Effect of caloric restriction on the expression of heat shock protein 70 and the activation of heat shock transcription factor 1

The regulation of heat shock protein 70 (hsp70) expression is an excellent example of a cellular mechanism that has evolved to protect all living organisms from various types of physiological stresses; therefore, the reported age-related alterations in the ability of cells to express hsp70 in response to stress could seriously compromise the ability of a senescent organism in respond to changes in its environment. Because caloric restriction (CR) is the only experimental manipulation known to retard aging and increase the survival of rodents, it was of interest to analyze the effect of CR on the age-related alteration in the induction of hsp70 expression in rat hepatocytes. The effect of CR on the nuclear transcription of hsp70 gene in rat hepatocytes in response to various levels of heat shock was determined, and it was found that the age-related decline in the transcription of hsp70 at all temperatures studied was reversed by CR. Because the heat shock transcription factor (HSF) mediates the heat-induced transcription of hsp70, the effect of CR on the induction of HSF binding activity by heat shock was studied and found to arise from HSF1, which has been shown to be involved in the induction of HSF binding activity in other cell types. The age-related decrease in the induction of HSF1 binding activity in rat hepatocytes was reversed by CR, and did not appear to be due to an accumulation of inhibitory molecules with age. Interestingly, the level of HSF1 protein was significantly higher in hepatocytes isolated from old rats fed ad libitum compared to hepatocytes obtained from rats fed the CR diet even though the levels of HSF1 binding activity were lower for hepatocytes isolated from the old rats fed ad libitum. The levels of the mRNA transcript for HSF1 was not significantly altered by age or CR. Thus, the changes in HSF1 binding activity with age and CR do not arise from changes in the level of HSF1 protein available for activation.