A new study suggests that low-intensity pulsed ultrasound (LIPUS) can be beneficial in eliminating senescent cells through the recruitment and activation of immune cells [1].
The double-edged sword of the SASP
One of the characteristics of an aging organism is the accumulation of senescent cells. Various approaches are being developed to remove or neutralize those cells.
Senescent cells produce the senescence-associated secretory phenotype (SASP), a cocktail of chemokines, pro-inflammatory cytokines, growth factors, and proteases [2]. While the SASP might have many deleterious effects, it can be a double-edged sword. Some of these many molecules have positive effects, such as attracting immune cells, which can eliminate senescent cells [3]. Precise modulation of the SASP can be a potent strategy for senescent cell elimination.
The authors of this study turned to ultrasound as a possible non-invasive therapeutic tool to eliminate senescent cells. Previous studies observed that LIPUS has positive effects on many types of tissues, including promoting wound healing or bone repair [4] and regulating the secretion of inflammation-associated cytokines [5].
Therefore, the authors of this study hypothesized that “LIPUS can modulate the secretion of SASP in senescent cells and thereby manipulate these cells” or aid in attracting immune cells.
Eating up the old cells
The authors cultured human male fibroblast cells as their research model. They made these cells replicatively senescent by allowing them to grow and divide multiple times. They divided the cells into two groups: ‘late cells,’ which had replicated many times but had not yet reached senescence, and ‘early cells,’ which had not replicated many times.
Following 20 minutes of stimulation with LIPUS, the researchers observed a marker of senescent cells, SA-β-gal, to be selectively increased in the ‘late cells’ but not in the ’early cells.’
When the researchers tested the impact of LIPUS on the expression of multiple SASP molecules, they learned that “LIPUS stimulation specifically increased the expression of immune cell attraction markers in the ‘late cells.’”
This increased expression led to an increased migration of immune cells, specifically monocytes and specific families of macrophages, towards these stimulated cells. Ultimately, it resulted in the ingestion and elimination of the ‘late cells’ by macrophages in a process called phagocytosis.
The molecules behind the scenes
In the next steps of their research, the authors investigated the molecular mechanism behind LIPUS’s selective stimulation of the SASP.
After excluding other possibilities, they tested the involvement of reactive oxygen species (ROS) since previous reports suggested increased ROS production following LIPUS stimulation [6]. These researchers confirmed that LIPUS stimulation increased intracellular ROS generation in the ‘late cells’ and observed that ROS production was required to increase SA-β-gal activity in LIPUS-stimulated ‘late cells.’
Further, they investigated the molecules that regulate the expression of SASP factors, focusing on two in particular: NF-κB and p38. NF-κB is a transcription factor family member that regulates gene expression, and p38 increases NF-κB activity.
Their experiments suggested that in ‘late cells,’ LIPUS stimulation leads to ROS-dependent activation of the p38-NF-κB pathway, activating immune cell-attracting SASP factors and leading to immune cell migration.
ROS plays a significant role in this process, but how did LIPUS stimulation cause the ROS generation? The researchers generated a few hypotheses. First, they learned that the LIPUS stimulation-generated production of extracellular ROS was not significant. Instead, LIPUS generated intracellular ROS via an enzyme called NOX4. NOX is a family of enzymes located in lipid rafts, special compartments on the plasma membranes that surround cells.
Further experiments showed that LIPUS stimulation led to perturbations in the structure and organization of the cellular membrane, creating transient pores and resulting in increased permeability, affecting the formation and localization of lipid rafts. This leads to NOX activation and ROS generation. This occurred only in ‘late cells’, whose membrane composition differs from that of ‘early cells.’
Reversing skin aging
At the end of their study, the researchers used an in vivo model of mouse skin aging to test whether LIPUS could be an efficient tool to remove senescent cells by regulating the SASP in a living organism.
After UVA-induced skin aging, LIPUS was applied for five days, and skin tissue was analyzed 10 days later. Neither UVA nor LIPUS treatment were found to impact body weight nor major organs of these mice. However, as expected, UVA resulted in extensive and deep wrinkles on the applied area and increased the levels of senescence markers.
LIPUS treatment increased SASP markers for immune cell attraction in a UVA-induced skin aging model. This increase translated into more attraction of immune cells than with UVA irradiation alone. At the same time, LIPUS treatment significantly reduced the number of cells with senescence markers, suggesting a decrease in these cells.
The researchers summarized that “these data suggest that senescent cells could be eliminated by macrophage infiltration via LIPUS stimulation.”
Optimizing for clinical use
LIPUS is a technology that can be easily applied in the clinic. Those researchers propose that it can be used to help remove senescent cells, possibly combined with senolytic treatment.
However, before this therapy can enter the clinic, LIPUS parameters need to be optimized and tested for side effects, as it is known that different LIPUS parameters can elicit different effects in different types of cells. Additionally, its limitations, such as penetration efficiency, and patients’ aged immune systems, which might not be as effective in clearing senescent cells, need to be considered.
Literature
[1] Gwak, H., Hong, S., Lee, S. H., Kim, I. W., Kim, Y., Kim, H., Pahk, K. J., & Kim, S. Y. (2025). Low-Intensity Pulsed Ultrasound Treatment Selectively Stimulates Senescent Cells to Promote SASP Factors for Immune Cell Recruitment. Aging cell, e14486. Advance online publication.
[2] Watanabe, S., Kawamoto, S., Ohtani, N., & Hara, E. (2017). Impact of senescence-associated secretory phenotype and its potential as a therapeutic target for senescence-associated diseases. Cancer science, 108(4), 563–569.
[3] Burton, D. G. A., & Stolzing, A. (2018). Cellular senescence: Immunosurveillance and future immunotherapy. Ageing research reviews, 43, 17–25.
[4] Schortinghuis, J., Bronckers, A. L., Stegenga, B., Raghoebar, G. M., & de Bont, L. G. (2005). Ultrasound to stimulate early bone formation in a distraction gap: a double blind randomised clinical pilot trial in the edentulous mandible. Archives of oral biology, 50(4), 411–420.
[5] Li, J. K., Chang, W. H., Lin, J. C., Ruaan, R. C., Liu, H. C., & Sun, J. S. (2003). Cytokine release from osteoblasts in response to ultrasound stimulation. Biomaterials, 24(13), 2379–2385.
[6] Duco, W., Grosso, V., Zaccari, D., & Soltermann, A. T. (2016). Generation of ROS mediated by mechanical waves (ultrasound) and its possible applications. Methods (San Diego, Calif.), 109, 141–148.
