Cell growth discovery has implications for targeting cancer

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The way cells divide to form new cellsto support growth, to repair damaged tissues, or simply to maintain our healthy adult functioningis controlled in previously unsuspected ways UC San Francisco researchers have discovered. The findings, they said, may lead to new ways to fight cancer.

The steps leading a quiet cell to make and divvy up new parts to form daughter cells rely on some of the cell's most complex molecular machines. Different machines play key roles at different stages of this cell cycle. Each of these cellular machines consists of many proteins assembled into a functioning whole. They carry out such tasks as repairing DNA in the newly replicated gene-bearing chromosomes, for instance, or helping pull the chromosomes apart so that they can be allocated to daughter cells.

In a study published online on October 10, 2013 in the journal Molecular Cell, UCSF researchers led by molecular biologist Davide Ruggero, PhD, associate professor of urology, and computational biologist Barry Taylor, PhD, assistant professor of epidemiology and biostatistics, found that the production of entire sets of proteins that work together to perform such crucial tasks is ramped up together, all at oncenot due to the transcription of genes into messenger RNA, a phenomenon scientists often study to sort out cellular controlsbut at a later stage of gene expression that occurs within the cell's protein-making factories, called ribosomes.

"We have found that these proteins are regulated specifically and exquisitely during the cell cycle," Ruggero said. When this regulation falters, it wreaks havoc in the cell, he added. "Cell-cycle control is a process that is most often misregulated in human disease," he said.

More specifically, the researchers found that this coordinated timing of protein production during the cell cycle is largely governed at the tail end of gene expression, within the ribosome, where cellular machinery acts on messenger RNA to churn out the chains of amino acids that eventually fold into functional form as proteins.

In 2010 Ruggero reported key evidence suggesting that this stage of protein production, called "translation," might be an often-neglected process in many tumors, ranging from lymphomas, multiple myeloma and prostate cancer.

In the new study, the researchers examined translation of messenger RNA into protein at the classic phases of the cell cycle, before the cell actually divides. These are the G1 phase, when cells grow and make lots of proteins before replicating their DNA; the S phase, when cells replicated their DNA; and the G2 phase, when cells make internal components known as organelles, which they divvy up along with the chromosomes when the cell actually divides during mitosis.

The scientists used a technique know as ribosome profiling, originally developed for yeast cells in the lab of Jonathan Weismann, PhD, Howard Hughes Investigator at UCSF and professor of cellular and molecular pharmacology, to figure out which messenger RNA was being translated into protein by the ribosome during human cell division. They then used computational techniques developed by Taylor's lab team along with the lab team of Adam Olshen, PhD, professor of epidemiology and biostatistics, to better quantify which genes had been translated into proteins.

By conducting a genome-wide investigation of translation and interrogating the data with sophisticated computer algorithms, the researchers discovered that different groups of protein were made in abundance at a particular phase, only to be quieted during another phase of the cell cycle. Previous studies of translation of messenger RNA into protein focused on only one or just a few genes at a time, according to Ruggero and Taylor.

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Cell growth discovery has implications for targeting cancer