Biology of senescent liver peroxisomes: role in hepatocellular aging and disease.

Despite rising interest in the health problems of the elderly, information on senescence-related alterations in essential metabolic pathways and their responses to various chemicals is scarce. Although peroxisomal pathways are involved in a multitude of cellular functions, little attention has been given to the potential relationship between senescence of these organelles and the process of aging and disease. Although the prevailing experimental evidence points to a decline in liver peroxisomal enzyme activities and a muted response to peroxisome-proliferating chemicals in aged animals, it is also evident that aged animals are more susceptible, in comparison to their young counterparts, to the hepatocarcinogenic effects of these chemicals. Furthermore, little is known about extraperoxisomal effects of peroxisome proliferators in aged animals. This review evaluates published studies on the impact of aging on basal hepatic peroxisomal metabolism, response to peroxisome proliferators, and changes in signal transduction pathways involved in these processes, with the aim of stimulating research efforts in this important area. The potential intricate relationship among senescent peroxisomes, aged hepatocytes, and health are also discussed.

Aging has been defined as a progressive loss of physiologic capacities that culminates in death (1). Most physiologic functions decline with age, although to different extents (1,2). Evidence indicates that cells accumulate damage over a lifetime (3). This slow incremental damage results in the gradual loss of differentiated functions and growth rate accompanied by an increased probability for the development of cancer (3). Such changes are normally held to a minimum by the organized state of the tissue and homeostatic regulation of the organism (3).
Despite the myriad of peroxisomal enzymes involved in essential cellular metabolic pathways, little attention has focused on the relationship among changes in the basal activities of these enzymes, their response to peroxisome-proliferating chemicals, and the process of cellular aging. This paucity of information is particularly alarming in light of the fact that the elderly population, particularly in the industrialized world, is exposed over its life span to an ever-increasing number of peroxisome-proliferating chemicals in the form of therapeutic agents and environmental pollutants.

Aging of the Liver
The liver, like most organs, is altered morphologically and functionally in old organisms; many age-related changes in hepatocytes are similar to alterations observed in other cell types [ Table 1; (4,5)]. Therefore, common mechanisms of aging may operate at the cellular level and cause widespread decline in physiologic processes, eventually culminating in death (5). The aged liver has a reduced ability to regenerate (6,7). Increases in mitosis in response to hepatectomy or to chemicals are diminished in old animals, as compared to their young counterparts (6)(7)(8)(9). Furthermore, the pool of proliferating hepatocytes is smaller in old animals, and is more sharply localized to periportal hepatocytes (7). The importance of these changes in aging is unclear. Studies have produced conflicting data on the effect of aging on the size of hepatocytes. Although most studies described conspicuous enlarged as well as small hepatocytes in old rats (7), others found far fewer variations of cell size, especially in female rats (7). Interestingly, hepatocellular organelles age faster than the cell (7). Mitochondria appear to generally decrease in number and size as a function of aging; the smallest mitochondria with shortest cristae were observed in the oldest rats (5,7). Similar observations have been documented in both mice and humans (7,10,11). Overall mitochondrial phosphorylation as well as turnover of mitochondrial proteins are reduced in the aged liver (7,12). Similarly, the endoplasmic reticulum seems diminished in aged animals (7,13). In rats, studies have documented the existence of an age-related decline in several monooxygenase activities without any evidence of significant changes in cytochrome 450 or reduced nicotinamide adenine dinucleotide phosphate cytochrome reductase content (14). In contrast to the situation in aging rats, mice demonstrate a decline in several monooxygenase activities with age (14). However, benzo[a]pyrene hydroxylase and 7-ethoxycoumarin deethylase showed significant increases in senescent mice (14). Conversely, activity of liver cytosolic alcohol dehydrogenase increased with age in male Fischer 344 rats (15). However, activity of this enzyme was not altered with age in female rats, but was higher than in males in both young and old rats (15). In contrast, hepatic aldehyde dehydrogenase activity was similar in both male and female rats and was unchanged with age (15). Lysosomes are conspicuously increased in aged animals, with their volume density reaching approximately five times that at birth (7).

