Tryosine kinase and ornithine decarboxylase activation in children with Helicobactor pylori gastritis.

H. pylori infection has been considered a risk factor for the development of gastric malignancy. Ornithine decarboxylase and tyrosine kinases activities are increased in patients with colon or esophageal cancer. In this study we compared the ODC and tyrosine kinases activities in the gastric mucosa of children with H. pylori infection and normal mucosa. Gastric biopsies were prospectively collected from children during routine upper endoscopic procedure. H. pylori infection was determined histologically. Biopsies were analyzed for ODC activity, total tyrosine kinases activities, and for the activity of protooncogene tyrosine kinase pp60(c-src). The mean ODC activity (pmol 14CO2/mg. protein/hr) and total tyrosine kinases activity (pmol 32P/mg. protein) were 186 and 5877 for H. pylori infected mucosa; and 229 and 4300, for normal mucosa, respectively (p> 0.05). Tyrosine kinase pp60(c-src) protein levels were similar between H. pylori infected mucosa and normal mucosa (3.12 and 2.15 pmol 32P/mg. protein, respectively; p>0.05). There was no correlation between gastric inflammation and the level of ODC or tyrosine kinase activities. ODC and tyrosine kinase activities in the gastric mucosa are similar in children with H. pylori infection compared to normal mucosa. The data suggest that these enzymes cannot be used as markers for future cancer development in children.

Enhanced ligand-induced activation of EGF-receptor and overall tyrosine kinase and phospholipase C in colonocytes isolated from azoxymethane-treated rats.

BACKGROUND/AIMS: In order to evaluate the role of epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) in colorectal cancer, the present investigation examines changes in EGF and TGF-alpha-mediated activation of overall and EGF receptor (EGF-R) associated tyrosine kinase activity in isolated rat colonocytes after administration of the colonic carcinogen azoxymethane. METHODOLOGY: Five days after a single injection of azoxymethane (20 mg/kg) or saline solution to 3-4 month old Fischer-344 rats, colonocytes were isolated, exposed for 2 minutes to 1 x 10(8) M EGF and TGF-alpha, and assessed for overall and EGF-R associated tyrosine kinase and phospholipase C activity. RESULTS: In colonocytes isolated from control animals, incubation with EGF and TGF-alpha resulted in a small (21-35%) increase in overall tyr-k. However, a marked (113-127%) rise of this enzyme occurred in colonocytes from AOM-treated rats, when compared with the corresponding basal levels. These differences were even more pronounced in colonocytes isolated from the distal part of the colon, as regards to the proximal part. In addition, EGF and TGF-alpha activated EGF-R tyr-k by 40-60% in controls and by 84-85% in AOM-treated animals. Incubation of colonocytes with these growth factors also stimulated PLC activity (in controls by 120-150% and in AOM injected rats by 204-271%) when compared with corresponding basal values. CONCLUSIONS: We conclude that AOM enhances the responsiveness of colonocytes to EGF and TGF-alpha, which may be one of the mechanisms involved in colorectal carcinogenesis.

Increased expression of EGFR in 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 the intracellular events that regulate this process. The present investigation examines the age-related changes in gastric mucosal tyrosine kinase activity and expression of epidermal growth factor receptor(EGFR) and its structural and functional analog p185c-erbB-2, the protein product of c-erbB-2/c-neu protooncogene. We observed a significantly higher intrinsic tyrosine kinase activity and tyrosine phosphorylation of EGFR and p185c-erbB-2 in the gastric mucosa of 24-mo-old (aged) rats than in that of their 4- or 12-mo-old counterparts. This was associated with increased levels of EGFR protein and steady-state mRNA levels of EGFR and p185c-erbB-2. In addition, we also observed threefold higher steady-state mRNA levels of transforming growth factor-alpha (TGF-alpha; one of the primary ligands of EGFR) in the gastric mucosa of aged rats than in that of 4-mo-old (young) animals. This was accompanied by a fivefold increase in the relative concentration of the 18-kDa precursor form of TGF-alpha in gastric mucosal membranes but not in the cytosol. In conclusion, our data demonstrate that aging is associated with increased tyrosine kinase activity of EGFR and p185c-erbB-2 in the gastric mucosa. Moreover, the observation that aging results in increased accumulation of TGF-alpha in gastric mucosal membranes raises the possibility that the membrane-bound TGF-alpha could partly be responsible for the constitutively active EGFR-induced signaling pathway in the gastric mucosa of aged rats and, in turn, for stimulation of mucosal proliferative activity.

