SR‐BI regulates the synergistic mast cell response by modulating the plasma membrane‐associated cholesterol pool

Abstract The high‐affinity IgE receptor FcεRI is the mast cell (MC) receptor responsible for the involvement of MCs in IgE‐associated allergic disorders. Activation of the FcεRI is achieved via crosslinking by multivalent antigen (Ag) recognized by IgE resulting in degranulation and proinflammatory cytokine production. In comparison to the T‐ and B‐cell receptor complexes, for which several co‐receptors orchestrating the initial signaling events have been described, information is scarce about FcεRI‐associated proteins. Additionally, it is unclear how FcεRI signaling synergizes with input from other receptors and how regulators affect this synergistic response. We found that the HDL receptor SR‐BI (gene name: Scarb1 / SCARB1 ) is expressed in MCs, functionally associates with FcεRI, and regulates the plasma membrane cholesterol content in cholesterol‐rich plasma membrane nanodomains. This impacted the activation of MCs upon co‐stimulation of the FcεRI with receptors known to synergize with FcεRI signaling. Amongst them, we investigated the co‐activation of the FcεRI with the receptor tyrosine kinase KIT, the IL‐33 receptor, and GPCRs activated by adenosine or PGE 2 . Scarb1 ‐deficient bone marrow‐derived MCs showed reduced cytokine secretion upon co‐stimulation conditions suggesting a role for plasma membrane‐associated cholesterol regulating respective MC activation. Mimicking Scarb1 deficiency by cholesterol depletion employing MβCD, we identified PKB and PLCγ1 as cholesterol‐sensitive proteins downstream of FcεRI activation in bone marrow‐derived MCs. When MCs were co‐stimulated with stem cell factor (SCF) and Ag, PLCγ1 activation was boosted, which could be mitigated by cholesterol depletion and SR‐BI inhibition. Similarly, SR‐BI inhibition attenuated the synergistic response to PGE 2 and anti‐IgE in the human ROSA KIT WT MC line, suggesting that SR‐BI is a crucial regulator of synergistic MC activation.