Nutrition interventions for healthy ageing across the lifespan: a conference report.

Thanks to advances in modern medicine over the past century, the world's population has experienced a marked increase in longevity. However, disparities exist that lead to groups with both shorter lifespan and significantly diminished health, especially in the aged. Unequal access to proper nutrition, healthcare services, and information to make informed health and nutrition decisions all contribute to these concerns. This in turn has hastened the ageing process in some and adversely affected others' ability to age healthfully. Many in developing as well as developed societies are plagued with the dichotomy of simultaneous calorie excess and nutrient inadequacy. This has resulted in mental and physical deterioration, increased non-communicable disease rates, lost productivity and quality of life, and increased medical costs. While adequate nutrition is fundamental to good health, it remains unclear what impact various dietary interventions may have on improving healthspan and quality of life with age. With a rapidly ageing global population, there is an urgent need for innovative approaches to health promotion as individual's age. Successful research, education, and interventions should include the development of both qualitative and quantitative biomarkers and other tools which can measure improvements in physiological integrity throughout life. Data-driven health policy shifts should be aimed at reducing the socio-economic inequalities that lead to premature ageing. A framework for progress has been proposed and published by the World Health Organization in its Global Strategy and Action Plan on Ageing and Health. This symposium focused on the impact of nutrition on this framework, stressing the need to better understand an individual's balance of intrinsic capacity and functional abilities at various life stages, and the impact this balance has on their mental and physical health in the environments they inhabit.

Healthy ageing: the natural consequences of good nutrition-a conference report.

Many countries are witnessing a marked increase in longevity and with this increased lifespan and the desire for healthy ageing, many, however, suffer from the opposite including mental and physical deterioration, lost productivity and quality of life, and increased medical costs. While adequate nutrition is fundamental for good health, it remains unclear what impact various dietary interventions may have on prolonging good quality of life. Studies which span age, geography and income all suggest that access to quality foods, host immunity and response to inflammation/infections, impaired senses (i.e., sight, taste, smell) or mobility are all factors which can limit intake or increase the body's need for specific micronutrients. New clinical studies of healthy ageing are needed and quantitative biomarkers are an essential component, particularly tools which can measure improvements in physiological integrity throughout life, thought to be a primary contributor to a long and productive life (a healthy "lifespan"). A framework for progress has recently been proposed in a WHO report which takes a broad, person-centered focus on healthy ageing, emphasizing the need to better understand an individual's intrinsic capacity, their functional abilities at various life stages, and the impact by mental, and physical health, and the environments they inhabit.

[Polymyalgia rheumatica: new developments and challenges]. / Polymyalgia rheumatica.

- Polymyalgia rheumatica (PMR) is an inflammatory rheumatic disorder in which inflammation markers, both erythrocyte sedimentation rate (ESR) and CRP values, are often elevated. However, a non-abnormal ESR or CRP value does not preclude the diagnosis.- PMR is an arbitrary diagnosis and presents both diagnostic and therapeutic challenges.- Imaging diagnostics, such as echography, MRI or FDG-PET/CT, may potentially be applied more frequently as a second-line investigation when there is doubt concerning the diagnosis. Currently these additional imaging techniques are not applied in first line diagnostics.- Glucocorticoids remain the cornerstone treatment for polymyalgia rheumatica. Often patients react swiftly to this, but in 29-45% of cases an effect is only observed 3-4 weeks later. The treatment course typically lasts 1-3 years.- More research has been conducted into potential glucocorticoid-sparing treatments. Most of the scientific evidence concerns the effectiveness of methotrexate; there is some evidence regarding the effectiveness of azathioprine and leflunomide. Tocilizumab, an IL-6 receptor inhibitor, has shown promise as a treatment, but further evidence is required.

Do peroxisome proliferating compounds pose a hepatocarcinogenic hazard to humans?

