Profile of David J. Mangelsdorf

Two decades ago, David Mangelsdorf did not expect his basic research findings to help launch an entire field of biology with far-reaching implications for a raft of human diseases, including cancer, metabolic disorders, and parasitic infections. Mangelsdorf, a Howard Hughes Medical Institute investigator and a professor of pharmacology at the University of Texas Southwestern Medical Center at Dallas, began his career by cloning the cellular receptor for vitamin D, which later helped other researchers uncover the genetic basis of osteoporosis and rickets (1, 2). Insights from that initial work veered Mangelsdorf in a direction that spawned the field of orphan nuclear hormone receptors.

Raised in Kingman, a sunswept town in the Mojave Desert of northwestern Arizona, Mangelsdorf says his love of nature preceded his love of science. Although much of his work has focused on molecules hidden in cells, his interest in biology began as a passion for marine life in open oceans. Impelled by the breathtaking imagery of French naval explorer Jacques Cousteau’s films of marine life, Mangelsdorf decided to become a marine biologist. “During my sophomore year in high school, I wrote to The Scripps Institution of Oceanography [in San Diego] to explore the option of marine biology as a career. I naively thought I could go there straight from high school, but Scripps did not take undergraduate students,” he says. However, Mangelsdorf learned that one of the best schools in the United States for undergraduate aquatic biology, Northern Arizona University at Flagstaff, was a mere 150 miles from his hometown. In the fall of 1977, he enrolled in the school’s aquatic biology program.

While taking a beginner’s course in chemistry to fulfill the program’s requirements, Mangelsdorf met one of his early mentors, John Wettaw, a chemistry professor who introduced him to biochemistry. During the early 1980s, researchers were making strides in the march against cancer. Smitten with a 1980 Time magazine cover story on the promise of interferon treatment for cancer, Mangelsdorf soon found that his fascination with biochemistry supplanted his passion for marine biology. After graduating with a bachelor’s degree in aquatic biology in 1981, he decided to spend the next 6 years in graduate school at the University of Arizona in Tucson, studying biochemistry in the laboratory of Mark Haussler, who discovered the hormonal form of vitamin D called calcitriol. “It was Mark Haussler who really changed my mind about marine biology. I fell in love with his work,” Mangelsdorf says.

When Mangelsdorf was casting about for a doctoral project upon his arrival at Haussler’s laboratory, Haussler offered him a choice: biochemically identify the hypothetical vitamin A receptor or clone the gene for the biochemically identified vitamin D receptor. Despite the allure of the unknown in the former, Mangelsdorf picked the latter. Armed with antibodies to the vitamin D receptor, Mangelsdorf set out to clone the gene for the receptor by using a then-novel, bacteriophage-based cloning technique described in a now highly cited PNAS paper (3). A fruitful collaboration with a graduate student in the group of Baylor College of Medicine endocrinologist Bert O’Malley culminated in a 1987 paper in Science describing the drawn-out cloning of the chicken gene for the vitamin D receptor (4). Mangelsdorf’s PhD thesis won the biochemistry department’s dissertation of the year award.

The award was not the only recognition that Mangelsdorf’s graduate work garnered. In 1985, NASA accepted Mangelsdorf’s proposal to conduct experiments with rodents aboard the Challenger spacecraft to study the receptor’s role in the alarming loss of bone density suffered by some astronauts returning from space. Mangelsdorf’s idea was to compare the expression of the receptor’s gene in the kidneys of rats sent into orbit with that of rats kept on Earth. “Unfortunately, it turned out that the space shuttle had to sit on the ground for almost 12 hours before we could harvest the rats’ kidneys. By that time, whatever happened in space might have been reversed by being back on Earth,” he recalls. “Nevertheless, I got to watch the space flight, send the rats up, and keep the NASA mission patch,” he chuckles.

The NASA experiment may have led nowhere, but Mangelsdorf’s work bore fruit in bigger ways. Because the vitamin D receptor controls the excretion of calcium by the kidneys, Mangelsdorf’s graduate work held medical promise (5). “Until you had the sequence for the receptor’s gene, it was impossible to understand the genetic basis of diseases like vitamin D-resistant rickets, which is caused by mutations in the receptor’s ligand-binding, DNA-binding, and dimerization domains,” Mangelsdorf explains. “The cloning of the receptor was the biggest thing in the field since the discovery of the hormonal form of vitamin D,” he adds, proudly.

