Growth regulation in sarcomas : studies in vivo and in cell culture

• Sarcomas are malignant tumors of mesenchymal origin, and can arise in soft-tissue and in bones. It has been suggested that the abnormal growth regulation in sarcoma cells may be due to an autocrine mechanism, in which the cells are stimulated by an endogenous production of growth factors. Platelet-derived growth factor (PDGF) has been detected in sarcomas, and may be one of the growth factors important for sarcoma growth.PDGF, originally discovered in platelets, is produced by, and binds to, a variety of cells. PDGF plays several roles both in normal conditions and in disease.Suramin is a polyanionic drug with antineoplastic activities, and is known to dissociate growth factors from their receptors. Suramin has been shown to inhibit growth in several tumors and tumor cell lines; however, some tumor cells have been unaffected, or even stimulated, by suramin.The present work was performed in order to a) examine the effects of suramin on sarcoma growth in vivo; b) investigate the kinetics of extravascularly administered PDGF in vivo; c) establish and characterize human sarcoma cells in vitro, including their relation to PDGF; d) evaluate the effects of suramin on sarcoma growth in vitro; e) compare the effects of PDGF on sarcoma growth in vivo and in vitro.Suramin was shown to inhibit growth of two different human osteosarcoma xenografts grown in nude mice. The action is believed to be mainly cytostatic, as the tumors continued to grow, albeit at a lower pace: the tumors of the suramin treated mice had a volume of one-third or less than the untreated ones. The percentage of cells in S and G2-M cell cycle phases was increased by suramin treatment, suggesting a selective effect of the drug in the S and G2 period.Blood and serum levels of 125I, after extravascular administration of 125I-PDGF-AB by intraperitoneal, intramuscular or subcutaneous injection in mice, were found to rise to a maximum 2-4 hours after injection. The levels of radioactivity persisted over several hours. Precipitiation of serum with 10% trichloracetic acid revealed that more than 50% of the radioactivity was in a macromolecular form. Gel chromatography of the serum showed that a major portion of the radioactive material in the circulation had the same molecular size as the original 125I-PDGF-AB.Eight cell lines derived from malignant fibrous histiocytomas (MFH) were established and characterized. A heterogeneity in the morphology of the MFH cell lines was noted. This heterogeneity was also reflected in the expression of mRNA for PDGF, transforming growth factor-alpha (TGF-/alpha/) and their receptors, ability to grow in serumfree media and secretion of PDGF into growth media. Two cell lines, able to grow in serum-free medium, coexpressed MRNA for PDGF, TGF-/aplpha/ and their receptors, suggesting that they may be regulated in an autocrine manner. However, other cell lines, unable to grow in a serum-free medium, also displayed this coexpression of mRNA. The simultaneous expression of a growth factor and its receptor is therefore not generally indicative of an autocrine mechanism.All cell lines, unable to grow in a serum-free medium, were growth inhibited by high-dose suramin (200 ug/ml). The two cell lines, previously noted to grow under serum-free conditions, were not affected by the high-dose suramin treatment. The finding that only serum-dependent human MFH cell lines were inhibited by high doses of suramin indicates that serum dependence in vitro may predict sensitivity of sarcoma cells to suramin.Two human sarcoma xenografts, one osteosarcoma and one malignant fibrous histiocytoma, were treated with human PDGF-AB when grown in nude mice. No effects on tumor growth were noted, although immunohistochemical studies revealed an expression of PDGF receptors. Furthermore, both sarcomas were markedly stimulated by PDGF-AB in vitro. It is concluded that mechanisms or factors other than available PDGF were limiting the growth of the examined tumors in vivo.

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MEDICINE

MEDICIN

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