The purpose of the workshop "Do Peroxisome Proliferating Compounds Pose a Hepatocarcinogenic Hazard to Humans?" was to provide a review of the current state of the science on the relationship between peroxisome proliferation and hepatocarcinogenesis. There has been much debate regarding the mechanism by which peroxisome proliferators may induce liver tumors in rats and mice and whether these events occur in humans. A primary goal of the workshop was to determine where consensus might be reached regarding the interpretation of these data relative to the assessment of potential human risks. A core set of biochemical and cellular events has been identified in the rodent strains that are susceptible to the hepatocarcinogenic effects of peroxisome proliferators, including peroxisome proliferation, increases in fatty acyl-CoA oxidase levels, microsomal fatty acid oxidation, excess production of hydrogen peroxide, increases in rates of cell proliferation, and expression and activation of the alpha subtype of the peroxisome proliferator-activated receptor (PPAR-alpha). Such effects have not been identified clinically in liver biopsies from humans exposed to peroxisome proliferators or in in vitro studies with human hepatocytes, although PPAR-alpha is expressed at a very low level in human liver. Consensus was reached regarding the significant intermediary roles of cell proliferation and PPAR-alpha receptor expression and activation in tumor formation. Information considered necessary for characterizing a compound as a peroxisome proliferating hepatocarcinogen include hepatomegaly, enhanced cell proliferation, and an increase in hepatic acyl-CoA oxidase and/or palmitoyl-CoA oxidation levels. Given the lack of genotoxic potential of most peroxisome proliferating agents, and since humans appear likely to be refractive or insensitive to the tumorigenic response, risk assessments based on tumor data may not be appropriate. However, nontumor data on intermediate endpoints would provide appropriate toxicological endpoints to determine a point of departure such as the LED10 or NOAEL which would be the basis for a margin-of-exposure (MOE) risk assessment approach. Pertinent factors to be considered in the MOE evaluation would include the slope of the dose-response curve at the point of departure, the background exposure levels, and variability in the human response.

Rat 17 beta-hydroxysteroid dehydrogenase type IV is a novel peroxisome proliferator-inducible gene.

To better understand the molecular mechanisms of the pleiotropic responses induced by exposure to peroxisome proliferator chemicals (PPCs), we conducted a systematic search for genes whose mRNA levels are modulated by the PPC WY-14,643 (WY) in rat liver. The sequence of one up-regulated cDNA (2480 bp) was predicted to encode a protein of 735 aa with 82% identity to the porcine 17 beta-hydroxysteroid dehydrogenase type IV (HSD IV). Like the porcine enzyme, the rat HSD IV contains' a region homologous to yeast hydratase-dehydrogenase-epimerases and to sterol carrier proteins, indicating that the rat HSD IV has broad substrate specificity and contributes to cholesterol metabolism. The rat HSD IV was regulated by diverse PPCs via two distinct mechanisms. Induction of HSD IV and acyl-CoA oxidase (ACO) proteins in rat liver at different treatment times and concentrations of gemfibrozil and di-n-butyl phthalate were almost identical, indicating that HSD IV mRNA induction involves the peroxisome proliferator-activated receptor alpha, a regulator of ACO. In contrast, HSD IV protein levels were only weakly induced by WY, a strong inducer of ACO protein, even though the levels of HSD IV and ACO mRNA were strongly stimulated by WY and gemfibrozil. Thus, HSD IV protein levels were uniquely regulated pretranslationally by WY via a novel mechanism. Increased conversion of estradiol to the less-active estrone by HSD IV induction may explain how phthalate exposure leads to decreases in serum estradiol levels and suppression of ovulation.

Chronic nephropathy and renal carcinogenicity of o-benzyl-p-chlorophenol in F344/N rats and B6C3F1 mice.

