Age-related changes in inducible mouse liver enzymes: ornithine decarboxylase and tyrosine aminotransferase

Effect of dietary restriction on hepatic and renal phosphoenolpyruvate carboxykinase induction in young and old Fischer 344 rats

Food restriction is known to ameliorate many of the adverse physiologic effects of age. In this study, we have examined the effect of food restriction on the induction of the gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK) in liver and kidney following a 12-h fasting period in young (6 month) and old (24 month) ad libitum-fed and food-restricted male Fischer 344 rats. In the liver, following the 12-h fast, the activity of PEPCK increased approximately 2-fold in the young ad libitum fed rats and 3-fold in the young restricted animals. However, PEPCK activity remained unchanged in response to the 12-h fast in the 24 month old ad libitum fed rats. In the old restricted rats, the induction of PEPCK mimicked that of the young rats (PEPCK activity increased 2-fold within the 12-h fasting period). Therefore, dietary restriction not only enhanced the induction response in the liver in young rats, but also restored the induction of hepatic PEPCK in the old animals. In the kidney, there was no effect of age or dietary restriction on the induction of PEPCK as the activity of renal PEPCK did not change in response to the 12-h fast in any of the four groups of rats.

Effect of age on the activity of phosphoenolpyruvate carboxykinase in the kidney following fasting and refeeding

The activity of renal phosphoenolpyruvate carboxykinase (PEPCK) was measured in 3-18 month old male Fischer 344 rats after alternate periods of fasting and refeeding. To compare the induction of renal PEPCK activity in response to fasting in young and old rats, 3, 6, 12 and 18 month old animals were fasted for 30 h followed by a 24 h ad libitum refeeding period to reduce PEPCK activity toward basal levels. The refeeding period was followed by a second 24 h fasting period during which the time course of PEPCK induction was monitored in the young and old animals. The first fast resulted in over a 20% increase in renal PEPCK activity in the 3 month old and slightly over a 70% increase in the 6 month old animals. In contrast, the activity did not increase significantly in the 12 or 18 month old animals during this fasting period. Therefore the induction of PEPCK in the kidney in response to fasting appears to be altered in the older animals. Refeeding for 24 h resulted in a decrease in PEPCK activity in all four age groups; therefore there was no indication of an age-related impairment in the response of renal PEPCK to refeeding. After the refeeding period, the food was removed again and the activity was measured at short intervals over the next 24 h to determine the time course of the induction in PEPCK activity. Interestingly, during the second fast, the activity of renal PEPCK was not significantly induced in either the young or the older animals. However, the activity measured in the older 18 month rats was consistently lower during the first 12 h of the second fast as compared to the activity in the 6 month old rats. In summary, the induction of PEPCK activity in the kidney is altered with age during an initial fast; in addition, PEPCK activity is not induced in either young or old rats during a second fasting period.

The effect of exercise, diet restriction, and aging on the pituitary--adrenal axis in the rat

The effects of calorie restriction, exercise, and aging on the pituitary-adrenal axis were studied in male Wistar rats from 12 to 28 months of age. There were 4 experimental groups: sedentary, ad libitum fed (A); sedentary, diet restricted by feeding on alternate days (R); exercised by swimming on alternate days, ad libitum fed (AE); exercised as AE, diet restricted as R (RE). Pituitary-adrenal function was assessed by measuring serum ACTH and corticosterone concentrations, adrenal weight, hepatic glucocorticoid receptor concentration, and hepatic tyrosine aminotransferase (TAT) activity. Serum corticosterone concentration increased with age while serum ACTH decreased from 12 to 20 months of age and then increased thereafter. TAT activity decreased and receptor concentration remained constant with age. Adrenal weights increased with age; those of AE rats increased dramatically. Analyses for relationships between variables revealed a quadratic relationship between serum ACTH and corticosterone concentrations. There tended to be an inverse relationship between TAT activity and corticosterone concentration. These observations may be indicative of a loss of feedback loop integrity with aging. Neither calorie restriction nor exercise were able to maintain the integrity of pituitary-adrenal function during aging, though dietary restriction did slow age-associated decrements of TAT activity.

