A phosphorylation defective retinoic acid receptor mutant mimics the effects of retinoic acid on EGFR mediated AP-1 expression and cancer cell proliferation.
BACKGROUND: The effects of the vitamin A metabolite retinoic acid (RA) are mediated at the transcriptional level by retinoic acid receptors (RAR). These proteins are part of a superfamily of transcription factors which activate target gene expression when bound to their respective ligands. In addition to ligand binding, heterodimerization with transcriptional cofactors and posttranslational modification such as phosphorylation are also critical for transactivation function. Previous studies have shown that phosphorylation of a serine residue at amino acid 77 in the RARalpha amino terminus was required for basal activation function of the transcription factor. RESULTS: We have determined that RA inhibits cyclin H and cdk7 expression thereby decreasing levels of phosphorylated RARalpha in human cancer cell lines. To determine the effects of decreased RARalpha phosphorylation in human cancer cells, we stably transfected a phosphorylation defective mutant RARalpha expression construct into SCC25 cultures. Cells expressing the mutant RARalpha proliferated more slowly than control clones. This decreased proliferation was associated with increased cyclin dependent kinase inhibitor expression and decreased S phase entry. In the absence of ligand, the RARalpha mutant inhibited AP-1 activity to an extent similar to that of RA treated control clones. Levels of some AP-1 proteins were inhibited due to decreased EGFR expression upstream in the signaling pathway. CONCLUSIONS: These results indicate that hypophosphorylated RARalpha can mimic the anti-AP-1 effects of RA in the absence of ligand.
Crowe
David L
DL
Center for Craniofacial Molecular Biology, University of Southern California, 2250 Alcazar Street, Los Angeles, CA 90033, USA. dcrowe@hsc.usc.edu
Kim
Randie
R