o-Benzyl-p-chlorophenol, an aryl halide biocide, was evaluated in male and female F344/N rats and B6C3F1 mice in a series of subchronic and 2-year toxicity and carcinogenicity studies. Kidney was the primary target of toxicity in the 13-week gavage studies in rats and mice, with increased nephropathy noted as low as 240 mg/kg in male rats. Considering the nephropathy to be doselimiting, the chronic (2-year) study was conducted at lower doses (male rats: 30, 60, or 120 mg/kg; female rats: 60, 120, or 240 mg/kg; male and female mice: 120, 240, or 480 mg/kg; in corn oil; n = 50/group). Survival and body weights of dosed rats were similar to controls in the 2-year study. Survival of high-dose male and female mice, and body weights of all dosed male and mid- and high-dose female mice, were lower than controls. The incidence and severity of nephropathy increased with dose and length of treatment in both rats and mice. There was an increased incidence of renal tubule adenomas or carcinomas in both the mid- and high-dose male mice. Despite similar evidence of nephropathy, however, there were no increased incidences of neoplasms in female mice or in male or female rats. This study suggests therefore that while nephrotoxicity may have been a necessary component, factors other than the marked nephrotoxicity of o-benzyl-p-chlorophenol were critical to the development of renal carcinogenesis induced in only male mice.

TGF alpha differentially expressed in liver foci induced by diethylnitrosamine initiation and peroxisome proliferator promotion.

Transforming growth factor alpha (TGF alpha) is a mitogenic growth factor for hepatocytes that may play a role in the development of liver cancer in rodents and humans. Epidermal growth factor receptor (EGFR) is the receptor for TGF alpha; its expression is also altered in mitogenic and carcinogenic processes. Homogeneous basophilic foci (HBF), the precursor lesion to hepatocellular carcinomas in peroxisome proliferator (PP)-treated rats, have labeling indices much greater than surrounding liver (approximately 5- to 20-fold) and other types of foci (approximately 2-fold greater than eosinophilic foci; EF). To test the hypothesis that PP-induced HBF over-express TGF alpha and/or EGFR, male F344 rats were treated with the PP WY-14,643 for 22 weeks (1000 p.p.m. in the diet) with (DEN-WY) and without (WY) prior diethylnitrosamine initiation (150 mg/kg body wt i.p.). Serial paraffin sections of liver were stained for TGF alpha or EGFR and with hematoxylin and eosin. DEN-WY and WY abrogated the small amount of centrilobular TGF alpha staining observed in livers from control rats. Increased staining for TGF alpha was not observed in HBF induced by WY (0/22) or DEN-WY (0/101). Additionally, increased EGFR expression was not observed in HBF induced by WY (0/22) or DEN-WY (0/19) or in EF induced by DEN-WY (0/30). An unexpectedly large proportion of EF induced by DEN-WY (6/38) and DEN-control (3/11) were TGF alpha-positive. None of the tumors induced by WY (0/13) or DEN-WY (0/11) over-expressed TGF alpha. All 13 WY-induced tumors also lacked increased expression of EGFR. TGF alpha over-expression noted in a significant proportion of EF was associated only with those regimens including DEN initiation. In conclusion, TGF alpha or EGFR over-expression is not associated with early appearing, rapidly proliferating HBF or tumors induced by PP.

Apoptosis and chemical carcinogenesis.

Long recognized as a normal component of organogenesis during development, apoptosis (programmed cell death) has recently been implicated in alterations of cell growth and differentiation. Tissue homeostasis is normally maintained by a balance between cell division and cell death, with apoptosis often functioning in complement to cell growth. Thus, antithetical parallels in chemical carcinogenesis can be drawn between apoptosis and the proliferative events more commonly addressed. While enhanced cell replication may contribute to an increased frequency of mutation, apoptosis within a tissue may counteract chemical carcinogenesis through loss of mutated cells. Many strong carcinogens act as tumor promoters, selectively expanding an initiated cell population advantageously over surrounding cells. Similarly, chemicals with a selective inhibition of apoptosis within an initiated population would offer a growth advantage. In contrast, chemicals causing selective apoptosis of initiated cells would be expected to have an anticarcinogenic effect. Selective apoptosis, in concert with cell-specific replication, may explain the unique promoting effects of different carcinogens such as the peroxisome-proliferating chemicals, phenobarbital, and 2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin (TCDD). Cell turnover, both cell growth and cell death, is central to the process of chemically induced carcinogenesis in animals and understanding its impact is a critical determinant of the relevance of chemically induced effects to man.

