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

Is stomach spontaneously ageing? Pathophysiology of the ageing stomach

During ageing, histological and physiological modifications occur in the stomach leading to a confirmed state of gastric frailty defined by a decreased capacity of tissue repairing after mucosa gastric aggression. The process of intrinsic gastric ageing may play a role in inducing abnormalities of gastric epithelial proliferation against injury but, most of the time, pathophysiological modifications observed in older people are the consequences of chronic insults, such as chronic Helicobacter pylori infection, polymedication, especially Proton-pump inhibitors (PPI) and Non-steroidal Anti-inflammatory Drugs (NSAID), and co morbidities. One of the major clinical consequences of the gastric frailty is the higher vulnerability to the occurrence of peptic ulcer disease during ageing. In this review recent data on gastric changes during ageing, focussing especially on histological modifications and motility disorders are summarized.

Regulation of TGF-alpha-induced activation of AP-1 in the aging gastric mucosa

Although the age-related activation of EGF receptor (EGFR) in the gastric mucosa of Fischer 344 rats is associated with increased DNA binding activity of activator protein-1 (AP-1), little is known about the EGFR signaling cascades that regulate this process. The primary objective of this investigation was to determine the role of signaling pathways initiated by EGFR in regulating the transforming growth factor-alpha (TGF-alpha)-induced activation of AP-1 in the gastric mucosa in aged rats. Freshly isolated gastric mucosal cells from male young (4-5 mo) and aged (22-24 mo) rats were used. We have observed that although exposure of mucosal cells from young (4-5 mo) and old (22-24 mo) rats to 1 nM TGF-alpha for 20 min stimulates the DNA binding activity of AP-1 in both age groups, the magnitude of stimulation is substantially higher in aged (131%) than in young (35%) rats. This stimulation in the aged is associated with a concomitant activation of MEKs and ERKs, but not JNKs and p38. The TGF-alpha induction of AP-1 transcriptional activity in gastric mucosal cells from aged rats could be totally abrogated by either PD153035, a specific inhibitor of EGFR tyrosine kinase, or PD98059, a specific inhibitor of MEKs, but not by Wortmannin, which inhibits phosphatidylinositol kinase. PP2, a specific inhibitor of Src kinase, produces a 50% inhibition of the TGF-alpha-induced activation of AP-1 transcriptional activity. Our results suggest that the TGF-alpha-induced stimulation of DNA binding activity of AP-1 in the gastric mucosa of aged rats is primarily through a signaling pathway involving MEKs and ERKs, whereas Src kinase pathways play a minor role.

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 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.

Innervation of the gastric mucosa

A plethora of neuronal messengers ("classical" transmitters, gaseous messengers, amino acid transmitters, and neuropeptides) are capable of mediating or modulating gastric functions. Accordingly, the stomach is richly innervated. Gastric nerves are either intrinsic to the gastric wall, i.e., they have their cell bodies in the intramural ganglia and thus belong to the enteric nervous system, or they reach the stomach from outside, originating in the brainstem, in sympathetic ganglia, or in sensory ganglia. Topographically, the nerve fibers in the stomach reach all layers from the most superficial portions of the gastric glands to the outer smooth muscle layer. This wide distribution implies that virtually all different cell types may be reached by neuronal messengers. Within the gastric mucosa endocrine and paracrine cells (e.g., gastrin cells, ECL cells, somatostatin cells), exocrine cells (parietal cells, chief cells, mucous cells), smooth muscle cells, and stromal cells are regulated by neuronal messengers. The sensory innervation, responding to capsaicin, plays an important role in mucosal protection, and in ulcer healing. Presumably also other nerves are involved and a plasticity in the neuropeptide expression has been demonstrated at the margin of gastric ulcers. Taken together, available data indicate a complex interplay between hormones, paracrine messengers and neuronal messengers, growth factors and cytokines in the regulation of gastric mucosal activities such as secretion, local blood flow, growth, and restitution after damage.

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.

Localization and significance of pp55, a gastric mucosal membrane protein with tyrosine kinase activity

