In the Jan. 28 issue of Cell, scientists led by Bruce Spiegelman, PhD, report that the harmful effects of saturated and trans fats are set in motion by a biochemical switch, or co-activator, in liver cells called PGC-1beta.
Until now, scientists lacked a detailed explanation of how saturated and trans fats caused an increase in blood cholesterol and triglycerides, while diets high in unsaturated and polyunsaturated fats did not. Evidence pointed to the liver, which is responsible for the body's synthesis of triglycerides and cholesterol, but the molecular chain of events from eating fats to the buildup of cholesterol and triglycerides in the blood were unknown.
"What we have found is a missing link, a mechanism by which saturated fats and trans fats can do their dirty work," said Spiegelman, who carries out basic research on fat cells and metabolic pathways in diabetes and cancer at Dana-Farber. "It offers the opportunity to try to understand what makes these fatty acids so deleterious, and what we need to avoid."
Moreover, it is possible that in the future, drug therapy might be used to "turn down" the mechanism, decreasing cholesterol levels and heart disease risk, explained Spiegelman, who is also a professor of cell biology at Harvard Medical School.
Saturated fats are found in fatty cuts of meat, whole-milk dairy products, butter, and palm and coconut oils; they are associated with higher risk of coronary disease. The healthier polyunsaturated fats are those that remain liquid at room temperature, such as various types of vegetable oils.
Trans-unsaturated fatty acids, or trans fats, are artificially prod uced solid fats used in shortening and margarine, baked goods, and the oils used to cook french fries and other fast food. Studies have shown that trans fats not only raise LDL levels in the bloodstream but lower high-density lipoproteins (HDL, or "good" cholesterol) and may have even stronger adverse effects than do saturated fats.
The researchers report that when activated by harmful fats, PGC-1beta alters liver metabolism through a cascade of biochemical signals. The result is an upsurge in the liver's production of very low-density lipoprotein (VLDL) cholesterol, the precursor of low-density lipoprotein (LDL) cholesterol (known as the "bad" form of cholesterol) and triglycerides ?another fatty substance ?that are secreted into the bloodstream.
PGC-1beta belongs to a specific family of co-activators, proteins that interact with other proteins to turn genes on and off and adjust their activity, like a dimmer switch that varies the brightness of a light. Co-activators join with other regulatory proteins called transcription factors in controlling the expression of genes. Spiegelman and Jiandie Lin, PhD, the paper's lead author and a researcher at Dana-Farber, have previously discovered the PGC-1 co-activator family and several of its biochemical activities.
The Dana-Farber researchers made the discovery in searching for the function of PGC-1beta co-activator that was isolated in 2002. Experiments including the measurement of gene activity by microarrays showed that saturated and trans fats caused greater activity of the gene that makes PGC-1beta co-activator than did unsaturated fats.
The research also showed that when the fats triggered PGC-1beta, the co-activator interacted physically and turned up the function of sterol responsive element binding proteins. These important parts of the mechanism activate many key genes of lipid biosynthesis involving the pathways of cholesterol and triglycerides; these genes directed the liver to m anufacture more cholesterol, which it does in the form of very low-density lipoproteins. The investigators noted that in mice fed high-fat diets, the PCG-1beta mechanism actually decreased cholesterol in the liver while increasing it in the bloodstream.
"Before this report, it wasn't clear what the differences were between saturated fats and unsaturated fats in their ability to raise cholesterol blood levels," commented Jeffrey Flier, MD, an obesity specialist at Beth Israel Deaconess Medical Center. "These are important findings in a long-established area of medicine."