In This Issue

See article, p. 598.

Acquired resistance to the anti-EGFR antibodies cetuximab and panitumumab in patients with colorectal cancer is frequently mediated by upregulation of EGFR ligands or mutations in EGFR, indicative of a continued dependence on EGFR signaling and suggesting that more potent EGFR inhibition may be effective. Dienstmann and colleagues found that, compared with cetuximab, Sym004, a mixture of two recombinant monoclonal antibodies targeting nonoverlapping EGFR epitopes, induced more significant EGFR degradation and exhibited enhanced growth-inhibitory activity in a panel of colorectal cancer cell lines in the presence of EGFR ligands. To evaluate Sym004 clinically, the authors performed a first-in-human phase I trial, consisting of a dose-escalation phase in 20 patients with advanced epithelial tumors, followed by a dose-expansion phase in 42 patients with metastatic colorectal cancer who had previously received anti-EGFR monoclonal antibody treatment. Consistent with known toxicities of anti-EGFR drugs, the most common grade 3 treatment-related adverse events included skin toxicities and hypomagnesemia, which were manageable with dose reductions and supportive care. Pharmacodynamic analyses revealed sustained downregulation of EGFR in tumor samples. Sym004 treatment induced tumor shrinkage in 17 (44%) patients with metastatic colorectal cancer, including 5 (13%) patients who achieved a partial response, and an overall disease control rate of 67%. Partial responses occurred in patients with wild-type RAS tumors, as well as in a patient with an acquired EGFR^S492R mutation. These findings demonstrate that Sym004 has clinically relevant antitumor activity and may overcome cetuximab resistance in patients with metastatic colorectal cancer.

See article, p. 610.

Integrative genomic analyses have identified putative genetic subsets of squamous cell lung cancer (SQCLC), but these studies have been largely limited to early-stage tumors, and associations between genomic and clinical characteristics have not been assessed. Paik and colleagues evaluated archival material from 79 patients with stage IV SQCLC and identified amplification of FGFR1 or PI3K pathway aberrations (PIK3CA or PTEN mutation or PTEN protein loss) in 61% of tumors. Notably, patients whose tumors exhibited PI3K pathway aberrations had a significantly shorter overall survival than patients without PI3K pathway aberrations or FGFR1 amplification, and patients whose tumors had PI3K aberrations had a significantly higher metastatic burden and increased incidence of metastasis to the brain, suggesting that alterations in the PI3K pathway distinguish a particularly aggressive subtype of SQCLC with a unique course of disease. Genomic profiling of matched primary SQCLCs and brain metastases showed that although PTEN loss and PIK3CA mutations were maintained between primary and metastatic sites, there was a high degree of intratumor heterogeneity and the brain metastases generally showed significant genetic divergence from the primary tumors, suggesting that brain metastatic clones may be distinct from dominant clones within primary SQCLCs. Although further studies are needed to determine whether the PI3K pathway plays a functional role in promoting metastasis, particularly to the brain, these data provide insight into evolutionary processes during tumor metastasis and suggest that PI3K pathway status should be considered in the management of patients with SQCLC.

See article, p. 622.

The gene encoding the proapoptotic protein SIVA is a direct target of p53, and SIVA mediates p53-dependent apoptosis in vitro; however, SIVA has also been shown to inhibit p53 and promote cell proliferation. To clarify the role of SIVA in tumorigenesis, Van Nostrand and colleagues generated mice with conditional deletion of Siva in a model of KRAS-driven non–small cell lung cancer (NSCLC). Surprisingly, loss of SIVA inhibited NSCLC tumor development, suggesting that SIVA exhibits protumorigenic activity. Consistent with this idea, SIVA knockdown in both mouse and human KRAS-mutant NSCLC cell lines inhibited cell proliferation and transformation without affecting the expression levels of p53 or activation of other p53 target genes, and knockdown of p53 did not rescue this proliferation inhibition. Furthermore, high expression of SIVA correlated with decreased overall survival in patients with NSCLC. SIVA depletion resulted in decreased mitochondrial DNA and mitochondrial respiration capacity and triggered increased autophagic flux, suggesting that SIVA promotes proliferation via stimulation of ATP production and suppression of autophagy. Mechanistically, SIVA knockdown was associated with reduced phosphorylation of key mTOR substrates, indicating that SIVA is essential for maximal mTOR activity. Reactivation of mTOR signaling partially rescued cell proliferation in SIVA-depleted cells, suggesting that induction of mTOR is required for SIVA-driven tumorigenesis. These results identify a critical role for SIVA in regulating metabolism and promoting KRAS-driven NSCLC and suggest that SIVA may be a potential therapeutic target.

See article, p. 636.

