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Abstract
Macropinocytosis is a highly conserved endocytic process which characterizes the engulfment of extracellular fluid and its contents into cells via large, heterogeneous vacuoles known as macropinosomes. Tubeimoside-1 (TBM1) is a low toxic triterpenoid saponin extracted from a traditional Chinese herb Bolbostemma paniculatum (Maxim.). TBM1 stimulates a quick accumulation of numerous phase-lucent cytoplasmic vacuoles in multiple colorectal cancer (CRC) cell lines. These vacuoles can be termed as macropinosomes that efficiently engulf lucifer yellow. These vesicles are not overlaps with endocytic organelle tracers, such as ERTracker, LysoTracker and mitoTracker. These vacuoles induced by TBM1 partially incorporate into lysosomes. Transmission electron microscope indicates membrane ruffling to form lamellipodia. Protrusions collapse onto and then fuse back with the plasma membrane to generate these large endocytic vacuoles. Notably, TBM1 efficiently trafficks dextrans into heterotopic xenografts in vivo, thus provide consolidated evidence that the vacuolization can be mainly defined as macropinocytosis. TBM1 downregulates cell viability and increases PI-positive, but not highlighted Hoechst 33342-positive cells. TBM1 induced cell death can be ascribed as methuosis by hyperstimulation of macropinocytosis which can be compromised by amiloride derivative 5-(Nethyl-N-isopropyl). Light chain 3 II is recruited to these vesicles to stimulate macropinocytosis. The cell death and vacuoles can be significantly neutralized by chloroquine, but can not be the inhibited by 3-methyladenine. TBM1 can coordinate with 5-FU to exert toxicity reducing and efficacy enhancing effects in vivo by increasing the uptake of the latter without hepatic injury. In conclusion, TBM1 effectively induces in vitro and in vivo macropinocytosis which can traffick small molecules into CRC cells. It is an attractive drug transporter and can be harnessed as a chemotherapeutic synergist with translational potential.
Highlights
Macromolecules are carried into the cell by invagination and pinching-off of pieces of the plasma membrane in a process termed endocytosis
To explore the underlying mechanism of TBM1 induced cell death, the morphological changes of SW480 cells were continuously monitored by phase contrast microscope
Similar results were obtained when the cells were stained with MitoTracker Green (MTG), a mitochondrial-selective fluorescent label (Figure 2A, central panel)
Summary
Macromolecules are carried into the cell by invagination and pinching-off of pieces of the plasma membrane in a process termed endocytosis. Endocytosis can be largely devided into two broad categories, ‘phagocytosis’ (the uptake of large particles) and ‘pinocytosis’ (the uptake of fluid and solutes) (Conner and Schmid, 2003). Macropinocytosis is a highly conserved endocytic process by which massive membrane is ruffled and extracellular fluid is ingested concurrently (Henson, 2005). Macropinocytosis has been described as a character of Ras-transformed cancer cells. Macropinocytotic cells uptake ATP and serum album, that are abundant in the tumor microenvironment. It provides additional sources of energy and amino acids, glutamine, for growth, metabolic robustness, and stress tolerance (Commisso et al, 2013; Qian et al, 2014)
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