The transforming growth factor (TGF)-beta family of proteins drive normal embryonic development, while dysregulation of TGF-beta signaling contributes to developmental disorders, cancer progression and dissemination, and fibrosis. Signaling by TGF-beta initiates upon ligand binding to transmembrane receptor proteins, which then promote gene expression changes through phosphorylation and activation of intracellular Smad proteins. Functional activation of the TGF-beta receptors is carefully regulated through integration of post-translational modifications, spatial regulation at the cellular level, and receptor availability at the cell surface.
While the majority of TGF-beta receptors reside intracellularly, they can be rapidly mobilized to the cell surface in response to Akt activation, thereby increasing the cell’s responsiveness to TGF-beta. Because TGF-beta is known to induce phosphorylation of Akt, I investigated whether TGF-beta regulates translocation of its own receptors to the cell surface and thus amplifies its own response. Through selective biotinylation of cell surface proteins, I found that TGF-beta induced a rapid increase of type I and type II TGF-beta receptors at the plasma membrane. This receptor upregulation was inhibited by blocking Akt phosphorylation, or in the presence of TGF-beta type I receptor kinase inhibitors. Furthermore, attenuation of the Akt-mediated increase in cell surface receptor presentation decreased Smad activation and TGF-beta-induced gene expression responses. I also observed that while TGF-beta treatment caused an overall increase in cell surface TGF-beta receptors, it also induced an increased rate of receptor endocytosis, suggesting an overall amplification of TGF-beta receptor cycling. Together, these data outline a novel response amplification mechanism, through a ligand-induced, rapid mobilization of cell surface receptors from intracellular stores.
Furthermore, BMP-4 caused cells to upregulate TGF-beta receptors, leading to subsequent autocrine activation of TGF-beta-responsive Smads and gene expression. Inhibition of Akt phosphorylation or inhibition of TGF-beta type I receptor kinase attenuated the autocrine TGF-beta signaling activation in response to BMP. The balance between BMP and TGF-beta signaling pathways is essential for proper spatial and temporal regulation of tissue specification during development, and has been observed biochemically, although the underlying mechanisms are not clear. This work provides context for activation of TGF-beta signaling by BMP, presenting a means by which cells could gain sensitivity to TGF-beta in the presence of BMP.