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Disruption of transforming growth factor beta signaling by a novel ligand-dependent mechanism
Tania Fernandez1, Stephanie Amoroso, Shellyann Sharpe
1Laboratory of Cell Regulation and Carcinogenesis, The National Cancer Institute, The National Institutes of Health, Bethesda, MD 20892, USA.
Abstract:
Transforming growth factor (TGF)-beta is the prototype in a family of secreted proteins that act in autocrine and paracrine pathways to regulate cell development and function. Normal cells typically coexpress TGF-beta receptors and one or more isoforms of TGF-beta, thus the synthesis and secretion of TGF-beta as an inactive latent complex is considered an essential step in regula-ting the activity of this pathway. To determine whether intracellular activation of TGF-beta results in TGF-beta ligand-receptor interactions within the cell, we studied pristane-induced plasma cell tumors (PCTs). We now demonstrate that active TGF-beta1 in the PCT binds to intracellular TGF-beta type II receptor (TbetaRII). Disruption of the expression of TGF-beta1 by antisense TGF-beta1 mRNA restores localization of TbetaRII at the PCT cell surface, indicating a ligand-induced impediment in receptor trafficking. We also show that retroviral expression of a truncated, dominant-negative TbetaRII (dnTbetaRII) effectively competes for intracellular binding of active ligand in the PCT and restores cell surface expression of the endogenous TbetaRII. Analysis of TGF-beta receptor-activated Smad2 suggests the intracellular ligand-receptor complex is not capable of signaling. These data are the first to demonstrate the formation of an intracellular TGF-beta-receptor complex, and define a novel mechanism for modulating the TGF-beta signaling pathway.
Insights
Transforming growth factor-beta (TGF-β) can bind to its receptor intracellularly in plasma cell tumors. This binding impacts receptor trafficking but does not appear to activate downstream signaling pathways.
Area of Science:
- Cell Biology
- Molecular Biology
- Cancer Biology
Background:
- Transforming growth factor-beta (TGF-β) is a key regulator of cell development and function, acting through autocrine and paracrine pathways.
- TGF-β is secreted as an inactive latent complex, and its regulation is crucial for pathway activity.
- Normal cells express both TGF-β isoforms and their receptors, maintaining controlled signaling.
Purpose of the Study:
- To investigate intracellular TGF-β ligand-receptor interactions within plasma cell tumors (PCTs).
- To determine if active TGF-β1 binds to intracellular TGF-beta type II receptor (TbetaRII) in PCTs.
- To elucidate the impact of intracellular TGF-β-TbetaRII binding on receptor trafficking and signaling.
Main Methods:
- Studied pristane-induced plasma cell tumors (PCTs) to analyze TGF-β signaling.
- Utilized antisense TGF-β1 mRNA to disrupt TGF-β1 expression and observe TbetaRII localization.
- Employed retroviral expression of a dominant-negative TbetaRII (dnTbetaRII) to compete for intracellular ligand binding.
- Analyzed TGF-β receptor-activated Smad2 phosphorylation to assess signaling activation.
Main Results:
- Demonstrated that active TGF-β1 binds to intracellular TbetaRII in PCTs.
- Showed that disrupting TGF-β1 expression restores TbetaRII to the cell surface, indicating ligand-induced trafficking impediment.
- Confirmed that dnTbetaRII competes for intracellular TGF-β1 binding and restores cell surface TbetaRII.
- Found that the intracellular TGF-β-TbetaRII complex does not appear to activate Smad2-mediated signaling.
Conclusions:
- Established the first evidence of an intracellular TGF-β-receptor complex formation.
- Defined a novel mechanism for modulating TGF-β signaling through intracellular interactions.
- Highlighted a potential regulatory pathway impacting receptor trafficking in cancer cells.