Consistent with the excellent clinical results in testicular germ cell tumors (TGCT), most cell lines derived from this cancer show an exquisite sensitivity to Cisplatin. It is well accepted that the high susceptibility of TGCT cells to apoptosis plays a central role in this hypersensitive phenotype. The role of the tumor suppressor p53 in this response, however, remains controversial. Here we show that siRNA-mediated silencing of p53 is sufficient to completely abrogate hypersensitivity not only to Cisplatin but also to non-genotoxic inducers of p53 such as the Mdm2 antagonist Nutlin-3 and the proteasome inhibitor Bortezomib. The close relationship between p53 protein levels and induction of apoptosis is lost upon short-term differentiation, indicating that this predominant pro-apoptotic function of p53 is unique in pluripotent embryonal carcinoma (EC) cells. RNA interference experiments as well as microarray analysis demonstrated a central role of the pro-apoptotic p53 target gene NOXA in the p53-dependent apoptotic response of these cells. In conclusion, our data indicate that the hypersensitivity of TGCT cells is a result of their unique sensitivity to p53 activation. Furthermore, in the very specific cellular context of germ cell-derived pluripotent EC cells, p53 function appears to be limited to induction of apoptosis.

Mantle cell lymphoma (MCL) is an aggressive lymphoid neoplasm with transient response to conventional chemotherapy. We here investigated the role of the Bcl-2 homology domain 3-only protein NOXA for life–death decision in MCL. Surprisingly, NOXA (PMAIP1) mRNA and NOXA protein levels were extremely discrepant in MCL cells: NOXA mRNA was found to be highly expressed whereas NOXA protein levels were low. Chronic active B-cell receptor signaling and to a minor degree cyclin D1 overexpression contributed to high NOXA mRNA expression levels in MCL cells. The phoshatidyl-inositol-3 kinase/AKT/mammalian target of rapamycin pathway was identified as the major downstream signaling pathway involved in the maintenance of NOXA gene expression. Interestingly, MCL cells adapt to this constitutive pro-apoptotic signal by extensive ubiquitination and rapid proteasomal degradation of NOXA protein (T½∼15–30 min). In addition to the proteasome inhibitor Bortezomib, we identified the neddylation inhibitor MLN4924 and the fatty acid synthase inhibitor Orlistat as potent inducers of NOXA protein expression leading to apoptosis in MCL. All inhibitors targeted NOXA protein turnover. In contrast to Bortezomib, MLN4924 and Orlistat interfered with the ubiquitination process of NOXA protein thereby offering new strategies to kill Bortezomib-resistant MCL cells. Our data, therefore, highlight a critical role of NOXA in the balance between life and death in MCL. The discrepancy between NOXA transcript and protein levels is essential for sensitivity of MCL to ubiquitin-proteasome system inhibitors and could therefore provide a druggable Achilles' heel of MCL cells.

Testicular germ cell tumors (TGCT) are considered a paradigm of chemosensitive tumors. Embryonal carcinoma cells represent the pluripotent entity of TGCTs and are characterized by expression of Oct-4, a key regulator of pluripotency and a determinant of their inherent hypersensitivity to cisplatin. However, the mechanisms underlying this Oct-4-mediated sensitivity are poorly understood. We previously showed that p53 is a major player in cisplatin hypersensitivity and therefore investigated whether Oct-4 may directly affect p53 activity. Despite a significant decrease in sensitivity, depletion of Oct-4 neither did alter cisplatin-induced transactivation of p53 target genes nor its subcellular localization. These data indicate that, rather than directly modulating p53 activity, Oct-4 provides a cellular context that augments the proapoptotic activity of p53. As mitochondrial priming by the Bcl-2 family is a known determinant of chemosensitivity, we compared the constitutive levels of these proteins in Oct-4-positive and -depleted cells. We identified Noxa as the only Bcl-2 family protein to be highly correlated with Oct-4 status and cisplatin sensitivity. Compared with differentiated cells, constitutive Noxa levels were significantly higher in Oct-4-positive cell lines and cancer patient samples. Furthermore, RNA interference-mediated knockdown of Oct-4 resulted in reduced Noxa transcript, in an almost complete loss of constitutive Noxa protein and decreased cisplatin hypersensitivity to a similar extent as did Noxa depletion. In conclusion, our study indicates that Noxa is a central determinant of hypersensitivity to cisplatin. Oct-4-dependent high constitutive levels of this BH3-only protein prime embryonal carcinoma cells to undergo rapid and massive apoptosis in response to p53 activation. Cancer Res; 73(5);

