Endocrine Abstracts

The adrenal glands serve as central organs of the endocrine system by producing steroid hormone essential for organismal homeostasis. Mechanisms ensuring proper adrenal homeostasis and function are therefore crucial for maintaining human life. ACTH, released by the pituitary corticotrope, is required for the differentiation of the inner part of the adrenal cortex (the zona fasciculata) and the resultant stimulation of cortisol production. Perturbation of ACTH signaling can lead to diverse pathologic manifestations such as adrenal hyperplasia, hypertrophy, hormone overproduction or adrenal insufficiency. For example, elevated ACTH is a characteristic of patients suffering from Congenital Adrenal Hyperplasia (CAH), a set of defects in cortisol synthesis due to autosomal recessive mutations in genes encoding steroidogenic enzymes. To study the complexity of the ACTH-responsive cell population in the adrenal cortex, we performed single-cell RNAseq of the steroidogenic lineage in the adult mouse adrenal. We identified Hhex as a transcription factor with a restricted expression to the ACTH-responsive zona fasciculata. Although the role of HHEX in adrenal biology is completely unknown, a meta-analysis identified a germ-line variant of uncertain significance near the gene HHEX associated with increased adrenal androgen production in humans. Interestingly, we have also observed an increase in Hhex expression in a mouse model of CAH. Together with our scRNAseq data, we hypothesized that HHEX contributes to the unique function of the ACTH-responsive inner cortex. To define the role of HHEX in adrenal homeostasis, we generated Hhex knockout mouse models. KO mice exhibited progressive adrenomegaly by 15 weeks of age, accompanied by hypertrophy, most prominent in the inner cortex. Expression of the transcription factor Nr5a1/Sf1 and steroidogenic enzymes involved in corticosterone production were significantly upregulated at 6 weeks old, prior to adenomegaly. For insights into the signaling pathways controlled by HHEX, we analyzed global transcriptional changes during loss of Hhex and found dramatic downregulation of members of the membranous progesterone receptor family (PAQR). PAQR signaling has been implicated in the downregulation of cAMP, a primary mediator of ACTH signaling. Thereby, we speculate that down-regulation of PAQR and subsequent increased ACTH sensitivity could drive the increase in steroidogenesis in Hhex KO mice. These findings suggest that HHEX provides a unique autocrine/paracrine intra-adrenal feedback to ACTH-driven cAMP signaling by upregulating PAQR in a unique progesterone-responsive cell population in the inner cortex. As a result, we are currently assessing the implication of HHEX in contributing to the hypertrophy and steroidogenic phenotype observed in CAH.