T-cell protein tyrosine phosphatase (TCPTP) is irreversibly inhibited by etoposide-quinone, a reactive metabolite of the chemotherapy drug etoposide

Etoposide is a widely prescribed anticancer drug that is, however, associated with an increased risk of secondary leukaemia. Although the molecular basis underlying the development of these leukaemias remains poorly understood, increasing evidence implicates the interaction of etoposide metabolites (i.e. etoposide quinone, EQ) with topoisomerase II enzymes. However, effects of etoposide quinone on other cellular targets could also be at play. We investigated whether TCPTP, a protein tyrosine phosphatase that plays a key role in normal and malignant haematopoiesis through regulation of JAK/STAT signalling could be a target of EQ. We report here that EQ is an irreversible inhibitor of TCPTP phosphatase (IC50= ~7 μM, second-order rate inhibition constant of ~810 M-1. min-1). No inhibition was observed with the parent drug. The inhibition by EQ was found to be due to the formation of a covalent adduct at the catalytic cysteine residue in the active site of TCPTP. Exposure of human hematopoietic cells (HL-60 and Jurkat) to EQ led to inhibition of endogenous TCPTP and concomitant increase in STAT1 tyrosine phosphorylation. Our results suggest that in addition to alteration of topoisomerase II functions, EQ could also contribute to ETOP-dependent leukaemogenesis through impairment of key hematopoietic signalling enzymes such as TCPTP. SIGNIFICANCE STATEMENT Not Applicable