Altered controls of proliferation in proximal small intestine of the senescent rat

The Aged Intestine: Performance and Rejuvenation

Owing to the growing elderly population, age-related problems are gaining increasing attention from the scientific community. With senescence, the intestine undergoes a spectrum of changes and infirmities that are likely the causes of overall aging. Therefore, identification of the aged intestine and the search for novel strategies to rescue it, are required. Although progress has been made in research on some components of the aged intestine, such as intestinal stem cells, the comprehensive understanding of intestinal aging is still limited, and this restricts the in-depth search for efficient strategies. In this concise review, we discuss several aspects of intestinal aging. More emphasis is placed on the appraisal of current and potential strategies to alleviate intestinal aging, as well as future targets to rejuvenate the aged intestine.

Fast digestive, leucine-rich, soluble milk proteins improve muscle protein anabolism, and mitochondrial function in undernourished old rats

SCOPE

One strategy to manage malnutrition in older patients is to increase protein and energy intake. Here, we evaluate the influence of protein quality during refeeding on improvement in muscle protein and energy metabolism.

METHODS AND RESULTS

Twenty-month-old male rats (n = 40) were fed 50% of their spontaneous intake for 12 weeks to induce malnutrition, then refed ad libitum with a standard diet enriched with casein or soluble milk proteins (22%) for 4 weeks. A 13C-valine was infused to measure muscle protein synthesis and expression of MuRF1, and MAFbx was measured to evaluate muscle proteolysis. mTOR pathway activation and mitochondrial function were assessed in muscle. Malnutrition was associated with a decrease in body weight, fat mass, and lean mass, particularly muscle mass. Malnutrition decreased muscle mTOR pathway activation and protein FSR associated with increased MuRF1 mRNA levels, and decreased mitochondrial function. The refeeding period partially restored fat mass and lean mass. Unlike the casein diet, the soluble milk protein diet improved muscle protein metabolism and mitochondrial function in old malnourished rats.

CONCLUSIONS

These results suggest that providing better-quality proteins during refeeding may improve efficacy of renutrition in malnourished older patients.

The life and death of epithelia during inflammation: lessons learned from the gut

Epithelial cells form protective barriers that physically separate an organism from the outside world. Rather than being merely static, impregnable shields, epithelia are highly dynamic structures that can adjust their proliferation, differentiation, and death in response to intrinsic and extrinsic signals. The advantages as well as pitfalls of this flexibility are highlighted in inflammatory disorders such as inflammatory bowel diseases and psoriasis, which are characterized by a chronically dysregulated homeostasis of the epithelium. In recent years, it has become increasingly apparent that epithelial cells communicate with their surroundings through converging, integrated signaling cascades and that even minor alterations in these pathways can have dramatic pathologic consequences. In this review, we discuss how inflammatory cytokines and other signaling molecules, directly or through cross talk, regulate epithelial homeostasis in the intestine, and we highlight parallels and differences in a few other organs.

EGFR(s) in aging and carcinogenesis of the gastrointestinal tract

Cells of the gastrointestinal (GI) mucosa are subject to a constant process of renewal which, in normal adults, reflects a balance between the rates of cell production and cell loss. Detailed knowledge of these events is, therefore, essential for a better understanding of the normal aging processes as well as many GI diseases, particularly malignancy, that represent disorders of tissue growth. In general, many GI dysfunctions, including malignancy, increase with advancing age, and aging itself is associated with alterations in structural and functional integrity of the GI tract. Although the regulatory mechanisms for age-related increase in the incidence of GI-cancers are yet to be fully delineated, recent evidence suggests a role for epidermal growth family receptors and its family members {referred to as EGFR(s)} in the development and progression of carcinogenesis during aging. The present communication discusses the involvement of EGFR(s) in regulating events of GI cancers during advancing age and summarizes the current available therapeutics targeting these receptors. The current review also describes the effectiveness of ErbB inhibitors as well as combination therapies. Additionally, the involvement of GI stem cells in the development of the age-related rise in GI cancers is emphasized.

Age-related changes in vagal afferents innervating the gastrointestinal tract

Recent progress in understanding visceral afferents, some of it reviewed in the present issue, serves to underscore how little is known about the aging of the visceral afferents in the gastrointestinal (GI) tract. In spite of the clinical importance of the issue-with age, GI function often becomes severely compromised-only a few initial observations on age-related structural changes of visceral afferents are available. Primary afferent cell bodies in both the nodose ganglia and dorsal root ganglia lose Nissl material and accumulate lipofucsin, inclusions, aggregates, and tangles. Additionally, in changes that we focus on in the present review, vagal visceral afferent terminals in both the muscle wall and the mucosa of the GI tract exhibit age-related structural changes. In aged animals, both of the vagal terminal types examined, namely intraganglionic laminar endings and villus afferents, exhibit dystrophic or regressive morphological changes. These neuropathies are associated with age-related changes in the structural integrity of the target organs of the affected afferents, suggesting that local changes in trophic environment may give rise to the aging of GI innervation. Given the clinical relevance of GI tract aging, a more complete understanding both of how aging alters the innervation of the gut and of how such changes might be mitigated should be made research priorities.

