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.