MEK Inhibition Enhances Paclitaxel-induced Tumor Apoptosis

Abstract

The anti-cancer drug paclitaxel (Taxol) alters microtubule assembly and activates pro-apoptotic signaling pathways. Previously, we and others found that paclitaxel activates endogenous JNK in tumor cells, and the activation of JNK contributes to tumor cell apoptosis. Here we find that paclitaxel activates the prosurvival MEK/ERK pathway, which conversely may compromise the efficacy of paclitaxel. Hence, a combination treatment of paclitaxel and MEK inhibitors was pursued to determine whether this treatment could lead to enhanced apoptosis. The inhibition of MEK/ERK with a pharmacologic inhibitor, U0126, together with paclitaxel resulted in a dramatic enhancement of apoptosis that is four times more than the additive value of the two drugs alone. Enhanced apoptosis was verified by the terminal transferase-mediated dUTP nick end labeling assay, by an enzyme-linked immunosorbent assay for histone-associated DNA fragments, and by flow cytometric analysis for DNA content. Specificity of the pharmacologic inhibitor was confirmed by the use of (a) a second MEK/ERK inhibitor and (b) a transdominantnegative MEK. Enhanced apoptosis was verified in breast, ovarian, and lung tumor cell lines, suggesting this effect is not cell type-specific. This is the first report of enhanced apoptosis detected in the presence of paclitaxel and MEK inhibition and suggests a new anticancer strategy.Paclitaxel is a promising frontline chemotherapy in the treatment of patients with ovarian, breast, and nonsmall cell lung carcinomas (1, 2). Paclitaxel is isolated from the bark of the pacific yew (Taxus brevifolia) and functions by binding and stabilizing microtubules (3). Binding of paclitaxel to microtubules blocks normal cell cycle progression during the merger of mitotic metaphase and anaphase. This prevents chromosome segregation, leading to tumor cell death.Combination therapy of paclitaxel and Herceptin, an antiHer2-neu antibody, has produced impressive responses among breast cancer patients (4), although this combination is obviously limited to Her2-neuϩ tumors. Combination therapy with other drugs, preferably via a rational molecular basis that is widely applicable to many tumor types, is essential for improved cancer treatment. A combination of paclitaxel with reagents that activate additional apoptotic signals, or inhibit survival signals, may provide a rational molecular basis for novel chemotherapeutic strategies.A rational molecular target is the ERK 1 mitogen-activated protein (MAP) kinase pathway that may serve as an opposing force to Jun N-terminal kinase (JNK/SAPK). Previous reports have shown that JNK/SAPK leads to cell death, while MEK activation contributes to cell differentiation, proliferation, and survival (5, 6). Activated Raf-1, a serine-threonine kinase, initiates the signaling cascade through MEK, which in turn phosphorylates a second serine-threonine kinase ERK. ERK phosphorylates additional kinases and specific transcription factors, such as Elk-1 and c-Fos, which are important in cell proliferation. However, the ...

MKK signaling and vascularization

“…Also, it is apparent from in vitro studies that there is ample cross-talk between MKK signaling pathways (see e.g. Xia et al, 1995;MacKeigan et al, 2000). These results indicate that in vitro cellular responses to stimuli may result from the coordinated activities of multiple MKK pathways.…”

Paclitaxel (Taxol) upregulates expression of functional interleukin-6 in human ovarian cancer cells through multiple signaling pathways

“…The paclitaxel-activated JNK signaling pathway has been correlated with tumor cell death in many types of ovarian cancer (Lee et al, 1996;Wang et al, 1999Wang et al, , 1998MacKeigan et al, 2000), breast cancer (Wang et al, 1998;MacKeigan et al, 2000) and lung cancer (MacKeigan et al, 2000), suggesting that it may be a common mechanism of chemotherapy. Using DNA microarray (from 4 to 1625 pg/1000 cells in 24 h, as specified in parentheses) IL-6 into culture media.…”

“…In addition to its stabilizing effects on the microtubules and the subsequent cell cycle arrest in the G2-M phase, paclitaxel also induces the vast activation of signal-transduction pathways that may be associated with proapoptotic signaling (Blagosklonny and Fojo, 1999;Taxman et al, 2003). Among the proapoptotic signaling networks activated by paclitaxel (Meikrantz and Schlegel, 1996;Lee et al, 1997), the c-Jun Nterminal kinase (JNK) signaling pathway played an important role (Lee et al, 1998;Wang et al, 1999Wang et al, , 2000MacKeigan et al, 2000;Vivat-Hannah et al, 2001;Mingo-Sion et al, 2004).…”

Inhibitors of the PI3‐kinase/Akt pathway induce mitotic catastrophe in non‐small cell lung cancer cells

“…Interestingly, MEK inhibition also sensitizes H157 cells to death induced by the microtubule stabilizing drug paclitaxel that blocks cell cycle progression in mitosis. 25 The reason why PD 98059 could not sensitize U1810 cells is not clear but might be explained by cellular differences in the level of checkpoint aberration. The two cell lines also seem to have differences in ERK regulation since PKC 412 increases the phosphorylation of ERK in H157 cells and decreases it in U1810 cells.…”