Selective fractionation of hypoxic B16 melanoma cells by density gradient centrifugation.

Cells of the mouse B16 melanoma growing in monolayer culture and as tumors were fractionated by isopycnic density centrifugation in a linear-density (1.02-1.20 g/ml) metrizamide gradient. Cultured cells concentrated into one or two distinct bands, with densities of 1.02-1.04 g/ml and 1.06-1.10 g/ml, depending on growth conditions. Cells subjected to extreme hypoxia (less than 0.02% O2) banded predominantly at the lower density, and normally-oxygenated cells banded at the higher density. Fractionated tumor cells concentrated at both densities. Compared with cells at the higher density, lower-density cells incorporated more of the hypoxic cell radiosensitizer [14C]misonidazole and less [3H]thymidine in vivo, were less clonogenic but more resistant to X-irradiation in situ, and labeled to a lesser extent with intravenously-delivered Hoechst 33342 fluorochrome, a marker for cells proximal to tumor blood vessels. Lower-density tumor cells were, therefore, enriched in non-proliferating radioresistant hypoxic cells from tumor regions remote from blood vessels.