Retrospective assessment of liver cell proliferation via PCNA: a comparison with tritiated thymidine.

Cell proliferation (S phase response) in archival liver tissues of partially hepatectomized rats was determined via proliferating cell nuclear antigen (PCNA) immunohistochemistry. These results were compared with the S phase response assessed previously in the same tissues via tritiated thymidine (Tdr) autoradiography. The effect of prolonged tissue fixation on PCNA immunohistochemistry was compared in two studies: study A, the liver was fixed for a maximum of 7 days and then embedded in paraffin and stored for approximately 18 months, while in study B, the liver was fixed in formalin for 7 years and then embedded in paraffin and stored for approximately 18 months until sectioning and immunostaining. PCNA immunostaining was successful in the liver sections of both studies, irrespective of the length of formalin fixation. Furthermore, the S phase labeling indices (LI) determined via PCNA and Tdr were comparable, although not identical, in the two studies. Therefore, use of PCNA immunohistochemistry should allow retrospective staining of rodent tissues for the assessment of cell proliferative activity in formalin-fixed organs from previously conducted toxicity and carcinogenicity studies.

Biological potential of basophilic hepatocellular foci and hepatic adenoma induced by the peroxisome proliferator, Wy-14,643.

The biological potential of hepatic foci and tumors induced by peroxisome proliferators such as Wy-14,643 has been poorly characterized. In this study, male F-344 rats (n = 20/group/time point) were fed Wy-14,643 (0.1%) for 22, 37 or 52 weeks ('W-22', 'W-37' or 'W-52' respectively). At each time point some rats were killed and additional Wy-14,643-fed rats were switched to basal diet (Wy-14,643/'stopped') for up to 104 weeks (referred to as 'W-22/S', 'W-37/S' and 'W-52/S'). Homogeneous basophilic foci, but not clear cell foci, increased in number and size in W-37 and W-52 rats. In W-37/S rats, clear cell foci replaced basophilic foci as the most frequent phenotype. In serial section overlays, adenosine triphosphatase deficient foci accounted for only 16% of basophilic foci in W-52 rats and 16% of clear cell foci in W-37/S rats at 52 weeks. The replication of basophilic foci of W-37 rats was markedly increased (focal labeling index, FLI = 61.8% versus non-focal labeling index, LI = 11.4%; control LI = 0.8%). Clear cell foci from W-37/S rats at 52 weeks had a FLI of 1.6% (non-focal LI = 0.6%). Hepatocellular adenomas were increased in W-37 (11/20 rats and 0.8 tumors/rat) and W-52 groups (19/20 rats and 2.8 tumors/rat). Prevalence of hepatocellular carcinomas was elevated in W-52 rats (6/20 rats) but not in W-22 or W-37 rats. Following removal of Wy-14,643, prevalence of animals with malignant, metastatic hepatocellular carcinomas in W-52/S rats was similar to the prevalence in W-52 rats. However, Wy-14,643-induced adenomas completely regressed in W-37/S and W-52/S groups. In summary, significant morphological continuity between highly proliferative basophilic foci and hepatocellular tumors was identified, emphasizing the superiority of basophilia as a marker for lesions leading to development of hepatocellular neoplasia in rats fed Wy-14,643. An important biological distinction was noted between regressive hepatic adenomas and progressive hepatocellular carcinomas induced by a peroxisome proliferator.

Lack of comitogenicity by the peroxisome proliferator hepatocarcinogens, Wy-14,643 and clofibric acid.

