High constitutive Akt2 activity in U937 promonocytes: effective reduction of Akt2 phosphorylation by the histamine H2-receptor and the β2-adrenergic receptor

Histamine (HA) is approved for the treatment of acute myeloid leukemia (AML). Its antileukemic activity is related to histamine H-2-receptor (H2R)-mediated inhibition of reactive oxygen species (ROS) production in myeloid cells facilitating survival of antineoplastic lymphocytes. The phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, which plays a crucial role in cell survival and proliferation, is constitutively activated in leukemic cells of most AML patients resulting in poor survival prognosis. In a proof-of-principle experiment using a human phosphorylated mitogen-activated protein kinase (MAPK) array, we found high phosphorylation levels of Akt2 in U937 promonocytes that was abrogated by HA or selective H2R agonists. The H2R and the beta(2)-adrenergic receptor (beta(2)AR) are G(s)-protein-coupled receptors. Stimulation results in adenylyl cyclase activation followed by generation of the second messenger adenosine 3',5'-cyclic monophosphate (cAMP). In our present study, we evaluated the pharmacological profile of the H2R and the beta(2)AR regarding Akt2 phosphorylation at Ser(474) via western blot analysis and ELISA and cAMP accumulation via HPLC-MS/MS in U937 promonocytes. H2R and beta(2)AR agonists concentration-dependently decreased Akt2 phosphorylation at Ser(474). Deviations of potencies and efficacies of agonists in Akt2 phosphorylation and cAMP accumulation assays indicated participation of cAMP-independent signaling in GPCR-induced reduction of Akt2 phosphorylation. Accordingly, our study supports the concept of functional selectivity of the H2R and the beta(2)AR in U937 promonocytes. In summary, we extended the antileukemic mechanism of HA via H2R and revealed the potential of beta(2)AR agonists, which are already approved in the treatment of bronchial asthma and chronic obstructive pulmonary disease, as antileukemic drugs.