Protein Kinase A (PKA) Isoform RIIβ Mediates the Synergistic Killing Effect of cAMP and Glucocorticoid in Acute Lymphoblastic Leukemia Cells

Zhenyu Ji,Fang C Mei,Aaron L Miller,E.Brad Thompson,Xiaodong Cheng

doi:10.1074/jbc.m803193200

Zhenyu Ji, Fang C Mei + Show 3 more

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https://doi.org/10.1074/jbc.m803193200

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Abstract

Protein kinase A (PKA) or cAMP-dependent protein kinase (cAPK) mediates the synergistic effects of cAMP- and glucocorticoid (GC)-induced apoptosis in lymphoid cells. Using two human acute lymphoblastic leukemia cell (CEM) clones with respective GC-sensitive and GC-resistant phenotypes, we discovered that the PKA regulatory subunit isoform RII(beta) is preferentially expressed in the GC-sensitive clone C7-14 cells, whereas other intracellular cAMP receptors, including the exchange proteins directly activated by cAMP (Epac), are expressed at similar levels in both GC-sensitive and GC-resistant clones. High RII(beta) expression level in C7-14 cells is associated with elevated total PKA cellular activity and cAMP sensitivity, which consequently lead to an increased basal PKA activity. cAMP analogs that selectively activate type II PKA recapitulate the effects of forskolin of promoting apoptosis and antagonizing AKT/PKB activity in both GC-sensitive and GC-resistant clones, whereas type I PKA-selective agonists do not. Furthermore, down-regulation of RII(beta) leads to increased AKT/PKB activation and enhanced GC resistance in C7-14 cells. These results demonstrate that PKA RII(beta) is responsible for increased GC sensitivity, critical for cAMP-mediated synergistic cell killing in CEM cells, and may represent a novel therapeutic target for GC-resistant lymphoid malignancy.

Highlights

Glucocorticoids (GCs)3 induce apoptosis and/or cell growth arrest in lymphoid cells and have been used widely as a mainstay therapy for lymphoid malignancies, especially for acute lymphoblastic leukemia (ALL) [1]
Using two human acute lymphoblastic leukemia cell (CEM) clones with respective GC-sensitive and GC-resistant phenotypes, we discovered that the protein kinase A (PKA) regulatory subunit isoform RII␤ is preferentially expressed in the GC-sensitive clone C7–14 cells, whereas other intracellular cAMP receptors, including the exchange proteins directly activated by cAMP (Epac), are expressed at similar levels in both GC-sensitive and GC-resistant clones
Results from our group show that the proapoptotic effect of cAMP is mediated by the classic intracellular cAMP receptor, protein kinase A (PKA)/ cAMP-protein kinase, and not by the newly discovered exchange protein directly activated by cAMP (Epac) [9]

Summary

Introduction

Glucocorticoids (GCs) induce apoptosis and/or cell growth arrest in lymphoid cells and have been used widely as a mainstay therapy for lymphoid malignancies, especially for acute lymphoblastic leukemia (ALL) [1]. GC resistance of several leukemia cell lines can be overcome by increasing cellular cAMP levels [8]. The mechanism underlying the synergy between PKA and GC is poorly understood. It is not clear which PKA isoform(s) mediates the synergistic effects of cAMP on GC-induced apoptosis, nor is it completely understood upon which pathway(s)/effector(s) PKA and GC converge to exert their effects. We used human GC-sensitive and -resistant CEM cells as model systems to investigate mechanisms of PKA potentiated apoptosis. We report that RII␤ subunit of PKA is reduced in GC-resistant cells, and this reduction/condition is partially responsible for the decreased GC sensitivity. We demonstrate that activation of PKA(II), but not PKA(I), recapitulates the synergistic proapoptotic effect of cAMP through the inhibition of the AKT/MCL-1 prosurvival pathway

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