Copper-imidazo[1,2-a]pyridines differentially modulate pro- and anti-apoptotic protein and gene expression in HL-60 and K562 leukaemic cells to cause apoptotic cell death

Abstract

Despite the availability of a myriad targeted treatments, resistance and treatment failures remains common in cancer treatment. Moreover, the high cost of targeted antibodies excludes a large cohort of patients from their benefits. In this context, copper-imidazo[1,2-a]pyridines were evaluated as alternative drug candidates against two common leukaemias, represented by HL-60 and K562 cells. A previous study identified JD88(21), JD47(29) and JD49(28) to be active against these cell lines with IC50 values between 1.9 and 6 μM and low leukocyte toxicity. To better understand their mechanism of action, their mode of cell death, effects on expression of apoptotic regulatory proteins and their respective genes were investigated. In both cell lines, the copper-imidazo[1,2-a]pyridines, at IC75 concentrations, caused membrane blebbing, raised phosphatidyl-serine levels on cell membranes and increased caspase-3 activity. A loss of mitochondrial membrane potential and activation of caspase-9, combined with poor caspase-8 activity indicated activation of intrinsic apoptosis. Apoptotic proteome analysis showed that the copper-imidazo[1,2-a] pyridines elevated protein levels of pro-apoptotic Bax and Smac/DIABLO in both cell lines, confirming their importance in apoptotic cell death. Conversely, though survivin was increased, this was counteracted by high levels of HTRA2/Omi expression. Effects on apoptotic regulatory proteins Bad, Bcl-2, XIAP and cIAP-1 was inconsistent between the copper-imidazo[1,2-a]pyridines and between the two cell lines, suggesting that the effect of the complexes was modulated by the molecular signature of each cell line. Analysis of mRNA transcripts showed a poor correlation between mRNA levels and associated proteins, implying that copper-imidazo[1,2-a]pyridines compromised protein synthesis and degradation.