In Fischer 344 rats, induction of gastric mucosal proliferative activity, whether the result of aging or injury or occurring after administration of epidermal growth factor, gastrin, or bombesin, is associated with a rise in tyrosine kinase activity and tyrosine phosphorylation of several mucosal proteins, including a protein with a molecular mass of 53-55 kDa. We hypothesized that this phosphotyrosine membrane protein (referred to as pp55) may play a role in regulating gastric mucosal cell proliferation and differentiation. Purification and subsequent immunoprecipitation studies now show that pp55 is a tyrosine kinase. In addition, the enzyme activity in the gastric mucosa is found to be fourfold higher in aged rats than in young rats. Incubation of gastric mucosal membranes with transforming growth factor-alpha (2 x 10(-8) M) stimulates tyrosine kinase activity of pp55. Immunolocalization studies reveal that pp55 immunoreactivity is predominantly present in mucous cells that are located just above the proliferative zone and spasmolytic peptide-immunoreactive mucous neck cells. Tyrosine kinase activity as well as expression of pp55 are also greatly increased in the gastric mucosa after hypertonic saline-induced injury, a condition that results in stimulation of surface mucosal cell proliferation and differentiation. Our current data suggest that pp55 is a tyrosine kinase, likely localized to pre-surface cells. The presence of pp55 in pre-surface mucous cells and the expression and tyrosine kinase activity of this protein, which can be stimulated during mucosal cell proliferation and differentiation, strongly suggest a role for pp55 in differentiation of gastric surface mucous cells.

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.

Identification and evaluation of the role of endogenous tyrosine kinases in azoxymethane induction of proliferative processes in the colonic mucosa of rats

Although tyrosine kinases (Tyr-k) are known to play a role in regulating proliferation of normal, preneoplastic and neoplastic cells, little is known about the identity of different species of Tyr-k involved in this process. Utilizing a non-denaturing polyacrylamide gel electrophoresis system, in which the separated proteins from tissue extracts are assayed directly for Tyr-k, we attempted to identify the species of Tyr-k that may be involved in azoxymethane (AOM) induction of colonic mucosal ornithine decarboxylase (ODC) activity, an enzyme whose activity is known to rise in rapidly proliferating cells. We have observed that 5 days after a single injection of the colonic carcinogen AOM (20 mg/kg body wt) to 3-4-month old rats, a significant 230% rise in colonic mucosal proliferative activity (as evidenced by 5-bromo-2'-deoxyuridine (BrdU) immunoreactivity) was also accompanied by a 550% increase in ODC activity. This was also associated with a marked rise (140-240%) in the relative activity of Tyr-k of three mucosal proteins with MI of 165, 145 and 125 kDa. Since the molecular mass of one of the Tyr-k (165 kDa) corresponded to that of EGF-receptor (EGF-R), this led us to examine the role of EGF-R Tyr-k in AOM induction of colonic mucosal ODC. We observed that a 320% increase in mucosal ODC activity, 5 days after AOM injection, was accompanied by over 200% rise in Tyr-k activity of EGF-R. Daily injection of tyrphostin (300 micrograms/kg body wt.), a Tyr-k inhibitor with a higher specificity for EGF-R Tyr-k, significantly attenuated AOM-induced stimulation of both ODC and Tyr-k activity of EGF-R. Administration of AOM also stimulated the rate of synthesis and secretion of TGF-alpha in isolated colonocytes. In addition, the levels of TGF-alpha and its mRNA in the colonic mucosa were also found to be 100% and 250% higher, respectively, in AOM-treated rats when compared with the controls. We suggest that (a) activation of intrinsic Tyr-k of EGF-R is an important event in AOM induction of colonic mucosal proliferative processes, and (b) this activation is thought to be mediated by TGF-alpha through an autocrine mechanism.

Novel gastrin receptors mediate mitogenic effects of gastrin and processing intermediates of gastrin on Swiss 3T3 fibroblasts. Absence of detectable cholecystokinin (CCK)-A and CCK-B receptors

We have reported previously mitogenic effects of gastrin on several immortalized and neoplastic cell lines, including Swiss 3T3 fibroblasts. Receptor subtypes, cholecystokinin (CCK)-A and CCK-B, for a closely related peptide, cholecystokinin, were recently cloned. These studies were undertaken to investigate if CCK-A- and CCK-B receptors were perhaps mediating the mitogenic effects of gastrin on Swiss 3T3 cells. Receptor antagonists that inhibit the biological effects and binding of peptides to the CCK-A (L-364,718 (L18)) and CCK-B (L-365,260 (L60)) receptors were ineffective toward inhibiting the binding and proliferative effects of gastrin on Swiss 3T3 cells. Radiolabeled L18 and L60 demonstrated no binding to the cells, indicating that CCK-A and CCK-B receptors may be absent on Swiss 3T3 cells. Radiolabeled CCK-8, gastrin, L18, and L60, on the other hand, demonstrated specific binding to a pancreatic cancer cell line (AR42J cells) (used as a positive control). In cross-linking studies the molecular mass of the major band of gastrin receptors (GR) on Swiss 3T3 cells was determined to be approximately 45 kDa. The mitogenic potency of 0.1-1.0 nM gastrin-like peptides on Swiss 3T3 cells was in the order of G1-17 > or = G1-17-Gly > G5-17 > or = G5-17-Gly > G2-17 > CCK-8-Gly > or = G1-17-Lys > or = CCK-8. The relative binding affinity of the peptides (based on the dose-dependent inhibition of binding of 125I-G1-17 to Swiss 3T3 cells) was similar to the relative mitogenic potency of the peptides as given above. Furthermore, G1-17-Gly was equally effective as G1-17 in displacing the binding of 125I-G1-17 to the 45-kDa GR from the Swiss 3T3 cells. Based on these studies it became evident that the novel gastrin preferring GR, expressed by Swiss 3T3 cells, binds and mediates the mitogenic effects of not only the mature (amidated) forms of gastrin-like peptides but also binds and mediates the mitogenic effects of glycine-extended forms of gastrin-like peptides. Possible mRNA expression of CCK-A and CCK-B receptor subtypes by gastrin-responsive rodent intestinal and fibroblast cell lines (Swiss 3T3, IEC-6, CA) was measured by the methods of Northern blot analysis and reverse transcriptase-polymerase chain reaction. mRNA from rat pancreas, AR42J cells, and rat antrum served as positive controls.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell signalling pathway involved in PACAP-induced AR4-2J cell proliferation

