In This Issue

See article, p. 415.

Lung adenocarcinomas in never smokers often harbor activating kinase gene mutations that confer kinase dependency and kinase inhibitor sensitivity, but a large subset of tumors lack tractable kinase targets. Fernandez-Cuesta and colleagues probed for alternative druggable oncogenic alterations in driver-negative lung tumors by performing gene copy number analysis and transcriptome sequencing of 25 lung adenocarcinomas of never smokers without KRAS or EGFR mutations and identified one invasive mucinous adenocarcinoma with a fusion between CD74 and the exons encoding the EGF-like domain of the neuron-specific neuregulin 1 (NRG1) III-β3 isoform. Screening of a larger cohort of 102 pan-driver–negative lung adenocarcinomas of never smokers identified 4 additional fusion-positive invasive mucinous tumors. Biochemical analysis of CD74-NRG1–positive cells indicated that the fusion generates a membrane-bound protein that exposes the EGF-like domain of NRG1 on the outer cell surface. In addition to high extracellular levels of the EGF-like domain of NRG1 III-β3, CD74–NRG1–positive tumors also had high levels of ERBB2, ERBB3, and phosphorylated ERBB3, suggesting that CD74–NRG1 may induce oncogenic activation of ERBB3 and downstream pathways by providing a ligand for ERBB2–ERBB3 heterodimers. Consistent with this possibility, CD74–NRG1 expression in lung cancer cells activated the PI3K–AKT pathway in an EGF-like domain-dependent manner and enhanced anchorage-independent growth of lung cancer cells. Together, these results implicate CD74–NRG1 as a potential driver of invasive mucinous lung adenocarcinomas that may be targetable with clinically available inhibitors of ERBB3–PI3K–AKT signaling.

See article, p. 423.

Increased glycolysis is a common feature of cancer cells that can be driven by oncogenes to support increased energy requirements. Activating BRAF^V600 mutations that are frequently observed in melanoma and other cancers are associated with increased glycolysis and glucose transporter 1 expression. Treatment with the BRAF inhibitor vemurafenib leads to decreased glucose uptake, prompting Parmenter and colleagues to investigate whether inhibition of glycolysis is required for vemurafenib sensitivity. Vemurafenib led to decreased glucose uptake in BRAF-mutant, but not BRAF–wild-type, melanoma cell lines, and the degree of glucose uptake inhibition and reduction of glycolytic gene expression was significantly correlated with vemurafenib sensitivity in BRAF-mutant melanoma cell lines and tumor biopsies. In support of the hypothesis that suppression of glycolysis plays a role in sensitivity to BRAF inhibition, expression of oncogenic NRAS, a common mechanism of BRAF inhibitor resistance, was able to restore glucose uptake in BRAF-mutant cells, and combined inhibition of BRAF and glycolytic metabolism synergized to induce cell death in vemurafenib-resistant melanoma cells. Mechanistically, BRAF inhibition led to decreased expression and stability of MYC and HIF1α, which normally promote glucose uptake, and increased target gene promoter occupancy of MONDOA, a negative regulator of glycolysis. The identification of a transcriptional network operating downstream of BRAF^V600 that regulates glycolysis illustrates how BRAF inhibition suppresses glycolysis and provides a rationale for combined use of inhibitors of BRAF and glycolysis in BRAF inhibitor–resistant cancers.

See article, p. 434.

Germline loss-of-function mutations of KIF1B in neuroblastomas and pheochromocytomas and frequent deletion of the KIF1B locus in neural crest–derived tumors implicate KIF1B as a tumor suppressor, but the underlying mechanism is unknown. The KIF1Bβ splice isoform has been shown to induce apoptosis in sympathetic neurons when nerve growth factor (NGF) is limiting, suggesting that failure to cull neuronal progenitors in the absence of KIF1Bβ may predispose to tumorigenesis. Chen and colleagues performed a proteomic screen for interactors of the pro-apoptotic domain of KIF1Bβ and identified the transcriptional regulator RNA helicase A (DHX9). Through its interaction with Exportin-2, a regulator of nuclear import, KIF1Bβ promoted DHX9 nuclear localization, which was necessary for KIF1Bβ-induced cell death. KIF1Bβ-dependent nuclear DHX9 subsequently induced expression of the pro-apoptotic gene X-linked inhibitor of apoptosis protein-associated factor 1 (XAF1). Upon NGF withdrawal, neural cell apoptosis was induced in association with increased levels of KIF1Bβ protein, DHX9 nuclear localization, and XAF1 expression, but inappropriate neuronal survival occurred when KIF1Bβ or DHX9 was knocked down. In vivo, nuclear localization of DHX9 coincided with KIF1Bβ expression in the sympathetic nervous system at the peak of developmental apoptosis, further pointing to a role of KIF1Bβ in neural pruning. In primary human neuroblastomas, DHX9 nuclear localization was specifically reduced in KIF1Bβ-deficient tumors, indicating that defects in DHX9-dependent apoptosis caused by KIF1Bβ loss might promote tumor formation. Together, these data indicate that DHX9 is a critical downstream effector of KIF1Bβ and suggest a potential mechanism of KIF1Bβ-mediated tumor suppression.

See article, p. 452.

