Cellular Basis Of Action Of Gastrointestinal Peptides/Growth factors

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Recent studies show that gastrointestinal hormones/growth factors may stimulate cell growth by stimulating multiple intracellular tyrosine phosphorylation (TyrP) signaling cascades. However at present little is known about the ability of many gastrointestinal hormones/growth factors to activate these cascades in GI tissues. We have investigated the ability and mechanism of various GI hormones to cause activation of the novel serine /threonine kinase protein kinase D(PKD). We found only hormones activating phospholipase C(PLC) activated PKD1 cause phosphorylation of S916, S744/748 and activation required both limbs of the PLC cascade. Using dominant negative PKCs, only PKC delta was involved in mediating this effect. The GI hormone CCK, caused activation OF PKD through both a PKC-dependent mechanism, as is widely reported by other stimulants in other tissues, and also by a novel cascade which was PKC-independent. The role of PKC delta is difficult to define in the various tissues due to the presence of multiple PKCs. The inhibitor rottlerin has been widely used in the literature for this purpose. We found in a detailed study of its action in pancreatic acini the inhibition caused by rottlerin was not specific for PKC delta and in fact most of the inhibition of various cellular signaling cascades was due to it acting as an uncoupler of oxidative phosphorylation. Therefore results with this compound should be carefully interpreted in each system it is used and its specificity established. HGF has been reported to play an important role in growth as well as the recovery from injury of acinar cells as well as function as an important growth factor in pancreatic cancer. Our studies show HGF stimulates GAB1 TyrP rapidly and potently at both the Y307 and Y627 Tyr loci of GAB1. This not only caused a redistribution of GAB1 to the membrane it also stimulated the association of GAB1 with numerous downstream effectors including SHP2, PI3K, Shc and Crk isoforms, but not PLC gamma. This stimulation did not depend on the ability of HGF to stimulate changes in cellular calcium or activate PKC. These results show GAB1 plays a central role in the ability of HGF but not different G protein coupled receptors to alter pancreatic acinar cell function. Additional studies done include the the identification of the somatostatin receptor subtypes meditating contractility in human colonic cells, and studies of the ability of Bombesin receptor ligands in combination with histone deactylase inhibitors to inhibit lung cancer proliferation.