The structure of organometals determines cytotoxicity and alteration of calcium homeostasis in HL-60 cells

Abstract

There is increasing concern about the degradation and metabolisation as well as the biochemical mechanisms of action of organometallic compounds. They are known to be immunotoxic and/or neurotoxic. Because of their different toxic capacities, the development of a reliable correlation between molecular parameters and biochemical effects, which could be helpful in risk assessment, was an aim of this study. The tested organolead and -tin compounds decrease the viability of human cells in culture in a time- and concentration-dependent manner. Parabolic QSAR^{1 1} The abbreviations used are: TMT, trimethyltin chloride; TET, triethyltin bromide; TPT, tripropyltin chloride; TBT, tri-n-butyltin chloride; DBT, di-n-butyltin dichloride; TEL, triethyllead chloride; DEL, diethyllead dichloride; TML, trimethyllead chloride; TPhL, triphenyllead chloride; QSAR, quantitative structure-activity relationships; TSA, total surface area; MW_ion, ionic molecular weight; fMLP, N-formyl-L-methionyl-L-leucyl-L-phenylalanine; fluo-3, fluo-3 free acid; fluo-3 AM, fluo-3 acetoxymethyl ester; Me₂SO, dimethyl sulfoxide; PLA₂, phospholipase A₂ (EC 3.1.1.4); FCS, fetal calf serum; HEPES, 4-(2-hydroxy-ethyl)-1-piperazineethanesulfonic acid; EGTA, [ethylene-bis(oxyethylenenitrilo)]tetraacetic acid; [Ca²⁺]_i, cytosolic free Ca²⁺ concentration models yield an adequate correlation between toxicity expressed as LC₅₀ and structural parameters like ionic molecular weight (MW_ion) or total surface area (TSA). Two main chemical attributes of the organometals are probably responsible for such a parabolic relationship: the hydrophobic side chain and the polar metal atom. Furthermore, all tested organometal compounds evoke a persistent increase of the cytosolic free calcium concentration [Ca²⁺]_i. This effect is mainly due to an influx from the extracellular space. Further results suggest that Ca²⁺ enters the cell via opened calcium channels. Based on the essential role of Ca²⁺ within cellular signalling, the perturbation of calcium homeostasis appears to be an important event in final cell killing by organometals and it is most likely that other biochemical mechanisms, e.g. activation of phospholipase A₂, are possibly mediated by an increase of [Ca²⁺]_i.