In This Issue

See article by Corcoran et al., p. 227.

Selective RAF inhibitors have produced dramatic response rates in melanomas harboring BRAF mutations but have been largely ineffective in the treatment of BRAF-mutant colorectal cancers. To identify the underlying causes of vemurafenib insensitivity in these tumors, Corcoran and colleagues compared the effects of vemurafenib treatment in BRAF-mutant melanoma and colorectal cancer cells. In contrast to the BRAF-mutant melanoma cells, ERK phosphorylation was only transiently suppressed in the BRAF-mutant colorectal cancer cells and was specifically associated with RAS-induced CRAF activation. To determine how RAS was activated in these cells, the authors examined activation of receptor tyrosine kinases (RTK) in the presence or absence of vemurafenib using an antibody array and found that, unlike melanomas, BRAF-mutant colorectal cancer cells exhibited high basal levels of several RTKs. Only inhibition of EGFR suppressed ERK phosphorylation and blocked RAS and CRAF activation in the presence of vemurafenib, suggesting that combined RAF and EGFR inhibition might be more effective in blocking the growth of BRAF-mutant colorectal cancer cells. Indeed, combination vemurafenib and gefitinib therapy led to significant inhibition and regression of BRAF-mutant colorectal cancer cell xenografts compared with vemurafenib alone. An analysis of a panel of BRAF-mutant melanomas and colorectal cancers further showed that 60% of BRAF-mutant colorectal cancers expressed high levels of active EGFR compared with 18% of melanomas, indicating that EGFR levels may identify colorectal cancer patients who are likely to respond to combination therapy.

See article by Benassi et al., p. 236.

Ubiquitin-specific protease (USP2a) is a deubiquitinase that is overexpressed in approximately half of human prostate adenocarcinomas and promotes oncogenic transformation, but the underlying mechanism is unclear. Given that microRNA (miRNA) deregulation is thought to contribute to oncogenesis, Benassi and colleagues evaluated miRNA expression in immortalized prostate epithelial cells and prostate cancer cells and noted that USP2a overexpression consistently downregulated multiple miRNAs predicted to target the 3′-untranslated region of MYC, including miR-34b/c. Consistent with these findings, exogenous miR-34b/c directly bound MYC mRNA and decreased MYC protein levels in a dose-dependent manner, and USP2a overexpression increased MYC protein levels and led to global upregulation of MYC target genes. Because miR-34b/c is a known p53 target and USP2a has previously been shown to bind and stabilize MDM2, the authors analyzed the effect of USP2a overexpression on MDM2/p53-mediated regulation of miR-34b/c transcription. The authors found that USP2a overexpression led to an increase in the cellular levels of MDM2, p53 downregulation, impaired recruitment of p53 to the miR-34b/c promoter, and a subsequent increase in MYC expression. MYC expression was required for the growth and tumorigenicity of USP2a-expressing prostate cancer cells, which activated a gene expression program that was strongly enriched in locally invasive prostate cancers compared with prostatic intraepithelial neoplasia and in various in vitro models of invasion. Together, these findings identify a link between miRNA expression and deubiquitinase activity and reveal a MYC regulatory mechanism that may potentially be exploited as a therapeutic strategy in prostate cancer.

See article by Xue et al., p. 248.