Differential degradation of intracellular proteins in human skin fibroblasts of mitotic and mitomycin-C (MMC)-induced postmitotic differentiation states in vitro

Rates of degradation of short- and long-lived proteins were analysed in homogeneous fibroblast cultures of mitotic or mitomycin C (MMC)-induced postmitotic states. When the highly mitotic MFII type cells--the major cell type of so called "early passage" or "young" fibroblasts--differentiate into MFIII type cells, the last mitotic fibroblast type, and further into MMC-induced postmitotic fibroblasts, the degradation of short-lived proteins increases by a factor of 1.4, resulting in significantly reduced half-lives of these proteins in the postmitotic fibroblasts. From the highly mitotic MFII to the final postmitotic PMFVI-type cells via the intermediates MFIII, PMFIV and PMFV, the half lives (t1/2) of short-lived proteins decrease by a total of 122 min in average, from 362 to 240 min. Degradation of long-lived proteins did not change significantly from cell type MFII to PMFVI. As analysed by two-dimensional (2D)-gel electrophoresis the half-lives of the mitotic and postmitotic cell-type-specific proteins except one, protein PIVa (33 kDa; Pi 5.0), range between 33.2 h and 62.9 h. Protein PIVa, the first protein specific for postmitotic cells, is initially expressed 18 h after the induction of the postmitotic state by mitomycin C (MMC) and has a half-life of approximately 66 min. This may indicate that protein PIVa could function as one possible regulatory factor controlling the postmitotic differentiation state.

Aging in the AXC/SSh rat: characterization of moderately abundant ventral prostate proteins showing age-dependent diminution and one protein exhibiting age-invariant content

To determine whether prior demonstrations of age-related decrements in prostate content of minor, androgen regulated proteins represent a generalized phenomenon, we validated a denaturing polyacrylamide gel electrophoretic protocol for separation and quantification of moderately abundant ventral prostate cytoplasmic proteins. We established age-related, progressive 3- to 3.5-fold decreases in prostate content of proteins of 90, 79, 63, and 58 kDa and found that content of a 46 kDa protein was age-invariant. The amount of 90 and 46 kDa proteins was not significantly altered, whereas the level of 79, 63 and 58 kDa proteins decreased during 72 h post-orchiectomy of 3-month-old rats. Testosterone injection of intact 26-month-old rats caused an average 2-fold increase in 90, 79, 63, and 58 kDa protein content and did not affect 46 kDa protein level. Because we demonstrated the 46 kDa protein is not a secretory protein, absence of an affect of aging or testosterone on prostate content is not due to secretion mediated inaccessibility to intracellular processing. The apparent relation between age and prostate content of these proteins is not a consequence of potential age-related changes in ventral prostate cell content or distribution because biochemical and histologic analyses show this does not significantly occur. Our studies establish age-related decreases in ventral prostate content of moderately abundant, androgen responsive proteins and show that content of at least one protein is age- and androgen-independent. It remains to be determined whether these findings reflect direct effects of gene regulation.

The effect of age on the induction of tyrosine aminotransferase and tryptophan oxygenase genes by physiological stress

Expression of the genes coding for tyrosine aminotransferase and tryptophan oxygenase in rats is induced in response to cold stress. We have studied the effect of ageing on this induction. The induction of tyrosine aminotransferase activity in young adult rats (10 months) was about twice that observed with old rats (25 months). This difference between the two age groups was also observed when the steady-state level of tyrosine aminotransferase mRNA was measured by hybridization with a specific DNA probe. However, when the transcription rate of the gene was measured by in vitro elongation of nascent RNA in isolated nuclei, no difference was detected. In contrast to the results with tyrosine aminotransferase, induced tryptophan oxygenase enzyme and mRNA levels did not show an age-dependent difference. These results suggest that there is, with ageing, an impairment in post-transcriptional regulation of the synthesis of tyrosine aminotransferase. The regulation of tryptophan oxygenase, on the other hand, is similar in the two age groups.

Age-dependent modifications of rat heart succinate dehydrogenase

The activity of rat heart succinate dehydrogenase (SDH) increases 1.7-fold in old animals (Vitorica, J., Cano, J., Satrústegui, J. and Machado, A., Mech. Ageing Dev., 16 (1981) 105-116). This increase is due to an increase in enzyme protein itself because: (a) the activation state of the enzyme does not vary with age; and (b) the increase of activity is paralleled by an increase in immunoprecipitable SDH in old rat heart mitochondria. SDH from old rat heart mitochondria differs in a number of ways from that of young animals: (a) The km value for succinate increases with age. (b) The thermostability decreases, and the activation energy in the 20-40 degrees C interval is higher in old animals. (c) The breaking points of the Arrhenius plots of SDH are shifted to higher values. (d) Reactivity towards N-ethylmaleimide in succinate protected mitochondria decreases with age.