Differential activation of total and EGF receptor (EGF-R) tyrosine kinase (tyr-k) in the rectal mucosa in patients with adenomatous polyps, ulcerative colitis and colon cancer.

BACKGROUND/AIMS: Tyrosine kinase and a number of growth factors, especially EGF and TGF-alpha are known to stimulate proliferation in much of the gastrointestinal tract, including colon. In humans increased colonic mucosal proliferative activity has been observed in numerous premalignant lesions including adenomatous polyps and ulcerative colitis. The aim of the present study was to determine the differences of proliferative patterns in patients with adenomatous polyps, ulcerative colitis and colonic adenocarcinoma as reflected by rectal mucosa tyrosine kinase, EGF receptor tyrosine kinase and PCNA and to evaluate the role of tyr-k in colonic mucosal cell proliferation during carcinogenic process. MATERIALS AND METHODS: The study population comprised 40 patients, aged 17-74 years (mean 57), in which 10 patients had adenomatous polyps, 10-ulcerative colitis in remission phase, 10- colon adenocarcinoma and 10 healthy controls. After informed consent 6-8 rectal mucosal biopsy specimen were obtained at 10 cm from the anal verge at the beginning of the colonoscopy examination and at least 10 cm away from any macroscopic mucosal changes. RESULTS: Mean PCNA labeling indices in patients with colon adenocarcinoma, adenomatous polyps, ulcerative colitis ulcerosa and healthy controls were respectively: 27.6% +/- 5.75; 12.18% +/- 6.76; 10.9% +/- 5.34 and 1.5% +/- 0.97. PCNA labeling index in rectal mucosa of patients with adenomatous polyps, ulcerative colitis and colon cancer was significantly higher (p < 0.01) than in the control group. An upward expansion of the proliferative compartment was also observed in patients with premalignant and malignant colon conditions as regards to the control group. Total tyrosine kinase activity in the rectal mucosa of patients with polyps was elevated by 219%, with ulcerative colitis by 224% and with colorectal carcinoma by 600% as regards to the control group. EGF receptor tyrosine kinase was increased in colonic mucosa by 35.2% in patients with adenomatous polyps, by 40.6% in patients with ulcerative colitis and by 123% in patients with colon cancer. CONCLUSIONS: Increased values of this enzyme in the above mentioned group of patients may suggest that tyrosine phosphorylation represents an early sign of colonic mucosa susceptibility for cancer development. We conclude, that overall an EGF receptor-associated tyrosine kinase plays an important role in the development of hyperproliferative state of the colonic mucosa and colon carcinogenesis.

Inhibition of gastric mucosal regeneration by tyrphostin: evaluation of the role of epidermal growth factor receptor tyrosine kinase.

Although induction of mucosal cell proliferation is a crucial event in gastric mucosal regeneration after injury, intracellular regulatory processes have not been fully elucidated. We hypothesize that tyrosine kinases (Tyr-k)--specifically the enzyme associated with epidermal growth factor receptor (EGF-R)--play an important role in mucosal regeneration. Utilizing tyrphostin--a Tyr-k inhibitor with a greater specificity for EGF-R Tyr-k than for other Tyr-ks--we have examined the role of EGF-R Tyr-k in gastric mucosal regeneration after injury. Gastric mucosal injury in 3-to 4-month-old rats was induced by orogastric administration of 2 mol/L NaCl, whereas the control animals received an equivalent volume of water. The animals were killed 24 hours later. During this 24-hour experimental period (reparative phase), one of the groups was also injected (IP) with tyrphostin-51 (0.65 mg/kg in 30% dimethyl sulfoxide), whereas the control group received the vehicle. In the absence of tyrphostin, the gastric mucosa showed signs of extensive regeneration, whereas in its presence the degree of regeneration was greatly attenuated. These changes were accompanied by parallel alterations in the number of proliferating cell nuclear antigen-immunoreactive cells and the Tyr-k activity of EGF-R. In water-fed control animals, tyrphostin also caused a significant 30% reduction in proliferating cell nuclear antigen-immunoreactive cells. In these animals, the Tyr-k activity of EGF-R was also decreased by 30%. At 24 hours after injury, EGF-R mRNA levels were increased 36-fold over the water-fed controls, and this increase was not significantly affected by tyrphostin. Our current data suggest that activation of EGF-R is an important event in mucosal regeneration.

