The Character Of Plk1 Signaling Pathway Involved In The Quiescence Regulation Of CancerStem-like Cells

Artemia are small crustaceans that distribute widely in extreme conditions on Earth such as salt lakes, salt ponds and so on. It remains unchanged morphologically, physiologically and biochemically during evolution since its first appearance 4 million years ago. However, to cope with such harsh habitats and various different environments, Artemia evolved two reproductive pathways. Under favorable conditions, Artemia produce and release swimming nauplius larvae in an ovoviviparous route. Under unfavorable environments, mature Artemia produce and release diapause embryos, which are encased in a complex external shell that protect them from certain extreme environments. Inner cell masses in diapause cysts remain in quiescent state (GO phase). After the activation by low temperature for several months, cell division and development resume in Artemia embryos as environment become favorable, results in the generation of nauplii. The initiation and termination of cell quiescence of Artemia embryo provides a unique model for cell-cycle research.Previous research showed that Plkl and RSK1 signaling pathway were involved in the formation of Artemia embryos that the activities of Plkl and RSK1 were suppressed in diapause cysts. In order to clarify the mechanisms underlying the function of Plkl and RSK1 signaling pathway in cell-cycle regulation and quiescence formation, we use Nicotinamide to artificially maintain cells in quiescent state during the hatching post-diapause procedure of cysts. However, removing the Nicotinamide, the cysts were re-activated, During the process, we test the expression of Plkl, MEK-ERK-RSK1 signaling pathway. The results show that the expression of Plkl and MEK signaling pathway were very low in Artemia embryonic cells with the treatment of Nicotinamide. After removing the Nicotinamide, the expression of Plkl and MEK start to rise again. These results indicate that Plkl and the RSK1 signaling pathway are involved in the quiescence initiation and termination of Artemia embryonic cells.The roles of Plkl and RSK1 signaling pathway in the regulation of quiescence were further studied in cancer stem-like cells. We sorted gastric cancer stem-like cells from human gastric cancer cell line MKN45 using cancer stem cell marker CD44. The quiescent subpopulation of cancer stem-like cells were then detected by the PKH26 label-retaining assay. The Western blotting assay showed that the expression of Plkl and RSK1 is significantly lower in quiescent cancer stem-like cells compared with proliferating cells. These results indicate that Plkl and the RSK1 signaling pathway are involved in the regulation of the quiescence in the cancer stem-like cells. In cancer stem-like cells, Plkl inhibitor leads to cell cycle arrest and sphere-formation ability. Similarly, when the RSK1 signaling pathway is inhibited by the RSK1 inhibitor, the expression of Plkl was increased, indicating that RSK1 can feedback to promote the expression of Plkl. As a upstream regulator MEK-ERK-RSK1 signaling pathway, Plkl controls the proliferation of cancer stem-like cells.Cancer is now one of the most severe disease. Traditional tumor therapy contains radio-chemotherapy and surgical resection, which cannot completely eradicate tumor. Research have showed that quiescent cancer stem-like cells exist in solid tumor, which are resistant to radio-chemotherapy. Artemia, as a new model organism for the research of cell cycle, provides new ideas and methods for the study of quiescence regulation mechanism of cancer stem-like cells.