The absence of extracellular calcium potentiates the killing of cultured hepatocytes by aluminum maltolate.

ABSTRACT

Dose- and time-dependent killing of cultured rat hepatocytes was produced by aluminum maltolate (AlM), a neutral, water-soluble complex of aluminum 3-hydroxy-2-methyl-4H-pyran-4-one. Treatment with 10 mM AlM for 1 h killed 50% or more of the cells within 3 h. Removal of calcium from the culture medium or treatment with calcium channel blockers (verapamil, nifedipine, diltiazem) potentiated the cell killing. By contrast, inhibition by thapsigargin of the sequestration of intracellular calcium by the endoplasmic reticulum reduced the toxicity of AlM. In turn, activation of protein kinase C with 12-O-tetradecanoylphorbol 13-acetate or activation of protein kinase A with 8-[4-chlorophenyl-thio]adenosine 3',5'-cyclic monophosphate also reduced the toxicity of AlM. By contrast, inhibition of protein kinase activity by staurosporine potentiated the cell killing. Staurosporine, however, did not reverse the protection afforded by thapsigargin. Hepatocytes treated with AlM for 1 h were rescued by adding deferoxamine as late as 90 min following the removal of AlM, whereas pretreatment for 1 h with deferoxamine did not prevent the toxicity of AlM. ATP depletion did not precede loss of viability. Pharmacologic probes excluded oxidative stress as a mechanism of lethal injury by AlM, and inhibition of protein synthesis by cycloheximide did not protect the hepatocytes, thereby excluding activation of a cell death program. These data define a new model in which aluminum kills liver cells by a mechanisms distinct from previously recognized pathways of lethal cell injury. It is hypothesized that aluminum binds to cytoskeletal proteins intimately associated with the plasma membrane. This interaction eventually disrupts the permeability barrier function of the cell membrane, an event that heralds the death of the hepatocyte. The intracellular calcium ion concentration and protein phosphorylation may modify the interaction of aluminum with its critical targets. Alternatively, aluminum may inhibit the phosphorylation of cytoskeletal elements, thereby interfering with their function.