SMAD2 sets divergent thresholds for TGF-β-induced SMAD1/5 signaling and IgE-mediated pro-inflammatory activation of mast cells

TGF-β-mediated signaling controls mast cell (MC) development and exerts anti-inflammatory functions, while antigen/allergen (Ag)-triggered FcϵRI activation commands pro-inflammatory reactions. TGF-β induces strong C-terminal and low linker phosphorylation of SMAD2. In contrast, Ag triggers immediate, MEK-dependent SMAD2 linker phosphorylation only. Both stimuli can positively or negatively influence each others effects on MC activation in a gene-dependent manner. However, the molecular and cellular mechanisms of SMAD2 in MCs still need to be elucidated. To decipher the role(s) of SMAD2 in MCs, SMAD2 was ablated in PMC-306 MCs using CRISPR/Cas9, and the effects were studied after TGF-β and/or Ag stimulation. The absence of SMAD2 led to increased proliferation and survival, as well as decreased transcription of target genes like Smad7 and Jun in steady state and after TGF-β treatment. Interestingly, SMAD2 was found to regulate the strength and kinetics of TGF-β-mediated SMAD1/5 activation, resulting in augmented expression of genes like Id2 and Id3 in SMAD2-deficient MCs. Unexpectedly, SMAD2 was observed to license Ag-triggered production of pro-inflammatory cytokines, such as IL-6 and TNF, by monitoring expression of secondary repressive signaling elements. Re-introducing SMAD2 restored these events with varying sensitivity depending on the receptor system triggered. Our findings reveal SMAD2 as an initial hub in TGF-β-SMAD1/5 and Ag-FcϵRI signaling, offering new possibilities for therapeutic intervention in both TGF-β-controlled and Ag-triggered MC functions using potential SMAD2 activators or inhibitors.