Francesca Rapino

Oncogenic BRAF induces chronic ER stress condition resulting in increased basal autophagy and apoptotic resistance of cutaneous melanoma

The notorious unresponsiveness of metastatic cutaneous melanoma to current treatment strategies coupled with its increasing incidence constitutes a serious worldwide clinical problem. Moreover, despite recent advances in targeted therapies for patients with BRAFV600E mutant melanomas, acquired resistance remains a limiting factor and hence emphasises the acute need for comprehensive pre-clinical studies to increase the biological understanding of such tumours in order to develop novel effective and longlasting therapeutic strategies. Autophagy and ER stress both have a role in melanoma development/progression and chemoresistance although their real impact is still unclear. Here, we show that BRAFV600E induces a chronic ER stress status directly increasing basal cell autophagy. BRAFV600E-mediated p38 activation stimulates both the IRE1/ASK1/JNK and TRB3 pathways. Bcl-XL/Bcl-2 phosphorylation by active JNK releases Beclin1 whereas TRB3 inhibits the Akt/mTor axes, together resulting in an increase in basal autophagy. Furthermore, we demonstrate chemical chaperones relieve the BRAFV600E-mediated chronic ER stress status, consequently reducing basal autophagic activity and increasing the sensitivity of melanoma cells to apoptosis. Taken together, these results suggest enhanced basal autophagy, typically observed in BRAFV600E melanomas, is a consequence of a chronic ER stress status, which ultimately results in the chemoresistance of such tumours. Targeted therapies that attenuate ER stress may therefore represent a novel and more effective therapeutic strategy for BRAF mutant melanoma.

BAG3 induction is required to mitigate proteotoxicity via selective autophagy following inhibition of constitutive protein degradation pathways

Simultaneous inhibition of the two major constitutive protein quality control (PQC) pathways, that is, the ubiquitin-proteasome system (UPS) and the aggresome-autophagy system, has been suggested as a promising strategy to trigger cell death in cancer cells. However, we observed that one third of rhabdomyosarcoma (RMS) cells survives parallel inhibition of the UPS by Bortezomib and the aggresome-autophagy pathway by the cytoplasmic histone deacetylase 6 inhibitor ST80, and is able to regrow upon drug removal, thus pointing to the induction of compensatory pathways. Here, we identify Bcl-2-associated athanogene 3 (BAG3) as a critical mediator of inducible resistance in surviving cells after concomitant blockage of constitutive PQC pathways by mitigating ST80/Bortezomib-triggered proteotoxicity via selective autophagy. ST80/Bortezomib cotreatment upregulates BAG3 mRNA and protein levels in surviving cells in addition to triggering the accumulation of insoluble protein aggregates. Intriguingly, knockdown of BAG3 by RNA interference severely impairs clearance of protein aggregates, significantly increases cell death and reduces long-term survival and clonogenic growth during recovery after ST80/Bortezomib cotreatment. Similarly, inhibition of autophagy by inducible autophagy-related protein 7 knockdown prevents removal of protein aggregates and cell regrowth during recovery after ST80/Bortezomib cotreatment. Also, the inhibition of lysosomal degradation using the V-ATPase pump inhibitor Bafilomycin A1 enhances accumulation of protein aggregates, and completely abolishes regrowth after Bortezomib/ST80-induced proteotoxic stress. By identifying BAG3 as a key mediator of inducible resistance by mitigating proteotoxicity via selective autophagy after inhibition of constitutive PQC systems, our study provides new insights into the regulation of PQC pathways in cancer cells and identifies new targets for therapeutic intervention.

