Reduced thermotolerance in aged cells results from a loss of an hsp72-mediated control of JNK signaling pathway.

Aged organisms exhibit a greatly decreased ability to induce the major heat shock protein, Hsp72, in response to stresses, a phenomenon that can also be observed in cell cultures (Heydari AR, Takahashi R, Gutsmann A, You S and Richardson A (1994) Hsp70 and aging. Experientia 50: 1092-1098). Hsp72 was shown to protect cells from a variety of stresses. The protective function of Hsp72 has been commonly ascribed to its chaperoning ability. However, recently we showed that Hsp72 protects cells from heat shock by suppression of a stress-kinase JNK, an essential component of the heat-induced apoptotic pathway (Gabai VL, Meriin AB, Mosser DD, Caron AW, Rits S, Shifrin VI and Sherman MY (1997) Hsp70 prevents activation of stress kinases. A novel pathway of cellular thermotolerance. J Biol Chem 272: 18033-18037). Here we demonstrate that because of the diminished inducibility of Hsp72 in aged cells, Hsp72-mediated control of JNK signaling pathway is compromised. This results in increased rate of apoptotic cell death following heat shock. We show that forced expression of Hsp72 in aged cells from an adenovirus-based vector completely suppresses activation of JNK by heat shock and consequently protects from heat-induced apoptosis. We also demonstrate for the first time that it is possible to restore endogenous expression of Hsp72 in aged cells. This can be achieved by treatment with the proteasome inhibitor MG132. Induction of Hsp72 in aged cells under these conditions leads to suppression of JNK activation by a heat shock and restoration of thermotolerance manifested in a lower rate of apoptosis.