Interactions of STAT5b-RARα, a novel acute promyelocytic leukemia fusion protein, with retinoic acid receptor and STAT3 signaling pathways

Abstract

AbstractSignal transducer and activator of transcription (STAT) 5b-retinoic acid receptor (RAR) α is the fifth fusion protein identified in acute promyelocytic leukemia (APL). Initially described in a patient with all-trans retinoic acid (ATRA)–unresponsive disease, STAT5b-RARα resulted from an interstitial deletion on chromosome 17. To determine the molecular mechanisms of myeloid leukemogenesis and maturation arrest in STAT5b-RARα+ APL and its unresponsiveness to ATRA, we examined the effect of STAT5b-RARα on the activity of myeloid transcription factors including RARα/retinoid X receptor (RXR) α, STAT3, and STAT5 as well as its molecular interactions with the nuclear receptor corepressor, SMRT, and nuclear receptor coactivator, TRAM-1. STAT5b-RARα bound to retinoic acid response elements (RAREs) both as a homodimer and as a heterodimer with RXRα and inhibited wild-type RARα/RXRα transactivation. Although STAT5b-RARα had no effect on ligand-induced STAT5b activation, it enhanced interleukin 6–induced STAT3-dependent reporter activity, an effect shared by other APL fusion proteins including promyelocytic leukemia-RARα and promyelocytic leukemia zinc finger (PLZF)–RARα. SMRT was released from STAT5b-RARα/SMRT complexes by ATRA at 10−6 M, whereas TRAM-1 became associated with STAT5b-RARα at 10−7 M. The coiled-coil domain of STAT5b was required for formation of STAT5b-RARα homodimers, for the inhibition of RARα/RXRα transcriptional activity, and for stability of the STAT5b-RARα/SMRT complex. Thus, STAT5b-RARα contributes to myeloid maturation arrest by binding to RARE as either a homodimer or as a heterodimer with RXRα resulting in the recruitment of SMRT and inhibition of RARα/RXRα transcriptional activity. In addition, STAT5b-RARα and other APL fusion proteins may contribute to leukemogenesis by interaction with the STAT3 oncogene pathway.