Transcription factor EGR-1 suppresses the growth and transformation of human HT-1080 fibrosarcoma cells by induction of transforming growth factor β1

Abstract

The early growth response 1 (EGR-1) gene product is a transcription factor with roles in differentiation and growth. We have previously shown that expression of exogenous EGR-1 in various human tumor cells unexpectedly and markedly reduces growth and tumorigenicity and, conversely, that suppression of endogenous Egr-1 expression by antisense RNA eliminates protein expression, enhances growth, and promotes phenotypic transformation. However, the mechanism of these effects remained unknown. The promoter of human transforming growth factor β1 (TGF-β1) contains two GC-rich EGR-1 binding sites. We show that expression of EGR-1 in human HT-1080 fibrosarcoma cells causes increased secretion of biologically active TGF-β1 in direct proportion (r(Pearson) = 0.96) to the amount of EGR-1 expressed and addition of recombinant human TGF-β1 is strongly growth-suppressive for these cells. Addition of monoclonal anti-TGF-β1 antibodies to EGR-1-expressing HT-1080 cells completely reverses the growth inhibitory effects of EGR-1. Reporter constructs bearing the EGR-1 binding segment of the TGF-β1 promoter was activated 4- to 6-fold relative to a control reporter in either HT-1080 cells that stably expressed or parental cells cotransfected with an EGR-1 expression vector. Expression of ΔEGR-1, a mutant that cannot interact with the corepressors, nerve growth factor-activated factor binding proteins NAB1 and NAB2, due to deletion of the repressor domain, exhibited enhanced transactivation of 2- to 3.5-fold over that of wild-type EGR-1 showing that the reporter construct reflected the appropriate in vivo regulatory context. The EGR-1-stimulated transactivation was inhibited by expression of the Wilms tumor suppressor, a known specific DNA-binding competitor. These results indicate that EGR-1 suppresses growth of human HT-1080 fibrosarcoma cells by induction of TGF-β1.