Dynamic Effect of Heat Shock Pretreatment on Apoptotic Responses to $TNF-α$ in Liver Cells

The heat shock (HS) response is a protective mechanism for cells to protect themselves against subsequent lethal stress. HS upregulated heat shock protein (HSP) expression reduced apoptosis following tumor necrosis factor-$α$ (⁠$TNF-α$⁠) stimulation. However, vector-mediated overexpression of HSP70 failed to provide similar protection but rather sensitized cells to $TNF-α$ induced apoptosis. This may be due to the fact that the kinetics of vector-mediated HSP overexpression is totally different from that of HSP upregulation by HS. We hypothesized that the response depends on the timing of $TNF-α$ challenge relative to HSP expression dynamics after HS. Therefore, we investigated the correlation between the dynamic change of HSP expression and the levels of apoptosis induced by $TNF-α$ after HS. Hepatoma cells were subjected to mild heat shock at $42°C$ for 2 h followed by varied recovery times and then treated with $TNF-α$ to induce apoptosis. The results from quantitative apoptosis assays using the TUNEL reaction reveal an optimal HS protection window centered around 5 h post-HS against $TNF-α$ induced apoptosis. In addition, we found a window extending up to 2 h after HS where HS sensitized cells to $TNF-α$ stress. Importantly, the correlation between apoptosis and HSP expression kinetics demonstrates that both high levels of HSPs and proper timing between HS and $TNF-α$ stress were critical for optimal protection. Our study establishes a dynamic experimental model for further investigation of HS as a potential clinical approach to target tissue survival or death.