Abstract
Purpose: :
Uveal melanoma (UM) cells with increased metastatic capability have an undifferentiated molecular signature indicative of a more primitive cellular phenotype. Given mounting evidence for the existence of cancer stem cells (CSC), which are able to self renew, differentiate into diverse progeny and drive continuous growth, we hypothesise that a tumorigenic stem cell-like population exists in UM. Studies of this kind, however, are hampered by problems associated with the long term culture of primary human tissues. We investigated whether, similar to other cancer cell lines, UM cell lines retain a population of self-renewing tumorigenic cells and could serve as models to study CSC.
Methods: :
UM cell lines from the primary tumour (Mel270) and liver metastasis (Omm2.5) of the same patient were used. Single cell serial colony forming assays were performed to assess self renewal. The expression of differentiation and lineage markers were examined in the different types of colony units formed by immunofluorescence. Mel270 cells were also treated with cisplatin to test selective resistance to chemotherapy.
Results: :
Mel270 and Omm2.5 cells exhibited distinct clonal morphologies akin to holoclones (primitive), meroclones (intermediate) and paraclones (differentiated). Mel270 and Omm2.5 holoclones were large colonies of tightly packed small cells with low expression levels of MelanA/HMB45 and high levels of MITF; paraclones were small colonies of larger, flatter cells that expressed high levels of MelanA/HMB45. Vimentin staining was strong in paraclones but was only observed in the peripheral cells of holoclones. Mel270 holoclones could be serially passaged (>10 generations) to produce colonies of all 3 types. Mel270 paraclones, however, could be passaged only once to produce further paraclones. Mel270 cells remaining in cisplatin-treated cultures were able to form colonies of all 3 types with a significantly higher proportion of holoclones formed compared with control cultures (45±4% vs 36±3%; p<0.02).
Conclusions: :
These data suggest the presence of CSC-like populations in cell lines from both the primary tumour and metastatic lesion and validate their use as models to identify novel therapeutics aimed at eradicating CSC.
Keywords: tumors • differentiation • cell survival