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.