Transit-amplifying cell frequency and cell cycle kinetics are altered in aged epidermis

Aged epidermis is less proliferative than young, as exemplified by slower wound healing. However, it is not known whether quantitative and/or qualitative alterations in the stem and/or transit-amplifying (TA) compartments are responsible for the decreased proliferation. Earlier studies found a normal or decreased frequency of putative epidermal stem cells (EpiSCs) with aging. We show, using long-term repopulation in vivo and colony formation in vitro, that, although no significant difference was detected in EpiSC frequency with aging, TA cell frequency is increased. Moreover, aged TA cells persist longer, whereas their younger counterparts have already differentiated. Underlying the alteration in TA cell kinetics in the aged is an increase in the proportion of cycling keratinocytes, as well as an increase in cell cycle duration. In summary, although no significant difference in EpiSC frequency was found, TA cell frequency was increased (as measured by in vivo repopulation, growth fraction, and colony formation). Furthermore, the proliferative capacity (cellular output) of individual aged EpiSCs and TA cells was decreased compared to that of young cells. Although longer cell cycle duration contributes to the decreased proliferative output from aged progenitors, the greater number of TA cells may be a compensatory mechanism tending to offset this deficit.

Schlafen 3, a novel gene, regulates colonic mucosal growth during aging

Although aging is associated with increased proliferation and decreased apoptosis in the colonic mucosa of Fischer 344 rats, the regulatory mechanisms are poorly understood. Gene expression profiling (Illumina platform) was carried out in freshly isolated colonic mucosal cells from young (4-6 mo old) and aged (22-24 mo old) Fischer 344 rats. Sixty-six genes were differentially expressed in the colonic mucosa between young and old animals (P<0.05). In particular, the expression of schlafen 3, a negative regulator of proliferation, was decreased by 8- to 10-fold in the colonic mucosa of aged rats. Administration of wortmannin, which inhibited colonic mucosal proliferation in the colonic mucosa of aged rats, stimulated the expression of schlafen 3, indicating a growth regulatory role of this gene. To further determine the growth regulatory properties of schlafen 3 gene, schlafen 3 cDNA was transfected in colon cancer HCT-116 cells. This resulted in a 30-40% inhibition of cellular growth, accompanied by decreased expression of PCNA and cyclin D1 and reduced phosphorylation of retinoblastoma protein. In conclusion, our present study demonstrates that several genes involved in proliferation and apoptosis are differentially expressed in the colonic mucosa of young and aged rats. Schlafen 3, a novel negative regulator of growth, which is markedly downregulated in the colonic mucosa of the aged, may play a role in regulating colonic mucosal growth during aging.

Starvation-induced proximal gut mucosal atrophy diminished with aging

BACKGROUND

Starvation induces small bowel atrophy with increased intestinal epithelial apoptosis and decreased proliferation. The authors examined these parameters after starvation in aged animals.

METHODS

Sixty-four 6-week-old and 26-month-old C57BL/6 mice were randomly assigned to either an ad libitum fed or fasted group. The small bowel was harvested at 12, 48, and 72 hours following starvation. Proximal gut mucosal height was measured and epithelial cells counted. Apoptosis was identified by terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) staining. Proliferation was determined by immunohistochemical staining for proliferating cell nuclear antigen. Comparison of fed vs fasted and adult vs old groups was done by one-way ANOVA with Tukey's test and unpaired Student's t test. Significance was accepted at P < .05.

RESULTS

Aged mice had higher proximal gut weights, mucosal heights, and cell numbers at baseline compared with the adult group (P < .05). The rate of apoptosis was lower in the aged (P < .05), but proliferation was not different between groups before starvation. After starvation, proximal gut wet weight decreased only in adult mice (P < .05). Gut mucosal height and mucosal cell number decreased more in adult than in aged mice (P < .05). This was related to decreased proliferation only in the adult group (P < .05). The fold of epithelial apoptosis that increased was higher in the aged group than in the adult group after starvation (P < .05).

CONCLUSIONS

Gut mucosal kinetics change with age and have lower rates of apoptosis and greater mucosal mass; the character of starvation-induced atrophy is diminished with aging.

Cell cycle and apoptosis regulatory protein-1: a novel regulator of apoptosis in the colonic mucosa during aging

Although the regulatory mechanisms for the age-related rise in proliferation and reduction in apoptosis in the colonic mucosa are yet to be fully delineated, we have demonstrated that these events are associated with increased expression and activation of epithelial growth factor receptor (EGFR)/ErbB-1 and some of its receptor family members (EGFRs), indicating their involvement in these processes. However, the downstream signaling events of EGFR and/or its family members regulating age-related changes in mucosal proliferation and apoptosis remain to be delineated. Cell cycle and apoptosis regulatory protein-1 (CARP-1), a novel growth signaling regulator that we isolated, participates in EGFR-dependent signaling. In the current investigation, we examined the involvement of CARP-1 in colonic mucosal growth-related processes during aging. We report that the age-related reduction in apoptosis in the colonic mucosa is associated with increased expression and tyrosine phosphorylation of not only EGFR but also ErbB-2 and ErbB-3. In contrast, protein and mRNA levels of CARP-1 as well as tyrosine phosphorylation of CARP-1 are decreased. Additionally, we have observed that administration of wortmannin, an inhibitor of phosphatidylinositol 3-kinase activity that accelerates apoptosis in the colonic mucosa of aged rats, causes a marked increase in expression and tyrosine phosphorylation of CARP-1. The age-related decline in CARP-1 expression could partly be attributed to increased methylation of the CARP-1 promoter. Taken together, our data suggest that not only EGFR but also its other members are involved in regulating colonic mucosal growth during aging and that CARP-1 may play a crucial role in transducing EGFRs signals.

Intestinal malabsorption in the elderly

BACKGROUND

Intestinal malabsorption in the elderly is infrequent, and clinical features are muted so that the diagnosis is often missed. Physiologic changes with aging are restricted to altered absorption of calcium and perhaps zinc and magnesium; however, achlorhydria can lead to impaired absorption of vitamin B(12), folic acid, and calcium.

