Study Of Function And Mechanism Of Nucleolar Protein Lyar In Embryonic Stem Cells

Embryonic stem (ES) cells are pluripotent cells derived from the inner cell mass of mammalian blastocyst-stage embryos. These cells can proliferate infinitely while maintaining pluripotency and normal karyotype. These unique characteristics make them attractive sources not only for biological research, but also for cell therapy to treat patients with degenerative diseases. However, ES cells have a propensity to produce tumours called teratoma, when they are transplanted, which may preclude their therapeutic usage. Why ES cells possess tumor-like properties is largely unknown. Many nucleolar proteins have close relationship with ribosomal assemble, protein synthesis and cell growth. Recently, it has been shown that they are also involved in the regulation of cell division, cell-cycle and aging control. Here we show that a nucleolar protein, Lyar which has been reported to be highly expressed in mouse early development and certain leukemia cells, plays an important role in proliferation of ES cells. We found that Lyar expression was high in undifferentiated ES cells and decreased when ES cells went differentiated. Importantly, the down-regulation of Lyar in mouse ES cells by tetracycline (Tc)-inducible small interference RNA (siRNA) led to a significant reduction in ES cell proliferation. Furthermore, cell cycle blockage in G2/M phase and apoptosis were induced by the decrease in Lyar expression. In addition, the ability of ES cells to form teratoma, when implanted, was impaired. These observations strongly indicate that Lyar is a critical nucleolar protein in regulation of ES cell proliferation, cell cycle, and survival. In addition, the expression level of Lyar might be related to ability of ES cells to form tumors. To elucidate molecular mechanisms by which Lyar regulates ES cell proliferation, we performed affinity chromatography. Lyar was found to interact with Nucleolin, another important nucleolar protein. Interestingly, Down-regulation of Lyar expression caused the decrease in nucleolin protein level, but not mRNA level, in ES cells. It is worthy to notice that down-regulation of Nucleolin expression resulted in a reduced proliferation in ES cells, suggesting that Lyar may regulate ES proliferation through stabilizing Nuceolin protein. However, Lyar may regulate cell cycle and apoptosis through different mechanisms, since down-regulation of Nucleolin expression did not affect cell cycle and survival in ES cells. Further investigations are needed to elucidate the molecular mechanisms for Lyar to function. The study will provide new clues for understanding how ES cells proliferate and tumor develops.