PMA up‐regulates the transcription of Axl by AP‐1 transcription factor binding to TRE sequences via the MAPK cascade in leukaemia cells

Background . Axl is a receptor tyrosine kinase promoting anti‐apoptosis, invasion and mitogenesis, and is highly expressed in different solid cancers. Axl basal transcriptional activity is driven by Sp1/Sp3, and overexpression of MZF‐1 (myeloid zinc‐finger 1) induces Axl transcription and gene expression. Furthermore, Axl expression is epigenetically controlled by CpG hypermethylation; however, little is known about inducible Axl gene expression and Axl regulation in haematopoetic malignancies. Results . In the present study, we studied Axl transcriptional regulation under PMA‐stimulated conditions in leukaemia cells. Luciferase analysis with sequential 5′‐deletion constructs revealed that the −660/−580 region of the Axl promoter is indispensable for induced promoter activity under PMA stimulation. This region includes AP‐1 (activator protein 1)/CREB [CRE (cAMP‐response‐element)‐binding protein] motifs, five times partially overlapping TGCGTG repeats and multiple GT repeats. Mutational, supershift and ChIP (chromatin immunoprecipitation) analysis determined that AP‐1 family members bind to AP‐1 motifs and to the 5 × TGCGTG overlapping repeats, thus transactivating Axl promoter activity. Furthermore, specific inhibitors of PKC (protein kinase C), ERK1/2 (extracellular‐signal‐regulated kinase 1/2) and p38 reduced Axl expression. Additionally, mithramycin treatment abolished constitutive and PMA‐induced Axl expression. Conclusions . Taken together the results of the present study suggest that PMA‐induced Axl gene expression in leukaemia cells is mediated by AP‐1 motifs and 5 × TGCGTG repeats within the promoter region −660/−580, and through the PKC/ERK1/2/AP‐1 or PKC/p‐38/AP‐1 signalling axis.