ROS Induction Targets Persister Cancer Cells with Low Metabolic Activity in NRAS-Mutated Melanoma.

Clinical management of melanomas with NRAS mutations is challenging. Targeting MAPK signaling is only beneficial to a small subset of patients due to resistance that arises through genetic, transcriptional, and metabolic adaptation. Identification of targetable vulnerabilities in NRAS-mutated melanoma could help improve patient treatment. Here, we used multiomics analyses to reveal that NRAS-mutated melanoma cells adopt a mesenchymal phenotype with a quiescent metabolic program to resist cellular stress induced by MEK inhibition. The metabolic alterations elevated baseline reactive oxygen species (ROS) levels, leading these cells to become highly sensitive to ROS induction. In vivo xenograft experiments and single-cell RNA sequencing demonstrated that intratumor heterogeneity necessitates the combination of a ROS inducer and a MEK inhibitor to inhibit both tumor growth and metastasis. Ex vivo pharmacoscopy of 62 human metastatic melanomas confirmed that MEK inhibitor-resistant tumors significantly benefited from the combination therapy. Finally, oxidative stress response and translational suppression corresponded with ROS-inducer sensitivity in 486 cancer cell lines, independent of cancer type. These findings link transcriptional plasticity to a metabolic phenotype that can be inhibited by ROS inducers in melanoma and other cancers. SIGNIFICANCE: Metabolic reprogramming in drug-resistant NRAS-mutated melanoma cells confers sensitivity to ROS induction, which suppresses tumor growth and metastasis in combination with MAPK pathway inhibitors.

Phosphorylated cingulin localises GEF-H1 at tight junctions to protect vascular barriers in blood endothelial cells.

Dysfunction of vascular barriers is a critical step in inflammatory diseases. Endothelial tight junctions (TJs) control barrier function, and the cytoplasmic adaptor protein cingulin connects TJs to signalling pathways. However, local events at TJs during inflammation are largely unknown. In this study, we investigate the local response of TJ adaptor protein cingulin and its interaction with Rho guanine nucleotide exchange factor H1 (GEF-H1, also known as ARHGEF2) upon vascular barrier disruption to find a new approach to counteract vascular leak. Based on transendothelial-electrical-resistance (TEER) measurements, cingulin strengthened barrier integrity upon stimulation with histamine, thrombin and VEGF. Cingulin also attenuated myosin light chain 2 (MLC2; also known as MYL2) phosphorylation by localising GEF-H1 to cell junctions. By using cingulin phosphomutants, we verified that the phosphorylation of the cingulin head domain is required for its protective effect. Increased colocalisation of GEF-H1 and cingulin was observed in the vessels of vasculitis patients compared to those in healthy skin. Our findings demonstrate that cingulin can counteract vascular leak at TJs, suggesting the existence of a novel mechanism in blood endothelial cells that protects barrier function during disease.

Inhibition of p38/MK2 Signaling Prevents Vascular Invasion of Melanoma.

Melanoma cells can switch between distinct gene expression profiles, resulting in proliferative or invasive phenotypes. Signaling pathways involved in this switch were analyzed by gene expression profiling of a cohort of 22 patient-derived melanoma cell lines. CDH1 negativity was identified as a surrogate marker for the invasive phenotype. CDH1 expression could be turned on and off by modulating activity of p38 or its downstream target MK2, suggesting that this pathway controls melanoma progression. Mechanistically, MK2 inhibition prevented melanoma-induced vascular barrier disruption, reduced the expression of PODXL and DEL-1, and prevented vascular dissemination in vivo. PODXL and DEL-1 expression in patients with melanoma were associated with poor survival and thus can be used as prognostic markers. Downstream targets of MK2 may thus serve as candidate therapeutics.

Co-existence of BRAF and NRAS driver mutations in the same melanoma cells results in heterogeneity of targeted therapy resistance.

Acquired chemotherapeutic resistance of cancer cells can result from a Darwinistic evolution process in which heterogeneity plays an important role. In order to understand the impact of genetic heterogeneity on acquired resistance and second line therapy selection in metastatic melanoma, we sequenced the exomes of 27 lesions which were collected from 3 metastatic melanoma patients treated with targeted or non-targeted inhibitors. Furthermore, we tested the impact of a second NRAS mutation in 7 BRAF inhibitor resistant early passage cell cultures on the selection of second line therapies.We observed a rapid monophyletic evolution of melanoma subpopulations in response to targeted therapy that was not observed in non-targeted therapy. We observed the acquisition of NRAS mutations in the BRAF mutated patient treated with a BRAF inhibitor in 1 of 5 of his post-resistant samples. In an additional cohort of 5 BRAF-inhibitor treated patients we detected 7 NRAS mutations in 18 post-resistant samples. No NRAS mutations were detected in pre-resistant samples. By sequencing 65 single cell clones we prove that NRAS mutations co-occur with BRAF mutations in single cells. The double mutated cells revealed a heterogeneous response to MEK, ERK, PI3K, AKT and multi RTK - inhibitors.We conclude that BRAF and NRAS co-mutations are not mutually exclusive. However, the sole finding of double mutated cells in a resistant tumor is not sufficient to determine follow-up therapy. In order to target the large pool of heterogeneous cells in a patient, we think combinational therapy targeting different pathways will be necessary.

Opposing Roles of JNK and p38 in Lymphangiogenesis in Melanoma.

In primary melanoma, the amount of vascular endothelial growth factor C (VEGF-C) expression and lymphangiogenesis predicts the probability of metastasis to sentinel nodes, but conditions boosting VEGF-C expression in melanoma are poorly characterized. By comparative mRNA expression analysis of a set of 22 human melanoma cell lines, we found a striking negative correlation between VEGF-C and microphthalmia-associated transcription factor (MITF) expression, which was confirmed by data mining in GEO databases of human melanoma Affymetrix arrays. Moreover, in human patients, high VEGF-C and low MITF levels in primary melanoma significantly correlated with the chance of metastasis. Pathway analysis disclosed the respective c-Jun N-terminal kinase and p38/mitogen-activated protein kinase activities as being responsible for the inverse regulation of VEGF-C and MITF. Predominant c-Jun N-terminal kinase signaling results in a VEGF-C(low)/MITF(high) phenotype; these melanoma cells are highly proliferative, show low mobility, and are poorly lymphangiogenic. Predominant p38 signaling results in a VEGF-C(high)/MITF(low) phenotype, corresponding to a slowly cycling, highly mobile, lymphangiogenic, and metastatic melanoma. In conclusion, the relative c-Jun N-terminal kinase and p38 activities determine the biological behavior of melanoma. VEGF-C and MITF levels serve as surrogate markers for the respective c-Jun N-terminal kinase and p38 activities and may be used to predict the risk of metastasis in primary melanoma.