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Plasticity of neoplasia, whereby cancer cells attain stem-cell-like properties, is required for disease progression and represents a major therapeutic challenge. We report that in breast cancer cells NANOG , SNAIL and NODAL transcripts manifest multiple isoforms characterized by different 5’ Untranslated Regions (5’UTRs), whereby translation of a subset of these isoforms is stimulated under hypoxia. The accumulation of the corresponding proteins induces plasticity and “fate-switching” toward stem cell-like phenotypes. Mechanistically, we observe that mTOR inhibitors and chemotherapeutics induce translational activation of a subset of NANOG , SNAIL and NODAL mRNA isoforms akin to hypoxia, engendering stem-cell-like phenotypes. These effects are overcome with drugs that antagonize translational reprogramming caused by eIF2α phosphorylation (e.g. ISRIB), suggesting that the Integrated Stress Response drives breast cancer plasticity. Collectively, our findings reveal a mechanism of induction of plasticity of breast cancer cells and provide a molecular basis for therapeutic strategies aimed at overcoming drug resistance and abrogating metastasis. Protein synthesis suppression protects breast cancer cells from clinically relevant stresses like hypoxia. Here, the authors show that unique mRNA isoforms that govern stem cell-like phenotypes escape translational repression to drive tumor progression and chemoresistance.

Overlapping or convergent stress-activated molecular pathways exist to coordinate cell fate in response to stimuli such as hypoxia, oxidative stress, DNA damage, and unfolded proteins. Cells can remodel the splicing and translation machineries to mount a specialized gene expression response to certain stresses. Here, we show that hypoxic human cells in 2D and 3D culture models increase the relative abundance by 1.7- to 2.6-fold and 4.7- to 11.5-fold, respectively, of a longer mRNA variant of ribosomal protein S24 (RPS24L) compared to a shorter mRNA variant (RPS24S) by favoring the inclusion of a 22 bp cassette exon. Mechanistically, RPS24L and RPS24S are induced and repressed, respectively, by distinct parallel pathways in hypoxia: RPS24L is induced in an autophagy-dependent manner, while RPS24S is reduced by mTORC1 repression and in a HIF-dependent manner. RPS24L is a more stable mRNA in hypoxia and produces a more stable protein isoform compared to RPS24S. Cells overexpressing RPS24L display improved survival and growth in hypoxia relative to control cells and cells overexpressing RPS24S, which display impaired survival. Previous work from our group showed a correlation between RPS24L levels and tumor hypoxia in prostate cancer. These data highlight RPS24L as a stress-induced alternative splicing event that favors hypoxic cell survival, which could be exploited by cancer cells in the tumor microenvironment.

The human neurotensin 1 receptor (hNTS1) is a G protein-coupled receptor involved in many physiological functions, including analgesia, hypothermia, and hypotension. To gain a better understanding of which signaling pathways or combination of pathways are linked to NTS1 activation and function, we investigated the ability of activated hNTS1, which was stably expressed by CHO-K1 cells, to directly engage G proteins, activate second messenger cascades and recruit eta-arrestins. Using BRET-based biosensors, we found that neurotensin (NT), NT(8-13) and neuromedin N (NN) activated the Gq-, Gi1-, GoA-, and G13-protein signaling pathways as well as the recruitment of eta-arrestins 1 and 2. Using pharmacological inhibitors, we further demonstrated that all three ligands stimulated the production of inositol phosphate and modulation of cAMP accumulation along with ERK1/2 activation. Interestingly, despite the functional coupling to Gi1 and GoA, NT was found to produce higher levels of cAMP in the presence of pertussis toxin, supporting that hNTS1 activation leads to cAMP accumulation in a Gs-dependent manner. Additionally, we demonstrated that the full activation of ERK1/2 required signaling through both a PTX-sensitive Gi/o-c-Src signaling pathway and PLCb-DAG-PKC-Raf-1- dependent pathway downstream of Gq. Finally, the whole-cell integrated signatures monitored by the cell-based surface plasmon resonance and changes in the electrical impedance of a confluent cell monolayer led to identical phenotypic responses between the three ligands. The characterization of the hNTS1-mediated cellular signaling network will be helpful to accelerate the validation of potential NTS1 biased ligands with an improved therapeutic/adverse effect profile.

Plasticity of neoplasia, whereby cancer cells attain stem-cell-like properties, is required for disease progression and represents a major therapeutic challenge. We report that in breast cancer cells NANOG, SNAIL and NODAL transcripts manifest multiple isoforms characterized by different 5' Untranslated Regions (5'UTRs), whereby translation of a subset of these isoforms is stimulated under hypoxia. This leads to accumulation of corresponding proteins which induce plasticity and fate-switching toward stem-cell like phenotypes. Surprisingly, we observed that mTOR inhibitors and chemotherapeutics induce translational activation of a subset of NANOG, SNAIL and NODAL mRNA isoforms akin to hypoxia, engendering stem cell-like phenotypes. Strikingly, these effects can be overcome with drugs that antagonize translational reprogramming caused by eIF2alpha phosphorylation (e.g. ISRIB). Collectively, our findings unravel a hitherto unappreciated mechanism of induction of plasticity of breast cancer cells, and provide a molecular basis for therapeutic strategies aimed at overcoming drug resistance and abrogating metastasis.