As reported in Nature Genetics this week, the Myc protein can stop the production of at least 13 microRNAs, small pieces of nucleic acid that help control which genes are turned on and off.

Furthermore, in several instances, re-introducing repressed miRNAs into Myc-containing cancer cells suppressed tumor growth in mice, raising the possibility that a gene-therapy approach could be an effective therapy for treating certain cancers.

Andrei Thomas-Tikhonenko, an associate professor in the Department of Pathobiology in Penn's School of Veterinary Medicine, and a research team led by Joshua Mendell, assistant professor at the McKusick-Nathans Institute of Genetic Medicine at Johns Hopkins, analyzed more than 300 miRNAs in both human and mouse lymphoma cells.

Mendell's team had previously found that Myc could turn on one particular group of growth-promoting miRNAs called the miR-17-92 cluster in lymphoma cells. In those cells that had high amounts of Myc protein, the researchers found significant changes in the quantities of at least 13 miRNAs.

"The surprising aspect, considering our miR-17-92 results," Tsung-Cheng Chang and Duonan Yu, lead co-authors on the study, wrote, "is that lots of Myc turns everything off, not on."

When they looked closer at the DNA of the lymphoma cells, the team also found that Myc was directly attaching to the DNA at the miRNA genes.

"This was further evidence that the decrease in miRNA levels was directly due to the action of Myc," says Chang said.

"This study expands our understanding of how Myc acts as such a potent cancer-promoting protein," Mendell said. "We already knew that it can directly regulate thousands of genes. Through its repertoire of miRNAs, Myc likely influences the expression of thousands of additional genes. Activation of Myc therefore profoundly changes the program of genes that are expressed in cancer cells."

"Still, we needed to determine whether any of these Myc-regulated microRNAs played a direct role in cancer," Thomas-Tikhonenko said.

The Penn team individually reintroduced several of the repressed miRNAs into mouse lymphomas that also had high levels of Myc and measured the effect on lymphoma progression in animals. They found that at least five of the miRNAs could stop cancer growth.

"While this result was not entirely surprising, we had no idea that cancer suppression by microRNAs could be so powerful," Thomas-Tikhonenko said.

Mendell also notes that RNA-based therapies have had some success in animal models, and researchers might potentially find a wide range of miRNAs that can stop cancers in their tracks.

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For their studies, the Salk researchers turned to the fruit fly Drosophila, a powerful model organism, whose genetics can easily be manipulated. When initial experiments indicated that the expression of several autophagy genes decreased over the normal lifespan of fruit flies, the Salk researchers focused on one particular protein, Atg8a. This protein is an essential component needed for the formation of new autophagosomes. Finley and her team found that levels of Atg8a were significantly reduced by four weeks of age, a time when the flies are considered middle aged. At the same time, protein aggregates were not efficiently cleared by the cellular clean-up crew and started to accumulate.

Without Atg8a, things went from bad to worse. Damaged proteins tagged for degradation started to pile up early and life expectancy plummeted. The abnormal accumulation of protein aggregates had striking similarities to those seen in the most common human neurodegenerative diseases, says first author Anne Simonsen, Ph.D., a visiting scientist from the University of Oslo, Norway.

When the researchers kept the neuronal levels of Atg8a high, the genetically engineered flies were spared the ravages of time. Promoting the pathway not only prevented the accumulation of protein aggregates but also significantly extended the average lifespan. Our experiments show for the first time genetically that autophagy can sequester and eliminate misfolded and damaged proteins, which accumulate in neurons as normal part of the aging process, says Simonsen, but most importantly they demonstrate that enhancing the clearance of damaged proteins and protein aggregates increases longevity.

Insulin signaling and caloric restriction are two major determinants of longevity and they also impact the activity level of autophagy. Therefore, regulating autophagy, the pathway that directly does the cleanup work, may be the key factor in controlling the aging process, the researchers say. By maintaining the expression of a rate-limiting autophagy gene in the aging nervous system there is a dramatic extension of lifespan and resistance to age-associated oxidative stress, says Finley.

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