The pyridinyl imidazole inhibitor SB203580 blocks phosphoinositide-dependent protein kinase activity, protein kinase B phosphorylation, and retinoblastoma hyperphosphorylation in interleukin-2-stimulated T cells independently of p38 mitogen-activated protein kinase

Pyridinyl imidazole inhibitors, particularly SB203580, have been widely use d to elucidate the roles of p38 mitogen-activated protein (MAP) kinase (p38 /HOG/SAPKII) in a wide array of biological systems. Studies by this group a nd others have shown that 8B203580 can have antiproliferative activity on c ytokine-activated lymphocytes, However, we recently reported that the antip roliferative effects of 8B203580 were unrelated to p38 MAP kinase activity. This present study now shows that SB203580 can inhibit the key cell cycle event of retinoblastoma protein phosphorylation in interleukin-a-stimulated T cells. Studies on the proximal regulator of this event, the phosphatidyl inositol 3-kinase/protein kinase B (PKB)(Akt/Rac) kinase pathway, showed th at SB203580 blocked the phosphorylation and activation of PKB by inhibiting the PKB kinase, phosphoinositide-dependent protein kinase 1. The concentra tions of SB203580 required to block PKB phosphorylation (IC,, 3-5 mu M) are only approximately 10-fold higher than those required to inhibit p38 MAP k inase (IC50 0.3-0.5 mu M). These data define a new activity for this drug a nd would suggest that extreme caution should be taken when interpreting dat a where SB203580 has been used at concentrations above 1-2 mu M.