Taurolidine and oxidative stress: a rationale for local treatment of mesothelioma.

Malignant mesothelioma is an asbestos-related, aggressive tumour, resistant to most anticancer therapies. Akt is a key mediator of mesothelioma cell survival and chemoresistance. This study aimed to clarify the mechanism by which taurolidine (TN), a known synthetic compound with antimicrobial and antineoplastic properties, leads to mesothelioma cell death. Apoptosis was studied by annexin V binding, cell cycle analysis, caspase-8 activation, poly(ADP-ribose) polymerase (PARP) cleavage and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labelling (TUNEL). Oxidative stress was measured by nitrite production and DNA oxidative damage. Protein expression and phosphorylation were evaluated by immunoprecipitation and immunoblotting. TN induces cell death of mesothelioma cells, but not of non-neoplastic human mesothelial cells. After TN treatment of mesothelioma cells, Akt but not extracellular signal-regulated kinase (Erk) 1/2 activity is inhibited a in time- and dose-dependent manner. Protein phosphatase (PP)1alpha and PP2A are activated several hours after drug addition. Apoptosis induced by TN is driven by oxidative stress and cell exposure to sulfydryl donors, such as glutathione monoethylester and l-N-acetylcysteine, significantly reduced pro-apoptotic effects and Akt inhibition. Conversely, expression of constitutively activated Akt did not affect cytoxicity elicited by TN, which retained its ability to inhibit the kinase. TN induces mesothelioma cell death via oxidative stress, accompanied by inhibition of Akt signalling. This provides a promising molecular rationale for TN as local treatment of malignant mesothelioma.

Inhibition of autophagy sensitizes malignant pleural mesothelioma cells to dual PI3K/mTOR inhibitors.

Malignant pleural mesothelioma (MPM) originates in most of the cases from chronic inflammation of the mesothelium due to exposure to asbestos fibers. Given the limited effect of chemotherapy, a big effort is being made to find new treatment options. The PI3K/mTOR pathway was reported to be upregulated in MPM. We tested the cell growth inhibition properties of two dual PI3K/mTOR inhibitors NVP-BEZ235 and GDC-0980 on 19 MPM cell lines. We could identify resistant and sensitive lines; however, there was no correlation to the downregulation of PI3K/mTOR activity markers. As a result of mTOR inhibition, both drugs efficiently induced long-term autophagy but not cell death. Autophagy blockade by chloroquine in combination with the dual PI3K/mTOR inhibitors significantly induced caspase-independent cell death involving RIP1 in the sensitive cell line SPC212. Cell death in the resistant cell line Mero-82 was less pronounced, and it was not induced via RIP1-dependent mechanism, suggesting the involvement of RIP1 downstream effectors. Cell death induction was confirmed in 3D systems. Based on these results, we identify autophagy as one of the main mechanisms of cell death resistance against dual PI3K/mTOR inhibitors in MPM. As PI3K/mTOR inhibitors are under investigation in clinical trials, these results may help interpreting their outcome and suggest ways for intervention.

The Bcl-2 repertoire of mesothelioma spheroids underlies acquired apoptotic multicellular resistance.

Three-dimensional (3D) cultures are a valuable platform to study acquired multicellular apoptotic resistance of cancer. We used spheroids of cell lines and actual tumor to study resistance to the proteasome inhibitor bortezomib in mesothelioma, a highly chemoresistant tumor. Spheroids from mesothelioma cell lines acquired resistance to bortezomib by failing to upregulate Noxa, a pro-apoptotic sensitizer BH3-only protein that acts by displacing Bim, a pro-apoptotic Bax/Bak-activator protein. Surprisingly, despite their resistance, spheroids also upregulated Bim and thereby acquired sensitivity to ABT-737, an inhibitor of anti-apoptotic Bcl-2 molecules. Analysis using BH3 profiling confirmed that spheroids acquired a dependence on anti-apoptotic Bcl-2 proteins and were 'primed for death'. We then studied spheroids grown from actual mesothelioma. ABT-737 was active in spheroids grown from those tumors (5/7, ∼70%) with elevated levels of Bim. Using immunocytochemistry of tissue microarrays of 48 mesotheliomas, we found that most (33, 69%) expressed elevated Bim. In conclusion, mesothelioma cells in 3D alter the expression of Bcl-2 molecules, thereby acquiring both apoptotic resistance and sensitivity to Bcl-2 blockade. Mesothelioma tumors ex vivo also show sensitivity to Bcl-2 blockade that may depend on Bim, which is frequently elevated in mesothelioma. Therefore, mesothelioma, a highly resistant tumor, may have an intrinsic sensitivity to Bcl-2 blockade that can be exploited therapeutically.

Erionite and asbestos differently cause transformation of human mesothelial cells.

Malignant mesothelioma (MM) is an aggressive tumor associated with environmental or occupational exposure to asbestos fibers. Erionite is a fibrous zeolite, morphologically similar to asbestos and it is assumed to be even more carcinogenic. Onset and progression of MM has been suggested as the result of the cooperation between asbestos and other cofactors, such as SV40 virus infection. Nevertheless, several cases of MM were associated with environmental exposure to erionite in Turkey, where SV40 was never isolated in MM specimens. We show here that erionite is poorly cytotoxic, induces proliferating signals and high growth rate in human mesothelial cells (HMC). Long term exposure to erionite, but not to asbestos fibers, transforms HMC in vitro, regardless of the presence of SV40 sequences, leading to foci formation in cultured monolayers. Cells derived from foci display constitutive activation of Akt, NF-kappaB and Erk1/2, show prolonged survival and a deregulated cell cycle, involving cyclin D1 and E overexpression. Our results reveal that erionite is able per se to turn HMC into transformed highly proliferating cells and disclose the carcinogenic properties of erionite, prompting for a careful evaluation of environmental exposure to these fibers. The genetic predisposition to the effect of erionite is a separate subject for investigation.