Calorie restriction, stress and the ubiquitin-dependent pathway in mouse livers

Calorie restriction (R) is the only known method to delay the aging process and extend mean and maximal lifespan in rodents. R has been shown to delay the age-related accumulation of damaged proteins and to protect organisms from various stresses which can produce damaged proteins. Such stresses inc lude irradiation, heat shock, and oxidative stress. The ubiquitin- and ATP- dependent proteolytic pathway (UPP) has been associated with the degradatio n of abnormal and/or damaged proteins. We examined the effect of diet and o xidative stress on activities of the UPP in supernatants from livers taken from 23-month-old Emery mice which had been exposed to an in-vivo injection of paraquat. Paraquat induces oxidative stress by generating superoxide ra dicals. In livers from non-stressed animals, steady-state levels of endogen ous ubiquitin conjugates, de novo conjugate formation, and El and E2 activi ties were significantly lower in R animals than in control (C) animals. How ever, after exposure to paraquat, levels of endogenous ubiquitin conjugates were significantly higher in R versus C animals, and de novo conjugate for mation and E1 and E2 activities in R animals rose to levels which were indi stinguishable from levels of these activities noted in C animals. R was ass ociated with an increased ability to degrade P-lactoglobulin by the UPP aft er an oxidative stress was imposed. Ability to degrade beta-lactoglobulin b y the C or R livers in non-stressed animals was not significantly different . Taken together, these data indicate that oxidative stress in R animals is associated with enhanced levels of ubiquitin conjugates and that this enha ncement may be due to an increase in UPP activity. These data also indicate that the ability to form ubiquitin conjugates and the UPP system does not change with oxidative stress in C animals. The latter is consistent with pr ior reports that suggests that older C animals may already be in a state of enhanced oxidative stress and that activities of the UPP provide a sensiti ve indicator of levels of cellular redox status. (C) 1998 Elsevier Science Ltd. All rights reserved.