We studied the transcriptional regulation of the HL gene by USF1 and USF2 in HepG2 cells. The transcriptional activity of the HL(-685/+13) promoter construct was increased up to 25-fold by co-transfection with USF1 and USF2. Silencing of USF1 by RNA interference reduced promoter activity by 30-40%. Chromatin immunoprecipitation assays showed binding of endogenous USF1 and USF2 to the proximal HL promoter region. In gel shift assays, USF1 and USF2 bound to E-boxes at -307/-312 and -510/-516, and to the TATA-Inr region. Although the -514C-->T substitution abolished in vitro USF binding to the -510/-516 E-box, the increase in HL promoter activity by USF1 and USF2 was unaffected. Deletion and mutation analysis of the HL promoter region, and insertion of multiple E-box copies in front of a heterologous promoter, revealed that upregulation by USFs was mainly mediated through the -307/-312 E-box and the TATA-Inr region. We conclude that in HepG2 cells USF1 and USF2 regulate transcriptional activity of the HL gene through their binding to the E-box at -307/-312 and the TATA-Inr region.