Peroxisomes: Function and Proliferation
The peroxisome is a ubiquitous single membrane-limited cytoplasmic organelle present in animal, plant, and fungal cells (16). Peroxisomes are most abundant in the liver and kidney (17) and morphometric studies show that there are between 370 and 620 peroxisomes per rat hepatocyte, occupying 1.5-2% of the cell volume (18). The average peroxisome is approximately onefifth the volume of a mitochondrion (19). Peroxisomal proteins are synthesized on free polyribosomes, with most of them made at their final size. These proteins are then posttranslationally imported from the cytosol into peroxisomes, with half-lives ranging from 1 to 15 min or longer (16).
Peroxisomefunction. Peroxisomes are respiratory organelles that play a role in cellular oxygen consumption as well as in production and removal of reactive oxygen species (20). Peroxisomes are involved in a number of essential cellular functions, including, but not limited to, cellular respiration, gluconeogenesis, lipid metabolism, thermogenesis, and synthesis of plasmologens (21). Among the oxidases identified in peroxisomes are urate oxidase, acyl-CoA oxidase, amino acid oxidase, polyamine oxidase, trihydroxycholestanoyl-CoA oxidase, and pristanoyl-CoA oxidase (22). Peroxisomal 1-oxidation of fatty acids is among the best characterized peroxisomal metabolic pathways. This system is not a functional duplicate of the mitochondrial system, as it is now clear that peroxisomes are involved in the degradation of a distinct set of compounds such as the very long-chain fatty acids and branched-chain fatty acids (22). Oxidation of erucic acid, arachidonic acid, and tetracosatetraenoic acid is initiated in peroxisomes (22. Mammalian peroxisomes are also a site for cholesterol and ether lipid synthesis (22).
Peroxisomeprohferation. Various chemicals, conditions, and factors cause peroxisome proliferation. This phenomenon was first described in the 1960s. It was observed that numerous electron dense single membrane-limited structures appeared in rat liver following clofibrate feeding (23). Since that time, a myriad of therapeutic agents, industrial chemicals, and environmental pollutants ( Figure 1 and Table 2) of diverse chemical structure have been shown to cause peroxisome proliferation, particularly in rodent livers (17,24). This group of chemicals has thus been referred to collectively as peroxisome proliferators. In addition to increasing the number of peroxisomes, these chemicals induce the activities of peroxisomal enzymes and cause hepatomegaly (17). These effects are not necessarily coupled, although they are dose and time dependent (17). Furthermore, these effects are tissue and species specific (25)(26)(27)(28)(29)(30), with the liver being the most responsive tissue (17). Although rats and mice are extremely responsive to the effects of peroxisome proliferators, guinea pigs are not and hamsters display an intermediate response (17,(26)(27)(28). Humans and other primates are either insensitive or respond marginally to peroxisome proliferators, both in vivo and in vitro (17,26,28,29).
In addition to peroxisome-proliferating chemicals, various other conditions and factors cause peroxisome proliferation, albeit not to the same degree as with xenobiotics. Among these conditions and factors are diabetes (31,32), changes in thyroid hormone levels (33,34), cold adaptation (17), endotoxin exposure (35), high-fat diets (36), and partial hepatectomy (37). These conditions and factors are all believed to produce an effect similar to that initiated by the peroxisome-proliferating xenobiotics.
Mechanisms involved in the response to such a diverse group of compounds, factors, and conditions are unclear. However, a receptor-based mechanism for the pleiotropic response to peroxisome proliferators in rodents has been suggested, where a nuclear peroxisome proliferator receptor belonging to the steroid hormone receptor superfamily has been identified (38,3,9. This receptor is activated by several peroxisome proliferators; thus, it is termed the peroxisome proliferator-activated receptor (PPAR). Activation of PPAR stimulates the expression of genes encoding peroxisomal proteins (40). Recently, three isoforms of the PPAR have been found and doned (41).
and the antidiabetic thiazolidinediones are ligands for the PPARy subtype of this receptor ( Figure 1) (42). These chemicals promote differentiation of preadipocytes and act as an insulin-sensitizing agent (43). Activation of PPAR6 subtype, however, was not enough to potentiate preadipocyte differentiation, nor did it result in modulation of glucose or triglyceride levels in vivo (43). On the other hand, leukotriene B4 and several known peroxisome-proliferating agents including the hypolipidemic and hypocholestermic WY-14,643 and fibrates, as well as fatty acids and eicosanoids, bind specifically to and activate PPARa ( Figure 1) (44)(45)(46)(47). Activation of this receptor subtype significantly reduces serum triglycerides, but with minimal effects on hyperglycemia (43).