Hepatocyte replication is an early biological response in rodents following administration of peroxisome proliferators (PP); however, the mechanism for this response is unknown. This study examined two PP, Wy-14,643 and clofibric acid (CA), for mitogenic activity on primary hepatocyte cultures, both as direct acting mitogens and as indirect modulators (co-mitogens) of two hepatocyte growth factors, epidermal growth factor (EGF), and transforming growth factor-beta (TGF-beta). Primary hepatocytes were isolated from male F344 rats, seeded on collagen-coated coverslips, and incubated with increasing concentrations of Wy-14,643 or CA. Tritiated thymidine was coadministered to identify the percentage of hepatocytes involved in replicative DNA synthesis (labeling index, LI). In the absence of PP, rat hepatocytes in vitro were markedly sensitive to the mitogenic stimulus of EGF (LI, approximately 55%; versus control, approximately 1%), a mitogenic response abolished by increasing concentrations of TGF-beta. Marginal, direct mitogenicity (LI twofold over controls) was observed for Wy-14,643 (mitogenic as low as 0.1 microM). CA, a PP considerably less mitogenic in vivo than Wy-14,643, was not mitogenic in primary hepatocytes. Neither Wy-14,643 nor CA exhibited comitogenic activity with EGF or EGF/TGF-beta. Thus, while some PP may exhibit minor direct mitogenic activity in primary hepatocyte cultures, factors other than the interactions with the potent hepatocyte growth factors EGF or TGF-beta appear to modulate the strong mitogenic response of these hepatocarcinogens following administration to rodents.

The nongenotoxic hepatocarcinogen Wy-14,643 is an uncoupler of oxidative phosphorylation in vivo.

Wy-14,643 is a potent nongenotoxic hepatic carcinogen and peroxisome proliferator in rodents; however, the mechanism by which it causes tumors remains unknown. In previous work it was demonstrated that Wy-14,643 caused a dose-dependent uncoupling of oxidative phosphorylation (half-maximal effect = 100 microM) in isolated mitochondria (Keller et al., 1992, Biochim. Biophys. Acta, 1162, 237-244); therefore, the purpose of this study was to determine if uncoupling occurred in vivo under conditions which lead ultimately to tumors. Rats were fed Wy-14,643 (0.1%) in ground laboratory chow for 1, 21, 75, and 105 days. As expected, activity of the peroxisomal marker enzyme, acyl-CoA oxidase, was increased about eightfold in liver homogenates during the first 3 weeks of treatment, confirming the induction of peroxisomes. Basal rates of oxygen uptake by the perfused liver were increased significantly by Wy-14,643 treatment at all time points studied, consistent with the hypothesis that oxidative phosphorylation was uncoupled. Basal rates of oxygen uptake of about 130 mumol/g/hr were increased by over 20 mumol/g/hr in rats fed Wy-14,643 in their diet for 10 weeks. Concomitantly, rates of urea synthesis from ammonia, a process highly dependent on ATP supply, were reduced significantly in the perfused liver from 104 mumol/g/hr in control livers to 13 mumol/g/hr in livers from rats treated with Wy-14,643 for 75 days. Taken together, these data indicate that energy supply is disrupted in vivo due to uncoupling of oxidative phosphorylation by Wy-14,643.

Several nongenotoxic carcinogens uncouple mitochondrial oxidative phosphorylation.

A number of plasticizers and lipid-lowering drugs induce peroxisomes and cause hepatocellular carcinoma in rodents by mechanisms which remain unknown. In this study, seven structurally dissimilar peroxisome proliferating agents were shown to uncouple oxidative phosphorylation in isolated rat liver mitochondria. For example, perfluorooctanoate (0.5 mM) increased succinate-induced (state 4) mitochondrial respiration by over 50% while stimulation of state 3 respiration with ADP was minimal (i.e., uncoupling occurred). Interestingly, compounds which are potent carcinogens in vivo (e.g., Wy-14,643 and perfluorooctanoate) were more powerful uncouplers of oxidative phosphorylation in vitro than weak tumor-causing agents (e.g., valproate). Uncoupling also occurred in vivo. Basal rates of oxygen uptake in perfused livers from chronically treated rats were increased from 137 +/- 7 mumol g-1/h in pair-fed controls to 153 +/- 5 mumol g-1/h after 2.5 months of feeding Wy-14,643 (0.1% w/v in diet). Concomitantly, rates of urea synthesis from ammonia, a process highly dependent on ATP supply, were reduced almost completely from 104 +/- 10 mumol g-1/h to 13 +/- 6 mumol g-1/h. Bile flow, another energy-dependent process, was also reduced significantly by treatment with Wy-14,643 in vivo for 24 h. Taken together, these data indicate that energy supply for cellular processes such as urea synthesis and bile flow was disrupted in vivo due to uncoupling of oxidative phosphorylation by Wy-14,643. It is proposed that peroxisomal proliferators accumulate in the liver where they uncouple mitochondrial oxidative phosphorylation and interfere with cellular energetics.

