Aging and the liver: functional aspects

Comprehensive transcriptional profiling of aging porcine liver

Background

Aging is a major risk factor for the development of many diseases, and the liver, as the most important metabolic organ, is significantly affected by aging. It has been shown that the liver weight tends to increase in rodents and decrease in humans with age. Pigs have a genomic structure, with physiological as well as biochemical features that are similar to those of humans, and have therefore been used as a valuable model for studying human diseases. The molecular mechanisms of the liver aging of large mammals on a comprehensive transcriptional level remain poorly understood. The pig is an ideal model animal to clearly and fully understand the molecular mechanism underlying human liver aging.

Methods

In this study, four healthy female Yana pigs (an indigenous Chinese breed) were investigated: two young sows (180-days-old) and two old sows (8-years-old). High throughput RNA sequencing was performed to evaluate the expression profiles of messenger RNA, long non-coding RNAs, micro RNAs, and circular RNAs during the porcine liver aging process. Gene Ontology (GO) analysis was performed to investigate the biological functions of age-related genes.

Results

A number of age-related genes were identified in the porcine liver. GO annotation showed that up-regulated genes were mainly related to immune response, while the down-regulated genes were mainly related to metabolism. Moreover, several lncRNAs and their target genes were also found to be differentially expressed during liver aging. In addition, the multi-group cooperative control relationships and constructed circRNA-miRNA co-expression networks were assessed during liver aging.

Conclusions

Numerous age-related genes were identified and circRNA-miRNA co-expression networks that are active during porcine liver aging were constructed. These findings contribute to the understanding of the transcriptional foundations of liver aging and also provide further references that clarify human liver aging at the molecular level.

Molecular alterations in proteasomes of rat liver during aging result in altered proteolytic activities

Aging induces alterations of tissue protein homoeostasis. To investigate one of the major systems catalysing intracellular protein degradation we have purified 20S proteasomes from rat liver of young (2 months) and aged (23 months) animals and separated them into three subpopulations containing different types of intermediate proteasomes with standard- and immuno-subunits. The smallest subpopulation ΙΙΙ and the major subpopulation Ι comprised proteasomes containing immuno-subunits β1i and β5i beside small amounts of standard-subunits, whereas proteasomes of subpopulation ΙΙ contained only β5i beside standard-subunits. In favour of a relative increase of the major subpopulation Ι, subpopulation ΙΙ and ΙΙΙ were reduced for about 55 % and 80 %, respectively, in aged rats. Furthermore, in all three 20S proteasome subpopulations from aged animals standard-active site subunits were replaced by immuno-subunits. Overall, this transformation resulted in a relative increase of immuno-subunit-containing proteasomes, paralleled by reduced activity towards short fluorogenic peptide substrates. However, depending on the substrate their hydrolysing activity of long polypeptide substrates was significantly higher or unchanged. Furthermore, our data revealed an altered MHC class I antigen-processing efficiency of 20S proteasomes from liver of aged rats. We therefore suggest that the age-related intramolecular alteration of hepatic proteasomes modifies its cleavage preferences without a general decrease of its activity. Such modifications could have implications on protein homeostasis as well as on MHC class I antigen presentation as part of the immunosenescence process.

Association of age-dependent liver injury and fibrosis with immune cell populations

BACKGROUND & AIMS

The liver's response to injury is fibrosis, and when chronic, cirrhosis. Age is a critical factor impacting many immune-mediated processes, potentially including the liver's wounding response to injury.

METHODS

The effects of age on acute and chronic liver injury were evaluated using a carbon tetrachloride model in mice. Lymphocyte and macrophage populations were assessed by flow cytometry and immunohistochemical analysis.

RESULTS

Acute liver injury was greater in 18-month-old (old) mice than in 9-month-old (middle-aged) mice as judged by changes in aminotransferases. Similarly, livers of 18-month-old mice had a significantly greater fibrogenic response to injury than did livers of 9-month-old mice after chronic injury (assessed by col1α1 mRNA expression, morphometric analysis and hydroxyproline measurement). Interestingly, livers from young mice (6 weeks old) also exhibited an increase in fibrogenesis compared to 9-month-old mice, albeit not to the same degree as in old mice. Consistent with a role for macrophages in fibrogenesis, the number of liver macrophages in young and 9-month-old mice increased, while in chronically injured livers of 18-month-old mice, the number of macrophages was reduced, and was less than in the livers of young and 9-month-old injured livers.

