Heart disease a new perspective, can fats from bacteria in mouth be responsible for atherosclerosis?
For almost half century, we have blamed dietary fats for creating heart disease. We then created statin drugs to block the production of fats. Meta-analysis of many studies shows that statins completely failed to deliver the results and rather created a slew of side effects which compensated quality of life. Atherosclerosis is the congestion and hardening of the arteries with particular types of fats. A recent study published in the Journal of Lipid Research in August 2017 issue explains that the lipids which clog the arteries, causing atherosclerosis, might not be derived from our food but instead bacteria in our mouth.
Scientists performed an analysis of atheroma samples of patients. Atheromas are accumulations of bodily debris that occur when blood vessels become inflamed and thicken. The genomic signatures of these fats analyzed were from a specific family of bacteria known as Bacteroidetes. These bacteria make peculiar fat molecules that have uncommon fatty acids with branched chains and an odd number of carbons. Mammals do not normally produce lipids that look like these.
The researchers feel that the immune cells, which primarily attach to the blood vessel walls and gather the fat, view the bacterial fats as foreign from the body. Hence, the immune cells react to the bacterial fats, creating inflammation, and therefore atherosclerosis, the basis for heart disease and strokes.
A closer look at the research study
Summary
A carotid endarterectomy is a surgical procedure that fixes narrowing of the common carotid artery. When the researchers analyzed lipid extracts from human carotid endarterectomy and carotid artery samples from a group of young participants, they found some specific bacterial serine dipeptide lipids, including Lipid 654 and Lipid 430. The concentrations of Lipid 654 and Lipid 430 were also established in common oral and intestinal bacteria from the phylum Bacteroidetes as well as brain samples and human serum from healthy adults. The average Lipid 430 to Lipid 654 ratio for carotid endarterectomy samples was greater than the average ratio in lipid extracts of common oral and intestinal Bacteroidetes bacteria, as well as brain and serum samples from healthy participants. What they found was that the average Lipid 430 to Lipid 654 ratio was substantially elevated in carotid endarterectomies in contrast to the control artery samples. It was inferred that deacylation of Lipid 654 to Lipid 430 takes place in diseased artery walls because of phospholipase A2 enzyme activity. Therefore, the Bacteroidetes bacteria of the gut and the oral cavity might lead to the development of atherosclerosis via serine dipeptide lipid deposition and metabolism in arterial walls. (Nimadi, et al., 2017)
Background
Gum disease is often linked to elevated levels of specific Bacteroidetes species. Some of the predominant genera of the oral Bacteroidetes (phylum) include Porphyromonas, Prevotella, Tannerella, and Capnocytophaga. Porphyromonas gingivalis is known as one of the major disease-causing microorganisms for gum disease, which has also been associated with atherosclerosis in lab animal models. The oral Bacteroidetes can actually invade human arterial endothelial cells.
Many oral and intestinal Bacteroidetes bacteria produce serine dipeptide lipids, and the serine dipeptide lipids formed by P. gingivalis involve human and mouse Toll-like receptor (TLR)2. The serine lipids of P. gingivalis are: Lipid 430 and Lipid 654. Via Electrospray Ionization ESI-multiple reaction monitoring (MRM), the researchers conducting this study showed that Lipid 654 is present in human blood serum samples as well as lipid extracts of diseased periodontal tissues. An excess of Lipid 654 in human tissues indicates the presence of an external TLR2 ligand created by bacteria in the oral cavity or intestinal tract. Many animal studies have shown that TLR2 is a significant factor in the development of atherosclerosis. (Nemati, et al., 2017)
Findings of the Investigation
Lipid extracts from fourteen oral Bacteroidetes species and four intestinal Bacteroidetes species were analyzed for Lipid 430 and Lipid 654 along with lipid extracts from 12 carotid endarterectomies. The lipid extracts of human carotid endarterectomies were contaminated with Lipid 654. The Lipid 654 levels in lipid extracts of 10 carotid artery samples were quantified. The average amount of Lipid 654 per milligram of total lipid was four-fold less in carotid endarterectomy lipid samples in contrast to the control artery samples. The atheroma samples from carotid endarterectomy, on the other hand, had a Lipid 430 to Lipid 654 ratio that was at the bare minimum 10-fold higher than the control carotid artery samples. (Nemati, et al., 2017)
An analysis of the results
When the serine dipeptide lipids are created by oral and intestinal Bacteroidetes bacteria, they can be recovered in lipid extracts of carotid arteries, and even atherosclerotic artery samples. This study discovered that the Lipid 654 as a percentage of the total arterial lipid was four-fold less in the carotid endarterectomies in contrast to the control carotid artery samples. This could show that Lipid 654 has no significance in atherogenesis (a disorder of the artery wall). Nevertheless, the low level of Lipid 654 recovered in endarterectomy lipid extracts might be associated with the presence of triglycerides, cholesterol, and other atherogenic fats in the carotid endarterectomies. In comparison, if the atherogenic lipids and Lipid 654 get collected by chance, the Lipid 654 levels do not alter as a fraction of the total existing fat.
