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

Abstract

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 < 0.025) rise in gastric mucosal overall Tyr-K activity and a 71% ( p < 0.001) increase in Tyr-k activity associated with pp60 c-src, when compared with the corresponding controls. The bombesin-induced stimulation of pp60 c-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< 0.001) increase in ODC activity, a 150% ( p< 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. We conclude that aging is associated with a loss of responsiveness of the gastric mucosa to the growth-promoting action of bombesin. This is in part due to the peptide's inability to activate certain tyrosine kinases, including pp60 c-src.