SUPPRESSION OF RELAXIN GENE-EXPRESSION BY RETINOIDS IN SQUAMOUS DIFFERENTIATED RABBIT TRACHEAL EPITHELIAL-CELLS

Northern blot analysis of total RNA from a variety of rabbit tissues i ndicated that placenta is the primary site of expression of the protei n hormone relaxin (previously called SQ10) in rabbits. Relaxin was not detected by this method in other rabbit tissues, including normal tra chea and several squamous tissues. However, relaxin is highly induced during squamous cell differentiation in cultured rabbit tracheal epith elial (RbTE) cells. Retinoic acid and retinoids that selectively bind to the nuclear retinoid receptors, the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), and induce RARE- or RXRE-dependen t transactivation as well as repression of AP-1-dependent transactivat ion, were all effective in suppressing relaxin expression. In addition , the retinoid SR11302, which exhibits only anti-AP-1 activity but doe s not induce RARE- or RXRE-dependent transactivation, was also able to inhibit relaxin expression. These results suggest that the suppressio n of relaxin expression is related to the anti-AP-1 activity of retino ids. To determine whether the relaxin gene is regulated by retinoids a t the level of transcription, a 4.3 kb fragment of the 5' flanking reg ion of the rabbit relaxin gene was cloned and analyzed. This regulator y region included a classic TATA-box as well as consensus sequences fo r several transcription factors, including CREB, NF-kappa B and AP-1. The ability of the 4.3 kb regulatory region to control the transcripti on of a luciferase reporter gene was analyzed in transiently transfect ed, squamous-differentiated RbTE cells. The results demonstrated that this regulatory region caused strong transactivation of the reporter g ene. This transactivation was inhibited by retinoic acid, suggesting r etinoid control at the transcriptional level. Deletion analysis indica ted that multiple regulatory elements are involved in the regulation o f relaxin gene expression during squamous differentiation as well as i n the suppression by retinoids. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.