Protein kinase Cδ mediates retinoic acid and phorbol myristate acetate–induced phospholipid scramblase 1 gene expression: its role in leukemic cell differentiation

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Abstract
Although phospholipid scramblase 1 (PLSCR1) was originally identified based on its capacity to promote transbilayer movement of membrane phospholipids, subsequent studies also provided evidence for its role in cell proliferation, maturation, and apoptosis. In this report, we investigate the potential role of PLSCR1 in leukemic cell differentiation. We show that all-trans retinoic acid (ATRA), an effective differentiation-inducing agent of acute promyelocytic leukemic (APL) cells, can elevate PLSCR1 expression in ATRA-sensitive APL cells NB4 and HL60, but not in maturation-resistant NB4-LR1 cells. ATRA- and phorbol 12-myristate 13-acetate (PMA)–induced monocytic differentiation is accompanied by increased PLSCR1 expression, whereas only a slight or no elevation of PLSCR1 expression is observed in U937 cells differentiated with dimethyl sulfoxide (DMSO), sodium butyrate, or vitamin D3. Cell differentiation with ATRA and PMA, but not with vitamin D3 or DMSO, results in phosphorylation of protein kinase Cδ (PKCδ), and the PKCδ-specific inhibitor rottlerin nearly eliminates the ATRA- and PMA-induced expression of PLSCR1, while ectopic expression of a constitutively active form of PKCδ directly increases PLSCR1 expression. Finally, decreasing PLSCR1 expression with small interfering RNA inhibits ATRA/PMA-induced differentiation. Taken together, these results suggest that as a protein induced upon PKCδ activation, PLSCR1 is required for ATRA- and PMA-triggered leukemic cell differentiation.

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