Bettina Mittendorfer, Ph.D.
Project Overview:
The goal of this three-year LRP project is to evaluate the effect of dietary protein intake on cardiovascular health in people and to determine the physiological and cellular mechanisms involved. This project involves a new translational collaboration among several clinical and basic science investigators and will also leverage the resources of a newly initiated, NIH-funded (R01 DK121560; PI: Mittendorfer) randomized clinical trial that will evaluate the effect of high protein intake from different sources (animal and plant) on several key metabolic functions involved in the pathogenesis of type 2 diabetes. We hypothesize that high protein intake will increase blood concentrations of proatherogenic secondary gut metabolites, increase platelet aggregation, stimulate proatherogenic metabolic functions in blood monocytes, which are the precursors for atherosclerotic plaque, and impair vascular function (assessed as endothelial function and vascular compliance). We also hypothesize that the adverse effects of high protein intake will be greater with high protein intake from animal foods (e.g., meats and dairy) than from plant foods. The results from the proposed studies will increase our understanding of the influence of dietary protein on healthy (disease-free) aging and could identify novel biomarkers of cardiometabolic health. This issue has become particularly important because high protein intake and consumption of protein-fortified foods is now a popular trend.
Progress Report (Year 2):
Since the initiation of the award, we have completed a series of multi-disciplinary translational research projects that involved experiments in atherosclerosis-prone ApoE-/- mice, human monocyte-derived macrophages, and mechanistic assays in isolated primary macrophages to evaluate the effect of high protein intake on the pathogenesis of atherosclerosis. We found that a high-protein diet, compared with a standard Western diet, increased atherosclerotic lesion size in atherosclerosis-prone ApoE-/- mice. Moreover, high protein intake increased apoptosis and necrotic core formation in atherosclerotic plaques; these features are two characteristics of complex/unstable plaques that are most likely to rupture. We also found that the adverse effects of high protein intake on atherosclerotic plaques were mediated by the increase in plasma amino acids after high protein intake because amino acids increased mTORC1 signaling in macrophages and blocking mTORC1 signaling in macrophages prevented the development of atherosclerosis in mice fed a high protein Western diet.
We subsequently evaluated the dose response relationship between protein intake and this signaling pathway in both mice and humans. We found that the amino acid-mTORC1-autophagy mechanism translates to human monocytes-macrophages. We also found that there is a threshold effect of high protein intake on this deleterious signaling pathway. Dietary protein contents equal to or greater than about 22% of dietary energy content, an amount that is consumed by nearly 1/4th of the population in Western societies, acutely activates mTORC1 signaling in monocytes/macrophages in humans and causes atherosclerosis in mice (Figure 1).
We have now identified leucine as the critical amino acid modulator of this response. This has important implications, because the leucine content is greater in dietary proteins from animal than plant sources.
In addition, we have evaluated the effect of high protein meals on endothelial function. We found that high-protein but not standard meals reduce the reactive hyperemia index, a measure of endothelial function that correlates with disease progression and predicts cardiovascular events. The adverse effect of high protein intake was observed after consuming meals with both high protein content from animal and plant sources.
These results from these studies have contributed preliminary data for a collaborative (Razani/Mittendorfer) grant application to the NIH (R01 Dissecting the Impact of Dietary Protein on Macrophage mTOR Signaling and Atherosclerosis; pending) to further evaluate the effects of high protein intake on vascular health (both in people with atherogenic dyslipidemia and mechanistically in vitro and in vivo in mice). In addition, the data collected from these studies have provided preliminary data for two career development awards that have been submitted to the American Heart Association by Dr. Xiangyu Zhang, Research Instructor in the Cardiovascular Division at WashU, and to the Institute of Clinical and Translational Sciences at WashU by Dr. Jeremiah Stitham, Senior Research Fellow in the Cardiovascular Division at WashU.
Project Overview (year 3):
In Year Three, we will continue to perform metabolic testing to evaluate the acute and chronic (12-week diet intervention) effects of high protein intake from animal and plant sources on metabolic and cardiovascular function. We will also develop assays to examine the mechanisms involved in the adverse effects of high protein intake on postprandial endothelial function, which will include assays in cultured human umbilical vein endothelial cells and endothelial cells harvested from veins of study participants consuming high protein and standard meals. We also plan to submit a new R01 application to evaluate the effect of weight loss induced with a standard-protein calorie-reduced vs. a high-protein calorie-reduced diet on cardiovascular function in people with type 2 diabetes.
