Vitamin D and Retinoic Acid Require Protein Kinase C Activity and Reactive Oxygen Species as Opposing Signals Regulating PEIG-1/GPRC5A Expression in Caco-2 and T84 Colon Carcinoma Cells.

PEIG-1/GPRC5A (phorbol ester induced gene-1/G-protein Coupled Receptor Class C Group 5 Member A) was the first identified member of the orphan G protein-coupled receptor family GPRC5. Deregulation of its expression is associated with the development and progression of various types of tumours, particularly colon carcinoma. In this work, we study the effects of vitamin D (VD, cholecalciferol) and retinoic acid (RA) on GPRC5A mRNA expression in the colorectal cancer cell lines Caco-2 and T84. Both VD (10 µM) and all-trans retinoic acid (ATRA, atRA, RA) (10 µM) increased GPRC5A mRNA levels. Protein kinase C (PKC) inhibition with Gö6983 (10 µM) completely abolished the effects of VD and RA on GPRC5A expression. In parallel, VD and RA increased cytosolic and mitochondrial ROS levels (cROS and mtROS). However, the antioxidants NAC (10 mM) and MitoTEMPO (10 µM) raised GPRC5A gene expression levels in the presence of VD or RA, suggesting that elevated ROS may inhibit GPRC5A expression. In conclusion, both VD and RA stimulate GPRC5A expression. The mechanisms involve a common and essential PKC signalling pathway, as Gö6983 inhibited both VD- and RA-induced signalling.