HSP70-mediated acceleration of translational recovery after stress is independent of ribosomal RNA synthesis

HSP70 is known to protect cells against stressful events. In the present study, the hypothesis was investigated that elevated HSP70 levels protect RNA pollymerase I during stress, leading to decreased inhibition of ribosomal RNA (rRNA) synthesis and accelerated recovery of protein translation after stress. To this end, transcriptional and translational activity was studied in H9c2 cells during recovery after a severe heat treatment (SHT, 1 h 45 degreesC) in the presence of elevated HSP70 levels. The latter was achieved by heat pretreatment or by adenovirus-mediated hsp70 gene transfer. Rates of transcription and translation were determined by measuring cellular H-3-labelled uridine and leucine incorporation, respectively. The two types of pretreatment did not affect basal rates of transcription and translation, immediately before SHT. During SHT, both transcriptional and translational rates dropped to less than 10% of basal levels in pretreated as well as non-pretreated cells. Two and four h after SHT, both transcriptional and translational rates were significantly higher in HSP70-overexpressing cells compared to nonpretreated cells. However, immediately after SHT, transcription rates were similarly depressed in non-pretreated and pretreated cells, showing that increased levels of HSP70 did not protect RNA polymerase, I activity during SHT. Thus, the HSP70-mediated acceleration of translational recovery is not preceded in time by an enhanced recovery of rRNA synthesis. Therefore, the HSP70-mediated early recovery of protein synthesis after heat stress is independent of rRNA synthesis.