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Signaling of transforming growth factor-beta family members through Smad proteins
1Division of Cellular Biochemistry, The Netherlands Cancer Institute, Amsterdam, The Netherlands.
Abstract:
Smads are pivotal intracellular nuclear effectors of transforming growth factor-beta (TGF-beta) family members. Ligand-induced activation of TGF-beta family receptors with intrinsic serine/threonine kinase activity trigger phosphorylation of receptor-regulated Smads (R-Smads), whereas Smad2 and Smad3 are phosphorylated by TGF-beta, and activin type I receptors, Smad1, Smad5 and Smad8, act downstream of BMP type I receptors. Activated R-Smads form heteromeric complexes with common-partner Smads (Co-Smads), e.g. Smad4, which translocate efficiently to the nucleus, where they regulate, in co-operation with other transcription factors, coactivators and corepressors, the transcription of target genes. Inhibitory Smads act in most cases in an opposite manner from R- and Co-Smads. Like other components in the TGF-beta family signaling cascade, Smad activity is intricately regulated. The multifunctional and context dependency of TGF-beta family responses are reflected in the function of Smads as signal integrators. Certain Smads are somatically mutated at high frequency in particular types of human cancers. Gene ablation of Smads in the mouse has revealed their critical roles during embryonic development. Here we review the latest advances in our understanding of the Smad mechanism of action and their in vivo functions.
Insights
Smad proteins are key intracellular signals for the transforming growth factor-beta (TGF-beta) family. This review details Smad functions in gene regulation, development, and cancer.
Area of Science:
- Molecular Biology
- Cell Signaling
- Genetics
Background:
- Smad proteins are crucial intracellular mediators of transforming growth factor-beta (TGF-beta) superfamily signaling.
- TGF-beta receptors initiate signaling cascades through phosphorylation of receptor-regulated Smads (R-Smads).
Purpose of the Study:
- To review recent advancements in understanding Smad protein mechanisms of action.
- To explore the in vivo functions of Smads in biological processes.
Main Methods:
- Literature review of Smad protein research.
- Analysis of Smad involvement in TGF-beta signaling pathways.
Main Results:
- R-Smads complex with common-Smads (Co-Smads) like Smad4 to regulate gene transcription in the nucleus.
- Inhibitory Smads counteract R-Smad and Co-Smad functions.
- Smad activity is tightly regulated and Smads integrate diverse TGF-beta family signals.
Conclusions:
- Smads play critical roles in embryonic development, as evidenced by gene ablation studies in mice.
- Somatic mutations in Smad genes are frequent in certain human cancers, highlighting their oncogenic relevance.
- Smads are multifunctional signal integrators with context-dependent roles in TGF-beta signaling.