Activation of both protein kinase A (PKA) type I and PKA type II isozymes is required for retinoid-induced maturation of acute promyelocytic leukemia cells.

Acute promyelocytic leukemia (APL) is characterized by granulopoietic differentiation arrest at the promyelocytic stage. In most cases, this defect can be overcome by treatment with all-trans-retinoic acid (ATRA), leading to complete clinical remission. Cyclic AMP signaling has a key role in retinoid treatment efficacy: it enhances ATRA-induced maturation in ATRA-sensitive APL cells (including NB4 cells) and restores it in some ATRA-resistant cells (including NB4-LR1 cells). We show that the two cell types express identical levels of the Cα catalytic subunit and comparable global cAMP-dependent protein kinase A (PKA) enzyme activity. However, the maturation-resistant NB4-LR1 cells have a PKA isozyme switch: compared with the NB4 cells, they have decreased content of the juxtanuclearly located PKA regulatory subunit IIα and PKA regulatory subunit IIβ, and a compensatory increase of the generally cytoplasmically distributed PKA-RIα. Furthermore, the PKA regulatory subunit II exists mainly in the less cAMP-responsive nonautophosphorylated state in the NB4-LR1 cells. By the use of isozyme-specific cAMP analog pairs, we show that both PKA-I and PKA-II must be activated to achieve maturation in NB4-LR1 as well as NB4 cells. Therefore, special attention should be paid to activating not only PKA-I but also PKA-II in attempts to enhance ATRA-induced APL maturation in a clinical setting.