β 1 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 β 1 integrins in Jurkat cells. Jurkat cells expressed α 4 β 1 and α 5 β 1 , with significant levels of constitutively activated β 1 integrins as assessed by labeling with mAb 15/7 that distinguishes between activation states. Adhesion to fibronectin (Fn) was mediated equally through α 4 and α 5 subunits, and was potentiated by the β 1 integrin activating mAb 8A2. Fn adhesion was decreased by okadaic acid through effects on both α 4 β 1 and α 5 β 1 . Tyrphostin A23 also decreased adhesion but was less potent. Neither inhibitor had any effect on the surface expression of total or activated β 1 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 β 1 integrins, and implicate both tyrosine kinases and serine‐threonine phosphatases in integrin function. J. Leukoc. Biol. 64: 753–758; 1998.