Functional properties of metallothionein expression in mitochondrial NADH:ubiquinone oxidoreductase deficiency
NADH:ubiquinone oxidoreductase (complex I) deficiency is one of the most frequently encountered causes of mitochondrial respiratory chain disorders. One of the major consequences of such a complex I deficiency is the production of high levels of reactive oxygen species (ROS) and its deleterious effects on the mitochondria and induction of apoptosis. Metallothioneins have been identified as scavengers of ROS, probably due to its unique structural characteristics that provide the ability to bind and reduce ROS. The study investigated the putative protective role of metallothionein overexpression in a complex I deficient cell line and establish whether this protection was targeted against ROS or ROS-related consequences. It was also necessary to establish whether different MT isoforms would lead to different levels of protection in complex I deficient cells. MT-1B and MT-2A cDNA sequences were respectively cloned into the plRESneo2 expression vector and transfected into HeLa cells. The expression levels were characterised and selected biochemical assays conducted. Caspase 3/7 activity measurement and cell viability assays of rotenone-induced complex I deficient cells revealed MT-1 B and especially MT-2A to protect against apoptosis induction, whilst MT-2A also showed additional protection against ROS-induced necrosis. Direct ROS quantification could not confirm MT protection in rotenone-induced complex I deficient cells, but showed lower levels of ROS production in t-BHP treated cells. Decreases in membrane potential also appeared to be much less in MT-over expressed complex I deficient cells. These cells also showed a tendency towards better cellular morphology. Hence, the results presented in this study support the hypothesis that both MT-1B and MT-2A has some protective effect in complex I deficient cells. It does appear, however, that MT-2A seems to be somewhat more effective in protection against ROS, mtPTP formation, apoptosis and ROS-induced necrosis than MT-1 B.