Contrasting hepatocytic peroxisome proliferation, lipofuscin accumulation and cell turnover for the hepatocarcinogens Wy-14,643 and clofibric acid.

Earlier studies indicated that the hepatocarcinogenic activity of two peroxisome proliferators (PP) Wy-14,643 and di(2-ethylhexyl)phthalate (DEHP) correlated to the degree of lipofuscin accumulation and sustained cell replication rather than the level of peroxisome induction. This study extends the comparison of peroxisome proliferation, lipofuscin accumulation and cell replication responses in rats fed (i) clofibric acid at 5000 p.p.m. (CA), a regimen of moderate hepatocarcinogenicity; (ii) Wy-14,643 at 50 p.p.m. (WYLD), a dose of unknown hepatocarcinogenicity; and (iii) Wy-14,643 at 1000 p.p.m. (WYHD), as the highly hepatocarcinogenic regimen. Adult male F344 rats were fed the experimental diets for 1, 2, 5, 11 or 22 weeks. Relative liver weights (% of body weight) were increased in rats fed CA (1.6- to 1.7-fold), WYHD or WYLD (2.0- to 2.7-fold), compared to controls (approximately 3%) at all time points. All rats fed CA, WYHD or WYLD had similar hepatic peroxisome proliferation at all time points with large elevations in peroxisomal enzyme activities and number, size and mean volume of peroxisomes. In contrast, hepatocytic lipofuscin accumulation differed between treatments, with a decreasing order of accumulation observed in WYHD greater than WYLD approximately equal to CA greater than controls. Replicative DNA synthesis (as assessed by nuclear labeling index, LI) in nonlesion hepatocytes was markedly elevated at 1 week by both WYHD and WYLD (45- and 40-fold over controls respectively) while CA induced a 10-fold response over controls (control LI less than or equal to 1%). From week 2 to week 22 the hepatocytic LI was sustained in WYHD and WYLD rats (8- and 4-fold over controls respectively) but not in CA-rats, as compared to controls. In contrast to the cell replication response, apoptosis was elevated only in WYHD at 22 weeks. Collectively, this study supports the conclusion that neither hepatomegaly nor peroxisome proliferation are accurate predictors of carcinogenic activity for PP. Further, these results suggest that if lipofuscin accumulation or sustained cell turnover are indicators of PP-induced carcinogenesis, then WYLD should be at least as carcinogenic as CA. The moderate carcinogenic activity of CA also suggests that additional factor(s) may be necessary besides lipofuscin accumulation and sustained cell replication to determine the ultimate carcinogenic activity of PP.

Dose-related effects of the hepatocarcinogen, Wy-14,643, on peroxisomes and cell replication.

The dose and time dependency of peroxisome proliferation and hepatocyte replication was evaluated in the liver of rats fed the peroxisome proliferator and hepatocarcinogen, Wy-14,643. Male F344 rats were fed NIH07 diet blended with Wy-14,643 at 0, 5, 10, 50, 100, or 1000 ppm for 1, 3, 6, or 13 weeks. Hepatomegaly was induced by Wy-14,643 at all doses and at all time points. Peroxisome proliferation was present in rats fed 5 ppm Wy-14,643 as early as 1 week, as determined by the peroxisome-specific NAD+ reduction of palmitoyl CoA (PCO) and the peroxisome-associated activity of carnitine acetyltransferase (CAT) (5- and 11-fold over control, respectively). The elevations of PCO and CAT were dose-dependent from 5 to 50 ppm and then plateaued from 50 to 1000 ppm throughout the treatment period. Hepatocellular replication, evaluated by nuclear histoautoradiography ([3H]thymidine labeling, 6-day infusion), was increased in all Wy-14,643 dose groups after 1 week of treatment (5 ppm, 4-fold; 10 ppm, 5-fold; 50 ppm, 13-fold; 100 ppm, 12-fold; and 1000 ppm, 13-fold over controls). However, in 5 and 10 ppm groups this cell replication returned to control levels by 3 weeks. In contrast, 50, 100, and 1000 ppm groups had sustained increases in cell replication up to 13 weeks (13 weeks: 6-, 7-, and 9-fold over controls, respectively). We have demonstrated that Wy-14,643 can induce peroxisome proliferation at 5 ppm, a dose 200 times lower than the dose shown to be highly hepatocarcinogenic in rats (100% incidence by 60 weeks).(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related susceptibility to the carcinogenic effect of the peroxisome proliferator WY-14,643 in rat liver.

