Metabolic cooperation between mouse mammary tumor subpopulations in three-dimensional collagen gel cultures

A series of subpopulation lines derived from a single mouse mammary tumor were tested for their ability to interact with each other in metabolic cooperation assays in three-dimensional collagen gel cultures. Inhibition of growth in the presence of selective drug (6-thioguanine or 2-fluoroadenine) of two 6-thioguanine-resistant cell lines, 66cl4 and 44FTO, as well as a 2-fluoroadenine-resistant line, 168FAR, occurred in the presence of other sensitive cells, demonstrating the transfer of drug sensitivity to drug-resistant cells. Transfer of drug resistance was shown by using selective medium containing hypoxanthine, aminopterin, and thymidine plus ouabain in which mutual metabolic cooperation between line 66cl4 (also ouabain resistant) and "wild-type" lines is required for growth. Effective communication leading to the transfer of drug resistance and ultimate survival of both cell types occurred in mixtures containing as little as 10% of either cell type. The requirement for mutual metabolic cooperation in hypoxanthine, aminopterin, and thymidine plus ouabain medium prevented line 66 from overgrowing line 66cl4 to the extent that it did in control medium. The ranked abilities of lines 66, 410.4, and 168 to promote growth when mixed with line 66cl4 in collagen cultures in hypoxanthine, aminopterin, and thymidine plus ouabain medium were correlated with the ranked cell densities required for these same cell lines to inhibit growth of line 66cl4 by metabolic cooperation in the presence of 6-thioguanine in monolayer cultures. These ranked abilities may be related to the effectiveness with which each of these cell lines is able to form communicating junctions with line 66cl4.