EXTRACELLULAR CA2+ STIMULATES THE ACTIVATION OF MITOGEN-ACTIVATED PROTEIN-KINASE AND CELL-GROWTH IN HUMAN FIBROBLASTS

In serum-free medium containing serum replacements but totally lacking in protein growth factors, diploid human fibroblasts remained quiescent if the extracellular Ca2+ concentration was only 0.1 mM. However, when the Ca2+ concentration in this medium was increased to 1 mM, the cells replicated as rapidly as they do in medium supplemented with protein growth factors. When quiescent cells in medium with only 0.1 mM Ca2+ were exposed to 1 or 10 mM Ca2+ or 100 ng/ml epidermal growth factor (EGF), the 42 kDa and 44 kDa forms of mitogen-activated protein kinase (MAPK) were rapidly activated, as demonstrated by a characteristic electrophoretic mobility shift of these proteins and by their enhanced ability to phosphorylate myelin basic protein (MBP). Analysis of fractions from Mono Q anion-exchange chromatography of lysates of cells exposed to 10 mM Ca2+ or 100 ng/ml EGF revealed a peak of MBP phosphorylation activity that was coeluted with p42 and p44 MAPK as shown by immunoblot analysis. Activation of MAPK by extracellular Ca2+ was dose-dependent and biphasic, with a peak of activation at 5-10 min after exposure, followed by a period of sustained activation of MAPK at a lower level. This pattern has been shown [Vouret-Craviari, Van Obberghen-Schilling, Scimeca, Van Obberghen and Pouyssegur (1993) Biochem J. 289, 209-214] to correlate with the re-entry of mammalian cells into the cell cycle.