Abstract
The silencing mediator for retinoid and thyroid hormone receptors (SMRT)
serves as a platform for transcriptional repression elicited by several
steroid/nuclear receptors and transcription factors. SMRT exists in two major
splicing isoforms, α and τ, with SMRTα containing only an
extra 46-amino acid sequence inserted immediately downstream from the
C-terminal corepressor motif. Little is known about potential functional
differences between these two isoforms. Here we show that the pregnane X
receptor (PXR) interacts more strongly with SMRTα than with SMRTτ
both in vitro and in vivo. It is interesting that the PXR-SMRTα
interaction is also resistant to PXR ligand-induced dissociation, in contrast
to the PXR-SMRTτ interaction. SMRTα consistently inhibits PXR
activity more efficiently than does SMRTτ in transfection assays, although
they possess comparable intrinsic repression activity and association with
histone deacetylase. We further show that the mechanism for the enhanced
PXR-SMRTα interaction involves both the 46-amino acid insert and the
C-terminal corepressor motif. In particular, the first five amino acids of the
SMRTα insert are essential and sufficient for the enhanced binding of
SMRTα to PXR. Furthermore, we demonstrate that Tyr2354 and Asp2355
residues of the SMRTα insert are most critical for the enhanced
interaction. In addition, expression data show that SMRTα is more
abundantly expressed in most human tissues and cancer cell lines, and together
these data suggest that SMRTα may play a more important role than
SMRTτ in the negative regulation of PXR.