Different Invasive Behaviour of Human Uveal Melanoma Cells at Different Stages of the Metastatic Cascade

Abstract

Abstract: : Purpose: An immunosuppressed rabbit model of uveal melanoma has enabled us to study the natural progression of this disease from intraocular tumor to the establishment of metastasis. The invasive ability of cells at each progressive step has not previously been investigated. Vascular endothelial growth factor (VEGF) has previously been shown to be the most potent inducer of invasion in the uveal melanoma cell line. The aim of this study was to evaluate the degree of invasion of human uveal melanoma cells cultured from the primary ocular tumor, peripheral blood and metastatic site in an immunosuppressed rabbit model. Methods: Two New Zealand albino rabbits were immunosuppressed with 15mg/kg/day intra–muscular cyclosporin A injections for three days before 1x106 human uveal melanoma cells were injected into the suprachoroidal space. Upon sacrifice of the animals, cells were harvested from the intraocular tumour, and lung metastases. Malignant cells in the peripheral blood were isolated using the Ficoll–plaque method. Cells were re–cultured in RPMI 1640 medium supplemented with 10% fetal bovine serum, and were utilized for the invasion assay within two passages. The invasion assay was performed with a modified Boyden chamber system consisting of a membrane with 8 µm pores pre–coated with Matrigel, an artificial basement membrane. 1x10⁵ cells were seeded onto the top portion of the chamber, with VEGF as chemo–attractant at a concentration of 20 ng/ml in the bottom of the wells. Wells were incubated for 48 and 72 hours. Non–invading cells were removed from the top of the Matrigel, the membrane was stained and invading cells were counted using twenty high–power fields. All experiments were performed in triplicate. Results: At both 48 and 72 hours, the cells cultured from both intraocular tumors had a higher rate of invasion than those cultured from the peripheral blood. The cultured circulating malignant cells had a higher rate of invasion than those cultured from the metastasis. In all cases the results were statistically significant (p < 0.05). Conclusions: This in vitro experiment showed that as the human uveal melanoma cells progressed from the intraocular tumors to peripheral blood to metastases they became less invasive due to loss of response to VEGF as a chemoattractant. It is possible that these findings may correlate with changes the human uveal melanoma cells must undergo to survive in haematogenous circulation and to begin proliferating at the site of metastasis.

Keywords: melanoma • tumors