Studies have documented the existence of a human form of PPARa (hPPARa) (48,49) and PPARy (hPPARy) (50). The tissue distribution pattern of hPPARa mRNA is similar to that of the rat PPARa. Both are highly expressed in the liver and kidneys, whereas both are expressed at low levels in the brain and lung, with lower levels in most human tissues as compared to rats (49,51). Interestingly, the relative expression of PPARa mRNA in human skeletal muscle is higher than in rodent skeletal muscle (51). This finding may be significant because studies focus almost exclusively on the liver as a site to compare the role of PPARa in gene transcription in humans and rodents.

Effect of Aging on Peroxisomal Metabolism
Information on the relationship between aging and peroxisomal metabolism is scarce. Studies point to a general decrease in peroxisomal function with aging (52,53). However, several studies failed to document measurable differences in peroxisomal enzyme activities  between young and old animals (54,55).
Peroxisomes become smaller but more numerous in older rats (7). In aged rats, peroxisomes show decreased catalase and acyl-CoA oxidase content, but increased content of thiolase and urate oxidase (56). The decrease in catalase activity coupled with the net increase in the activities of oxidases are thought to contribute to oxidative stress (57). It has been shown that peroxisome proliferation was not accompanied by enhanced levels of oxidative damage in young mature animals (24,58). However, whether aging lowers the threshold for peroxisome proliferator-induced oxidative stress is not known, but may be implied from the enhanced susceptibility of aged animals to the hepatocarcinogenic effects ofperoxisome proliferators (55,59).
Basal peroxisomal enzyme activities in aged animals. Apparent inconsistencies in age-related effects on peroxisomal 5-oxidation (Table 3) may have to do with species differences and the age of animals at the time of sacrifice, as opposed to their age at the beginning of the experiment. For example, in some studies animals were kept for extended periods, which resulted in younger and older animals being senescent at the time of sacrifice (Table 3). Another potential reason for these apparent disparate results may reside in the ages of animal groups used, as peroxisomal enzyme activities appear to change dramatically and abruptly at certain age points during development and aging (52). The Female OFi mice 100 (11) 61 (24) 'Results are expressed as percent of specific activity in young animals within the same study. bNumbers in parentheses indicate age of animals, in months, at time of sacrifice.
dedine of catalase activity with aging in male Fischer 344 rats has been observed in various studies (Table 4). In our laboratories, hepatic catalase activity declined by 40% between the ages of 4 and 100 weeks, with most of the decline (30%) observed between the ages of 50 and 100 weeks (Table 4). Similar findings were also observed by others (52) for both peroxisomal P-oxidation and catalase in both CSWV and OFi mice strains (Tables 3 and  4), and similar findings were reported for the enzymatic activity and mRNA coding for catalase in rat liver (63,64). Transcription of the catalase gene decreased by approximately 60% between 6 and 29 months of age (64). Thus, the age-related decline in catalase mRNA levels appears to arise primarily from a decrease in the transcription of the gene (64). Quantitative analysis of kinetic parameters (rates of protein synthesis and degradation) revealed that the aged rat liver exhibited 'Results are expressed as percent of specific activity in young animals within the same study. bNumbers in parentheses are age of animals in months. a decrease in rates of catalase synthesis without significant changes in degradation rates (65). Peroxisomeproliferation in agedanimals. Aging also interferes with the ability of the liver to respond to peroxisome-proliferating chemicals (60). Peroxisomal area relative to hepatocellular cytoplasmic area increased by approximately 7-fold in both 8-and 52-week-old rats following 14 days of daily 200mg/kg clofibrate doses in diet; it increased only by 4-fold in 117-week-old rats (60). In the same rats, increases in hepatic peroxisomal f-oxidation showed a similar pattern, with increases approaching 15-and 11-fold in 8-and 52-week-old rats, but only 3-fold in the 117-week-old rats (60). In our laboratory, perfluorooctanoic acid (PFOA) induced peroxisomal P-oxidation significantly in male Fischer 344 rats of varying age groups to different levels. Forty-eight hours after 150 mg/kg PFOA administration, hepatic peroxisomal P-oxidation increased in 100-week-old rats to levels equaling approximately 60% of those achieved in 10-week-old rats (Table 5).
