Tumor Suppressor MicroRNA Blocks Cancer Growth in Model

MicroRNAs (miRNAs) and short interfering RNAs (siRNA) comprise a class of about 20 nucleotides-long RNA fragments that block gene expression by attaching to molecules of messenger RNA in a fashion that prevents them from transmitting the protein synthesizing instructions they had received from the DNA. MiRNAs resemble siRNAs of the RNA interference (RNAi) pathway, except miRNAs derive from regions of RNA transcripts that fold back on themselves to form short hairpins, whereas siRNAs derive from longer regions of double-stranded RNA. With their capacity to fine-tune protein expression via sequence-specific interactions, miRNAs help regulate cell maintenance and differentiation.

In the current study, which was published in the December 4, 2018, online edition of the journal Scientific Reports, investigators at the University of California, San Diego (USA) worked with the microRNA miR-335, which specifically inhibits the SOX4 transcription factor that promotes tumor growth.

To deliver miR-335 to tumor cells in a mouse model system, the investigators loaded B-cells growing in culture with a miR-335 precursor. The B-cells converted the precursor into mature, active miR-335 and packaged it into small, membrane-coated vesicles that budded off from the cell as induced extracellular vesicles (iEVs). Each B-cell was able to produce about 100,000 miR-335-containing vesicles per day.

The investigators demonstrated that iEVs-335 efficiently and durably restored the endogenous miR-335 pool in human triple negative breast cancer cells, downregulated the expression of the miR-335 target gene SOX4 transcription factor, and markedly inhibited tumor growth in vivo. For this study, human breast cancer cells growing in culture were treated with miR-335-containing vesicles or sham vesicles. The cancer cells were transplanted into mice. After 60 days, 100% (5/5) of the mice with mock-treated cancer cells had large tumors. In contrast, only 44% (4/9) of the mice with miR-335 vesicle-treated cancer cells had tumors. On average, the tumors in the treated mice were more than 260 times smaller than those in the mock-treated mice.

The iEVs-335 mediated transcriptional effects persisted in tumors for more than 60 days following implantation. Genome-wide RNASeq analysis of cancer cells treated in vitro with iEVs-335 showed the regulation of a discrete number of genes only, without broad disruption of the transcriptome.

"Once further developed, we envision this method could be used in situations where other forms of immunotherapy do not work," said senior author Dr. Maurizio Zanetti, professor of medicine at the University of California, San Diego. "The advantages are that this type of treatment is localized, meaning potentially fewer side effects. It is longlasting, so a patient might not need frequent injections or infusions. And it would likely work against a number of different tumor types, including breast cancer, ovarian cancer, gastric cancer, pancreatic cancer, and hepatocellular carcinoma."

"Ideally, in the future we could test patients to see if they carry a deficiency in miR-335 and have an overabundance of SOX4," said Dr. Zanetti. "Then we would treat only those patients, cases where we know the treatment would most likely work. That is what we call personalized, or precision, medicine. We could also apply this technique to other microRNAs with other targets in cancer cells and in other cell types that surround and enable tumors."

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