HOMOCYSTEINE-DEPENDENT ALTERATIONS IN MITOCHONDRIAL GENE-EXPRESSION, FUNCTION AND STRUCTURE - HOMOCYSTEINE AND H2O2 ACT SYNERGISTICALLY TO ENHANCE MITOCHONDRIAL DAMAGE

Mitochondrial abnormalities have been identified in hepatocytes of pat ients with hyperhomocysteinemia and in endothelial cells from the aort as of rats with diet-induced hyperhomocysteinemia. However, the mechan ism by which homocysteine affects mitochondria is unknown. In this rep ort, homocysteine-induced expression of the mitochondrial electron tra nsport chain gene, cytochrome c oxidase III/ATPase 6,8 (CO3/ATPase 6,8 ), was identified in a human megakaryocytic cell line DAMI using mRNA differential display. Steady-state mRNA levels of CO3/ATPase 6,8, as w ell as other mitochondrial transcripts, were increased in DAMI cells b y homocysteine in a concentration- and time-dependent manner. Despite an increase in mitochondrial RNA levels and changes in mitochondrial u ltrastructure, no effect on either cell growth or mitochondrial respir ation rates was observed in DAMI cells exposed to homocysteine at conc entrations up to 1 mM. In contrast, 1 mM homocysteine in the presence of Cu2+, which is known to generate H2O2, significantly decreased mito chondrial RNA levels, caused gross morphological changes in mitochondr ial ultrastructure, and inhibited both cell growth and mitochondrial r espiration rates. However, precursors of cellular glutathione and pree xposure to heat shock blocked the decrease in mitochondrial RNA levels caused by homocysteine and Cu2+. The observations that (i) homocystei ne and H2O2, but not H2O2, alone, caused a decrease in mitochondrial R NA levels, (ii) intracellular levels of H2O2 were significantly increa sed in the presence of homocysteine and Cu2+, and (iii) catalase, but not free radical scavengers, prevented a decrease in mitochondrial RNA levels, provide evidence that homocysteine and H2O2 act synergistical ly to cause mitochondrial damage. Furthermore, our findings suggest th at intracellular glutathione and heat shock proteins play a role in pr otecting mitochondria against the adverse effects elicited by homocyst eine and H2O2.