Tumor cell variants were established in vitro by treating a metastatic variant of murine colon adenocarcinoma 26 (P‐26‐select) in serum‐free RPMI 1640 medium for 6 to 10 weeks. Five tumor cell lines were established from independent cultures and designated PS‐I to PS‐5. The PS cells showed higher growth potential in vitro under serum‐free or low‐serum conditions than the parental P‐26‐select. In comparison to P‐26‐select, the PS cell lines possessed enhanced lung‐colonizing ability after i. v. inoculation and metastasized spontaneously to the lung following s. c. inoculation into the flank or the right fore‐footpads of BALB/c mice. Metastatic nodules were also observed in the liver. Spleens and livers of the tumor‐bearing mice were enlarged mainly as a result of hematopoiesis, suggesting the production of CSF‐like substance(s) by the tumor cells. The PS cells secreted increased amounts of putative autocrine growth factor(s) and CSF‐like substance(s) in vitro. These characteristics might be related to the in vivo metastatic ability of the PS cells.
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