Anti-leukemic Role Of Phospholipid Scramblase 1 And Its Molecular Mechanisms

Phospholipid scramblase 1 (PLSCR1) is a multiply palmitoylated, calcium-binding type II transmembrane protein, while unpalmitoylated PLSCR1 was also found to be localized into the nucleus where it binds to genomic DNA. However, the exact mechanisms of expression regulation of PLSCR1 gene and its biological functions remain largely unclear. More recently, our laboratory found that protein kinase Cδ(PKCδ) upregulates PLSCR1 expression via the sequential activation of c-Jun N-terminal kinase (JNK) and singnal transducer and activator of transcription 1 (STAT1). What's more, PLSCR1 acts a role in the all-trans retinoic acid (ATRA) and phorbol 12-myristate 13-acetate (PMA) -induced leukemic cell differentiation. In the present work, we try to establish an inducible PLSCR1-expressing myeloid leukemic cell line to explore direct evidence of potential roles of PLSCR1 in the leukemic cell differentiation. Towards this end, the following original and interesting results were gotten:(1) An inducible human PLSCR1-expressing leukemic cell line U937plscr1 was generated using tet-off gene inducible system and overexpressing PLSCR1 protein was found to be distributed largely in the plasma membrane and cytoplasm with a little in the nucleus.(2) Upon PLSCR1 induction, the proliferation of U937plscr1 cells was arrested at G1 phase of the cell cycle. More importantly, PLSCR1 overexpression alone could also drive leukemic U937 cells to undergo differentiation towards granulocyte-like cells, as determined by morphology, differentiation markers and differentiation-related gene expressions. In aggrement with changes of these cellular behaviors, PLSCR1 expression also increased p27Kip1/p21Cip1 proteins and reduced c-Myc, SKP2 proteins, which induce growth arrest and cell differentiation.(3) PLSCR1 induction increased cellular sensitivity to etoposide-induced apoptosis with decreased Bcl-2 protein and enhanced proteolytic cleavage of PKCδ, the ?Ψm collapse, caspase-3 activation, apoptotic nuclear changes, and PARP cleavage as well.Taken together, this work proposes that PLSCR1 exerts anti-leukemic effects through inducing growth arrest and differentiation as well as increasing sensitivity to apoptosis induction. These findings shed new insights for understanding the biological functions of PLSCR1 and provide the theoretic basis for further exploring the molecular mechanisms of leukemic cell differentiation and for recognizing new drug targets of differentiation therapy.