Abstract Body: Plitidepsin (APL), an antitumor agent originally isolated from the marine tunicate Aplidium albicans, is being tested in multiple myeloma (MM) patients in a phase III pivotal trial in combination with dexamethasone and a phase I trial in combination with bortezomib and dexamethasone. eEF1A2 is one of two isoforms of the alpha subunit of the eEF1 complex. In mammals, eEF1A2 has a selective pattern of expression in those tissues that do not express the A1 isoform, namely brain and muscle.
Nonetheless, eEF1A2 is aberrantly expressed in many cancers, including solid tumors (1-3) and MM (4), and has been shown to hold pro-oncogenic activities (5). Here we analyze the interaction of plitidepsin with its target, eEF1A2. DARTS assays, either with whole cell extracts or with purified eEF1A2 protein, indicate that plitidepsin binds to eEF1A2 and protects it from digestion by subtilisin (EC 3.4.21.62). Moreover, a [14C]-APL binding-guided fractionation of K562 cell extracts through differential centrifugation and three chromatographic steps demonstrated that eEF1A2 is the only cellular protein that can be retrieved through specific binding to plitidepsin.
A saturation binding experiment with [14C]-APL and purified GTP-bound eEF1A2 (from rabbit muscle) allowed us to calculate a Kd of around 80 nM for the interaction, while a dissociation experiment revealed a residence time of around 9 minutes. Indeed, we have found that plitidepsin exclusively binds to the GTP-bound form of eEF1A2. HeLa-APL-R cells, ≥1000 fold more resistant to plitidepsin than parental HeLa wt cells (6), are now shown to have lower eEF1A2 mRNA and protein levels than parental cells.
Furthermore, when eEF1A2 levels were restored to normal in HeLa-APL-R cells through ectopic expression of an eEF1A2-GFP construct, they were rendered partially sensitive to plitidepsin. Interestingly, transfected cells recovered most of the signaling events typically induced by the drug in HeLa wt cells. NCI-H460 (lung) and HGC-27 (stomach) cancer cell lines were rendered resistant to plitidepsin following the same procedure described in (6) for HeLa-APL-R cells.
When we analyzed the levels of eEF1A2 in this two new cell lines we observed that both of them were lacking eEF1A2. Altogether, our results demonstrate that plitidepsin targets the pro-oncogenic eEF1A2 protein, a new druggable target for anticancer therapy.
El trabajo ahonda en las particularidades de la interacción directa de Aplidin® con la forma unida a GTP de eEF1A2.
Abstract Number: 3015
Presentation Title: Plitidepsin targets the GTP-bound form of eEF1A2 in cancer cells .
Presentation Time: Tuesday, Apr 19, 2016, 8:00 AM -12:00 PM
Author Block: Alejandro Losada, Maria J. Munoz-Alonso, Juan F. Martínez-Leal, Juan M. Dominguez, Carlos M. Galmarini. PharmaMar S.A., Madrid, Spain .