Mitochondria-derived oxidative stress induces a heat shock protein response

In addition to minimizing native and non-native protein aggregations, heat shock proteins (HSPs) regulate programmed cell death pathways. Members of this conserved protein family are in general activated by oxidative stress, and likely help maintain viability under this stress condition. To further our understanding of heat shock protein physiology, we studied whether a specific subtype of oxidative stress, namely that arising from mitochondrial electron transport chain inhibition, induced a heat shock protein response. We exposed human teratocarcinoma (NT2) cells to varying concentrations of the cytochrome oxidase inhibitor sodium azide. Micromolar exposures resulted in a cytoplasm to nucleus translocation of the inducible Hsp70 and of Hsp40, and this was followed by an overall upregulation. The response did not coincide temporally with the onset of azide exposure, but rather was activated when the degree of cytochrome oxidase inhibition (which was progressive over time) surpassed a threshold. Azide did not affect either Hsp70 or Hsp40 dynamics in NT2 rho0 cells, which lack functional electron transport chains. For the azide-exposed native cell line, addition of the antioxidant trolox to the medium abrogated both Hsp70/Hsp40 translocation and upregulation. We conclude that mitochondrial electron transport chain dysfunction activates a heat shock protein response, and that this response is mediated by oxidative stress.