Abstract
Furin is recognized as being one of the main convertases of the cellular constitutive secretion pathway but the mechanisms regulating its expression are still unknown. We have previously demonstrated that TGFβ1 up‐regulates its own converting enzyme, furin, creating a novel activation/regulation cycle of potential importance in a variety of physiological and pathophysiological conditions. The fur (fes upstream region) gene is regulated via three alternative promoters; P1, P1A, and P1B. To gain insight into the molecular mechanism(s) underlying this up‐regulation, we performed transient cell transfections with P1, P1A, and P1B promoter luciferase constructs. Transfection experiments in HepG2 cells revealed that fur P1 promoter is the strongest and the most sensitive to TGFβ1 stimulation (5 ng/ml) (3.2‐fold vs. 2.4‐fold for P1A and 2.1‐fold for P1B). Cotransfection with either a dominant negative mutant form of Smad2 [Smad2(3SA)] or a known Smad inhibitor [Smad7] inhibit constitutive and TGFβ1‐induced luciferase activity indicating the participation of endogenous Smads. Increased levels of TGFβ1‐induced transcriptional activation of the P1 promoter by overexpression of Smad2 and/or Smad4 is greatly reduced in the presence of Smad2(3SA) and completely inhibited by Smad7, suggesting the participation of endogenous Smad2/Smad4 complexes. Furthermore, the fork‐head activin signal transducer (FAST‐1), known to interact with Smad2/Smad4 complexes, is a potent stimulator of TGFβ1‐induced transactivation of the fur P1 promoter. Five prime‐deletion analysis of this promoter identified the proximal region (between positions −8734 and −7925), as the nucleotide stretch that carries most of the transcriptional activation of fur P1 promoter by Smad2. Overall, the present data demonstrate that Smad2 and Smad4 possibly in complex with FAST‐1 or other DNA binding partners participate in the constitutive and inducible transactivation of the fur P1 promoter. This represents the first detailed study of the transcriptional regulation of the fur gene. © 2001 Wiley‐Liss, Inc.