Unbiased cell surface proteomics identifies SEMA4A as an effective immunotherapy target for myeloma.

The accessibility of cell surface proteins makes them tractable for targeting by cancer immunotherapy, but identifying suitable targets remains challenging. Here we describe plasma membrane profiling of primary human myeloma cells to identify an unprecedented number of cell surface proteins of a primary cancer. We used a novel approach to prioritize immunotherapy targets and identified a cell surface protein not previously implicated in myeloma, semaphorin-4A (SEMA4A). Using knock-down by short-hairpin RNA and CRISPR/nuclease-dead Cas9 (dCas9), we show that expression of SEMA4A is essential for normal myeloma cell growth in vitro, indicating that myeloma cells cannot downregulate the protein to avoid detection. We further show that SEMA4A would not be identified as a myeloma therapeutic target by standard CRISPR/Cas9 knockout screens because of exon skipping. Finally, we potently and selectively targeted SEMA4A with a novel antibody-drug conjugate in vitro and in vivo.

RITA (Reactivating p53 and Inducing Tumor Apoptosis) is efficient against TP53abnormal myeloma cells independently of the p53 pathway.

BACKGROUND: The aim of this study was to evaluate the efficacy of the p53-reactivating drugs RITA and nutlin3a in killing myeloma cells. METHODS: A large cohort of myeloma cell lines (n = 32) and primary cells (n = 21) was used for this study. This cohort contained cell lines with various TP53 statuses and primary cells with various incidences of deletion of chromosome 17. Apoptosis was evaluated using flow cytometry with Apo2.7 staining of the cell lines or via the loss of the myeloma-specific marker CD138 in primary cells. Apoptosis was further confirmed by the appearance of a subG1 peak and the activation of caspases 3 and 9. Activation of the p53 pathway was monitored using immunoblotting via the expression of the p53 target genes p21, Noxa, Bax and DR5. The involvement of p53 was further studied in 4 different p53-silenced cell lines. RESULTS: Both drugs induced the apoptosis of myeloma cells. The apoptosis that was induced by RITA was not related to the TP53 status of the cell lines or the del17p status of the primary samples (p = 0.52 and p = 0.80, respectively), and RITA did not commonly increase the expression level of p53 or p53 targets (Noxa, p21, Bax or DR5) in sensitive cells. Moreover, silencing of p53 in two TP53(mutated) cell lines failed to inhibit apoptosis that was induced by RITA, which confirmed that RITA-induced apoptosis in myeloma cells was p53 independent. In contrast, apoptosis induced by nutlin3a was directly linked to the TP53 status of the cell lines and primary samples (p < 0.001 and p = 0.034, respectively) and nutlin3a increased the level of p53 and p53 targets in a p53-dependent manner. Finally, we showed that a nutlin3a-induced DR5 increase (≥ 1.2-fold increase) was a specific and sensitive marker (p < 0.001) for a weak incidence of 17p deletion within the samples (≤ 19%). CONCLUSION: These data show that RITA, in contrast to nutlin3a, effectively induced apoptosis in a subset of MM cells independently of p53. The findings and could be of interest for patients with a 17p deletion, who are resistant to current therapies.

Bendamustine and melphalan kill myeloma cells similarly through reactive oxygen species production and activation of the p53 pathway and do not overcome resistance to each other.

Because the old alkylating drug bendamustine (BDM) is currently under evaluation in patients with multiple myeloma, we compared its efficacy to that of melphalan in 29 human myeloma cell lines (HMCLs). The concentrations of BDM and melphalan that killed 50% of cells (LD50) in HMCLs were linearly correlated (p < 0.001), and reactive oxygen (ROS) scavengers similarly inhibited cell death induced by both drugs. Sensitivity of HMCLs to both drugs was correlated to p53: the BDM and melphalan median LD50 values of TP53(wild-type) HMCLs were more than two-fold lower than those of TP53(abnormal) HMCLs (p < 0.001), and p53 silencing in TP53(wt) NCI-H929 cells inhibited BDM- and melphalan-induced cell death. Both drugs induced expression of p53 targets, p21, Puma and DR5, only in TP53(wt) HMCLs. In primary cells, both drugs induced an increase in DR5 expression in cells without del(17p). Finally, we demonstrated that the combined effect of BDM and melphalan was additive, and that BDM did not overcome melphalan resistance and vice versa.

Uncovering the role of p53 splice variants in human malignancy: a clinical perspective.

