e18522 Background: Malignant Pleural Mesothelioma (MPM) is an aggressive cancer that originates from the mesothelium of the pleural cavities. Due to its resistance to all chemotherapies available, news therapies against MPM are urgently needed. Bortezomib is a potent Proteasome Inhibitors (PI) that induces apoptosis in different cancers and it was recently approved for treatment of Multiple Myeloma (MM). Since in MM an unbalance between a reduced proteasomal activity (capacity) and an increased proteins degradation (load) was shown to determine apoptotic sensitivity to PI, we challenged this paradigm also in MPM. Methods: Four different MPM (MM98, REN, MMB, MSTO) and a normal (TERT) pleural lines were treated with increasing doses (from 1 to 1000 nM) of Bortezomib and after 24h apoptosis was assessed by flow cytometric analysis of AnnexinV and propidium iodide positive cells. Levels of polyubiquitinated proteins were evaluated by immunofluorescence microscopy and western blotting with the specific monoclonal Ab Fk2 (BIOMOL International). The three main proteasome activities were measured in continuoin cellular extracts at 37°C with a Carry Eclipse spectrofluorometer (Varian, Palo Alto) by means of specific fluorogenic substrates (Suc-LLVY-amc, Bz-VGR-amc, Suc-YVAD-amc). Results: The sensitivity towards Bortezomib greatly differs between the 5 pleural lines analyzed. Specifically the EC50value was higher for the TERT line (130 nM) while for MPM lines ranged between 17 nM (MM98) and 71 nM (MSTO). Importantly, for all the 5 lines analyzed enhanced sensitivity to Bortezomib perfectly correlates with increased levels of ubiquitinated proteins and reduced proteasome specific activity. Conclusions: Our results prove that MPM lines are significantly more sensitive towards the pro-apoptotic effect of Bortezomib than normal pleural cells. Moreover, as in MM, also in MPM PI-sensitivity stems from an unbalance of the load/capacity ratio. Our data, therefore, establish a rationale for the use of Bortezomib (and new second generation PI) in MPM and indicate that combinatorial therapies with other drugs active on the proteostasis network (like HSP inhibitors) might be highly effective against this cancer.
TRAIL and proteasome inhibitors combination induces a robust apoptosis in human malignant pleural mesothelioma cells through Mcl-1 and Akt protein cleavages