Bortezomib antagonizes microtubule-interfering drug-induced apoptosis by inhibiting G2/M transition and MCL-1 degradation

Inhibition of the proteasome is considered as a promising strategy to sensitize cancer cells to apoptosis. Recently, we demonstrated that the proteasome inhibitor Bortezomib primes neuroblastoma cells to TRAIL-induced apoptosis. In the present study, we investigated whether Bortezomib increases chemosensitivity of neuroblastoma cells. Unexpectedly, we discover an antagonistic interaction of Bortezomib and microtubule-interfering drugs. Bortezomib significantly attenuates the loss of cell viability and induction of apoptosis on treatment with Taxol and different vinca alkaloids but not with other chemotherapeutics, that is, Doxorubicin and Cisplatinum. Importantly, Bortezomib inhibits G2/M transition by inhibiting proteasomal degradation of cell cycle regulatory proteins such as p21, thereby preventing cells to enter mitosis, the cell cycle phase in which they are most vulnerable to antitubulin chemotherapeutics. Consequently, Bortezomib counteracts Taxol-induced mitotic arrest and polyploidy, as shown by reduced expression of PLK1 and phosphorylated histone H3. In addition, Bortezomib antagonizes Taxol-mediated degradation of MCL-1 during mitotic arrest by preventing cells to enter mitosis and by inhibiting the proteasome. Downregulation of MCL-1 is critically required for Taxol-induced apoptosis, as overexpression of a phosphomutant MCL-1 variant, which is resistant to degradation, significantly diminishes Taxol-triggered apoptosis. Vice versa, attenuation of Bortezomib-mediated accumulation of MCL-1 by knockdown of MCL-1 significantly enhances Taxol/Bortezomib-induced apoptosis. Thus, Bortezomib rescues Taxol-induced apoptosis by inhibiting G2/M transition and mitigating MCL-1 degradation. The identification of this antagonistic interaction of Bortezomib and microtubule-targeted drugs has important implications for the design of Bortezomib-based combination therapies.

Elp3 links tRNA modification to IRES-dependent translation of LEF1 to sustain metastasis in breast cancer

Delaunay et al. reveal the role of U34 tRNA-modifying enzymes in the regulation of specific mRNA translation to support cell invasion and metastasis.

Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine

Tumor initiation in the intestine can rapidly occur from Lgr5+ crypt columnar stem cells. Dclk1 is a marker of differentiated Tuft cells and, when coexpressed with Lgr5, also marks intestinal cancer stem cells. Here, we show that Elp3, the catalytic subunit of the Elongator complex, is required for Wnt-driven intestinal tumor initiation and radiation-induced regeneration by maintaining a subpool of Lgr5+/Dclk1+/Sox9+ cells. Elp3 deficiency dramatically delayed tumor appearance in Apc-mutated intestinal epithelia and greatly prolonged mice survival without affecting the normal epithelium. Specific ablation of Elp3 in Lgr5+ cells resulted in marked reduction of polyp formation upon Apc inactivation, in part due to a decreased number of Lgr5+/Dclk1+/Sox9+ cells. Mechanistically, Elp3 is induced by Wnt signaling and promotes Sox9 translation, which is needed to maintain the subpool of Lgr5+/Dclk1+ cancer stem cells. Consequently, Elp3 or Sox9 depletion led to similar defects in Dclk1+ cancer stem cells in ex vivo organoids. Finally, Elp3 deficiency strongly impaired radiation-induced intestinal regeneration, in part because of decreased Sox9 protein levels. Together, our data demonstrate the crucial role of Elp3 in maintaining a subpopulation of Lgr5-derived and Sox9-expressing cells needed to trigger Wnt-driven tumor initiation in the intestine.

NIK is required for NF-κB-mediated induction of BAG3 upon inhibition of constitutive protein degradation pathways.