In contrast to the PFOA findings, and in agreement with results reported in the literature (54), young and old male Fischer 344 rats given various concentrations of the peroxisome proliferator WY-14,643 in the diet exhibited comparable hepatic peroxisomal Ioxidation activity (Table 5). Similarly, a study in male Wistar rats showed that age did not seem to influence either basal or nafenopin-induced induction of peroxisomal P-oxidation (55). In these long-term feeding studies (54,55), animals were given diets containing peroxisome proliferators for 22-59 weeks, which placed the age of young and old rats between 68 and 116 weeks at the conclusion of the experiment. Potentially, these are ages where responses to peroxisome proliferators are similar. In a more detailed study in our laboratories (66), striking results showing compound-specific effects were observed ( Table 6). Although aging-related differences appeared to exist in the response of the liver to certain peroxisome proliferators, aging did not seem to influence the response to other peroxisome proliferators studied ( Table 6). The lack of uniformity in the response to various peroxisome proliferators may be due to potential aging-related alterations in specific pathways  (24)f (53) &Results are expressed as percent of peroxisome proliferator-induced specific activity in young animals within the same study. bNumbers in parentheses are age of animals in months. cAnimals were given perfluorooctanoic acid (150 mg/kg, orally in corn oil 48 hr prior to sacrifice). dAnimals were given clofibric acid in the diet (various concentrations to correspond to 200 mg/kg/day for 14 days). "Animals received WY-14,643 in the diet (0.1% for 22 weeks). fAnimals received clofibrate-supplemented diet (0.5% for 5 days). involved in the metabolism of specific proliferators. In other words, peroxisomal signal transduction pathways are most likely altered in old animals, as evidenced by the significant differences observed between young and old animals in response to the nonmetabolizable PFOA.
Mechanisms Involved in the Aging-Related Decrease in Peroxisomal Enzyme Activities Factors responsible for the reported decline in peroxisomal enzyme activities and the diminution of their response to peroxisomeproliferating chemicals in aged animals are unclear. A schematic depiction of a proposed mechanism involved in this effect is presented in Figure 2. The aged liver suffers from a defect in gene transcription and translation of certain messengers (67). This defect may be the result of deficiency in important receptors and/or impaired signal transduction pathways. Recent findings show that expression of nuclear thyroid hormone receptor mRNA is reduced by 50% in the livers of male Wistar rats 24 months old as compared to their 6-month-old counterparts (68). This decrease was not accompanied by changes in the binding characteristics of the receptors, although Bma and Kd decreased by approximately 2-fold in the older rats.
Similar changes were also observed for the mRNA of retinoic acid receptors (68). An overlap between the gene networks regulated by peroxisome proliferators and those for retinoids and thyroid hormones has been documented (57). PPARs, thyroid receptors, and retinoid receptors (RXR) recognize the same DNA response sequence and there is cross-talk among these receptors (57,69). PPAR binds to peroxisome proliferatorresponse elements as heterodimers with RXR and activate gene transcription in response to activators (70). Furthermore, RXR ligands produce effects similar to those produced by PPAR ligands (70). Peroxisome proliferators induce malic enzyme gene transcription, which is a known response to treatment with thyroid hormones (68), through the action of heterodimers of PPARot and RXR (71). In addition to changes in receptor abundance and affinity, aging also causes modifications in cellular transduction pathways, leading to altered gene expression, which may be responsible for some important impairment in physiologic functions (67). The decrease in receptor affinity in aged animals is hypothesized to be due to changes in the phosphorylation-dephosphorylation cycle in these animals (68). The potential impact of aging on the expression of the various peroxisome proliferator-activated receptor isoforms and/or binding of peroxisome-proliferating chemicals to these receptors is shown by a decline in the abundance of PPARa mRNA expression in the liver of aged rats (72. Aging, Peroxisomes, and Health The importance of the peroxisome to animal as well as to human life is evidenced by the fact that their absence, or the dysfunctionality of one or more peroxisomal enzymes, is invariably fatal (73,74). In addition, inhibiting peroxisomal enzyme activities resulted in the accelerated death of animals (75), which suggests the existence of a linkage between peroxisomal metabolism, aging, and death. However, the exact nature of a potential relationship is not known.

Inherited peroxisomal disorders.
