Abstract #5236: Lenalidomide affects F-Actin cytoskeleton in mantle cell lymphoma cells in vitro

AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO Lenalidomide is an anticancer drug approved in the US and Europe in combination with dexamethasone for the treatment of multiple myeloma in patients who have received \#8805;1 prior therapy. Lenalidomide has shown promising clinical activity in various B-cell lymphomas. Actin filaments (F-actin) play important roles in biological functions, e.g. cell cycle progression, cell signaling, cell-to-cell contact, intracellular transport and cell motility. F-actin are dynamic structures that undergo rapid cycles of polymerization and depolymerization controlled by associated proteins regulated by intra- and extracellular cues. Key upstream regulators of the F-actin cytoskeleton include the Rho family guanosine-5'-triphosphatases (GTPases). Downstream of Rho GTPases are the ezrin-radixin-moesin (ERM) proteins, essential for Rho- and Rac-induced cytoskeletal effects. ERMs act as cross-linkers between integral plasma membrane proteins and F-actin, and control polarity and cell shape. To understand the F-actin cytoskeleton effects of lenalidomide in lymphoma we treated two mantle cell lymphoma cell lines, Jeko-1 and Mino, with 1 \#956;M lenalidomide. A Cellomics high-content screening platform was used to quantify changes in cytoskeletal structure and cellular morphology using fluorescence probes. Lenalidomide induced actin polymerization and polarization within minutes, independently of other stimuli. Lenalidomide increased the amounts of F-actin and caused cytoskeleton rearrangement primarily manifested as re-localization of F-actin to a polarized region of the plasma membrane 15 minutes post-treatment. Pre-treatment with GTPase inhibitors prevented this effect on F-actin, indicating that Rho and Rac GTPases activities are required for lenalidomide effects on F-actin. The localization of the ERM protein Moesin was also monitored. Lenalidomide induced the co-localization of Moesin protein to the sites of F-actin accumulation which also depended on Rho GTPase. We also studied the effect of lenalidomide on the cytoskeleton dynamics of immune cells isolated from PBMCs: Lenalidomide had similar effects increasing F-actin polymerization and inducing polarization within minutes. Recently, lenalidomide has been shown to improve immune function against tumor cells by repairing F-actin polymerization and increasing immune synapse in clinical samples from CLL patients (Ramsay et al., 2008). Similarly, lenalidomide enhanced the number of \#947;\#948; T-MCL immune synapses and the lytic activity of \#947;\#948; T cells against the tumor cells. We propose that the regulation of cytoskeleton dynamics constitutes a key mechanism of lenalidomide action on tumor and immune cells and the participation of Rho GTPases in this process may be considered one of the earliest effects of lenalidomide in these cells. The implications of these findings for the efficacy of lenalidomide in patients are currently under investigation in our laboratory. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5236.