METHODS AND RESULTS

Small bowel bacterial overgrowth occurs more commonly in elderly than in younger patients, accompanying gastric hypochlorhydria, altered intestinal motor activity, or diseases such as Parkinson's disease. Changes in pancreatic anatomy and secretion occur but are insufficient to produce macronutrient malabsorption. In addition to pancreatic cancer and pancreatic stones, older patients may present with severe pancreatic insufficiency of unknown etiology. Celiac disease is recognized as very common at all ages and may not become evident until late in life. Manifestations of celiac disease in the elderly are occult and the diagnosis often is not considered until serologic tests are performed and confirmed by upper small intestinal biopsy. Associated intestinal lymphoma, esophageal carcinoma, intestinal pseudo-obstruction, and splenic atrophy may be more common in the elderly. Treatment of older patients with celiac disease with a gluten-free diet may be difficult, and intensive vitamin and micronutrient replacement is mandatory. A pragmatic approach to the evaluation of malabsorption in elderly patients is discussed.

Phosphatidylinositol 3-kinase/Akt signaling stimulates colonic mucosal cell survival during aging

Although aging is shown to be associated with decreased apoptosis and increased survival of cells in the colonic mucosa of Fischer 344 rats, the regulatory mechanisms are poorly understood. The current investigation examines the involvement of phosphotidylinositol 3-kinase (PI3K)/Akt signaling pathway in mediating the events of colonic mucosal cell survival during aging. We have observed that aging is associated with activation of PI3K/Akt signaling, as evidenced by the higher levels of phosphorylated forms of p85, the regulatory subunit of PI3K and of Akt in the proximal and distal colonic mucosa, of aged (21-23 mo) than in young (4-7 mo) rats. These increases are accompanied by a concomitant rise in phosphorylation of proapoptotic protein Bad, which is sequestered by the 14-3-3 family of proteins following phosphorylation by Akt, resulting in a reduction in nonphosphorylated Bad. The amount of antiapoptotic Bcl-xL bound to nonphosporylated Bad in the colonic mucosa is found to be substantially lower in aged than in young rats, resulting in a marked rise in the unbound/free form of Bcl-xL in the aging colon. The age-related activation of PI3K and the reduction in caspase-3 activity could be reversed by wortmannin, a specific inhibitor of PI3K. Increased levels of Bcl-xL and phosphorylated forms of Akt and Bad and reduction in caspase-3 activity were observed throughout the entire length of the colonic crypt of aged rats. We conclude that the constitutive activation of the PI3K/Akt-signaling pathway is partly responsible for the age-related increase in colonic mucosal cell survival. This is evident throughout the entire length of the colonic crypt.

Differences in the effects of age on intestinal proliferation, crypt fission and apoptosis on the small intestine and the colon of the rat

The increase in gastrointestinal epithelial tissue mass and the development of the gut can occur through three main mechanisms, namely elevated cell production from the intestinal crypts, by raised crypt number, which occurs through the process of crypt fission or by altered apoptosis. The small bowel and the colon have various rates of these, which were studied in rats of various ages. Wistar rats were fed ad libitum, and were killed at 3, 4, 6, 9, 12, 18, 26 and 48 weeks of age. Tissue was later stained and microdissected and the number of native mitoses and apoptotic figures per crypt and the percentage of crypts in fission were determined. There was an almost linear increase in body weight from 3 to 9 weeks, followed by a more gradual rise until 18 weeks. The weight of the stomach and the small intestine reached maximum values at 9 weeks, whereas the caecum and the colon approached this at 12 weeks. Mitotic activity per crypt in the small intestine increased from 3.8 +/- 0.1 at 3 weeks to 7.8 +/- 0.4 mitoses per crypt (P < 0.001) at 9 weeks and then decreased slightly; crypt fission increased from 4.6% +/- 0.8 at 3 weeks to 8.4 +/- 0.9% at 6 weeks and then decreased gradually reaching a value of 1.5 +/- 0.4% at 48 weeks. Apoptosis also peaked at 6 weeks and was then very low. In the colon, the proliferation decreased from 4.2 +/- 0.2 mitoses per crypt in the young (3 weeks) rat and reached a plateau by 9 weeks (2.5 +/- 0.1 mitoses per crypt, P < 0.001). Crypt fission also declined rapidly in the first 9 weeks (from 67.6 +/- 4.2 to 23.1 +/- 4.6%, P < 0.01) and then continued to decline, although at a lower rate. The crypt fission index at 48 weeks was 9.8 +/- 1.0. Apoptosis in the colon persisted throughout the duration of the study, 0.19 +/- 0.06 apoptotic bodies per crypt were seen at week 48. The development of the small intestine is more dependent on cell proliferation, whereas in the colon crypt fission is far more predominant, with the colon having fission indices approximately six times greater than those of the small intestine. Proliferative activity in the colon was approximately half that of the small intestine.

