Heat shock cellular stress on aged gut-associated lymphocytes; mRNA expression of inducible heat shock protein gene and protooncogenes

To investigate the effects of senescence and cellular stress on gut mucosal lymphocytes, we used heat shock (HS) to determine the mRNA expression of an inducible HSP70-family gene (HSP68) and two nuclear protooncogenes (c-fos and c-myc) from gut-associated (GA) lymphocytes of aged (≥24 month) and young (3–4 month) mice. First, with temperatures of 37–44°C, the maximal HS effect on expression of inducible HSP68 mRNA was at 41°C, and that of c-fos and c-myc at 42°C; with HS durations of 0–6 h at 42°C, the maximal effect was at 1 or 2 h in the young GA lymphocytes. Second, without HS (37°C), inducible HSP68 mRNA was not expressed in the GA lymphocytes of both age groups. The expression of c-fos and c-myc mRNA was grater in the aged GA lymphocytes than in the young, and c-myc mRNA was more highly expressed than c-fos mRNA in both age groups. Third, with HS (42°C) for 30 min to 6 h, followed by recovery (37°C) for up to 6 h, the peak expression of inducible HSP68 mRNA in the aged GA lymphocytes was reduced, but occurred at the same time as seen in the young (2 h) and declined faster during the recovery period. The peak expression of c-fos mRNA in the aged GA lymphocytes was earlier than that of the HSP68 mRNA in both age groups (1 h); the intensity of expression was decreased at all time points in the aged and declined more rapidly during the recovery period, whereas the peak expression of c-myc mRNA in the aged GA lymphocytes after HS was retarded in time (2 h) but also reduced in amount, when compared to that of the young, and during recovery the expression declined more rapidly in the aged than in the young. However, the recovery kinetics of the above three mRNAs remained unaltered in the lymphocytes from two different ages. To summarize our conclusions: First, prior to HS cellular stress, aged GA lymphocytes did not show mRNA expression of HSP68 but augmented mRNA expression of the nucleoprotooncogenes c-fos and c-myc. These findings suggest that senescent lymphocytes are more active metabolically in vivo than are young ones without significant activation of an inducible HSP70 family gene. Second, the effect of HS stress in vitro is to evoke reduction in the peak expression of the inducible HSP mRNA in aged GA lymphocytes, and the kinetic of a decline of mRNA expression during recovery did not alter in both age groups. Third, HS stress in vitro diminishes both the peak expression of c-fos and c-myc in aged GA lymphocytes; their post-HS recoveries were rapid, but with the similar kinetics of decay to that of the young, suggesting a declined responsiveness of aged GA lymphocytes to cellular stresses, and that the expression of inducible HSP70 family mRNA intimately correlates with that of the nucleoprotooncogene (c-fos and c-myc) mRNA. The latter cell activation-related nucleoprotooncogene genes, particularly c-fos, may play a role in the induction of HSP68 mRNA in the early response to HS stress. The hyporesponsiveness of HSP mRNA induction in the aged lymphocytes disfavors its protective capacity against exogenous cellular stresses.