Protein kinase C modulates aryl hydrocarbon receptor nuclear translocator protein-mediated transactivation potential in a dimer context

Protein kinase C (PKC)- and protein kinase A (PKA)mediated modulation of th e transactivation potential of human aryl hydrocarbon receptor nuclear tran slocator (hARNT), a basic helix-loop-helix (bHLH)-PAS transcription factor, and the bHLH-ZIP transcription factors USF-1 (for upstream regulatory fact or 1) and c-Myc were examined, An 81 nM dose of the PKC activator phorbol-1 2-myristate-13-acetate (PMA), shown here to specifically activate PKC in CO S-l cells, or a 1 nM dose of the PKA activator 8-bromoadenosine-3',5'-cycli c monophosphate (8-Br-cAMP) results in 2.6- and 1.9-fold enhancements, resp ectively, in hARNT-mediated transactivation of the class B, E-box-driven re porter pMyc3E1bLuc relative to identically transfected, carrier solvent-tre ated COS-l cells. In contrast, 81 nM PMA and 1 nM 8-Br-cAMP did not enhance transactivation of pMyc3E1bLuc-driven by USF-1 and c-Myc expression relati ve to identically transfected, carrier-treated COS-1 cells. Co-transfection of pcDNA3/ARNT-474-Flag, expressing a hARNT carboxyl-terminal transactivat ion domain deletion, and pMyc3E1bLuc does not result in induction of report er activity, suggesting PMA's effects do not involve formation of unknown h ARNT-protein heterodimers. Additionally, PMA had no effect on hARNT express ion relative to Me2SO-treated cells. Metabolic P-32 labeling of hARNT in ce lls treated with carrier solvent or 81 nM PMA demonstrates that PMA does no t increase the overall phosphorylation level of hARNT. These results demons trate, for the first time, that the transactivation potential of ARNT in a dimer context can be specifically modulated by PKC or PKA stimulation and t hat the bHLH-PAS and bHLH-ZIP transcription factors are differentially regu lated by these pathways in COS-1 cells.