Integrated pituitary adaptations to the low estrogen state

Estrogen's broad trophic effects on anterior pituitary function are not fully understood. The compound 4-vinyl cyclohexene diepoxide (VCD) induces early ovarian failure and reduced E2 production, which may differentially affect anterior pituitary cells. We recently identified transcriptomic changes in pituitary gonadotropes at the single-cell level in females treated with VCD for 20 days, which were acyclic, with low serum estradiol (E2) levels and 8-12-fold increases in gonadotropins 138 days later.

We tested vehicle-control and VCD-treated female mice in metabolic cages and assayed serum hormone levels by EIA, and pituitary cell transcripts by qRT-PCR and single-cell RNA-seq.

VCD-treated mice showed normal metabolic health. Compared with controls, VCD-treated females exhibited a 5-fold elevation in serum pituitary thyroid-stimulating hormone (TSH) levels and near-normal thyroid hormone levels. Pituitary Tshb mRNA levels were reduced, and pituitary Gh, Prl, and Pomc mRNA levels were unchanged. Serum growth hormone (GH) levels were higher in VCD-treated mice than in controls 62 days after the start of treatment (perimenopause) and then reduced to control levels after 118-138 days. Ingenuity Pathway Analyses (IPA) of upregulated differentially expressed gene (DEG) transcripts predicted activation of G protein-mediated signaling pathways in one scRNA-seq somatotrope subcluster and in the thyrotrope and lactotrope clusters. In contrast, downregulated DEGs in corticotropes and the remaining two somatotrope clusters predicted downregulation of critical signaling pathways.

The unchanged pituitary Gh, Prl, and Pomc mRNA levels correlated with unchanged serum levels. The interim rise in serum GH 62 days after treatment agrees with reports of perimenopausal responses due to estrogen spikes. Reduced pituitary Tshb correlates with the recently reported reduced expression in gonadotropes. IPA analysis of upregulated DEGs in thyrotrope clusters predicted the activation of canonical signaling pathways that support elevated serum TSH levels. The near-normal thyroid hormone levels suggest subclinical hypothyroidism in menopausal women. Downregulated DEGs in corticotropes and in two somatotrope subclusters predicted compromised stress and metabolic responses. Finally, reduced multihormone gene expression suggested compromised plasticity in a low-estrogen environment. These different responses to a sustained low-estrogen state, such as that seen in menopause, reflect the diversity of estrogen's trophic actions.