Smad-Dependent MicroRNA Processing
Authors: Gough Nancy R. 
Published: September 2008

Signaling initiated by binding of ligands of the transforming growth factor-β (TGF-β) and bone morphogenetic protein (BMP) families to their cognate receptors stimulates the activity of Smad proteins (see Connections Map by Wrana and Attisano). In the canonical pathway, receptor Smads (R-Smads) are phosphorylated by the ligand-bound receptor complex and then interact with the common Smad, Smad4, which leads to nuclear translocation of the R-Smad/Smad4 complex and regulation of specific genes. Davis
et al
. show that a Smad4-independent pathway for TGF-β and BMP signaling is mediated by the interaction of R-Smads with specific primary microRNA (miRNA) transcripts and the miRNA processing complex called DROSHA, which contains the RNase III DROSHA, the DiGeorge syndrome critical region gene 8 (DGCR8), and RNA helicases p68 and p72, and that this interaction allows TGF-β and BMP to stimulate the production of the mature miRNAs, which then repress the expression of specific genes. In vascular smooth muscle cells, BMP or TGF-β signaling leads to the increased abundance of smooth muscle cell markers, such as smooth muscle α-actin (SMA). Davis
et al
. show that BMP2, 4, or 7 stimulated the abundance of mature
miR-21
and
miR-199a
through a nontranscriptional mechanism in human primary pulmonary artery smooth muscle cells (PASMCs). The abundance of the primary
miR-21
transcripts did not increase, only the abundance of the precursor and mature
miR-21
increased in response to BMP or TGF-β. Expression of the
miR-21
target
PDCD4
(encoding programmed cell death 4) was decreased in response to BMP. Forced expression of
PDCD4
prevented the induction of smooth muscle cell markers by BMP, and knockdown of PDCD4 with small-interfering RNA (siRNA) prevented BMP from stimulating SMA abundance. Knockdown of both BMP-specific R-Smads, Smad1 and Smad5, which are present in PASMCs, prevented BMP-induced increase in the precursor and mature
miR-21
; however, knockdown of Smad4 had no effect. In vitro assays suggested that Smad1, Smad5, and the TGF-β-specific Smad3 interacted with p68 through an RNA-independent mechanism and that BMP stimulated the coimmunoprecipitation of DROSHA and p68 with Smad1 or 5 from PASMCs. RNA chromatin immunoprecipitation experiments revealed that BMP4 stimulated the association of Smad1, but not Smad2 or Smad3, with primary
miR-21
or
miR-199a
, but not
miR-214
, and that TGF-β stimulated the association of Smad2 or Smad3, but not Smad1, with primary
miR-21
or
miR-199a
, but not
miR-214
. Thus, the interaction between the Smads and the microRNA precursor is ligand specific and microRNA specific. Nuclear extracts from Cos7 cells treated with BMP4 or TGF-β exhibited enhanced primary
miR-21
processing activity. Promotion of microRNA processing may be a mechanism by which TGF-β signaling contributes to oncogenesis. In the Smad4-deficient breast cancer cell line MDA-MB-468, TGF-β stimulation increased precursor and mature
miR-21
abundance, and expression of a dominant-negative TGF-β receptor subunit in these cells decreased the abundance of the precursor
miR-21
, suggesting that autocrine signaling by TGF-β through the Smad4-independent miRNA processing pathway may contribute to the cancerous phenotype.

B. N. Davis, A. C. Hilyard, G. Lagna, A. Hata, SMAD proteins control DROSHA-mediated microRNA maturation.
Nature
454
, 56-61 (2008).
[PubMed]