EXPRESSION OF TRANSDUCED TROPOMYOSIN-1 CDNA SUPPRESSES NEOPLASTIC GROWTH OF CELLS TRANSFORMED BY THE RAS ONCOGENE

Synthesis of certain members of the tropomyosin family of microfilament-associated proteins is suppressed in fibroblasts neoplastically transformed by a number of retroviral oncogenes, by transforming growth factor a, and by chemical mutagens. To test whether tropomyosin suppression is a required event in neoplastic transformation, expression of one of two suppressed tropomyosins in NIH 3T3 mouse cells transformed by the ras oncogene was restored by retro-virally mediated cDNA transfer. Cells expressing the inserted cDNA showed partial restoration of microfilament bundle formation (which is typically deranged in transformed cells) together with increased cytoplasmic spreading. More importantly, they lost anchorage-independent growth capability, and the onset of tumor growth in athymic mice was delayed. When tumors arose they no longer expressed the inserted cDNA. These observations support the conclusion that tropomyosin suppression is a necessary event for the expression of components of the transformed phenotype, particularly with respect to anchorage-independent growth and tumorigenesis, which correlate closely with neoplastic potential. This potentially reversible requirement may link different initial events produced by a variety of oncogenic modalities to a common pathway leading to neoplastic growth.