Anticancer effect of retinoic acid via AP-1 activity repression is mediated by retinoic acid receptor α and β in gastric cancer cells

Abstract

To uncover the mechanisms relating to the anticancer effect of retinoic acids in gastric cancer cells, the mediation of activator protein-1 (AP-1) activity repression by retinoic acid receptors (RARs) was investigated. All-trans retinoic acid (ATRA) inhibited AP-1 activity in BGC-823 cells (RARα +, RARβ +), but not in MKN-45 cells (RARα lo, RARβ −). Transient transfection of RARβ expression vector into MKN-45 cells significantly resulted in direct repression of AP-1 activity in a receptor concentration-dependent manner, and this could be strengthened by ATRA. Stable transfection of RARβ into MKN-45 cells directly inhibited cell growth and colony formation, and ATRA also enhanced these effects. Transient transfection of RARα into MKN-45 cells however, displayed receptor concentration-dependent AP-1 activity inhibition only in the presence of ATRA. Stable transfection of RARα into MKN-45 cells resulted in ATRA-dependent inhibition of cell growth and colony formation. For AP-1 binding activity induced by TPA, the repressive effect of ATRA was only observed in BGC-823 and RARα and RARβ stably transfected MKN-45 cells, but not in intact MKN-45 cells. This indicates the necessity for sufficient cellular RARα and/or RARβ in order for AP-1 activity repression to occur. Deletion of DNA binding domain (DBD) of RARβ, but not ligand binding domain (LBD), eliminated the anti-AP-1 function of RARβ. It is therefore concluded that both RARα and RARβ are mediators in the anticancer function of ATRA via AP-1 activity inhibition, and that RARβ, not RARα, can inhibit AP-1 activity to a certain extent directly by itself. Thus DBD, not LBD, is critical for anti-AP-1 activity.