Dietary lipids modify the age-associated changes in intestinal uptake of fructose in rats

Because reduced nutrient absorption may contribute to malnourishment in the elderly, age and diet modulate fructose uptake in mice, and alterations in fructose uptake may be paralleled by changes in the abundance of fructose transporters, the objectives of this study were to determine 1) the effects of aging on fructose absorption in rats, 2) the effect of feeding diets enriched with saturated fatty acids (SFA) vs. polyunsaturated fatty acids (PUFA), and 3) the mechanisms of these age-and diet-associated changes. Male Fischer 344 rats aged 1, 9, and 24 mo received isocaloric diets enriched with SFA or PUFA. The uptake of (14)C-labeled D-fructose was determined in vitro using the intestinal sheet method. Northern and Western blot analyses and immunohistochemistry were used to determine the abundance of sodium-independent glucose and fructose transporters (GLUT)2 and GLUT5. When expressed on the basis of mucosal surface area, jejunal fructose uptake was increased in 9 and 24 mo compared with 1-mo-old animals fed SFA. PUFA-fed animals demonstrated increased fructose uptake at 24 mo compared with younger animals. Ileal fructose uptake was increased with SFA vs. PUFA in 9-mo-old rats but was reduced with SFA in 1- and 24-mo-old rats. Variations in GLUT2 and GLUT5 abundance did not parallel changes in uptake. These results indicate that 1) age increases fructose uptake when expressed on the basis of mucosal surface area, 2) age influences the adaptive response to dietary lipid modifications, and 3) alterations in fructose uptake are not explained by variations in GLUT2 or GLUT5.

Selective pressure for a decreased rate of asymmetrical divisions within stem cell niches may contribute to age-related alterations in stem cell function

Most mammalian continuously renewing tissues are maintained by stem cells located within stem cell niches. Each niche contains a number of stem cells that replicate asymmetrically to give differentiated cells and also undergo periodic deletion and compensatory replacement by symmetrical "self-renewal" divisions of stem cells remaining within the niche. It has been recognized that there is selective pressure for an increased rate of self-renewal divisions and that the increasingly likely consequence is neoplasia. However, to date it has not been appreciated that there is also an independent selective pressure for a decreased rate of asymmetrical divisions. In this article, the origin of this second type of selective pressure is explained and its consequences explored through the use of computer modeling. It is shown that age-related changes in a range of mammalian stem cell compartments can be understood in the context of a decreased rate of asymmetrical stem cell divisions with an increased propensity for self-renewal divisions. It is proposed that a decreased rate of asymmetrical divisions impairs the ability of old stem cell compartments to respond effectively to stress.

Expression of EGF-receptor related protein (ERRP) decreases in gastric mucosa during aging and carcinogenesis

Aging and gastrointestinal malignancies, including that of the stomach are associated with increased activation of EGF-receptor (EGFR). Although the intracellular events that regulate this process are poorly understood, we hypothesize that loss of ERRP (EGFR-related protein; GenBank accession number AF187818), a recently identified negative regulator of EGFR, that possesses a substantial homology to the ligand binding extracellular domain of EGFR, may contribute to this event. In support of our hypothesis, we have observed that in Fischer-344 rats, whereas aging is associated with increased activation of EGFR in the gastric mucosa, expression of ERRP decreases inthis tissue during this period. The latter is accompanied by a concomitant reduction in the amount of TGF-alpha bound to ERRP. In contrast, the amount of TGF-alpha bound to EGFR is found to be higher in the gastric mucosa of aged than in young rats. This is accompanied by a concomitant rise in EGFR levels. In the gastric mucosa, EGFR and ERRP are found to be colocalized. Gastric adenocarcinoma in humans, which has been shown to be associated with increased activation of EGFR, shows a substantial reduction in ERRP expression, when compared with benign tissues. We conclude that increased activation of EGFR in the gastric mucosa during aging and carcinogenesis may partly be due to the loss of ERRP.

Aging is associated with increased proliferation and decreased apoptosis in the colonic mucosa

Although the incidence of colon cancer increases with advancing age, reasons for this increase are not fully understood. Earlier studies have demonstrated that in Fischer-344 rats, aging is associated with increased crypt cell production in the colon, an event considered to be central to the initiation of carcinogenesis. Apoptosis also plays a critical role in the development and progression of colon cancer. Therefore, we have examined the age-related changes in proliferation and apoptosis in the colonic mucosa of 4-5, 12-14, and 22-24 month-old Fischer-344 rats. We have observed that proliferative activity in the colon, as assessed by proliferating cell nuclear antigen immunoreactivity, is higher (50-80%) in 12-14 and 22-24 month-old rats than in their 4-6 month-old counterparts. In contrast, the number of apoptotic cells, (as determined by TdT-mediated dUTP nick-end labeling assay) in the colonic mucosa of 12-14 and 22-24 month-old rats are considerably lower (50-60%) than in 4-6 month-old animals. These changes are accompanied by a concomitant reduction (75%) in pro-apoptotic Bak and stimulation (200%) of anti-apoptotic Bcl-xL levels. Since activation of caspases is associated with initiation and maintenance of apoptosis, we also analyzed the levels of pro and active forms of caspase-3, 8 and 9. The levels of active forms of caspase-3, 8 and 9 are found to be considerably (60-80%) lower in the colonic mucosa of 22-24 month-old rats, compared to their younger counterparts. This is accompanied by decreased cleavage of poly(ADP-ribose) polymerase, a substrate for caspases. In conclusion, our data show that aging enhances proliferation, but attenuates apoptosis in the colonic mucosa. These changes may partly be responsible for the age-related rise in colorectal cancer.

