Inhibition of 5‐HT 1A receptor‐dependent cell survival by cAMP/protein kinase A: Role of protein phosphatase 2A and Bax

Serotonergic 5‐HT 1A receptor signaling leading to nuclear factor‐κB (NF‐κB) activation appears to be critical for cell survival. Adenylyl cyclase and protein kinase A (AC/PKA) are effectors of the 5‐HT 1A receptor that are inhibited by Gα i subunits. Conversely, Gβγ i subunits downstream from the 5‐HT 1A receptor participate in the activation of extracellular signal‐regulated kinases (ERK1/2), phosphatidylinositol 3‐kinase (PI3K), Akt, and NF‐κB. To model the contribution of pro‐ and antiapoptotic signaling cascades downstream of activated 5‐HT 1A receptor in cell survival, Chinese hamster ovarian (CHO) cells were employed that exogenously overexpress 5‐HT 1A receptors. Stimulation with the 5‐HT 1A receptor agonist 8‐OH‐DPAT and pharmacological agonists of AC induced PKA and protein phosphatase 2A (PP2A) activity, which in turn inhibited: Akt activity, IκBα degradation, nuclear translocation of NF‐κB, and expression of X‐linked inhibitor of apoptosis protein (XIAP/BIRC4). Pharmacological inhibition of PP2A with calyculin A potentiated Akt activity while attenuating ERK1/2 signaling via increased inhibitory phosphorylation of Raf (pSer259). In contrast, increased cAMP levels enhanced Bax translocation to the mitochondria, resulting in the release of cytochrome c , caspase‐3 activation, and apoptosis induction. Our data suggest a central role of cAMP/PKA‐dependent PP2A in shifting the homeostasis of intracellular signaling downstream of activated 5‐HT 1A receptor toward cell death in biological systems linked to neuropsychiatric disorders. © 2008 Wiley‐Liss, Inc.