Thirty-five years of research on p53 gave rise to more than 68,000 articles and reviews, but did not allow the uncovering of all the mysteries that this major tumor suppressor holds. How p53 handles the different signals to decide the appropriate cell fate in response to a stress and its implication in tumorigenesis and cancer progression remains unclear. Nevertheless, the uncovering of p53 isoforms has opened new perspectives in the cancer research field. Indeed, the human TP53 gene encodes not only one but at least twelve p53 protein isoforms, which are produced in normal tissues through alternative initiation of translation, usage of alternative promoters, and alternative splicing. In recent years, it became obvious that the different p53 isoforms play an important role in regulating cell fate in response to different stresses in normal cells by differentially regulating gene expression. In cancer cells, abnormal expression of p53 isoforms contributes actively to cancer formation and progression, regardless of TP53 mutation status. They can also be associated with response to treatment, depending on the cell context. The determination of p53 isoform expression and p53 mutation status helps to define different subtypes within a particular cancer type, which would have different responses to treatment. Thus, the understanding of the regulation of p53 isoform expression and their biological activities in relation to the cellular context would constitute an important step toward the improvement of the diagnostic, prognostic, and predictive values of p53 in cancer treatment. This review aims to summarize the involvement of p53 isoforms in cancer and to highlight novel potential therapeutic targets.

A functional, new short isoform of death receptor 4 in Ewing's sarcoma cell lines may be involved in TRAIL sensitivity/resistance mechanisms.

Ewing's sarcoma (ES) is a high-grade neoplasm arising in bones of children and adolescents. Survival rate decreases from greater than 50% to only 20% after 5 years for patients not responding to treatment or presenting metastases at diagnosis. TRAIL, which has strong antitumoral activity, is a promising therapeutic candidate. To address TRAIL sensitivity, 7 human ES cell lines were used. Cell viability experiments [3'[1-(phenylaminocarbonyl)-3,4-tetrazolium]-bis(4-methoxy-6-nitro-)benzene sulfonic acid hydrate (XTT) assay] showed that 4 of the 7 ES cell lines were resistant to TRAIL. Western blotting and flow cytometry analyses revealed that DR5 was uniformly expressed by all ES cell lines, whereas DR4 levels were higher in sensitive cell lines. In TRAIL-sensitive TC-71 cells, knockdown of TNFRSF10A/DR4 by short hairpin RNA (shRNA) was associated with a loss of sensitivity to TRAIL, in spite of DR5 presence. Interestingly, we identified a new transcript variant that results from an alternative splicing and encodes a 310-amino acid protein which corresponds to the 468 aa of DR4 original isoform but truncated of aa 11 to 168 within the extracellular TRAIL-binding domain. According to modeling studies, the contact of this new DR4 isoform (bDR4) with TRAIL seemed largely preserved. The overexpression of bDR4 in a TRAIL-resistant cell line restored TRAIL sensitivity. TRAIL resensitization was also observed after c-FLIP knockdown by shRNA in two TRAIL-resistant cell lines, as shown by XTT assay and caspase-3 assay. The results presented in this study showed that DR4, both as the complete form or as its new short isoform, is involved in TRAIL sensitivity in ES.

Cell death via DR5, but not DR4, is regulated by p53 in myeloma cells.

Myeloma cells are sensitive to TRAIL through the two death receptors DR4 and DR5. Because p53 directly modulates expression of death receptors, we investigated here whether p53 can modulate myeloma sensitivity to TRAIL. We found that p53 affects the sensitivity of myeloma cells to the DR5 agonistic human antibody lexatumumab but not the DR4 antibody mapatumumab. TP53 wild-type myeloma cells overexpressed DR5 in correlation with sensitivity to lexatumumab. Both nongenotoxic (nutlin-3a) and genotoxic (melphalan) p53-inducing stresses increased DR5 expression only in TP53 wild-type cells and synergistically increased lexatumumab efficiency yet did not increase DR4 expression, nor sensitivity to mapatumumab. Silencing of p53 strongly decreased DR5 expression and induced resistance to nutlin-3a and lexatumumab but did not modulate DR4 expression or sensitivity to mapatumumab. Increase of lexatumumab efficiency induced by nutlin-3a was related to a p53-dependent increase of DR5 expression. In primary myeloma cells, nutlin-3a increased DR5 expression and lexatumumab efficiency but did not increase mapatumumab efficiency. Taken together, our findings indicate that p53 controls the sensitivity of myeloma through DR5 but not DR4 and suggest that a subset of patients with multiple myeloma may benefit from DR5 therapy.

Mcl-1(128-350) fragment induces apoptosis through direct interaction with Bax.

Mcl-1 full-length (Mcl-1(1-350)), a tightly regulated protein, plays an important role in protecting cells against apoptosis. Cleavage of Mcl-1 at Asp127 by caspase (Mcl-1(C1)) contributes to the regulation of Mcl-1 expression, but its pro-apoptotic function remains controversial. Here, we reported that Mcl-1(128-350) expression induced caspase-dependent apoptosis. We demonstrated that Mcl-1(128-350) but not Mcl-1(1-350) interacts with Bax. This interaction required an intact BH3 Mcl-1(128-350) domain and leads to Bax activation and translocation to mitochondria. The silencing of Bax, but not of Bak, prevented Mcl-1(128-350) induced apoptosis. In conclusion, Mcl-1(128-350) exerts a pro-apoptotic function governed by its capacity to interact with Bax.