Pyridinyl imidazole inhibitors, particularly SB203580, have been widely used to elucidate the roles of p38 mitogen-activated protein (MAP) kinase (p38/HOG/SAP-KII) in a wide array of biological systems. Studies by this group and others have shown that SB203580 can have antiproliferative activity on cytokine-activated lymphocytes. However, we recently reported that the antiproliferative effects of SB203580 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-2-stimulated T cells. Studies on the proximal regulator of this event, the phosphatidylinositol 3-kinase/protein kinase B (PKB)(Akt/Rac) kinase pathway, showed that SB203580 blocked the phosphorylation and activation of PKB by inhibiting the PKB kinase, phosphoinositidedependent protein kinase 1. The concentrations of SB203580 required to block PKB phosphorylation (IC 50 3-5 M) are only approximately 10-fold higher than those required to inhibit p38 MAP kinase (IC 50 0.3-0.5 M). These data define a new activity for this drug and would suggest that extreme caution should be taken when interpreting data where SB203580 has been used at concentrations above 1-2 M.
Interleukin-2 (IL-2)1 is a potent T cell growth factor that mediates its effects via a high affinity heterotrimeric receptor comprising ␣, , and ␥ c subunits. Several intracellular signaling pathways are known to be activated by IL-2, including the p42/44 mitogen-activated protein kinase (MAP kinase, also known as ERK2/1), the p38 and p54 MAP kinases (also called stress kinases, or HOG and JNK, respectively), the phosphatidyl inositol 3Ј (PI) 3-kinase pathway and the Jak/STAT (signal transducer and activator of transcription) pathways. Our earlier studies using the MEK (mitogen-activated protein kinase/ extracellular signal-regulated kinase kinase) inhibitor PD-098059 (1) and those of others (2, 3) have indicated that the p42/44 MAP kinase pathway is not required for IL-2-driven proliferation. In contrast, a pyridinyl imidazole inhibitor of p38 MAP kinase, SB203580, inhibited IL-2-driven T cell proliferation with an IC 50 of 3-5 M, suggesting a possible role for p38 MAP kinase in this process (4). Recently, we have further investigated the role of p38 MAP kinase in proliferation by mapping the subdomains of the IL-2 receptor  chain involved in the activation of the kinase. As previously shown for p42/44 MAP kinase, activation of p38 and p54 MAP kinases required the acidic rich A region of the IL-2 receptor  chain (5). However, the A region is not required for proliferation (2, 5), indicating that neither p38 MAP kinase nor p54 MAP kinase is essential for this function. Furthermore, CNI-1493 (6, 7), an inhibitor of p38 and p54 MAP kinase activation by IL-2 was unable to inhibit proliferation (5). Surprisingly, SB203580 was still able to inhibit proliferation in the absence of IL-2 stimulated p38 MAP kinase activation. It has already been reported that SB203580 does inhibit p54...