Polo-like kinase-1 (Plk-1) is expressed in neoplastic mast cells and is a potential new target in advanced systemic mastocytosis.

Systemic mastocytosis (SM) is a myeloid neoplasm characterized by abnormal growth and accumulation of mast cells (MC) in one or more internal organs. In most patients, the D816V-mutated variant of KIT that confers resistance against several tyrosine kinase inhibitors, is detectable. In aggressive SM or mast cell leukemia (MCL) the response to conventional drugs is poor and the prognosis is grave. Therefore, current research is seeking novel drug targets in neoplastic MC. Polo-like kinase 1 (Plk-1) is a serine/threonine kinase that plays an essential role in mitosis and has recently been introduced as a potential new target in myeloid leukemias. We analyzed the expression and function of Plk-1 in neoplastic human MC in patients with SM (ISM, n=27; ASM, n=3, MCL, n=2). As assessed by immunostaining, primary neoplastic MC were found to display activated/phosphorylated Plk-1 (pPlk-1) in all patients examined. The human MC leukemia cell line HMC-1 was also found to exhibit pPlk-1. In addition, we found that primary neoplastic MC as well as HMC-1 cells express Plk-1 mRNA. A Plk-1-specific siRNA induced apoptosis in neoplastic MC, whereas no effect was seen with a control siRNA. As assessed by 3H-thymidine uptake, the Plk-1-targeting drug BI 2536 was found to inhibit proliferation in HMC-1 cells (IC50 5-10 nM) and in the canine mastocytoma cell line C2 (IC50 10-50). The effect of BI 2536 was seen in both subclones of HMC-1, i.e. in HMC-1.1 cells harbouring KIT G560V but not KIT D816V, and in HMC-1.2 cells exhibiting KIT G560V as well as KIT D816V, with comparable IC50 values. Moreover, BI 2536 was found to inhibit the proliferation of primary neoplastic MC. The growth-inhibitory effects of BI 2536 on MC were found to be associated with mitotic arrest and G2-M cell cycle arrest as well as consecutive apoptosis. In consecutive experiments, we found that pPlk-1 is expressed in neoplastic MC independent of KIT D816V. We also asked whether combined targeting of KIT D816V and Plk-1 would lead to synergistic drug-effects. In these experiments, BI 2536 was found to synergize with midostaurin (PKC412) in counteracting the proliferation of HMC-1 cells, C2 cells, and primary neoplastic MC. In summary, our data show that activated Plk-1 is expressed in neoplastic MC in SM. Targeting of Plk1 in neoplastic MC may be an attractive pharmacologic concept in advanced SM.

Medizinische Universität Wien - AKH
Veterinärmedizinische Universität Wien