Lymphocyte proliferation is a critical step in the immune response to antigen. Previously we demonstrated that β-catenin is constitutively expressed in hematologic malignancies and that β-catenin regulates leukemia cell proliferation. Here we report that β-catenin is highly regulated in peripheral blood lymphocytes via posttranslational mechanisms, and that β-catenin signaling promotes lymphocyte proliferation. β-catenin protein was undetectable by western blot in resting PBL and readily detectable within 30 min after activation by TCR ligation, mitogen or PMA/ionomycin. Proteasome inhibitor alone induced β-catenin accumulation in resting PBL. Transfection of β-TrCP1, an adaptor protein that recruits serine-phosphorylated β-catenin to the Skp1-Cullin-F box ubiquitin ligase complex down-regulated β-catenin in activated cells and conversely, dominant-negative β-TrCP1 upregulated β-catenin in resting cells. β-catenin was serine/threonine phosphorylated in resting PBL and rapidly dephosphorylated after activation, consistent with regulation by an activation-induced phosphatase. In resting PBL, elevation of intracellular Ca2+ stabilized β-catenin protein. In activated PBL, β-catenin stabilization was inhibited either by blocking Ca2+ flux with EGTA and BAPTA/AM, or by incubation with cyclosporine A, an inhibitor of the calcium-activated phosphatase calcineurin. In activated PBL inhibition of β-catenin nuclear signaling with dominant-negative β-catenin, dominant-negative TCF or β-catenin antisense inhibited IL-2 expression and proliferation. Conversely, wild-type β-catenin increased IL-2 expression and stimulated proliferation. We conclude that

  1. β-catenin protein is continuously synthesized and efficiently degraded in resting lymphocytes,

  2. β-catenin is rapidly stabilized in response to activation-induced Ca2+ flux,

  3. β-catenin nuclear signaling plays a critical role in peripheral blood lymphocyte activation.

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