An article recently published in Scientific Reports explored how endothelial cells (ECs) absorb heme using Raman imaging. The study provided new insights into the physiological mechanisms that counteract the inflammatory effects of labile heme released from damaged red blood cells (RBCs). The researchers demonstrated heme uptake by ECs in both in vitro and ex vivo systems, highlighting its potential impact on cardiovascular health.
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Background
Heme, a component of hemoglobin (Hb) in RBCs, is released into the bloodstream when RBCs are damaged or age. Free heme can cause oxidative stress and inflammation, leading to cell damage.
ECs, which line blood vessels, play a crucial role in cardiovascular health by regulating blood flow, blood pressure, and nutrient exchange between blood and tissues. They also produce signaling molecules like nitric oxide and prostacyclin to maintain vascular balance. However, EC dysfunction can contribute to diseases such as atherosclerosis and hypertension.
A newly discovered function of ECs is their ability to uptake labile heme, which helps reduce the inflammatory effects of free heme and supports vascular health. Raman spectroscopy, a non-invasive technique that analyzes molecular structures through inelastic light scattering, is widely used to study molecular properties and detect specific biomolecules in cells and tissues.
About the Research
The authors aimed to investigate heme uptake by ECs using Raman imaging, a label-free technique that allows for in situ observation of molecular processes. They used human aortic endothelial cells (HAECs) as the cellular model and conducted experiments on both healthy and atherosclerotic mice. The concentration of labile heme was carefully adjusted to avoid toxicity.
Raman imaging was performed using two excitation wavelengths (488 nm and 532 nm) to capture heme uptake and analyze the oxidation state of the iron ion in heme.
In vitro experiments involved incubating HAECs with Hb lysate to track labile heme accumulation. Ultraviolet-visible (UV-Vis) absorption spectroscopy measured the concentration of Hb lysate, while Raman spectroscopy observed heme uptake.
Ex vivo experiments used murine aortic tissues, which were isolated, sectioned, and subjected to Raman imaging. K-means Cluster (KMC) analysis was used to process the Raman spectra and identify the distribution of heme within the cells and tissues.
Research Findings
The study showed that HAECs could uptake heme from Hb lysate, confirmed through Raman imaging. This uptake occurred in both healthy cells and cells affected by atherosclerosis, with heme primarily in its ferric state, which is linked to inflammation. Raman spectra revealed distinct bands for heme, allowing clear identification from other cellular components.
In vitro experiments indicated that heme uptake did not significantly affect HAEC shape or viability, even at higher Hb concentrations. Raman imaging showed that heme was mainly localized in the cytoplasm. Ex vivo experiments further demonstrated heme absorption in both the endothelial and subendothelial layers of the aorta, suggesting it can penetrate deeper into vascular tissue, potentially contributing to inflammation and tissue damage.
The study also highlighted the benefits of different excitation wavelengths for Raman imaging. The 488 nm wavelength was better at distinguishing between heme and cytochrome C (CytC) in cells, while the 532 nm wavelength was more effective for tissue imaging.
Heme accumulation in the endothelium was also associated with foam cell formation and activation of inflammatory pathways, key features of atherosclerosis.
Applications
This research has significant implications for understanding the role of heme in vascular health and disease, especially its uptake by ECs and the resulting oxidative stress and inflammation. Insights into these mechanisms could help develop therapies to mitigate the harmful effects of free heme in cardiovascular diseases like atherosclerosis.
Targeting heme uptake may offer a promising therapeutic approach, and the authors highlight the potential of Raman imaging as a diagnostic tool for detecting vascular inflammation and injury.
Conclusion
This study provided the first Raman-based evidence of heme uptake by ECs in both in vitro and ex vivo systems, demonstrating the potential of Raman imaging for studying molecular processes. The findings underscored the role of heme uptake in reducing oxidative damage and inflammation in vascular tissues.
Future research should explore the therapeutic potential of targeting these pathways to protect against cardiovascular diseases, with the aim of developing new strategies for enhancing vascular health by understanding the pro-inflammatory properties of ferric heme.
Journal Reference
Wajda, A., et al. (2024). Raman imaging unveils heme uptake in endothelial cells. Sci Rep. DOI: 10.1038/s41598-024-71600-2, https://www.nature.com/articles/s41598-024-71600-2
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