Peroxisomal disorders are a group of inherited metabolic diseases that are classified into 10 complementation groups (76). These diseases involve multiple genes (76) and are characterized by impairment of one or more functions of peroxisomes (73,74,76). Recent studies have suggested that at least 16 genes may be involved in the observed abnormalities in the assembly of peroxisomes (74,77), and the list of peroxisomal disorders continues to grow (76). The combined incidence for the occurrence of these diseases is estimated as 1 in 20,000 or greater (74). The cerebrohepatorenal syndrome known as Zellweger syndrome was the first peroxisomal disorder identified (74,78). This syndrome is characterized by the absence of morphologically distinguishable peroxisomes (74). In contrast to Zellweger syndrome, patients suffering from X-adrenoleukodystrophy have peroxisomes of normal number and morphology, and the disease is caused by a single enzymatic abnormality; lignoceroyl-CoA ligase is suggested as the missing enzyme (74,79). Other diseases such as rhizomelic chondrodysplasia are recognized as multiple peroxisomal enzymatic deficiencies, although they have intact peroxisome morphology, normal size, and number (74,80).
Aging, apoptosis, and response to peroxisome proliferators. Aged animals are more susceptible to the hepatocarcinogenic effect of peroxisome proliferators (55,59). Longterm treatment with either of the peroxisome-proliferating chemicals nafenopin or WY-14,643 produced numerous hepatocellular adenomas and carcinomas in old male Wistar and Fischer 344 rats while producing only a few changes in young animals (55,59). Because the enhanced susceptibility of old rats did not appear to result from enhanced hepatic oxidative damage, it was hypothesized that peroxisome proliferators may promote spontaneously occurring preneoplastic foci. These foci are more abundant Volume 107, Number 10, October 1999 * Environmental Health Perspectives in the livers of aged animals as a natural consequence of aging (59). Hypolipidemic drugs are preferentially prescribed to older individuals who may have high levels of preneoplastic hepatocytes. Promotion of these cells by peroxisome proliferators has the potential to result in liver cancer.
As an alternative mechanism responsible for the hepatocarcinogenic effect of peroxisome proliferators, several studies have shown that peroxisome proliferators suppress apoptosis in the liver in a process mediated by PPARa (81,8.). Apoptosis appears to be a safeguard to prevent cells with DNA damage from progressing to a tumor (83). In primary cultures of rat hepatocytes and in an FaO rat hepatoma cell line, apoptosis was induced by transforming growth factor P, an effect that was significantly reduced by the coadministration of the peroxisome proliferator nafenopin (84). Furthermore, nafenopin suppressed hepatic apoptosis in vivo and its Figure 2. Schematic depiction of the effect of aging on proposed signaling pathways involved in maintaining and proliferating hepatic peroxisomes. Abbreviations: GR, glucocorticoid receptor; PPAR, peroxisome proliferator-activated receptor; RXR, retinoic acid receptor; TR, thyroid hormone receptor. Aging is associated with a decline in GR, TR, and RXR. The compromised integrity of these systems, along with a potential decline in PPAR expression and/or activation with aging may be responsible for the diminished basal levels of peroxisomal enzyme activities and their response to peroxisome proliferators in aged animals. withdrawal resulted in a 100to 200-fold elevation in apoptosis (84). Hepatocytes generated during nafenopin-induced hyperplasia were not the same as those that underwent apoptosis upon nafenopin withdrawal (84). Hepatocytes resistant to apoptotic death may represent preferential targets for promotion by peroxisome proliferators; thus, suppression of apoptosis may play a role in the hepatocarcinogenicity of this class of nongenotoxic compounds. Current evidence indicates that the signal transduction pathways for apoptosis and the cell cycle overlap (85). Senescent cells are resistant to both proliferation and apoptotic stimuli, possibly because of their failure to induce the expression of G] genes, which are required for both outcomes, when subjected to stimulation (85). An alternative hypothesis to explain the resistance of senescent cells to apoptosis stipulates that these cells have the protein terminin predominantly in its 60-Kda form, in contrast to the 90 Kda in apoptotic-prone cells (85). Resistance to apoptosis may contribute to the enhanced vulnerability of the aged liver to the hepatocarcinogenic effect of peroxisome proliferators.

