PLC-β2 monitors the drug-induced release of differentiation blockade in tumoral myeloid precursors

The differentiation therapy in treatment of acute promyelocytic leukemia (APL), based on the administration of all-trans retinoic acid (ATRA), is currently flanked with the use of As,03, a safe and effective agent for patients showing a resistance to ATRA treatment. A synergy between ATRA and AS(2)O(3) was also reported in inducing granulocytic differentiation of APL-derived cells. We have demonstrated that phospholipase C-beta 2 (PLC-beta 2), highly expressed in neutrophils and nearly absent in tumoral promyelocytes, largely increases during ATRA treatment of APL-derived cells and strongly correlates with the responsiveness of APL patients to ATRA-based differentiating therapies. Here we report that, in APL-derived cells, low doses of As2O3 induce a slight increase of PLC-beta 2 together with a moderate maturation, and cooperate with ATRA to provoke a significant increase of PLC-beta 2 expression. Remarkably, the amounts of PLC-beta 2 draw a parallel with the differentiation levels reached by both ATRA-responsive and -resistant cells treated with ATRA/As2O3 combinations. PLC-beta 2 is not necessary for the progression of tumoral promyelocytes along the granulocytic lineage and is unable to overcome the differentiation block or to potentiate the agonist-induced maturation. On the other hand, since its expression closely correlates with the differentiation level reached by APL-derived cells induced to maturate by drugs presently employed in APL therapies, PLC-beta 2 represents indeed a specific marker to test the ability of differentiation agents to induce the release of the maturation blockade of tumoral myeloid precursors.