CONCLUSIONS

Our data indicate that the fibrogenic response to injury varies substantially with age, and moreover that macrophage recruitment and dynamics may be an important component in differential age-associated fibrotic disease.

The hydrogen sulfide signaling system: changes during aging and the benefits of caloric restriction

Hydrogen sulfide gas (H(2)S) is a putative signaling molecule that causes diverse effects in mammalian tissues including relaxation of blood vessels and regulation of perfusion in the liver, but the effects of aging on H(2)S signaling are unknown. Aging has negative impacts on the cardiovascular system. However, the liver is more resilient with age. Caloric restriction (CR) attenuates affects of age in many tissues. We hypothesized that the H(2)S signaling system is negatively affected by age in the vasculature but not in the liver, which is typically more resilient to age, and that a CR diet minimizes the age affect in the vasculature. To investigate this, we determined protein and mRNA expression of the H(2)S-producing enzymes cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), H(2)S production rates in the aorta and liver, and the contractile response of aortic rings to exogenous H(2)S. Tissue was collected from Fisher 344 × Brown Norway rats from 8-38 months of age, which had been maintained on an ad libitum (AL) or CR diet. The results demonstrate that age and diet have differential effects on the H(2)S signaling system in aorta and liver. The aorta showed a sizeable effect of both age and diet, whereas the liver only showed a sizeable effect of diet. Aortic rings showed increased contractile sensitivity to H(2)S and increased protein expression of CSE and CBS with age, consistent with a decrease in H(2)S concentration with age. CR appears to benefit CSE and CBS protein in both aorta and liver, potentially by reducing oxidative stress and ameliorating the negative effect of age on H(2)S concentration. Therefore, CR may help maintain the H(2)S signaling system during aging.

Ischemia-Reperfusion Injury is More Severe in Older Versus Young Rat Livers

BACKGROUND

Hepatic warm ischemia during surgery remains a significant problem, particularly in organs with possible baseline dysfunction. The objective of this study was to investigate whether age influences the degree of warm ischemia-reperfusion injury in rat livers.

MATERIALS AND METHODS

The left and median lobes of young (3 months) and adult (9 months) male rats were exposed to 75 min of ischemia followed by reperfusion. Each age group was divided into two sub-groups. One sub-group was observed for 8 h, whereas the other was allowed to survive. Animals in the 8-h groups (young and adult) were sacrificed, and blood and tissue were taken to determine liver enzymes, neutrophil accumulation, and blood metabolic profiles and to examine the histology.

RESULTS

Hepatocellular injury was significantly greater in adult rats after 8 h of reperfusion, as determined by hepatic enzyme levels and histology. Liver enzyme levels were massively elevated in adult rats and were significantly higher compared with those of young rats. The degree of necrosis and neutrophil accumulation was significantly higher in adult rats. After 8 h of reperfusion, the metabolic profiling of the blood revealed elevated levels of creatine, creatinine, allantoin, and amino acids (tyrosine, methionine) in the adult rats. At 24 h of reperfusion, all adult rats died, in contrast to young rats, which all survived.

CONCLUSIONS

Aging in rats is associated with greater hepatocellular injury and poor survival rate after 75 min of warm hepatic ischemia.

Age-related changes on parameters of experimentally-induced liver injury and regeneration

