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 im paired expression of the nuclear enzyme topoisomerase II (topo II) are well -known mechanisms for in vitro acquired drug resistance. The clinical relev ance 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 pg p, and in vitro sensitivity to etoposide in mononuclear blood cells from 24 patients with AML, 16 with chronic lymphocytic leukaemia (CLL) and five he althy blood donors. Following incubation with etoposide, AML cells showed more DNA damage, dete rmined by a DNA unwinding technique, than CLL cells (P = 0.001), whereas th ere was no difference in cellular etoposide accumulation. Pgp and topo ITP levels, determined by Western blot, showed a pronounced variation between p atients, but no correlation with induced DNA damage, whereas topo II alpha protein was undetectable, In the AML group, topo II beta expression correla ted with pgp expression (rho=0.7, P=0.001, n = 24), The topo II beta expres sion 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 prev iously unknown coexpression of topo II beta and pgp in AML, thereby suggest ing that topo II beta is a potentially interesting resistance factor in AML .