It is generally believed that mild forms of injury induce apoptosis, while more severe forms of insult result in necrosis. Moreover, in the liver, as in other tissues, many compounds can even cause apoptosis and necrosis simultaneously. The two most well-studied pathways of apotosis include the surface death receptor pathway (i.e. Fas) and the mitochondria-initiated pathway. Mitochondria are deeply involved in the regulation of cell death, undergoing membrane permeabilization which commits hepatocytes to apoptosis. Apoptosis may be a major event in chemical-induced injury and therefore the detection of apoptotic effects when developing new drugs is highly relevant in screening for pharmaco-toxicologic risk assessment. However, as apoptosis in vitro normally degenerates to secondary necrosis, it may be underestimated unless sensitive and early parameters of apoptosis are used. The markers selected include several biochemical parameters (down-regulation of the anti-apoptotic bclXL gene, caspase activation and cytochrome c release from mitochondria), and flow cytometry determinations (chromatin complexity and DNA integrity). The apoptotic effect can be generally detected at low concentrations of the drugs long before cell necrosis, but some compounds induce apoptosis concomitantly to necrosis. Among the markers evaluated caspase 3 activation, and nuclei and DNA analysis by flow cytometry fulfil the compromise between reliability, sensitivity and ease of performance.
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