A noncanonical endocytic pathway is involved in the internalization of 3 μm polystyrene beads into HeLa cells

Extracellular fine particles of various sizes and origins can be taken up by cells, affecting their function. Understanding the cellular uptake processes is crucial for understanding the cellular effects of these particles and the development of means to control their internalization. Although macropinocytosis is a possible pathway for the cellular uptake of particles larger than 0.2 μm, its contribution to cellular uptake in non-phagocytic cells is controversial. Using 3 μm polystyrene beads as a model particle, we aimed to assess the detailed modes of their cellular uptake by non-phagocytic HeLa cells. Cellular uptake was assessed using confocal, scanning electron, and scanning ion conductance microscopy analyses, together with inhibitor studies. Our results revealed that 3 μm beads were taken up by HeLa cells by an actin-, cholesterol-, and membrane protrusions-dependent noncanonical endocytic pathway, different from the canonical macropinocytic and phagocytic pathways. Our work provides a framework for studying the cellular uptake of extracellular fine particles.