The PI3K–Akt pathway is a central regulator of survival and growth signals across receptor systems, controlled at the membrane-proximal PDK1–Akt signaling node. Although selective serotonin reuptake inhibitors (SSRIs) primarily target the serotonin transporter (SERT), evidence suggests additional effects on intracellular signaling. Escitalopram, a widely used SSRI, was previously shown to inhibit Syk phosphorylation in platelets, raising the possibility of SERT-independent effects on kinase cascades. Here, we investigated whether escitalopram and related compounds interfere with PI3K–Akt signaling in Ramos B lymphoma cells stimulated through the B cell receptor (BCR). Escitalopram and its enantiomer R-citalopram selectively reduced phosphorylation of Akt and its downstream effector FoxO1 while sparing upstream Syk and parallel ERK and PLC–PKC pathways. To localize the site of interference, we combined analyses of PDK1-dependent targets with a PTEN-inhibition model. Escitalopram suppressed PLK1 and S6K1 phosphorylation without altering mTORC2 autophosphorylation, and it also reduced H₂O₂-induced Akt phosphorylation. These complementary results point to a downstream site of interference at the level of PDK1–Akt complex formation, beyond PI3K and PIP3 availability. By contrast, the structurally related analog N-methyl-citalopram showed no effect, reinforcing a structure-dependent mechanism of interference with PDK1–Akt signaling. Functionally, escitalopram enhanced apoptosis in BCR-stimulated cells. Similar inhibition under insulin and SDF-1 stimulation indicates a generalized, receptor-independent effect. These findings reveal a noncanonical, SERT-independent mechanism by which escitalopram disrupts PDK1–Akt activation, with potential relevance to the broader intracellular effects of SSRI treatment.
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