In Aging Cell, researchers have linked macrophage senescence to the failure of new blood vessel formation, finding a key target that might make it easier to treat arterial clogs.
Macrophages can impair blood vessel formation
Heart attack and stroke are not the only problems caused by clogged and unusable blood vessels; when this occurs in the extremities, it is known as peripheral arterial disease (PAD), which affects roughly 113 million people around the world [1]. It is possible to fix some cases of this through surgery, but conducting this kind of surgery in older people can be a dicey prospect [2]. Ideally, it would be possible to encourage the body to restore these blood vessels itself [3], but this approach has seen little success, as it is hampered by the processes of aging [4].
Previous research has pointed out clues as to why. Macrophages normally encourage the formation of new blood vessels (angiogenesis) [5], but these, like other cells, gradually become senescent [6], leading to a wide variety of other downstream disorders. PAD is characterized by changes in vascular endothelial growth factor A (VEGF-A), specifically a decrease in one of its isoforms, VEGF-A165A, and an increase in another isoform, VEGF-A165B [7]. As macrophages are responsible for this molecule, these researchers decided to determine if macrophage senescence was causing this shift.
When one cell’s senescence harms another
The researchers’ first experiment was a basic analysis of macrophage senescence. Here, everything was entirely as expected: the macrophages in the skeletal muscle of old mice were less able to proliferate, expressed more inflammatory factors, and had more senescence biomarkers, including p16, p21, and SA-β-gal.
These senescent macrophages were then tested on young mice. Three days after a hindlimb injury, young mice were injected with a control serum, non-senescent macrophages, or senescent macrophages. There were no differences between the angiogenesis of the first two groups. However, the mice in the third group were more like old mice subjected to the same injury: their toes were more likely to become necrotic, their muscles became more fibrotic, and there were fewer capillaries in the tissue after healing. Even a pre-existing artery that had not been part of the injury had its diameter reduced by the presence of senescent macrophages.
This was accompanied by negative effects on the endothelial cells that line blood vessel walls. The proliferation ability of these cells was harmed, and a crucial pathway required for angiogenesis was impaired. These findings were confirmed by a cellular experiment: driving mouse macrophages senescent through hydrogen peroxide exposure, then exposing endothelial cells to them, led to significant decreases in the endothelial cells’ angiogenesis-related protein expression and abilities.
Pinpointing the cause
As the researchers expected, these changes were found to be directly related to VEGF-A165B. In one group of macrophages, the researchers knocked down the gene responsible for producing this protein, and in another culture, they introduced an antibody against it. Both methods were sufficient to prevent senescent macrophages from harming endothelial cells’ abilities.
With this knowledge in hand, the researchers then returned to mice. Mice were given senescent macrophages that were modified not to produce VEGF-A165B, and their angiogenesis and tissue necrosis were practically identical to mice that were given non-senescent macrophages. Additionally, giving mice non-senescent macrophages that also could not produce VEGF-A165B appeared to be beneficial.
Finally, the researchers took a look at human beings. Older people have more VEGF-A165B, and more total VEGF-A, than younger people do, and this was directly and significantly correlated with smaller blood vessels.
The researchers note some of the limitations with this work; for example, they were not able to stratify their results by sex, which has an impact on PAD [8], and inflammatory molecules secreted by senescent macrophages might also be having an effect alongside VEGF-A165B. Still, this work presents a clear potential target and opportunities for future clinical work involving either macrophage-targeting senolytics or anti-VEGF-A165B drugs.
Literature
[1] Kim, M. S., Hwang, J., Yon, D. K., Lee, S. W., Jung, S. Y., Park, S., … & Seylani, A. (2023). Global burden of peripheral artery disease and its risk factors, 1990–2019: a systematic analysis for the Global Burden of Disease Study 2019. The Lancet Global Health, 11(10), e1553-e1565.
[2] Gornik, H. L., Aronow, H. D., Goodney, P. P., Arya, S., Brewster, L. P., Byrd, L., … & Wilkins, L. R. (2024). 2024 ACC/AHA/AACVPR/APMA/ABC/SCAI/SVM/SVN/SVS/SIR/VESS guideline for the management of lower extremity peripheral artery disease: a report of the American College of Cardiology/American Heart Association Joint Committee on Clinical Practice Guidelines. Journal of the American College of Cardiology, 83(24), 2497-2604.
[3] Annex, B. H., & Cooke, J. P. (2021). New directions in therapeutic angiogenesis and arteriogenesis in peripheral arterial disease. Circulation research, 128(12), 1944-1957.
[4] Cooke, J. P., & Losordo, D. W. (2015). Modulating the vascular response to limb ischemia: angiogenic and cell therapies. Circulation research, 116(9), 1561-1578.
[5] Takeda, Y., Costa, S., Delamarre, E., Roncal, C., Leite de Oliveira, R., Squadrito, M. L., … & Mazzone, M. (2011). Macrophage skewing by Phd2 haplodeficiency prevents ischaemia by inducing arteriogenesis. Nature, 479(7371), 122-126.
[6] Lin, J. B., Sene, A., Santeford, A., Fujiwara, H., Sidhu, R., Ligon, M. M., … & Apte, R. S. (2018). Oxysterol signatures distinguish age-related macular degeneration from physiologic aging. EBioMedicine, 32, 9-20.
[7] Kikuchi, R., Nakamura, K., MacLauchlan, S., Ngo, D. T. M., Shimizu, I., Fuster, J. J., … & Walsh, K. (2014). An antiangiogenic isoform of VEGF-A contributes to impaired vascularization in peripheral artery disease. Nature medicine, 20(12), 1464-1471.
[8] Pabon, M., Cheng, S., Altin, S. E., Sethi, S. S., Nelson, M. D., Moreau, K. L., … & Hess, C. N. (2022). Sex differences in peripheral artery disease. Circulation research, 130(4), 496-511.
