β₁ integrin-dependent binding of Jurkat cells to fibronectin is regulated by a serine-threonine phosphatase

We investigated the effects of signaling molecule inhibitors on the expression and function of b₁ integrins in Jurkat cells. Jurkat cells expressed α₄β₁ and α₅β_, with significant levels of constitutively activated β₁ integrins as assessed by labeling with mAb 15/7 that distinguishes between activation states. Adhesion to fibronectin (Fn) was mediated equally through α₄ and α₅ subunits, and was potentiated by the β₁ integrin activating mAb 8A2. Fn adhesion was decreased by okadaic acid through effects on both α₄β₁ and α₅β₁. Tyrphostin A23 also decreased adhesion but was less potent. Neither inhibitor had any effect on the surface expression of total or activated β₁ integrins. The effect of tyrphostin was completely reversed by 8A2; the effect of okadaic acid was only partially reversed. Using Calyculin A, we determined that Jurkat adhesion to Fn was regulated via protein phosphatase 1, independent of the levels of integrins or integrin activation epitopes. Activation of Jurkat cells with a CD3- stimulating mAb enhanced adhesion to Fn and was partially blocked by okadaic acid. These data demonstrate different regulatory pathways for constitutive versus activation-dependent adhesion via β₁ integrins, and implicate both tyrosine kinases and serine-threonine phosphatases in integrin function.