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 ex- pressed a4b1 and a5b1, with significant levels of constitutively activated b1 integrins as assessed by labeling with mAb 15/7 that distinguishes between activation states. Adhesion to fibronectin (Fn) was mediated equally through a4 and a5 subunits, and was potentiated by the b1 integrin activating mAb 8A2. Fn adhesion was decreased by okadaic acid through effects on both a4b1 and a5b1. Tyrphostin A23 also decreased adhesion but was less potent. Neither inhibitor had any effect on the surface expression of total or activated b1 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 inte- grins or integrin activation epitopes. Activation of Jurkat cells with a CD3-stimulating mAb enhanced adhesion to Fn and was partially blocked by oka- daic acid. These data demonstrate different regula- tory pathways for constitutive versus activation- dependent adhesion via b1 integrins, and implicate both tyrosine kinases and serine-threonine phospha- tases in integrin function. J. Leukoc. Biol. 64: 753-758; 1998.