P-Glycoprotein Is Downregulated in KG1a Primitive Leukaemia Cells by LDL Cholesterol Deprivation and by HMG-CoA Reductase Inhibitors.

P-glycoprotein (pgp) is a membrane transporter encoded by the multidrug resistance (MDR1, ABCB1) gene. Pgp is a poor prognostic factor in elderly patients with acute myeloid leukaemia (AML). In addition to its role in drug efflux, pgp has been implicated in cellular cholesterol homeostasis. We investigated the effects of exogenous cholesterol removal on pgp expression and function. KG1a drug-naïve, primitive leukaemia cells were cultured in serum free medium with or without the addition of low density lipoprotein (LDL) cholesterol. After 72 hours pgp expression and function was assessed by flow cytometry and total cholesterol content of the KG1a cells was determined by the Amplex Red^® cholesterol assay. The addition of clinically available cholesterol lowering agents, HMG-CoA (3-hydroxy-3-methylglutaryl coenzyme A) reductase inhibitors to KG1a cells was also assessed. There was a 39% (SEM 8.3% P=0.03) decrease in pgp protein expression after 3 days of serum free culture without HMG-CoA reductase inhibitors. Message was decreased by 40% (P=0.01) and pgp function was also reduced by 40% (P=0.005). The addition of low density lipoprotein (LDL) cholesterol restored pgp expression to 86% of the basal value. The addition of a HMG-CoA reductase inhibitor to KG1a cells in serum free culture resulted in a further 26% (lovastatin, P=0.03) and 16% (pravastatin, P=0.05) reduction in pgp respectively. Lovastatin also significantly reduced cellular cholesterol levels by 47% (P=0.002) under serum free conditions. Furthermore, the toxicity of the pgp substrate drug daunorubicin was significantly enhanced following lovastatin pre-culture (P=0.04). We conclude that LDL/cholesterol contributes to pgp expression and chemoresistance in primitive leukaemia cells. The use of HMG-CoA reductase inhibitors may be of clinical value in lowering pgp expression in AML.