An autocrine mitogenic activity produced by a pleural human mesothelioma cell line

The pleural human mesothelioma cell line ZL5 established in our laboratory exhibits an unusual phenotype with adherent and floating cells. ZL5 cells grow in a chemically defined medium (ACL3*) and can be maintained over 3 weeks in protein‐free basal medium alone (RPMI). Basal medium conditioned by ZL5 cells possesses a mitogenic activity with an autocrine effect, as measured by cell counting and by a 3H‐thymidine incorporation assay. Moreover, the conditioned medium affects the DNA synthesis of a variety of other lung‐derived cells. The active principle of medium conditioned by ZL5 cells is not identical to the defined growth‐factors EGF, PDGF, and TGF‐β, known to stimulate the growth of normal human mesothelial cells: treatment with these factors does not mimic the effect of conditioned medium on ZL5 cells. Our observations suggest that the mesothelioma cell line ZL5 produces an unknown autocrine mitogen.