GASTRIN AND GLYCINE-EXTENDED PROGASTRIN PROCESSING INTERMEDIATES INDUCE DIFFERENT PROGRAMS OF EARLY GENE ACTIVATION

We recently reported that gastrin and glycine-extended progastrin proc essing intermediates (G-Gly) exert growth promoting effects on AR4-2J cells (derived from rat pancreas) via interaction with distinct recept ors, In this study we sought to investigate the mechanisms by which ga strin and G-Gly stimulate cell proliferation, While gastrin increased [Ca2+](i) in AR4-2J cells, G-Gly had no effect. Similarly, G-Gly had n o effect either on basal and 10(-7) M vasoactive intestinal polypeptid e-stimulated cAMP generation, although gastrin is known to inhibit cAM P generation, Gastrin dose dependently stimulated AR4-2J cell mRNA con tent of both c-fos and c-jun, two genes known to function in regulatin g cell proliferation, but G-Gly had no effect, Gastrin also induced th e expression of luciferase in AR4-2J cells transfected with a construc t consisting of a luciferase reporter gene coupled to the serum respon se element of the c-fos gene promoter, In similar fashion, gastrin sti mulated the activity of mitogen-activated protein kinase, an enzyme kn own to mediate the induction of the c-fos serum response element in re sponse to growth factor stimulation. Although G-Gly had none of these effects of gastrin in AR4-2J cells, it stimulated activity of c-Jun am ino-terminal kinase, an enzyme known to phosphorylate and transcriptio nally activate c-Jun. These data support the notion that gastrin stimu lates cell proliferation by inducing c-fos and c-jun gene expression, while G-Gly acts by post-translationally regulating early gene transcr iptional activation, Our studies represent a novel model in which both the precursor and the product of a key processing reaction, peptide a lpha-amidation, act cooperatively to stimulate cell proliferation via distinct receptors linked to different signal transduction pathways.