EGF stimulates gastrin promoter through activation of Sp1 kinase activity

Epidermal growth factor (EGF) receptor activation stimulates gastrin gene e xpression through a GC-rich element called gastrin EGF response element (gE RE). This element is bound by Sp1 family members and is a target of the ras -extracellular signal-regulated kinase (Erk) signal transduction cascade. T his raised the possibility that Sp1 may be phosphorylated by kinases of thi s signaling pathway. Erk is capable of phosphorylating other mitogen-induci ble transcription factors, e.g., Elk and Sap, suggesting that Erk may also mediate EGF-dependent phosphorylation of Sp1. This possibility was tested b y studying Sp1-dependent kinase activity in extracts prepared from EGF-acti vated AGS cells by use of solid-phase kinase assays and immunoprecipitation of metabolically labeled Sp1. The results revealed that Sp1 kinase activit y (like gastrin promoter activation) is inhibited by PD-98059 and, therefor e, is dependent on mitogen-activated protein kinase kinase 1 (Mek 1). Howev er, EGF-dependent activation of endogenous Erk did not account for most of the Sp1 kinase activity, since Erk and additional Sp1 kinase activity analy zed in a solid-phase kinase assay eluted from an ion-exchange column in dif ferent fractions. Phosphoamino acid analysis of in vivo radiolabeled Sp1 de monstrated that the kinase phosphorylates Sp1 on Ser and Thr in response to EGF. Therefore, most EGF-stimulated Sp1 kinase activity is Mek 1 dependent and distinct from Erk.