Age-dependent changes related to liver injury and regeneration were studied in rats aged 2, 12, and 30 months in a time period of 96 hr following a sublethal dose of thioacetamide (6.6 mmoles/kg body wt). Serum aspartate aminotransferase activity increased earlier in young rats, but the severity of injury was higher in those aged 12 months when compared to young and to old. Microsomal hepatocyte FAD monooxygenase activity was induced earlier in 2-month-old rats following intoxication and the increase was significantly lower both in the youngest and in the oldest groups when compared to adults. As a parameter of hepatocellular postnecrotic regeneration, DNA synthesis (2C --> 4C) was evaluated. The population of hepatocytes in S phase peaked more sharply and earlier in young rat hepatocytes, and was 8 to 12 times higher than the initial in hepatocytes from 2- and 12-month-old rats, while the rise was only 3 times in the oldest group. At 96 hr of intoxication the restoration towards normal in all these parameters was complete in young, incomplete in adult, and slightly detected in the oldest. Serum proliferative activity, assayed on mouse NIH 3T3 fibroblast cultures, increased preceding the necrosis and this increase was higher in 2- and 12-month-old (171% and 224%, respectively), while in the oldest the increase was only 110%. This mitogenic activity decreased in all groups during necrosis, showing a second peak, nondetectable in rats aged 30 months, parallel to regeneration. Serum TNFalpha level was absent in untreated animals and increased markedly following intoxication, the highest values being recorded at 72 hr of intoxication in serum from rats aged 12 months (347 +/- 30 pg/ml) and the lowest at 30 months (4.1 +/- 0.3 pg/ml). The serum ability to induce nitric oxide synthase activity on peritoneal macrophages ex vivo showed significant time- and age-dependent changes in nitric oxide release: a decrease throughout necrosis and an increase during regeneration. We conclude that the main age-related changes in the sequenced process of liver injury and regeneration are the delayed response in the development of cell killing and regeneration and the decreased regenerative ability, which significantly delays the restoration of liver function.

c-PKC-dependent modulation of plasma fibrinogen levels during the acute-phase response in young and old rats

The expression and subcellular distribution of liver cPKC alpha and beta, nPKC delta and aPKC zeta isoenzymes and the plasma levels of fibrinogen were measured in young, 2- and 6-month-old, and aged, 24-month-old, normal and turpentine-treated rats, to induce an aseptic inflammatory condition and the acute-phase response. In young and old rats a down-regulated expression of cPKC alpha and, to a lesser extent, beta isoenzymes, was observed 8 h after turpentine administration, i.e. at times preceding the maximal expression of fibrinogen mRNA. Under these conditions, the plasma fibrinogen levels peaked by 24 h in young animals, being up to 7-fold over the values of untreated controls at 72 h. By contrast, old untreated control rats showed 4-fold increases of basal plasma fibrinogen levels compared with young controls, with down-regulated expression of cPKC alpha. In old rats, treatment with turpentine increased up to 1.9-fold over the basal control values the fibrinogen concentration within 72 h. Levels similar to those of young turpentine-treated animals were reached at this time. The results of this study suggest a prominent role for cPKC alpha in eliciting the synthesis of fibrinogen after inducing an acute-phase response with turpentine administration in young as well as old rats. This isoform may act by regulating the serine phosphorylation of Stat3 transcription factor, whose activation is under IL-6 control, a multifunctional cytokine that is proving to be a major contributor to the acute-phase response. No evidence for a role of aPKC zeta or of nPKC delta was demonstrated under these conditions.

Aging delays the post-necrotic restoration of liver function

Age-associated changes in liver injury and post-necrotic regeneration were studied in rats aged 6 and 30 months in a period of 96 h following a dose of thioacetamide (6.6 mmol/kg body weight). Hepatocellular necrosis was detected in both groups by serum aspartate aminotransferase, but the severity of injury was significantly lower (one fourth, p < 0.001) in the oldest. Differences were observed in hepatocyte FAD monooxygenase activity between 6 and 30 months old rats at 24 h (278 versus 170%, p < 0.001, respectively) and also in GSH/GSSG ratio, in protein thiol groups and in malondialdehyde. Glutathione peroxidase, glutathione reductase and glucose-6-phosphate dehydrogenase activities rose markedly in both groups, this increase being slightly lower in the oldest. Superoxide dismutase and catalase did not show significant changes between both groups. At the end of the 96 h experimental period the restoration towards normal of GSG/GSSG, protein thiols malondialdehyde and the activities of Cu-Zn superoxide dismutase and catalase were significantly lower in hepatocytes from 30 months old rats. We summarize that the main age-related changes in the sequenced process of liver injury and regeneration occurred to a lesser extent in severity of injury and delayed response in the post-necrotic restoration of liver function, probably due to a lower increase in antioxidant enzyme system.

Age-dependent modifications in rat hepatocyte antioxidant defense systems

BACKGROUND/AIMS

Age-dependent changes in the hepatic antioxidant systems were studied in hepatocytes from newly weaned (21 days) to 30-month-old rats.

