TRANSCRIPTION FACTOR EGR-1 SUPPRESSES THE GROWTH AND TRANSFORMATION OF HUMAN HT-1080 FIBROSARCOMA CELLS BY INDUCTION OF TRANSFORMING GROWTH-FACTOR-BETA-1

The early growth response 1 (EGR-1) gene product is a transcription fa ctor with roles in differentiation and growth. We have previously show n that expression of exogenous EGR-1 in various human tumor cells unex pectedly and markedly reduces growth and tumorigenicity and, conversel y, that suppression of endogenous Egr-1 expression by antisense RNA el iminates protein expression, enhances growth, and promotes phenotypic transformation. However, the mechanism of these effects remained unkno wn. The promoter of human transforming growth factor beta 1 (TGF-beta 1) contains two CC-rich EGR-1 binding sites. We show that expression o f EGR-1 in human HT-1080 fibrosarcoma cells causes increased secretion of biologically active TGF-beta 1 in direct proportion (T-Pearson = 0 .96) to the amount of EGR-1 expressed and addition of recombinant huma n TGF-beta 1 is strongly growth-suppressive for these cells. Addition of monoclonal anti-TGF-beta 1 antibodies to EGR-1-expressing HT-1080 c ells completely reverses the growth inhibitory effects of EGR-1. Repor ter constructs bearing the EGR-1 binding segment of the TGF-beta 1 pro moter was activated 4- to 6-fold relative to a control reporter in eit her HT-1080 cells that stably expressed or parental cells cotransfecte d with an EGR-1 expression vector. Expression of Delta EGR-1, a mutant that cannot interact with the corepressors, nerve growth factor-activ ated factor binding proteins NAB1 and NAB2, due to deletion of the rep ressor domain, exhibited enhanced transactivation of 2- to 3.5-fold ov er that of wild-type EGR-1 showing that the reporter construct reflect ed the appropriate in vivo regulatory context. The EGR-1-stimulated tr ansactivation was inhibited by expression of the Wilms tumor suppresse r, a known specific DNA-binding competitor. These results indicate tha t EGR-1 suppresses growth of human HT-1080 fibrosarcoma cells by induc tion of TGF-beta 1.