Novel Role for JNK as a Stress-activated Bcl2 Kinase

Abstract

Interleukin (IL)-3-induced Bcl2 phosphorylation at Ser(70) may be required for its full and potent antiapoptotic activity. However, in the absence of IL-3, increased expression of Bcl2 can also prolong cell survival. To determine how Bcl2 may be functionally phosphorylated following IL-3 withdrawal, a stress-activated Bcl2 kinase (SAK) was sought. Results indicate that anisomycin, a potent activator of the stress kinase JNK/SAPK, can induce Bcl2 phosphorylation at Ser(70) and that JNK1 can be latently activated following IL-3 withdrawal to mediate Bcl2 phosphorylation. JNK1 directly phosphorylates Bcl2 in vitro, co-localizes with Bcl2, and collaborates with Bcl-2 to mediate prolonged cell survival in the absence of IL-3 or following various stress applications. Dominant-negative (DN)-JNK1 can block both anisomycin and latent IL-3 withdrawal-induced Bcl2 phosphorylation (>90%) and potently enhances cell death. Furthermore, low dose okadaic acid (OA), a potent protein phosphatase 1 and 2A inhibitor, can activate the mitogen-activated protein kinases JNK1 and ERK1/2, but not p38 kinase, to induce Bcl2 phosphorylation and prolong cell survival in factor-deprived cells. Since PD98059, a specific MEK inhibitor, can only partially inhibit OA-induced Bcl2 phosphorylation but completely blocks OA-induced Bcl2 phosphorylation in cells expressing DN-JNK1, this supports the conclusion that OA may stimulate Bcl2 phosphorylation via a mechanism involving both JNK1 and ERK1/2. Collectively, these findings indicate a novel role for JNK1 as a SAK and may explain, at least in part, how functional phosphorylation of Bc12 can occur in the absence of growth factor.