THE TRANSCRIPTIONAL EFFECT OF WT1 IS MODULATED BY CHOICE OF EXPRESSION VECTOR

The WT1 Wilms' tumor suppressor gene encodes a zinc finger transcripti on factor which plays a critical role in renal and genitourinary devel opment, The WT1 protein was reported to both activate and repress tran scription. We found that the transcriptional effect of nrrl on the Egr 1 promoter could be modulated by the use of expression vectors contain ing different promoters. WT1 activated the Egr1 promoter when expressi on of WT1 was driven by the Rous sarcoma virus promoter, In contrast, a cytomegalovirus (CMV) promoter-containing WT1 expression vector repr essed the Egr1 promoter, However, WT1 activated transcription of a sim ple test promoter, EGR(3)tkCAT, regardless of the expression vector us ed, Co-transfection of the parental CMV-based vector strongly depresse d the basal activity of the Egr1-CAT reporter, suggesting that the CMV promoter competes with the Egr1 promoter for transcription factors or cofactors which may be required for activation by WT1. In support of this hypothesis, WT1 was converted from an activator to a repressor by co-transfection of an excess of the parental CMV-based vector, These results provide an important caveat to the interpretation of co-transf ection studies and confirm the hi-functional nature of the WT1 transcr iption factor.