Testing Uncharted Waters

Around the time Mangelsdorf announced the cloning of the vitamin D receptor, Salk Institute molecular biologist Ronald Evans cloned the glucocorticoid receptor, an important drug target in asthma and arthritis that is ancestrally related to the vitamin D receptor. “Ron Evans published the idea of a family of proteins of which the receptors for vitamin D, vitamin A, glucocorticoids, and mineralocorticoids were members,” Mangelsdorf says. Like a returning ghost, the vitamin A receptor, whose biochemical characterization he had passed up in Haussler’s laboratory at the start of his doctoral project, came hauntingly back. “This time, I wanted to identify the vitamin A receptor, and I thought his lab was the best place to work on it,” he adds. In 1987, Mangelsdorf joined Evans’s laboratory as a postdoc.

Members of Evans’s laboratory had been frantically cloning receptors of this family when Mangelsdorf decided to work with Evans. Progress came so swiftly that another of Evans’s postdocs had identified the vitamin A receptor by the time Mangelsdorf joined the laboratory in the fall. What could have been an unfortunate development for Mangelsdorf proved to be a propitious turn of fate; his gift for self-invention helped him forge a new field of research.

Mangelsdorf embarked on a quest to discover unknown receptors in the slowly growing superfamily. To find new receptors involved in hepatic function, Mangelsdorf sifted through a genetic library of DNA sequences expressed in liver cells by using as a probe the DNA-binding domain of the receptors that had already been cloned. The effort yielded more than a dozen potential candidates for receptors of mysterious function. Because the binding partners for these receptors were unknown at the time, Mangelsdorf called them “X-Rs,” a name that later morphed into “orphan receptors.” “This was a rich area of exploration, a golden age for receptor discovery. We cloned and stockpiled the receptors, calling them X-R 1, 2, 3, and so on,” Mangelsdorf says. “At that time, having just the sequence for one of these receptors gave you a Nature or Science paper,” he adds. Mangelsdorf picked one of the orphan receptor candidates for further study.

Mangelsdorf says serendipity, perspiration, and inspiration provide the narrative for much of what followed. “In retrospect, of all the candidates I found, I was lucky to have picked what turned out to be the grand prize, the one we called RXR, or retinoid X receptor,” he says. Using a seminal assay developed in Evans’s laboratory to identify ligands for orphan receptors, Mangelsdorf found that the vitamin A metabolite, 9-cis retinoic acid, was RXR’s natural ligand (6); the study was carried out together with researchers from California-based Ligand Pharmaceuticals, a spin-off from Evans’s studies that Mangelsdorf named. “That was the first demonstration that a compound that was not known to work through one of these receptors indeed did so,” Mangelsdorf says. “It truly opened up the field of orphan receptors.”

The discovery of RXR’s ligand meant an array of therapeutic applications for the receptor. The finding suggested that vitamin A, which was then being tested as an experimental cancer drug, likely acted through RXR, rendering the receptor a therapeutic target for cancer. Further, Mangelsdorf’s collaborators found that RXR paired up with other nuclear receptors to control many endocrine pathways, pointing to the receptor’s role in diabetes and metabolic syndrome (7).

Mangelsdorf’s trailblazing findings on orphan receptors established his reputation and earned him a faculty position at the University of Texas Southwestern Medical Center in Dallas. At a 1991 pharmacology conference organized by Johnson & Johnson, Mangelsdorf met Alfred Gilman, then chairman of the university’s pharmacology department, who discovered G protein-mediated signaling in cells. That meeting paved the way to Mangelsdorf’s independent academic career. “Gilman came to me and said, ‘The only reason I’m at this meeting is to invite you to come to UT Southwestern to give a talk for a job,’” Mangelsdorf says. Although he had applied to a handful of universities across the United States, Mangelsdorf had never considered going to Dallas. However, he knew Dallas was home to two Nobel Prize-winning pioneers in lipid biology, Michael Brown and Joseph Goldstein, whose work on cholesterol metabolism partly overlapped with his own research on nuclear receptors. “When I saw Dallas—the city and the university—it went from not even being on my radar to the top of my list. The environment for fostering scientific potential was astounding,” Mangelsdorf says.

