Investigating how TCR and TGFβR signaling cooperatively influence T cell differentiation, Cattley et al. developed a model in which TGFβR signaling promotes the formation of a SMAD3/4-PKA complex that activates CSK, which affects multiple kinase pathways. First, CSK interferes with TCR-mediated activation of LCK and thus downstream ZAP70. Second, while TCR signaling induces formation of a P85-P110 PI3K heterodimer, CSK interferes by phosphorylating p85, reducing PI3K activity and enhancing PTEN maintenance. The shift in balance between PI3K and PTEN shifts the isoforms of phosphatidylinositol, thereby regulating activation of AKT.
Contributed by Lauren Hitchings
ABSTRACT: The cytokine content in tissue microenvironments shapes the functional capacity of a T cell. This capacity depends on the integration of extracellular signaling through multiple receptors, including the T-cell receptor (TCR), co-receptors, and cytokine receptors. Transforming growth factor beta (TGF-beta) signals through its cognate receptor, TGFbetaR, to SMAD family member (SMAD) proteins and contributes to the generation of a transcriptional program that promotes regulatory T-cell differentiation. In addition to transcription, here we identified specific signaling networks that are regulated by TGFbetaR. Using an array of biochemical approaches, including immunoblotting, kinase assays, immunoprecipitation, and flow cytometry, we found that TGFbetaR signaling promotes the formation of a SMAD3/4-protein kinase A (PKA) complex that activates C-terminal Src kinase (CSK) and thereby down-regulates kinases involved in proximal TCR activation. Additionally, TGFbetaR signaling potentiated CSK phosphorylation of the P85 subunit in the P85-P110 phosphoinositide 3-kinase (PI3K) heterodimer, which reduced PI3K activity and down-regulated the activation of proteins that require phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3) for their activation. Moreover, TGFbetaR-mediated disruption of the P85-P110 interaction enabled P85 binding to a lipid phosphatase, phosphatase and tensin homolog (PTEN), aiding in the maintenance of PTEN abundance and thereby promoting elevated PtdIns(4,5)P2 levels in response to TGFbetaR signaling. Taken together, these results highlight that TGF-beta influences the trajectory of early T-cell activation by altering PI3K activity and PtdIns levels.