Loss of the tumor suppressors PTEN and TP53 frequently contributes to prostate cancer metastatic progression but the role of cell–cell communication remains less defined. Nowak and colleagues found that mouse embryonic fibroblasts (MEF) with combined Pten and Trp53 loss, but not MEFs with deletion of either gene alone, gained a significant growth advantage as a result of elevated IL6 secretion, which led to paracrine activation of STAT3 and induction of its target MYC. Disruption of IL6–STAT3–MYC signaling inhibited the proliferation of double-mutant MEFs. In addition, mice harboring prostate-specific deletion of Pten and Trp53 exhibited detectable IL6 in both prostate epithelial cells and the circulation, suggesting the potential utility of IL6 as a biomarker for MYC-driven metastasis. Consistent with the paracrine action of IL6, stromal fibroblasts surrounding Pten/Trp53-knockout prostate tissue were highly proliferative and displayed increased levels of phosphorylated STAT3 and MYC expression. In both primary and metastatic Pten/Trp53-mutant prostate tumors, the highest proliferation zones positively correlated with activation of STAT3/MYC and inversely correlated with phosphorylated AKT levels. Interestingly, MYC-mediated expression of PH domain and leucine rich repeat protein phosphatase 2 (PHLPP2) was necessary for the suppression of AKT phosphorylation and induction of cell growth both in vitro and in in vivo models of Pten/Trp53-null prostate metastasis. Taken together, these results indicate that Pten/Trp53 deficiency promotes prostate cancer cell proliferation and metastasis via the IL6–STAT3–MYC signaling axis independent of AKT.

See article, p. 652.

Clear-cell renal cell carcinoma (ccRCC) is characterized by constitutively active hypoxia-inducible factor (HIF) signaling and the presence of intracellular lipid droplets; however, the functional consequences of altered lipid metabolism in ccRCC tumorigenesis are largely unknown. Qiu and colleagues found that perilipin 2 (PLIN2), a lipid droplet coat protein, was transcriptionally elevated in primary ccRCC samples independent of tumor stage and was correlated with activation of HIF2α, but not HIF1α. Depletion of HIF2α reduced PLIN2 levels and decreased ccRCC cell viability, neutral lipid storage, and xenograft tumor growth, which were restored by PLIN2 overexpression. Consistent with these findings, PLIN2-depleted ccRCC cell lines exhibited similarly reduced lipid storage and cell survival. In addition, PLIN2 depletion resulted in activation of the unfolded protein response and cell death due to increased mTORC1-mediated protein synthesis and induction of endoplasmic reticulum (ER) stress, suggesting that PLIN2-dependent lipid storage maintains ER homeostasis. In support of this idea, exogenous PLIN2 expression restored lipid storage and protected HIF2α-depleted ccRCC cell lines from cytotoxic ER stress–inducing drugs such as tunicamycin and the proteasome inhibitor bortezomib. Taken together, these data outline a model for ccRCC progression in which HIF2α activation leads to increased lipid storage, protection against ER stress, and tumor cell survival via upregulation of PLIN2, which may be exploited in the clinic with combined HIF2α and proteasome inhibition in patients with ccRCC.

See article, p. 668.

Secondary mutations in the FMS-like tyrosine kinase 3 (FLT3) gatekeeper residue F691 and activation loop residue D835 have been identified as common resistance mechanisms to the FLT3 tyrosine kinase inhibitor quizartinib, which exhibits clinical efficacy in patients with acute myeloid leukemia (AML) harboring FLT3 internal tandem duplication (ITD) mutations. However, the structural mechanisms of resistance imparted by these mutations remain unclear. Smith and colleagues characterized the co-crystal structure of FLT3 bound to quizartinib, which highlighted the importance of the gatekeeper residue F691 and F830 within the conserved DFG motif of the activation loop in quizartinib binding via edge-to-face interactions, and prompted screening for small molecules with activity against FLT3 gatekeeper mutations. PLX3397, a type II kinase inhibitor, selectively inhibited oncogenic FLT3-ITD signaling in vitro, prevented AML xenograft tumor formation in vivo, and impaired the proliferation of AML cells expressing the quizartinib-resistant FLT3 F691L mutation. Importantly, 18 FLT3 mutations, including many located within the activation loop and several that have previously been shown to promote quizartinib resistance, were identified using an in vitro mutagenesis screen and shown to confer resistance to PLX3397. In line with this finding, sequencing of samples from patients with FLT3-ITD⁺ AML enrolled in a phase I/II clinical trial of PLX3397 confirmed an enrichment for polyclonal FLT3 activation loop mutations and the absence of F691L mutation upon relapse. Together, this work demonstrates that PLX3397 inhibits the gatekeeper FLT3 F691L mutation, but remains susceptible to FLT3 activation loop mutations.

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