A mathematical analysis of aging influences on enzyme deactivation/activation kinetics. Examples of the influence of regional brain development and drugs in rats

A series-type enzyme deactivation/activation model involving active enzyme states is utilized to theoretically quantify the influence of regional brain development and drugs on enzyme activity levels in rats. Continuous hexachlorobenzene administration with, or without, phenobarbitone pretreatment has different effects on the deactivation/activation kinetics of porphyrinogen carboxylase, delta-aminolaevulinate synthase and delta-aminolaevulinate dehydratase. The deactivation/activation kinetics exhibited by pyruvate dehydrogenase, citrate synthase, and D-3-hydroxybutyrate dehydrogenase during the development of the medulla oblongata, mid-brain, striatum, and hypothalamus sections exhibit similarities as well as discrepancies. These are identified and made more quantitative.

Changes during aging in rat lens endopeptidase activity

Protein degradation has been implicated in lens aging and cataract development. We propose that proteolytic enzymes are important in these processes. In this study, lens neutral proteinase activity and thermal stability have been measured as a function of cell age and animal age in rat lens. Epithelial, cortical, and nuclear lens regions from animals between 1 and 25 months were analyzed. Specific activity and thermal stability were found to decrease with lens cell age, that is, from epithelium to nucleus. Specific activity in the cortex increased with animal age, while specific activity in the nuclear region remained constant with animal age. In the epithelium, specific activity showed no correlation with animal age. In 15 day old rats, thermal stability curves for the cortical enzyme were linear, indicating only one form of activity hydrolyzing synthetic substrate was present. At 1.25 months of age, thermal stability curves for the cortical enzyme were non-linear indicating that multiple forms of the activity were present.

Polyamines in mammalian ageing: an oncological problem, too? A review

This review surveys the literature about changes in polyamine contents and levels of activity of the enzymes involved in the polyamine biosynthetic pathway in organs of ageing mammals. The literature about changes in the polyamine levels in physiological fluids in healthy ageing humans is also reviewed. Generally speaking, decreases in the levels of the main polyamines (noticeably putrescine and spermidine) are observed in different mammalian organs with ageing. The polyamine levels in serum and in urine of healthy human beings are also age-related, declining progressively with increasing age. Some major enzymes (i.e., ornithine decarboxylase (EC 4.1.1.17) and S-adenosyl-L-methionine decarboxylase (EC 4.1.1.50) involved in the polyamine biosynthetic pathway show similar trends. Hormonal induction of ornithine decarboxylase activity is strongly reduced in organs of aged animals, as it is in neoplastic organs. There is also some evidence for an age-related decrease in the level of ornithine decarboxylase and its inducibility in mammalian cells cultured in vitro. Some in vitro effects of spermidine and spermine on aged structures or systems are briefly summarized. There is no evidence yet that this generally reduced capacity of mammalian aged organs for polyamine biosynthesis is one of the factors responsible for the well known high incidence of some neoplasias in elderly humans. In view of the typical stimulatory effects of the tumour promoters on polyamine biosynthesis in target tissues and the effects of senescence on the same metabolic pathway, it can be excluded that the ageing process has a tumour promoting effect by itself. However, although the exact mechanism responsible for the increased occurrence of some tumors during mammalian senescence is still obscure, there are enough experimental data (both in humans and in animals) to indicate that the reduced polyamine biosynthetic capacity of aged mammals can account for the slower course of some tumors in elderly patients.

Altered degradation of intracellular proteins in aging human fibroblasts

Confluent cultures of fibroblasts at different population doubling levels were incubated with [14C]leucine for 2 days and with [3H]leucine for 2 h to label long-lived and short-lived proteins, respectively. Proteolysis was then measured in the presence of excess unlabeled leucine to prevent reutilization of the isotope. Catabolism of long-lived proteins was reduced in senescent cells when measured in media without fetal bovine serum, insulin, fibroblast growth factor, or dexamethasone. In contrast, degradation of short-lived proteins was increased in senescent cells but only when measured in the presence of serum, hormones, and growth factors. Further experiments with cells of varying ages indicate that in unsupplemented medium half-lives of long-lived proteins lengthened by as much as 20 min per population doubling and in supplemented media half-lives of short-lived proteins decreased by 4 min per population doubling. The reduced catabolism of long-lived proteins in senescent cells cannot be explained by age-related changes in protein secretion or cell death during degradation measurements. These alterations in proteolysis may have major effects on protein content and composition in senescent cells.

Current concepts: I. The relationship between age-related changes in gene expression, protein turnover, and the responsiveness of an organism to stimuli

A general decline in gene expression, translation and transcription, has been observed to occur with increasing age in a wide variety of organisms and tissues. Because the level of most enzymes and proteins remains relatively constant with increasing age, one would predict that the decline in gene expression would result in an age-related decline in protein turnover. Recent studies show that protein turnover in mouse liver and nematodes declines with increasing age. The decline in protein turnover could lead to an age-related decrease in the response of inducible enzymes to stimuli. This could explain the molecular basis for the decline in aging organisms to respond to a variety of environmental factors.