Significant advances have been made in the development of small molecules blocking the p53/MDM2 interaction. The Mdm2 inhibitor Nutlin-3 is restricted to tumors carrying wtp53. In contrast, RITA, a compound that binds p53, has recently been shown also to restore transcriptional functions of mtp53. As more than 50% of solid tumors carry p53 mutations, RITA promises to be a more effective therapeutic strategy than Nutlin-3. We investigated effects of RITA on apoptosis, cell cycle and induction of 45 p53 target genes in a panel of 14 cell lines from different tumor entities with different p53 status as well as primary lymphocytes and fibroblasts. Nine cell strains expressed wtp53, four harbored mtp53, and three were characterized by the loss of p53 protein. A significant induction of cell death upon RITA was observed in 7 of 16 cell lines. The nonmalignant cells in our panel were substantially less sensitive. We found that in contrast to Nultin-3, RITA is capable to induce cell death not only in tumor cells harboring wtp53 and mtp53 but also in p53-null cells. Importantly, whereas p53 has a central role for RITA-mediated effects in wtp53 cells, neither p53 nor p63 or p73 were essential for the RITA response in mtp53 or p53-null cells in our panel demonstrating that besides the known p53-dependent action of RITA in wtp53 cells, RITA can induce cell death also independently of p53 in cells harboring defective p53. We identified an important role of both p38 and JNK/SAPK for sensitivity to RITA in these cells leading to a typical caspase- and BAX/BAK-dependent mitochondrial apoptosis. In conclusion, our data demonstrate that RITA can induce apoptosis through p38 and JNK/SAPK not only in tumor cells harboring wtp53 and mtp53 but also in p53-null cells, making RITA an interesting tumor-selective drug.

Mantle cell lymphoma (MCL) is an aggressive type of non-Hodgkin lymphoma with comparatively short response to chemotherapy and frequent relapses. New treatment strategies for this malignancy are urgently needed. The genetic hallmark of MCL is the t(11;14)(q13;q32) translocation. This alteration leads to deregulated expression of the oncogene Cyclin D1 and is considered the primary event in the pathogenesis of MCL. Additionally, deregulations of different oncogenic signaling and cell death pathways have been described in MCL. In this study, we investigated the role of the BH3-only protein NOXA for life/death decision in MCL. We found a stunning discrepancy between constitutive Noxa (PMAIP1) mRNA and NOXA protein levels in MCL cell lines and primary cells. Noxa mRNA was found to be highly expressed whereas NOXA protein levels were low. Remarkably, constitutive high gene expression of this pro-apoptotic Bcl2 family member was dependent on Cyclin D1 overexpression and chronic active B cell receptor (BCR) signaling, two of the major oncogenic alterations in MCL. We identified the PI3K/AKT/mTOR pathway to be crucial in the maintenance of Noxa transcript levels downstream of BCR. Cyclin D1 overexpression contributed to the high Noxa mRNA levels by exerting a positive feedback loop on PI3K/AKT/mTOR signaling. Intriguingly, the high constitutive Noxa transcript levels do not impair cell viability. MCL cells adapt to this constitutive pro-apoptotic signal by keeping NOXA protein low due to extensive ubiquitination and rapid proteasomal degradation of NOXA protein (T½ ∼ 15-30 min). As expected, treatment of the cells with the proteasome inhibitor Bortezomib accumulated NOXA protein and efficiently induced cell death. Additionally, we identified the neddylation inhibitor MNL4924 and the fatty acid synthase (FASN) inhibitor Orlistat as potent inducers of NOXA protein, and thereby apoptosis, in MCL. Cell death upon Bortezomib as well as MLN4924 and Orlistat treatment was dependent on NOXA since RNAi mediated silencing of the pro-apoptotic protein significantly reduced induction of apoptosis. We found that all three inhibitors targeted the rapid NOXA protein turnover by stabilizing the preexisting pool of NOXA. In contrast to Bortezomib, however, MLN4924 and Orlistat inhibited the ubiqutination process of NOXA protein and stabilized the pro-apoptotic protein by a proteasome independent manner. These findings could be of great clinical relevance as Bortezomib resistance is a frequently observed phenomenon. Indeed, both inhibitors were still able to induce NOXA and cell death in Bortezomib resistant clones through targeting ubiquitine-proteasome system-mediated NOXA turnover upstream of the proteasome. Interestingly, active PI3K/AKT/mTOR signaling was needed for effective accumulation of NOXA protein and induction of cell death by all these compounds indicating that the high constitutive Noxa mRNA levels are essential for sensitivity of MCL cells. Together, our data highlight that NOXA regulation may represent an important Achilles heel of MCL cells. Stabilization of NOXA protein by inhibition of the ubiquitin-proteasome system on different levels might be an effective strategy to kill MCL cells and offer novel treatment options. Disclosures: No relevant conflicts of interest to declare.

<p>PDF file - 82K, TaqMan assays</p>

<p>PDF file - 39K, siGenome SMARTpool (Dharmacon) siRNA sequences</p>

<p>PDF file - 56K, qPCR primer pairs</p>