Increased in vitro activation of EGFR by membrane-bound TGF-alpha from gastric and colonic mucosa of aged rats

Although aging is associated with increased epidermal growth factor receptor (EGFR) tyrosine kinase activity in Fischer 344 rat gastric and colonic mucosa, the regulatory mechanisms for the age-related rise in EGFR tyrosine kinase are poorly understood. Transmembrane transforming growth factor-alpha (TGF-alpha) may modulate EGFR function through an autocrine/juxtacrine mechanism. The present study aimed to determine the contribution of membrane-bound precursors of TGF-alpha in enhancing EGFR activation in the gastric and colonic mucosa during aging. The extent of EGFR tyrosine phosphorylation, a measure of EGFR activation, was substantially higher (300--350%) in the gastric and colonic mucosa of 23- (aged) vs. 4-mo-old (young) Fischer 344 rats. This was accompanied by an increase (200--1,000%) in the relative concentration of 18- to 20-kDa membrane-bound precursor forms of TGF-alpha. The amount of TGF-alpha bound to EGFR was also higher (150-250%) in the gastric and colonic mucosa of aged vs. young rats. In vitro studies revealed that exposure of HCT 116 cells (a colon cancer cell line) to TGF-alpha from gastric and colonic mucosal membranes of aged rats caused a 200--250% higher activation of EGFR and extracellular signal-related kinases (p42/44) compared with young rats. Our data suggest that the membrane-bound precursor form(s) of TGF-alpha may partly be responsible for enhancing EGFR activation in the gastric and colonic mucosa of aged rats, probably though an autocrine/juxtacrine mechanism(s).

Aging: a barrier to renutrition? Nutritional and immunologic evidence in rats

BACKGROUND

Previous reports suggest that correcting the malnourished state is more difficult in elderly people than in younger ones and that protein requirements may be higher in elderly than in younger adults.

OBJECTIVE

The aim of this study was to establish whether malnourished old rats respond to protein-supplemented nutritional repletion as do young adult rats.

DESIGN

Adult (3 mo old) and old (22 mo old) rats were submitted to dietary restriction programs that induced similar metabolic and nutritional alterations. Malnourished adult and old rats were then killed (R groups) or refed for 1 wk with a high-protein diet (HPD; 23% protein) or a very-high-protein diet (VHPD; 27% protein). Control groups at both ages were fed ad libitum throughout the experiment. Effects of food repletion were evaluated in terms of protein metabolism, intestinal histomorphometry, and nonspecific immune status.

RESULTS

In adult rats, HPD sufficed to increase body weight and restore basal values of liver weight and protein content (P: < 0.01 compared with the R adult group), nitrogen balance (P: < 0.01 compared with the R adult group), and hydrogen peroxide production by polymorphonuclear neutrophils and monocytes (P: < 0.01 compared with the R group); VHPD had no supplementary effect except on nitrogen balance. In old rats, HPD was less effective and greater benefit was observed with VHPD in terms of body weight gain (10%; P: < 0.01 compared with the old group fed HPD), albuminemia, muscle weight and protein content, plasma arginine concentration, and hydrogen peroxide production by stimulated polymorphonuclear neutrophils and monocytes compared with the old R group (P: < 0.01).

CONCLUSION

Aging is a significant variable affecting the response to nutritional support.

Aging enhances G(1) phase in the colonic mucosa of rats

Although in Fischer-344 rats, aging has been shown to be associated with increased crypt cell production in the colonic mucosa, no information is available about the responsible intracellular mechanisms for the age-related rise in colonic mucosal cell proliferation. To determine whether cell cycling events are affected by aging, the present investigation examines the age-related changes in Cdk2 activity and the regulation of this process in the colonic mucosa. Colonic mucosae from 4-, 13- and 24-month-old Fischer-344 rats were assayed for Cdk2 activity and protein expression of Cdk2, cyclin D1 and E, as well as p21(Waf1/Cip1) (total and the fraction bound to Cdk2), p53 and phosphorylated Rb. Kinase activity and protein levels of Cdk2, as well as cyclin D1 concentration in the colonic mucosa, rose steadily with advancing age. However, the levels of cyclin E in the colonic mucosa were found to be higher in 24-month-old than 13-month-old rats, compared to their 4-month-old counterparts. On the other hand, levels of mucosal p21(Waf1/Cip1) (total and the fraction bound to Cdk2), one of the universal inhibitors of Cdks, were found to be lower in aged than in young rats. This was accompanied by a parallel decrease in mucosal p53, a tumor suppressor protein that is known to regulate p21(Waf1/Cip1). Additionally, we observed that the levels of phosphorylated Rb protein, a form which is involved in regulating progression of cells through the S phase, are increased in the colonic mucosa of 24-month-old rats, but not in 13-month-old animals, when compared with their 4-month-old counterparts. Our data suggest that, G(1) to S phase transition, as well as progression through the S phase of the cell cycle are accelerated in the colonic mucosa of aged rats.

Cell proliferation and apoptosis in the immune system in the elderly

Loss of the cell proliferative capability and involution of tissues and organs are among the most important phenomena that characterize the aging process. Some of the aged-linked immune dysfunctions could be partly due to a dysregulation of apoptotic processes and to a lower responsiveness of aged lymphoid cells to activation and proliferation signals. The main changes in proliferative activity and cell death during aging and their impact on the process of immunosenescence are discussed. In fact, a very important function that has been suggested to deteriorate with age and to play a major role in the aging process is the capability of cells from aged subjects to respond to mitogenic stimuli and, consequently, to undergo cell proliferation. However, the cellular activation processes are very complex and the proliferative responses can follow different interconnected signal transduction pathways, and only some of them appear to be modified during age. Moreover, cell growth, immunosenescence, and longevity are strictly interconnected and deeply related to programmed cell death or apoptosis. The cellular equilibrium between cell survival and proliferation, on the one hand, and programmed cell death, on the other hand, seems to be unbalanced with advancing age, although in each type of immune cell it could be differentially modulated, resulting in a variety of clinicopathological consequences. Thus, cell proliferation and cell death are two physiologically active phenomena closely linked and regulated and a failure of these mechanisms determines profound dysregulations of cell homeostasis with major consequences in immune functioning and the onset of autoimmune diseases and cancer, whose incidence appears to be increased in the elderly.

