The Rab GTPase family has been increasingly related to carcinogenesis and cancer biology during the past decade. In melanoma, Rab27a has been identified as a tumour dependency gene. Rab27a is thought to regulate exosome secretion in several cell types, however the exact role of Rab27a in melanoma biology and the underlying mechanisms are not well investigated.
To investigate the influence of Rab27a on melanoma biology and exosome secretion, Rab27a shRNA knockdown (KD) or CRISPR knockout (KO) was performed in human and murine melanoma cell lines and cell invasion and motility was analysed by in vitro 3D spheroid invasion and live cell imaging. The effect of Rab27a loss on metastasis was investigated using a mouse melanoma spontaneous metastasis model. Exosomes secreted by melanoma cell lines with/without Rab27a KD/KO were purified and characterised by electron microscopy, NanoSight, BCA assay, western blotting and mass spectrometry.
Our results indicate that Rab27a loss in Rab27a high metastatic melanoma cell lines reduced 3D spheroid invasion and cell motility and Rab27a loss also reduced spontaneous melanoma metastasis in vivo. Rab27a KD invasion phenotype can be partially rescued by addition of exosomes from Rab27a replete cell conditioned media, but not exosomes derived from Rab27a KD cells. Loss of Rab27a does not alter the number of exosomes secreted from melanoma cells, but does alter exosome protein composition and morphology. In summary, Rab27a promotes the invasion and metastasis of a subset of melanoma cells via the regulation of pro-invasive exosomes, which indicates Rab27a as a potential therapeutic target for preventing melanoma progression.