α-thrombin induces rapid and sustained Akt phosphorylation by β-arrestin1-dependent and -independent mechanisms, and only the sustained Akt phosphorylation is essential for G₁ phase progression

In Chinese hamster embryonic fibroblasts (IIC9 cells) α-thrombin activates the MAPK(ERK) and phosphatidyl-inositol 3-OH-kinase (PI 3-kinase)/Akt pathways, and both are essential for progression through the G₁, phase of the cell cycle. We investigated in IIC9 cells, the role of β-arrestin1 in α-thrombin signaling to these pathways. α-Thrombin stimulates rapid and sustained PI 3-kinase and Akt activities. Expression of a dominant negative 13-arrestin1 (β-arrestin1(V53D)) inhibits rapid but not sustained PI 3-kinase and Akt activities. Surprisingly, expression of β-arrestin1(V53D) does not block activation of the MAPK(ERK) pathway. PI 3-kinase and Akt activities are also inhibited by expression of a β-arrestin1 mutant, which impairs binding to c-Src (β-arrestin1(P91G-P121E)), indicating the involvement of c-Src in the rapid stimulation of the PI 3-kinase/Akt pathway. Consistent with these results, PP1, a selective inhibitor of c-Src family kinases, prevents α-thrombin-stimulated Akt phosphorylation. Expression of β-arrestin1(V53D) does not prevent G₁ progression, as its expression has no effect on [³H]thymidine incorporation into DNA. In agreement with the ineffectiveness of β-arrestin1(V53D) to block G₁, progression, cyclin D1 protein amounts and CDK4-cyclin D1 activity is unaffected by expression of β-arrestin1(V53D). Thus in IIC9 cells, α-thrombin activates rapid β-arrestin1-dependent and sustained β-arrestin1-independent Akt activity, suggesting that two mechanisms are involved. Furthermore, although blocking the β-arrestin1-independent PI 3-kinase/Akt pathway prevents G₁, progression, inhibition of the β-arrestin1-dependent pathway does not, indicating different roles for the rapid and sustained activities.