Pumping up

Researcher probe the diverse roles of mTOR proteins in growth, cancer and bodybuilding

Scientists from David Sabatini's lab gather in an MIT gym.

Photo: John Soares

Shomit Sengupta and Jan Reiling are flipping through a fitness magazine purchased as a gag gift when an ad catches their attention. A massive bodybuilder in a tank top stands behind a lab bench with a distinguished-looking man in a white lab coat.

“Genetic limitations are a thing of the past,” reads the headline. “Anator-p70 turns on the three major muscle-building master genetic regulators, mTOR, PKB, and p70^S6K.”

“We’ve been scooped,” joke the two graduate students, who work in Whitehead Member David Sabatini’s lab, which is largely devoted to the study of mammalian TOR (mTOR). This protein serves as a kind of traffic cop, allowing cell growth and proliferation to proceed when amino acids and growth factors are abundant and blocking these processes when nutrients are scarce. But how nutrients regulate mTOR signaling to control size remains a major mystery—to academic scientists, anyway.

According to the ad, Anator is the culmination of five years of research by Team MuscleTech and represents a “scientific breakthrough the likes of which the supplement world has never before seen.” The product purports to activate mTOR and other growth regulators with special ingredients called LeuciGene, PhenylGene and GeneTOR. The ad even includes a diagram of the mTOR signaling pathway to illustrate how these ingredients work.

Had Team MuscleTech solved the mystery that has eluded Sabatini and other mTOR experts for a decade? Had the supplement creators parsed out some missing players upstream of mTOR and unlocked the black box of nutrient sensing?

Probably not.

A close examination of the Anator label reveals that “LeuciGene” contains several derivatives of the amino acid leucine, which is already known to activate mTOR. Just to be safe, Sabatini lab technician Robert Lindquist hikes down to the local GNC and asks for some Anator. The salesman announces he’s in luck. A shipment has just arrived.

Back at Whitehead Institute, Lindquist and several of his equally lean colleagues engage in a brief workout and then prepare Anator shakes in the third-floor lounge. In addition, graduate student Yasemin Sancak tests the product on cells in culture. Results: Anator does activate mTOR, but the amino acid leucine works just as well.

What a relief!

“I’ve used this pseudoscience in talks to joke with competitors, and some do start to look nervous until they see the ad,” says Sabatini.

Sabatini has devoted much of his career to TOR. At Johns Hopkins in the mid-1990s, he led one of the teams that discovered the mammalian version of the protein while working on rapamycin, a drug that helps prevent organ rejection in transplant patients. Sabatini found that the drug works by blocking a previously unknown protein, which was eventually dubbed mTOR (for mammalian target of rapamycin).

Supersize me

Labs soon showed that mTOR serves as an important signaling hub, using information about the environment to regulate cell growth. This surprised many scientists, who assumed that all the major mysteries of metabolism had been solved by the 1930s, at which point biochemists had teased apart the details of the Krebs cycle and other major metabolic pathways.

Whitehead Member David Sabatini Photo: Justin Knight

In essence, the discovery of mTOR’s function stirred up a stagnant area of research, prompting scientists to reexamine how cells and organisms use energy and nutrients to grow (and providing fodder for protein supplement marketers).

“Our work brings us back to one of the most interesting, and obvious, questions out there,” says Sabatini. “How does biology regulate size?”

As it turns out, mTOR is likely involved at all levels, regulating size for cells, organs and organisms. Drosophila and mice with low levels of mTOR signaling are much smaller than usual. Their constituent cells are also smaller.

Thus mTOR research could eventually explain why organs change size in response to environmental cues or why mice are so much smaller than humans, despite the fact that we share thousands of the same genes. It could also provide new insights into diseases such as diabetes, which is characterized (in part) by abnormal metabolism.

A cellular relay race

Think of mTOR as the anchor runner in a complex relay race. It cannot sense nutrients and growth factors directly. Instead, it relies on other proteins, or runners, to gather information from the cellular environment and pass it along like a baton. Each baton changes hands a number of times before ending the race at mTOR. Given the right combination of batons, or signals, mTOR recognizes that conditions are optimal for growth and instructs the cell to make more proteins.

Over the past 10 years, Sabatini’s lab has identified some of the runners and batons, though many remain a mystery. In 2002, for example, Do-Hyung Kim (now a faculty member at the University of Minnesota) published a paper in Cell showing that mTOR typically sidles up to a protein called raptor. Without this essential binding partner, codependent mTOR loses its ability to sense nutrients and promote growth.

More recently, graduate students Yasemin Sancak and Carson Thoreen discovered that a protein called PRAS40 lounges on raptor, keeping mTOR’s binding partner in check. But it springs out of the way when levels of the hormone insulin rise. Insulin circulates through the body when an animal is well fed, instructing cells to absorb and store glucose. Thus PRAS40 allows raptor and mTOR to foster protein production when nutrients are abundant.

“The intricacies of this particular pathway are astonishing,” says Sancak. “And we’re just beginning to appreciate how mTOR interacts with a myriad of other pathways related to cell growth,” adds Thoreen.

The cancer connection

Sabatini’s lab dropped a bombshell in February 2005, when it reported on an unexpected role for mTOR in the journal Science. Dos Sarbassov, now a principal investigator at the University of Texas Medical School, showed that mTOR activates Akt, a prominent cancer protein involved in cell proliferation.

Researchers had overlooked this connection because they had focused on raptor, which enables the drug rapamycin to target mTOR. Scientists didn’t realize that mTOR sometimes “cheats” on raptor by cozying up to a different protein called rictor. When mTOR binds to rictor, it generally becomes immune to rapamycin, and assumes different functions.

“The first TOR complex regulates the size of a cell, while the second regulates cell division and cell survival,” explains postdoctoral researcher David Guertin. “Both complexes use information from the cellular environment to make decisions about growth, but the function of the second complex may be more closely linked to human cancers.”

Sarbassov relied on biochemical techniques to interfere with mTOR in a Petri dish, so some scientists remained skeptical of its cancer-causing role in animals. Guertin erased their doubts by knocking out rictor in mice and showing that Akt activity dropped significantly. His results appeared in Developmental Cell in December 2006.

“The discovery greatly increased the interest in the field, because many tumors exhibit deranged Akt signaling,” says Sabatini. “Many labs and pharmaceutical companies are now searching for ways to inhibit the second mTOR complex.”

Sabatini lab researchers are tackling this problem too. But they’re thinking more holistically about the connection between metabolism and cancer. Scientists have known for decades that animals live longer and develop fewer tumors when they’re fed low-calorie diets. mTOR may offer a mechanistic explanation.

“Research on mTOR in mice could help us connect the dots between metabolism and cancer,” explains postdoctoral researcher Nada Kalaany. “We may have found the missing link.”

And it turns out that the two pathways involving mTOR intersect. The rictor/mTOR complex runs before the raptor/mTOR complex in the giant cellular relay race. Thus the pathway that pumps you up by increasing cell mass coordinates with the pathway that controls cell division.

“I never thought that the work on rapamycin would lead to a new field,” remarks Sabatini. “It’s been gratifying for me to be part of something that is having an important impact on both our basic understanding of biology and our treatment of disease.”