The carcinogenicity of the peroxisome proliferator WY-14,643 was compared in young (starting age 2 months) and old (starting age 15 months) rats. Old rats had a 5- to 7-fold higher yield of grossly visible hepatic tumors following 22 weeks of dietary WY-14,643 when compared to young rats. Volume densities of foci with large cells and homogeneously basophilic cytoplasm, cytologically similar to adenomas and carcinomas, were also higher in old rats fed WY-14,643 when compared to young rats. Although peroxisome proliferation and sustained hepatocellular proliferation have been suggested as relevant for the mechanism of WY-14,643 carcinogenicity, neither response was exaggerated in old versus young rats. Since initiation is considered to occur spontaneously and irreversibly, old rats may have a greater accumulation of spontaneously initiated hepatocytes than young rats. If so, these results are consistent with the hypothesis that the carcinogenic mechanism of the peroxisome proliferator WY-14,643 is due to the promotion of spontaneously initiated hepatocytes.

Chemically induced cell proliferation in liver carcinogenesis.

Hepatocyte proliferation has been noted in several stages of the hepatocarcinogenic process. Immediately associated with DNA adducts, hepatocyte proliferation clearly has been shown to enhance initiation. The evidence is strong that enhanced cell proliferation, preceeding DNA repair of endogenous and exogenous lesions, thereby results in the fixation of mutagenic events. In contrast to the rather well-established role of hepatocyte proliferation in the initiation phase of carcinogenesis, the role of chemically induced hepatocyte proliferation in liver tumor development is supported by sparse information. Although the available data suggest that chronic cell proliferation in the liver may be associated with tumorigenesis, this information is limited to simple correlations and a relatively small number of chemicals. In the carcinogenic process, another aspect of chemically induced cell proliferation would be direct stimulation of the altered hepatocytes to proliferate and develop tumors rapidly. This latter hypothesized activity is the least well characterized and documented for cell proliferation at this time. Because a quantitative relationship between hepatocyte proliferation and carcinogenic potential has been established for only a few chemicals, general conclusions must await data from correlative studies with other chemicals. Some correlations have been noted, but correlations do not prove cause and effect. The underlying mechanisms of chemically induced cell proliferation must be understood before cause-and-effect relationships between hepatocyte proliferation and enhancement of multiple stages of liver tumor development can be drawn.

Failure of the peroxisome proliferator WY-14,643 to initiate growth-selectable foci in rat liver.

The ability of the peroxisome proliferator WY-14,643 to act as an initiator of hepatocarcinogenesis was examined using a modified growth-selection protocol in rats. Feeding rats 0.1% WY-14,643 in the diet for 3 weeks, coupled with partial hepatectomy after 10 days, failed to initiate the development of growth-selectable foci identifiable by 3 enzyme histochemical stains. Elevation of palmitoyl CoA oxidase activity in WY-14,643 rats from 1 to 11 days after partial hepatectomy suggested tht peroxisome proliferation, even when coupled with hepatocellular DNA replication, did not result in initiation. The failure of WY-14,643 to initiate growth-selectable foci may indicate that promotional activity is important in the hepatocarcinogenicity of the peroxisome proliferators.