Induction of transcriptional activity of AP-1 and NF-kappaB in the gastric mucosa during aging

Although aging enhances expression and tyrosine kinase activity of epidermal growth factor receptor (EGFR) in the gastric mucosa, there is no information about EGFR signaling cascades. We examined the age-related changes in mitogen-activated protein kinases (MAPKs) [extracellular signal-related kinases (ERKs), c-Jun NH(2)-terminal kinases (JNKs), and p38], an EGFR-induced signaling cascade, and activator protein-1 (AP-1) and nuclear factor-kappaB (NF-kappaB) transcriptional activity in the gastric mucosa of 4- to 6-, 12- to 14-, and 22- to 24-mo-old Fischer 344 rats. AP-1 and NF-kappaB transcriptional activity in the gastric mucosa rose steadily with advancing age. This can be further induced by transforming growth factor-alpha. The age-related activation of AP-1 and NF-kappaB in the gastric mucosa was associated with increased levels of c-Jun, c-Fos, and p52, but not p50 or p65. Total and phosphorylated IkappaBalpha levels in the gastric mucosa were unaffected by aging. Aging was also associated with marked activation of ERKs (p42/p44) and JNK1. In contrast, aging decreased p38 MAPK activity in the gastric mucosa. Our observation of increased activation of ERKs and JNK1 in the gastric mucosa of aged rats suggests a role for these MAPKs in regulating AP-1 and NF-kappaB transcriptional activity. These events may be responsible for the age-related rise in gastric mucosal proliferative activity.

Aging alters gastric mucosal responses to epidermal growth factor and transforming growth factor-alpha

Administration of pharmacological doses of epidermal growth factor (EGF) or transforming growth factor-alpha (TGF-alpha) in young rats stimulates gastric mucosal proliferation, but, in aged rats, the same treatment inhibits proliferation. This may be due to enhanced ligand-induced internalization of EGF receptor (EGFR). In support of this, we demonstrated that although a single injection of EGF (10 microg/kg) or TGF-alpha (5 microg/kg) in young (4-6 mo old) rats greatly increased membrane-associated EGFR tyrosine kinase activity, the same treatment slightly inhibited the enzyme activity in aged (24 mo old) rats. This treatment also produced a greater abundance of punctate cytoplasmic EGFR staining in gastric epithelium of aged rats, consistent with EGFR internalization. In vitro analyses demonstrated that exposure of isolated gastric mucosal cells from aged but not young rats to 100 pM TGF-alpha resulted in marked increases in intracellular EGFR tyrosine kinase activity and that induction of EGFR tyrosine kinase activity in mucosal membranes from aged rats occurred at doses 1,000-fold less than those required in young rats. Our data suggest that aging enhances sensitivity of the gastric mucosa to EGFR ligands. This may partly explain EGFR-mediated inhibition of gastric mucosal proliferation in aged rats.

Induction of G(1) checkpoint in the gastric mucosa of aged rats

Although in Fischer 344 rats aging is found to be associated with increased gastric mucosal proliferative activity, little is known about specific changes in the regulatory mechanisms of this process. To determine whether changes in cell cycling events could partly contribute to the age-related rise in gastric mucosal proliferative activity, the present investigation examines changes in cyclin-dependent kinase (Cdk2) activity and the regulation of this process in the gastric mucosa of Fischer 344 rats aged 4 (young), 13 (middle aged), and 24 (old) mo. We observed that aging is associated with a progressive rise in activity and protein levels of Cdk2 in the gastric mucosa. This is also found to be accompanied by a concomitant increase in cyclin E but not cyclin D1 levels. On the other hand, the levels of p21(Waf1/Cip1) (total as well as the fraction associated with Cdk2), a nuclear protein that is known to inhibit different cyclin-Cdk complexes, are found to decline in the gastric mucosa with advancing age. In contrast, with aging, there was a steady rise in p53 levels in the gastric mucosa. We have also observed that the levels of phosphorylated retinoblastoma protein, a form that participates in regulating progression through the S phase, are markedly elevated in the gastric mucosa of aged rats. In conclusion, our data suggest that, in the gastric mucosa, aging enhances transition of G(1) to S phase as well as progression through the S phase of the cell cycle. However, the age-related decline in p21(Waf1/Cip1) in the gastric mucosa appears to be independent of p53 status.

Altered stem cell regeneration in irradiated intestinal crypts of senescent mice

Ageing is associated with a progressive deterioration in the functions of many organs within the body. In tissue with high cell turnover, the maintenance of the stem cells is of particular importance. Any accumulation of damage in stem cells may affect their function and hence threaten the homeostasis and regenerative capacity of the tissue. The small intestine represents a good model for the study of stem cells because of its spatial and hierarchical organisation. We have examined the effect of age on stem cell regenerative capacity after irradiation, using the microcolony assay. Crypt survival levels, the growth rate of surviving crypts, and the number of cells able to repopulate a crypt have been investigated by irradiating groups of 6–7 month old and 28–30 month old ICRFa male mice. After high doses of irradiation, the surviving crypts in old mice were both smaller and fewer in number than in young mice. The growth rate of surviving crypts was determined by measuring the crypt area and the number of cells/crypt at various times after 14 Gy irradiation. There was a growth delay of between about one half and one day in the older mice. Surprisingly, the number of clonogenic cells per crypt was estimated to be greater in the older mice. These studies indicate important age-related alterations in the capacity to regenerate the crypts after radiation damage.

