Etoposide-induced DNA strand breaks in relation to p-glycoprotein and topoisomerase II protein expression in leukaemic cells from patients with AML and CLL

Elevated expression of the membrane transporter p-glycoprotein (pgp) and impaired expression of the nuclear enzyme topoisomerase II (topo II) are well-known mechanisms for in vitro acquired drug resistance. The clinical relevance of topo II remains unclear, whereas a relationship between pgp levels and treatment results has been shown in acute myelogenous leukaemia (AML). We have investigated the relationships between the levels of topo II and pgp, and in vitro sensitivity to etoposide in mononuclear blood cells from 24 patients with AML, 16 with chronic lymphocytic leukaemia (CLL) and five healthy blood donors.

Following incubation with etoposide, AML cells showed more DNA damage, determined by a DNA unwinding technique, than CLL cells (P  =  0.001), whereas there was no difference in cellular etoposide accumulation. Pgp and topo IIβ levels, determined by Western blot, showed a pronounced variation between patients, but no correlation with induced DNA damage, whereas topo IIα protein was undetectable. In the AML group, topo IIβ expression correlated with pgp expression (ρ  =  0.7, P  =  0.001, n  =  24). The topo IIβ expression was 147.4(±74.6)% in the pgp⁺ AML cells (n  =  10), compared to 33.4(±27.8)% in pgp⁻ AML cells (n  =  14) (P  =  0.0001). Our results show a previously unknown coexpression of topo IIβ and pgp in AML, thereby suggesting that topo IIβ is a potentially interesting resistance factor in AML.