the neighbouring keratinocytes. Melanin production has been described as a protective mechanism activated by melanocytes also against radiation damage, increasing radioresistance to conventional radiotherapy and suggesting C-ions as therapeutic option to overcome this intrinsic radioresistance. A very important mechanism that contributes to melanogenesis is the Ca²⁺ signaling pathway, the alterations of which are known to play a role in tumorogenesis.

Although there are a few studies that have reported an increase in dendrite formation and elongation in melanoma cells, they mainly concern cutaneous melanomas and describe the effects after exposure to UV irradiation, while the impact of high-LET irradiation on these features of mucosal melanoma, remains unexplored.

The purpose of our study was to investigate for the first time the effects of different doses of C-ions on the pigment appearance and dendrite formation of human mucosal melanoma cells of the vagina in relation to Ca²⁺ signaling alterations.

HMV-II cells were exposed to 0, 2, 4, 6, 8 and 10Gy of C-ions and the length of their dendrites was measured up to 48 hours after irradiation, while melanin production was estimated by quantification of brownish cells as well as by the qualitative evaluation of their pellets, up to 20 days after the exposure. To examine the effect of C-ions on Ca²⁺ signaling, the pyrazole derivative BTP-2, which selectively blocks the Store-Operated-Ca²⁺-Entry (SOCE) channel, was added in the medium of the cells after their exposure to 0, 2 and 4Gy. 

   Melanocyte dendricity and average dendrite length increase progressively after carbon ion radiation exposure. More specifically, already 24 hours after the irradiation, all the radiation doses induce a statistically significant elongation of dendrites in a dose-dependent way, which further increases at 48 hours. Regarding melanin synthesis, the quantification of cells presenting melanin pigments shows that there is an increase in their number, which is also supported by the qualification of pellets. Interestingly, the addition of BTP-2 leads to a decrease both in dendrite length and melanin production by the cells.

   In conclusion, taken together our results demonstrate that there is a correlation between radiation dose and vaginal mucosal melanoma cells activation in terms of dendricity and melanin synthesis, while they also highlight the involvement of Ca²⁺ signaling in this activation.