In cell cultures, suppressing PAK1 reduced fluid uptake and inflammatory gene activity in senescent cells
In cell cultures, suppressing PAK1 reduced fluid uptake and inflammatory gene activity in senescent cells
On 14 July, researchers from the Okinawa Institute of Science and Technology published a preprint on bioRxiv. In several models of cellular senescence, they observed sustained uptake of extracellular fluid and examined how disrupting this process changed the activity of specific inflammatory genes.
Senescence stops a damaged cell from dividing. The cell remains alive and often develops SASP, a set of secreted molecules that includes inflammatory signals. The composition of SASP depends on the cell type and the cause of senescence. This preprint tracks the activity of several genes and describes part of this program.
The authors compared dividing cells with human fibroblasts, which are connective tissue cells, that had stopped dividing after many replication cycles, DNA damage, or damage to the cell membrane. In every senescence model, large vesicles formed near the membrane.
This process is called macropinocytosis. The membrane forms a fold, closes around it, and takes in a large volume of extracellular fluid together with dissolved substances. Immune cells use this pathway to capture particles, while some tumor cells use it to obtain nutrients. Senescent fibroblasts continued this uptake even in serum-free medium.
The formation of these membrane folds depends on PAK1, a protein that regulates actin filaments, which form part of the cell's internal scaffold. The authors suppressed PAK1 using the inhibitor IPA-3 and a small interfering RNA that reduces the amount of the protein. Both interventions reduced the uptake of labeled dextran, which served as a fluid tracer, and made large vesicles less common.
The activity of IL6, CCL2, and IL12A also decreased in the same cells. However, the markers of senescence and cell cycle arrest examined by the authors remained present. In these cultures, PAK1 supports the inflammatory component of the senescent cell program without reversing the arrest of cell division.
The connection with TGFβ/SMAD, a signaling pathway that regulates gene activation, was only partial. Its activity weakened after PAK1 was reduced. Adding TGFβ1 restored only part of the IL6 activity and further reduced CCL2. This pathway therefore accounts for part of the effect of PAK1, but it affects these two genes differently.
The authors also disrupted macropinocytosis in several independent ways. They interfered with actin assembly and reduced proteins required for vesicle formation and maturation. These interventions also lowered inflammatory gene transcripts in different cell cultures. This supports the hypothesis that fluid uptake itself contributes to the inflammatory program.
A 2021 study linked macropinocytosis in senescent cells to the protein LY6D and to cell survival. The new preprint adds a link to inflammatory genes. In a study published yesterday, abemaciclib reduced inflammatory SASP in aged mice through a different pathway, CDK4/6–RARα–NF-κB. The new study identifies another cellular target for investigation, but it does not yet provide results from animals.
All experiments were performed in cell cultures. The study measured transcripts. The amount of secreted protein, the effects on tissues, and the effects in animals remain unknown. PAK1 contributes to actin function in normal cells, so the therapeutic value of this pathway will depend on whether inflammatory signals can be reduced without impairing tissue repair and protection.