The novel 38-amino acid neuropeptide PACAP (pituitary adenylate activating peptide) has recently been shown to induce the pancreatic acinar tumour AR4-2J cell growth. This growth promoting effect of PACAP was, however, independent of adenylate cyclase activation but suppressed by pertussis toxin and the somatostatin analog SMS 201-995. This study was undertaken to search for potential cell signalling pathways involved in the growth promoting effect of PACAP on AR4-2J cells. The AR4-2J cells were grown in Dulbecco's Modified Eagle's Medium containing 10% foetal calf serum. For studies on cell signalling pathways, all experiments were carried out on cells which have reached 50 to 75% confluency. At that point, they were transferred to serum free medium overnight with or without 1 microCi/ml myristic acid. The next morning, cells were harvested, washed and used for tyrosine kinase and phospholipase D (PLD) activities. For studies on growth, cells were grown for 2 days in the presence of 1 nM PACAP +/- the different inhibitors of tyrosine kinase and PLD. PACAP-38 and -27 caused a dose-dependent and parallel activation of tyrosine kinase and PLD an effect prevented by the antagonist PACAP 7-38. PACAP-38-stimulated tyrosine kinase and PLD activation are both dose-dependently inhibited by SMS 201-995. Finally, PACAP-stimulated tyrosine kinase and PLD activities are both inhibited by cell's preincubation with genistein and pertussis toxin. After 2 days, the PACAP-induced increase in AR4-2J cell growth was significantly inhibited by increasing concentrations of genistein and wortmannin, inhibitors of tyrosine kinase, PLD and phosphatidylinositol 3-kinase, respectively. PACAP can induce concomitant activation of tyrosine kinase and PLD; this finding and the observation that inhibition of these two enzymes inhibited PACAP-induced AR4-2J cell growth strongly suggests that they are intimately involved in the overall process of PACAP-induced AR4-2J cell proliferation.

Ornithine decarboxylase inhibitor attenuates bombesin enhancement of intestinal carcinogenesis and metastasis induced by azoxymethane

The effects of combined administration of bombesin (40 micrograms/kg body weight) and the ornithine decarboxylase (ODC) inhibitor, 1,3-diaminopropane (DAP), on the development of large and small intestinal tumors and the incidence of their metastasis to the peritoneum induced by azoxymethane (AOM, 7.4 mg/kg body weight), the ODC activity of the intestinal wall, and the labeling index of the intestinal mucosa and tumor were investigated in inbred Wistar rats. Rats received weekly s.c. injections of AOM for 10 weeks, s.c. injections of bombesin every other day, and drinking water containing DAP (2.5 g/l) until the end of the experiment at week 40. Administration of bombesin significantly increased the incidence of intestinal tumors at week 40. It had no influence on the location, size, histological features or depth of involvement of intestinal adenocarcinomas, but significantly increased the incidence of their metastasis to the peritoneum. It also resulted in a significant increase in the intestinal ODC activity and labeling index. Administration of DAP with bombesin significantly reduced the enhancement of intestinal carcinogenesis by bombesin. Although the combined use of DAP with bombesin had little or no influence on the location, size, histological features, or depth of involvement of intestinal cancers, the incidence of their metastasis was significantly reduced. DAP significantly attenuated bombesin enhancement of the intestinal ODC activity and labeling index. These findings indicate that ODC inhibition attenuated the enhancement of intestinal carcinogenesis and metastasis to the peritoneum.