Received for publication June 7, 2006.
Revised August 23, 2006.
Accepted for publication September 5, 2006.

Coadministration of Sorafenib with Rottlerin Potently Inhibits Cell Proliferation and Migration in Human Malignant Glioma Cells

Abstract

Mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) are activated in the majority of gliomas and contribute to tumor cell growth and survival. Sorafenib (Bay43-9006, Nexavar^®) is a novel dual-action Raf kinase and vascular endothelial growth factor receptor inhibitor that blocks receptor phosphorylation and MAPK-mediated signaling, and inhibits growth in a number of tumor types. Because our initial studies of this agent in a series of glioma cell lines showed only partial growth inhibition at clinically achievable concentrations, we questioned whether inhibition of PKC signaling using the PKC- inhibitor rottlerin might potentiate therapeutic efficacy. Proliferation assays, apoptosis induction studies, and Western immunoblot analysis were conducted in cells treated with sorafenib and rottlerin as single agents or in combination. Sorafenib and rottlerin reduced proliferation in all cell lines when used as single agents, and the combination produced marked potentiation of cell growth inhibition. Flow cytometric measurements of cells stained with annexin V-propidium iodide and immunocytochemical assessment of cytochrome c and apoptosis-inducing factor (AIF) release demonstrated that the addition of rottlerin resulted in significantly higher levels of apoptosis than were achieved with sorafenib alone. In addition, the combination of sorafenib and rottlerin reduced or completely inhibited the phosphorylation of ERK and Akt and downregulated cell cycle regulatory proteins such as cyclin D1, cyclin D3, cdk4 and cdk 6 in a dose and time-dependent manner. Our results clearly indicate that inhibition of PKC- signaling enhances the antiproliferative effect of sorafenib in malignant human glioma cell lines, and supports the examination of combinations of signaling inhibitors in these tumors.

Key words: Apoptosis, Cell cycle, Glioma, Migration, PKC delta, Sorafenib

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