Purification and characterization of a Mr ∼66,000 lung‐derived (paracrine) growth factor that preferentially stimulates the in vitro proliferation of lung‐metastasizing tumor cells

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Abstract
In medium containing low concentrations of serum, rat 13762NF mammary adeno-carcinoma cell lines and clones (MTPa and MTC; isolated from the locally growing tumor) of low metastatic potential to lung did not exhibit a growth response to lung-conditioned medium, whereas a highly metastatic cell clone isolated from a spontaneous lung metastasis (MTLn3) did. The major growth-promoting factor for MTLn3 cells from porcine and rat lung-conditioned media was isolated by using a five-step procedure (anion exchange chromatography, Affi-gel blue affinity chroma-tography, chromatofocusing, size exclusion chromatography, and preparative native gel electrophoresis). The lung-derived factor that stimulated the growth of highly metastatic MTLn3 cells was a glycoprotein of Mr ≈66,000 (non-reduced) or Mr ∼72,000 (reduced) and possessed a pI of 6.9–7.0. It preferentially promoted the growth of lung-metastasizing tumor lines over their poorly lung-metastasizing counterparts in three tumor systems: rat 13762NF mammary adenocarcinoma, murine B16 melanoma, and murine RAW 117 large-cell lymphoma. The factor's growth-stimulatory affect was inactivated by reduction or exposure to high temperature (95°C). Although the growth factor appears to be glycosylated, its molecular weight was not altered by treatment with the protein-deglycosylating agent, trifluo-romethane sulfonic acid. Cleavage of the protein by cyanogen bromide resulted in the formation of five fragments. Malignant cell response to this lung-derived paracrine growth factor may be important in the successful formation of lung metastases.