Ca2+ and protein kinase C-dependent mechanisms involved in gastrin-induced Shc/Grb2 complex formation and P44-mitogen-activated protein kinase activation

The proliferative effects of gastrin on normal and neoplastic gastro-intestinal tissues have been shown to be mediated by the gastrin/CCK_B (G/CCK_B) G-protein-coupled receptors. We have recently reported that gastrin stimulates the tyrosine phosphorylation of Shc proteins and their subsequent association with the Grb2/Sos complex, leading to mitogen-activated protein kinase (MAPK) activation, a pathway known to play an important role in cell proliferation. We undertook the present study to characterize the signalling pathways used by this receptor to mediate the activation of the Shc/Grb2 complex. Since G/CCK_B receptor occupancy leads to the activation of the phospholipase C (PLC)/protein kinase C (PKC) pathway, we examined whether PKC stimulation and Ca²⁺ mobilization contribute to the phosphorylation of Shc proteins and their association with Grb2 in response to gastrin. Our results indicate that Shc proteins are tyrosine phosphorylated and associate with Grb2 in response to phorbol esters, suggesting that activation of PKC is a potential signalling pathway leading to activation of the Shc/Grb2 complex. Inhibition of PKC by GF109203X completely blocked the effect of PMA on Shc tyrosine phosphorylation and its subsequent association with Grb2, but had a partial inhibitory effect on the response to gastrin. Depletion of the intracellular Ca²⁺ pools by treatment with thapsigargin blocked the increase in intracellular free calcium concentration induced by gastrin and diminished the ability of the peptide to stimulate Shc phosphorylation and recruitment of Grb2. In addition, removal of extracellular Ca²⁺ partially inhibited the effect of gastrin on Shc phosphorylation as well as its association with Grb2, indicating that the effects of gastrin are also mediated by Ca²⁺-dependent mechanisms. Furthermore, we show that blockage of the two major early signals generated by activation of PLC, which induced the activation of the Shc/Grb2 complex, also blocked gastrin-induced MAPK activation.