Expression of the spexin gene in the rat adrenal gland and evidences suggesting that spexin inhibits adrenocortical cell proliferation

Spexin (SPX, also called NPQ) is a recently identified, highly conserved peptide which is processed and secreted. We analysed the SPX gene and its protein product in the rat adrenal gland to ascertain whether SPX is involved in the regulation of corticosteroid secretion of and growth of adrenocortical cells. In adult rat adrenal glands the highest levels of SPX mRNA were present in the glomerulosa (ZG) and fasciculate/reticularis (ZF/R) zones. High SPX gene expression levels were found in freshly isolated adult rat ZG and ZF/R cells. In cultured adrenocortical cells the levels of SPX mRNA were lower than in freshly isolated cells. SPX mRNA expression levels were found to be 2-3 times higher during days 90-540 of postnatal development than found during days 2-45. Prolonged ACTH administration lowered and dexamethasone increased adrenal SPX mRNA levels in vivo. Adrenal enucleation produced a significant linear increase in SPX mRNA levels, with the highest value occurring at day 8 after surgery, with control values taken on day 30 after enucleation. Immunohistochemistry revealed SPX-like immunoreactivity in the entire cortex of the adult male rat and in enucleation-induced regenerating cortex. A concentration of 10-6M SPX peptide stimulated basal aldosterone secretion by freshly isolated ZG. In prolonged exposure of adrenocortical cell primary cultures to SPX (10-6M) resulted in a small increase in corticosterone secretion and a notable decrease in BrdU incorporation. The results suggest the direct involvement of SPX in the regulation of adrenocortical cell proliferation; however, the mechanism of action remains unknown.