OBJECTIVE We previously described that the cold-induced apoptosis of cultured hepatocytes and liver endothelial cells is mediated by an increase in the cellular chelatable iron pool-in the absence of any increase in O(2)(.-)/H(2)O(2) formation. As this is an unusual mechanism, we here set out to assess whether an increase in cellular chelatable iron per se is sufficient to trigger cell injury/apoptosis. METHODS Cultured rat hepatocytes were acutely loaded with iron using the membrane-permeable complex Fe(III)/8-hydroxyquinoline and incubated under otherwise 'physiological' conditions. RESULTS Incubation with Fe(III)/8-hydroxyquinoline (15 microM/30 microM) increased the cellular chelatable iron and induced strong hepatocellular injury with morphological features of apoptosis, but also of necrosis. The iron-induced cell injury was oxygen-dependent, and although it was not inhibitable by extracellular catalase, it was strongly inhibited by the novel membrane-permeable catalase mimic TAA-1/Fe. The experimentally induced increase in cellular chelatable iron triggered a mitochondrial permeability transition (MPT) as assessed using double-staining with calcein and tetramethylrhodamine methyl ester. The MPT inhibitor cyclosporine A partially and the well-known inhibitor combination trifluoperazine+fructose completely inhibited the iron-induced cell injury/apoptosis. CONCLUSIONS These results show that iron per se can induce cell injury/apoptosis and that this injury is mediated via an MPT.