Proliferating cell nuclear antigen expression is increased in small bowel epithelium in the elderly

Although previous studies suggest that in aging animals the small intestine is in a hyperproliferative state, no information is currently available on the influence of age on the proliferation pattern of human small bowel enterocytes. The immunohistochemical expression of the proliferating cell nuclear antigen (PCNA), the villous height to total mucosal thickness ratio and the enterocyte height were evaluated in a panel of duodenal biopsy specimens obtained from 18 subjects aged less and 14 subjects aged more than 65 years. There was a significant positive correlation (P < 0.001) between age in years and percent of positive PCNA enterocytes both at the level of crypts (rs = 0.50) and villi (rs = 0.77). Moreover, the percentage of PCNA+ enterocytes was significantly higher in elderly versus adult subjects, both at the level of villi (6.5 vs 0%; P < 0.001) and of crypts (40.0 vs 23.7%; P < 0.01). No correlation was found between the percentage of PCNA + enterocytes and enterocyte height or villous height to total mucosal thickness ratio. Our results show that PCNA reactivity increases with advancing age both in crypts and villi. This abnormality of the proliferation pattern may explain the coexistence of normal morphology and impaired absorptive function in the elderly.

Cell aging in vivo and in vitro

It has become a staple assumption of biology that there is an intrinsic fixed limit to the number of divisions that normal vertebrate cells can undergo before they senesce, and this limit is in some way related to aging of the organism. The notion of such a limited replicative lifespan arose from the often repeated observation that diploid fibroblasts cannot proliferate indefinitely in monolayer culture, and that the number of divisions before senescence is directly related to the in vivo lifespan of different species. The in vitro evidence is countered by estimates that the number of cell divisions in some organs of rodents and man are one or more orders of magnitude higher than the in vitro limit, with no indication of the degenerative changes seen in culture. Serial transplantation experiments in animals also exhibit many more cell divisions than the in vitro studies, with some indicating an indefinite replicative lifespan. I present evidence that vertebrate cells are severely stressed by enzymatic dispersion and sustain cumulative damage during serial subcultivations. The evidence includes large increases in cell size and its heterogeneity, reductions in replicative efficiency at low seeding densities, appearance of abnormal structures in the cytoplasm, changes in metabolism to a common cell culture type, continuous loss of methyl groups and reiterated sequences from DNA, and a constant rate of decline of growth rate with passage. This evidence is complemented by the reduction induced in the replicative life span of diploid cells by a large array of treatments which have different primary targets in the cells. The most consistent and general observation of cell behavior in aging animals, with only a few exceptions, is a reduction in the rate of cell proliferation. This reduction is perpetuated when the cells are grown in culture, indicating it is an enduring and intrinsic property of the cells rather than a systemic effect of the aging organism. A similar heritable reduction in growth rate can be induced in established cell lines by prolonged incubation at quiescence. The reduction can be exaggerated by subculturing the quiescent cells under suboptimal conditions, just as the effects of age are exaggerated under stress. The constant decline of growth rate that occurs during serial passage of diploid cells may represent a similar decay of cell function. I propose that the limit on replicative lifespan is an artifact that reflects the failure of diploid cells to adapt to the trauma of dissociation and the radically foreign environment of cell culture. It is, however, a useful artifact that has given us much information about cell behavior under stressful conditions. The overall evidence indicates cell in vivo accumulate damage over a lifetime that results in gradual loss of differentiated function and growth rate accompanied by an increased probability for the development of cancer. Such changes are normally held to a minimum by the organized state of the tissues and homeostatic regulation of the organism. The rejection of an intrinsic limit on the number of cell divisions eliminates the need for a cellular clock, such as telomere length, that counts mitoses. I offer a heuristic explanation for the gradual reduction of cell function and growth capacity with age based on a cumulative discoordination of interacting pathways within and between cells and tissues. I also make a case for the use of established cell lines as model systems for studying heritable damage to cell populations that simulates the effects of aging in vivo, and represents a relatively unexplored area of cell biology.

Total intestinal lactase and sucrase activities are reduced in aged rats

Lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) are intestinal microvillus membrane hydrolases that play important roles in carbohydrate digestion. Although the expression of these enzymes during postnatal development has been characterized, the effect of old age on disaccharidase activity is poorly understood. In the present investigation, we examined the effect of aging on lactase and sucrase activities and their mRNA levels in the small intestines of 3-, 12- and 24- mo-old rats by sampling from nine equidistant segments of small intestine. Total intestinal disaccharidase activity or mRNA abundance was determined from areas under the proximal-to-distal curves. Rats 24 mo of age had total intestinal lactase and sucrase activities that were 12 and 38% lower, respectively, than the 3-mo-old animals (P < 0.05). In contrast, total LPH and SI mRNA abundance did not change significantly. Thus, total intestinal lactase and sucrase activities decrease with age in a manner that likely involves a posttranscriptional process. The age-related decline in disaccharidase activity, if extrapolated to humans, may have important implications for the digestion of carbohydrate contained in the diet of the elderly.

