Fabio Petrocca

miR-200 Enhances Mouse Breast Cancer Cell Colonization to Form Distant Metastases

Background The development of metastases involves the dissociation of cells from the primary tumor to penetrate the basement membrane, invade and then exit the vasculature to seed, and colonize distant tissues. The last step, establishment of macroscopic tumors at distant sites, is the least well understood. Four isogenic mouse breast cancer cell lines (67NR, 168FARN, 4TO7, and 4T1) that differ in their ability to metastasize when implanted into the mammary fat pad are used to model the steps of metastasis. Only 4T1 forms macroscopic lung and liver metastases. Because some miRNAs are dysregulated in cancer and affect cellular transformation, tumor formation, and metastasis, we examined whether changes in miRNA expression might explain the differences in metastasis of these cells. Methodology/Principal Findings miRNA expression was analyzed by miRNA microarray and quantitative RT–PCR in isogenic mouse breast cancer cells with distinct metastatic capabilities. 4T1 cells that form macroscopic metastases had elevated expression of miR-200 family miRNAs compared to related cells that invade distant tissues, but are unable to colonize. Moreover, over-expressing miR-200 in 4TO7 cells enabled them to metastasize to lung and liver. These findings are surprising since the miR-200 family was previously shown to promote epithelial characteristics by inhibiting the transcriptional repressor Zeb2 and thereby enhancing E-cadherin expression. We confirmed these findings in these cells. The most metastatic 4T1 cells acquired epithelial properties (high expression of E-cadherin and cytokeratin-18) compared to the less metastatic cells. Conclusions/Significance Expression of miR-200, which promotes a mesenchymal to epithelial cell transition (MET) by inhibiting Zeb2 expression, unexpectedly enhances macroscopic metastases in mouse breast cancer cell lines. These results suggest that for some tumors, tumor colonization at metastatic sites might be enhanced by MET. Therefore the epithelial nature of a tumor does not predict metastatic outcome.

Promise and Challenge of RNA Interference–Based Therapy for Cancer

Cancer therapeutics still fall far short of our goals for treating patients with locally advanced or metastatic disease. Until recently, almost all cancer drugs were crude cytotoxic agents that discriminate poorly between cancer cells and normally dividing cells. The development of targeted biologics that recognize tumor cell surface antigens and of specific inhibitors of pathways dysregulated in cancer cells or normal cellular pathways on which a cancer cell differentially depends has provided hope for converting our increasing understanding of cellular transformation into intelligently designed anticancer therapeutics. However, new drug development is painfully slow, and the pipeline of new therapeutics is thin. The discovery of RNA interference (RNAi), a ubiquitous cellular pathway of gene regulation that is dysregulated in cancer cells, provides an exciting opportunity for relatively rapid and revolutionary approaches to cancer drug design. Small RNAs that harness the RNAi machinery may become the next new class of drugs for treating a variety of diseases. Although it has only been 9 years since RNAi was shown to work in mammalian cells, about a dozen phase I to III clinical studies have already been initiated, including four for cancer. So far there has been no unexpected toxicity and suggestions of benefit in one phase II study. However, the obstacles for RNAi-based cancer therapeutics are substantial. This article will discuss how the endogenous RNAi machinery might be harnessed for cancer therapeutics, why academic researchers and biotech and pharmaceutical companies are so excited, and what the obstacles are and how they might be overcome.

A Genome-wide siRNA Screen Identifies Proteasome Addiction as a Vulnerability of Basal-like Triple-Negative Breast Cancer Cells

Basal-like triple-negative breast cancers (TNBCs) have poor prognosis. To identify basal-like TNBC dependencies, a genome-wide siRNA lethality screen compared two human breast epithelial cell lines transformed with the same genes: basal-like BPLER and myoepithelial HMLER. Expression of the screens 154 BPLER dependency genes correlated with poor prognosis in breast, but not lung or colon, cancer. Proteasome genes were overrepresented hits. Basal-like TNBC lines were selectively sensitive to proteasome inhibitor drugs relative to normal epithelial, luminal, and mesenchymal TNBC lines. Proteasome inhibition reduced growth of established basal-like TNBC tumors in mice and blocked tumor-initiating cell function and macrometastasis. Proteasome addiction in basal-like TNBCs was mediated by NOXA and linked to MCL-1 dependence.

miR-200 promotes the mesenchymal to epithelial transition by suppressing multiple members of the Zeb2 and Snail1 transcriptional repressor complexes

The miR-200 family promotes the epithelial state by suppressing the Zeb1/Zeb2 epithelial gene transcriptional repressors. To identify other miR-200-regulated genes, we isolated mRNAs bound to transfected biotinylated miR-200c in mouse breast cancer cells. In all, 520 mRNAs were significantly enriched in miR-200c binding at least twofold. Putative miR-200-regulated genes included Zeb2, enriched 3.5-fold in the pull down. However, Zeb2 knockdown does not fully recapitulate miR-200c overexpression, suggesting that regulating other miR-200 targets contributes to miR-200’s enhancement of epithelial gene expression. Candidate genes were highly enriched for miR-200c seed pairing in their 3′UTR and coding sequence and for genes that were downregulated by miR-200c overexpression. Epidermal growth factor receptor and downstream MAPK signaling pathways were the most enriched pathways. Genes whose products mediate transforming growth factor (TGF)-β signaling were also significantly overrepresented, and miR-200 counteracted the suppressive effects of TGF-β and bone morphogenic protein 2 (BMP-2) on epithelial gene expression. miR-200c regulated the 3′UTRs of 12 of 14 putative miR-200c-binding mRNAs tested. The extent of mRNA binding to miR-200c strongly correlated with gene suppression. Twelve targets of miR-200c (Crtap, Fhod1, Smad2, Map3k1, Tob1, Ywhag/14-3-3γ, Ywhab/14-3-3β, Smad5, Zfp36, Xbp1, Mapk12, Snail1) were experimentally validated by identifying their 3′UTR miR-200 recognition elements. Smad2 and Smad5 form a complex with Zeb2 and Ywhab/14-3-3β and Ywhag/14-3-3γ form a complex with Snail1. These complexes that repress transcription assemble on epithelial gene promoters. miR-200 overexpression induced RNA polymerase II localization and reduced Zeb2 and Snail1 binding to epithelial gene promoters. Expression of miR-200-resistant Smad5 modestly, but significantly, reduced epithelial gene induction by miR-200. miR-200 expression and Zeb2 knockdown are known to inhibit cell invasion in in vitro assays. Knockdown of each of three novel miR-200 target genes identified here, Smad5, Ywhag and Crtap, also profoundly suppressed cell invasion. Thus, miR-200 suppresses TGF-β/BMP signaling, promotes epithelial gene expression and suppresses cell invasion by regulating a network of genes.

