Primary aldosteronism is the most common form of secondary hypertension. Aldosterone producing adenoma (APA) and bilateral adrenal hyperplasia together account for ~95% of cases. Recurrent somatic mutations in KCNJ5, CACNA1D, ATP1A1 and ATP2B3 have been identified in APA. This has allowed us to establish a pathophysiological model involving modifications in intracellular ionic homeostasis and membrane potential leading to the activation of calcium signaling, the major trigger for aldosterone production. In parallel, we have identified retinoic acid receptor (RAR) signaling as a central molecular network involved in APA formation independently of the mutation status. The aim of our study was to evaluate the role of RAR signaling in the development of APA.

To this purpose we have investigated the modulation of RAR signaling in two different cell lines derived from the human adrenal cortex as well as in different mouse models.

Whereas treatment of H295R cells with 10–6M of all trans retinoic acid (ATRA, a RARα agonist) and 9-cis retinoic acid (9-cis RA, a RXRα agonist) did not affect cell viability, 10–5M of ATRA dramatically decreased cell number in a time-dependent manner. This effect was due to decreased cell proliferation and increased cell apoptosis. However, 9-cis RA did not modify tumor growth in a mouse xenograft model. RARα invalidation by shRNA transduction in H295R cells did not affect their proliferative properties, but induced a major change in cellular phenotype with cells forming spheroid structures able to grow in suspension. Investigation of the adrenal phenotype of rarα knock-out mice demonstrated that in young (12 weeks) and old (40 weeks) rarα-/- mice the characteristic cellular arrangement of the adrenal cortex was replaced by an enlarged zona glomerulosa and a disorganized zona fasciculata, this effect being more pronounced in old mice.

Our results suggest that RAR signaling contributes to normal adrenal morphology and functional zonation. Disruption of RAR signaling could trigger abnormal proliferation of cells in the adrenal cortex, creating a propitious environment for the emergence of specific mutations affecting ionic channels and ATPases leading to increased aldosterone production.