BAP1 promotes Stem Cell-like features and three-dimensional spheres formation in Malignant Pleural Mesothelioma

Abstract Background. BRCA1-associated protein 1 (BAP1) is an ubiquitin C-terminal hydrolase member of the deubiquitinating enzyme superfamily, responsible for the reversible process of ubiquitin signaling through the removal of ubiquitin from protein substrates which changes their fate. Members of families with a high incidence of mesothelioma, as well as other cancers, carry heterozygous BAP1 mutations, a condition referred to as "BAP1 cancer syndrome". Germline BAP1 mutations predispose to malignant pleural mesothelioma (MPM), the most frequent histological type among malignant mesotheliomas, characterized by extremely poor prognosis and resistance to current therapies. The potent tumor suppressor activity of BAP1 has been linked to its dual role in the nucleus and cytoplasm. In the nucleus, BAP1 modulates the transcription of several genes involved in cell-cycle and promotes DNA repair by facilitating homologous recombination. In the cytoplasm, BAP1 regulates Ca2+ signaling-mediated cell death. BAP1 mutations are an early event in several cancers, but the involvement of BAP1 in malignant progression is less clear. In MPM, despite BAP1 loss of expression predisposes to tumor onset, patients bearing BAP1 mutations have longer survival. Aim of the study. So, the open question is: why do MPM patients with mutations or reductions in BAP1 expression have a higher survival than MPM patients carrying BAP1 wt? To address this question we silenced BAP1 in mesothelioma cell lines and assessed how this affected various tumorigenic features. Results. We report that BAP1 plays positive roles in cell proliferation and migration in MPM cells. BAP1 depletion, by Lentiviral particles, makes cells unable to aggregate into 3D spheres and reduces KLF5 protein level, a transcription factor that in breast cancer regulates the expression of target genes which promote cell proliferation and metastasis. Moreover, it is possible that BAP1 can regulate mitochondrial Ca2+ levels by Inositol 1,4,5-trisphosphate receptor type 3 (IP3R3) deubiquitination BAP1-mediated in MPM cells. Conclusion. Our data show that BAP1 silencing in MPM cells reduces stem cell features in addition to reduced capacity to form 3D spheres and reduced Ca2+ flux after IP3R3 stimulation in vitro. Overall our data suggest that wild type BAP1 could favor tumor progression by sustaining cancer stem cell features which could help to explain its seeming paradoxical role in MPM pathogenesis.