Etoposide-induced Smad6 expression is required for the G1 to S phase transition of the cell cycle in CMT-93 mouse intestinal epithelial cells.

The inhibitory Smad6 and Smad7 are responsible for cross-talk between TGF-betabone morphogenic protein (BMP) signaling and other cellular signaling pathways, as well as negative feedback on their own signaling functions. Although inhibitory Smads are induced by various stimuli, little is known about the stimuli that increase Smad6 transcription, in contrast to Smad7. Here we demonstrate that etoposide, which induces double strand breaks during DNA replication, significantly up-regulates the transcription of the Smad6 gene in CMT-93 mouse intestinal cells by increasing specific DNA binding proteins. In addition, endogenous inhibition of the Smad6 gene by RNAi interference led to transient accumulation of G1 phase cells and reduction in incorporation of bromodeoxyuridine (BrdU). These findings strongly suggest that Smad6 plays a distinct role in the signaling of etoposide-induced DNA damage.