Receptors Aplenty

At UT Southwestern, Mangelsdorf continued to mine the mother lode of receptors he had cloned in Evans’s laboratory. He picked one, called LXR, which he later found was activated by cholesterol metabolites called oxysterols (8). To Mangelsdorf, the finding seemed a mixed blessing: Although he realized he was in a suitable place to study a receptor triggered by a sterol—UT Southwestern was a veritable mecca for aficionados of lipid metabolism—he recalls, “It was a goldmine, but I was nervous because Brown and Goldstein were working on similar things.”

Part of what propels Mangelsdorf is his drive, discipline, and doggedness. However, it was his knack for turning stumbling blocks into stepping stones that helped him broker a partnership with Brown and Goldstein instead of competing with them. That partnership, in turn, led to the discovery that LXR is the master regulator of a transcription factor called SREBP1, uncovering an unknown regulatory interplay between cholesterol and fatty acid metabolism (9). The discovery also helped vault LXR to the vanguard of investigational drug treatment for cholesterol-related diseases. Because LXR helps removes cholesterol from macrophages by turning on the gene for a cholesterol transporter protein (10), the reasoning goes, compounds stimulating the receptor could help slow the progression of coronary heart disease, which is caused by an inflammatory reaction to cholesterol-laden macrophages in the arteries.

Mangelsdorf’s contributions to orphan receptor research earned him the coveted 1997 John J. Abel Award from the American Society for Pharmacology and Experimental Therapeutics, an accolade historically recognized as a harbinger of higher honors.

Yet another receptor from Mangelsdorf’s repertory became the focus of the next few months of his research. Mangelsdorf teamed up with his erstwhile competitor Steven Kliewer, a former postdoc of Evans whom Mangelsdorf helped lure from GlaxoSmithKline to UT Southwestern, to study the role of FXR, or the bile acid receptor, in lipid metabolism. “Steve came to the department as a full professor, already renowned for his work on nuclear receptors, without a single research tool because he was returning from industry. I opened up my lab to him, and we started collaborating,” Mangelsdorf says.

The collaboration helped researchers understand how FXR and LXR use a byzantine signaling network to control lipid metabolism immediately after we eat a meal. During a meal, cholesterol activates LXR, which drives the activity of genes that regulate two metabolic pathways—one that turns lipids and sugars into stored fat, and another that removes cholesterol by helping excrete most of it and turning the rest into bile acids. After the meal, most of the bile acids, which help digest food, are recycled to the liver. During this process, some bile acids latch onto FXR in intestinal cells. The binding of bile acids to FXR triggers the synthesis of a hormone, called fibroblast growth factor 15, which turns off both bile acid synthesis and the LXR target genes in the liver, thus closing a complex regulatory loop and resetting the system for the next meal (11).

Mangelsdorf’s findings on this tortuous feedback loop won him the 2004 Gerald D. Aurbach Award from the Endocrine Society—not least because the findings pointed to a seemingly endless array of therapeutic possibilities. First, compounds activating LXR could help prevent and treat atherosclerosis by hastening the removal of cholesterol from the body (10, 12, 13). Next, Mangelsdorf and others showed that an experimental compound that activated RXR, the receptor he had characterized as a postdoc in Evans’s laboratory, could help treat metabolic syndrome by stimulating FXR, LXR, and PPARs, all of which are dimerization partners of RXR (10, 14, 15). Further, Mangelsdorf published a 2004 Nature Medicine report showing that a compound that stimulated FXR could help curb cholesterol gallstone disease, a painful condition caused by cholesterol crystals in the gall bladder (16). In addition, his work on the vitamin D receptor showed that compounds that activate the receptor could help neutralize potentially cancer-causing bile acids, such as lithocholic acid, providing a likely explanation for the protective effect of vitamin D against colon cancer (17). Add to these therapeutic targets a slew of cellular processes that could be modulated by directly targeting PPAR, which controls aspects of cell division and metabolism (18, 19).

In 2000, Mangelsdorf cofounded a California-based biotechnology firm, X-Ceptor Therapeutics Inc., now a part of Exelixis, to discover compounds modulating nuclear receptors. Whereas a few of X-Ceptor’s experimental compounds failed to result in drugs because of side effects, others are being tested in clinical trials. “In general, the problem with nuclear receptors is that potent activators of the receptors can do both good and bad. What we want are modulators to tickle the receptors just enough to do the good but not the bad,” Mangelsdorf says. “That concept is just beginning to be appreciated.”

