Fine Tuning and Cross-talking of TGF-β Signal by Inhibitory Smads

Transforming Growth Factor (TGF)-β family, including TGF-β, bone morphorgenic protein (BMP), and activn, plays an important role in essential cellular functions such as proliferation, differentiation, apoptosis, tissue remodeling, angiognesis, immune responses, and cell adhesions. TGF-β predominantly transmits the signals through serine/ threonine receptor kinases and cytoplasmic proteins called Smads. Since the discovery of TGF-β in the early 1980s, the dysregulation of TGF-β/Smad signaling has been implicated in the pathogenesis of human diseases. Among signal transducers in TGF-β/Smad signaling, inhibitory Smads (I-Smads), Smad6 and Smad7, act as major negative regulators forming autoinhibitory feedback loops and mediate the cross-talking with other signaling pathways. Expressions of I-Smads are mainly regulated on the transcriptional levels and post-translational protein degradations and their intracellular levels are tightly controlled to maintain the homeostatic balances. However, abnormal levels of I-Smads in the pathological conditions elicit the altered TGF-β signaling in cells, eventually causing TGF-β-related human diseases. Thus, exploring the molecular mechanisms about the regulations of ISmads may provide the therapeutic clues for human diseases induced by the abnormal TGF-β signaling.