Cellular localization of HSF1 and Hsp70 in skeletal muscle following acute exercise

Exercise induction of the inducible isoform of the 70 kDa heat shock protein (Hsp70) requires activation of the Heat Shock Transcription Factor (HSF1), possibly via its phosphorylation by intracellular protein kinases. However, phosphorylation of HSF1 (pHSF1), its localization and the potential fiber specific relationship between Hsp70 and pHSF1 in skeletal muscle following exercise (EX), is unknown. To address this issue, the plantaris (Plt) and the white portion of the vastus lateralis (WV) were harvested from adult male Sprague‐Dawley rats either 30mins or 24hr post‐EX (1hr treadmill run at 30m/min). These muscles were chosen for analysis because of previous reports indicating that the Plt is relatively refractory to exercise whereas the WV demonstrates a robust response. Indeed, western blots from the WV revealed an ∼4 fold increase in Hsp70 (24hr after EX) whereas Plt was unresponsive. Co‐localization of pHSF1 and Hsp70 with confocal microscopy, revealed strong nuclear staining of pHSF1 in those fibers inducing Hsp70 following EX. However, pHSF1 was also found in nuclei of fibers which did not demonstrate an increase in Hsp70. This suggests that nuclear localization of pHSF1 is essential but not sufficient to induce Hsp70 in skeletal muscle fibers post‐EX and supports a role for additional downstream regulation of this response. Supported by CIHR, CCT‐83029 and NSERC, 8170‐05 RGPIN.