A newly discovered mechanism regulates cholesterol metabolism.

Gökhan Hotamisligil is on a mission to help us survive our affluence and its attendant cardiometabolic diseases. His prolific laboratory at the Harvard T.H. Chan School of Public Health and Sabri Ülker Center for Nutrient, Genetic, and Metabolic Research has recently generated another line of inquiry that could lead to treatments or prevention strategies for heart disease, stroke, and other disorders.

Led by postdoctoral fellow Scott Widenmaier, the team has found a “molecular guardian,” as Hotamisligil has dubbed it, which senses the toxic buildup of cholesterol within cells and triggers powerful protective responses.

The research, published last November in Cell, represents a natural outgrowth of studies that Hotamisligil has conducted for more than 25 years on metabolism, nutrition, obesity, and the immune system. A decade ago, this line of inquiry led him to the endoplasmic reticulum, a distribution center within cells where proteins are processed, stored, and packaged for delivery to their ultimate cellular destinations.

“We made a series of discoveries which told us that the endoplasmic reticulum may have a much bigger function in the cell, especially on metabolic health, and operate almost like an environmental surveillance device,” says Hotamisligil, who is chair of the Department of Molecular Metabolism, the J.S. Simmons Professor of Genetics and Metabolism, and director of the Sabri Ülker Center.

“Cholesterol is a two-edged sword,” Hotamisligil says. “It is absolutely necessary—you cannot live without it. At the same time, it is highly reactive and toxic, so you cannot have it floating around, either.”

Cholesterol’s vital roles

Hotamisligil’s discoveries have shed light on the molecular basis of obesity, diabetes, and other cardiometabolic disorders, as well as age-associated pathologies such as neurodegeneration. As he and his colleagues uncovered the role of the endoplasmic reticulum in protecting cells from shortages or surpluses of certain molecules, the researchers turned their attention to cholesterol. “One thing which dawned on us was a huge gap in the understanding of cholesterol homeostasis,” says Hotamisligil.

For all the bad press it gets, cholesterol is one of the most important molecules in the body, comprising 25 to 30 percent of our cell membranes and helping give them the perfect balance of stability and flexibility that animal cells need. But since evolution is a resourceful tinkerer, plentiful molecular actors like cholesterol have been drafted for many roles: hormone and vitamin production, digestion, signaling, and others. While most cells in the body can generate some cholesterol, the main source is the liver, and to a smaller extent, cholesterol-containing foods. With so much riding on cholesterol, the body has mechanisms to ensure that there is enough, including a sensor in the membrane of the endoplasmic reticulum called SREBP2, a protein that signals any need to boost cholesterol synthesis.

Too much cholesterol, however, can be toxic to cells, and through its various biological roles, excessive cholesterol contributes to a range of health disorders, the most well-known being atherosclerosis. When cells called macrophages reach their capacity for scavenging and recycling LDL cholesterol, (the “bad” cholesterol), the overburdened cells settle in the walls of blood vessels, seeding atherosclerosis.

“Cholesterol is a two-edged sword,” Hotamisligil says. “It is absolutely necessary—you cannot live without it. At the same time, it is highly reactive and toxic, so you cannot have it floating around, either.”

Scientists have known for a while that there are several mechanisms to reduce cholesterol levels. But it was clear that they didn’t tell the whole story.

A thoughtful approach to a big question

When he arrived in Boston from Saskatchewan, Canada, postdoctoral fellow Widenmaier immediately impressed the Hotamisligil lab with his scholarship. “He’s extremely meticulous in his thinking and reading, and very broadly knowledgeable,” says Hotamisligil.

Widenmaier’s first challenge was to define a path and identify a candidate for the role of “molecular cholesterol guardian.” He was looking for molecules that reside in the membrane of the endoplasmic reticulum—which is involved in stress responses to protect the cell—with structural elements that would allow binding to cholesterol. Such were the requirements for the job of a molecule to sense and defend against the harmful effects of cholesterol.

As Widenmaier was perusing the literature and databases one day, he came across a protein called nuclear factor erythroid 2-related factor 1 (Nrf1), which has regions that looked like docking sites for cholesterol. “That was my ‘Eureka!’ moment,” says Widenmaier. “I thought, ‘Well, maybe Nrf1 is a cholesterol sensor.’”

“When Scott treated cells with cholesterol, we saw this very nice, clear, and beautiful regulation of the NRF1 protein,” says Hotamisligil. “That was our first big moment.”

The second big moment was when they treated cells that lack Nrf1 and found that cholesterol accumulated in the cells, which eventually died. This set the stage for several years of work in which Widenmaier and his colleagues explored how Nrf1 plays the guardian role. “We had to find new reagents that could allow us to measure and look at Nrf1 activity,” says Widenmaier. “That took a lot of trial and error. Generating genetic models alone took almost two years.”

After Widenmaier’s Canadian Institutes of Health Research Fellowship ran out, Hotamisligil continued to support his work. “Gökhan thought there was potential in going forward and basically said, ‘I’ll keep you here as long as it takes to get this to where it needs to be.’ That is unique for a PI [principal investigator],” says Widenmaier. Hotamisligil returns the compliment: “Scott spent six years in my lab, and this remarkable discovery was the result. That requires extreme stamina and faith, and he has it.”

The team established that Nrf1 detects excess cholesterol to protect the endoplasmic reticulum and the cell that it resides in. It orchestrates a tamping down of inflammatory pathways through the targeting of a molecule called CD36 and also by activating other cholesterol-clearance systems that are normally kept inactive.

The model that Hotamisligil’s group has proposed is one in which NRF1 is on one end of a seesaw, with SREBP2 on the other. When cholesterol levels are low, SREBP2 boosts production, while NRF1 maintains the balance when cholesterol levels exceed safe limits. Together, these systems ensure a precise range of cholesterol concentration for the healthy functioning of the cells.

Could tweaking either end of this seesaw translate to treatments? “It’s very, very early. We have to do a lot of work to see whether it is going to add to the translational part,” says Hotamisligil. One auspicious discovery for human applications is that large-scale genetic studies have found the Nrf1 gene to be a contributor to human metabolic diseases.

“We now have handles to understand the entirety of cholesterol homeostasis, which is one of the most profound processes in human metabolism,” notes Hotamisligil. “I’m not saying we solved this problem, but we have discovered the other side of the coin. Controlling this lever carries great promise.”

—Hakon Heimer, MS, is a freelance editor and writer based in Providence, Rhode Island.

Photo: Kent Dayton/Harvard Chan