Current perspectives on inhibitory SMAD7 in health and disease

Abstract

Transforming growth factor beta (TGF-beta) family members play an extensive role in cellular communication that orchestrates both early development and adult tissue homeostasis. Aberrant TGF-beta family signaling is associated with a pathological outcome in numerous diseases, and in-depth understanding of molecular and cellular processes could result in therapeutic benefit for patients. Canonical TGF-beta signaling is mediated by receptor-regulated SMADs (R-SMADs), a single co-mediator SMAD (Co-SMAD), and inhibitory SMADs (I-SMADs). SMAD7, one of the I-SMADs, is an essential negative regulator of the pleiotropic TGF-beta and bone morphogenetic protein (BMP) signaling pathways. In a negative feedback loop, SMAD7 inhibits TGF-beta signaling by providing competition for TGF-beta type-1 receptor (T beta RI), blocking phosphorylation and activation of SMAD2. Moreover, SMAD7 recruits E3 ubiquitin SMURF ligases to the type I receptor to promote ubiquitin-mediated proteasomal degradation. In addition to its role in TGF-beta and BMP signaling, SMAD7 is regulated by and implicated in a variety of other signaling pathways and functions as a mediator of crosstalk. This review is focused on SMAD7, its function in TGF-beta and BMP signaling, and its role as a downstream integrator and crosstalk mediator. This crucial signaling molecule is tightly regulated by various mechanisms. We provide an overview of the ways by which SMAD7 is regulated, including noncoding RNAs (ncRNAs) and post-translational modifications (PTMs). Finally, we discuss its role in diseases, such as cancer, fibrosis, and inflammatory bowel disease (IBD).