Recently, we reported that induction of the co-chaperone Bcl-2-associated athanogene 3 (BAG3) is critical for recovery of rhabdomyosarcoma (RMS) cells after proteotoxic stress upon inhibition of the two constitutive protein degradation pathways, that is, the ubiquitin-proteasome system by Bortezomib and the aggresome-autophagy system by histone deacetylase 6 (HDAC6) inhibitor ST80. In the present study, we investigated the molecular mechanisms mediating BAG3 induction under these conditions. Here, we identify nuclear factor-kappa B (NF-κB)-inducing kinase (NIK) as a key mediator of ST80/Bortezomib-stimulated NF-κB activation and transcriptional upregulation of BAG3. ST80/Bortezomib cotreatment upregulates mRNA and protein expression of NIK, which is accompanied by an initial increase in histone H3 acetylation. Importantly, NIK silencing by siRNA abolishes NF-κB activation and BAG3 induction by ST80/Bortezomib. Furthermore, ST80/Bortezomib cotreatment stimulates NF-κB transcriptional activity and upregulates NF-κB target genes. Genetic inhibition of NF-κB by overexpression of dominant-negative IκBα superrepressor (IκBα-SR) or by knockdown of p65 blocks the ST80/Bortezomib-stimulated upregulation of BAG3 mRNA and protein expression. Interestingly, inhibition of lysosomal activity by Bafilomycin A1 inhibits ST80/Bortezomib-stimulated IκBα degradation, NF-κB activation and BAG3 upregulation, indicating that IκBα is degraded via the lysosome in the presence of Bortezomib. Thus, by demonstrating a critical role of NIK in mediating NF-κB activation and BAG3 induction upon ST80/Bortezomib cotreatment, our study provides novel insights into mechanisms of resistance to proteotoxic stress in RMS.

tRNA modification: is cancer having a wobble?

Translational control of protein synthesis supports tumor development and progression to metastasis. Wobble tRNA modifications are required during translation elongation and sustain proteome homeostasis. Recent work has highlighted the surprising upregulation of the wobble uridine 34 (U34) tRNA cascade in cancer, which underlies the specific requirement for this pathway in tumor development.

Codon-specific translation reprogramming promotes resistance to targeted therapy

Reprogramming of mRNA translation has a key role in cancer development and drug resistance1. However, the molecular mechanisms that are involved in this process remain poorly understood. Wobble tRNA modifications are required for specific codon decoding during translation2,3. Here we show, in humans, that the enzymes that catalyse modifications of wobble uridine 34 (U34) tRNA (U34 enzymes) are key players of the protein synthesis rewiring that is induced by the transformation driven by the BRAFV600E oncogene and by resistance to targeted therapy in melanoma. We show that BRAFV600E-expressing melanoma cells are dependent on U34 enzymes for survival, and that concurrent inhibition of MAPK signalling and ELP3 or CTU1 and/or CTU2 synergizes to kill melanoma cells. Activation of the PI3K signalling pathway, one of the most common mechanisms of acquired resistance to MAPK therapeutic agents, markedly increases the expression of U34 enzymes. Mechanistically, U34 enzymes promote glycolysis in melanoma cells through the direct, codon-dependent, regulation of the translation of HIF1A mRNA and the maintenance of high levels of HIF1α protein. Therefore, the acquired resistance to anti-BRAF therapy is associated with high levels of U34 enzymes and HIF1α. Together, these results demonstrate that U34 enzymes promote the survival and resistance to therapy of melanoma cells by regulating specific mRNA translation.Enzymes that catalyse modifications of wobble uridine 34 tRNA are essential for the survival of melanoma cells that rely on HIF1α-dependent metabolism through codon-dependent regulation of the translation of HIF1A mRNA.

Correction: Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine

Correction: Elp3 drives Wnt-dependent tumor initiation and regeneration in the intestine Aurélie Ladang, Francesca Rapino, Lukas C. Heukamp, Lars Tharun, Kateryna Shostak, Damien Hermand, Sylvain Delaunay, Iva Klevernic, Zheshen Jiang, Nicolas Jacques, Diane Jamart, Valérie Migeot, Alexandra Florin, Serkan Göktuna, Brigitte Malgrange, Owen J. Sansom, Laurent Nguyen, Reinhard Büttner, Pierre Close, and Alain Chariot Vol. 212, No. 12, November 16, 2015. Pages 2057–2075.

Wobble uridine tRNA modification: a new vulnerability of refractory melanoma

ABSTRACT The enzymes catalysing the modification of the wobble uridine (U34) of tRNAs (U34-enzymes) play an important role in tumor development. We have recently demonstrated that the U34-enzymes are crucial in the survival of glycolytic melanoma cultures through a codon-specific regulation of HIF1α mRNA translation. Moreover, depletion of U34-enzymes resensitizes resistant melanoma to targeted therapy. These results indicate that targeting U34-enzymes represents a new therapeutic opportunity for melanoma patients.