Altered polyamine biosynthesis with aging after massive proximal small bowel resection in rat

We examined the effect of aging on polyamine biosynthesis in the small intestine. Two groups of male Wistar rats (young; 10-week-old, n = 40; old; 24-month-old, n = 40) underwent either a jejunal transection and reanastomosis or 90% proximal small bowel resection. The rats were sacrificed on the 1st, 2nd, 4th, and 7th postoperative day (POD). The mucosa was submitted for histological examination, weighed, and assayed for protein, DNA, RNA, and polyamine content. Ornithine decarboxylase (ODC) activity was measured and ODC mRNA in the mucosa was determined by Northern blot analysis. Compared with the values for wet weight and protein content in old rats, young rats showed significantly higher values for wet weight on the 1st and 2nd POD, and for protein content on the 1st POD, but there were no differences between young and old rats after the 4th POD. The values for ODC activity and ODC mRNA were significantly lower in old rats than in young rats on the 1st POD, but there were no differences between young and old rats after the 2nd POD. The value for putrescine in old rats was significantly lower on the 2nd POD, but was significantly higher on the 4th POD than that in young rats. The present study showed that, in old rats, the residual intestine after small bowel resection preserved sufficient adaptive capacity, but that the adaptive response was decreased. The findings in this study also suggest that a decrease in ODC mRNA expression is involved in the decreased adaptive response that occurs with aging.

Age-related changes in duodenal adaptation after distal small bowel resection in rat

Two groups of male Fisher 344 rats (young: 4 months old; aged: 25 months old) underwent either 70% distal small bowel resection or sham operation (small bowel transection). Rats from each treatment group of each age were sacrificed on the 10th (N = 15: young rats; N = 13: aged rats) or 20th (N = 15: young; N = 13: aged) postoperative day (POD), and the duodenal mucosa was weighed and assayed for DNA, RNA, and protein contents, as well as for specific activities of the disaccharidase, sucrase, maltase, and lactase. Compared to the sham operation, distal small bowel resection significantly increased DNA by 48%, RNA by 122%, and protein by 75% in young rats and DNA by 40%, RNA by 92%, and protein by 71% in aged rats on the 20th POD. Both young and aged rats showed similar adaptive hyperplasia on the 10th POD. On the 20th POD after distal small bowel resection, specific activities of all tested enzymes were significantly increased in young rats (sucrase +86%, maltase +110% and lactase +64%), but showed no significant changes in aged rats. These findings suggest that the duodenum of aged rats may have sufficient proliferative potential to respond to distal small bowel resection, but that the mechanisms governing return of function in response to distal small bowel resection are inhibited in aged rats, compared to those mechanisms in the young.

Colonic cell proliferation and growth fraction in young, adult and old rats

Colonic epithelial proliferation was investigated in three groups of rats, aged 3, 60 and 121 weeks. As reported in previous work, the crypts were markedly longer in the young rats, and the number of labelled cells per crypt was significantly greater. There was an upward movement of the marker positions derived from the distribution of labelled cells within the crypt of the young rats. This was a consequence of the increased crypt length, so that the growth fraction, as expressed as a percentage of crypt length, was the same. The proliferative changes between the young rats and the other aged rats were therefore effected by altering the size of the crypts, while maintaining the kinetic organisation. There was no evidence of any proliferative changes or changes in the growth fraction when the colons of the old rats were compared with those of the 60 week old rats.

In vivo immediate early gene expression induced in intestinal and colonic mucosa by feeding

Since the gut responds rapidly to food intake, the levels of expression of several immediate early genes were measured in mucosa from small and large intestine of rats starved for 3 days or refed. Within 1 h of refeeding, jejunal and ileal c-fos, jun B and zif/268 mRNA and colonic zif/268 dramatically increased. The zif/268 gene in jejunum corresponded in size to the full-length cDNA but, in ileum, an RNA band of about 1.2 kb in size increased greatly after feeding. This represents a physiologic in vivo model for the study of gene regulation associated with intestinal epithelial cellular responses to feeding.

Changes in intestinal cell proliferation, absorptive capacity and structure in young, adult and old rats

Intestinal epithelial morphology, crypt cell production and absorptive function were investigated in 9 groups of rats, aged from 3 to 121 weeks. Absorption per unit length of intestine peaked in the first weeks of life and this was associated with increased villus size, crypt length and absorption per mg tissue weight. Absorption per gut continued to increase throughout life, which could be attributed to a similar increase in intestinal length and weight per cm, despite a decreased absorption per mg in the adult animals. Crypt-villus ratio increased rapidly between 6 and 8 weeks, but then appeared to reach a plateau. Crypt cell production started off at high rate in all sites and then decreased in the young adult. While there was some evidence for a small, non-significant, increase in proliferation in the very old rats, there was no evidence of the marked elevation reported by some workers.

Colonic proliferation is increased in senescent rats

Our previous studies suggested that crypt size enlarged and that proliferation rate might be greater in the small intestine of rats during senescence. Crypt cell numbers and crypt cell proliferation rates, using the vincristine-induced metaphase arrest technique, now have been measured in the colon of aging and young Fischer 344 rats. The proximal colon of 26-28-mo-old unfasted rats had 10% more crypt cells and a higher proliferative rate than 3-4-mo-old young controls. In the distal colon, the crypt cell proliferation rate in aging rats was 56% greater than in the young. A 3-day fast reduced crypt cell proliferation about fourfold in young rats but only by 20% in aging rats. One-day refeeding abruptly increased the crypt cell population and proliferation rate in rats of both age groups. The crypt zone of proliferating cells from aging rats was broader than that seen in young rats. In addition, starvation lowered colonic crypt cell cycling rate much less in aging than in young animals. We conclude that the colons of aging rats demonstrate a hyperproliferative state and a failure to adapt appropriately to changes in food intake. These observations may be relevant to states of altered proliferation that occur in the premalignant colon.