Activation of ERK by Ca 2ϩ store depletion in rat liver epithelial cells. Am. J. Physiol. 276 (Cell Physiol. 45): C221-C230, 1999.-In rat liver epithelial (WB) cells, Ca 2ϩ pool depletion induced by two independent methods resulted in activation of extracellular signal-regulated protein kinase (ERK). In the first method, Ca 2ϩ pool depletion by thapsigargin increased the activity of ERK, even when rise in cytosolic Ca 2ϩ was blocked with the Ca 2ϩ chelator BAPTA-AM. For the second method,

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... on of extracellular EGTA at a concentration shown to deplete intracellular Ca 2ϩ pools also increased ERK activity. In each instance, ERK activation, as measured by an immunocomplex kinase assay, was greatly reduced by the tyrosine kinase inhibitor genistein, suggesting that Ca 2ϩ store depletion increased ERK activity through a tyrosine kinase pathway. The intracellular Ca 2ϩ -releasing agent thapsigargin increased Fyn activity, which was unaffected by BAPTA-AM pretreatment, suggesting that Fyn activity was unaffected by increased cytosolic free Ca 2ϩ . Furthermore, depletion of intracellular Ca 2ϩ with EGTA caused inactivation of protein phosphatase 2A and protein tyrosine phosphatases. ANG II-induced activations of Fyn, Raf-1, and ERK were augmented in cells pretreated with BAPTA-AM, but ANG II-induced expression of the dualspecificity phosphatase mitogen-activated protein kinase phosphatase-1 was blocked by BAPTA-AM pretreatment. Together these results indicate that ERK activity is regulated by the balance of phosphorylation vs. dephosphorylation reactions in intact cells and that the amount of Ca 2ϩ stored in intracellular pools plays an important role in this regulation.