Isolation of a novel USF2 isoform: repressor of cathepsin B expression

We previously demonstrated that upstream stimulatory factor 1 (USF1) and USF2 regulate transcription of cathepsin B. Here, we have cloned a novel transcript variant of USF2 from a human DU145 prostate cancer cell line by reverse transcription-polymerase chain reaction (RT-PCR). This new transcript variant, designated USF2c, results from alternative splicing of the primary USF2 transcript using a cryptic splicing acceptor site within exon 6. As a consequence, USF2c is missing exons 4, 5, and part of exon 6. USF2c can be transcribed and translated to a protein of approximately 29 kDa in vitro, and the resulting USF2c protein can bind as a homodimer to the E-box of the cathepsin B promoter. USF2c is expressed in two other prostate cancer cell lines (LNCaP, PC3), and U87 human glioblastoma cells as are USF2a and USF2b, two previously identified isoforms of USF2. Cotransfection experiments in DU145 and U87 cells demonstrate that USF2c can down-regulate expression of cathepsin B. These results suggest that USF2c regulates expression of cathepsin B by binding to the E box element in the cathepsin B promoter as a repressor.