Conclusion
The importance of peroxisomes to normal cell functions and cellular survival makes the investigation of whether these organelles play a role in aging and aging-related diseases an urgent task. Research in the field of peroxisomes has until recently focused on these organelles and their proliferation, particularly in the rodent liver. Because peroxisomes do not seem to proliferate to significant levels in the livers of humans exposed to peroxisomeproliferating chemicals, pre-mature conclusions were drawn that dismissed the potential risk these chemicals pose to humans. However, recent advances show that signal transduction pathways linked to peroxisomes are involved in a myriad of extraperoxisomal effects. Examples of these effects include adipocyte differentiation, regulation of glucose homeostasis, inhibition of macrophage and monocyte activation, and inhibition of angiogenesis (42,43,86,87). Although several of these effects appear beneficial, others may potentially be harmful. Therefore, it is imperative that a concerted and comprehensive effort is channeled toward investigating a wide array of potential effects due to peroxisome-proliferating chemicals. A significant portion of this effort should be directed toward the elderly population, which might be more vulnerable to these effects.
Membrane structure, fluidity, and permeability to ions undergo some age-dependent alterations (53,88,89). These alterations have serious inhibitory effects on the enzymatic catalysis rates involved in protein Reviews a Youssef and Badr synthesis that may be responsible for the decline in protein synthetic capacity in aged cells and may also contribute to the onset of age-related diseases (53,88,89). For example, membrane fatty acid composition affects membrane structure and function (90). Membrane fatty acid composition is maintained through a balanced process of synthesis and degradation. Although degradation of long-chain fatty acids occurs in both mitochondria and peroxisomes, very long-chain fatty acids are metabolized exclusively in peroxisomes (91). Thus, peroxisomes appear to play an important role in the maintenance of membranes and membrane function by maintaining fatty acid balance. Consequently, the decline in peroxisomal catabolism of fatty acids with aging may represent a potential mechanism by which these organelles are involved in the process of aging.
An aging-related decline in peroxisomal protein synthesis, among other proteins, is expected to diminish peroxisomal capacity to metabolize very long-chain fatty acids. This effect may be manifested in an altered composition of the structure of cellular and organellar membranes. For example, it has been reported that aging causes an increase in the cholesterol:phospholipid ratio in rat liver microsomal, mitochondrial, and cellular membranes (88,90). Phospholipids are essential for the activation of various signal transduction pathways. This includes the modulation of protein kinase C activity by phosphatidylserine and various other membrane phospholipids (92). Accordingly, changes in membrane phospholipids as a result of age-related diminished peroxisomal metabolism may trigger a cascade of harmful events and/or interrupt normal important cellular functions.
Aging-associated changes in membrane phospholipid composition are also reflected by an increase in Na, K-ATPase activity, and intracellular potassium content (88). Increased intracellular ion concentrations are postulated to reduce the activity of the whole translational machinery and increase aggregation of macromolecules, which retard all enzymatic catalysis (88,93) and interfere with mitotic activity (94).
The impact of aging on extracellular matrix, growth factors, cytokines, and nonparenchymal cells, among other factors, will be of utmost importance to our understanding of the relationship between aging and peroxisomal metabolism. Because these factors play important roles in liver cell division (95,96), their alterations with aging may explain the potential interrelationship between peroxisomal metabolism and agingrelated disease and death. Peroxisomes grow by the posttranslational incorporation of new content and membrane proteins into preexisting peroxisomes, which then divide to form daughter peroxisomes in a dynamic fission process (16). Consequently, a vicious cycle involving hepatocellular senescence and a decline in peroxisomal metabolism may ensue, leading to the demise of the liver and ultimately to death.
Peroxisome-proliferating chemicals produce a myriad of extraperoxisomal effects in the liver and other tissues of experimental animals (24). Such effects include metabolic as well as hormonal alterations, in addition to effects on ion homeostasis and interference with metabolism and effects of other chemicals (24). These extraperoxisomal effects may be directly or indirectly related to the effects on peroxisomes or may be totally independent of these events. Whether any or all of these effects occur in humans exposed to peroxisome proliferators is not known. Focusing only on the peroxisomal effects of these chemicals may grossly underestimate the harmful potential of peroxisome proliferators to humans, especially the elderly, who are more susceptible to toxic insults. Furthermore, because of the essential roles played by peroxisomal metabolism, the possibility that modulation of peroxisomal functions may be a contributing factor to the process of aging should not be ignored.