Cancer remains a major challenge in global healthcare, primarily due to its high incidence rate and complex pathophysiological mechanisms. Chemotherapy is one of the three main strategies for cancer treatment, however, many patients suffer from drug resistance (Papalazarou et al., 2018, Liu et al., 2021), which significantly reduces therapeutic efficacy and often leads to treatment failure. Therefore, understanding the factors that promote chemoresistance is important for improving cancer treatment.

The tumor microenvironment (TME) refers to the complex local environment in which tumor cells reside. It includes cellular components (e.g., fibroblasts, immune cells, and endothelial cells) and non-cellular components (e.g., the extracellular matrix and tumor interstitial fluid) (De Visser and Joyce, 2023). The abnormal biochemical cues within the tumor microenvironment—such as acidity, hypoxia, and elevated cytokines/growth factors—are considered important contributors to chemoresistance in cancer cells (Peppicelli et al., 2025, Senthebane et al., 2017, Chen et al., 2023). In recent years, increasing evidence shows that mechanical cues in the tumor microenvironment can influence chemoresistance (Fig. S1) (Kalli et al., 2023). For example, substrate stiffness can promote drug resistance via the YAP/TAZ and RhoA–ROCK pathways (Drain et al., 2021, Qin et al., 2020, Xu et al., 2021, Shou et al., 2023, Wang et al., 2021). Fluid shear stress can enhance drug resistance largely through PI3K/Akt and STAT3 signaling (Triantafillu et al., 2019, Hassan et al., 2022, Ip et al., 2016). In addition, the increased osmotic pressure can enhance aquaporin 5 (AQP5) expression and promote cellular sensitivity to drugs (Chen et al., 2015). This indicates that the effects of different mechanical cues may differ. For example, extracellular matrix stiffness primarily modulates cell–ECM adhesion and cytoskeletal tension (Drain et al., 2021, Qin et al., 2020, Xu et al., 2021, Shou et al., 2023, Wang et al., 2021), whereas osmotic pressure mainly alters transmembrane water transport and cell volume regulation (Chen et al., 2015). Overall, the tumor mechanical microenvironment is an important regulator of chemoresistance.

Recently, extracellular fluid viscosity (EFV) has been recognized as an important physical cue in cell biology (Bera et al., 2022, Pittman et al., 2022). The EFV of normal tissue is around 0.7 cP, whereas the EFV of tumor can exceed 8 cP, which can be attributed to the excessive secretion of mucins and hyaluronic acid by tumor and epithelial cells. Also, the enhanced compression of lymphatic vessels and degradation of extracellular matrix may lead to the accumulation of macromolecules, increasing EFV (Bera et al., 2022, Pittman et al., 2022). Such a high-viscosity microenvironment has been shown to promote cancer cell migration (Bera et al., 2022, Pittman et al., 2022) by activating TRPV4 and enhancing Rho function. Yet whether such a high viscosity can affect drug resistance or not is still unknown.

Here, we found that a high-viscosity culture medium increases the chemoresistance of SKOV-3 cells. Mechanistically, the high-viscosity mechanical cue activates mechanosensitive pathways and is involved in P-gp upregulation, thereby reducing intracellular DOX accumulation. These findings suggest that lowering viscosity may help enhance chemotherapy efficacy.