Overexpression of TWIST1, a transcription factor that normally regulates cell migration during embryonic development, promotes the epithelial–mesencyhmal transition (EMT) and metastasis of cancer cells. TWIST1 has previously been shown to be phosphorylated on serine-42 (S42) by AKT, but it remains uncertain whether TWIST1 phosphorylation contributes to tumor dissemination. Xue and colleagues observed TWIST1 phosphorylation in 90% of over 1,500 invasive breast tumors analyzed on a high-throughput tissue microarray, suggesting that phosphorylation of TWIST1 may play an important role in metastasis. The authors utilized a serine-to-alanine S42A TWIST1 mutant and the PI3K/AKT pathway inhibitor LY294002 to directly examine the consequences of AKT-mediated TWIST1 phosphorylation and found that S42 phosphorylation by AKT was necessary for cell migration and upregulation of proteins required for EMT and metastasis. Furthermore, in a metastatic breast cancer xenograft mouse model, ablation of the TWIST1 S42 phosphorylation site specifically blocked the formation of metastatic lung nodules. Gene expression and chromatin immunoprecipitation analysis revealed that phosphorylated TWIST1 directly regulated expression of TGF-β2, with increased SMAD2 phosphorylation in the lung nodules of TWIST1 wild-type mice confirming the correlation between TWIST1 phosphorylation and TGF-β pathway activation in metastatic cells. Inhibition of TGF-β signaling not only significantly decreased metastasis in the mouse model but suppressed AKT and TWIST1 phosphorylation, indicating that a feedback loop between TWIST1 phosphorylation and PI3K/AKT hyperactivation is a key driver of breast cancer metastasis.

See article by Frese et al., p. 260.

Pancreatic ductal adenocarci-noma (PDA) is a highly lethal cancer that is inherently chemoresistant. A recent phase I/II trial indicated that the combination of gemcitabine, a nucleoside analog that is the current standard of care for PDA, and nab-paclitaxel, a water-soluble, albumin-bound form of paclitaxel, may extend survival in patients with PDA, but the underlying mechanism is unknown. To better understand the efficacy and mechanism of action of gemcitabine and nab-paclitaxel, Frese and colleagues used a genetically engineered mouse model of PDA and treated tumor-bearing mice with vehicle, gemcitabine, nab-paclitaxel, or combination therapy. Mice treated with combination therapy had significantly smaller tumors than either gemcitabine or nab-paclitaxel alone, with 2 of 8 tumors regressing, and exhibited a trend toward increased survival and decreased metastasis compared with mice treated with monotherapy. Combination treatment with nab-paclitaxel also elevated the intratumoral levels of gemcitabine and its activated metabolite gemcitabine triphosphate (dFdCTP), suggesting that direct regulation of gemcitabine metabolism by nab-paclitaxel may underlie the synergistic antitumor effects of combination therapy. In tumor epithelial cells, nab-paclitaxel treatment led to decreased levels of cytidine deaminase (CDA) that could be reversed in vitro by treatment with either the proteasome inhibitor MG132 or the free radical scavenger N-acetylcysteine. These findings suggest nab-paclitaxel generates an oxidized intratumoral environment that improves gemcitabine stability via CDA degradation and suppression of gemcitabine catabolism.

See article by Sennino et al., p. 270.

Antiangiogenic therapies targeting vascular endothelial growth factor (VEGF) aim to suppress tumor growth by inhibiting the formation and survival of blood vessels. However, some preclinical studies suggest that inhibition of VEGF signaling can promote tumor invasiveness and metastasis, which could explain why some patients fail to respond to anti-VEGF therapy. To better understand why inhibition of VEGF signaling increases the aggressiveness of tumors, Sennino and colleagues evaluated tumor progression in transgenic and orthotopic mouse models of pancreatic cancer in response to treatment with a VEGF blocking antibody or sunitinib, which targets VEGF receptors (VEGFR). Anti-VEGF therapy significantly decreased tumor size and vascularity, yet the smaller tumors had more irregular borders, a more mesenchymal phenotype, and were accompanied by more numerous liver metastases. Furthermore, the areas of vascular pruning in tumors were more hypoxic, which increased the expression and activation of the hepatocyte growth factor receptor (c-MET). When combined with anti-VEGF therapy, selective c-MET inhibitors reduced tumor invasiveness, strongly suppressed the formation of liver metastases, and increased survival. Treatment with cabozantinib (XL184), a receptor tyrosine kinase inhibitor with potent activity against c-MET and VEGFR, was accompanied by a more epithelial tumor cell phenotype, a greater reduction in tumor size, vascularity, invasion, and metastasis, and prolonged survival. These findings suggest that combined inhibition of c-MET and VEGF signaling may be more effective in suppressing tumor growth, angiogenesis, and metastasis than targeting VEGF alone.

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