Gastrin activates tyrosine kinase and phospholipase C in isolated rat colonocytes.

Postreceptor regulation of the trophic action of gastrin is not fully elucidated. Tyrosine kinase (Tyr-kinase) has been associated with receptors of a number of growth factors and plays an important role in regulation of cellular growth within the gastrointestinal tract. The aim of this study was to determine, whether Tyr-kinase plays a role in mediating the growth promoting action of gastrin and whether phospholipase C (PLC) is involved in the signal transduction pathway. Colonocytes isolated from Fischer 344 rats were incubated for 2 min with gastrin (10(-8) M) and assayed for Tyr-kinase and PLC activities. Incubations with gastrin resulted in 60%-70% rise in Tyr-kinase and 150%-200% rise in PLC activities over the corresponding basal levels. When processed separately, in proximal colon Tyr-kinase activation by gastrin was 15%-20%, while in distal colon 70%-80% as compared to the buffer control. Gastrin activation of both Tyr-kinase and PLC was abolished by Tyr-kinase inhibitor, tyrphostin-25 (3.2 microM) and was not affected by staurosporine (20 ng/ml). We conclude that Tyr-kinase is involved in the mechanism of trophic action of gastrin, and PLC activation appears to be the next step in the signal transduction pathway.

Ornithine decarboxylase transformation of NIH/3T3 cells is mediated by altered epidermal growth factor receptor activity.

Ornithine decarboxylase (ODC) has been shown to be oncogenic in transfected NIH/3T3 cells overexpressing the enzyme from a heterologous promoter. These cells, designated as NODC-2 cells, acquire proliferative properties associated with tumorigenic transformation such as loss of contact inhibition, decreased population doubling time, anchorage-independent growth, and tumor production in nude mice. At least one of these parameters, loss of contact inhibition, remains dependent on elevated ODC levels. We have used these cells to investigate the molecular mechanisms by which ODC overexpression drives cell transformation and to examine the involvement of other proto-oncogene products in this process. An interaction between ODC overexpression and the epidermal growth factor receptor (EGF-R) was suggested initially by the elevation of both basal (300%) and ligand-induced (457%) EGF-R tyrosine kinase activities in NODC-2 cells compared to similarly treated control NLK cells. Disruption of EGF-R mediated signal transduction in NODC-2 cells both by treatment with tyrphostin-25 or by transfection with a vector expressing a dominant negative EGF-R mutant resulted in reacquisition of contact-inhibited growth and suppression of anchorage-independent, clonogenic growth in soft agar. We conclude that ODC-induced transformation of NIH/3T3 cells is mediated, at least partly, by alterations in EGF-R signal transduction activity.

Induction of EGF-receptor tyrosine kinase during early reparative phase of gastric mucosa and effects of aging.

BACKGROUND: Although the gastric mucosa of adult healthy animals possesses a remarkable capacity to promptly repair its mucosal architecture after an acute injury, aging attenuates this process. We hypothesize that certain tyrosine kinases (Tyr-k), specifically the enzyme associated with EGF-receptor (EGF-R), may play a role in this process. The present investigation was undertaken to evaluate the role of this enzyme in the early reparative phase of the gastric mucosa in young and aged rats. EXPERIMENTAL DESIGN: In our initial effort to test the hypothesis, we examined the changes in both total and EGF-R-associated Tyr-k activities in the gastric mucosa of young adult rats (4-months old) during the first 60 minutes after hypertonic saline (2 M NaCl; 1.5 ml/130 g body weight)-induced injury. Because the maximal stimulation (90-100% over the controls) in both total and EGF-R-associated Tyr-k occurred at 30 minutes after injury, we used this time point to perform the next experiment, in which groups of young and aged rats were given (intragastically) 2 M NaCl or water. One of the young and aged groups of rats was also injected (i.p.) with the Tyr-k inhibitor tyrphostin-51 (300 micrograms/kg body weight) 60 minutes before injury. The gastric mucosa was assayed for EGF-R Tyr-k activity and tyrosine phosphorylation and expression of EGF-R, phospholipase C (PLC) activity and relative concentration and tyrosine phosphorylation of PLC-gamma 1, as well as transforming growth factor-alpha (TGF-alpha) levels. RESULTS: Basal EGF-R Tyr-k activity and the extent of tyrosine phosphorylation of EGF-R, as well as PLC activity, were all found to be higher in the gastric mucosa of aged than in young rats. Although 30 minutes after injury, EGF-R Tyr-k activity, tyrosine phosphorylation of EGF-R, and relative abundance of the receptor were all increased in the gastric mucosa of both young and aged rats, the magnitude of stimulation of each of the parameters was found to be considerably lower in aged than in young rats, compared with the corresponding basal levels. A similar phenomenon was also observed for PLC activity and tyrosine phosphorylation of PLC-gamma 1. The relative concentration of mucosal PLC-gamma 1 level was, however, not affected by injury in either young or aged rats. Tyrphostin greatly attenuated the injury-induced increases in the above mentioned parameters in both young and aged rats. In young but not in aged rats, injury caused a significant increase in mucosal TGF-alpha levels. CONCLUSIONS: We conclude that (a) activation of EGF-R Tyr-k is an important event in the early reparative process of the gastric mucosa, and (b) local production of TGF-alpha may play an important role in regulating the activation of EGF-R Tyr-k.