In this experiment, the average Lipid 654 level in endarterectomy samples lessened while the average Lipid 430 to Lipid 654 ratio arose in contrast to the control artery samples. This means that Lipid 654 became deacylated to Lipid 430, leading to a net loss of Lipid 654. The conversion of Lipid 654 to Lipid 430 was increased in carotid endarterectomies in contrast to carotid artery samples. Also, the average Lipid 430 to Lipid 654 ratio in Bacteroidetes is not consistent with the direct invasion of artery walls by live bacteria. If that was the case, the Lipid 430 to Lipid 654 ratios between carotid artery and bacterial samples would be comparable. It is also worth pointing out that the average Lipid 430 to Lipid 654 ratio in carotid endarterectomy samples was larger than the human brain samples, keeping in mind the vascularity of brain tissue. One can infer from the increased level of Lipid 430 compared to Lipid 654 noticeable in carotid endarterectomy samples that it is a conversion process in diseased arteries.
As a means to evaluate the enzymatic deacylation of Lipid 654 to Lipid 430, the scientists studied various enzyme preparations to understand the hydrolysis of Lipid 654 to Lipid 430. The enzymes were selected on the basis of their expression in activated macrophages or due to their link to atherosclerosis. Secretory PLA2 enzyme is known to become elevated in atherosclerotic arteries as well as with the formation of atheroma. An increased PLA2 expression could lead to an increased conversion of Lipid 654 to Lipid 430 in diseased arteries. The shift in the Lipid 430 to Lipid 654 ratio was most apparent in carotid endarterectomies in this study. Data demonstrates that Lipid 654 hydrolysis to Lipid 430 via PLA2 enzyme activity might promote atherogenesis through the involvement of Toll-like receptor (TLR)2.
Research shows that TLRs, especially, TLR2 and TLR4, play a significant role in the development of atherosclerosis. Lipid 654 does not involve TLR4. The contact of Lipid 654 with the artery walls is plausibly significant in atherogenesis for the following reasons: 1.) An increased TLR2 expression is seen at the site of atheromas; 2.) Genetically engineered mice deficient in LDL or ApoE receptor function have shown increased susceptibility to atherosclerosis which can be reversed if mice are simultaneously changed to TLR2 deficient; 3.) TLR2 involvement increases PLA2 expression in immune cells. The PLA2 enzymes have been associated with atherogenesis, but current research shows that pharmacological inhibition of PLA2 does not prevent severe cardiovascular events secondary to atherosclerosis. This study provides evidence that indicates that bacterial serine lipids accumulate in carotid arteries early onwards in life before significant atherogenesis is present and the conversion to Lipid 430 increases with atheroma formation, hinting greater PLA2 activity in atheromas.
Hence, as shown in this research investigation, Lipid 654 and related lipids are formed by many members of the phylum Bacteroidetes within the human microbiome, and in the scenario of the gastrointestinal tract, the biomass of Bacteroides species exposed to the host is crucial. Oral species in the Bacteroidetes play an important role in the buildup of serine dipeptide lipids in arteries, but the total oral Bacteroidetes phylum biomass is quite less than the gut. Preventing atherosclerosis using antibiotics will not be effective in atherogenesis. There have been no reports of dietary restrictions or modifications that lower colonization of the oral cavity or gut with Bacteroidetes.
Since serine dipeptide lipids of Bacteroidetes can be detected in endarterectomies and are metabolized in mammalian tissues, it might be worthwhile learning whether these lipids promote atherogenesis in experimental animals. Moreover, one must determine whether vascular and immune cells can metabolize these lipids and characterize the biological properties of these lipids on macrophages and cells of the vascular wall. Since these lipids appear in lipid extracts of carotid arteries in young persons, it implies that the process of bacterial lipid deposition is followed by consequent metabolism in life in link with the process of atherogenesis.
The expression of TLR2 in various cell types related to atheroma growth could change based on the locality of the deposited bacterial lipids. We know that bacterial serine dipeptide lipids can lead to the development of atherosclerosis. Further studies are required to determine how these lipids are actually carried and deposited in our arteries. (Nemati, et al., 2017)
References
Nemati, R., Dietz, C., Anstadt, E. J., Jorge, C., Liu, Y., Dewhirst, F. E., . . . Nichols, F. C. (2017, August). Deposition and hydrolysis of serine dipeptide lipids of Bacteroidetes bacteria in human arteries: relationship to atherosclerosis. Journal of Lipid Research, 58.