17β-estradiol mediates sex-biased progesterone receptor expression via estrogen receptor α in chicken pituitary

Sex-biased in gene expression is a pervasive biological phenomenon in animals and plays critical roles in sex-dependent physiological activities. While previous studies have identified numerous genes displaying sex-biased expression at pre- or post-sexual maturity, the regulatory mechanisms driving such differential expression remain poorly understood. Our earlier work showed that progesterone receptor (PGR) expression in the chicken pituitary exhibits sex bias. Herein, we aimed to identify key gonadal steroids and their regulatory mechanisms mediating female-enriched PGR expression in the chicken pituitary. Our results showed that: (1) Pituitary PGR expression in sexually mature chickens was confirmed to be sex-biased; (2) Both in vitro assays and subcutaneous injections demonstrated that 17β-estradiol (E₂) potently stimulated PGR expression in the chicken pituitary in a time- and dose-dependent manner, whereas progesterone and dihydrotestosterone showed no such effect; (3) E₂-induced PGR transcription relies on estrogen receptor α (ERα) rather than ERβ or G protein-coupled estrogen receptor; (4) Single-cell RNA-seq analysis revealed 26.3 % co-expression of ESR1 (the ERα-encoding gene) and PGR in chicken gonadotrophs; (5) E₂-elevated PGR expression may involve both membrane-anchored ERα (mERα)-triggered rapid non-genomic pathways and ERα-associated long-lasting genomic actions; (6) mERα-mediated regulation of pituitary PGR expression in chickens might be linked to PLC-dependent dual messenger systems (IP3/Ca²⁺ and DAG/PKC), the MEK/ERK cascade, and L-type voltage-gated Ca²⁺ channels. These findings provide novel insights into the estrogen-dependent regulatory mechanisms of sex-biased PGR expression in the avian pituitary, advancing our understanding of estrogen-mediated modulation of the reproductive axis in birds.