Ornithine decarboxylase and tyrosine kinase activity in juvenile polyps of childhood.

Juvenile polyps (JP) are the most common colonic tumor in children. Although considered benign, malignant transformation has been reported in JP. Ornithine decarboxylase (ODC) and tyrosine kinase (TyK) enzymes are markers for a rapid cell proliferation index. DNA aneuploidy score and p53 gene expression are late malignant changes seen in patients with colon cancer. In this study, we investigated ODC and TyK activities as well as DNA aneuploidy score and p53 expression in juvenile polyps compared with the adjacent normal colonic mucosa. Results showed that ODC was significantly increased in JP compared with the adjacent normal colonic mucosa. TyK activity was increased in 3/5 polyps and decreased in 2/5 polyps compared with the mucosa. Mean TyK activity was higher in JP compared with normal mucosa but did not reach significance (707 and 632 pmol/mg pmol, respectively). Moreover, changes in phosphorylization of TyK proteins was also observed in JP but not in normal mucosa. JP had a normal DNA aneuploidy score and showed no expression of p53 gene. We conclude that JP do not express p53 gene and aneuploidy but had higher activity of ODC and TyK enzymes, suggesting a higher stage of cell proliferation.

Aging diminishes gastric mucosal regeneration: relationship to tyrosine kinases.

BACKGROUND: Increased incidence of gastric ulcer observed in the aged could be partly attributed to increased susceptibility of the mucosa to various damaging agents together with impediment of the repair process. The present investigation was undertaken to compare the rate of mucosal regeneration and the role of tyrosine kinases in regulation of this process between young (4-month-old) and aged (24-month-old) rats during the first 24 hours after injury. EXPERIMENTAL DESIGN: Groups of young and aged rats were given intragastrically with either 2 M NaCl (1.5 ml/130 gm body weight), or an equivalent volume of water and killed 1, 6, and 24 hours later. Each animal was injected intraperitoneally with 5-bromo-2'-deoxyuridine (BrdU; 50 mg/kg) 1 hour before killing to assess proliferative activity by immunocytochemistry. The stomach (oxyntic gland area) was also evaluated by light microscopy for the extent of injury and subsequent regeneration, and mucosa assayed for ornithine decarboxylase and tyrosine kinase (Tyr-k) activity and tyrosine phosphorylation of membrane proteins. RESULTS: Although 2 M NaCl caused extensive damage to the gastric mucosa in both young and aged rats, as evidenced by the total loss of the surface epithelium at 1 hour postinjury, the degree of regeneration was faster in young animals. In young rats, gastric epithelium showed signs of regeneration at 6 hours postinjury and was essentially complete by 24 hours. In contrast, in aged rats, only intermittent surface cells were seen 24 hours after injury. In both age groups, injury resulted in stimulation of mucosal proliferative activity. However, whereas ornithine decarboxylase activity in both age groups was maximally stimulated (350% in young versus 80% in aged) at 6 hours after injury, the number of BrdU-positive cells in young rats increased steadily with time after injury. In contrast, aged rats showed a biphasic pattern in that the number of BrdU-positive cells/gland remained decreased for up to 6 hours, whereafter a steep rise occurred. At 24 hours after injury, the number of BrdU-positive cells/gastric gland in aged rats were found to be higher than in young rats (6 +/- 1.5 cells/gland in young rats versus 9 +/- 2.1 cells/gland in aged rats). The pattern of Tyr-k activity in young and aged rats after injury was found to be quite different from that observed for proliferative activity. In young rats, mucosal Tyr-k activity increased by about 60% at 1 hour after injury, then decreased slightly over the next 5 hours and increased again revealing a 120% rise at 24 hours postinjury. This was associated with a concomitant change in tyrosine phosphorylation of six membrane proteins with molecular weight (in kilodalton) of 30, 35, 50, 55, 60 and 70. In contrast, in aged rats, Tyr-k activity was increased only marginally (about 20%) during the first 6 hours, but at 24 hours postinjury it was found to be 70% above the control. In aged rats, injury produced no significant stimulation in tyrosine phosphorylation of gastric mucosal membrane proteins. CONCLUSIONS: We conclude that aging is associated with the diminished regenerative capacity of the gastric mucosa. This could partly be attributed to diminished activation of mucosal Tyr-k and decreased tyrosine phosphorylation of certain membrane proteins.

