Honokiol enhancement of paclitaxel induced apoptosis involves the inhibition of the EGFR-STAT3 signaling pathway.

<p>(<b>a</b>) Honokiol enhances paclitaxel induced PARP and caspase-3 cleavage in KB-8-5 cells. The cells were treated with 5 µg/ml of honokiol, 15 ng/ml of paclitaxel, or the combination of the two drugs for 24, 48, and 72 h. Full-length and cleaved PARP, cleaved caspase-3 and β-actin as a loading control were detected by immunoblotting using whole cell lysates. (<b>b</b>) Honokiol enhances paclitaxel induced release of cytochrome <i>c</i> in the cytoplasm in KB-8-5 cells. The cells were treated with 5 µg/ml of honokiol, 15 ng/ml of paclitaxel, or the combination for 48 h. Cytoplasmic and mitochondrial fractions were separated and immunoblotted with cytochrome c (Cyto C) antibody. COX4 (a mitochondrial protein) was used to show efficiency of cell fractionation. (<b>c</b>) Honokiol inhibits the EGFR-STAT3 signaling pathway and downregulates STAT3 target gene expression in KB-8-5 cells. The cells were treated with 5 µg/ml of honokiol, 15 ng/ml of paclitaxel, or the combination for 48 h. Whole cell lysates were immunoblotted for the indicated proteins. (<b>d</b>) Paclitaxel induces microtubule polymerization in KB-8-5 cells. Drug-induced stabilization of microtubules as evidenced by an increase in microtubule polymer mass resulting in bundling was observed upon treatment with both paclitaxel and the combination; however, honokiol alone was not effective in microtubule stabilization (magnification 400x). The cells were treated with 5 µg/ml of honokiol, 15 ng/ml of paclitaxel, or the combination for 24 h. All experiments were repeated at least three times, and representative data are presented.