The Parasite Puzzle

Another concept just being appreciated is the role of orphan nuclear receptors in parasitic infections. Although it was known that invertebrates had the receptors, their function in this group of animals had long remained a mystery. Mangelsdorf identified the hormonal ligand for a receptor dubbed DAF-12 in the free-living roundworm Caenorhabditis elegans, a finding that not only shed light on the evolution of hormonal pathways in animals but unearthed a potential drug target for parasitic diseases, such as hookworm and threadworm. “Of all the things I’ve done, I find this to be one of the most exciting—if not the most exciting,” Mangelsdorf says. Whereas mammals have 48 different nuclear receptors, and fruit flies have about 20, C. elegans harbors 284 such receptors, a bewilderingly large number for a tiny worm. When Mangelsdorf first learned of the worm receptors, not a single one had a known ligand, let alone well-defined functions. “That was too tempting a target to go after especially because C. elegans is an experimental system used by most life scientists. Because I was a ligand hunter, I wanted one of those as a notch in my belt,” he says. Mangelsdorf knew that the worm needed cholesterol to pass through a phase of dormancy in its life cycle called dauer diapause, suggesting the involvement of a nuclear receptor triggered by a steroid. What was more, the DAF-12 gene showed telltale sequence similarities with LXR and FXR.

In a 2006 Cell paper, Mangelsdorf and his graduate students described how steroid ligands of DAF-12, called dafachronic acids, help the worm resume metabolism and reach reproductive maturity after diapause; overcrowding, food scarcity, and harsh temperatures drive the worm toward diapause (20). “Dauer diapause is one of the reasons for the longevity of these worms,” Mangelsdorf says.

Mangelsdorf’s findings on the regulation of the worm’s fasting response by DAF-12 harkens back to the roles of LXR and FXR in controlling dietary cholesterol metabolism in people, underscoring the evolutionary conservation of this hormonal pathway among animals. More importantly, the findings paved the way for research that identified DAF-12 as a drug target in parasitic nematode infection, a worldwide scourge that afflicts more than a billion people. “The infectious stage of many parasitic nematodes resembles the C. elegans diapause—nonreproductive, nonfeeding, and long-lived. Something in the nematodes’ hosts helps the worms exit this stage and reach reproductive maturity. Our hunch was that it was the activation of DAF-12,” Mangelsdorf says.

In his PNAS inaugural article, published in June 2009, Mangelsdorf proved his hunch correct: he demonstrated that administering dafachronic acid to the threadworm Strongyloides stercoralis noticeably reduced the number of pathogenic worms; the ligand activated DAF-12, nudged the slumbering worms out of diapause, and disrupted the worms’ normal development (21). “Parasitic nematodes are estimated to infect nearly one third of the human population, and there is no known drug that attacks their infectious stage,” Mangelsdorf says. Because DAF-12 controls both entry into and exit from diapause, parasitic worms would likely need more than one resistance-conferring mutation to survive drugs targeting DAF-12. “That’s why I think it’s a huge therapeutic target,” he says.

The long list of physiological roles for nuclear receptors bolsters Mangelsdorf’s belief that building profiles of their synthesis and activity for individual patients could have therapeutic benefits. “Our work on orphan receptors has demonstrated that they work as a group, not individually, to regulate physiology. They seem to work as a hierarchical transcriptional network that controls metabolism,” he says. Profiling this network in cancer patients, for example, could provide biomarkers that could not only help diagnose cancer but also direct treatment.

Mangelsdorf says the high point of his career was his election in 2008 to the National Academy of Sciences. “The election is an award that was given for my entire career, which is shaped by all the people who stand behind me. I am most grateful for the election because it acknowledges the importance of the work,” he says.

Over nearly three decades, Mangelsdorf has illuminated the once-obscure world of orphan nuclear hormone receptors. However, lofty goals, he says, lie ahead: “Al Gilman always said one of his greatest desires was for the pharmacology department at UT Southwestern to discover a drug. My goal is to be the first to discover a widely used drug that goes from the department into the clinic.”

David J. Mangelsdorf.