Increased expression of pp60c-src in gastric mucosa of aged rats.

The relationship between proliferative activity and the expression of pp60c-src in gastric mucosa (oxyntic gland area) of young (4-month) and aged (24-month) Fischer 344 rats was examined. Gastric mucosal proliferative activity, as assessed by 5-bromo-2'-deoxyuridine (BrdU) immunoreactive cells, was found to be 115% (p < .001) higher in aged than in young rats. This was associated with a 47% rise (p < .025) in overall tyrosine kinase (Tyr-k) activity and a 5-7-fold increase in autophosphorylation of four prominent protein bands with M(r) of 40, 55, 60, and 70 kDa in gastric mucosal membranes. A similar phenomenon was also observed for Tyr-k activity of pp60c-src in that the aged rats revealed a 69% (p < .025) higher enzyme activity and a 5-fold rise in the extent of autophosphorylation of this protein when compared with the corresponding values from young animals. Increased Tyr-k activity of pp60c-src in the gastric mucosa of aged rats could in part be due to higher levels of this protein because the relative concentration of pp60c-src, as assessed by Western blot analysis, showed a 2-5-fold increase over the young animals. In addition, the relative concentration of c-src mRNA in the gastric mucosa of aged rats was also found to be 5-6-fold higher than in young animals. We suggest that pp60c-src may play a role in regulating gastric mucosal proliferative processes in the gastric mucosa of aged rats.

Changes in gastric mucosal ornithine decarboxylase and tyrosine phosphorylation of proteins in postgastrectomy patients.

There is evidence to suggest that partial gastrectomy is associated with an increased risk of developing gastric carcinoma in humans. Since ornithine decarboxylase (ODC) and tyrosine phosphorylation of proteins are involved in the regulation of cell proliferation, the present study was undertaken to examine the time-dependent changes of these variables in the postgastrectomy stomach. Thirty-seven postgastrectomy patients (Billroth I (BI), n = 7, and Billroth II (BII), n = 30) underwent gastroscopy. For comparison, five patients with intact stomachs (three healthy and two postvagotomy and pyloroplasty) were also studied. Gastric mucosal biopsy specimens were obtained within 5 cm of the anastomosis and analyzed for ODC activity. In addition, tyrosine phosphorylation of membrane proteins was also determined in representative samples of BII patients. Gastric mucosal ODC activity was significantly higher in BII patients in whom gastrectomy had been performed > 15 years earlier compared with those in whom it had been performed < 15 years earlier (p < 0.001) or controls (p = 0.004). Although the mean ODC activity was higher in BII than in BI patients, the difference was not significant (p = 0.103). Isolated patients with high ODC activity demonstrated increased phosphorylation of tyrosine membrane proteins with M(r) of 55-60.

Role of tyrosine kinases in bombesin regulation of gastric mucosal proliferative activity in young and aged rats.

