Activation of the aryl hydrocarbon receptor by the widely used Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(dimethylethyl)pyrazolo[3,4-d]pyrimidine (PP2)

Small molecular weight protein kinase inhibitors are frequently used tools to unravel the complex network of cellular signal transduction under certain physiological and pathophysiological conditions. 4-amino-5-(4-chlorophenyl)-7-(dimethylethyl)pyrazolo[3,4-d]pyrimidine (PP2) is a widely used compound to block the activity of Src family kinases, the major group of non-receptor tyrosine kinases, which trigger multiple cellular signaling pathways. Here, we show that PP2 induces cytochrome P450 1A1 mRNA expression and enzyme activity in a dose-dependent manner in human HepG2 hepatoma cells and NCTC 2544 keratinocytes. By means of reporter gene assays, RNA interference, electrophoretic mobility shift assay, and competitive ligand-binding assay, we further demonstrate that PP2 is a ligand for the aryl hydrocarbon receptor (AHR), an intracellular chemosensor that regulates xenobiotic metabolism, environmental stress responses, and immune functions. Upon ligand-dependent activation, the AHR translocates into the nucleus and dimerizes with the AHR nuclear translocator (ARNT) to modulate the expression of its target genes. In addition, AHR activation is frequently accompanied by an activation of the tyrosine kinase c-Src, resulting in stimulation of cell-surface receptors and downstream signal transduction. As PP2 activates the AHR/ARNT pathway by simultaneously blocking c-Src-mediated alternative signaling routes, this compound may be a suitable tool to study the contribution of the different AHR-dependent signaling pathways to biological processes and adverse outcomes. On the other hand, the unexpected property of PP2 to stimulate AHR/ARNT signaling should be carefully taken into account in future investigations in order to avoid a false interpretation of experimental results and molecular interrelations.