Hsp70 chaperone rescues C6 rat glioblastoma cells from oxidative stress by sequestration of aggregating GAPDH

The Hsp70 chaperone is known to elicit cytoprotective activity and this protection has a negative impact in anti-tumor therapy. In cancer cells subjected to oxidative stress Hsp70 may bind damaged polypeptides and proteins involved in apoptosis signaling. Since one of the important targets of oxidative stress is glyceraldehyde-3-phospate dehydrogenase (GAPDH) we suggested that Hsp70 might elicit its protective effect by binding GAPDH. Microscopy data show that in C6 rat glioma cells subjected to hydrogen peroxide treatment a considerable proportion of the GAPDH molecules are denatured and according to dot ultrafiltration data they form SDS-insoluble aggregates. Using two newly developed assays we show that Hsp70 can bind oxidized GAPDH in an ATP-dependent manner. Pharmacological up- or down-regulation of Hsp70 with the aid of U133 echinochrome or triptolide, respectively, reduced or increased the number of C6 glioma cells containing GAPDH aggregates and dying due to treatment with hydrogen peroxide. Using immunoprecipitation we found that Hsp70 is able to sequester aggregation-prone GAPDH and this may explain the anti-oxidative power of the chaperone. The results of this study led us to conclude that in cancer cells constantly exposed to conditions of oxidative stress, the protective power of Hsp70 should be abolished by specific inhibitors of Hsp70 expression.