In vivo and in vitro experiments were performed to examine the responsiveness of the gastric mucosa to the growth-promoting action of bombesin in young (4 months) and aged (22 months) Fischer 344 rats. In addition, the role of tyrosine kinase (Tyr-K) in regulating this action of bombesin was also examined. In young rats, infusion of bombesin (300 ng/kg/h) by osmotic minipump for 2 weeks resulted in a significant 100% increase in mucosal DNA synthesis and ornithine decarboxylase (ODC) activity. These increases were accompanied by a 32% (p less than 0.025) rise in gastric mucosal overall Tyr-K activity and a 71% (p less than 0.001) increase in Tyr-k activity associated with pp60c-src, when compared with the corresponding controls. The bombesin-induced stimulation of pp60c-src Tyr-k activity was also associated with a 25% increase in phosphorylation of this protein. In contrast, in aged rats, none of these parameters were affected by bombesin. A similar phenomenon was also observed when mucosal explants from young and aged rats were exposed to bombesin in an organ culture system. Exposure of gastric mucosal explants from young, but not from aged, rats to 10(-8) M bombesin for 8 h resulted in a 300% (p less than 0.001) increase in ODC activity, a 150% (p less than 0.001) rise in Tyr-k activity, and a marked increase (400-600%) in tyrosine-specific phosphorylation of three membrane proteins with M(r) of 55, 44, and 41 kDa, when compared with the corresponding controls. However, these increases were totally abolished by genistein, a specific irreversible inhibitor of Tyr-k.(ABSTRACT TRUNCATED AT 250 WORDS)

Supplemental calcium suppresses colonic mucosal ornithine decarboxylase activity in elderly patients with adenomatous polyps.

Epidemiological and animal studies suggest a role for calcium in the chemoprevention of colorectal neoplasia. This study was designed to investigate whether supplemental oral calcium has a suppressant effect on colonic mucosal ornithine decarboxylase (ODC) and tyrosine kinase activities in patients with adenomatous polyps or a history of adenomatous polyps and whether this is affected by age. ODC and tyrosine kinase activities were measured in rectal mucosal biopsies of 19 male patients (age, years 46-85 years; mean, 66 years) with adenomatous polyps or a history of adenomatous polyps before and after 1 week of calcium supplementation p.o. (CaCO3; 2500 mg/day) and 2 weeks after cessation of calcium treatment. The basal rectal mucosal ODC activity of patients greater than or equal to 64 years old was nearly 4-fold higher than that of patients less than 64 years old (P less than 0.005). In patients greater than or equal to 64 years old, there was a significant decrease in rectal mucosal ODC activity following 1 week of calcium p.o. compared to those age less than 64 years (P less than 0.05). Overall tyrosine kinase activity did not differ significantly in either patient group before or after calcium supplementation p.o. However, the concentration of phosphotyrosine membrane proteins with molecular weights between 40,000 and 60,000 and between 80,000 and 100,000 were suppressed in patients age greater than or equal to 64 years after 1 week of calcium treatment p.o. These patients also had a corresponding decrease in their rectal mucosal ODC activity. Alternatively, patients whose ODC was not affected by calcium showed no apparent change in the relative concentration of rectal mucosal phosphotyrosine membrane proteins. Our data indicate that there is an age-related increase in basal rectal mucosal ODC activity in patients with adenomatous polyps which can be suppressed with calcium supplementation p.o., suggesting a role for dietary calcium in the chemoprevention of colorectal neoplasia.

Morphological and biochemical changes in gastric mucosa of aging rats.

Although previous data from this laboratory have indicated that aging is associated with increased gastric mucosal proliferative activity, no direct assessment of proliferative potential of the tissue has been made during aging. In order to assess this, and to determine whether changes in mucosal proliferative potential would be reflected in growth of the tissue, we have examined the labeling index (LI), height and morphology of the gastric mucosa in young (4-month-old) and aged (24-month-old) Fischer-344 rats. In addition, tyrosine kinase (Tyr-k) activity and the levels of phosphotyrosine proteins were determined to evaluate their relationship to mucosal proliferative activity. Histologic evaluation revealed a marked atrophy of the mucosal glandular component with 32% reduction in height in aged rats when compared with young animals. In aged rats, there was also a decrease in gland density, resulting in a reduction in the number of epithelial cells of all types with evidence of decreased secretory activity. Despite the occurrence of mucosal atrophy in aged rats, LI in these animals was significantly increased by 28%. This was associated with a parallel rise in mucosal Tyr-k activity, and a two- to threefold increase in the relative concentrations of seven phosphotyrosine membrane proteins with Mr of 120, 105, 90, 60, 55, 48 and 32 kDa. We conclude that (1) although aging is associated with increased gastric mucosal proliferative activity, this does not result in mucosal growth and that (2) Tyr-k and tyrosine phosphorylation of certain proteins play a role in the regulation of gastric mucosal cell proliferation during aging.