The Aging Endoplasmic Reticulum. (Paperback)

Hallmarks of aging include the accumulation of aberrant proteins and a lower resistance to stresses. Because the endoplasmic reticulum (ER) functions to fold proteins and possesses unique stress sensing pathways, the central hypothesis of this dissertation is that the ER is a target of cellular aging and significantly underlies these age-related phenotypes. This hypothesis was tested using isolated microsomes from young (3-6 mo) and old (24-26 mo) rats and a novel primary hepatocyte cell culture model that maintains the aging phenotype. The ER luminal oxidation/reduction (redox) environment, exemplified by the glutathione redox ratio (GSH: GSSG), is vital for both ER stress signaling and the regulation of ER oxidoreductase activity. Therefore, the hepatic microsomal GSH: GSSG ratio from young and old rats was determined. Our results showed that the GSH: GSSG ratio in young rats was 3.8:1 and that, in contrast to the age-related loss of GSH redox in whole liver, microsomal GSH: GSSG increased to 6.4:1 in old rats. This increase is sufficient to convert ER oxidoreductases from folding proteins to enzymes involved in protein unfolding. These results show that the ER is under an increased stress with age, yet no activation of ER stress response pathways was detected. Using the aging cell culture model noted above to examine the loss of ER stress response with age, we show that following induction of an ER stress by tunicamycin, ER-mediated "pro-apoptotic" pathways were in intact with age, but induction of "pro-survival" genes were significantly delayed. Taken together, these show that aging leads to: (1) a reductive shift in ER thiol redox, and (2) an attenuation of ER stress response. Lastly, previous work showed that (R)-alpha-lipoic acid (R-LA) restores GSH levels and redox status in aged rat livers. To determine whether R-LA also reversed the age-related changes in ER GSH redox status, young and old rats were supplemented with R-LA. Results showed that R-LA reversed both the age-related increase in the ER GSH redox state and induced protective ER stress response pathways in a manner similar to young animals. Thus, R-LA may be an effective therapeutic to maintain ER homeostatic mechanisms with age.