Gene Knockdown by EpCAM Aptamer–siRNA Chimeras Suppresses Epithelial Breast Cancers and Their Tumor-Initiating Cells

Effective therapeutic strategies for in vivo siRNA delivery to knockdown genes in cells outside the liver are needed to harness RNA interference for treating cancer. EpCAM is a tumor-associated antigen highly expressed on common epithelial cancers and their tumor-initiating cells (TIC, also known as cancer stem cells). Here, we show that aptamer–siRNA chimeras (AsiC, an EpCAM aptamer linked to an siRNA sense strand and annealed to the siRNA antisense strand) are selectively taken up and knock down gene expression in EpCAM+ cancer cells in vitro and in human cancer biopsy tissues. PLK1 EpCAM-AsiCs inhibit colony and mammosphere formation (in vitro TIC assays) and tumor initiation by EpCAM+ luminal and basal-A triple-negative breast cancer (TNBC) cell lines, but not EpCAM− mesenchymal basal-B TNBCs, in nude mice. Subcutaneously administered EpCAM-AsiCs concentrate in EpCAM+ Her2+ and TNBC tumors and suppress their growth. Thus, EpCAM-AsiCs provide an attractive approach for treating epithelial cancer. Mol Cancer Ther; 14(10); 2279–91. ©2015 AACR.

Micromanipulating cancer: microRNA-based therapeutics?

Because microRNAs regulate cancer cell differentiation, proliferation, survival, and metastasis, manipulating microRNA function, either by mimicking or inhibiting miRNAs implicated in cancer, could provide a powerful therapeutic strategy to interfere with key pathways for cancer progression. This review will explore some of the opportunities and obstacles to harnessing microRNA biology for cancer therapy.

Micromanagers of immune cell fate and function.

MicroRNAs (miRNAs) regulate cell fate during development and in response to environmental cues. Here, we review the emerging story of how miRNAs regulate immune cell development and function.

Regional Delivery of Chimeric Antigen Receptor (CAR) T-Cells for Cancer Therapy

Chimeric Antigen Receptor (CAR) T-cells are T-cells with recombinant receptors targeted to tumor antigens. CAR-T cell therapy has emerged as a mode of immunotherapy and is now being extensively explored in hematologic cancer. In contrast, CAR-T cell use in solid tumors has been hampered by multiple obstacles. Several approaches have been taken to circumvent these obstacles, including the regional delivery of CAR-T cells. Regional CAR-T cell delivery can theoretically compensate for poor T-cell trafficking and tumor antigen specificity while avoiding systemic toxicity associated with intravenous delivery. We reviewed completed clinical trials for the treatment of glioblastoma and metastatic colorectal cancer and examined the data in these studies for safety, efficacy, and potential advantages that regional delivery may confer over systemic delivery. Our appraisal of the available literature revealed that regional delivery of CAR-T cells in both glioblastoma and hepatic colorectal metastases was generally well tolerated and efficacious in select instances. We propose that the regional delivery of CAR-T cells is an area of potential growth in the solid tumor immunotherapy, and look towards future clinical trials in head and neck cancer, mesothelioma, and peritoneal carcinomatosis as the use of this technique expands.

RIG-ing an antitumor response.

A small interfering RNA has been engineered to silence an oncogene and activate the immune response simultaneously. The approach shrinks tumors in mice ( pages 1256–1263 ).

Basal-A Triple-Negative Breast Cancer Cells Selectively Rely on RNA Splicing for Survival

Prognosis of triple-negative breast cancer (TNBC) remains poor. To identify shared and selective vulnerabilities of basal-like TNBC, the most common TNBC subtype, a directed siRNA lethality screen was performed in 7 human breast cancer cell lines, focusing on 154 previously identified dependency genes of 1 TNBC line. Thirty common dependency genes were identified, including multiple proteasome and RNA splicing genes, especially those associated with the U4/U6.U5 tri-snRNP complex (e.g., PRPF8, PRPF38A). PRPF8 or PRPF38A knockdown or the splicing modulator E7107 led to widespread intronic retention and altered splicing of transcripts involved in multiple basal-like TNBC dependencies, including protein homeostasis, mitosis, and apoptosis. E7107 treatment suppressed the growth of basal-A TNBC cell line and patient-derived basal-like TNBC xenografts at a well-tolerated dose. The antitumor response was enhanced by adding the proteasome inhibitor bortezomib. Thus, inhibiting both splicing and the proteasome might be an effective approach for treating basal-like TNBC. Mol Cancer Ther; 16(12); 2849–61. ©2017 AACR.