Gastrin Induces IP3 Formation Through Phospholipase C Gamma 1 and Pp60c-src Kinase

We have previously reported that gastrin induces a rapid and transient tyrosine phosphorylation of phospholipase C gamma 1 (PLC gamma 1) in association with inositol 1,4,5-trisphosphate (IP3) formation in rat colonic epithelial cells (34). In this study, we demonstrate that gastrin regulates IP3 formation mainly through PLC gamma 1 isozyme. Immunoblotting analysis revealed the expression of PLC beta 3 and -gamma 1, but not PLC beta 1, -beta 2, or -beta 4 in the rat colonic epitheliums. To explore what PLC isozyme(s) modulates gastrin effect on IP3, immunoneutralizing antibody to PLC beta 1, -beta 3, or -gamma 1 was introduced into the colonic cells using a lipid carrier. The gastrin-stimulated increase in IP3 concentration was specifically prevented by anti-PLC gamma 1 but not by anti-PLC beta 1 or -beta 3 antibody. Immunoprecipitation assays have also revealed that gastrin promoted an increase in tyrosine phosphorylation and co-precipitation of a 60 kDa src kinase with PLC gamma 1. Administration of antibody specific to pp60c-src into the colonic cells prevented the gastrin-stimulated increases in IP3. Tyrosine phosphorylation of PLC gamma 1 may be a major mechanism through which gastrin regulates IP3 level in the colonic cells. Pretreatment of cells with the tyrosine kinase inhibitor genistein abrogated gastrin's effect on IP3, while extended pretreatment with pertussis toxin, a G-protein inhibitor, did not affect the ability of gastrin to stimulate IP3 formation. Colonic cells expressed the G alpha i subunits1-3; however, immunoblotting analysis did not reveal any difference in G alpha i proteins' expression between control and gastrin treated cells. The results provide direct evidence that gastrin regulates IP3 level by a signaling mechanism that involves PLC gamma 1 and pp60c-src kinase.