Primitive Yeast Yields Secrets of Human Cholesterol and Drug Metabolism
By first probing the way primitive yeast make cholesterol, a team of scientists has discovered a long-sought protein whose human counterpart controls cholesterol production and potentially drug metabolism.
The collaborative study by investigators at Johns Hopkins University School of Medicine, Vanderbilt University, Indiana University and Eli Lilly Co., was published in the February issue of Cell Metabolism.
"Dap1 controls the activity of a clinically important class of enzymes required for cholesterol synthesis and drug metabolism," says Peter Espenshade, Ph.D., assistant professor of cell biology at Johns Hopkins. "We're excited because although we originally identified this protein in yeast, humans not only have the same protein, but it works the same way."
The search for Dap1 began with the hunt for factors that influence the actions of a large family of enzymes called cytochrome P450. These enzymes control many life-sustaining chemical reactions in humans and other animals.
Happily, Espenshade says, yeast have only two P450 enzymes, and both play roles in making cholesterol, narrowing down the territory for their search and giving them a telltale marker (the cholesterol) to track.
Reasoning that whatever controls the P450s likely would be turned on and off at roughly the same time as the P450 enzymes themselves, the researchers found that Dap1 does just that in the yeast cell.
To figure out what Dap1 does, Espenshade and colleagues genetically altered yeast cells to lack Dap1. Those cells predictably were unable to make cholesterol and instead contained a build-up of cholesterol precursors.
The research team then tracked Dap1's counterpart in humans by looking for similar proteins in a computer database and repeated their experiments in human kidney cells engineered to lack the human version of Dap1. As in yeast, the altered human cells accumulated cholesterol precursors and died because cholesterol is essential for cell survival.
To show that Dap1 directly works with P450s and not through some other biochemical steps, Espenshade's team tested the ability of human Dap1 protein to bind to four of the 57 known human P450 enzymes, essentially challenging Dap1 to bind to P450s that perform totally different functions in different cells as a way to see how far-reaching its control might be.
Dap1 locked on to all four P450s, including one required for clearing half of all known drugs from the body, another involved in making bile and one required for making natural steroid hormones in the adrenal glands.
"Collectively, our experiments suggest that Dap1 acts as a common regulator of cytochrome P450s in mammals," says Espenshade.
Because Dap1 affects one particular P450 responsible for drug metabolism, Espenshade suspects that genetic variations in the genetic blueprint coding for Dap1 may provide clues to how and why different people react differently to certain drugs.
"Understanding the molecular underpinnings of so-called pharmacogenetic variation will have a big impact on the future of medicine," he says.
The research was funded by the National Institutes of Health, American Heart Association and Burroughs Wellcome Fund.
Authors on the paper are Adam Hughes and Espenshade of Hopkins; David Powell and Andrew Link of Vanderbilt University School of Medicine; Martin Bard of Indiana University-Purdue University Indianapolis; James Eckstein and Robert Barbuch of Eli Lilly and Company.