Smad6 Is a Smad1/5-induced Smad Inhibitor

Abstract

Smad6 is an inhibitory Smad that is induced by bone morphogenetic proteins (BMPs) and interferes with BMP signaling. We have isolated the mouse Smad6 promoter and identified the regions responsible for transcriptional activation by BMPs. The proximal BMP-responsive element (PBE) in the Smad6 promoter is important for the transcriptional activation by BMPs and contains a 28-base pair GC-rich sequence including four overlapping copies of the GCCGnCGC-like motif, which is a binding site for Drosophila Mad and Medea. We generated a luciferase reporter construct (3GC2-Lux) containing three repeats of the GC-rich sequence derived from the PBE. BMPs and BMP receptors induced transcriptional activation of 3GC2-Lux in various cell types, and this activation was enhanced by cotransfection of BMP-responsive Smads, i.e. Smad1 or Smad5. Moreover, direct DNA binding of BMP-responsive Smads and common-partner Smad4 to the GC-rich sequence of PBE was observed. These results indicate that the expression of Smad6 is regulated by the effects of BMP-activated Smad1/5 on the Smad6 promoter.