Abstract

Retinoic acid (RA) promotes terminal differentiation of both normal hematopoietic cells and acute promyelocytic leukemia (APL) blasts by transcriptional regulation of myeloid genes. To identify additional RA target genes, we used representational difference analysis (RDA) with RNA derived from a PML/RARĪ± inducible U937 myeloid cell line. From this screen we identified a novel early responsive RA target gene, RTP801 (REDD1). Recent studies showed that RTP801 is a critical transducer of several cellular stress signals, including hypoxia and energy depletion, through the TSC-mTOR pathway. We show that All-trans retinoic acid (ATRA) induces RTP801 mRNA in AML cell lines in a dose- and time-dependent manner. ATRA regulation of RTP801 is direct and does not require protein synthesis. Inhibition of endogenous RTP801 in U937 cells by siRNA abrogates ATRA-induced dephosphorylatioin of 4E-BP1, a key mTOR substrate. Overexpression of RTP801 in these cells results in growth arrest and apoptosis. RTP801 is differently expressed during maturation of normal CD34+ cells, suggesting it is involved in this process. We performed a yeast two-hybrid screen using a leucocyte cDNA library and identified the myeloid-specific protease, neutrophil elastase, as a binding partner of RTP801. Taken together, RTP801 is a novel ATRA target gene possibly involved in ATRA-induced differentiation of myeloid cells.

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