Delineating three-dimensional behavior of uveal melanoma cells under anchorage independent or dependent conditions

Background: Although rare, uveal melanoma (UM) is a life-threatening malignancy. Understanding its biology is necessary to improve disease outcome. Three-dimensional (3D) in vitro culture methods have emerged as tools that incorporate physical and spatial cues that better mimic tumor biology and in turn deliver more predictive preclinical data. Herein, we comprehensively characterize UM cells under different 3D culture settings as a suitable model to study tumor cell behavior and therapeutic intervention.

Methods: Six UM cell lines were tested in two-dimensional (2D) and 3D-culture conditions. For 3D cultures, we used anchorage-dependent (AD) methods where cells were embedded or seeded on top of basement membrane extracts and anchorage-free (AF) methods where cells were seeded on agarose pre-coated plates, ultra-low attachment plates, and on hanging drops, with or without methylcellulose. Cultures were analyzed for multicellular tumor structures (MCTs) development by phase contrast and confocal imaging, and cell wellbeing was assessed based on viability, membrane integrity, vitality, apoptotic features, and DNA synthesis. Vascular endothelial growth factor (VEGF) production was evaluated under hypoxic conditions for cell function analysis.

Results: UM cells cultured following anchorage-free methods developed MCTs shaped as spheres. Regardless of their sizes and degree of compaction, these spheres displayed an outer ring of viable and proliferating cells, and a core with less proliferating and apoptotic cells. In contrast, UM cells maintained under anchorage-dependent conditions established several morphological adaptations. Some remained isolated and rounded, formed multi-size irregular aggregates, or adopted a 2D-like flat appearance. These cells invariably conserved their metabolic activity and conserved melanocytic markers (i.e., expression of Melan A/Mart-1 and HMB45). Notably, under hypoxia, cells maintained under 3D conditions secrete more VEGF compared to cells cultured under 2D conditions.

Conclusions: Under an anchorage-free environment, UM cells form sphere-like MCTs that acquire attributes reminiscent of abnormal vascularized solid tumors. UM cells behavior in anchorage-dependent manner exposed diverse cells populations in response to cues from an enriched extracellular matrix proteins (ECM) environment, highlighting the plasticity of UM cells. This study provides a 3D cell culture platform that is more predictive of the biology of UM. The integration of such platforms to explore mechanisms of ECM-mediated tumor resistance, metastatic abilities, and to test novel therapeutics (i.e., anti-angiogenics and immunomodulators) would benefit UM care.