Melanoma is one of the most aggressive and treatment-resistant human cancers. It has been shown that the ion channel TPC2, activated by the second messenger NAADP and typically found on the membrane of acidic organelles, is present in melanosomes and involved in pigmentation. In addition, our laboratory has demonstrated NAADP/TPC2/Ca2+signalling is involved in melanoma progression and metastasis. To date, it is well known that the microenvironment has a key role in cancer progression. It was shown very recently that melanosomes release pro-metastatic factors. It is therefore important to understand the mechanism of melanosome release. Rab proteins have an important role in intracellular trafficking. In particular some of them are associated to melanosomes such as Rab 7 and Rab 27A. We have observed that melanin release in the B16-F0 melanoma cell line is markedly reduced by the TPC2 inhibitor Naringenin. In addition in this murine melanoma cell line we observed TPC2/Rab 7 and TPC2/Rab27 co-localization. These data suggest that TPC2 might be a novel therapeutic target in melanoma.
To describe TPC2 role in melanosomes release, we produced a stable TPC2 knockout B16-F0 cell line, using CRISPR-Cas9 technique.
Further to understand exosomes and melanosomes release mechanism in cancer cells which still remains a fundament goal in the research she is analyzing the cross-talk between TPC2 and Rabs. Rab proteins are small GTPases and function as regulators of vesicles transport, protein trafficking, membrane targeting and fusion (Tzeng and Wang 2016). Rab 27a controls peripheral trafficking and binds melanosomes; Rab7 is present and cycling on early and intermediate stage melanosomes when Rab27a is not present (Jordens I. et al 2006). We showed in B16-F0 melanoma cell line after TPC-2 over-expression, a common spatial localization between Rab 7 and TPC2 and Rab 27a and TPC2, in addition TPC2 pharmacological inhibition with Naringenin increase Rab 27a protein levels after 24h treatment, without changing Rab 7 levels.
These data strongly indicate that TPC2 is involved in melanosome release, which is important for the creation of tumor microenvironment. So TPC2 is running to be a possible new target for melanoma cells and that Naringenin might be a promising new tool for the control of melanosome progression.