RESULTS

Biphasic changes were observed in superoxide dismutase (SOD), glucose-6-phosphate dehydrogenase (G6PDH) and malic enzyme (ME), in which noticeable decreases were detected in hepatocytes from newly weaned to 6-month-old rats: Cu-Zn SOD decreased to 46% (p < 0.001), Mn SOD to 41% (p < 0.001), G6PDH to 71% and ME to 19% (p < 0,001), and significant increases were observed from 6 to 30 months. In hepatocytes from 6- to 30- month-old rats the enzymes involved in antioxidant defense underwent increases in their activities as well in their mRNA: Cu-Zn SOD (142%, p < 0.001), catalase (182%, p < 0.001) and glutathione peroxidase (325%, p < 0.001). However, chronological decreases were observed in the levels of reduced glutathione (69%, p < 0.001), in the GSH/GSSG ratio (78%) and in protein thiol groups (55%, p < 0.001), with concomitant increases in peroxides (155%, p < 0.001) and malondialdehyde (142%, p < 0.001) levels. DNA ploidy was also assayed by flow cytometry; a sharp increase in tetraploid (2.5-40.1%, p < 0.001) and octoploid (0.1-16.1%; p < 0.001) populations, and a noticeable decrease in diploid hepatocytes (92.9-34.3%; p < 0.001), were observed. Populations involved in 2C-->4C DNA synthesis decreased from 3.6 to 0.9% (p < 0.001), while those involved in 4C-->8C increased from 0.9% to 5.2% (p < 0.001). A hypodiploid population (apoptotic cells) was detected from 12 months, increasing thereafter.

CONCLUSIONS

These results show that the antioxidant cell defense system increases with age but the rate of reactive oxygen species generation exceeds the induced antioxidant ability, generating a situation that favors oxidative stress and peroxidation. The progressive polyploidization is accompanied by changes in the proliferative potential that decreases from 2C to 4C and increased from 4C to 8C. The relationship between the modifications of the oxidant/antioxidant system and increased polyploidy is not clear and may be interpreted as two independent manifestations of the aging process.

The influence of age and some inducers on UDP-glucuronyltransferase activity

UDP-glucuronyltransferase (UDP-GT) activity was examined in male Wistar rats aged 0.5, 1, 2, 4, 8, 12, 20, and 28 months. The rats were treated with phenobarbital (75 mg/kg, 72 and 48 h before death), beta-naphthoflavone or dexamethasone (40 mg/kg and 20 mg/kg, respectively, for three days before death). Prior to decapitation the rats were fasted for 12 h. Hepatic microsomes were prepared according to the method of Dallner. UDP-GT activity was determined by the method of Burchell and Weatherill. p-Nitrophenol was used as an aglucone. UDP-GT activity decreased rapidly in the control rats aged from two weeks to four months. In the older control rats the activity tended to increase. Two-week-old rats treated with phenobarbital showed a slightly increased UDP-GT activity. In the older animals (up to one year) UDP-GT activity increased to 150% of the control value and stayed at this level in the remaining age groups. beta-Naphthoflavone was a more potent inducer of UDP-GT than phenobarbital. The activity of beta-naphthoflavone-induced UDP-GT was low in the youngest rats. It was about 180% in two-month-old rats and reached 260% of the control value in eight-month-old rats. Although the activity decreased in the older rats, it still exceeded 200%. Dexamethasone did not affect UDP-GT activity. Only in two-week-old and two-month-old rats did we observe a slight increase in the activity of UDP-GT.

Endogenous antioxidant systems of two teleost fish, the rainbow trout and the black bullhead, and the effect of age

Antioxidant enzyme activities and glutathione status were determined in different tissues of two teleost species, the rainbow trout (Oncorhynchus mykiss) and the black bullhead (Ameiurus melas) to establish whether age-related changes exist between mature and immature individuals. Glutathione reductase and superoxide dismutase activities were significantly lower in hepatic and extrahepatic tissues of 3+ year than 1+ year trout and bullheads. Activities of glutathione peroxidase, catalase and glutathione S-transferase did not exhibit a clear pattern, with decreases in liver and kidney, but increases in gill and muscle tissues. Glutathione concentrations were significantly higher in most tissues of 3+ year than in 1+ year trout, but remained unchanged or decreased in tissues of older bullheads. The results imply an age- or maturation-dependent effect on key antioxidant enzymes in various tissues of these 2 teleost species. Thus, age and maturation may impact upon the use of oxidative stress parameters as indicators of contaminant exposure in environmental studies.