FcαRI Dynamics Are Regulated by GSK-3 and PKCζ During Cytokine Mediated Inside-Out Signaling

Abstract

IgA binding to FcαRI (CD89) is rapidly enhanced by cytokine induced inside-out signaling. Dephosphorylation of serine 263 in the intracellular tail of FcαRI by PP2A and PI3K activation are instrumental in this process. To further investigate these signaling pathways, we targeted downstream kinases of PI3K. Our experiments revealed that PI3K activates PKCζ, which subsequently inhibits GSK-3, a constitutively active kinase in resting cells and found here to be associated with FcαRI. We propose that GSK-3 maintains FcαRI in an inactive state at homeostatic conditions. Upon cytokine stimulation, GSK-3 is inactivated through a PI3K-PKCζ pathway, preventing the maintenance of phosphorylated inactive FcαRI. The concomitantly activated PP2A is then able to dephosphorylate and activate FcαRI. Moreover, FRAP and FLIP studies showed that FcαRI activation coincides with an increased mobile fraction of the receptor. This can enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes. This tightly regulated inside-out signaling pathway allows leukocytes to respond rapidly and efficiently to their environment and could be exploited to enhance the efficacy of future IgA therapeutics.

Antibody-Dependent Cellular Phagocytosis of HIV-1-Infected Cells Is Efficiently Triggered by IgA Targeting HIV-1 Envelope Subunit gp41

“…Indeed, an IgA molecule, even monomeric, binds to two FcαRI molecules whereas an IgG molecule binds only to one FcγR molecule (45). IgA binding to FcαRI is also rapidly enhanced by cytokine induced inside-out signaling, and can in turn enhance FcαRI valency and contribute to stronger avidity for IgA immune complexes (46), thus increasing ADCP efficacy compared to IgG. Furthermore, in a model of opsonized cancer cell killing by neutrophils, IgA-mediated binding to neutrophils is more stable compared to that of IgG, resulting in an IgA engagement of neutrophils capable of eliciting a stronger Fc receptor signaling than IgG, and in turn higher killing (43).…”

Fc Engineering Strategies to Advance IgA Antibodies as Therapeutic Agents

“…It is expressed on myeloid cells such as neutrophils, eosinophils, monocytes, subsets of dendritic cells, and macrophages [13]. Soluble monomeric IgA only transiently interacts with the receptor due to low affinity but can stably interact with high avidity once complexed, thereby crosslinking the receptor [14,15]. FcαRI engagement with IgA-opsonized targets or immune complexes induces signaling via its associated FcR γ-chain bearing ITAM motifs, which become phosphorylated, allowing docking of cellular proteins such as Syk and Src family protein tyrosine kinases that on their end induce a signaling cascade.…”

Inside-Out Control of Fc-Receptors

“…Next to integrins various studies show that also FcγR's and FcαR are subjected to inside-out control (39–43). In contrast to integrins where a consensus is present that this mechanism is important, this concept has not yet been generally accepted for FcR function.…”

“…Ten Broeke et al recently provided evidence that the identity of this constitutively active kinase is glycogen synthase kinase-3 (GSK-3) (43), a kinase that is constitutively active in resting cells such as leukocytes and its activity is inhibited by phosphorylation (43). Interestingly, such phosphorylation can be mediated by cytokine-induced activation of the PI-3K and protein kinase-Cζ (PKCζ) -axis (43). This leads to a model were the function of FcαRI is actively suppressed by phosphorylation by GSK-3 in unactivated cells.…”

“…Not much is known regarding the distribution of FcRs in these different domains, but recent studies support the importance of lateral mobility of FcRs in the plain of the membrane and the importance of co-localization in these domains (54). In addition, Ten Broeke et al provided evidence that dephosphorylation of FcαRI and functional activation of the receptor is associated with enhanced lateral movement of the receptor and possibly an increase in valency of the receptor (43). Moreover, data of Yang et al implied that cross-linking of FcγRIIIb (CD16b) leads to lipid raft mediated activation of SHP2 (51).…”