In addition to activating the RAF–MAPK and PI3K–AKT mitogenic signaling pathways, KRAS induces inflammatory cytokines that can promote oncogenic transformation and tumor cell survival, though the underlying mechanism remains unclear. Zhu and colleagues found that murine embryonic fibroblasts lacking TANK-binding kinase 1 (TBK1), which is activated downstream of KRAS and required for KRAS-induced transformation, showed impaired colony formation that was attributable to reduced production of CCL5 and IL-6, suggesting that TBK1-dependent autocrine cytokine signaling may promote proliferation downstream of KRAS. Indeed, TBK1 activation was increased in KRAS-dependent non–small cell lung cancer (NSCLC) cell lines compared with KRAS-independent NSCLC cell lines, and CCL5 and IL-6 neutralization completely blocked KRAS-dependent NSCLC cell proliferation and migration. Noting that CYT387, a Janus kinase (JAK) inhibitor in clinical development, also potently inhibited the kinase activity of TBK1 and its homolog IκB kinase epsilon (IKKϵ), the authors treated NSCLC cell lines with CYT387 and found that CYT387 preferentially reduced proliferation of KRAS-dependent cell lines and more potently inhibited CCL5- and IL-6–mediated autocrine signaling than TBK1-specific or JAK-specific inhibitors. CYT387 also induced significant and durable regression of established murine lung tumors driven by mutant Kras, and synergized with the MEK inhibitor selumetinib to induce tumor regression in a treatment-refractory Kras-mutant, Trp53-deficient NSCLC model. These findings indicate that autocrine cytokine signaling promotes KRAS-driven tumorigenicity and suggest that simultaneous inhibition of JAK, TBK1, IKKϵ, and MEK may be an effective approach for treatment of KRAS-driven cancers.

See article, p. 466.

Autophagy has been implicated as a prosurvival mechanism utilized by cancer cells in certain contexts in response to stress. RAS activation is known to enhance basal autophagic activity, but the mechanisms by which autophagy supports RAS-induced tumorigenesis are not fully understood. Lock and colleagues found that knockdown of autophagy-related (ATG) genes significantly decreased HRAS^V12-driven invasion of mammary epithelial cells in three-dimensional culture and in an in vivo mouse lung metastasis model. Autophagy inhibition had no effect on the proliferation or death of HRAS^V12-expressing cells, nor did it alter RAS downstream signaling, but rather was correlated with decreased extracellular matrix proteolysis and cell migration. Interestingly, conditioned media from autophagy-competent HRAS^V12-expressing cells rescued the invasion of ATG-deficient HRAS^V12-expressing cells, suggesting that autophagy regulates the production of secreted factors that promote invasion. Indeed, autophagy inhibition significantly reduced the amount of secreted interleukin 6 (IL6), a pro-invasive cytokine, and exogenous IL6 treatment partially restored ATG-deficient HRAS-mutant cell invasion. Consistent with these findings, an IL6-blocking antibody prevented autophagy-competent conditioned media from inducing invasion, indicating that the autophagy-dependent secretion of IL6 is necessary for this process. Analysis of invasion-associated genes showed that the expression of WNT5A and MMP2 were also significantly suppressed by autophagy inhibition in HRAS-mutant cells, implicating autophagy in the production of multiple secreted promigratory factors. Collectively, these results highlight a survival-independent role for autophagy in RAS-driven invasion and metastasis.

See article, p. 480.

The NF-κB pathway plays a critical role in the survival and therapy-resistant nature of activated B cell-like diffuse large B cell lymphoma (ABC DLBCL). Polyubiquitination of IκB kinase γ (IKKγ)/NEMO by the linear ubiquitin chain assembly complex (LUBAC) facilitates NF-κB activation. To investigate the role of LUBAC in ABC DLBCL, Yang and colleagues searched for mutations in the LUBAC subunit genes RNF31, RBCK1, and SHARPIN among lymphoma biopsy samples. Previously reported rare single-nucleotide polymorphisms (SNP) in RNF31 that reside in the RBCK1 interaction domain were specifically enriched in patients with ABC DLBCL, with an overall frequency of 7.8%. Notably, the RNF31 variants were more effective than wild-type RNF31 at stimulating NF-κB transcriptional activity as they enhanced binding to RBCK1 and led to increased LUBAC complex formation and ubiquitin ligase activity, raising the possibility that LUBAC, and particularly the RNF31-RBCK1 interface, may represent therapeutic targets. Indeed, RNF31 depletion was selectively cytotoxic in ABC DLBCL cell lines and led to reduced IKKγ/NEMO polyubiquitination and NF-κB transcriptional activity. Moreover, stapled RNF31 α-helical peptides inhibited LUBAC formation and downstream NF-κB signaling and selectively induced ABC DLBCL cell death. Importantly, RNF31 stapled peptides sensitized refractory ABC DLBCL cells to cytotoxic chemotherapy or targeted inhibition of the B-cell receptor signaling pathway with ibrutinib. Taken together, these findings identify LUBAC as a therapeutic target in ABC DLBCL due to its integral role in promoting NF-κB pathway activation via linear polyubiquitination.

Note: In This Issue is written by Cancer Discovery Science Writers. Readers are encouraged to consult the original articles for full details.